What is air mass? In how many categories are air masses categorized on the basis of their source regions?

Explanation

The air with distinctive characteristics in terms of temperature and humidity is called an air mass. In other words, it is defined as a large body of air having little horizontal variation in temperature and moisture.
The homogenous surfaces, over which air masses form, are called the source regions. The air masses are classified according to the source regions into five major source regions. These are:
(i) Warm tropical and subtropical oceans: Maritime tropical (mT),
(ii) The subtropical hot deserts: Continental tropical (cT),
(iii) The relatively cold high latitude oceans: Maritime polar (mP),
(iv) The very cold snow covered continents in high latitudes: Continental polar (cP),
(v) Permanently ice covered in the Arctic and Antarctica: Continental arctic (cA).