General circulation, part 2 Download

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MAR 110: Introductory
Oceanography
The atmosphere and ocean
Oceans and climate, part 1
• In the middle latitudes, westerly winds moderate the
climates of continents on eastern sides of the ocean
basins.
– Western Europe’s climate is milder than that of Eastern
North America in that the difference between summer and
winter temperatures is less.
• SSTs change little over the course of the year.
• Gulf stream adds heat and water vapor to atmosphere above,
especially during winter.
• Cork, Ireland (51° 54′ N): 4.5 °C in January; 15.5 °C in July.
• Saskatoon, Saskatchewan (52° 8′ N): -18.5 °C in January; 18.0 °C
in July.
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Oceans and climate, part 2
• What would happen if the influence of the westerlies
weakened?
– Winters were much colder in Western Europe during some
periods of the past.
– In the Little Ice Age (1400 to 1850 CE), sea ice expanded
over the North Atlantic, montane glaciers advanced, and
growing seasons shortened.
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Oceans and climate, part 3
• Proxy evidence shows a number of long cold spells in
the last 10,000 years.
– Average temperatures have been as much as 7 °C colder.
– Weakening circulation in the North Atlantic may have
played a role.
– Wallace Broecker (of Lamont-Doherty Earth Observatory)
first proposed that changes in thermohaline circulation
caused by fluctuating amounts of fresh water in the North
Atlantic may trigger major climate swings (as in the movie
The Day After Tomorrow.
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Earth-atmosphere system
• We will examine radiational heating and cooling of
the earth-atmosphere system.
• The oceans support the atmosphere by playing a vital
role in the Earth’s heat budget, transporting solar
energy from the tropics – where there is a surplus – to
the poles – where there is a deficit.
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Weather
• Weather is the instantaneous state of the atmosphere
at a given place and time.
– Weather may be described in terms of variables such as
temperature, humidity, cloudiness, precipitation, and wind
speed and direction.
– Weather varies continuously from place to place and time
to time.
• “If you don’t like the weather, wait a minute.”
– Meteorology is the study of the atmosphere and the
processes that cause weather.
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Climate, part 1
• Climate refers to the pattern or average of weather
conditions over a long period, encompassing mean
characteristics, variability, and extremes.
– By international convention, climate variables are averaged
over a 30-year period beginning with the first year of a
decade (e.g., 1971-2000). The averaging period is shifted
forward 10 years with the beginning of a new decade.
– These 30-year averages (normals) of monthly temperature
and precipitation variables are used to describe climate.
• Seasonal variables, length of growing season, percent of possible
sunshine, and number of days with dense fog among other
important variables.
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Climate, part 2
• Climate is probably the most important
environmental control, affecting agriculture, water
supply, heating and cooling requirements for
buildings, weathering and erosion processes, and
much, much more.
• Climatology is the study of climate, its controls, and
its spatial and temporal variability.
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The role of the sun
• How does energy flow into and out of the Earthatmosphere system?
– The sun drives the atmosphere, providing the energy that
powers atmospheric and oceanic circulation and storms.
• The sun emits electromagnetic energy which strikes
the Earth’s atmosphere – some of which is absorbed
and converted into other forms of energy, like heat
and the kinetic energy of wind and water currents.
– First law of thermodynamics: Energy can be transferred
and transformed, but not created nor destroyed.
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Electromagnetic spectrum
• The energy from the sun that bathes the Earth is part
of the electromagnetic spectrum, which is energy that
possesses electrical and magnetic properties.
• Electromagnetic energy exhibits properties of waves,
thus wavelength or frequency are effective measures
to use to classify energy bands within the spectrum.
– Electromagnetic energy ranges from radio waves to gamma
waves.
– One energy band grades into another.
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Wave properties
• Wavelength is the distance between successive wave
crests or successive wave troughs.
• Frequency is the number of waves that pass a given
point in a specified amount of time.
– Passage of a complete wave is called a cycle.
– The typical measure of frequency is the number of cycles
per second, or hertz (Hz).
• Wavelength and frequency are inversely proportional.
– The higher the frequency, the shorter the wavelength.
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Radiation bands, part 1
• There are seven primary bands of electromagnetic
radiation.
– Gamma rays, x-rays, and ultraviolet radiation: These three
bands occur naturally, but can also be produced artificially.
• Gamma rays have the shortest wavelengths (and highest
frequencies) and contain the most energy.
• X-rays have slightly longer wavelengths (and shorter frequencies)
• Ultraviolet waves are the short-wavelength band adjacent to the
visible spectrum.
• All three are too short to be seen by the human eye, and they can
cause considerable damage to living organisms if they reached the
surface, but the atmosphere filters them out.
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Radiation bands, part 2
• Seven primary bands (continued):
– The visible spectrum, ranging from wavelengths of from
0.4 to 0.7 micrometers, makes up only 3 percent of the
electromagnetic spectrum, but it represents a large portion
of solar energy.
• Visible light is necessary for many activities of plants and animals,
such as photosynthesis and daylength-induced control of
reproduction.
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Radiation bands, part 3
• Seven primary bands (continued):
– Infrared light, the portion of the electromagnetic spectrum
with wavelengths slightly longer than the visible spectrum,
cannot be seen by the human eye.
• It is emitted by hot objects and is thus sometimes called heat rays.
• Radiation emitted by the Earth radiation is entirely in the infrared
region, but it represents only a small fraction of total solar radiation
– Microwave and radio waves are low wavelength, low
energy waves (radio waves have the longest wavelengths)
that are useful in communications, cooking, and weather
radar..
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Blackbody
• Blackbody: A blackbody is a perfect absorber and
emitter of energy.
• At a constant temperature, blackbodies absorb all
wavelengths of energy incident upon them, and emit
all wavelengths that they absorb.
– Emissivity is a measure of how closely an object resembles
a blackbody.
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Wien’s displacement law, part 1
• Wien’s displacement law: The wavelength of the most
intense radiation emitted by an object is proportional
to its temperature.
– max = C/T, where
• max is the wavelength of most intense radiation emitted by an
object
• C is a proportionality constant, and equals 2,897 if max is
expressed in micrometers
• T is the temperature expressed in degrees Kelvin
– In other words, hot objects emit radiation that peaks at
short wavelengths, while cool objects do the opposite.
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Wien’s displacement law, part 2
• The spectrum emitted by the sun is similar to that of a
blackbody at about 6,000 K.
• The spectrum emitted by the Earth is similar to that of
a blackbody at 288 K.
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Stefan-Boltzmann law
• Stefan-Boltzmann law: The total energy flux, or flow,
emitted by a blackbody across all wavelengths (E) is
proportional to the absolute temperature of the object
raised to the fourth power:
– E = T4, where the temperature is given in degrees Kelvin.
