Download Atmosphere 2014 2015 - Republic School District

The Atmosphere:
Weather and Climate
Chapter 19 – The atmosphere
Chapter 20 – water in the atmosphere
Chapter 21 – weather
Chapter 22 - climate
Chapter 19 Section 1: Characteristics of the Atmosphere
Key Ideas
Describe the composition of Earth’s atmosphere.
Explain how two types of barometers work.
Identify the layers of the atmosphere.
Identify two effects of air pollution.
NGSS for Atmosphere
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HS-ESS2-1. Develop a model to illustrate how Earth’s internal and surface processes operate at different spatial and temporal scales to form continental
and ocean-floor features. [Clarification Statement: Emphasis is on how the appearance of land features (such as mountains, valleys, and plateaus) and seafloor features (such as trenches, ridges, and seamounts) are a result of both constructive forces (such as volcanism, tectonic uplift, and orogeny) and
destructive mechanisms (such as weathering, mass wasting, and coastal erosion).] [Assessment Boundary: Assessment does not include memorization of
the details of the formation of specific geographic features of Earth’s surface.]
HS-ESS2-2. Analyze geoscience data to make the claim that one change to Earth’s surface can create feedbacks that cause changes to other Earth
systems. [Clarification Statement: Examples should include climate feedbacks, such as how an increase in greenhouse gases causes a rise in global
temperatures that melts glacial ice, which reduces the amount of sunlight reflected from Earth’s surface, increasing surface temperatures and further
reducing the amount of ice. Examples could also be taken from other system interactions, such as how the loss of ground vegetation causes an increase in
water runoff and soil erosion; how dammed rivers increase groundwater recharge, decrease sediment transport, and increase coastal erosion; or how the
loss of wetlands causes a decrease in local humidity that further reduces the wetland extent.]
HS-ESS2-4. Use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate. [Clarification
Statement: Examples of the causes of climate change differ by timescale, over 1-10 years: large volcanic eruption, ocean circulation; 10-100s of years:
changes in human activity, ocean circulation, solar output; 10-100s of thousands of years: changes to Earth's orbit and the orientation of its axis; and 10100s of millions of years: long-term changes in atmospheric composition.] [Assessment Boundary: Assessment of the results of changes in climate is limited
to changes in surface temperatures, precipitation patterns, glacial ice volumes, sea levels, and biosphere distribution.]
HS-ESS2-5. Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes. [Clarification Statement:
Emphasis is on mechanical and chemical investigations with water and a variety of solid materials to provide the evidence for connections between the
hydrologic cycle and system interactions commonly known as the rock cycle. Examples of mechanical investigations include stream transportation and
deposition using a stream table, erosion using variations in soil moisture content, or frost wedging by the expansion of water as it freezes. Examples of
chemical investigations include chemical weathering and recrystallization (by testing the solubility of different materials) or melt generation (by examining
how water lowers the melting temperature of most solids).]
HS-ESS2-6. Develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere. [Clarification
Statement: Emphasis is on modeling biogeochemical cycles that include the cycling of carbon through the ocean, atmosphere, soil, and biosphere
(including humans), providing the foundation for living organisms.]
HS-ESS2-7. Construct an argument based on evidence about the simultaneous co-evolution of Earth’s systems and life on Earth. [Clarification Statement:
Emphasis is on the dynamic causes, effects, and feedbacks between the biosphere and Earth’s other systems, whereby geoscience factors control the
evolution of life, which in turn continuously alters Earth’s surface. Examples of include how photosynthetic life altered the atmosphere through the
production of oxygen, which in turn increased weathering rates and allowed for the evolution of animal life; how microbial life on land increased the
formation of soil, which in turn allowed for the evolution of land plants; or how the evolution of corals created reefs that altered patterns of erosion and
deposition along coastlines and provided habitats for the evolution of new life forms.] [Assessment Boundary: Assessment does not include a
comprehensive understanding of the mechanisms of how the biosphere interacts with all of Earth’s other systems.]
NGSS for Atmosphere
Disciplinary Core Ideas
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ESS1.B: Earth and the Solar System Cyclical changes in the shape of Earth’s orbit around the sun, together with changes in the tilt of the
planet’s axis of rotation, both occurring over hundreds of thousands of years, have altered the intensity and distribution of sunlight falling on the
earth. These phenomena cause a cycle of ice ages and other gradual climate changes. (secondary to HS-ESS2-4)
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ESS2.A: Earth Materials and Systems
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Earth’s systems, being dynamic and interacting, cause feedback effects that can increase or decrease the original changes. (HSESS21),(HS-ESS2-2)
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The geological record shows that changes to global and regional climate can be caused by interactions among changes in the sun’s energy
output or Earth’s orbit, tectonic events, ocean circulation, volcanic activity, glaciers, vegetation, and human activities. These changes can occur
on a variety of time scales from sudden (e.g., volcanic ash clouds) to intermediate (ice ages) to very long-term tectonic cycles. (HS-ESS2-4)
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ESS2.C: The Roles of Water in Earth’s Surface Processes.
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The abundance of liquid water on Earth’s surface and its unique combination of physical and chemical properties are central to the planet’s
dynamics. These properties include water’s exceptional capacity to absorb, store, and release large amounts of energy, transmit sunlight,
expand upon freezing, dissolve and transport materials, and lower the viscosities and melting points of rocks. (HS-ESS2-5)
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ESS2.D: Weather and Climate
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The foundation for Earth’s global climate systems is the electromagnetic radiation from the sun, as well as its reflection, absorption, storage,
and redistribution among the atmosphere, ocean, and land systems, and this energy’s re-radiation into space. (HS-ESS2-2),(HS-ESS2-4)
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Gradual atmospheric changes were due to plants and other organisms that captured carbon dioxide and released oxygen. (HS-ESS2-6),(HSESS2-7)
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Changes in the atmosphere due to human activity have increased carbon dioxide concentrations and thus affect climate. (HS-ESS2-6),(HSESS2-4)
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ESS2.E: Biogeology
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The many dynamic and delicate feedbacks between the biosphere and other Earth systems cause a continual co-evolution of Earth’s surface
and the life that exists on it. (HS-ESS2-7)
Gathering Data for Predictions
• From now until the end of the school year, two students will
record the weather conditions displayed on the Vantage
Pro 2 Weather Console.
• Rotation is alphabetical.
• Make sure you are looking at the readout for OUTSIDE
CONDITIONS…. Not inside conditions.
• Students will write down the temperature, pressure, wind
speed, wind direction, and observed cloud cover (if any) in
a station model format in the weather logbook found next
to the weather console.
Composition of the Atmosphere
• atmosphere a mixture of gases
that surrounds a planet, such as
Earth
• The most abundant elements in
air are the gases nitrogen (N2)
and oxygen (as O2 and O3).
• The two most abundant
compounds in air are the gases
carbon dioxide, CO2, and water
vapor, H2O.
