It is convenient to think of the lower atmosphere, or troposphere, as one uniform and consistent mass of air. In reality this is not the case. Large pockets of air commonly form that are distinct from the surrounding atmosphere. We call these moving pockets of air air masses.
All air masses have three things in common. Without these three characteristics we would not consider a body of air an air mass. These three things are as follows:
Firstly, an air mass must be large in size. Often they are more than 1,000 miles (1,600 km) across. In addition to being large horizontally across the landscape, they can also extend several miles, or kilometers, upward in altitude, extending high into the atmosphere.
Secondly, an air mass must have a uniform and consistent makeup at all points within the air mass. This means that temperature, humidity and stability will be relatively the same at any point within the air mass.
Thirdly, an air mass must be physically bound together, traveling across the atmosphere as a single unit. It must be sufficiently strong so as to not break apart as it is pushed along its course.