• In other words, a small change in the temperature of a
blackbody results in a large change in the total
amount of energy emitted by the object.
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Inverse square law
• Inverse square law: The intensity of radiation falling
on an object diminishes with increasing distance from
the source.
– Doubling the distance reduces the intensity by 75 percent.
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Global radiation equilibrium
The global radiation equilibrium is a balance between
radiation received by the Earth-atmosphere system and
that energy lost from the Earth-atmosphere system.
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The sun, part 1
• The sun is more than 80 percent hydrogen (by mass),
with most of the rest of its mast as helium.
– Its internal temperatures may exceed 20 million degrees C.
– It produces energy by the fusion of four hydrogen atoms to
form one helium atom.
• The mass of the four hydrogens exceeds that of one helium by 0.7
percent; the remaining mass is converted to energy.
– E = mc2, where
• E is energy
• m is mass
• c is the speed of light (300,000 km per second)
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The sun, part 2
• The visible surface of the sun is called the
photosphere.
– At about 6,000° C, the photosphere is much cooler than the
sun’s interior.
– Irregularly shaped convection cells, or granules, give the
photosphere its honeycomb-like appearance.
• Granules are about 1,000 km in diameter, with supergranules up to
50,000 km in diameter.
– The photosphere also features dark, cool areas called
sunspots.
• They may be as much as 1,800° C cooler than the photosphere.
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The sun, part 3
• The photosphere (continued):
– Faculae are bright areas near sunspots.
• The chromosphere, consisting of hydrogen and
helium ions, extends outward from the photosphere.
– Chromosphere temperatures range from 4,000° C to
40,000° C.
• The outermost portion of the sun is the corona, a
region of highly ionized gases with temperatures as
high as 4 million degrees C.
– The corona extends millions of kilometers into space.
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The sun, part 4
• The solar wind originates in the corona and sweeps
the solar system.
• Solar flares that originate in the photosphere intensify
the solar wind.
• The Earth intercepts only a portion of the energy
emitted by the sun.
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Solar altitude
• The intensity of solar radiation striking the Earth is a
function of the solar altitude, or the angle that the sun
is above the horizon.
– The higher the angle of the sun, the more intense the
radiation, with the most intense radiation striking areas
where the sun is directly overhead (at a 90° angle).
– It is the primary determinant of the amount of solar
radiation reaching a particular place on Earth.
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Daylength
• Daylength is important because the longer the day,
the more solar radiation can be received and the more
heat can be absorbed.
– Middle and high latitudes have pronounced seasonal
variations in day length, while tropical areas have little
variation.
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Atmospheric obstructions
• Atmospheric obstructions – such as clouds,
particulate matter, and gas molecules – absorb,
reflect, or scatter solar radiation.
– How much effect they have depends on path length, the
distance a ray must travel.
• Because angle of incidence determines path length, atmospheric
obstruction reinforces the pattern established by the varying angle
of incidence.
• Because they must pass through more atmosphere than high-angle
rays, low-angle rays are subject to more depletion through
reflection, scattering, and absorption.
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Latitudinal differences
• The latitudinal radiation balance occurs because the
belt of maximum solar energy swings back and forth
through tropics as the direct rays of sun shift
northward and southward in course of a year.
– Low latitudes (about between 28° N and 33° S) receive an
energy surplus, with more incoming than outgoing
radiation.
– There is an energy deficit in latitudes north and south of
these low latitudes.
• This simple latitudinal pattern is interrupted principally by
atmospheric obstruction.
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Earth’s revolution, part 1
• The tropical year is the time it takes Earth to
complete one revolution around the Sun; for practical
purposes it can be simplified to 365.25 days.
• Earth’s revolution is an ellipse, along which varies
the Earth-Sun distance.
– The varying distance between Earth and the Sun is not an
important determinant of seasonal temperature fluctuations.
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Earth’s revolution, part 2
• The two “end” points of the ellipse are the perihelion
and aphelion.
– The perihelion is the point in an orbit that takes a planet
nearest to the Sun (for Earth, it is 147,166,480 kilometers
or 91,455,000 miles, on January 3).
– The aphelion is the point in an orbit that takes a planet
furthest away from the Sun (for Earth, it is 152,171,500
kilometers or 94,555,000 miles, on July 4).
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The seasons, part 1
• The plane of the ecliptic is the imaginary plane that
passes through the Sun and through every point of
Earth’s orbit around the Sun.
– It is not perpendicular to Earth’s rotation axis, which allows
for seasons to occur.
• Inclination is the degree to which Earth’s rotation axis
is tilted (about 23.5 degrees away from the
perpendicular).
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The seasons, part 2
• Polarity, also called parallelism, occurs because
Earth’s axis always points toward Polaris, the North
Star, no matter where Earth is in its orbit.
• Insolation is incoming solar radiation.
– The angle at which the Sun’s rays strike Earth determines
the amount of insolation reaching any given point on Earth.
• That angle is a result of the combined effect of rotation, revolution,
inclination, and polarity.
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The seasons, part 3
• Solstices
– A solstice is one of two times during year in which the
Sun’s perpendicular (vertical) rays hit the northernmost or
southernmost latitudes (23.5°).
• On or about December 21 (called the winter solstice in Northern
Hemisphere).
• On or about June 21 (called the summer solstice in Northern
Hemisphere).
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The seasons, part 4
• Solstices (continued)
– The tropic of Cancer is the parallel of 23.5° north latitude,
which marks the northernmost location reached by the
vertical (perpendicular) rays of the Sun; occurs on or about
June 21.
– The Tropic of Capricorn is the parallel of 23.5° south
latitude, which makes the southernmost location reached by
the vertical (perpendicular) rays of the Sun; occurs on or
about December 21.
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The seasons, part 5
• Solstices (continued)
– The Arctic Circle is the parallel of 66.5° north latitude;
experiences 24 hours of either light (circa June 21) or dark
(circa December 21).
– The Antarctic Circle is the parallel of 66.5° south latitude;
experiences 24 hours of either light (circa December 21) or
dark (circa June 21).
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The seasons, part 5
• Equinoxes
– The Equinoxes are the times of year when the
perpendicular rays of the Sun strike the equator, the circle
of illumination just touches both poles, and the periods of
daylight and darkness are each 12 hours long all over Earth.
• On or about March 20 (called vernal equinox in Northern
Hemisphere).
• On or about September 22 (autumnal equinox in Northern
Hemisphere).
– The equinoxes represent the midpoints in the shifting of
direct rays of the Sun between the Tropic of Cancer and the
Tropic of Capricorn.
