Download Ch. 3 Sec. 1 Notes

Ch. 3 Sec. 1 Notes
Air Masses and Fronts
*A huge body of air that has similar temperature, humidity, and air pressure at any
given height is called an air mass
-A single air mass may spread over millions of square km and be up to 10 km
deep
Types of Air Masses
*Scientists classify air masses according to 2 characteristics:
1. Temperature
2. Humidity
*4 major types of air masses influence the weather in North America:
1. Maritime tropical
2. Continental tropical
3. Maritime polar
4. Continental polar
*Temperature affects air pressure
-Cold, dense air has high pressure
-Warm, less dense air has low pressure
*Tropical, or warm, air masses form in the tropics and how low air pressure
*Polar, or cold, air masses form north of 50° north latitude and south of 50° south
latitude
-Polar air masses have high air pressure
*Whether an air mass is humid or dry depends on whether it forms over water or
land
*Maritime air masses form over oceans
-Water evaporates from the oceans, so the air can become very humid
*Continental air masses form over land
-Continental air masses have less exposure to large amounts of moisture from
bodies of water
-Therefore, their masses are drier than maritime air masses
Why is MN so humid in the summer?
Answer: Air masses get their moisture from all the lakes
Maritime Tropical
*Warm, humid air masses form over tropical oceans
*Gulf of Mexico and Atlantic Ocean air masses invade US first
-These air masses affect the weather on the East coast and Central US
*Pacific Ocean air masses affect the weather on the West coast and the Rocky
Mountains
*In summer, maritime air masses cause hot, humid air and thunderstorms
*In winter, maritime air masses cause heavy rain or snow
Maritime Polar
*Cool, humid air masses form over the icy cold North Pacific and North Atlantic
oceans
*Affect West Coast
-Provide fog, rain, and cool temperatures
Continental Tropical
*Hot, dry air masses form mostly in summer over dry area of the Southwest and
northern Mexico
*Cover a smaller amount of land
*Can move to the southern Great Plains
Continental Polar
*Large, form over central and northern Canada and Alaska
*Provide freezing cold weather with low humidity
*In summer, produce huge thunderstorms if they collide with maritime tropical air
masses moving north
How Air Masses Move
*When air mass moves into an area and interacts with other air masses, it causes
the weather to change
*In the continental US, air masses are commonly moved by the prevailing westerlies
and jet streams
Prevailing Westerlies
*Major wind belts over the continental US, push air masses from west to east
Jet Streams
*Bands of high-speed winds about 10 km above Earth's surface, push air masses from
west to east
Fronts
*As huge masses of air move across the land and the ocean, they collide with each
other, but do not mix
*The boundary where two air masses with different temperatures meet is called a
front
*Storms and changeable weather occur at fronts
Types of Fronts
*Colliding air masses can form 4 types of fronts:
1. Cold Fronts
-Cold air is dense and sinks and moves fast
-When a rapidly moving cold air mass runs into a slowly moving warm
air mass, the denser cold air slides under the lighter warm air
-The warm air is pushed upward and a cold front is formed
-Clouds form from cold fronts
-If the air is super moist, then rain or snow may fall
-After a cold front passes through, colder, drier air moves in, often
bringing clear skies, a shift in wind, and lower temperatures
2. Warm Fronts
-Fast-moving warm air mass overtakes a slowly moving cold air mass
-If warm air is humid, then rain or snow may fall
-If warm air is dry, then clouds form
-Warm fronts move slowly, so it can rain for days
-After a warm front passes, weather will be warm and humid
3.Stationary Fronts
-When cold and warm air masses meet, but they can't move each other
-If the fronts cannot move each other, then it will bring many days of
precipitation and clouds
4. Occluded Fronts
-Warm air mass is caught between 2 cold air masses
-Temperatures drop and rain or snow may fall
Cyclones and Anticyclones
*As air masses collide to form fronts, jet streams can cause the boundaries of air
masses to become bent causing swirling air
-Swirling air creates low-pressure centers
Cyclones
*If a weather map has an "L" on it, then the area will have low air pressure
*A swirling center of low air pressure is called a cyclone
*As air at the center of a cyclone rises, air pressure decreases
*Cyclones and decreasing air pressure are associated with clouds, wind, and
precipitation
Anticyclones
*High-pressure centers of dry air
*Weather maps will have an "H" on it
*The descending air in an anticyclone generally causes dry, clear weather