Last reviewed on: 25th November, 2020
To realize what cause the wind blow, we first require considering what atmospheric pressure is. Pressure at the earth's surface is a degree of the weight of air compressing down on it. The larger the mass of air above us, the greater the pressure we feel, and vice-versa. The significance of this is that air at the surface will need to move from high to low pressure to balance the dissimilarity, which is what we recognize as wind.
In the atmosphere the compression at the earth's surface imitates the weight of air above it, which sequentially is determined mostly by its temperature, and as people usually know from daily life, hot air is lighter than cold. This turns with the fact that depressions (low pressure systems) usually take along warm air.
Wind that is initiated by dissimilarity in pressure covering a large area (from about 100 km and beyond) does not run directly from the area of high compression to the depression as in the instance of the balloon. As an alternative, the wind blows anti-clockwise nearby the low compression area in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is the effect of the earth's circling, which produces a force known as Coriolis that rebounds the wind from its path.
The Coriolis force rebounds air to the right in the northern hemisphere and to the left in the southern hemisphere. Nearby the high and low pressure systems one can obviously see on weather charts, e.g. on the TV, there is a scheme of equilibrium between Coriolis force and the force that draws air in the way of lower pressure. Such equilibrium is attained when wind blows surrounding low pressure systems, and not straight into them.
Thus wind is caused by variation in atmospheric pressure, but why do we get these variation? It is rising and sinking of air in the atmosphere. Somewhere air is rising we perceive lower pressure at the earth's surface, and somewhere air is sinking we perceive higher pressure. Actually if there is no rising and sinking motion in the atmosphere then we will have no wind and weather.
This mounting and sinking of air in the atmosphere occurs both on a global scale and a local scale. The simple example of a local wind is the sea breeze. During the summer (sunny days) the sun's rays heat the ground up rapidly. Unlike the ground, the sea surface has a greater capability to absorb the sun's rays and is more demanding to warm up, this give rise to temperature difference between the warm land and the cooler sea.
As the ground heats up, it warms the air above it. The warmer air turns out to be less dense than the nearby cooler air and commences to rise, like bubbles in a pot of boiling water. The rising leads to lower pressure over the land. The air above the sea remains cooler and denser, and then pressure is higher than inland. Therefore we now have a pressure variation set up, and air transfers inland from the sea to try and equalize this variation (Sea breeze), this clarifies why beaches are frequently much cooler than inland areas on a hot sunny day.
The same process takes place on a global scale. The sun rays reach the earth surface in polar regions at a much more tilted angle than at equatorial regions. This establishes a temperature variation amongst the hot equator and cold poles. Therefore the heated air rises at the equator (give rise to low pressure), while the cold air sinks above the poles (give rise to high pressure). This pressure variation sets up a global wind flow as the cold polar air tries to shift southwards to replenish the rising tropical air. Though, this is complex by the earth's rotation (coriolis effect).
This is chiefly because the sun heats the earth's surface unequally. As heat moved to the air, we obtain regions of warm and cool air which can lead into regions of low and high pressure. This variation in pressure creates a force that causes the wind to blow.
Posted by: Lusubilo A. Mwaijengo
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