3. Air mass

An air mass is an extensive body of air that is located in a specific region on the surface of the Earth (i.e. the origin of the air mass) and stays for a substantial period of time with horizontally uniform physical characteristics, such as temperature and humidity.

Air masses can be classified as continental and maritime air masses such as continental polar air mass and maritime tropical air mass, according to the latitude of their originating region. They can also be classified as cold and warm air masses, according to the nature of heat exchange with the region they pass through after leaving their origin. When the air mass is colder than the land surface it passes through, it will be called a cold air mass. For example, an air mass originating in Siberia during winter is classified as a continental cold air mass. When it moves southward to Taiwan, Taiwan will often experience a drastic temperature decline in a short period of time about 24-48 hours and a severely cold weather will be expected.