In order to build the big picture you need to know what creates it.
The three basic elements of weather are:
- temperature – warm or cold
- wind – direction and speed
- moisture or humidity
Air masses can be described as warm, cold, moist or dry. Air masses are named according to where they originate from, and each has its own characteristic temperature and humidity.
- A tropical air mass consists of air flowing from the tropics, and is generally warm.
- A polar air mass consists of air flowing from polar regions, and is cold.
- A maritime air mass is one flowing over a large sea area, and is moist.
- A continental air mass is one flowing over a large land area, and is dry.
The air masses reaching New Zealand are generally maritime-polar or maritime-tropical.
Temperature differences support the development of low pressure systems. Surface low pressure systems usually have fronts associated with them. A front is the zone between two air masses that contain different combinations of the three basic elements: temperature, wind, and moisture.
A front marks the boundary between two air masses and appears on the weather map as a line with triangles or semicircles attached. See later in this course for more information.