Weather varies considerably from day to day, especially in middle and high latitudes. It is controlled by weather systems. Everywhere in the world, these are found in various states of development, some intensifying, some mature, others decaying.

The rain-bearing weather systems of the tropics and sub-tropics range from weak features that bring thick cloud and heavy showers to the powerful cyclones that bring torrential downpours and hurricane-force winds. They tend to follow a sequence building from a tropical depression, into a tropical storm, and if the conditions are right into a truly powerful cyclone. Tropical weather systems will grow through this sequence creating new systems as they die back again. Near equatorial and rain systems in the ITCZ contain systems of developing, mature and decaying cumulonimbus clouds.

Weather in mid and high latitudes is controlled by systems called depressions and anticyclones. Anticyclones, or 'highs', usually bring fine, settled weather. Depressions, or 'lows', bring rain, strong winds and changeable conditions. Changeable weather is a feature of British weather, depressions are responsible for much of this. In Britain, most depressions cross the country from west to east, starting in the Atlantic Ocean. As a depression passes there are distinct changes in weather conditions. Air moves in clockwise and anti-clockwise whirls like water in a rocky stream. These whirls move around warm and cold air in the form of air masses. (with uniform temperature and humidity characteristics often from the region in which they formed). Fronts develop at the boundaries of the air masses because masses of air at different temperatures do not mix but move up and over each other.