This document does not pretend to be established expertise on the subject. It is an attempt by a self-taught student to bring together ideas about meteorology as part of a preparation for cruising in the South Pacific.

Weather can be best understood by thinking at several levels: global, synoptic, intermediate and local.

The Anatomy of the Atmosphere

Imagine the earth as a large smooth billiard ball with a circumference of 40,000 km through the poles. Slightly more round the equator.

Remember it is, by original definition, 10,000 km from the equator to the pole.

Remember also that a minute of latitude is one nautical mile. 90° X 60' = 5,400 nautical miles. Nautical miles X 1.8518 = kilometres, so 5,400nm = 10,000 km.

The atmosphere consists of different layers: the Troposphere, the Stratosphere, the Mesosphere and the Thermosphere. Most weather occurs in the Troposphere. It is about 20,000 ft deep over the poles, and about 60,000 ft deep over the Equator.

Over the equator, maximum sunlight lands. The air is heated and rises. Over the poles, minimum sun lands. The air is cold and sinks. If the atmosphere were infinitely deep, there would probably be a single circulation, rising at the equator and sinking at the poles.

In fact, the circulation on Earth involves three cells in the Troposphere, rotating like cogs in a clock. Cold air sinks over the poles, and moves towards the 60° parallel in the Polar cell, where it rises. Also rising at the 60° parallel is air from the Ferrel cell, which rotates in the opposite direction, sinking at the 30° parallel, where it meshes with the sinking air from the Hadley cell. Because of the Coriolis effect, (see below) the winds are all bent to the right in the northern hemisphere, and to the left in the southern hemisphere.

The complete diagram summarizing atmospheric circulation. Note that rising warm, moist air results in a low pressure zone and descending cold, dry air results in a high pressure zone. These zones of high and low pressure along with the Coriolis force create the major wind belts. Note: The Hadley cell extends from the equator to about 30 degrees N and S latitude, the Ferrel cell extends from about 30 to 60 degrees N and S latitude and the Polar cell extends from 60 degrees latitude to the poles (90 degrees N and S latitude).

Other planets have different numbers of Hadley cells. Jupiter has many more, with jetstreams between them.