1. Earth Science 20.1 Weather Patterns & Severe Storms

2. Earth Science 20.1 Weather Patterns & Severe Storms
Severe storms are among natures most destructive forces.
The forces associated with these storms can be incredibly strong.
During late summer and early fall, severe storms known as hurricanes strike the United States.
Strong winds and heavy rains from hurricanes can do major damage and have destroyed entire American cities in a single storm.
Thunderstorms as well can trigger more intense weather such as tornados which can leave a path of death and destruction in their wakes as they strike with little warning.

3. Air Masses and Weather:
Earth Science 20.1 Weather Patterns & Severe Storms
Air Masses and Weather:
For many people who live in the middle latitudes, which includes much of the United States, summer heat waves and cold winter spells are familiar experiences.
During summer heat waves, several days of high temperatures and high humidity often end when a series of strong storms pass through an area.
This stormy weather is generally followed by several days of cool sunny weather. By contrast, winter brings periods of frigid temperatures under clear skies.

4. Earth Science 20.1 Weather Patterns & Severe Storms
These bitter cold periods are usually followed by cloudy, snowy, or relatively warm days that seem mild when compared with the previous deep freeze.
In both these cases, periods of fair weather are followed by short periods of changes in the weather.
What do we think triggers these changes?

5. Air Masses
Air Masses
The weather patterns associated with the weather we 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 latitude.
An air mass can be 1600 kilometers across or larger and several kilometers thick.

6. Air Masses
Air Masses
Because of it’s 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 events in adjacent air-masses.

7. Movement of Air Masses
When an air mass moves out of a region over which it formed, it carries its temperature and moisture conditions with it.
An example of a cold dry air mass from Northern Canada is shown in blue in the example at right moving southward.
The initial temperature of the air mass in northern Canada is -46C. It warms 13 degrees by the time it reaches Winnipeg. The air continues to warm as it moves southward through the US Great Plains and into Mexico. As it moves, the characteristics of an air mass change and so does the weather in the area over which the air mass moves.

8. Classifying Air Masses
The area over which an air mass gets its characteristic properties of temperature and moisture is called its source region.
The source regions that produce air masses that influence weather in North America are shown in the figure at right.
Air masses are named according to their source region. Polar (P) air masses form at high latitudes near Earth’s poles.

9. Classifying Air Masses
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.

10. Classifying Air Masses
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 content of the air masses. Continental air masses are typically dry while maritime air masses are humid and moist.

11. 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 (continental polar) air masses begin in northern Canada, the interior of Alaska, and the Arctic areas.
The mT (maritime tropical) air masses most often begin over the warm waters of the Gulf of Mexico or the adjacent Caribbean waters.

12. Continental Polar Air Masses
Continental Polar (cP) 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, these continental polar air masses bring the clear skies and cold temperatures we associate with cold waves.

13. Continental Polar Air Masses
Continental Polar (cP) air masses are not, usually, associated with heavy precipitation.
However, those that cross the Great Lakes during late autumn and winter sometimes bring snow to leeward shores.
These localized storms, which are known as lake-effect storms, make Buffalo and Rochester New York among the snowiest cities in the US.

14. Continental Polar Air Masses
What causes lake-effect storms? During late autumn and early winter, the difference in temperature between the lakes and adjacent land areas can be very large.
This temperature contrast can be very great when a very cold cP (continental polar) 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 and heavy snow is possible.

15. Maritime Tropical Air Masses
Maritime tropical air masses (mT) 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.

16. Maritime Tropical Air Masses
The heavy precipitation that arrives with hurricanes is the result of maritime tropical air masses moving through an area.
In summer, when an mT (maritime tropical) air mass invades the central or eastern United States , it brings the high temperatures and excessive humidity associated with it’s source region, the tropics.

17. Maritime Polar Air Masses
During the winter, maritime polar air masses that affect weather in North America come from the Pacific Northwest.
Such air masses often began as cP (continental polar) air masses in Siberia.
The cold, dry continental polar air changes into a relatively mild, humid, unstable maritime polar air during it’s long journey across the North Pacific as it picks up moisture.
As this maritime polar air arrives at the western shore of North America, it is often accompanied by low clouds and showers.

18. Maritime Polar Air Masses
When this maritime polar air (mP) advances inland against the western mountains, uplift of the air produces heavy rain or snow on the windward sides 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 New England.

19. Maritime Polar Air Masses
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 regionally as a nor’easter.

20. Continental Tropical Air Masses
Continental tropical air masses have the least influence on the weather of North America.
These hot, dry masses begin in the southwestern United States and Mexico during the summer.
Only occasionally do cT (continental tropic) air masses affect the weather outside their source regions.

21. Continental Tropical Air Masses
However, when a cT (continental tropic) air mass does move from it’s source region, it can cause extremely hot, drought like conditions in the Great Plains of the United States in the summer months.
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.