• In addition to containing gaseous
elements and compounds, the
atmosphere commonly carries
various kinds of tiny solid
particles, such as dust and
pollen.
Atmospheric Nitrogen (N2)
• Nitrogen makes up about 78% of Earth’s atmosphere and is
maintained through the nitrogen cycle.
• Nitrogen is removed from the air mainly by the action of
nitrogen-fixing bacteria.
• The bacteria chemically change nitrogen from the air into
nitrogen compounds that are vital to the growth of all plants.
• Decay releases nitrogen back into the atmosphere.
Atmospheric Oxygen O2
• Oxygen makes up about 21% of Earth’s atmosphere.
• Animals, bacteria, and plants remove oxygen from the air as
part of their life processes.
• Land and ocean plants produce large quantities of oxygen
in a process called photosynthesis.
• The amount of oxygen produced by plants each year is
about equal to the amount consumed by all animal life
processes.
Atmospheric Carbon Dioxide (a primary greenhouse gas)
• Carbon Dioxide enters the atmosphere in many ways, to
include: volcanic outgassing and eruption, cellular
respiration, decay, and the burning of fossil fuels.
• Like water vapor, CO2 is an important greenhouse gas
which allows our planet to trap heat at the surface.
Atmospheric Water Vapor (a primary greenhouse gas)
• As water evaporates from oceans, lakes, streams, and soil,
it enters air as the invisible gas water vapor.
• Plants and animals give off water vapor during transpiration
or respiration. But as water vapor enters the atmosphere, it
is removed by the processes of condensation and
precipitation.
• The percentage of water vapor in the atmosphere varies
depending on factors such as time of day, location, and
season.
• ADD…. Energy input is required to vaporize water,
therefore water vapor in the air is energy in the air!!
Ozone in the Atmosphere O3
• ozone a gas molecule that is made up of three oxygen
atoms
• Ozone in the upper atmosphere forms the ozone layer,
which absorbs harmful ultraviolet radiation from the sun.
• Without the ozone layer, living organisms would be severely
damaged by the sun’s ultraviolet rays.
• Unfortunately, a number of human activities has damaged
the ozone layer.
The Ozone (Shield) Layer
• The ozone layer is a part of the stratosphere where
incoming ultraviolet solar radiation is absorbed by the tri
atomic molecule of oxygen called ozone.
• UV light is harmful to organisms because it can damage the
genetic material in living cells.
• By shielding the Earth’s surface from most of the sun’s UV
light, the ozone in the stratosphere acts like a sunscreen for
the Earth’s inhabitants.
Chemicals That Cause Ozone Depletion
• Chlorofluorocarbons (CFCs) are hydrocarbons in which
some or all of the hydrogen atoms are replaced by chlorine
and fluorine.
• They are used in coolants for refrigerators and air
conditioners and in cleaning solvents. They were also used
as a propellant in spray cans of everyday products such as
deodorants, insecticides, and paint.
• Their use is now restricted because they destroy ozone
molecules in the stratosphere.
• Once CFC molecules break apart, the chlorine atoms
chemically break down ozone.
Chemicals That Cause Ozone Depletion
• Each CFC molecule contains from one to four chlorine
atoms, and scientists have estimated that a single chlorine
atom in the CFC structure can destroy 100,000 ozone
molecules.
The Ozone Hole
• The ozone hole is a thinning of stratospheric ozone that
occurs over the poles during the spring. It was first
discovered in 1985, studies by scientists working in
Antarctica.
• This was the first news of the hole, and was published in
an article in the scientific journal Nature.
• After the results were published, NASA scientists reviewed
data that had been sent to Earth by the Nimbus 7 weather
satellite. They were able to see the first signs of ozone
thinning in the data from 1979.
The Ozone Hole
• Although the concentration of ozone fluctuated during the
year, the data showed a growing hole.
• Ozone levels over the Arctic have decreased as well. In
March 1997, ozone levels over part of Canada were 45
percent below normal.
Composition of the Atmosphere
Particulates in the Atmosphere
• The atmosphere contains various tiny solid and liquid
particles, called particulates.
• Particulates can be volcanic dust, ash from fires,
microscopic organisms, or mineral particles lifted from soil
by winds. Pollen from plants and particles from meteors
that have vaporized are also particulates.
• Large, heavy particles remain in the atmosphere only
briefly, but tiny particles can remain suspended in the
atmosphere for months or years.
5 Layers of the Atmosphere
• Earth’s atmosphere as a distinctive pattern of temperature
changes with increasing altitude.
• The temperature differences mainly result from how solar
energy is absorbed as it moves through the atmosphere.
• Scientists identify five main layers of the atmosphere based
on these differences.
– Troposphere
– Stratosphere
– Mesosphere
– Thermosphere
– Exosphere
The Troposphere (sphere of change)
• troposphere the lowest layer of the atmosphere, in which
temperature drops at a constant rate as altitude increases;
the part of the atmosphere where weather conditions exist
• At an average altitude of 12 km, the temperature stops
decreasing. This zone is called the tropopause and
represents the upper boundary of the troposphere.
The Stratosphere (sphere of layers)
• stratosphere the layer of the atmosphere that lies between the
troposphere and the mesosphere and in which temperature
increases as altitude increases; contains the ozone layer.
• In the lower stratosphere, the ozone layer absorbs most of the
Sun's UV radiation. It contains high concentrations of ozone (O3)
relative to other parts of the atmosphere
• In the upper stratosphere, the temperature increases as altitude
increases because air in the stratosphere is heated from above
by absorption of solar radiation by ozone.
• The stratopause, about 50 km above Earth’s surface, marks the
upper boundary of the stratosphere.
The Mesosphere (the middle sphere)
• mesosphere the coldest layer of the atmosphere, between
the stratosphere and the thermosphere, in which the
temperature decreases as altitude increases
• The mesosphere is located above the stratopause and
extends to an altitude of about 80 km.
• The upper boundary of the mesosphere, called the
mesopause, has an average temperature of nearly 90°C,
which is the coldest temperature in the atmosphere.
The Thermosphere
• thermosphere the uppermost layer of the atmosphere, in
which temperature increase as altitude increases; includes
the ionosphere
• Temperature increases as altitude increases because
nitrogen and oxygen atoms absorb solar radiation.
• The lower region of the thermosphere, at an altitude of 80 to
400 km, is commonly called the ionosphere.
• Interactions between solar radiation and the ionosphere
cause the phenomena known as auroras.
• There are not enough data about temperature changes in
the thermosphere to determine its upper boundary.
The Exosphere
• Above the ionosphere
is the region where
Earth’s atmosphere
blends into the almost
complete vacuum of
space.
• This zone of indefinite
altitude, called the
exosphere, extends
for thousands of
kilometers above the
ionosphere.
Chapter 21 Section 3: Weather Instruments
Station model: Recording Lower-Atmospheric Conditions
• station model a pattern of meteorological symbols that
represent the weather at a particular observing station and
that is recorded on a weather map.
