Wind results from the movement of air due to atmospheric pressure gradients. Wind flows from regions of higher pressure to regions of lower pressure. The larger the atmospheric pressure gradient, the higher the wind speed.
The Sun is the key source of winds on the earth’s planet. Winds are mainly caused by the uneven heating of the earth’s surface by the sun.
The region close to the equator receives a greater incidence of solar rays in relation to the polar regions, giving rise to temperature gradients (differences).
Thus, the warm air found at low altitudes in tropical regions tends to rise(because it is lighter and less dense), being replaced by a mass of cooler air that moves from the polar regions.
The regions near the poles have a greater volume of air, creating a difference in atmospheric pressure (pressure gradients), pushing cold air to lower regions towards the equator. These displacements of air masses determine the formation of winds. Therefore, there are places on earth where the winds never cease.
The movement of air around the Earth alleviates the extreme temperature and produces surface winds with constant energy transfer.
The above simple airflow illustration (Fig 2) is not so accurate as the generation and movement of air are much more complicated and influenced by the Coriolis forces (due to the rotation of the earth) and local geographical conditions.
The Coriolis force that arises from the rotation of the earth deflects the wind to the right direction (relative to the travel direction) in the northern hemisphere and to the left in the southern hemisphere.
The Coriolis force depends on the earth’s latitude; it is zero at the equator and reaches maximum values at the poles. In addition, the amount of deflection on wind also depends on the wind
speed; slowly blowing wind is deflected only a small amount, while stronger wind
The combination of pressure gradient and Coriolis force causes the meridional atmospheric circulation to break down into three cells(Polar cell, Ferrel cell, and Hadley cell ) in each hemisphere with distinct circulation patterns. The Ferrel cell is indirectly driven by the polar and Hadley cell.
The distance along the surface of the Earth between the North Pole and the equator is about 10 000 km. Air moves within the troposphere region that are 5-15km in thickness from the earth’s surface.
The Three-Cell Model
RISING MOTION (air movement from the equator to the pole) is associated with THUNDERSTORMS, and so that belt of rising motions along the Equator explains why the tropics are so wet.
SINKING MOTION (from pole to the equator) is associated with DRY CONDITIONS, and so that belt of sinking motions along about 30°N explains all the deserts at those latitudes.
Most parts of the tropics experience winds from the east (or northeast or southeast) more or less all the time we call those the “trade winds”
The polar front is a belt where cold air from the north keeps meeting warm air from the south. Sinking motion at poles explains why the north and south poles are so dry.
Atmospheric flow for Wind Energy
Within wind energy, we are mainly concerned with the atmospheric flow closest to the ground,
perhaps only the first few hundred meters often denoted the atmospheric boundary layer.
In the atmospheric boundary layer, the wind is largely influenced by the presence of the ground: vegetation, buildings, orograph (changes in elevation), land-sea interfaces, etc. all contribute to the character of the wind.