BACKGROUND:

The type of air mass over an area is dependent on the (1) latitude and (2) whether it is over a continental or marine area. This exercise is just to familiarize students with the different type of air masses that occur over the United States.

You may want to discuss that warm air causes low pressure areas and that cold air causes high pressure areas. Generally a high pressure area moves toward a low pressure area, When the two air masses meet, they will form a FRONT, which will cause weather changes.

Another term for a low pressure center is called a cyclone and for high pressure area it can be called anticyclones. The pressure decreases from the outside toward the center in a cyclone. In an anticyclone the pressure increases from the outside toward the center. Winds blow from high pressure to low pressure and are deflected to the right or left by the earth's rotation. This deflection is similar to the deflection in the oceans. In the Northern Hemisphere the result is that winds blow in and counterclockwise around a low and out and clockwise around a high as shown on the diagram on the right.

Local winds may "adjust" the situation. Local winds may be classified as slope winds, coast winds and mountain winds. Slope winds represent the slow drainage into the valleys of air which have been chilled by contact with the cool ground. Sea breezes are caused by land becoming considerably warmer than the sea. The breeze reaches its maximum strength in the early afternoon and usually persists into the evening before dying away. Mountain winds are caused when a "funnel" is produced by the topography of the mountain sometimes causing a very persistent wind for days.

The underlying cause of winds is the unequal heating of the Earth's surface and the fact that the Earth rotates. But the amount of heat varies considerably, and there are many other factors. This is the reason why weather persons do not have a good record for predicting and forecasting the weather. It’s a hard job!

PROCEDURE:

1. The key objective is for students to see that the atmosphere is  very complicated.  The worksheet shows students where stable air masses are located in North America.  Other factors like Coriolis effect that cause movement of the air masses set up air masses that collide, causing fronts.
      
2. Students should look at the worksheet and determine the type of stable air masses found in North America. 
     
3. Ask students about the weather patterns that occurs at the boundaries of the air masses.  Usually they are very unstable and cause severe weather patterns, except for the hot, dry air masses.
     
4. Have the student try and predict the climate for specific areas.  
   1. Students could compare the temperature and/or precipitation the year they were born and last year.  

   ANSWERS:
   The air masses listed here reflect the average climate.  Notice in the central portion of the United States the air masses are too unstable to generalize.  The answers are: 1.(cA);  2.(mP);  3.(cP);  4.(mP);  5.(mT);  6.(cT); and  7.(mT).

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