• A simple station model, like the ones you will be recording
in the weather journal, is shown here.
Station model: Air Temperature
• thermometer an instrument that
measures and indicates air
(atmospheric) temperature
• A common type of thermometer uses
a liquid-usually mercury or alcoholsealed in a glass tube to indicate
temperature.
• A rise in temperature causes the liquid
to expand and fill more of the tube
(kinetic theory again). A drop in
temperature causes the liquid to
contract and fill less of the tube.
Station model: Air Temperature
• Another type of thermometer is an electrical thermometer.
• As the temperature rises, the electric current that flows
through the material of the electrical thermometer increases
and is translated into temperature readings.
• A thermistor, or thermal resistor, is a type of electrical
thermometer that responds very quickly to temperature
changes.
Station model – Temp, Relative Humidity, and Dew Point
• Dew point Temperature can be determined using the Air Temperature and
Relative Humidity and the chart below.
Station model – Temp, Relative Humidity, and Dew Point
• The temperature and dew point are plotted to left of
center on the station model and are expressed in
degrees Fahrenheit in the US.
• The dew point indicates how high the humidity of the
air is. When the air temperature drops to the dew point,
condensation forms.
• This knowledge can help determine location of fronts,
where storms and precipitation can occur.
• A front is a boundary separating two masses of air of
different densities.
Station model - Cloud formation/types
• cloud a collection of small water droplets or ice crystals
suspended in the air, which forms when the air is cooled
and condensation occurs
• For water vapor to condense and form a cloud, a solid
surface on which condensation can take place must be
available.
• The lowest layer of the atmosphere, the troposphere,
contains millions of suspended particles of ice, salt, dust,
and other materials that serve as solid surfaces.
• Because the particles are so small—less than 0.001 mm in
diameter—they remain suspended in the atmosphere for a
long time.
Station model - Cloud formation/types
• condensation nucleus a solid particle in the atmosphere
that provides the surface on which water vapor condenses
• In addition, for clouds to form, the air must be “saturated”
with water vapor. When the temperature of the air drops,
condensation occurs more rapidly than evaporation does.
Cloud Formation - Adiabatic Lapse Rate
• The rate at which the temperature of a parcel of air changes as the air
rises or sinks is called the adiabatic lapse rate. The adiabatic lapse rate
of clear air is about –1 °C per every 100 m that the air rises.
• The slower rate of cooling of moist air results from the release of latent
heat as the water vapor condenses into liquid water droplets.
Cloud Formation - Adiabatic Cooling
• As a mass of air rises, the surrounding atmospheric
pressure decreases.
• The molecules in the rising air move further apart, and
fewer collisions between the molecules happen.
• The resulting decrease in the amount of energy that
transfers between molecules decreases the temperature of
the air.
• adiabatic cooling the process by which the temperature of
an air mass decreases as the air mass rises and expands
Cloud Formation - Condensation Level
• When air cools to a temperature that is below the dew
point, net condensation causes clouds to form.
• The altitude at which this net condensation begins is called
the condensation level.
• The condensation level is marked by the base of the
clouds.
• Further condensation allows clouds to rise and expand
above the condensation level.
Cloud Formation - Mixing and Lifting of Air Masses
Mixing
• Some clouds form when one body of moist air mixes with another body
of moist air that has a different temperature.
• The combination of the two bodies of air causes the temperature of the
air to change.
• This temperature change may cool the combined air to below its dew
point, which results in cloud formation.
Lifting
• Air can be forced upward when a moving mass of air meets sloped
terrains, such as a mountains range.
• As the rising air expands and cools, clouds form.
• The large cloud formations associated with storm systems also form by
lifting.
• These clouds form when a mass of cold, dense air enters an area and
pushes a less dense mass of warmer air upward.
Cloud Formation - Advective Cooling
• advective cooling the process by which the temperature of
an air mass decreases as the air mass moves over a cold
surface
• As air moves over a surface that is colder than air is, the
cold surface absorbs heat from the air and the air cools.
• If the air cools below its dew point, clouds form.
Station model - Cloud formation/types
Adiabatic Cooling – reaching the dew point
• As a mass of air rises, the surrounding atmospheric
pressure decreases.
• The molecules in the rising air move further apart, and
fewer collisions between the molecules happen.
• The resulting decrease in the amount of energy that
transfers between molecules decreases the temperature of
the air.
• adiabatic cooling the process by which the temperature of
an air mass decreases as the air mass rises and expands
Station model - Cloud formation/types
Condensation Level
• When air cools to a temperature that is below the dew point,
net condensation causes clouds to form.
• The altitude at which this net condensation begins is called
the condensation level.
• The condensation level is marked by the base of the clouds.
• Further condensation allows clouds to rise and expand
above the condensation level.
Station model - Cloud formation/types
• Clouds are classified by their
shape and altitude.
• The three basic cloud forms or
shapes are:
– stratus clouds,
– cumulus clouds, and
– cirrus clouds.
• There are also three altitude groups:
– low clouds (0-2,000 m),
– middle clouds (2,000 to 6,000 m),
“alto”
– high clouds (above 6,000 m)
“cirro”
Station model - Cloud formation/types
Stratus Clouds - gray clouds that have a flat uniform base
and that commonly form at very low altitudes
• Stratus means “sheet-like” or “layered.”
• Stratus clouds form where a layer of warm, moist air lies
above a layer of cool air.
• Stratus clouds cover large areas of sky and often block out
the sun. Usually, very little precipitation falls from most
types of stratus clouds.
Station model - Cloud formation/types
• Three variations of stratus clouds are known as
nimbostratus, altostratus, and cirrostratus.
– Unlike other stratus clouds, the dark nimbostratus clouds
can cause heavy precipitation.
– Altostratus clouds form at middle altitudes and usually
produce very little precipitation.
– Cirrostratus clouds form at higher altitudes and usually
produce very little precipitation.
Station model - Cloud formation/types
(clockwise)
• cirrostratus.
• Altostratus
• Nimbostratus
Station model - Cloud formation/types
Cumulus Clouds are low-level, billowy clouds that commonly
have a top that resembles cotton balls and have a dark, flat
base which represents the condensation level.
• Cumulus means “piled” or “heaped.”
• These clouds form when warm, moist air rises and cools. As
the cooling reaches its dew point, the clouds form.
Station model - Cloud formation/types
• If the base of cumulus clouds begins at higher altitudes, the
clouds are called cirrocumulus clouds.
• If the base of cumulus clouds begins at middle altitudes, the
clouds are called altocumulus clouds.
• Low clouds that are a combination of stratus and cumulus
clouds are called stratocumulus clouds.
Station model - Cloud formation/types
• High, dark storm clouds known as cumulonimbus clouds, or
thunderheads, are often accompanied by rain, lightning,
thunder, and severe weather.