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The seasons, part 6
• Changes in daylight and darkness
– The period of daylight varies throughout the year,
increasing everywhere north of the equator from the
shortest day of the year on the December solstice until the
longest day of the year of the June solstice. Then days
begin to shorten again in Northern Hemisphere. (Southern
Hemisphere experiences an opposite effect.)
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The seasons, part 7
• Changes in daylight and darkness (continued)
– Both day length and the angle at which the Sun’s rays strike
Earth are principal determinants of the amount of insolation
received at any particular latitude.
• Tropic latitudes are always warm/hot because they always have
high Sun angles and consistent days close to 12 hours long.
• Polar regions are consistently cold because they always have low
sun angles.
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The solar constant, part 1
• The solar constant is the fairly constant amount of
solar insolation received at the top of the atmosphere
at a place perpendicular to the solar beam.
• The solar constant – which actually fluctuates slightly
during the year – is about 1.97 Langleys per minute,
or 1368 Watts per square meter (W/m2).
– A Langley is a unit of measure of radiation intensity that is
1 calorie per square centimeter (a calorie is the amount of
heat required to raise the temperature of 1 gram of water by
1°C).
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The solar constant, part 2
• The solar constant varies with distance of the Earth
from the sun, with the value is at the perihelion 2.04
Langley/min (or 1417 W/m2), and lowest at the
aphelion, 1.91 Langley/min (or 1326 W/m2).
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Radiation in the atmosphere
• Solar radiation reacts with gases and aerosols as it
travels through the atmosphere.
– Some will be either reflected back into space or scattered.
– Some will be absorbed.
• Radiation budget:
– Incoming solar radiation = percentage of radiation absorbed
(absorbtivity) + percentage reflected or scattered (albedo)
+ percentage transmitted to Earth’s surface (transmissivity)
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Reflectivity, part 1
• Reflection is the ability of an object to repel waves
without altering either the object or the waves.
– Reflectivity: The percentage of incoming solar radiation
that is reflected back into space.
– Albedo: the fraction of incoming solar radiation reflected
by a surface; it is calculated as the ratio of the ratio between
relflected radiation and insiden
• albedo = (reflected radiation/incident radiation)
• Dark surfaces have a low albedo and reflect relatively little solar
radiaiton; light surfaces have a high albedo and reflect a lot.
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Reflectivity, part 2
• Reflection (continued):
– Law of reflection: Angle of incident radiation = the angle
of reflected radiation
– Clouds are the most important reflector of solar radiation.
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Scattering
• Scattering is the process by which light waves change
in direction, but not in wavelength. Occurs in the
atmosphere when particulate matter and gas
molecules deflect wavelength and redirect them.
– Sometimes when insolation is scattered, the waves are
diverted into space; but most continue through atmosphere
in altered, random directions.
– Amount of scattering depends on wavelength of wave and
the size, shape, and composition of the molecule or
particulate.
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Absorption, part 1
• Absorption is the ability of an object to assimilate
energy from the electromagnetic waves that strike it.
• The absorbed energy is converted into heat.
• Different objects vary in their capabilities to absorb
radiant energy (and thus increase in temperature).
– Color plays a key role in an object’s absorption ability;
dark-colored surfaces more efficiently absorb the visible
portion of the electromagnetic spectrum than light-colored
surfaces.
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Absorption, part 2
• Oxygen, ozone, water vapor, and some aerosols
absorb solar heat.
– Stratospheric oxygen absorbs ultraviolet radiation at
wavelengths shorter than 0.2 micrometers.
– Stratospheric ozone absorbs ultraviolet radiation at
wavelengths between about 0.2 and 0.3 micrometers.
– The absorption by oxygen and ozone reduces the intensity
of ultraviolet light at the surface of the Earth and results in
a marked warming of the stratosphere.
– Water vapor absorbs some wavelengths greater than 0.8
micrometers.
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Absorption, part 3
• The atmosphere is more or less transparent to
wavelengths between 0.3 and 0.8 micrometers.
• Clouds absorb relatively little solar radiation.
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Albedo
• Light surfaces have high albedo.
– Fresh fallen snow may be as high as 95 percent.
• Dark surfaces have low albedo.
– Blacktop roads or spruce forests may be as low as 5
percent.
• Albedo may vary on some surfaces depending on the
angle of the sun above the horizon.
– The higher the sun is, the lower the albedo.
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Solar radiation and oceans
• While the atmosphere is relatively transparent to solar
radiation, the oceans are otherwise, absorbing much
solar radiation and transporting that heat to greater
depths than land surfaces.
– The various wavelengths are absorbed at differing depths.
• In clear water, reds are completely absorbed within 15 m, wheras
greens and blues penetrate to about 250 m.
– Content of particles and dissolved substances affect
absorption, with the colors that penetrate deepest shifting
toward yellow-green in coastal areas to red in very turbid
estuarine waters.
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Phase changes
• Tremendous amounts of energy are involved in phase
changes of water.
– Latent heat of melting: 80 cal are required to convert 1 g of
frozen water to liquid water at the freezing/melting point
• Temperature remains at 0 °C until all ice melts
– Latent heat of vaporization: varies, depending on initial
temperature of water
• 600 cal required to evaporate 1 g of liquid water at 0 °C
• 540 cal required to evaporate 1 g of liquid water at 100 °C
– Latent heat of sublimation: equals sum of latent heat of
melting plus latent heat of vaporization
• 680 cal required to evaporate 1 g of frozen water at 0 °C
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Sensible heating, part 1
• Heat transfer by conduction and convection can be
measured (sensed) by temperature changes.
• Sensible heating incorporates both conduction and
convection.
– Heating reduces the density of air, causing it to rise above
cooler, denser air.
• Convection thus transports heat from surface to troposphere
• Convection is more important than conduction
because air is a poor conductor of heat.
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Sensible heating, part 2
• Sensible and latent heating often work together.
– As air cools by convection, the water vapor in the air
condenses, thus releasing its latent heat as sensible heat –
and leading to the formation of cumulus clouds.
– The latent heat released by water vapor is converted into
sensible heat in the air. This in turn can lead to stronger
updrafts, as is seen in cumulonimbus clouds.
– By these processes, heat can also be transferred from the
atmosphere to the surface, such as on cold nights when
radiational cooling causes the surface to have a lower
temperature than the air above.
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Bowen ratio
• The Bowen ratio describes how heat energy received
at the Earth is partitioned into sensible and latent
heat.
– Bowen ration = [(sensible heating)/(latent heating)]
• Globally
– Bowen ratio = [(7 units)/(23 units)] = 0.3
• The Bowen ratio varies considerably by region and
surface type.
