The world's weather is carried around the earth by huge swirling weather systems called highs and lows - areas of high and low pressure. Areas of high pressure - called anticyclones, are made by falling air. They move slowly, causing the weather to be settled. the air is dry, bringing hot dry weather in summer and cold clear weather in winter. Areas of low pressure are called depressions. They are caused by rising air. The air is moist, bringing clouds, rain or maybe snow.

A depression is formed where a belt of warm and cool air collide. The two do not mix, but push into each other. Fronts form at the boundaries of the air masses and the weather becomes unsettled. A depression can be hundreds of miles across but usually passes across within 24 hours. A warm front is usually the first to arrive. When it has passed, a cold front follows close behind.

A warm front has warm moist air behind it. This warm air rises up over the cold air and forms clouds along the front. When the warm front has passed, there will be dry weather before the cold front arrives.

A cold front has cold air behind it. The front is much steeper than a warm front. The cold air pushes underneath the warm air and water vapor rises and condenses into clouds and rain. The air pressure drops and the wind gets stronger. As the fronts moves on, there are often showers from rain clouds trailing behind.

An occluded front is the area where the warm and cold front meet. The weather is then a mixture of the two.