Moving masses of air

It is difficult to construct a model of atmospheric circulation, because very many factors influence the displacement of masses of air through the atmosphere; however, the fundamental principle is that the gases that make up the atmosphere tend to seek a condition of equilibrium that implies a uniform distribution of energy, equalizing the temperature and the pressure on the entire planet. The ‘motor’ of atmospheric circulation is therefore given by the redistribution of the energy received from the Sun. Solar radiation, in fact, varies at different latitudes with the result that the equatorial regions are warmer than the polar ones. The atmosphere tends to re-balance this difference moving masses of warm air from the regions where there is an excess of heat towards colder regions, in an attempt to reduce the difference in temperature between the Equator and the Poles.
Differences in temperature bring about immediate differences in the pressure within the masses of air: it is these differences in pressure that cause air displacement. Low pressure areas attract air from areas where the pressure is higher (vedi grafico 10: Alte e basse pressioni). Vice versa, air tends to move away from high pressure areas towards areas with lower pressure. The speed at which this displacement takes place is directly proportional to the difference in pressure between the two points. In ideal conditions, if the Earth were motionless and if there were no friction or obstacles, air would flow perpendicularly to the isobars (the lines that join points with equal pressure), according to the so-called pressure gradient, following a path that makes the air travel along the shortest route from the high pressure area to the low pressure one. However, many factors contribute to deviating the flow of air from the ideal condition.

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