1. Air Masses and Fronts

2. Air Masses An air mass is an immense body of air (1600 km/1000 mi or more across and several km/2 mi thick) that is characterized by similar temperatures and amounts of moisture at any given altitude.

3. Movement of Air Masses Because of its size, it may take several days for an air mass to move over an area, so that area experiences fairly constant weather during its passage. Remember last week’s very cool weather? We were in a large cold air mass.

4. When an air mass moves out of the region over which it is formed, it carries its temperature and moisture conditions with it. As it moves, its characteristics change and so does the weather it causes. How much warmer was the air mass when it reached Tampico, Mexico, than when it formed?

5. Classifying Air Masses The area over which an air mass gets its characteristic properties of temperature and moisture is called its source region. Polar (P) air masses form at high latitudes Tropical (T) air masses form at low latitudes

6. Moisture Classification In addition to their overall temperature, air masses are classified according to the surface over which they form and describe the air mass’ moisture characteristics: Continental (c) air masses form over land and are likely to be dry Maritime (m) air masses form over water and are humid or moist.

7. Four Basic Types of Air Masses Using this classifi- cation, there are 4 basic types of air masses: (embedded movie)

8. Continental Polar (cP) SOURCES: Canada, Alaska, northern US Are uniformly cold and dry in winter and cool and dry in summer Along with maritime tropical air masses, responsible for much of the weather in North America. Can bring heavy snow to the leeward shores of the Great Lakes (downwind from the Lakes)

10. Maritime Polar (mP) SOURCES: North Pacific, North Atlantic Cool, humid, unstable maritime air Often starts as cP air over Siberia but becomes more humid and warmer in its long journey over the Pacific Ocean mP air masses in the Atlantic affect weather in the northeastern states when New England is north or northwest of a passing low pressure center, leading to winter storms called nor’easters

11. Continental Tropical (cT) SOURCES: Mexico, southwestern US Hot, dry air masses that usually do not influence weather outside of their source region (have the least influence on US weather) In the summer, can cause extremely HOT and dry conditions in the Great Plains region In the fall, when moving north toward the Great Lakes region, causes warm and mild conditions called “Indian summer”

12. Maritime Tropical (mT) SOURCE: Gulf of Mexico, Pacific off Mexico Warm to hot, very moist, usually unstable Along with cP air masses, play a dominant role in North American weather Responsible for most of the precipitation that falls in the eastern two-thirds of the US (east of the Rocky Mountains) In summer, can bring high temperatures and oppressive humidity

13. Weather Fronts

14. Air Mass Review We learned that air masses are Large (1600 km by several km deep) Maintain the temperature and moisture characteristics of their source regions Labeled by their source region So, what happens when air masses meet?

15. What is a front? A boundary that separates two air masses Very narrow—between 15 and 200 km wide Warmer air overlies cooler, denser air Both air masses can move at the same speed and direction, but most often, one moves faster than the other, advancing into the slower mass with some mixing of air

16. Types of fronts Classified by the temperature of the ADVANCING air mass Four kinds:  Warm  Cold  Stationary  Occluded

17. Cold Front Cold, dense air moves into an area occupied by warm air Twice as steep as warm front, moves faster If the warm air is unstable, violent weather occurs with cumulonimbus clouds, heavy downpours and gusty winds Behind the front, cumulus or stratocumulus clouds form where the cold, stable air mass dominates.

18. Warm air moves into an area formerly covered by cooler air Gradual, low slope Cloud sequence: cirrus, cirrostratus, altostratus, finally stratus and nimbostratus about 300 km ahead of front Moves slowly Light-to-moderate precipitation over an extended period Winds shift from EAST to SOUTHWEST Warm Front

19. Stationary Front Sometimes there is a standoff, and neither air mass moves The resulting surface front doesn’t move Can have gentle to moderate precipitation

20. Occluded Front An active cold front overtakes a warm front, forcing the warm air upward so that it is above the surface

21. Mid-Latitude Cyclones Apply your new under- standing of fronts to weather patterns in the contiguous US The main weather producers are mid-latitude cyclones or low pressure systems that travel from west to east and cause stormy weather These storms usually have a cold front and a warm front associated with them

22. The mid-latitude cyclone starts with a frontal boundary, with wind moving in opposite directions. Over time, a ‘kink’ or ‘wave’ develops, and warm air moves poleward and cold air moves equatorward. This develops into a counterclockwise rotation with falling pressure at the ‘kink.’ As the cold air overtakes the warm air, an occlusion begins, strengthening the storm. Pressure at the center of the storm continues to drop and wind speeds increase.

23. During the winter, this phase of the storm can bring heavy snowfalls and blizzard-like conditions. During the summer, this phase of the storm can bring severe thunderstorms and tornadoes.

24. Airflow Aloft Maintains Cyclones Cyclones often exist for a week or longer. Remember that lows are areas of surface convergence, so for them to stay as storms this long, there must be divergence aloft that maintains the surface storm. Most of the time air aloft is what fuels a mid-latitude cyclone.

25. Lastly, where does the diverging air aloft go? Where does the inflowing surface air come from? Notice that cyclones (lows) and anticyclones (highs) occur together. Like cyclones, anticyclones depend on air aloft to maintain their circulation.

26. Reviewing Concepts Answer these questions on a sheet of paper: 1.What happens when two air masses meet? 2.Describe a warm front (clouds, precip., timing, symbol on weather map) 3.What are the characteristics of a cold front? 4.What are the stages in the formation of an occluded front? 5.What is a mid-latitude cyclone? 6.What sustains a mid-latitude cyclone?