Weather: A full frontal view of the skies

The atmosphere consists of huge, continent-sized air masses. The frontal systems you see on weather maps form when a cold air mass bumps into a warm one.
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The are vast regions of the earth which hardly have any weather at all. Above the oceans, and the frozen poles, and in the centre of large land masses, the sun warms the atmosphere (slightly, because air does not retain heat very well) and heats the earth and sea (which do retain the heat), and the heat from the ground and seas then add to the warmth of the air, and the whole system reaches an equilibrium. Locally, the weather hardly changes, but it is these areas (known as "source areas") which are responsible for forming the air masses that determine the weather for the rest of us.

There are four main types of air mass.

Continental polar: cold and dry; developing over the most northerly lands of Greenland, Canada, Europe and Asia.

Continental tropical: hot and dry; formed over the land masses of North Africa and North Australia.

Maritime polar: cool and moist; coming from the North and South Atlantic and Pacific oceans.

Maritime tropical: warm and moist; formed over the mid-Atlantic and mid- Pacific, and the Indian oceans.

The major weather fronts form a cold air mass meets a warm air mass, and the type of front - and whether it is depicted with spikes or curves on the chart - depends on how they meet. The spiky or rounded symbols, incidentally, are always placed on the side that corresponds to the direction of movement of the front.

A cold front occurs when the edge of an advancing mass of cold air supplants warm air near the ground by moving underneath it. Since cold air is heavier than warm air, this is a natural state of affairs.

A warm front occurs when a mass of warm air moving in one direction replaces a mass of cold air retreating in front of it. The warm air also rises and moves above the cold.

Winds tend to push fronts along at a speed of about 40 kph, though it is important to notice that because cold air is heavier and more compact than warm air, cold fronts tend to move at about twice the speed of warm fronts. When a cold front catches up with a warm front, we get an occluded front, which is when the weather map begins to look really messy.

The general picture of the large-scale weather is predictable from the frontal systems. Cold fronts, roughly speaking, tend to bring heavy rain as the warm air mass is chilled by the overtaking cold one below its condensation point. When the rains are over, colder weather follows.

Warm fronts are associated with lighter, steadier rain, as the edge of the warm air drifts slowly into the cold and releases its excess moisture. After the rain comes warmer and more humid weather. That's when we feel stickiest, because the air is saturated and our perspiration can't evaporate.

The general pattern is that the fast-moving cold fronts cause quick changes in the weather, while warm fronts produce gradual change. Occluded fronts divide into cold-front occlusions and warm-front occlusions according to whether the air behind the cold front is colder or warmer than that in front of the warm front. Cold-front and warm-front occlusions behave similarly to cold and warm fronts, but in a less extreme manner. Finally, we sometimes encounter a stationary front, when a cold front meets a warm front, and both just stay where they are for days, bringing moderate weather because not much is happening.

Finally, before leaving the weather map, let's take a quick look at those areas of high and low pressure. One of the apparent paradoxes of weather is that warm air expands and produces low pressure, while cold air is heavier and produces high pressure. So why does low pressure tend to be associated with poor weather?

The answer comes by considering the effects of the pressure systems, not their causes. Air flows from high to low pressure, moving into the low pressure area and forcing air upwards. The warm air, going upwards, cools and cannot hold as much water vapour. So the vapour condenses into water droplets, held up by the rising air. Result: cloudy weather.