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Latitudinal differences, part 1
• There is unequal heating of different latitudinal zones
for four basic reasons, angle of incidence, day length,
atmospheric obstruction, and latitudinal radiation
balance:
• The angle of incidence is the angle at which rays
from the Sun strike Earth’s surface; always changes
because Earth is a sphere and Earth rotates on own
axis and revolves around the Sun.
– Angle of incidence is the primary determinant of the
intensity of solar radiation received on Earth.
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Latitudinal differences, part 2
• The angle of incidence (continued).
– Heating is more effective the closer to 90°, because the
more perpendicular the ray, the smaller the surface area
being heated by a given amount of insolation.
• Angle is 90° if Sun is directly overhead.
• Angle is less than 90° if ray is striking surface at a glance.
• Angle is 0° for a ray striking Earth at either pole.
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Latitudinal differences, part 3
• Day length is important because the longer the day,
the more insolation can be received and the more heat
can be absorbed.
– Middle and high latitudes have pronounced seasonal
variations in day length, while tropical areas have little
variation.
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Latitudinal differences, part 4
• Atmospheric obstructions – such as clouds,
particulate matter, and gas molecules – absorb,
reflect, or scatter insolation.
– How much effect they have depends on path length, the
distance a ray must travel.
• Because angle of incidence determines path length, atmospheric
obstruction reinforces the pattern established by the varying angle
of incidence.
• Because they must pass through more atmosphere than high-angle
rays, low-angle rays are subject to more depletion through
reflection, scattering, and absorption.
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Latitudinal differences, part 5
• The latitudinal radiation balance occurs because the
belt of maximum solar energy swings back and forth
through tropics as the direct rays of sun shift
northward and southward in course of a year.
– Low latitudes (about between 28° N and 33° S) receive an
energy surplus, with more incoming than outgoing
radiation.
– There is an energy deficit in latitudes north and south of
these low latitudes.
• This simple latitudinal pattern is interrupted principally by
atmospheric obstruction.
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Global radiation budget
• The Earth’s planetary albedo is 31 percent.
– The moon’s albedo is seven percent.
• The atmosphere absorbs 20 percent of the total solar
radiation.
• The Earth’s surface absorbs 49 percent of incoming
solar radiation.
• Reconciling the budget:
– 31 + 30 + 49 = 100
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Radiation budget, part 1
• The global and annual average energy budget (for
every 100 units incoming solar radiation):
–
–
–
–
31 units scattered and reflected to space
20 units absorbed by the atmosphere
49 units absorbed at the Earth’s surface
100 units total
• At the Earth’s surface
– 19 units lost due to infrared cooling
– 49 units gained by solar heating
– 30 units net heating
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Radiation budget, part 2
• The atmosphere
– 50 units lost due to infrared cooling
– 20 units gained by solar heating
– 30 units net cooling
• Heat transfer from Earth’s surface to atmosphere
– 7 units sensible heating (conduction plus convection)
– 23 units latent heating
– 30 units net transfer
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Heat transfer, part 1
• The tropics would become progressively warmer (and
less habitable) until the amount of heat energy
absorbed equaled the amount radiated from Earth’s
surface if not for two specific mechanisms moving
heat poleward in both hemispheres:
– Atmospheric circulation is the most important mechanism,
accomplishing 75 to 80 percent of all horizontal heat
transfer.
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Heat transfer, part 2
• Heat transfer mechanisms (continued):
– Oceanic circulation (ocean currents) reflect average wind
conditions over a period of several years.
• Current refers to various kinds of oceanic water movements.
• The atmosphere and oceans serve as thermal engines; their currents
are driven by the latitudinal imbalance of heat.
• There is a direct relationship between these two mechanisms:
– Air blowing over ocean is the principal driving force of major surface
ocean currents;
– Heat energy stored by ocean affects atmospheric circulation.
• Waters cooler than the overlying air act as a heat sink.
• Waters cooler than the overlying air act as a heat source.
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Whither weather?
• The sun heats the Earth.
• The variations in heating through time and space
generate radiation imbalances.
• The imbalances generate energy redistribution
mechanisms that are among the fundamental causes
of weather and climate variations.
– Weather systems do not last indefinitely, however, as
kinetic energy is dissipated in the form of frictional heat as
winds blow across the Earth’s surface.
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Air mass advection
• Air mass advection refers to the movement of air
masses from one region to another.
– Cold air advection occurs when wind transports colder air
over a warmer land surface.
– Warm air advection occurs when wind transports warmer
air over a colder land surface.
• The significance of air mass advection depends on the
initial temperature of the air mass and the degree of
modification it undergoes as it is transported.
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Air masses, part 1
• An air mass is a large parcel of air that has relatively
uniform properties in the horizontal dimension and
moves as an entity. Such extensive bodies are distinct
from one another and compose the troposphere.
• Characteristics
– An air mass must meet three requirements:
• It must be large (horizontal and vertical).
• Its horizontal dimension must have uniform properties
(temperature, humidity, and stability).
• It must be distinct from surrounding air, and when it moves, it must
retain that distinction (not be torn apart.
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Air masses, part 2
• Origin
– Air mass formation occurs if air remains over a uniform
land or sea surface long enough to acquire uniform
properties.
• Source regions are parts of Earth’s surface that are particularly
suited to generate air masses because they are
– Extensive
– Physically uniform
– Associated with air that is stationary or anticyclonic.
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Air masses, part 3
• Classification
– Because the source region determines properties of air
masses, it is the basis for classifying them.
– Classifications use a one- or two-letter code.
– The following table provides a simplified classification of
air masses, along with the properties associated with each.
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Air masses, part 4
• Movement and modification
– Some air masses remain in source region indefinitely.
– Movement prompts structural change:
• Thermal modification – heating or cooling from below;
• Dynamic modification – uplift, subsidence, convergence,
turbulence;
• Moisture modification – addition or subtraction of moisture.
– A moving air mass modifies the weather of the region it
moves through.
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Air masses, part 5
• North American air masses
– The physical geography of U.S. landscape plays a critical
role in air-mass interaction.
• There are no east-west mountains to block polar and tropical air
flows, so they affect U.S. weather and climate.
• North-south mountain ranges in the west modify the movement,
therefore the characteristics, of Pacific air masses.
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Air masses, part 6
• North American air masses (continued)
– Maritime tropical (mT) air from the Atlantic,
Caribbean/Gulf of Mexico strongly influences climate east
of the Rockies in the United States, southern Canada, and
much of Mexico.
• Primary source of precipitation. Also brings periods of
uncomfortable humid heat in summer.
– Continental tropical (cT) air has insignificant influence on
North America, except for bringing occasional heat waves
and drought conditions to the southern Great Plains
– Equatorial (E) air affects North America only through
hurricanes.
