|MadSci Network: Earth Sciences|
It's great that you still have an interest in the answer even after the test is finished. Curiosity, and caring about the things you learn, are the most important qualities of a good scientist. I think your teacher should help you find the answer, as long as it won't affect your grade. On the other hand, you should be able to do enough research yourself to find the answer.
The National Weather Servicedefines an air mass as
Air Mass- A large body of air that has similar horizontal temperature and moisture characteristics.
How could such a body be created? Well, the temperature and humidity of air are set by the amount of incoming sunlight and the properties of the ground. So the air at a place where the sunlight and ground properties are the same over a large area will tend to have the same properties. For example, air over an ocean will tend to become warm and humid, air over a desert will become hot and dry, and air over the Arctic will become cold and dry. Thus, an air mass is formed. These air masses are then carried to other places by the wind. Usually we only call it an "air mass" after it's been carried from somewhere else by the wind: when the air is typical for an area, it's usually not worth mentioning.
Try the links in the MadSci Library for more information on Earth Sciences.