Figure 25 – Airmasses

1. Maritime tropical (mT)

2. Continental tropical (cT)

3. Maritime polar (mP)

4. Continental polar (cP)

5. Continental arctic (cA).

Where ‘m’ stands for Maritime; ‘c’ stands for continental; ‘T’ stands for tropical; ‘P’ stands for polar and ‘A’ stands for arctic region.

As these air masses move around the earth they can begin to acquire additional attributes. For example, in winter an arctic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar air mass (mP) - one that is still fairly cold but contains moisture. If that same polar air mass moves south from Canada into the southern U.S. it will pick up some of the warmth of the ground, but due to lack of moisture it remains very dry. Another way of changes is internal modification in the airmasses. The resultant air mass by these processes is termed as secondary air mass. Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.