Download Weather NOTES

Characteristics of the
Atmosphere
•The atmosphere is a mixture of gases and particles that
surrounds the earth, like an ocean of gases
•The study of the atmosphere is called meteorology
•Weather is the general daily condition of the atmosphere…it
includes, air temperature movements, and moisture content
•Climate is the general weather conditions over time…yearly or
seasonally
Composition of the Atmosphere
• The atmosphere, or air, is a mixture of chemical elements and
compounds
• The most abundant elements in the air are Nitrogen, Oxygen, and
Argon…the most abundant compounds are CO2 and water vapor
• Most water vapor comes from oceans…plants, lakes, etc. account for
the rest
• As water is added to the atmosphere by evaporation, it is being
removed through condensation and precipitation…the % varies based
on location, time of day, season
Layers of the
Atmosphere
• As altitude increases, air
pressure decreases rapidly, as
well as the density of the air
• There are no sharp pressure
changes that divides the
atmosphere into layers
• There are distinct
temperature changes though
• Based on temperature
changes, scientists identify
four layers to the atmosphere
Tropos
phere
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Closest to the earth’s surface
Almost all weather changes occur in this layer
Nearly all water vapor and Carbon Dioxide is found here
Temperatures go down as altitude goes up
The temperature remains constant in an area called the
tropopause…at about 10 km up…it stops at 20 km up.
Stratosphere
• Extends upward from the tropopause to an altitude of 50
km (30 mi)
• Temperature begins to increase with altitude…result of
absorption of energy by the ozone layer…almost all ozone is
found in the stratosphere
• The stratopause marks the upper boundry of the
stratosphere
Ozone in the Stratosphere
• Ozone is an important substance in the
atmosphere
• It has three oxygen atoms per molecule O3
• Protects the Earth’s inhabitants by absorbing UV
rays from the sun
• Ozone is located in the stratosphere
• Humans are damaging the ozone layer
Mesosphere
• Extends to an
altitude of 80 km
(50 miles)
• Temperature once
again decreases with
altitude
• It is the coldest
layer of the
atmosphere
• (-1000 C)
• The upper boundary
is called the
mesopause
Thermosphere
• Temperature increases
with altitude
• Nitrogen and oxygen
atoms absorb solar
energy…explains the
high temperatures
• The air is very thin the
lower region (80 km to
550km) is often called
the ionosphere
• Above the ionosphere
is the region where the
earth’s atmosphere
blends to
interplanetary
space…this is called the
exosphere
Air Pressure
• Air pressure is the force
of air molecules pushing
on an area
• More force = more
pressure
• Pushes in all directions
• Air pressure decreases
with altitude
• The density of air also
decreases with altitude
because the molecules
aren’t as compacted
• Barometers measure air
pressure
Variations in
Temperature
• Radiation from the sun does
not heat the earth the same
everywhere
• Location is one factor
• Temps are higher near the
equator than the poles
• Slanting rays spread their
energy
• Elevation is another factor
• Water vapor in the air
• The sun’s rays are highest
and most direct in the
northern hemisphere from
march 21 to sept. 21
• Uneven heating of the Earth
causes global winds
Convection and
Wind
• Convection is the movement of gas and liquids when they are
heated unevenly. Heated air rises, cools, then falls
• The atmospheric pressure is generally lower beneath a body of
warm air than it is under cool air
• As dense (heavy) cool air moves into a low pressure region, the
less dense warn air is pushed upward
• Movement of cool air is always toward regions of lower pressure
• These pressure differences, which results from unequal heating
from convection, causes wind or air moving parallel to the
ground
Global Winds
• If Earth didn’t rotate, winds would
flow from the poles to the equator
• Coriolis Effect is the influence of
Earth’s rotation on winds, causing
them to curve
• In the Northern hemisphere they
curve right, in the south, left
• Causes global wind belts
• The doldrums are low-pressure
zones near the equator-fuels
tropical storms
• The horse latitudes are highpressure zones at about 300 N and
S
• Trade winds blow from the east
• Westerlies blow from the west,
bring storms to the USA
• Easterlies blow from the east
Winds (cont.)