Station model - Cloud formation/types
• cirrus cloud a feathery cloud that is composed of ice crystals
and that has the highest altitude of any cloud in the sky
• Cirro– and cirrus mean “curly.”
• Cirrus clouds form at altitudes above 6,000 m. These clouds
are made of ice crystals because the temperatures are low at
such high altitudes.
• Because these clouds are thin, light can easily pass through
them
Station model - Cloud formation/types
• Cirrocumulus clouds are highaltitude, billowy clouds
composed entirely of ice
crystals. Cirrocumulus clouds
commonly appear just before
a snowfall or a rainfall.
• Long, thin clouds called
cirrostratus clouds form a
high, transparent veil across
the sky.
• A halo may appear around the
sun or moon when either is
viewed through a cirrostratus
cloud.
Station model – Cloud cover and type
• Along with wind direction
cloud cover is one of the
oldest atmospheric
conditions to be coded on a
station model.
• The circle in the middle of
the station model represents
cloud cover.
Station model - Cloud formation/types
Fog - water vapor that has condensed very near the surface
of Earth because air close to the ground has cooled
• The obvious difference between fog and clouds is that fog
is very near the surface of Earth. However, fog also differs
from clouds because of how fog forms.
Radiation Fog - One type of fog forms from the nightly
cooling of Earth.
• The layer of air in contact with the ground becomes chilled
to below the dew point, and the water vapor in that layer
condenses into droplets.
• This type of fog is called radiation fog because it results
from the loss of heat by radiation.
Station model - Air Pressure (barometric pressure)
• Atmospheric pressure is the
force per unit area exerted on a
surface by the weight of
atmosphere above it.
• 1013.25 millibars or hectopascals
= 1 atm (atmosphere)
• barometer an instrument that
measures atmospheric pressure.
•“We live submerged at the bottom of an
ocean of elementary air, which is known by
incontestable experiments to have weight" Evangelista Torricelli, 1644
Station model - Air Pressure (barometric pressure)
Air Pressure (barometric pressure)
• Highest and lowest recorded
– 1,085.7 hPa (32.06 inHg)
– lowest non-tornadic pressure was 870 hPa
• The approach of a front is usually indicated by a drop in
air pressure as surface air is lifted (convected).
Station model - Air Pressure (barometric pressure)
Air Pressure (continued)
• 2 commonly used types of
barometers mercury and aneroid
• Mercury barometers
– A glass tube with a height of at
least 84 cm, closed at one end,
with an open mercury-filled
reservoir at the open end.
Station model - Air Pressure (barometric pressure)
Air Pressure (continued)
• Aneroid barometers (no liquid)
– uses a small, flexible metal box called an aneroid cell
(capsule)
– The evacuated capsule is prevented from collapsing by a
strong spring. Small changes in external air pressure
cause the cell to expand or contract.
Station model - Air Pressure (barometric pressure)
• The atmospheric pressure is plotted on the top right corner
of the model.
• It shows the last two integer digits of the pressure in
millibars, or hectopascals (hPa), along with the first digit
after the decimal.
• For instance, if the pressure at a certain location is 999.7
hPa, the pressure portion of the station model will read 997.
• Most of the time, choosing first digits that would lead to
a value nearest to 1000 works out best.
• The plotting of this value within the station model allows for
the analysis of isobars on weather maps.
Station model – pressure tendency
•The pressure tendency figure is plotted below the pressure will lie and indicates the
pressure change over the past three hours.
•The number depicting the pressure change will usually have two digits and indicate
the pressure change in 0.1 millibar increments.
• Steady rises can indicate improving conditions and approaching high pressure,
and normally occur in the wake of a cold front.
•Steady falls can indicate deteriorating conditions and an approaching low pressure
area, with the largest falls occurring in advance of a surface cyclone and its
attendant warm front.
Station model – Wind speed and direction
Wind Speed
• anemometer an instrument used to measure wind speed
• A typical anemometer consists of small cups that are
attached by spokes to a shaft that rotates freely.
• The wind pushes against the cups and causes them to
rotate. This rotation triggers an electrical signal that
registers the wind speed in meters per second or in miles
per hour.
Station model – Wind speed and direction
Wind Direction
• wind vane an instrument used to determine direction of the
wind
• The wind vane is commonly an arrow-shaped device that
turns freely on a pole as the tail catches the wind.
• Wind direction may be described by using one of 16
compass directions, such as north-northeast. Wind
direction also may be recorded in degrees by moving
clockwise and beginning with 0° at the north.
Station model – wind speed and direction
• The station model uses a wind
barb to show both wind direction
and speed. The wind barb
shows the speed using "flags"
on the end.
– Each half of a flag depicts
five knots
– Each full flag depicts 10
knots
– Each pennant (filled triangle)
depicts 50 knots[4]
• Winds are depicted as blowing
from the direction the flags are
facing.
Station model – weather conditions
• To the left of the cloud
shape in the center of
the station model is
the symbol depicting
present weather.
• The visibility is shown
as a number to the left
of the present weather
symbol. It describes
how far the observer
can see at that time.
Phases of Water and Latent Heat
• Latent heat the heat energy that is absorbed or released
by a substance during a phase change.
Three processes which absorb latent heat
• Evaporation – Atmospheric heat is absorbed
when water changes from a liquid to a gas. The
greatest amount of water enters the atmosphere
through the evaporation of ocean water near the
equator.
• Melting – Atmospheric heat is absorbed by ice,
causing it to melt.
• sublimation the process in which a solid changes
directly into a gas. When the air is dry and the
temperature is below freezing, ice and snow may
sublimate into water vapor.
Three processes which release latent heat
• Condensation - the change of water vapor to liquid water
releases latent heat in the atmosphere. This is the source of
the energy for storms.
• Freezing - the change of liquid water to solid water (ice)
releases latent heat in the atmosphere.
• Deposition - the change of water vapor directly into a solid
releases latent heat in the atmosphere as frost forms.
Humidity
• Water vapor in the atmosphere is known as humidity.
• Humidity is controlled by rates of condensation and
evaporation.
• The rate of evaporation is determined by the temperature of
the air. The higher the temperature is, the higher the rate of
evaporation is.
• The rate of condensation is determined by vapor pressure.
When vapor pressure is high, the condensation rate is high.
• When the rate of evaporation and the rate of condensation
are in equilibrium, the air is said to be “saturated.”
Dew Point – water saturated air
• dew point - at constant pressure and water vapor content,
the temperature at which the rate of condensation equals
the rate of evaporation.
• At temperatures below the dew point, net condensation
occurs, and liquid water droplets form.
Absolute Humidity
•absolute humidity the mass of water vapor per unit volume
of air that contains the water vapor, usually expressed as
grams of water vapor per cubic meter of air
•However, as air moves, its volume changes as a result of
temperature and pressure changes.
