8.36 Air-masses and Fronts

8.36.2 Define an ‘air-mass’.

An air mass is a large body of air with relatively uniform temperature and moisture characteristics.

Air masses take on these characteristics from their source region, such as tropical or polar areas, and then influence the weather as they move across another region.

8.36.4 State the two air-masses that routinely affect the New Zealand region.

The two air masses that routinely affect the New Zealand region are:

• tropical air
• polar air

Tropical air is generally warmer and often moist.

Polar air is generally colder and may become unstable, especially when it moves over warmer sea surfaces.

8.36.6 Define a ‘front’.

A front is the boundary between two air masses with different temperature and moisture characteristics.

Fronts are commonly associated with changes in:

• temperature
• pressure
• wind direction and speed
• cloud
• precipitation
• visibility
• turbulence

8.36.8 Describe the formation processes of the following frontal types:

(a) cold;

A cold front forms when a cold air mass advances into an area occupied by warmer air.

The cold air is denser, so it undercuts the warm air and forces it to rise.

This lifting may produce: