Weather Patterns Severe Storms Chapter 20 Section 1

Weather Patterns & Severe Storms Chapter 20

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

1. Air Masses & Weather • Air Masses • Weather patterns result from air masses on the move v. 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

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

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

Classifying Air Masses (cont. ) v. 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 (c. P) air mass is dry and cool • A continental tropical (c. T) air mass is dry and warm • A maritime polar (m. P) air mass is humid and cool • A maritime tropical (m. T) air mass is humid and warm

3. Weather in North America v. Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (c. P) and maritime tropical (m. T) air masses. • Continental Polar Air Masses • Uniformly cold and dry in winter and cool and dry in summer • In summer, c. P 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

Weather in North America (cont. ) • Not associated with heavy precipitation • However, if c. P crosses 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 c. P 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

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 m. T air mass invades the central and eastern United States, it brings the high temperatures and oppressive humidity typically associated with its source region.

Weather in North America (cont. ) • Maritime Polar Air Masses • During the winter, m. P air masses that affect weather in North America come from the North Pacific • Often begin as c. P air masses in Siberia. • The cold, dry c. P air changes into relatively mild, humid, unstable m. P air during its long journey across the United States • As the m. P 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

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

Weather in North America (cont. ) • Continental Tropical Air Masses • c. T air mass begin in the southwestern United States and Mexico during the summer v. Only occasionally do c. T air masses move outside their source region • However, when a c. T 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