•Therefore, meteorologists prefer to describe humidity by
using the mixing ratio of air.
mass of water vapor (grams)
absolute humidity =
volume of air (cubic meters)
Mixing Ratio
• The mixing ratio of air is the mass of water vapor in a unit
of air relative to the mass of the dry air.
• Because this measurement uses only units of mass, it is
not affected by changes in temperature or pressure.
Relative Humidity
• relative humidity the ratio of the amount of water vapor in
the air to the amount of water vapor needed to reach
saturation at a given temperature
• If the temperature does not change, the relative humidity
will increase if moisture enters the air.
• Relative humidity can also increase if the moisture in the air
remains constant but the temperature decreases.
• If the temperature increases as the moisture in the air
remains constant, the relative humidity will decrease.
Measuring Relative Humidity
Using a Psychrometer
• A psychrometer consists of two identical thermometers. The
bulb of one thermometer is covered with a damp wick. The
bulb of the other thermometer remains dry.
• When the psychrometer is held by a handle and whirled
through the air, the air circulates around both thermometers.
• The difference between the dry-bulb temperature and the
wet-bulb temperature is used to calculate relative humidity.
Reaching the Dew Point
• When the air is nearly saturated with a relative humidity of almost 100%,
only a small temperature drop is needed for air to reach its dew point.
• Air may cool to its dew point by conduction when the air comes in
contact with a cold surface.
• During the night, grass, leaves, and other objects near the ground lose
heat. When the temperature of air cools below the dew point,
condensation called dew is formed.
• If dew point falls below the freezing temperature of water, water vapor
may change directly into solid ice crystals, or frost.
• Because frost forms when water vapor turns directly into ice, frost is not
frozen dew.
• Frozen dew is relatively uncommon. Unlike frost, frozen dew forms as
clear beads of ice.
Forms of Precipitation
• precipitation any form of water that falls to Earth’s surface
from the clouds; includes rain, snow, sleet, and hail
• The four major types of precipitation are rain, snow, sleet,
and hail.
– Rain is a liquid precipitation. If the raindrops are smaller than 0.5
mm in diameter, the rain is called drizzle.
– The most common form of solid precipitation is snow, which consists
of ice particles. These particles may fall as small pellets, as
individual crystals, or as crystals that combine to form snowflakes.
– When rain falls through a layer of freezing air near the ground, clear
ice pellets, called sleet, can form.
– Hail is a solid precipitation in the form of lumps of ice. Hail usually
forms in cumulonimbus clouds.
2 Causes of Precipitation
• Most cloud droplets have a diameter of about 20
micrometers, which is smaller than the period at the end of
this sentence.
• A droplet must increase in diameter by about 100 times to
fall as precipitation.
• Two natural processes cause cloud droplets to grow large
enough to fall as precipitation: coalescence and
supercooling.
Causes of Precipitation - Coalescence
• coalescence the formation of large droplets by the
combination of smaller droplets
• Larger droplets fall much faster through the air than small
ones do.
• As these large droplets drift downward, they collide and
combine with smaller droplets.
• Each large droplet continues to coalesce until it contains a
million times as much water as it did originally.
Measuring Precipitation
Amount of Precipitation
• In one type of rain gauge, rainwater passes through a
funnel into a calibrated container, where the amount of
rainfall can then be measured.
• Snow depth is simply measured with a measuring stick.
• The water content of the snow is determined by melting a
measured volume of snow and by measuring the amount of
water that results.
A more complete station model.
Station model practice exercises
Global Weather Monitoring
• Weather observers at stations around the world report
weather conditions frequently, often several times per hour.
• The World Meteorological Organization (WMO) sponsors a
program called World Weather Watch to promote the rapid
exchange of weather information.
• This organization helps developing countries establish or
improve their meteorological services and also offers advice
on the effect of weather on natural resource and on human
activities, such as farming and transportation.
Weather Forecasts
• To forecast the weather, meteorologists regularly plot to the intensity
and path of weather systems on maps.
• Meteorologists then study the must recent weather map and compare it
with maps from previous hours.
• By following the progress of weather systems, meteorologist can
forecast the weather.
Severe Weather Watches and Warnings
• One main goal of meteorology is to reduce the amount of destruction
caused by severe weather by forecasting severe weather early.
• A watch is issued when the conditions are ideal for severe weather.
• A warning is given when severe weather has been spotted or is
expected within 24 hours.
Measuring Upper-Atmospheric Conditions
Radiosonde
• radiosonde a package of instruments that is carried aloft by
a balloon to measure upper atmospheric conditions,
including temperature, dew point, and wind velocity
• A radiosonde sends measurements as radio waves to a
receiver that records the information.
• When the balloon reaches a very high altitude, the balloon
expands and bursts, and the radiosonde parachutes back to
Earth.
Measuring Upper-Atmospheric Conditions
Radar - (radio detection and ranging) a system that uses reflected
radio waves to determine the velocity and location of objects like
large particles of water in the atmosphere.
• The newest Doppler radar can indicate the precise location,
movement, and extent of a storm. It can also indicate the
intensity of precipitation and wind patterns within a storm.
Weather Satellites
• Satellite images provide weather information for regions where
observations cannot be made from ground.
• The direction and speed of the wind at the level of the clouds can
also be measured by examining a continuous sequence of cloud
images.
• Satellite instruments can also measure marine conditions.
Measuring Upper-Atmospheric Conditions
Computers
• Before computers were available, solving the mathematical
equations that describe the behavior of the atmosphere
was very difficult, and sometimes impossible.
• In addition to solving many of these equations, computers
can store weather data from around the world. These data
can provide information that is useful in forecasting weather
changes.
Global Weather Monitoring
• Weather observers at stations around the world report
weather conditions frequently, often several times per hour.
• The World Meteorological Organization (WMO) sponsors a
program called World Weather Watch to promote the rapid
exchange of weather information.
• This organization helps developing countries establish or
improve their meteorological services and also offers
advice on the effect of weather on natural resource and on
human activities, such as farming and transportation.
Chapter 21 Section 4: Forecasting the Weather
• Explain how weather stations communicate weather data.
• Explain how a weather map is created.
• Explain how computer models help meteorologists
forecast weather.
• List three types of weather that meteorologists have
attempted to control.
Weather Maps - Weather Symbols
• station model a pattern of meteorological symbols that
represent the weather at a particular observing station and
that is recorded on a weather map
• Common weather symbols describe cloud cover, wind
speed, wind direction, and weather conditions, such as type
of precipitation and storm activity.
• Other information included in the station model are the air
temperature and the dew point.
• The dew point indicates how high the humidity of the air is.
• The station model also includes the atmospheric pressure,
indicated by a three-digit number in the upper right hand
corner.
Weather Maps - Plotting Temp and Pressure
• Lines that connect points of equal temperatures are called
isotherms.
• Lines that connect points of equal atmospheric pressure are
called isobars.
• The spacing and shape of the isobars help meteorologists
interpret their observations about the speed and direction of
the wind.
• Centers that are marked H represent high pressure.
