The factors that affect wind direction and intensity are very similar to those that influence ocean currents. The main variables in both cases are the Coriolis force, pressure, and temperature. Another important factor for winds is friction. All of these are interconnected. Wind systems are very complex and unpredictable. It is still hard for example for meteorologists to predict exactly what path a hurricane will take.
In simple terms wind is the movement of air from areas of high pressure to areas of low pressure. Pressure itself is affected by gravity and temperature. Gravity means the air at ground level is under much greater pressure than that further up.
The atmosphere consists of gases that expand and contract at different temperatures. Since the temperature is not uniform throughout the world this means that areas of high and low density are created. The higher the density the higher the pressure force and this will move air towards areas of low pressure.
Hot air rises, decreasing pressure, and cold air sinks, increasing pressure. As the sun heats an area the pressure decreases, leading to air moving in. The uneven heating of the earth’s surface indirectly creates winds and dictates their direction.
A very simple illustration of pressure causing air flow is seen when you fan yourself with a piece of paper. By pushing the air you are temporarily increasing the pressure and creating wind on a small scale.
The rotation of the earth pulls air along with it. However the higher the air the less it is affected so different layers of the atmosphere are moving at different speeds. This creates turbulence and produces additional winds at ground level.
The rotation of the Earth also creates what is known as the Coriolis force. This affects the direction of winds by deflection. Winds and currents are directed to the right in the northern hemisphere and to the left in the southern hemisphere. The Coriolis force is highest at the Poles then decreases with latitude to zero at the equator.
Friction has the effect of slowing winds down. There is little friction over the oceans and it varies over land. Forests and mountain ranges in particular slow down winds. Manmade structures such as cities have a similar effect.
Despite recent advances in meteorology, particularly with regard to computer modelling, weather systems are so complex that predicting what will happen on any particularly day is still notoriously difficult. We can have a good idea how intense and in what direction winds will be blowing but completely accurate weather forecasts remain a thing of the future.