Weather presenters are apt to turn to an “H” on their charts with a beaming demeanor. Fair Weather Systems, they like to call them. Blue skies, mild temperatures, very little possibility of rain and almost none of storms. This “H” comes bundled in a few concentric isobars, and represents a bundle of delight for viewers. How do these systems come into being?
H of course stand for “high”, and the weather system being indicated is called a high pressure system. The weight of the air presses down on the surface of the earth, giving rise to what is known as atmospheric pressure. But this is not necessarily the same everywhere. The earth is a big place and the availability of heat is not the same everywhere. Other factors influencing the pressure are local geographical features, cloud formations and rain. The two pressure extremes, low and high, give rise to characteristic weather patterns.
It is advisable to start with a very common law of chemistry – Charles’ Law. According to this law, applying heat to some gas makes it expand, as long as the pressure stays the same. It is all about the molecules of gas acquiring more speed and therefore requiring more space to move about. The reverse is that when the gas cools it contracts.
The case of contraction lays down the scenario of the formation of high pressure systems. A body of air moving across a part of the ocean that is cooler than its surroundings, or over a cooler land mass, is subjected to contraction. Now, by itself, such contraction does not necessarily lead to high pressure. Charles’ law works adiabatically, i.e. the pressure does not change. Only the volume shrinks. The air molecules are not moving about so rapidly and therefore require less space. The layer of air that is affected experiences contraction as a whole.
But the interesting thing happens not on the surface of the earth where the cooling and contraction has taken place, but instead high above in the atmosphere, in the troposphere. The cooled air, requiring less space, drops down, leaving a vacuum above the layer of air, which is then filled by the even cooler air from the surrounding troposphere in a process called subsidence. It is this tropospheric air pushing down the layers beneath it that raises the overall pressure. The net effect is a heavier column of air in which the relatively cooler molecules are more densely packed. The point where the pressure is highest is said to be the centre of the system, the point marked “H” on a weather chart.
The high pressure dictates most of the features found in such systems. Air absorbs more water at high pressure and so clouds tend to disappear. This usually happens during subsidence, and the result is clear skies. The mild winds felt are result of the air moving away from high center towards the surrounding regions of lower pressure. The alternative name of “anticyclone” for a high pressure center is suggested by the fact that air moving outwards begins to rotate around the high center, becoming more violent the further the distance. Due to the earth’s rotation, anticyclones rotate clockwise is the north and anticlockwise in the south (the Coriolis Effect).