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Physics
• Force: a push or pull that can cause an object at rest
to move, or can affect the motion of an object already
in motion.
– Newton’s first law of motion says that an object in motion
tends to stay in motion and an object at rest tends to stay at
rest unless acted upon by an unbalanced force.
– The terms force and acceleration can be used
interchangeably, according to Newton’s second law of
motion:
• force = (mass) x (acceleration)
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Forces
• Forces that affect the motion of air parcels:
– Air pressure gradients
– Centripetal forces (actually occurs as a consequence of
other forces)
– Coriolis effect
– Friction
– Gravity
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Pressure gradient force
• The pressure gradient force results from the
difference in air pressure between two locations.
– The steeper the gradient, the greater the force, and vice
versa.
– Air flows from where the pressure is greatest to where it is
lowest.
– Horizontal pressure gradients at the surface can be denoted
by the spacing of isobars (lines of equal pressure).
– Pressure gradients are measured along lines perpendicular
to the isobars.
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Centripetal force
• Isobars plotted on weather maps are curved, thus the
wind blows in curved paths.
• The centripetal force is a force that confines an air
parcel (or any object to a curved path).
– If the force dissipates, the parcel flies off in a straight line
in keeping with Newton’s first law of motion (that an
object in motion stays in motion and an object at rest stays
at rest until being acted upon by some force.
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The Coriolis effect, part 1
• The Coriolis effect is the apparent deflection of free
moving objects to the right in the Northern
Hemisphere and to the left in the Southern
Hemisphere, in response to the rotation of Earth.
– The objects, which are moving in a straight line in the
atmosphere, appear to move a long a curved path as the
Earth’s surface rotates out from under the object.
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The Coriolis effect, part 2
• The Coriolis effect can significantly influence longrange movements.
• There are four basic points to remember:
– A free moving object appears to deflect to right in Northern
Hemisphere and to left in Southern Hemisphere;
– The apparent deflection is strongest at the poles and
decreases progressively toward the equator where there is
zero deflection;
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The Coriolis effect, part 3
• Four basic points to remember (continued):
– Fast-moving objects seem to be deflected more than slower
ones because the Coriolis effect is proportional to the speed
of the object;
– The Coriolis effect influences direction only, not speed.
• The Coriolis effect influences winds and ocean
currents, in particular serving as important
component of general circulation of oceans.
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The Coriolis effect, part 4
• It does not affect the circulation pattern of water
draining out of a washbowl – the time involved is too
short and water speed so slow; instead draining
direction is determined by the characteristics of the
plumbing system, shape of washbowl, and pure
chance.
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Friction, part 1
• Friction is the resistance an object encounters as it
moves against another object.
• Viscosity: Friction of fluid flow
– Molecular viscosity: results from the random motion of
molecules in a liquid or gas
– Eddy viscosity: results from the large, irregular motions
that develop within fluids
• Example: Effect of rocks in a fast-moving stream
• Snow fences demonstrate effects of frictional slowing of wind.
– Eddy velocity most important in meteorological processes
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Friction, part 2
• The rougher the surface of the Earth, the greater is the
eddy viscosity of the wind.
– Forest-covered landscapes have more eddy viscosity than
grass-covered ones.
• Horizontal wind speed increases with altitude, up to
about 1,000 m above the surface.
– The portion of the atmosphere below 1,000 m is called the
atmospheric boundary layer (ABL).
• Turbulence is fluid flow caused by eddy motion.
– Turbulence is often demonstrated as gusts of wind.
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Gravity
• All air parcels are subject to gravity.
• Gravity results from the interaction of two forces, the
centripetal force, and gravitation.
– Gravitation is the force of attraction between two objects.
• The magnitude of gravitation is the directly proportional to the
masses of the two objects and inversely proportional to the distance
between their centers of mass.
– The force (acceleration) of gravity is about 9.8 m/sec2
– Gravity acts directly downward (toward the heaviest
object’s center of mass.
– Gravity does not modify the horizontal wind.
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Joining forces, part 1
• The five forces (pressure gradient, centripetal,
Coriolis effect, friction, and gravity) interact to
control the horizontal and vertical motions of the
atmosphere.
–
–
–
–
Hydrostatic equilibrium
Geostrophic wind
Gradient wind
Surface winds
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Joining forces, part 2
• The hydrostatic equilibrium is point at which the
gravitational force is equals the vertical pressure
gradient force, such that the net vertical acceleration
of a parcel of air is zero.
• The geostrophic wind is a wind above the ABL that
moves parallel to isobars as a result of balance
between the pressure gradient force and the Coriolis
effect.
– Air parcels move in an oscillatory pattern which dampens
as they approach geostrophic equilibrium, this is called an
inertial oscillation.
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Joining forces, part 3
• Gradient wind is similar to geostrophic wind except
that it blows in a curved path as a result of
interactions among the pressure gradient force,
Coriolis effect, and centripetal forces.
– Centripetal forces prevent equilibrium, however.
– Gradient winds blow around anticyclones and cyclones
– Idealized anticyclone (Northern Hemisphere)
• Pressure gradient force: away from center of cyclone
• Coriolis effect: inward, slightly greater than pressure gradient force,
leading to inward centripetal force
• Clockwise flow (above ABL, parallel to isobars)
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Joining forces, part 3
• Gradient wind (continued):
– Idealized cyclone (Northern Hemisphere)
• Pressure gradient force: in toward center of cyclone, slightly greater
than Coriolis effect, leading to inward centripetal force
• Coriolis effect: outward
• Counterclockwise flow (above ABL, parallel to isobars)
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Surface winds, part 1
• Geostrophic and gradient winds are frictionless (they
occur at altitudes above the ABL).
• In the ABL, friction combines with the Coriolis effect
to balance the horizontal pressure gradient force.
– Friction works in a direction opposite to the wind direction.
– Coriolis effect operates at an angle perpendicular to the
wind direction.
– Friction slows wind velocity, thus weakening the Coriolis
effect, with the result that wind direction shifts toward low
pressure.
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Surface winds, part 2
• The effect of friction decreases with altitude to the
point where it is nil at the top of the ABL.
• Friction also affects winds flowing around
anticyclones and cyclones, shifting wind directions
toward low pressure.
– Anticyclone (Northern Hemisphere): clockwise circulation
that spirals outward
– Cyclone (Southern Hemisphere): counterclockwise
circulation that spirals inward
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General circulation, part 1
• According to an idealized circulation pattern (on a
non-rotating earth with a uniform surface), unequal
heating would create a two-cell circulation pattern.
– The cells, one in each hemisphere, would have rising air at
the equator and descending air at the poles.
