In meteorology, an air mass is a large volume of air having fairly uniform characteristics, covering many thousands of square miles, and taking on the charcteristics of the Earth's surface over which it lies.
An air mass is typically referred to as a large scale event. Whereas, a microburst is a small scale event. An ideal air mass has a characteristic temperature, atmospheric pressure, and water vapor content. Also, the air mass can have well defined borders or poorly defined borders with other air masses. At these borders, where air masses collide, weather events can occur. Depending on the characteristics of the colliding air masses, different types of weather can be produced. These collisions are called fronts.
In a real air mass, its characteristic properties can vary with time. For instance, as an air mass moves over a warmer land area, its temperature will increase slowly. Also, if an air mass moves over large water region, it can acquire more water vapor. Air masses can not be defined by perfect lines or borders. Usually, there is a small region of interaction of two or more air masses where mixing occurs. But on a large scale map, good approximations can be made to locate the general region of these frontal zones.
See Weather front.