• The jet stream flows in the
upper troposphere from west
to east at high speeds
• Like global winds it forms
from uneven heating
• Each hemisphere usually has
2 jet streams, polar and
subtropical
• Local winds change daily –
sea breezes and valley
breezes
• Monsoons change with the
seasons – cause flooding
Humidity
• Humidity is the amount of water
vapor in the air
• As water evaporates, humidity
increases
• When the air holds all the water
vapor it can at a given temp, it is
saturated
• The amount of water vapor the air
can hold depends on temperature.
The hotter it is, the more it can hold
• Relative humidity is the amount of
water vapor in the air at a given
temperature, divided by the amount
it can hold at its saturation point
• It also changes if the humidity of
the air remains the same and the
temperature goes up or down…if
temp goes up, humidity goes down &
vise versa
Dew Point
• Warm air can hold more water
vapor than cold air
• Dew point is the temperature
to which air must be cooled to
reach its saturation point
• If the air is nearly saturated,
only a small drop in
temperature is needed to
reach the dew point
• If the dew point falls below
the freezing point of water,
frost forms
Fog
• Condensation occurs when water
vapor turns into a liquid
• May be in the form of dew, fog, or
clouds
• They all require saturated air to
develop
• Saturation occurs when water
vapor is added to the air, or when
the air is cooled to its dew point
• In fog, the ground exchanges heat
with the air
• 3 common: advection – moist air
over a warm surface; radiation –
surfaces cool rapidly; upslope –
humid air moves up a slope
• During the night the ground cools
rapidly, along with the air above it
causing condensation…this forms
fog
• The difference between fog and
clouds is the manner and location
that they are formed – fog has a
base near the ground
Air & Clouds
• When air is compressed it warms, when it
expands it cools…this is called adiabatic
cooling
• Process responsible for most cloud
formation
• As air rises it is under less pressure and it
expands, therefore cooling down
• Orographic lifting occurs when elevated
terrains, such as mountains, act as barriers
to flowing air, the must go up! (most of the
rainiest places on earth are located on the
windward slope)
• On the leeward side of a mountain, most of
the moisture is lost, creating a rain-shadow
desert
• Frontal wedging also contributes to the
lifting of air…cold air lifts warm air
• Convergence happens when air masses flow
together…generates an upward
movement…when they meet the air has to
go somewhere so it creates a column of air
Clouds
• Remember the forms of
condensation…dew, fog, and clouds
• For any of these to occur, the air
must be saturated
• Saturation occurs when air is
cooled to its dew point, or water
vapor is added to the air
• There must be a surface on which
the water vapor can
condense…condensation nuclei
(dust, smoke, salt)
• Condensation alone is not
responsible for rain, several
processes may cool the air
Types of Clouds
• There are 3 major types of clouds,
classified by their altitude and
shape
• They are stratus, cumulus, and
cirrus clouds
• Stratus (sheet/layered) clouds are
the lowest…form when a layer of
warm, moist air lies above a layer
of cool air…2 variations of these
are nimbostratus, which bring
heavy rain and snow, and
altostratus, meaning “high”
• Cumulus (piled/heaped) are above
stratus…they are thick with high
tops…form when air rises and
cools…the lowest are called
stratocumulus…they usually form
in fair weather…cumulonimbus are
thunderclouds
• Cirrus are the
highest…cirrocumulus clouds are
extremely high and form before
rain or snow
Precipitation
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Any moisture that falls from
the air to the earth’s surface
is called precipitation
Liquid precipitation falls in the
form of drizzle and rain. Rain
is the size of .5mm to 5mm in
diameter…drizzle is anything
less than .5mm
Solid precipitation is in the
form of snow, sleet, and hail
Snowflakes form ice crystals
and can be small pellets or
large snowflakes
Sleet is produced when rain
falls through a layer of
freezing air
Hail usually forms in
cumulonimbus clouds.
Convective currents within the
clouds carry raindrops to high
levels, where they freeze.