– Winds at the surface would blow from the poles toward the
equator, and winds aloft would blow from the equator
toward the poles.
• The rotation of the Earth and its variable surfaces
create a complex atmospheric circulation pattern.
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General circulation, part 2
• Only the tropical regions have complete vertical
circulation cells, called Hadley cells.
– In a Hadley cell, heating at the equator warms the air
above, causing it to rise to elevations of about 15 km,
where it cools, moves poleward, then subsides. The air
descends at roughly 30° north or south latitude.
– There are two Hadley cells.
• Outside the tropical and subtropical latitudes, vertical
cells do not exist or are weakly and sporadically
developed.
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General circulation, part 3
• Global pressure patterns drive global wind patterns.
• The patterns observed migrate north and south with
the patterns of solar heating produced as the Earth
orbits the sun.
– The north-south migration of climate patterns is enhanced
over the continents and reduced over the oceans.
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General circulation, part 4
• The general circulation of the atmosphere has seven
surface components:
–
–
–
–
–
–
–
Subtropical highs
Intertropical convergence zone
Polar highs
Subpolar lows
Trade winds
Midlatitide westerlies
Polar easterlies
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General circulation, part 5
• Subtropical highs
– The subtropical latitudes serve as the “source” of the major
surface winds of the planet.
– The subtropical highs (STHs) are large semipermanent
anticyclones centered at about 30° latitude over the oceans
– Their average diameter is about 3,200 kilometers.
– They develop from the descending limbs of Hadley cells.
– The location of the subtropical highs are coincident with
most of the world’s major desert belts.
– Migration of the anticyclones affects weather of
midlatitudes.
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General circulation, part 6
• Subtropical highs (continued)
– The Horse latitudes are areas in the subtropical highs
characterized by warm, tropical sunshine and an absence of
wind.
• They exist because the weather within a subtropical high is nearly
always clear, warm, and calm.
– The subtropical highs serve as source for two of the world’s
three major surface systems:
• Trade winds
• Westerlies
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General circulation, part 7
• Intertropical convergence zone
– The intertropical convergence zone (ITCZ) is a belt of calm
air where the northeast trades and southeast trades
converge, generally in the vicinity of the equator (or at least
the heat equator).
• The zone is also called the equatorial front, the intertropical front,
and the doldrums.
• Intertropical convergence zone thunderstorms provide the updrafts
where all the rising air in of the tropics ascends.
• The zone often appears as a narrow band of clouds over oceans, but
it is less distinct over continents.
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General circulation, part 8
• Polar highs
– The polar highs are anticyclones centered over the polar
regions.
• The Antarctic high is quite different from the Arctic high because it
forms over an extensive, high-elevation, and very cold continent,
while the Artic high forms primarily over sea ice.
• The Antarctic high is strong, persistent, and almost permanent,
while the Arctic high is much less pronounced and more ephemeral.
• The polar highs are the source of the polar easterlies, which blow
toward the equator and toward (not from) the west.
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General circulation, part 9
• Subpolar lows
– The subpolar lows are a zone of low pressure situated at
about 50° to 60° of latitude in both the Northern and
Southern hemispheres.
• They often contain the polar front.
• The characteristics vary in either hemisphere because the continents
modify circulation in the Northern hemisphere, while circulation in
the Southern hemisphere is over a virtually continuous expanse of
ocean, the Southern Ocean.
• The polar front is the meeting ground of the cold polar easterlies
and the warm midlatitude westerlies, and is the site of genesis of
many midlatitude weatehr systems.
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General circulation, part 10
• Trade winds
– The trade winds are the major wind system of the tropics,
originating from the equatorward sides of the subtropical
highs and blowing toward the west as well toward the
equator.
– The trades are the most reliable of all winds in terms of
both direction and speed.
– They are named for the direction they blow from.
• In the Northern Hemisphere, they blow from the northeast, so are
called the northeast trades.
• In the Southern Hemisphere, they blow from the southeast, so are
called the southeast trades.
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General circulation, part 11
• Trade winds (continued)
– The trades are warming, drying winds, but are capable of
holding enormous amounts of moisture.
• They generally do not release moisture unless forced by a
topographic barrier or a pressure disturbance.
– The winds typically pass over low-lying islands, drying them of
moisture and turning them into desert islands.
– On the other hand, windward slopes exposed to the trades, as in the
mountains of Hawai’i, are some of the wettest places on Earth.
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General circulation, part 12
• Midlatitude westerlies
– The westerlies are the great wind system of the
midlatitudes, flowing from west to east around the world in
a latitudinal zone between about 30° and 60˚ both north and
south of the equator.
– They originate from the poleward side of the subtropical
highs, blowing toward the poles and toward the east.
– There are two cores of high-speed winds at high altitudes in
the westerlies:
• Polar front jet stream
• Subtropical front jet stream
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General circulation, part 13
• Midlatitude westerlies (continued)
– A major feature of the midlatitude westerlies are the
Rossby waves, sweeping north-south undulations that
frequently develop aloft.
• The undulating motion of the Rossby waves, coupled with the
migratory pressure systems and storms associated with the
westerlies, give the middle latitudes more short-run weather
variability than any other place on Earth.
• Anticyclonic circulation at the surface is associated with ridges in
the waves, while cyclonic circulation is associated with troughs in
the waves.
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General circulation, part 14
• Polar easterlies
– The polar easterlies are a global wind system that occupies
most of the area between the polar highs and about 60° of
latitude.
• The winds move generally from east to west and are typically cold
and dry.
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General circulation, part 15
• Vertical patterns of the general circulation
– Winds in the upper elevations of troposphere differ from
surface winds.
• The most dramatic difference occurs between surface trade winds
and the upper-elevation antitrade winds.
• Antitrade winds are tropical upper air winds that blow toward the
northeast in the Northern Hemisphere and toward the southeast in
the Southern Hemisphere.
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General circulation, part 16
• Vertical patterns (continued):
– Trade wind inversion
• Forms over tropical and subtropical oceans
• The air just over the surface forms the marine boundary layer.
• Air subsiding in the descending limb of the Hadley circulation
undergoes compressional warming.
• Where the descending air meets the marine boundary layer, a
temperature inversion, in which the upper air is warmer than the
lower air; this is the trade wind inversion.
• The air in the trade wind inversion is very stable, as the inversion
acts as a cap over the vertical movement of air, thus the convective
development of clouds and precipitation.
– It also limits orographic precipitation.
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Midlatitude cyclones, part 1
• A midlatitude cyclone is a large migratory lowpressure system that occurs within the middle
latitudes and moves generally with the westerlies;
midlatitude cyclones are also called lows or wave
cyclones, depressions.
– Midlatitude cyclones are probably most significant of all
atmospheric disturbances.
