1. CHAPTER 20.1 Air Masses

2.  Severe storms can be one of nature’s most destructive forces.  During spring time there are tornadoes, which or short, violent storms.  During late summer and early fall there are hurricanes – have strong winds and heavy rains.  Thunderstorms produce heavy rains, loud thunder, and lightning.

3. AIR MASSES AND WEATHER  In many places in the US summer will have heat waves, several days with high temperatures and high humidity. This will be followed by storms and then cooler weather for a few days.  Winter cold spells have periods of very cold temperatures with clear skies followed by cloudy, snowy, warm days.

4. AIR MASSES  The weather before takes place because of air masses.  An air mass is a large body of air that has similar temperatures and amounts of moisture at any altitude.  Air masses can very large so it may take several days for an air mass to move over an area. This gives fairly constant weather.

5. MOVEMENT OF AIR MASSES  When an air mass moves out of an area where it formed it takes the temperature and moisture it has with it.  What this means is that as an air mass moves it will become warmer as it moves southward.  As an air mass moves the characteristics and weather in the air over the air mass changes.

6. CLASSIFYING AIR MASSES  The source region is the area where an air mass forms and where it gets is temperature and moisture.  Air masses are named according to their source regions.  Polar air masses form at high latitudes near the poles. They tend to be cold.  Tropical air masses form at low latitudes. They tend to be warm.

7.  Air masses are also classified by the surface over which they form.  Continental air masses form over land and tend to be dry.  Maritime air masses form over water and will be humid.

8.  There are 4 basic types of air masses: continental polar (cP) which is dry and cool.  Continental tropical (cT) which is dry and warm or hot.  Maritime polar (mP) which is colder.  Maritime tropical (mT) which is warmer.

9. WEATHER IN NORTH AMERICA  Most of the weather in North America is influenced by continental polar and maritime tropical air masses (especially east of the Rocky mountains)

10. CONTINENTAL POLAR AIR MASSES  Continental polar air masses are usually cold and dry in the winter and cool and dry in the summer. They usually do not bring large amounts of rain. They can bring snow in areas with large lakes. This is called lake-effect snow.

11. MARITIME TROPICAL AIR MASSES  These air masses usually have warm air with a lot of moisture that is unstable.  This air brings a majority of the precipitation in the eastern United States.  This type of air mass also gives very high temperatures and high humidity in the summer in the central and eastern United States.

12. MARITIME POLAR AIR MASSES  In the North Pacific we get air masses that begin in Siberia. This air moves over the ocean so it becomes mild, humid, unstable air. This air will produce heavy rain or snow.  These air masses also start in the North Atlantic and move into the northeastern United States. This air brings in storms that has snow and cold temperature – a nor’easter.

13. CONTINENTAL TROPICAL AIR MASSES  This type of weather does not affect much of the United States. It can cause extremely hot, drought-like conditions in the Great Plains in the summer. This also gives what is called an Indian Summer – where you have unseasonably warm and mild weather.