There are areas of different air pressure all over the earths surface. Generally, because of the cold air, the air tends to sink at the poles, causing a high pressure, while at the equator the air is warmer, and rises, causing a low pressure. Different air masses are therefore formed. These air masses move in certain directions. Polar air masses originate from the poles, and tropical air masses originate from the equator.
There are two main kinds of weather systems:
1) Depressions
2) Tropical Cyclones
1) Mid-latitude Cyclones or Depressions
When warm tropical air from the equator moves towards the poles and cold polar air moves towards the equator, the two air masses meet along a line known as a polar front. The two air masses will then move together in a clockwise direction in the Southern Hemisphere and an anti-clockwise direction in the Northern Hemisphere.
In the above picture, (Northern Hemisphere), the warm tropical air is moving north from the equator, and is at the bottom of the picture, while the cold, polar air is on the top. The arrows indicate the movement of the air masses around a low pressure cell. The solid line with triangles indicates the leading edge of the cold air, and is known as the cold front. The solid line with semi-circles is the leading edge of the warm air and is known as the warm front. The cold air moves faster than the warm air therefore, the cold air mass will eventually overtake the warm air mass - the section labelled warm air will get narrower.
Because the cold air mass is heavier, when it overtakes the warm air it forces the warm air upwards along the cold front and rain can form (above left). Similarly, at the warm front (above right) the warm air rises over the cold air mass. Eventually, when the warm air is occluded by the cold air, the warm air will be above the cold air.
A depression moves from west to east, and can occur in families of 2 to 5. A depression covers a large area (a few thousand km) and can last for up to 5 days. As the depression moves over an area, weather conditions associated with the warm front will be experienced first and this is an indication of the approaching cold front. When the warm front approaches the wind direction changes, dew point temperature rises, humidity increases, there is extensive cloud cover and the atmospheric pressure drops. The cold front has similar surface characteristics - the wind changes direction and blows more strongly, forming a line squall, temperature decreases, humidity decreases, precipitation can occur and the atmospheric pressure decreases.
View Synoptic Chart and Satellite Photo's of a Depression
2) Tropical Cyclone or Anticyclone
The common names for Tropical Cyclones are hurricanes, typhoons, cyclones, or willy-willies. There are several conditions that need to be met before a cyclone can form. It must form over a sea, because it needs the moisture, and the temperature of the sea must be higher than 27 C. They must form 5 North of South of the equator, because the Coriolis Force is necessary. They occur mainly in late summer. They are slow moving, and move from east to west away from the equator. They only affect a small area (300 - 500 km). The isobars are close together, which show a sharp change in atmospheric pressure, resulting in hurricane force winds. In the centre of the cyclone is a calm eye. Air moves upwards, around the eye, clockwise in the Southern Hemisphere and anti-clockwise in the Northern Hemisphere. They develop on the eastern sides of continents and are accompanied by heavy rain, thunder and lightning, which can cause devastation because of high winds and flooding. They are named alphabetically during the season.
The main cause of a cyclone is the extreme drop in atmospheric pressure, because of the heat of the ocean. As soon as the system moves over land it starts to dissipate, or break up, because it has been removed from its source, namely the sea.
The above picture is a cross-section of a cyclone in the Northern Hemisphere. The amount of rainfall and wind speed, increase towards the eye, while atmospheric pressure decreases. As soon as the eye is reached there is a sudden change in conditions - no rain and no wind But once the eye has passed, there is another sudden change is conditions, as the rain and wind increase to what they were before the eye passed over. The amount of rain and wind speed now slowly decrease as the system moves over and the atmospheric pressure slowly increases.
View synoptic charts and satellite photo's of Cyclones