An air mass is a large body of air with a generally uniform temperature and moisture level. Such a mass can extend over 100 miles or more horizontally and have a height of between 9,000 and 16,000 kilometers (5600 - 10,000 miles).
There are three main factors which will determine the characteristics of an air mass: the type of area over which it has passed, changes that have occurred to the mass over time, and its age.
In general terms air masses are defined and described according to their source areas. For example a mass moving away from Equatorial areas will be known as a tropical mass, whereas one moving down from the poles will be called a polar mass. Similarly an air mass originating over an ocean will be a maritime (moist) mass, whereas one passing over long distances of land will be a Continental (dry) mass.
Together these features define the four most common types of air mass: tropical (warm)maritime, tropical (warm) continental, polar (cool or cold) maritime and polar (cool or cold) continental.
When two air masses collide, they tend to stay separate in the first instance, as oil remains separate from water. What happens when two masses collide relates to the differing characteristics of each mass.
Warm air is lighter than cool air, therefore when a mass of warm air meets a mass of cooler air, it tends to rise up over it. How quickly this rising takes place will be affected by the type of collision between the masses.
If a warm air mass collides with a cool or cold mass, it will ride gradually up and over it. The line which separates the warmer air from the cooler air is called a front and the front will rise from the ground at an angle with the warmer air above the cooler air. Normally, if it is warm air which approaches the cooler air the front will form a gentle angle and the warmer air will consequently rise gently upwards. However, if cooler air approaches and collides with warmer air, the heavy, denser cool air will tend to undercut the warmer air, pushing it rather like a bulldozer, forming a steeply rising front.
The name of each front is derived from the type of air which follows behind it at ground level. For example when warm air is approaching cool air, the passing of the front will presage a rise in temperature as an observer passes from the cool mass to the warm mass, and consequently this is a warm front, marked with red semi-circles on a weather map.
On the other hand, if cool air is approaching a warmer air mass the front will mark a fall in temperature and be called a cold front, marked with blue triangles on a weather map.
In each case, with warm air rising over cool air, the air will expand and cool as it rises. If the rising air contains a large amount of moisture, this moisture will tend to condense into water droplets when the temperature of the rising air reaches dew point. This is not a fixed temperature, but varies due to different meteorological characteristics.
The formation of water droplets will cause clouds to develop, and if the clouds grow to a sufficient extent, rain will fall. In general terms, the faster the air rises, the taller will be the clouds, and the heavier will be the rain.
In conclusion, masses of air take their characteristics mainly from the regions over which they pass, being either warm or cool, moist or dry. When two masses collide the warmer air will ride up and over the cooler air, causing condensation of the water vapour it contains, as well as clouds and rain.