Air Masses 4:Weather Patterns and Severe Storms

A mass of cold, dry air brings temperatures of -10°C to Minneapolis, Minnesota. Which temperature will the air mass most likely bring when it reaches Memphis, Tennessee, a city farther south?

2°C

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 or hot. Maritime polar (mP) and maritime tropical (mT) air masses both form over water. But a maritime polar air mass is much colder than a maritime tropical air mass.

North America Maritime

As this maritime polar air arrives at the western shore of North America, it is often accompanied by low clouds and showers. When this maritime polar air advances inland against the western slopes of the Rocky Mountains, uplift of the air produces heavy rain or snow on the windward slopes of the mountains. Maritime polar air masses also originate in the North Atlantic off the coast of eastern Canada. These air masses influence the weather of the 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.

Continental Polar Air Masses

Continental polar air masses are 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, this continental polar air brings the clear skies and cold temperatures normally associated with a cold wave. Continental polar air masses are not, as a rule, associated with heavy precipitation. However, those that cross the Great Lakes during late autumn and winter sometimes bring snow to the leeward shores. These localized storms, which are known as lake-effect snows, make Buffalo and Rochester, New York, among the snowiest cities in the United States.

What causes large amounts of snow to fall on the southern and eastern shores of the Great Lakes?

Continental polar air masses, crossing the Great Lakes with its associated warmer temperatures, cause heavy lake-effect snows.

Continental Tropical Air Masses

Continental tropical air masses have the least influence on the weather of North America. These hot, dry air masses begin in the southwestern United States and Mexico during the summer. Only occasionally do cT air masses affect the weather outside their source regions. However, when a cT air mass does move from its source region, it can cause extremely hot, drought-like conditions in the Great Plains in the summer. The movement of such air masses in the fall results in mild weather in the Great Lakes region, often called Indian summer. Conditions during Indian summer are unseasonably warm and mild.

Maritime Polar Air Masses

During the winter, maritime polar air masses that affect weather in North America come from the North Pacific. Such air masses often begin as cP air masses in Siberia. The cold, dry continental polar air changes into relatively mild, humid, unstable maritime polar air during its long journey across the North Pacific.

Air Masses and Weather

For the many people who live in the middle latitudes, which include much of the United States, summer heatwaves and winter cold spells are familiar experiences. During summer heatwaves, several days of high temperatures and high humidity often end when a series of storms pass through the area. This stormy weather is followed by a few days of relatively cool weather. By contrast, winter cold spells are often characterized by periods of frigid temperatures under clear skies. These bitter cold periods are usually followed by cloudy, snowy, relatively warm days that seem mild when compared to those just a day earlier. In both of these situations, periods of fairly constant weather conditions are followed by a short period of changes in the weather.

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Continental and Maritime

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 humid.

Maritime Tropical Air Masses

Maritime tropical air masses also play a dominant role in the weather of North America. These air masses are warm, loaded with moisture, and usually unstable. Maritime tropical air is the source of much, if not most, of the precipitation received in the eastern two-thirds of the United States. Heavy precipitation may be the result of maritime tropical air masses moving through the area. In summer, when an mT 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

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. The cP air masses begin in northern Canada, the interior of Alaska, and the Arctic areas. The mT air masses most often begin over the warm waters of the Gulf of Mexico, the Caribbean Sea, or the adjacent Atlantic Ocean.

Introduction

Severe storms are among nature's most destructive forces. Every spring and summer, for example, newspapers and newscasts report the damage caused by tornadoes, which are short but violent windstorms that move quickly over land. The forces associated with these storms can be incredibly strong. During late summer and early fall, you have probably heard reports about severe storms known as hurricanes. Unlike tornadoes, hurricanes form over Earth's tropical oceans. As they move toward land, the strong winds and heavy rains produced by these storms produce tremendous destruction along their paths. You are probably most familiar with a type of severe storm known as a thunderstorm.

source region

The area over which an air mass gets its characteristic properties of temperature and moisture

Classifying Air Masses

The source regions that produce air masses that influence the weather in North America are shown in the figure. Air masses are named according to their source region. Polar (P) air masses form at high latitudes toward Earth's poles. Air masses that form at low latitudes are tropical (T) air masses. The terms polar and tropical describe the temperature characteristics of an air mass. Polar air masses are cold, while tropical air masses are warm.

Air Masses

The weather patterns just described result from movements of large bodies of air called air masses. An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given altitude. An air mass can be 1,600 kilometers or more across and several kilometers thick. Because of its size, it may take several days for an air mass to move over an area. This causes the area to experience fairly constant weather, a situation often called air-mass weather. Some day-to-day variations may occur, but the events will be very unlike those in an adjacent air mass.

Lake-Effect Snow

What causes lake-effect snow? During late autumn and early winter, the difference in temperature between the lakes and adjacent land areas can be large. The temperature contrast can be especially great when a very cold cP air mass pushes southward across the lakes. When this occurs, the air gets 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. Heavy snow is not unusual during this time of the year.

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. A cold, dry air mass from northern Canada is shown moving southward. The initial temperature of the air mass is -46°C. It warms 13 degrees by the time it reaches Winnipeg. The air mass continues to warm as it moves southward through the Great Plains and into Mexico. Throughout its southward journey, the air mass becomes warmer. But it also brings some of the coldest weather of the winter to the places in its path. As it moves, the characteristics of an air mass change and so does the weather in the area over which the air mass moves.

Which type of air mass brings the hottest, driest weather?

Which type of air mass brings the hottest, driest weather?

Thunderstorms

a type of severe weather that produces heavy rains, loud noises you know as thunder, and flashes of light called lightning.

What is an air mass?

an immense body of air characterized by temperature and moisture content

Which two air masses most affect the weather of the eastern United States?

continental polar (cP) and maritime tropical (mT)

Which type air mass is responsible for much of the precipitation in the eastern two-thirds of the United States?

maritime tropical

Maritime air masses are typically _____ than continental air masses.

more humid