• Centers that are marked L represent low pressures.
Weather Maps - Plotting Fronts and Precipitation
• Most weather maps mark the locations of fronts and areas
of precipitation.
• Fronts are identified by sharp changes in wind speed and
direction, temperature or humidity.
• Areas of precipitation are commonly marked with colors or
symbols.
Weather Forecasts
• To forecast the weather, meteorologists regularly plot to the intensity
and path of weather systems on maps.
• Meteorologists then study the must recent weather map and compare it
with maps from previous hours.
• By following the progress of weather systems, meteorologist can
forecast the weather.
• Computers models can show the possible weather conditions for
several days. By using computers, scientists can manipulate data on
temperature and pressure to simulate errors in measuring these data.
• Some computer models may be better at predicting precipitation for a
certain area, while other computer models may be better at predicting
temperature and pressure. Comparing models helps meteorologists
better predict weather.
Reading
Why do meteorologists
compareCheck
models?
Meteorologists compare computer models because different
models are better at predicting different weather variables.
If information from two or more models matches, scientists
can be more confident of their predictions.
Weather Forecasts, continued
Types of Forecasts
Meteorologists make four types of forecasts.
– Daily forecasts predict weather conditions for a 48-hour
period.
– Extended forecasts look ahead 3 to 7 days.
– Medium range forecasts look ahead 8 to 14 days.
– Long-range forecasts cover monthly and seasonal
periods.
• Accurate weather forecasts can be made for 0 to 5 days.
However, accuracy decreases with each day.
Weather Forecasts, continued
Severe Weather Watches and Warnings
• One main goal of meteorology is to reduce the harm
caused by severe weather by forecasting it early.
• A watch is issued when the conditions are ideal for severe
weather.
• A warning is given when severe weather has been spotted
or is expected within 24 hours.
Controlling the Weather
• Some meteorologists are investigating methods of controlling rain, hail,
and lightning.
• Currently, the most researched method for producing rain has been cloud
seeding. Cloud seeding can also be used to prevent more severe
precipitation.
• Seeding of potential lightning storms with silver-iodide nuclei has seemed
to modify the occurrence of lightning. However, no conclusive results have
been obtained.
Hurricane Control
• Hurricanes have also been seeded with freezing nuclei in an effort to
reduce the intensity of the storm.
• During Project Stormfury, which took place from 1962 to 1983, four
hurricanes were seeded, and the project had mixed results.
• Scientists have, for the most part, abandoned storm and hurricane control
because it is not an attainable goal with existing technology
Conduction
• The molecules in a substance move faster as they become heated.
(kinetic theory of matter again)
• Collisions between the particles result in the transfer of energy, which
warms the substance.
• conduction the transfer of energy as heat through a material
• Thus, conduction heats only the lowest few centimeters of the
atmosphere, where air comes into direct contact with the warmed
surface of Earth.
Convection
• convection the movement of matter due to differences in
density that are caused by temperature variations; can result
in the transfer of energy as heat
• Convection occurs when gases or liquids are heated
unevenly.
Convection
• Because warm air is less dense than cool air is, warm air exerts
less pressure than the same volume of cooler air does. (the
warmer the more buoyant).
• The atmospheric pressure is lower beneath a mass of warm air.
• As dense, cool air moves into a low-pressure region, the less
dense, warmer air is pushed upward.
• These pressure differences, which are the result of the unequal
heating that causes convection, create winds.
• The continuous cycle in which cold air sinks and warm air rises
warms Earth’s atmosphere (provides a heat barrier to the
coldness of space).
Radiation
• All of the energy that Earth receives from the sun travels through
space between Earth and the sun as radiation.
• Radiation includes all forms of energy that travel through space
as waves.
• Radiation travels through space in the form of waves at a very
high speed—approximately 300,000 km/s.
• The distance from any point on a wave to the identical point on
the next wave, for example from crest to crest, is called the
wavelength of a wave.
• The various types of radiation differ in the length of their waves.
• electromagnetic spectrum all of the frequencies or wavelengths
of electromagnetic radiation
The Atmosphere and Solar Radiation
• As solar radiation passes through Earth’s atmosphere, the
atmosphere affects the radiation in several ways.
• Most of the solar rays that reach the lower atmosphere,
such as visible and infrared waves, have longer
wavelengths.
• Most incoming infrared radiation is absorbed by carbon
dioxide, water vapor, and other complex molecules in the
troposphere.
• As visible light waves pass through the atmosphere, only a
small amount of this radiation is absorbed.
The Atmosphere and Solar Radiation
Scattering
• Clouds, dust, water droplets, and as molecules in the
atmosphere disrupt the paths of radiation from the sun and
cause scattering.
• Scattering occurs when particles and gas molecules in the
atmosphere reflect and bend solar rays. This deflection
causes the rays to travel out in all directions without
changing their wavelength.
• As a result of scattering, sunlight that reaches Earth’s
surface comes from all directions. The sky appears blue,
and the sun appears red at sunset.
The Atmosphere and Solar Radiation
Reflection
• The amount of energy that is absorbed or reflected depends on
characteristics such as color, texture, composition, volume,
mass, transparency, state of matter, and specific heat of the
material on which the solar radiation falls.
• The intensity and amount of time that a surface material receives
radiation also affects how much energy is reflected or absorbed.
• albedo the fraction of solar
radiation that is reflected off
the surface of an object.
Absorption of Sunlight
• The Solar radiation that is not reflected is absorbed
by rocks, soil, water, and other surface materials.
• The absorption of this radiation causes the matter to
become energized. As the material is heated, it begins
to emit an increasing amount of infrared radiation.
Absorption and Infrared Energy
• Gas molecules, such as water vapor and carbon
dioxide, in the atmosphere absorb the infrared
rays.
• The absorption of thermal energy from the ground
heats the lower atmosphere and keeps Earth’s
surface much warmer than it would be if there were
no atmosphere.
The Greenhouse Effect
• One of the ways in which the gases of the atmosphere
absorb and reradiate infrared rays can be compared to the
process that keeps a greenhouse warm.
• Earth’s atmosphere slows the escape of energy that
radiates from Earth’s surface.
• greenhouse effect the warming of the surface and lower
atmosphere of Earth that occurs when carbon dioxide, water
vapor, and other gases in the air absorb and reradiate
radiation
The Greenhouse Effect
• As Earth’s surfaces absorb solar radiation, they become
energized and heated.
• The increasing heat of these surfaces causes an increase in
the amount of infrared radiation they give off.
• Much of this infrared (heat) radiation emitted by the Earth’s
surface never reaches space. Instead, it is absorbed by
greenhouse gases which trap this radiant heat at Earth’s
surface.
The Greenhouse Effect - Human Impact
• Generally, the amount of solar energy that enters Earth’s
atmosphere is about equal to the amount that escapes into
space.
• However, human activities, in addition to natural causes,
are changing this balance and are causing the average
temperature of the atmosphere to increase.
