An air flow coming from a particular (warm, cold, moist or dry) is called an air-mass.

* A tropical air-mass consists of air flowing from the tropics, like Polynesia. (It is WARM)

* A polar air-mass consists of air flowing from polar regions, like Antarctica. (It is COLD)

* A maritime air-mass is one flowing over a large sea area, like the Pacific. (It is MOIST)

* A continental air-mass is one flowing over a large land area, like Australia. (It is DRY).

The air-masses reaching New Zealand are generally either maritime polar or maritime tropical. Why?

A front marks the boundary between two air-masses.