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Earth Science
Chapter 7
Study of the Atmosphere
Introduction
• The atmosphere was most likely created
by the process of volcanic outgassing.
• Weather is the state or condition of the
atmosphere at a particular location for a
short period of time.
• The study of weather is called
meteorology.
Electromagnetic Energy
• Electromagnetic energy is energy that has
the properties of transverse waves.
Electromagnetic Energy
• All stars, including the sun constantly produce and
emit electromagnetic energy.
• Solar energy is the major source of energy for
Earth.
– Most of the energy from the Sun is in the form of
invisible waves (ultraviolet and infrared).
– The sun also gives off visible light that can be
separated into different wavelengths.
• The entire range of wavelengths is known as the
electromagnetic spectrum.
Electromagnetic Energy
– When electromagnetic energy comes in
contact with a material, it can be:
• refracted, or bent
• reflected to a new direction
• scattered (refracted and reflected) into
several directions
• absorbed by the material
Electromagnetic Energy
– Half of the energy absorbed by Earth is longwave (infrared) and half is short-wave (visible
and ultraviolet).
– Almost all of the energy released by Earth or
re-radiated is long-wave radiation.
Energy Transfer in the Atmosphere
•
There are 3 methods of energy transfer:
convection, conduction, and radiation.
– Convection is the transfer of heat energy by
movements of fluids (liquids and gasses).
– Conduction is the transfer of heat energy by
the collision of atoms with adjoining atoms.
– Radiation is the transfer of electromagnetic
energy through space in the form of waves.
Heat Energy and Phase Changes
• Normally when heat is added to a
substance the temperature will rise.
• The exception is during a change in phase.
• Latent heat is energy taken in or given off
by a substance during a change in phase.
– These transfers of energy DO NOT result in a
change in temperature.
Atmospheric Relationships
• Temperature
– Temperature is greatly affected by intensity
and duration of insolation
• INcoming SOLar radiATION.
– The atmosphere receives most of its heat
energy by conduction through direct contact
with Earth’s surface.
– The amount of reradiated energy absorbed is
directly related to the amount of water vapor,
carbon dioxide and various other pollutants.
Atmospheric Relationships
– Factors affecting local temperature
• Latitude
–Higher latitude = lower intensity of
insolation = lower temperatures
• Altitude
–Higher altitude = lower temperatures
• Closeness to large bodies of water
–Water heats and cools slower than land,
so large bodies of water regulate temp.
Atmospheric Relationships
• Moisture
– The warmer the air the more moisture it can hold
– When the air contains all the moisture it can hold
at a particular temperature it is saturated.
• The air is not usually saturated.
• To become saturated more water vapor must
be added or the air must be cooled.
• The temperature at which condensation
occurs is called the dewpoint temperature.
Atmospheric Relationships
– Absolute humidity is the actual amount of water
vapor in the air.
– Relative humidity is a comparison between the
amount of water in the air and the amount the
air can actually hold at that temperature.
• Example: If the air is holding half as much
water as it could, the relative humidity is 50%.
– An instrument called a psychrometer is used to
find dewpoint and relative humidity.
Atmospheric Relationships
• Air pressure (atmospheric pressure)
– The force, or weight, of the air pushing down
on a unit area of a surface.
– Air pressure is inversely proportional to
temperature changes.
– Air pressure is directly proportional to density
changes.
– The instrument used to measure air pressure
is called a barometer.
Atmospheric Relationships
– Air pressure is inversely proportional to the
amount of moisture in the air.
• When moist air moves into a region, air
pressure decreases and the barometer “falls”.
• A “falling” barometer is an indication that rainy
weather is on the way.
– As altitude increases, the number of air
molecules decreases and, thus, pressure
decreases.
Atmospheric Relationships
• Factors affecting rate of evaporation:
– Amount of energy available
• More heat energy = more evaporation
– Surface area
• More surface area = more evaporation
– Amount of moisture in the air
• More moisture = less evaporation
Atmospheric Relationships
• Large horizontal movements of air near
Earth’s surface are called winds.
• Smaller, local horizontal movements are
called breezes.
• Winds are named from the direction they
come from.
Atmospheric Relationships
• The primary causes of winds are differences
in air temperature, which causes differences
in air pressure.
• Air always moves from areas of high
pressure to areas of low pressure.
• The rate of change in pressure between two
locations is called the pressure gradient.
– Close isobars = steep gradient
Atmospheric Relationships
Local Breezes
Sea Breeze
Land Breeze
Atmospheric Relationships
Planetary Winds
Atmospheric Relationships
• Jet streams are winds at high altitudes that exert a
controlling influence over the direction traveled by
air-masses at Earth’s surface.
– 7 to 8 miles above Earth’s surface
– Avg. speed in summer 35 mph; in winter 75 mph
• Atmospheric transparency is how much or little the
Sun’s radiation is scattered or reflected.
– Varies inversely with the amount of aerosols
(dust and vapor) in the air.
Clouds and Precipitation
• Clouds - collections of tiny water droplets or
ice crystals suspended in the atmosphere.
– Form when moist air expands and cools as it
rises vertically in the atmosphere.
– When air cools to the dewpoint temperature, it
becomes saturated & condensation occurs.
