Winds are more or less rapid horizontal displacements of air masses caused by differences in the pressure distribution. Due to the fact that pressure variations are mainly caused by temperature variations, it follows that the wind’s main motor is the divergence in solar radiation in the different regions of the world.

Wind direction. Each wind is characterized by the direction in which it moves and by its speed. Normally, when one talks about wind direction, one means the direction the wind comes from: westerly winds, for example, are winds that blow from the West to the East.

Air masses would tend to move perpendicularly to the isobars, following the pressure gradient, i.e. the difference in pressure that brings about the displacement of masses of air, but the Coriolis effect modifies their course. In ideal conditions in which there is no friction, and in which the pressure gradient force and the Coriolis effect are equal and opposite, winds move parallel to the isobar lines, leaving the high pressure areas to their right in the Northern Hemisphere and to their left in the Southern Hemisphere: these are the so-called geostrophic winds (see the graph “geostrophic winds” for more information), winds that are “ideal” so to speak.

Generally, at ground level, friction with the Earth’s surface causes deviations in the wind’s direction and only winds at an altitude greater than 1,500 m are very similar to geostrophic winds. 

Winds at ground level

Winds can be divided into ground level winds, that blow in the lower strata of the troposphere, and high altitude winds. As far as winds at ground level are concerned, different types can be distinguished: local periodical winds, such as coastal and valley breezes, that form as a result of the unequal heating caused by unequal solar radiation, and global winds, whose direction and intensity depend on the distribution of the big pressure cells in the world. The trade winds blow from the subtropical anticyclonic cells towards the equatorial low pressure areas. 

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High altitude winds

In theory, one might expect ground level winds to be coupled with similar winds, blowing in the opposite direction, at high altitudes, in the higher strata of the troposphere. In practise, however, from observations that have been carried out, it has been shown that above an altitude of 4-5.000 m there are only westerly currents that move from West to East roughly following the route of the parallels. Only above the Equator there is a narrow band of easterly winds that are probably connected to the convergence zone of the trade winds.

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The Foehn wind

The name Föhn (or Foehn) comes from the dialect of Tyrol, and indicates a particular type of wind that is characteristic in the Alps, which can also be noted, naturally, in most of the other mountain ranges. It is generated when a moving mass of warm humid air meets a mountain on its path. By inertia, the mass of air moves against the mountain and the air is forced to rise along the sides of the mountain. As it rises, the air cools and expands, thus becoming saturated with water vapour. 

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Jet streams

Jet streams are particular evolutions of high altitude winds. They are rapidly moving air currents caused by pressure differences that result from the temperature divergences that occur when big air masses meet. They were discovered fortuitously by American aeroplanes flying towards Japan during the Second World War. To be called a jet stream, the wind speed has to be higher than 50 knots, about 90 km/h, however, jet streams usually have much greater speeds ranging from 160 to 250 km/h, with peaks of 320 km/h. 

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Clouds are formed by microscopic drops of water or by small ice crystals. The size of the droplets generally range from a couple of microns to 100 microns: this is the limit beyond which cloud drops become rain drops. The shape of the drops is usually spherical, but it can vary, especially in bigger drops that are deformed by gravity. 

Water in the atmosphere: evaporation and condensation. The evaporation of water bodies and evapotranspiration from soil and vegetation supply the atmosphere with great quantities of water vapour (vedi sezione acqua). Even though it is abundant, the amount of water vapour supplied to the atmosphere by evaporation processes generally is not sufficient to make the air reach saturation point. Saturation and the consequent condensation of vapour into drops of liquid water are therefore generally caused by the cooling of air masses at a temperature below condensation, also known as dew point. When condensation takes place, a part of the excess water vapour changes from the gaseous state to the liquid state, forming microscopic drops of water. For condensation to occur, however, it is not sufficient to reach the dew point: it is necessary that the so-called condensation nuclei should be present. These are small particles (whose dimensions range between 0.001 and 10 micron) on which the tiny drops of water can condense. The condensation nuclei normally present in the atmosphere are sodium chlorides, sulphates, carbonaceous particles and atmospheric dust and can be present as a result of natural causes (as for example, marine aerosols transported by the wind or ashes coming from volcanic activity) or due to human activities (the result of the combustion of fossil fuels, for example). The bigger and more hygroscopic (i.e. capable of attracting water molecules) the nuclei are, the more effective is their role in favouring condensation processes. 

If the temperature is low, water condenses in the form of ice crystals, but it still requires the above-mentioned nuclei that, in this case, are called crystallization nuclei. 

Cloud formation

Cloud formation is apparently a very simple process, caused by the condensation of atmospheric humidity into drops of water, but in actual fact it is a more complex phenomenon, in which different factors and mechanisms are involved. All the processes however are based on the ascent of a mass of humid air and its successive cooling up to dew point. In this process a body of water or the humidity of the ground evaporate as a result of solar radiation and a volume of hot, humid air forms and slowly starts to rise since hot, humid air is lighter than the air surrounding it. 

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How they move

The contribution of the Sun: lifting by convection. Solar radiation is the first and most evident cause of the lifting of warm and humid air masses: the air gets heated both by direct absorption of solar radiation (especially in the infrared band, that is preferentially absorbed by water vapour), and by convection above the ground that loses heat.

The contribution of the mountains: orographic lifting. The process is analogous to the previous one, but in this case, the mass of warm and humid air rises because of the topography: when a moving mass of air meets a mountain it is forced to move along its sides, rising and cooling.

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Different shapes

The first scientific classification of clouds was made by an English chemist, L. Howard, in 1803, and the classification system that he proposed is, with some modifications, still used today.  It is based on two principal groups, divided according to the development (vertical and horizontal) and three types: cirrus, cumulus and stratus clouds. The different combinations of groups and types lead to the different cloud formations.

Different shapes. Stratus (St) clouds, are widely extended horizontal sheets in the lower elevations, and are generally grey.

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Different altitudes

Low clouds reach a maximum limit of 1,800 m, while the low limit can even be at ground level. Stratus, nimbostratus, stratocumulus and cumulus clouds belong to this category. Average clouds are to be found at elevations ranging between 2,000 and 6,000 m. Altostratus and altocumulus clouds belong to this category, but at these heights we often find clouds that are moving from the lower layers to the higher ones.

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