JMJ
7-4 Atmosphere Outline and Study Guide
Vocabulary for the unit
Charles’ Law
Atmosphere
Barometric pressure High pressure system
Humidity
Front
Pressure gradient force
Wind
(PGF)
Evaporation
Condensation
Troposphere
Low pressure system
Cold front
Atmospheric pressure
Air mass
Warm front
Isobar
Barometer
Precipitation
Atmosphere must know:
where it is found
what it is made of (main two gases and approximate percentages)
Weather must know
Definition – what is weather?
Main factors involved in weather
Troposphere
Where is it?
What happens in the troposphere
What is the jet stream and where is it found?
How does energy from the sun heat the atmosphere?
How do land and water help heat the atmosphere?
Temperature and volume of a gas
What happens when a gas is heated?
Relationship between temperature and volume (Charles’ Law)
Density
Definition
Units of measure for density
How does density of air change when the air is heated?
How does density of air change when the air is cooled?
Calculate density given mass and volume
Unequal heating of Earth
Which parts of Earth gets more heat?
What happens to air at the equartor?
What happens to air at the poles?
Wind
What causes wind?
What is the relationship between the pressure gradient force and the direction the wind
blows?
Air Pressure
What is air pressure?
What is a low pressure system?
What is a high pressure system?
Where is air pressure greatest?
What happens to air pressure as you go up through the atmosphere?
Relationship between air pressure and density
Denser air will have a higher air pressure- there are more air molecules in a given
space to push down on you
Less dense air will have a lower air pressure- there are fewer air molecules to push
down on you.
Relationship between water content (humidity) and air
Density of moist air, pressure of moist air
Mass of a water molecule v. mass of gasses in air
Impact of Temperature on Air Pressure
Warm air is less dense than cold air. Therefore, warm air has a lower air pressure and
cold air has a higher air pressure.
The molecules in warm air are moving fast and are spread farther apart. Therefore
there are fewer air molecules in a given area to push down on you.
Barometer
What does a barometer measure?
What are the units of measure?
Relationship between air pressure and weather
High pressure area has less moisture and usually has clear, dry weather
Low pressure area has more moisture and can have cloudy, rainy weather
Air Pressure on a Weather Map
Areas of High and Low pressure on shown on a weather map with an H or an L.
Lines called isobars show areas of equal atmospheric pressure
Fronts
What is a front?
What is a warm front?
What is a cold front?
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low pressure
high pressure
warm or cold air
warm
cold
air rising or sinking
rising
sinking
clouds or no clouds
clouds
no clouds, clear weather
molecules move fast or slow
fast
slow
molecules are
packed together or far apart
far apart, less dense
packed together, dense
moist or dry
moist
dry
Animation of Fronts
CALCULATIONS:
Calculate density, given information data and the formula
Calculate pressure gradient force (given the formula) and the direction the wind will blow
Recognize the front symbols
Unit Summary
The volume and density of air change with changes in temperature. As the temperature
of air increases, its volume increases and its density decreases. As the temperature of air
decreases, its volume decreases and its density increases.
As air is heated, its density decreases because its mass remains constant but its volume
increases. As air is cooled, its density increases because its mass remains constant but its volume
decreases.
Air is a fluid. Fluids that are denser sink below those that are less dense. The decrease in
density as air is heated causes it to rise above cooler, denser air. The increase in density as air is
cooled causes it to sink below warmer, less dense air.
The warmer air in the atmosphere rises and cooler air in the atmosphere sinks. This
causes convection currents in the atmosphere (moving air).
Air that moves from the upper levels of the atmosphere downward towards Earth creates
an area of high pressure beneath the falling air. Air that rises or moves upward from the Earth or
from lower levels of the atmosphere toward the upper atmosphere creates an area of low pressure
beneath the rising air. Cold air sinking would increase the pressure in an area while warm air
rising would decrease the pressure in an area.
When there are pressure differences in the atmosphere, are moves from areas of high
pressure to areas of low pressure. This movement of air creates wind.
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Changes in pressure are measured with a barometer. One type of barometer contains a
liquid like mercury or water that moves up and down in narrow tube of the barometer. As the
pressure of an area increases, the increased downward force of air pushes the liquid in a
barometer up into the barometer tube. As the pressure of an area decreases, the level of liquid in
the barometer tube falls because the downward force of air on the liquid in the barometer has
decreased.
The temperature of the atmosphere decreases with the height of the atmosphere. Areas of
the atmosphere closer to the ground have a higher temperature than areas higher in the
atmosphere. When cold fronts form, air from the cold and warm air masses move as a result of
their densities. The denser cold air pushes under the less dense warm air ahead of it. The air in
the warm air mass rises. As the warm air rises, it encounters cooler temperatures and the rate of
evaporation decreases. As the rate of evaporation becomes slower than the rate of condensation,
water vapor in the warmer air condenses and forms clouds and precipitation. When warm fronts
form, air from the cold and warm air masses move as a result of their densities. The less dense
warm air behind the colder, denser air masses rises and slides over and above the cold air mass.
As the warm air rises, it encounters cooler temperatures and the rate of evaporation decreases.
As the rate of evaporation becomes slower than the rate of condensation, water vapor in the
warmer air condenses and forms clouds and precipitation.
In both cold and warm fronts, the rising of the warmer air mass is accompanied by a
decrease in the atmospheric pressure. In a cold front, there is a decrease in pressure as the front
forms because the cool air mass slides under the warmer air mass and pushes it upwards. The
rising warm air results in a decrease in atmospheric pressure. This decrease is followed by an
increase in pressure once the front has passed because of the cooler sinking air that moved in
behind the warmer rising air. In a warm front, there is a decline in the atmospheric pressure as
the front moves into an area because a warm air mass moves in behind a cooler air mass. As the
warm air mass slides over the cooler air mass there is a decrease in pressure. The pressure
continues to drop as the front passes because of the warmer, rising air that moves in behind the
cooler air mass.
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