– When the water droplets or ice crystals in a
cloud grow large enough to fall, precipitation
results.
Moisture and Energy Transfer
• Adiabatic temperature change – any
change in temperature of a system without
heat being added or removed from that
system.
– In the atmosphere, when air descends, it is
compressed by the air around and its
temperature increases.
– When air rises, it expands and its temperature
decreases.
Moisture and Energy Transfer
Forecasting the Weather
• Measurements of atmospheric variables,
when combined with similar measurements
taken earlier, can provide the information
needed to predict the weather.
– Since relationships between variables are often
complex, predictions are not always accurate.
– These relationships are expressed as the
probability of occurrence.
• Example: 40% chance of snow
Weather Maps and Forecasting
• A station model is a recording of weather
observations for a particular location
Weather Maps and Forecasting
• An air-mass is a huge body of air in the
troposphere (diameter up to 2000 km)
having similar pressure, moisture, wind,
and temperature characteristics throughout.
• Air-masses have definite characteristics
that depend on their source region.
• On a weather map air-masses are usually
labeled with two letters indicating the
moisture and temperature characteristics
Weather Maps and Forecasting
– Maritime air-masses (m)
• Develop over water and are moist
– Continental air-masses (c)
• Develop over land and are dry
– Polar air-masses (P)
• Develop in high latitudes and are cool
– Tropical air-masses (T)
• Develop in lower latitudes and are warm
– Arctic air-masses (A)
• Develop in very high latitudes & are very cold and dry
Weather Maps and Forecasting
• Cyclone - low-pressure air mass with winds
moving counterclockwise toward its center.
– When the moving air converges at the center
of a low, it rises vertically, often producing rain.
• Anticyclone - high-pressure air mass with
winds moving clockwise away from its
center.
– The air descending in their center is often dry
and they usually bring cool, clear weather.
Weather Maps and Forecasting
Weather Maps and Forecasting
• The boundary between two air-masses is
called a front.
– Atmospheric conditions at fronts:
• Unstable air
• Clouds
• Strong winds
• Precipitation
• Other weather changes
Weather Maps and Forecasting
• Warm fronts
– Occur when warm air meets and rises over cold
air on the ground.
– Have long gentle slopes.
• can be over 1000 km
– Bring predictable sequence weather and clouds.
– Precipitation may occur ahead of the front
Weather Maps and Forecasting
Weather Maps and Forecasting
• Cold Fronts
– Occur when cold air meets and pushes out
warmer air.
– Have short, steep slopes.
– Move faster than warm fronts.
– No sequence of clouds warning approach.
– Precipitation occurs all around the front
Weather Maps and Forecasting
Weather Maps and Forecasting
• Occluded front
– When a faster moving cold front overtakes a
slower moving warm front and lifts the warmer
air between the two fronts above the ground.
– Weather is characteristic of both fronts without
any gap in the sequence.
• Stationary front
– When a warm air-mass and a cold air-mass are
side-by-side, with neither one moving.
– Weather is similar to a warm front.
Weather Maps and Forecasting
• Mid-latitude cyclones
– Mix polar and tropical air-masses.
– First develop in areas of low pressure.
– The greater the pressure gradient, the faster
the winds move into the low, and the greater
the impact of the Coriolis Effect.
– The counterclockwise flow circulates the warm
air northward and the cold air southward.
– Fronts occur at the interfaces of air-masses.
Weather Maps and Forecasting
• Making predictions
– Decreasing air pressure often brings warm
unsettled air and rainy weather
– Increasing air pressure brings cool, clear
weather.
– Weather systems in the United States
generally move from west to east.
– Look at pressure and rate of movement of airmasses to the west to predict local weather.
Hurricanes and Tornadoes
• A hurricane is a doughnut shaped ring of
counterclockwise winds exceeding 75 mph
around an area of extremely low pressure.
– As the air moves closer to the center of the
storm, its velocity increases.
– The eye of the hurricane is a relatively calm
area of clear skies in the middle of the
hurricane.
Hurricanes and Tornadoes
Hurricanes and Tornadoes
• A hurricane is fueled by heat stored in
water vapor.
– The released heat warms the air and provides
lift for its upward flight.
– This reduces the pressure near the surface
and encourages a more rapid inflow of air.
– Hurricanes develop in late summer when high
temperatures provide the heat and moisture.
– Energy produced in 1 day is equal to US
electrical energy production for 1 year.
Hurricanes and Tornadoes
• Tornadoes are local storms of short
duration that are among nature’s most
destructive forces.
– Violent windstorms that take the form of a
rotating funnel of air that extends downward
from a cumulonimbus cloud.
– Winds can exceed 300 mph.
– Pressure drop is usually around 25 mb, but
drops of up to 200 mb have been observed.
Hurricanes and Tornadoes
Hurricanes and Tornadoes
• Tornadoes are extremely variable with a
small diameter and a short lifetime that are
impossible to forecast precisely.
– They generally form in the vicinity of intense
cold fronts and squall lines associated with
mid-latitude cyclones.
– Throughout spring, cold and dry cP air from
the north mix with warm humid and unstable
mT air from the Gulf spawning tornadoes.