Download 5.2.1 - Air Masses and Fronts

5.2.1 - Air Masses and Fronts
Background: Air masses, high and low pressure systems, and frontal boundaries are the major causes of weather
variations and storms. They move because of differences in pressure, caused by variations in temperature
throughout areas. Weather is the current, day-to-day conditions of the atmosphere, while climate is the month-tomonth and year-to-year conditions of the atmosphere.
Air Masses
There are several types of air masses that usually affect North America. Air masses are volumes of air defined by
their temperature and water vapor content (how much water vapor is in the air), which can modify according to the
surface(s) over which they travel. The two major air masses are high-pressure and low-pressure systems.
Recall, the Coriolis Effect causes fluids to rotate in terms
of the direction the air is traveling. High-pressure system
are associated with sinking air and are also called
anticyclones. With the Coriolis Effect, in the Northern
Hemisphere, sinking air circulates downward and to the
right (which will be clockwise) spreading away from the
center when it reaches Earth’s surface. Follow the
arrows in the image to the right and below. Highpressure systems are usually associated with fair
weather and generally pleasant conditions.
Low-pressure systems are associated with rising air and are also called cyclones. Rising air must be replaced, so
the flow of air is inward toward the center and then upward. A low-pressure system in the northern hemisphere
moves in a counterclockwise direction because of the Coriolis Effect. Low-pressure systems are usually associated
with clouds and precipitation.
High pressure and low pressure systems are going to work
oppositely in terms of the “spin direction”, in the Northern
Hemisphere, air moves around high pressure in a clockwise
direction and low pressure in an anticlockwise direction. This
becomes important in terms of determining how fronts will
move. In the Southern Hemisphere, these spin directions
reverse and low pressure is clockwise while high pressure is
anticlockwise.
Air Masses Review Questions:
1. What is an air mass?
2. What causes fluids to spin?
3. Make a table: What are the two major types of air masses, is it a cyclone or anticyclone, rising or sinking air,
good weather or storms?
4. Draw me a picture, and label it with clockwise or anticlockwise, of a high pressure system and the wind
traveling around it in the Northern Hemisphere.
5. Draw me a picture, and label it with clockwise or anticlockwise, of a low pressure system and the wind
traveling around it in the Northern Hemisphere.
Fronts: When Air Masses Interact
A front is the boundary separating two different air masses of differing densities. There are four major types of
fronts: cold front, warm front, occluded front, and stationary front. The types of fronts interacting determine the
direction of airflow through the frontal, which influences the direction of frontal movement. This determines the
characteristic cloud formations and weather patterns that result from each frontal boundary.
Clouds are often created when two different types of air masses run into each other -- a warm air mass and a cold air
mass. Typically, the less dense warm air gets pushed up over the more dense cold air. As warm air rises,
condensation occurs; the air cools to a point where water will condense from the gas state into a water state, forming
clouds and occasionally rain.
This simulation should walk you through the different fronts. Take notes on the four front types by making quick
drawings with arrows as you do this simulation.
http://www.phschool.com/atschool/phsciexp/active_art/weather_fronts/
Cold Front: A cold front occurs when a cold air mass moves forward into a warm air
mass. Because air masses don’t mix, the warm air mass is actually lifted up as the
cold air mass slides underneath. As the warm air mass rises is cools, causing
moisture in the air to condense, allowing clouds and sever weather patterns to form.
Warm Front: A warm air mass moves towards a cold air mass. As the warm air mass
is less dense than the cold air mass, it is forced to rise up above the cold air mass.
Again, this causes the warm air to cool, and any moisture in the air to condense. This
leads to clouds and less severe precipitation and weather.
Occluded Front: A warm air mass gets pinched between two cold air masses that are
colliding. This generally happens because cold air masses travel faster than warm air
masses and sometimes the cold air catches up with the warm air mass. Because the
warm air mass is the least dense, it is pushed upward into the troposphere. As the
warm air rises, moisture in the air condenses to form clouds and rain.
Stationary Front: A cold front and a warm front are both pushing against each other,
with neither being strong enough to move the other. This front can stay stationary for
days, and generally causes cloud formation and occasionally rain.
Fronts Review Questions:
6. Get the 4 drawings of fronts including arrows showing air movement listed in the above paragraphs from the
animation linked above. That is #3. If you put all 4 on one page in 4 opposite corners, you can probably
answer the review questions around them and stay super organized.
7. What is a front?
8. What could cause two different air masses to have different densities?
9. What is a cold front?
10. What happens when the two air masses interact? Explain using the word density.
11. What type of weather is found at a cold front? Are these conditions normally mild or severe?
12. What is a warm front?
13. What happens when the two air masses interact? Explain using the word density.
14. What type of weather is found at a warm front? Are these conditions normally mild or severe?
15. What is an occluded front?
16. Why does an occluded front form?
17. What type of weather is generally found in an occluded front?
18. What is a stationary front?
19. What type of weather is generally found in a stationary front?
20. How long can a stationary front last for?