Air is something that we all take for granted. We breathe it every day, we look through it, we pollute it. Most of the time, we don’t even notice it. But it can quickly make its presence known when wind is involved. Wind, in general is just moving air, and although in its stationary state it may seem harmless, once started, it is a force to be reckoned with; it can uproot trees, lift roofs of buildings, and generally wreak havoc. It is caused by differences in air pressure within the atmosphere, and the driving force behind all of that is the solar radiation.
Earth is almost spherical and because of its tilt, the sun’s rays hit the area near the equator at an almost right angle, while the areas further away are hit at a smaller angle. This means that the concentration of heat received at the equator is higher, and therefore the air warms quicker there than in the pole-ward regions. The air molecules expand and lift when they are heated, and that, coupled with differences in temperature sets the atmosphere in motion in a massive global chain reaction. The air lifted from the equator creates a low-pressure zone, and is replaced by winds from higher-pressure zones and then in turn, something has to move the air toward them. This air movement is affected by many factors, which makes it even more complicated and harder to understand, but scientists have come up with idealised models of general circulation.
They separated the sphere into three belts, or cells north and south of the equator – the Hadley cell ranging from the equator to about 30° N and S, the Ferrel cell from about 30° to about 60° N and S and the Polar cell from about 60° N and S toward the poles.
The polar cell is a relatively simple system. Warmer air rises at the subpolar low and then moves toward the pole where it descends because it has cooled and dried along the way. Then it moves away from the pole, twisting westward because of the Coriolis effect – the apparent deflection of moving objects if they are seen from a rotating reference frame, which, in this case is the Earth itself.
The Ferrel cell is secondary, and its existence depends on the existence of the other two cells. It is characterised by prevalent westerly winds.
The cell we are interested in for the purposes of this paper is the Hadley cell with the air rising along the equator creating an equatorial low, and the easterly trade winds blowing towards the equator from both north and south. This is where the South- and Southeast Asian monsoons originate.
In the past, the word monsoon has been defined exclusively in terms of either seasonal rainfall, or reversal of the direction of the prevailing surface winds between summer and winter. Ramage (1971) defines it by the following criteria:
The prevailing wind direction shifts by at least 120° between January and July;
The average frequency of prevailing wind directions in January and July exceeds 40%;