The weather depends on two main principals, Bonfires and beer! Bonfires: From the diagram below you can see that the fire heats up the air making it rise and cold air moves in to fill the gap, this is a simplified version of the atmosphere where the sun heats up the equator making the air rise in this region.

As the hot air rises it produces an area of low pressure. At the poles the cold air sinks producing an area of high pressure, the circular movement of the 'bonfire' effect produces bands of HIGH and LOW pressure, the wind on the surface will naturally flow from the high pressure to the low pressure. We in the UK find ourselves in the area between the warm wind coming up from the south and the colder wind from the Polar High region from the North. This band is known as the polar front.

The Polar Front: the cold 'beer' effect When a cold beer is taken from the fridge almost instantly the outside of it becomes wet, this is known as 'condensation', simply the cold air can not hold as much moisture as the warm air and condensates on the nearest cold surface. Where warm and cold air meet, (a front) condensation occurs producing clouds and later as the moisture gets heavier rain. This happens in the area where the moist warm air is coming from the 'Azores high' and cold air is coming from the polar region. We in Europe are affected by this weather, this meeting point is known as the POLAR FRONT.

In a perfect world this line would be continuous around the world giving a 'ring' of rain, in reality the 'spin' of the earth deflects the cold air to come from the North East and the warm air to come from the South West. The effect is the 'spin' of the air mass in a 'anti-clockwise' direction. This effect is the coriollis effect. As this air mass spins the cold air wedges under the warm air, and begins to catch up with it, until finally the warm air is raised off the ground. The diagram below illustrates the Depression in its mature stage and with the two warm fronts that have caught up with each other (occluded) to the right.

As the system spins anti clockwise the wind blows in from the lines of equal pressure (isobars) at an approximate direction of twenty degrees towards the middle. The closer the isobars are the windier it will be, if you are on a front it will probably rain. The diagram me below shows the cloud formation and rain associated with a typical depression.

As the depression or low pressure passes over you will experience the cloud formations and rain as the system moves from left to right. First you will experience the this wispy cirrus cloud which will build up with the Cirro stratus (stratus being layered). The cloud will build up as the Nimbo Stratus moves in. At about 200nm from the warm front it will start to rain lightly, this will build up to persistent rain. When the front passes the rain will stop and you will feel a change in temperature. At the end of the 'warm sector' the 'cold front' will be marked by heavy rain and a drop in temperature.