• Increases in the amount of carbon dioxide may intensify the
greenhouse effect and may cause Earth to become
warmer.
Insolation
• Radiation from the sun does not heat Earth equally at all
places at all times.
• Insolation (solar irradiation) is the total amount of solar
radiation energy received on a given surface area during a
given time.
• The unit recommended by the World Meteorological
Organization is megajoules per square meter (MJ/m2) or
joules per square millimeter (J/mm2).
Insolation
• Because Earth is a sphere, the sun’s rays do not strike all
areas at the same angle.
• This causes surface temperatures to be higher at the
equator and lower at the poles.
Variations in Temperature
• Earth’s surface must absorb energy for a time before
enough energy has been absorbed and reradiated from the
ground to change the temperature of the atmosphere.
• The temperature of the atmosphere in any region on Earth’s
surface depends on several factors, including:
– Latitude
– season (time of year)
– Surface features (mountains, cities, etc)
– Amount of humidity
– Wind
– Presence of large bodies of water
Variations in Temperature – Latitude and Season
• Latitude is the primary factor that affects the
amount of solar energy that reaches any point on
Earth’s surface.
• When sunlight hits Earth’s surface at an angle
smaller than 90°, the energy is spread out over a
larger area and is less intense.
Variations in Temperature – Latitude and Season
• Temperature also varies seasonally because the tilt of
Earth’s axis influences insolation.
• As Earth revolves around the sun once each year, the
portion of Earth’s surface that receives the most intense
sunlight changes.
Variations in Temperature – Presence of Water
• Because water vapor stores heat, the amount of
water in the air affects the temperature of a region.
• Land areas close to large bodies of water generally
have more moderate temperatures.
• Water heats up and cools down slower than land
does, so the temperature of water changes less
than the temperature of land does
Chapter 19 Section 3: Atmospheric Circulation
Key Ideas
Explain the Coriolis effect.
Name and Describe the global patterns of air circulation.
Identify two factors that form local wind patterns.
The Coriolis Effect
• The circulation of the atmosphere and of the ocean is
affected by the rotation of Earth on its axis.
• Points near the equator travel farther and faster than points
closer to poles do. When air moves toward the poles, it
travels east faster than the land beneath it does. As a result,
the air follows a curved path.
• Coriolis effect the curving of the path of a moving object
from an otherwise straight path due to Earth’s rotation
• In general, the Coriolis effect is detectable only on objects
that move very fast or that travel over long distances.
Global Winds
•
Each hemisphere
contains three
looping patterns
of flow called
convection cells.
•
Each convection
cell correlates to
an area of Earth’s
surface, called a
wind belt, that is
characterized by
winds that flow in
one direction.
These winds are
called prevailing
winds.
Global Winds, Trade Winds (the Hadley cell)
• trade wind prevailing winds that blow from east to west
from 30º latitude to the equator in both hemispheres
• Like all winds, trade winds are named according to the
direction from which they flow.
• In the Northern Hemisphere, the trade winds flow the
northeast and are called the northeast trade winds.
• In the Southern Hemisphere, they are the southeast trade
winds.
Global Winds, the Prevailing Westerlies
• Between 30º and 60º latitude, air moving toward the poles
is deflected by the Coriolis effect.
• westerlies prevailing winds that blow from west to east
between 30º and 60º latitude in both hemispheres
• In the Northern Hemisphere, the westerlies are the
southwest winds. In the Southern Hemisphere, they are the
northwest winds.
Global Winds, The Polar Easterlies
• Over the polar regions, descending cold air creates areas
of high pressure.
• Surface winds created by the polar high pressure are
deflected by the Coriolis effect and become the polar
easterlies.
• polar easterlies prevailing winds that blow from east to
west between 60 and 90 latitude in both hemispheres
• Where the polar easterlies meet warm air from the
westerlies, a stormy region known as a front forms.
Global Winds - The Doldrums and Horse Latitudes
• The trade wind systems of the Northern Hemisphere and
Southern Hemisphere meet at the equator in a narrow zone
called the doldrums, or the Intertropical Convergence Zone
(ITCZ).
• As the air approaches 30º latitude, it descends and a highpressure zone forms. These subtropical high-pressure
zones are called horse latitudes (or subtropical highs).
• Surface winds are weak and variable in both of these zones.
Global Winds – the Jet Streams
• jet streams a narrow band of strong winds that blow in the upper
troposphere.
• In the Northern Hemisphere, the jet streams move from west to
east because of the Coriolis Effect.
• Polar jet streams can reach speeds of over 400 km/h and can
affect airline routes and the paths of storms.
• Subtropical jet streams do not change much in speed or position.
Global Winds - Wind and Pressure Shifts
• As the sun’s rays shift northward and southward during
the changing seasons of the year, the positions of the
pressure belts and wind belts shift.
• Although the area that receives direct sunlight can shift
by up to 47º, the average shift for the pressure belts
and wind belts is only about 10º of latitude.
• However, even this small change causes some areas
of Earth’s surface to be in different wind belts during
different times of the year.
Local Winds
• Local winds are not part of the global wind belts.
• Movement of air are also influenced by local
conditions, and local temperature variations
commonly cause local winds.
• Winds that blow at speeds of less than 50 km/h are
called breezes.
• Two types of breezes
– Mountain and Valley Breezes
– Land and Sea Breezes
Local Winds - Mountain and Valley Breezes
Mountain and Valley Breezes
• A valley breeze forms when warm air from the valleys
moves upslope.
• At night, the mountains cool more quickly than the valleys
do, and cool air descends from the mountain peaks to
create a mountain breeze.
• Areas near mountains may experience a warm afternoon
that turns to a cold evening soon after sunset.
Local Winds, Land and Sea Breezes
Land and Sea Breezes
• Equal areas of land and water may receive the same
amount of energy from the sun. However, land surfaces
heat up faster than water surfaces do.
• The cool wind moving from water to land is called a sea
breeze.
• Overnight, the land cools more rapidly than water does, and
the sea breeze is replaced by a land breeze.
Air Masses – air pressure affected by temp
• Differences in air pressure are caused by
unequal heating of Earth’s surface.
• The region along the equator receives more solar
energy than the regions at the poles do.
– The heated equatorial air rises and creates a
low-pressure center.
– Conversely, cold air near the pole sinks and
creates high-pressure centers.
• Differences in air pressure at different
locations on Earth create wind patterns.
How Air Moves
• Air moves from areas of high pressure to areas of low pressure.
Therefore, there is a general, worldwide movement of surface air
from the poles toward the equator.
• Temperature and pressure differences on Earth’s surface create
three wind belts in the Northern Hemisphere and three wind belts
in the Southern Hemisphere.
• The Coriolis effect, which occurs when winds are deflected by
Earth’s rotation, also influences wind patterns.
Formation of Air Masses
• When air pressure differences are small, air remains
relatively stationary.
