In most cases these air masses do not merge with each other, but are clearly separated by relatively narrow transition zones, or fronts. 7.1 Definition The troposphere is not a single entity with uniform physical characteristics, as its horizontal segmentation shows. An air mass may be defined as a large section of the troposphere with uniform properties of temperature and moisture in the horizontal. They are separated by fronts, which form transition zones between two air masses, zones in which horizontal temperature and moisture fields undergo significant and rapid changes. An air mass is defined as a large section of the troposphere whose characteristics are influenced by the large regions it has crossed in its lifetime.
Air masses are often several thousands of kilometres across.
In winter, cold dry arctic air can cover more than half of North America and extend large fingers down into the warm and moist tropical air masses. Although the cold air is gradually warmed as it moves southward, there is still a marked difference between the two air masses, when they are side by side in the temperate zone. Since air masses generally do not merge, the transition zone between the warm air and the cold air is relatively narrow. This is the polar front. If the earth's surface were uniform, there could be just two air masses, one cold and the other warm, separated by a polar front.