This may happen numerous
times, while hail accumulates
layers of ice
Precipitation falls when the
droplets in a cloud become
large enough to fall…they
coalesce to one another
Fronts
• When two air masses meet,
temperature differences
prevent them from mixing
(however some does
mix)…warm air rises, cool
air stays low…a boundary
exists between the air
masses called a front
• Fronts are boundaries
between air masses of
different densities
• Changes in weather usually
occur along a front
• For a front to form, one
air mass must collide with
the other…no matter which
air mass is advancing, it is
always the warmer, less
dense air that is forced up
Types of Fronts
• When a cold air mass
overtakes a warm, a cold
front is formed. Cold fronts
generally move faster than
warm fronts
• The cold air lifts the warm
air…if the warm air is moist,
clouds (cumulus or
cumulonimbus) form…drop in
temp and increased winds
come with the front…this is
frontal wedging
• Storms created along a cold
front are usually short lived
and violent
• Fast moving fronts produces
more concentrated
cloudiness and precipitation
than a slow moving one
• A long line of heavy
thunderstorms is called a
squall line
Warm Fronts
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Warm air masses that overtake cold
ones are called warm fronts
Less dense warm air rises over the
cooler air, producing a distinct pattern
of clouds…it begins with cirrus, then
cirrostratus, followed by altostratus,
then nimbostratus, and finally stratus
A warm front generally produces
light/moderate precipitation over a
large area…violent storms may be
produced if the air is moist…a gradual
increase in temperature is accompanied
by a warm front
Sometimes when two fronts meet,
neither is displaced, this is called a
stationary front …they both move
parallel to one another…weather is
similar to a warm front
As occluded front is produced when a
fast moving cold front overtakes a warm
front, lifting it off the ground…weather
is complex here, precipitation usually
occurs due to the rising of warm air
Thunderstorms
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Result of vertical movement of warm
unstable air
Associated with cumulonimbus clouds that
generate heavy rainfall, thunder,
lightening, and occasionally hail and
tornadoes
At any given time there are 2000
thunderstorms going on…45,000 per
day…lightening strikes 100 times/second
Most occur with short lived cumulonimbus
clouds that produce precipitation…require
warm, moist air
Thunderstorms are most common in the
afternoon and early evening…due to the
increase in surface temperatures needed
for instability
A continuous supply of moist air is needed
to create a cumulonimbus cloud…which
create updrafts of 60 mph
After about an hour, the amount of
precipitation becomes too great for the
updraft to support, so a downdraft results
in part of the cloud, creating precipitation
Eventually a total downdraft develops
Tornadoes
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Storms of short duration but some of the
most destructive
A rotating column, or vortex, of air that
spirals down from a cumulonimbus cloud
Air from the ground rushes into the
tornado from all directions, spiraling
upward into the cumulonimbus tower
Wind speeds can reach 300 mph
Can occur anytime, but April through June
is the period of greatest frequency in the
United States
They form with severe thunderstorms,
producing high winds, heavy rain, and hail
Meteorologists are still not use what
triggers tornado formation but believe
that they are the product of strong
updrafts in the thunderstorm and winds in
the troposphere
Usually spawned along the cold front of a
middle-latitude cyclonic front…cold, dry
air from the poles meets warm, moist air
from the Gulf…the greater the
differences in air masses, the more
intense the storm
These air masses are most likely to meet
in the central United States, because
there is no natural barrier separating air
from the arctic or the Gulf
Hurricanes
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The greatest storms on Earth
Most hurricanes form between latitudes of 5
and 20 degrees
They have different names in different parts
of the world…in the western pacific, they are
called typhoons…in the Indian Ocean they are
called cyclones
Winds of 75 mph are considered category 1
hurricanes
Average 375 miles in diameter, and extend to
40,000 feet above sea level
Extremely low pressure area
The eye wall is an ring of cumulonimbus clouds
and the center of the storm…has the greatest
wind speed and heaviest rainfall
At the top of a hurricane the flow is outward,
providing more room for inward flow at the
surface
The eye is about 13 miles in diameter where
there is no rain or wind
Large quantities of warm, moisture rich air is
required to keep the storm going…the moisture
condenses
The ocean water needs to be at a temperature
of 800 F at a depth of 150 feet for hurricanes
to form…heated air rises, reducing the pressure
Start as depressions, then become tropical
storms