– They are basically responsible for most day-to-day weather
changes.
– They bring precipitation to much of the world’s populated
regions.
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Midlatitude cyclones, part 2
• Characteristics
– Typical mature midlatitude cyclone is 1,600 kilometers
(1,000 miles) in diameter; it has an oval shape.
– Patterns of isobars, fronts, and wind flow in the Southern
Hemisphere are mirror images of those in the Northern
Hemisphere.
– Northern Hemisphere patterns:
• Circulation pattern converges counterclockwise;
• Wind-flow pattern attracts cool air from north and warm air from
south; creates two fronts.
• These two fronts divide the cyclone into a cool sector north and
west of center and a warm sector south and east.
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Midlatitude cyclones, part 3
• Characteristics (continued)
– Northern Hemisphere patterns (continued)
• Circulation pattern converges counterclockwise;
• Wind-flow pattern attracts cool air from north and warm air from
south; creates two fronts.
• These two fronts divide the cyclone into a cool sector north and
west of center and a warm sector south and east.
• Size of sectors varies with location: on ground, cool sector is larger,
but in atmosphere, warm sector is more extensive.
• Warm air rises along both fronts, causing cloudiness and
precipitation, which follows patterns of cold and warm fronts.
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Midlatitude cyclones, part 4
• Characteristics (continued)
– Northern Hemisphere patterns (continued)
• Much of cool sector is typified by clear, cold, stable air, while air of
warm sector is often moist and tending toward instability, so may
have sporadic thunderstorms. May have squall fronts of intense
thunderstorms.
• Movements
– Midlatitude cyclones move throughout their existence.
• Movement is typically from West to East.
• Cold front moves faster than warm front.
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Midlatitude cyclones, part 5
• Lifecycle
– Origin to maturity typically takes 3 to 6 days, then another
3 to 6 days to dissipate.
– Cyclogenesis is the birth of cyclones.
– Most common cause believed to be upper-air conditions in
the vicinity of the polar-front jet stream.
– Most begin as waves along the polar front.
– Cyclogenesis can also occur on the leeward side of
mountains.
– Often bring heavy rain or snowstorms to the northeastern
United States and southeastern Canada.
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Midlatitude cyclones, part 6
• Lifecycle (continued)
– A comma cloud may be apparent at the peak of the storm.
– After cyclonic circulation is well developed, occlusion
begins.
– After occluded front is fully developed, cyclone dissipates.
• Occurrence and Distribution
– Occur at scattered but irregular intervals throughout the
zone of the westerlies.
– Route of cyclone is likely to be undulating and erratic, but
it generally moves west to east.
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Midlatitude cyclones, part 7
• Bomb cyclogenesis is the rapid development of a
cyclone.
– Bombs are defined as storms whose central pressure drops
by 24 mb in 24 hours.
– Most form in winter over warm water, such as over the
Kuroshio Current off Japan and the Gulf Stream off the
United States.
• The “Perfect Storm” of 1991 is a classic example of a bomb.
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The Fitz, part 1
• In 1976, Gordon Lightfoot, a Canadian
singer/songwriter, released a song, The Wreck of the
Edmund Fitzgerald, that recounts the November 10,
1975, sinking of the largest ore carrier on the Great
Lakes. The song, which became a huge hit, has a
haunting melody which seared the wreck of the ship,
with the loss of 29 lives, into the consciousness of
many who have heard Lightfoot’s recording.
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The Fitz, part 2
• The 222-meter-long ship, carrying 26,000 tons of iron
ore, departed the Duluth-Superior harbor on the
afternoon of November 9. The Fitz’s destination was
a plant at Zug Island on the Detroit River.
• At 0600 CST on November 9, a low-pressure system
began developing over central Kansas. It rapidly
intensified as it tracking toward the northeast.
• The storm passed near La Crosse, Wis., at 0600 on
November 10.
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The Fitz, part 3
• The storm was centered just west of Marquette,
Mich., at noon, with a central pressure of 982 mb.
– Gale-force northeast winds swept the eastern end of Lake
Superior, gusting to 115 km (71 mph) at Sault Ste Marie,
Mich.
• At 0100 on November 10, the Fitz reported northeast
winds of 97 km with winds to 3 m.
• At 0700, with the ship about 73 km north of Copper
Harbor, Mich., the ship reported northeast winds of
67 km.
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The Fitz, part 4
• The Fitz’s captain, Ernest McSorley, then chose a
course that took the ship through waters that sheltered
it from the strong northeast winds.
• The storm passed over the ship during the afternoon.
• The storm’s center approached Moosonee, Ontario, a
town on the shore of James Bay, that evening.
• By that time, winds over Lake Superior had shifted
from the northeast to the north, then to the northwest
and west.
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The Fitz, part 5
• The longer fetch (length of open lake surface exposed
to the wind) of the northwest and west wind fueled
the development of higher waves.
• A nearby ship, the Arthur M. Anderson, reported
winds of 95 km, gusting to 137 km, with waves of 3.5
m to 5 m.
• Sometime between 0615 and 0625, the Fitz
disappeared from the Anderson’s view as well as
radar, sinking in 163 m of water within 27 km of
Whitefish Point, Mich., and shelter from the wind.
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Midlatitude anticyclones, part 1
• A midlatitude anticyclone is an extensive migratory
high-pressure cell of the midlatitudes that moves
generally with the westerlies.
– They are typically larger than a midlatitude cyclone, but
also moves west to east.
– They travel at the same rate, or little slower, than
midlatitude cyclone.
• Anticyclones are prone to stagnate or remain over same region
(while cyclones do not).
• They can cause concentration of air pollutants.
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Midlatitude anticyclones, part 2
• Cold- and warm-core systems
– A cold-core anticyclone is a dome of cold air.
• In the Northern Hemisphere, continental polar (cP) or Arctic (A) air
masses are cold-core systems.
– cP air masses form from the polar high.
– A air masses form from the arctic high.
• Cold-core anticyclones are the most intense (with highest
pressures) in the winter.
– Powerful systems can bring freezing weather to South Florida.
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Midlatitude anticyclones, part 3
• Cold- and warm-core systems (continued):
– A warm-core anticyclone forms south of the polar front and
consists of subsiding warm, dry air.
• Warm-core anticyclones bring intense heat waves and droughts.
• The subtropical highs, such as the Bermuda High, are warm-core
anticyclones, but others may form over the interior of continents.
• Cyclones and anticyclones alternate with one another
in an irregular sequence.
– There is often a functional relationship between the two.
• An anticyclone can be visualized as a polar air mass with the cold
front of cyclone as its leading edge.
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Sea and land breezes
• Sea and land breezes are a common local wind
system along coastlines.