• If air remains stationary or moves slowly over a uniform
region, the air takes on characteristic temperature and
humidity of that region.
• air mass a large body of air throughout which temperature
and moisture content are similar
• Air masses that form over frozen polar regions are very cold
and dry. Air masses that form over tropical oceans are warm
and moist.
Types of Air Masses
• Air masses are classified according to their source
regions.
• The source regions for cold air masses are polar areas.
The source regions for warm air masses are tropical
areas.
• Air masses that form over the ocean are called
maritime. Air masses that form over land are called
continental.
• The combination of tropical or polar air and continental
or maritime air results in air masses that have distinct
characteristics.
Types of Air Masses, continued
Continental Air Masses
• There are two types of continental air masses: continental polar (cP) and
continental tropical (cT).
• Continental polar air masses are cold and dry. Continental tropical air masses
are warm and dry.
• An air mass may remain over its source region for days or weeks. However, the
air mass will eventually move into other regions because of global wind patterns.
Maritime Air Masses
• The two types of maritime air masses are maritime polar (mP) and maritime
tropical (mT).
• Maritime polar air masses are moist and cold. Maritime tropical air masses are
moist and warm.
• When these very moist masses of air travel to a new location, they commonly
bring precipitation and fog.
North American Air Mass
• The four types of air masses that affect the weather of North
America come from six regions.
• An air mass usually brings the weather of its source region,
but an air mass may change as it moves away from its
source region.
• For example, cold, dry air may become warm and more
moist as it moves from land to the warm ocean.
Fronts
• A cool air mass is dense and does not mix with the lessdense air of a warm air mass.
• Thus, a boundary, called a front, forms between air masses.
• Changes in middle-latitude weather usually take place along
the various types of fronts.
• Fronts do not exist in the tropics because no air masses
that have significant temperature differences exist there.
Types of Fronts - Cold Fronts
• cold front the front edge of a moving mass of cold air that
pushes beneath a warmer air mass like a wedge
• If the warm air is moist, clouds will form. Large cumulus and
cumulonimbus clouds typically form along fast-moving cold
fronts.
• A long line of heavy thunderstorms, called a squall line, may
occur in the warm, moist air just ahead of a fast-moving cold
front.
• A slow-moving cloud front typically produces weaker storms and
lighter precipitation than a fast-moving cold front does.
Types of Fronts, Warm Fronts
• warm front the front edge of advancing warm air mass that
replaces colder air with warmer air
• The slope of a warm front is gradual.
• Because of this gentle slope, clouds may extend far ahead
of the surface location, or base, of the front.
• A warm front generally produces precipitation over a large
area and may occasionally cause violent weather.
Types of Fronts, Stationary and Occluded Fronts
• Sometimes, when air masses meet, the cold moves parallel
to the front, and neither air mass is displaced.
• stationary front a front of air masses that moves either
very slowly or not at all
• occluded front a front that forms when a cold air mass
overtakes a warm air mass and lifts the warm air mass of
the ground and over another air mass
Polar Fronts and Midlatitudes Cyclones
• Over each of Earth’s polar regions is a dome of cold air that
may extend as far as 60° latitude.
• The boundary where this cold polar air meets the tropical
air mass of the middle latitudes, especially over the ocean,
is called the polar front.
• Waves commonly develop along the polar front. A wave is a
bend that forms in a cold front or stationary front.
• Waves are the beginnings of low-pressure storm centers
called mid-latitude cyclones or wave cyclones.
Polar Fronts and Mid-latitudes Cyclones
• Mid-latitude cyclone an area of low pressure that is
characterized by rotating wind that moves toward the rising
air of the central low-pressure region
• These cyclones strongly influence weather patterns in the
middle latitudes.
Polar Fronts and Mid-latitudes Cyclones
Stages of a Midlatitude Cyclone
• A midlatitude cyclone usually last several days.
• In North America, midlatitude cyclones generally travel
about 45 km/h in an easterly direction as they spin
counterclockwise.
• They follow several storm tracks, or routes, as they move
from the Pacific coast to the Atlantic coast.
• As they pass over the western mountains, they may lose
their moisture and energy.
Polar Fronts and Mid-latitudes Anticyclones
Anticyclones
• Unlike the air in the midlatitude cyclone, the air in an
anticyclone sinks and flows outward from a center of high
pressure.
• Because of the Coriolis effect, the circulation of air around
an anticyclone is clockwise in the Northern Hemisphere.
• Anticyclones bring dry weather, because their sinking air
does not promote cloud formation.
Severe Weather - Thunderstorms
• thunderstorm a usually brief, heavy storm that consists of rain,
strong winds, lightning, and thunder
• Thunderstorms develop in three distinct stages.
– In the cumulus stage, warm, moist air rises, and the water vapor within the air
condenses to form a cumulus cloud.
– In the mature stage, the cloud rises and becomes a dark, cumulonimbus cloud from
which heavy, torrential rain and hailstones may fall.
– During the dissipating stage, the strong downdrafts stop air currents from rising. The
thunderstorm dissipates as the supply of water vapor decreases.
Severe Weather, Lightning
• During a thunderstorm,
clouds discharge electricity in
the form of lightning. to
equalize the electrical
charges.
• For lightning to occur, the
clouds must have areas that
carry distinct electrical
charges.
• The released electricity heats
the air, and the air rapidly
expands and produces a
loud noise known as thunder.
Severe Weather, Tornadoes
• tornado a destructive, rotating column of air that has very
high wind speeds and that maybe visible as a funnel-shaped
cloud.
• The smallest, most violent, and shortest-lived severe storm.
• A tornado forms when a thunderstorm meets high-altitude
horizontal winds. These winds cause the rising air in the
thunderstorm to rotate.
Severe Weather, Tornadoes, continued
• If the funnel does touch the ground, it generally moves in a wandering,
haphazard path.
• The destructive power of a tornado is due to mainly the speed of the
winds. These winds may reach speeds of more than 400 km/h.
• The Fujita scale was introduced in 1971.
• The F-Scale was replaced with the Enhanced Fujita Scale (EF-Scale) in
the United States in February 2007.
Severe Weather, Tornadoes, continued
Severe Weather, Hurricanes
• hurricane a severe storm that develops over tropical
oceans and whose strong winds of more than 120 km/h
spiral in toward the intensely low-pressure storm center
• A hurricane begins when warm, moist air over the ocean
rises rapidly.
• When moisture in the rising warm air condenses, a large
amount of energy in the form of latent heat is released.
This heat increase the force of the rising air.
• A fully developed hurricane consists of a series of thick
cumulonimbus cloud bands that spiral upward around the
center of the storm.
Severe Weather, Hurricanes, continued
• Every hurricane is categorized on the Saffir-Simpson scale
by using several factors. These factors include central
pressure, wind speed, and storm surge.
• The most dangerous aspect of a hurricane is a rising sea
level and large waves, called a storm surge.