METEOROLOGY. – The etymology of the word indicates that originally it referred to the study, or at least the observation, of all supraterrestrial phenomena (τὰ μετέωρα = things suspended above the earth), which at the dawn of every civilization were considered expressions of one or more deities depending on the time, place, and phenomenon. Currently, meteorology proper is concerned almost exclusively with the lower layers of the atmosphere, from the ground up to a height of 25–30 km, and in particular the first 10–15 km.
In this layer, whose thickness is only a few thousandths of the Earth’s radius, a very dense network of meteorological stations—whose service is standardized on an international basis—tracks the horizontal and vertical movements of air masses (winds), the distribution of pressure, temperature, and water vapor content, as well as variations in these quantities, across the entire surface of the Earth. Observations are also made of phenomena related to the condensation of vapor: fogs, clouds, and precipitation of various kinds and intensities.
The reports of these observations, condensed into a few coded groups, are transmitted—mostly by radio—to national and international collection and redistribution centers, so that research centers can have all the observations plotted with appropriate symbols on synoptic charts within two hours of the time of observation (1, 4, 7, 10, 13, 16, 19, 22 GMT).
Only in this way is it possible to follow, as a whole, the development of atmospheric phenomena, which are generally very complex and occur on an intercontinental scale.
When these observations are analyzed using synoptic mapping and statistical methods, they reveal that the atmosphere possesses a kind of “general mean circulation” of a zonal nature, i.e., following the parallels, and that major meteorological phenomena can be considered more or less deep disturbances of this circulation. This circulation is roughly symmetrical with respect to the equatorial plane and consists mainly of currents flowing from east to west at high latitudes (above 60°); predominantly westerly currents (westerlies) at latitudes averaging between 60° and 30°; a zone of subtropical calms between 30° and 15°; and finally a zone of easterly currents (the trade winds) in the equatorial belt. Particularly noteworthy is the fact that the westerly currents at an altitude of about 10 km and a latitude of about 45° show a pronounced maximum (with winds up to 300 km/h), known as the “jet stream.”
This mean circulation, attributable in broad outline to the combined effect of the Earth’s rotation and the varying solar radiation received at different latitudes, ensures that air masses remain relatively long at the same latitude, acquiring at various altitudes the temperatures and moisture contents that characterize them. Thus, within certain limits—albeit rather broad ones—it is possible to speak of ancient (easterly) air masses, intermediate and tropical (westerly) air masses, and equatorial air masses (easterly currents: trade winds). These masses then exhibit further particular characteristics (maritime, continental), depending on the geographical surface over which they have traveled from their latitude of origin. Air masses retain, more or less for long periods and even through the vicissitudes of a disturbance, the characteristics of their zone of origin, so that, within certain limits, they can be identified. This is because, as observations have shown, when air masses come into contact and move against one another, they exhibit mostly very slight and slow mixing phenomena, so that one can speak of a surface of separation between the masses—a surface of discontinuity. These surfaces of separation are mostly nearly horizontal (with an inclination of 1° to 10°); they can undergo undulatory motions with waves in which the motion of the particles is predominantly horizontal, with wavelengths on the order of a thousand kilometers, and which, under conditions of instability of the two air masses between which they form, can evolve into the giant vertical vortices that constitute cyclones. In these vortices, the potential energy of the air masses in contact (the denser cold air sinks, the warm air rises) is converted into kinetic energy (wind).
The condensation phenomena (clouds, rain, etc.) that precede or accompany, in every case, sudden changes in temperature and wind are precisely due to the passage over the affected area of a discontinuity between two different air masses under unstable conditions. It is the warm masses that, rising above the cold ones and being lifted by them (a phenomenon that can be intensified or attenuated by orography or other causes), move from high pressure (at the ground) to low pressure (aloft); they thus undergo an approximately adiabatic expansion process, i.e., without heat exchange with other masses, and consequently cool until saturation is reached (at the altitude where the cloud base will form), condensing the vapor into droplets or ice crystals.
The surfaces of discontinuity, at which warm air comes to overlie cold masses, are called warm fronts, and their intersection with the ground is called a warm front. They typically present the following sequence of clouds: “cirrus” clouds, composed of ice crystals, very white, thin, and transparent, at an altitude of about 6–8 km; “altostratus” clouds of supercooled water droplets, white and denser than the first, at an altitude between 2 and 6 km; and “nimbostratus” clouds of relatively large water droplets, dark and with a very low base, of considerable thickness (2–5 km), giving continuous precipitation (rain, snow); below these, there are often banks of continuous or fragmented low clouds (stratus, fractostratus) or fog.
The surfaces of discontinuity, at which warm air is lifted by cold air, are called cold fronts, and their intersection with the ground is called a cold front. They typically present the following sequence of clouds: “altocumulus” clouds, composed of water droplets, white, in more or less large aggregates (popularly known as “sheep clouds”), at an altitude between 2 and 6 km; “cumulonimbus” clouds, typical thunderstorm clouds with very high, white-tipped summits (6–8–10 km), composed of ice crystals, with dense, dark bodies formed by water droplets; at their often very low base, the droplets are very large. These clouds are accompanied by showery precipitation, especially hail, and are followed by “cumulus” or “towering cumulus” clouds with bases around 1 km and summits between 2 and 5 km.
The passage of a warm front typically lasts about a day, while that of a cold front lasts a few hours. In reality, nature often deviates from this simple scheme; cold and warm fronts can already join and overlap in mixed systems called “occlusions,” but it is especially orographic obstacles, warm and cold seas, etc., that can bring about even very profound alterations in the evolution of a system of discontinuities, both from a dynamic and thermal standpoint, intensifying or attenuating frontal phenomena over very short time intervals. This is what makes forecasting so difficult for meteorologists, difficulties that are all the greater the more rugged the geography of the area studied, as is particularly the case in the Mediterranean region (cf.: METEOROLOGICAL ORGANIZATION).