Weather Patterns & Severe Storms

1. Weather Patterns & Severe Storms Chapter 20

2. Section 1: Air Masses Introduction • Section Objectives: • Define air mass • Explain how air masses are classified • Explain characteristics features of each air mass class • Explain the influence of continental polar and maritime tropical air masses on the majority of North America

3. 1. Air Masses & Weather • Air Masses • Weather patterns result from air masses on the move • An air mass is an extremely large body of air that is located in the troposphere and is characterized by similar temperatures and amounts of moisture at any given altitude • Can be 1600 km or more across and several km thick • An air mass may take several days to move over an area, which experiences fairly constant weather, called air-mass weather • Some day-to-day changes in the weather within an air mass may occur, but the events will be different from a nearby air mass

4. Air Masses & Weather (cont.) • Movement of Air Masses • When an air mass moves out of the region over which it formed, it carries its temperature and moisture conditions with it • As an air mass moves, its characteristics change and so does the weather in the area over which it moves

5. 2. Classifying Air Masses • The area over which an air mass gets its characteristic properties of temperature and moisture is called its source region • Air masses are named according to their source region • Polar (P) air masses form at high latitudes towards Earth’s poles • Low latitude air masses are Tropical (T) air masses • The terms polar and tropical describe the temperature characteristics of an air mass • Polar air masses are cold, tropical air masses are warm

6. Classifying Air Masses (cont.) • In addition to their overall temperature, air masses are classified according to the surface over which they form • Continental (c) air masses form over land • Maritime (m) air masses form over water • The terms continental and maritime describe the moisture characteristics of the air mass • Continental air masses are likely to be dry • Maritime air masses are likely to be humid • Using this classification scheme, there are four basic types of air masses • A continental polar (cP) air mass is dry and cool • A continental tropical (cT) air mass is dry and warm • A maritime polar (mP) air mass is humid and cool • A maritime tropical (mT) air mass is humid and warm

7. 3. Weather in North America • Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses. • Continental Polar Air Masses • Uniformly cold and dry in winter and cool and dry in summer • In summer, cP air masses may bring a few days of relatively cooler weather • In winter, it brings the clear skies and cold temperatures characteristic of a cold wave

8. Weather in North America (cont.) • Not associated with heavy precipitation • However, if cPcrosses over small bodies of water it brings snow storms to the other side of the water • These localized snow storms are known as lake-effect snows • The cold cP air mass picks up large quantities of heat and moisture from the relatively warm lake surface • By the time it reaches the opposite shore, the air mass is humid and unstable. • Lake-effect snows make certain cities among the snowiest cities in the United States • The areas that receive heavy snow are known as snowbelts

9. Weather in North America (cont.) • Maritime Tropical Air Masses • Also play a dominant role in the weather of North America • These air masses are warm and loaded with moisture • They are usually unstable • The source of much, if not most, of the precipitation received in the eastern two thirds of the United States • In the summer, when a mT air mass invades the central and eastern United States, it brings the high temperatures and oppressive humidity typically associated with its source region.

10. Weather in North America (cont.) • Maritime Polar Air Masses • During the winter, mP air masses that affect weather in North America come from the North Pacific • Often begin as cP air masses in Siberia. • The cold, dry cP air changes into relatively mild, humid, unstable mP air during its long journey across the United States • As the mP air arrives at the western shore of North America, it is often accompanied by low clouds and showers • When it advances inland against the western mountains, uplift of the air produces heavy rain or snow on the windward slopes of the mountains

11. Weather in North America (cont.) • Maritime polar air masses also originate in the North Atlantic off the coast of eastern Canada. • These air masses influence the weather of northeastern United States • In winter, when New England is on the northern or northwestern side of a passing low-pressure center, the counterclockwise winds draw in maritime polar air • The result is a storm characterized by snow and cold temperatures, known locally as a nor’easter

12. Weather in North America (cont.) • Continental Tropical Air Masses • cT air mass begin in the southwestern United States and Mexico during the summer • Only occasionally do cT air masses move outside their source region • However, when a cT air mass does move from its source region in the summer, it can case extreme heat and drought in the Great Plains • Movement of such air masses in the fall result in mild weather, often called Indian Summer, in the Great Lakes region