– They are essentially a convectional circulation caused by
differential heating of land and water surfaces.
– A land breeze is a local wind blowing from land to water,
usually at night (and normally considerably weaker flow
than that of sea breeze).
– A sea breeze is a local wind blowing from sea toward the
land, usually during the day.
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Greenhouse effect, part 1
• In the atmosphere, atmospheric gases, known as
greenhouse gases, transmit the incoming solar
shortwave radiation, which are absorbed by Earth’s
surface.
• Atmospheric gases do not transmit the outgoing
longwave terrestrial radiation, but instead absorb it,
then reradiate the terrestrial radiation back toward the
surface.
• Heat is then trapped in the lower troposphere.
• This is known as the greenhouse effect.
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Greenhouse effect, part 2
• It is more appropriately called the atmospheric effect,
because the warming of the atmosphere is not the
same as what happens in actual greenhouses, as
originally thought.
– Greenhouses stay warm because warm air is trapped inside
and does not mix with the cooler air outside, so the heat
does not dissipate.
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Greenhouse effect, part 3
• The warming up of the atmosphere is more similar to
what occurs in a closed automobile parked in the
sunlight.
– The window glass transmits shortwave radiation, which is
then absorbed by the upholstery.
– The car emits longwave radiation, which is not readily
transmitted through the glass.
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Greenhouse effect, part 4
• A natural greenhouse effect maintains a livable
temperature at the surface of the Earth.
– From space, the Earth radiates at -18° C.
– Average temperatures at the surface are about 15° C.
– The 33° C difference is a result of the natural greenhouse
effect.
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Greenhouse effect, part 5
• The gases in the atmosphere that trap and reradiate
longwave energy are called greenhouse gases.
– Major greenhouse gases include water vapor, carbon
dioxide, ozone, methane, and nitrous oxide.
• Water vapor is the principal greenhouse gas.
– These gases absorb relatively little radiation in wavelengths
near the Earth’s peak infrared intensity
– These windows of radiation transmission – in which little
or no radiation is absorbed – are called atmospheric
windows.
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Greenhouse effect, part 6
• Clouds are composed of water vapor, but their role in
the greenhouse effect are unclear.
– They can absorb and radiate infrared radiation, thus
warming the atmosphere.
– They can reflect solar radiation, thus cooling the
atmosphere.
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Global warming, part 2
• There are strong indications that this effect has been
intensified by human actions.
• According to climate data, the average global
temperature has increased about 0.6 degree C during
the 20th century, with the warmest records occurring
since 1990s.
• Measurements of this temperature increase, both
direct and proxy, have pointed toward a clear
warming trend on the Earth in recent decades.
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Global warming, part 3
• Although the climate changes do occur naturally, the
evidence is increasingly pointing to these changes
being caused by anthropogenic sources.
– The International Panes on Climate Change (IPCC)
released a report in 2001 discussing climatic changes on
both global and local scales and the strong evidence
pointing to this change being a result of human activities.
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Global warming, part 4
• Carbon dioxide and other “greenhouse gasses” appear
to be the principal offenders.
– Carbon dioxide is believed to be responsible for about 75
percent of the human-enhanced greenhouse effect.
– Since 1750 carbon dioxide levels have increased by more
than 30 percent.
– This increase in carbon dioxide is attributed to the
increased burning of fossil fuels in recent decades.
– The warming is further exacerbated by the burning of the
tropical rainforests which serve to absorb carbon from the
atmosphere.
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Global warming, part 5
• Principal offenders (continued):
– Carbon dioxide concentrations are the highest in 420,000
years and the rate of increase is greater than any in the past
20,000 years.
– Carbon dioxide concentrations may be double preindustrial
levels by the end of the century.
– A 50 percent decrease in fossil fuel consumption would be
required to eliminate the warming trend.
– The increased use of other gases (methane,
chloroflurocarbons, and nitrous oxides) have also
contributed to the increase in global temperatures.
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Global warming, part 6
• According to the IPCC Synthesis report, it is
estimated that:
– There will be an average global temperature increase of 1.4
to 5.8 degrees C between 1990 and 2100 – a climate
change greater in magnitude than anything in the past
10,000 years.
– Climate zones may shift poleward – by as much as 550 km
in North America.
– Sea levels may rise because of melting icecaps and thermal
expansion of seawater.
• Sea level rise will accelerate coastal erosion and flooding.
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Global warming, part 7
• Warming effects (continued):
– There will be an increase in climate variability.
– There will be increasing socioeconomic costs related to
weather damage.
• It’s not all bad news:
– Growing seasons may lengthen in higher latitudes.
– Savings in heating costs may offset cost of air conditioning
in middle and low latitudes.
– Seasonally dependent navigation may benefit from longer
ice- and snow-free periods.
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Global warming, part 8
• The warming has not been globally uniform, but
rather widespread.
– Warming so far has been greatest over North American and
Eurasia.
– Seven of the warmest years on record have occurred since
1990.
– On regional scales, there is clear evidence of changes in
variability or extremes, with a general trend in the
reduction of diurnal temperature range over more than 40
percent of the global landmass.
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Global warming, part 8
• Widespread warming (continued):
– There has been an increase in the number of frost free days,
and an increase in the number of days in the growing
season.
– Average sea level rise has been increasing at a rate of 1 to 2
mm per year.
– Arctic sea ice thickness had decreased between 10 percent
and 15 percent.
– Montane glaciers are in widespread retreat.
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Global warming, part 9
• The view from the crystal ball is obscure.
– Isolated events may affect climate in unforeseen ways.
• The eruption of Mount Pinatubo in the Philippines in 1991 emitted
massive amounts of sulfur dioxide (SO2) into the atmosphere.
• The sulfur dioxide mixed with water droplets to form sulfuric acid;
some formed sulfate aerosols.
• The sulfate aerosols, by absorbing, scattering, and reflecting solar
radiation back into space, reduced the amount reaching the Earth’s
surface, contributing to a reduction in global temperatures that
lasted until 1993.
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Global warming, part 10
• Some scientists do not believe global warming is
occurring.
– Some claim the observed warming is an artefact caused by
changes in instrumentation, urbanization, and gaps in the
global network of weather stations.
– Some argue that global climate models used to predict the
effects of a carbon-dioxide enriched atmosphere are flawed.
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Global warming, part 11
• Most scientists argue that the risks of doing nothing
outweigh the risks of taking steps to head off global
warming.
• Suggested solutions include:
–
–
–
–
Reducing fossil fuel consumption;
Increasing reliance on alternative energy sources;
Increasing energy efficiency;
Halting deforestation and reforesting areas previously
cleared.
• These suggested solutions have added benefits.
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