Climate (from ancient greek κλίμα, klíma, “region, stretch of country”) is the average state of weather at various spatial scales (local, regional, national, continental, hemispheric or global) detected over at least 30 years (according to the official definition provided by the World Meteorological Organization).

It is mostly a function of the inclination of the sun’s rays on the Earth’s surface as the latitude varies; each climatic-latitudinal band of the Earth corresponds to different physical-environmental characteristics in terms of flora and fauna called biomes (e.g. rainforests, deserts, temperate forests, steppes, taiga, tundra and polar ice floes), strongly influencing the economic activities, habits and culture of the populations that inhabit the territory.

The main characteristic of climate compared to the common “meteorological time”, in addition to the time interval of observation and study, is to have a trend that tends to remain stable over the years even with interannual climate variability due to the seasons and medium to long term that overlaps. The scientific attention in recent decades has shifted more and more on the understanding or in-depth research of the mechanisms that regulate the Earth’s climate, especially in relation to the dreaded climate changes observed in recent decades (e.g. global warming). The scientific discipline that studies all these aspects is climatology.

Climate is the synthesis of the fundamental meteorological characteristics observable over a long period of time (many decades or hundreds of years). Therefore, the climate of a given territorial area is the representation of the set of statistics, referred to a given time interval, of all those meteorological phenomena, individually taken or related to each other, which in their typical or characteristic trends constitute the “norm”.

Climatic factors

Climatic factors are called the conditions that produce variations on the elements of climate; we distinguish zonal factors that act with regularity from the equator to the poles and geographical factors that act differently for each location.

Zonal factors are: the latitude, for which from the equator to the poles the temperature decreases because the solar energy that the earth’s surface receives decreases; the general atmospheric circulation, which affects the exchange of heat between the warm intertropical regions and the colder regions of the middle and high latitudes.

Geographical factors are altitude, land and sea distribution, sea currents, vegetation, and human activity. Altitude has a great influence on climate because with height temperature, pressure and humidity decrease, while solar radiation and, up to a certain altitude, rainfall increase. Very important is also the disposition of the reliefs: a mountainous chain transversal to the prevailing direction of the winds can cause strong rainfall in the windward side and drought in the opposite side. For these reasons it has been defined a particular climate, called mountain climate, characterized by strong thermal excursions, diurnal and seasonal, by remarkable local variations and by atmospheric instability. The distribution of land and seas causes significant changes in temperature values at different latitudes. The soil has little thermal capacity and heats up quickly, but just as quickly gives up the heat to the atmospheric layers; the continents are therefore subject to considerable temperature changes both daily and during the year; on the contrary the sea, being able to transmit heat through convective movements, has a considerable thermal capacity and can therefore slowly give up the heat received, attenuating temperature oscillations.

We can therefore distinguish two types of climate: the continental and the maritime; the first is characterized by strong temperature changes, low humidity and limited rainfall, while the second is characterized by a certain uniformity between summer and winter, weak excursions and higher humidity and rainfall. The marine currents act on the climate of the coastal regions concerned: the warm currents make it constantly hot-humid, while the cold ones make it cold-humid.

The vegetation, when it is very abundant, causes a decrease in temperature and an increase in humidity especially during the hottest months. Human activity acts on climate as it is capable of modifying the natural environment and the balance of ecosystems. In particular, air pollution, urbanization, land use change, intensive agriculture, industrialization of certain areas, etc., are all factors capable of modifying the climatic characteristics of territorial areas more or less extensive and regional scale.

The emission and accumulation in the atmosphere of certain chemical compounds capable of absorbing the earth’s infrared radiation (greenhouse effect), such as carbon dioxide, methane, nitrous oxide, chlorinated and fluorinated hydrocarbons (such as freon), etc., are instead factors capable of modifying the climate at a global level because they act on the overall energy balance of the atmosphere or modify the characteristics of the stratosphere and especially of the ozone layer.

Not less important factors on climate at medium and large scale are also some sulfur and nitrogen compounds that chemically transforming in the atmosphere give rise to the phenomenon known as “acid precipitation” or acidification of hydrometeoras.

Causal climate factors

The causal factors that underlie the study of climate are called climate elements. The main elements of climate are: insolation, temperature, pressure, humidity, precipitation and cloudiness. The sun insolation is an element of fundamental importance because from it depend directly or indirectly all the other atmospheric phenomena: of the solar energy that arrives in the atmosphere, a part (shorter radiation) is diffused by clouds and by the molecules of the gases constituting the air, a part is absorbed by water vapor and carbon dioxide, a part (long radiation) arrives to the ground where it is absorbed and irradiated again in the atmosphere. The amount of insolation measurable on the Earth’s surface during the day depends on the angle of incidence of sunlight and the duration of the day; considerable influence is exerted by the cloudiness: the insolation, which theoretically decreases from the equator to the poles, does not have its maximum at the equator, but in correspondence of the tropics where the transparency of the air is greater.

The distribution of temperature on the earth’s surface is represented by the annual isotherms, lines that join all points of equal average annual temperature, reduced to sea level; they indicate that the temperature, following the trend of insolation, decreases from the equator to the poles: the hottest zone, however, is not the equatorial zone where the abundant vegetation and high humidity cause the lowering of average values, but a continental zone (thermal equator) shifted towards the Tropic of Cancer; thus the poles of cold do not coincide with the geographical poles, but are shifted to the continental circumpolar regions. For climate studies are also of great interest the temperature excursions between the average maximum and minimum diurnal, monthly and annual.

Pressure is important above all for the influence it exerts on many atmospheric phenomena such as winds, precipitation, convective air motions; it is also very important for the so-called altitude climates, since the decrease in pressure with height affects not only some physical parameters (temperature, humidity, etc.) but also has considerable physiological effects on living organisms.

As for humidity, both absolute and relative humidity can be considered; for the purposes of a climatic study, it is preferable to consider relative humidity, whose variation also has sensitive effects on organisms. Humidity varies continuously with evaporation and atmospheric precipitation; the least humid regions are those far from the seas and without vegetation. Humidity also decreases with height, absolute humidity very rapidly, while relative humidity decreases slowly as temperature decreases at the same time.

Precipitation is a climatic element, depending mainly on temperature and air humidity, very much used in climate classifications. The monthly and annual totals of all precipitation, the frequency and intensity of rainfall, the monthly mean values and, in particular, the precipitation regime, i.e., its monthly and seasonal distribution, are used. Cloudiness affects the temperature because it prevents most of the solar radiation from reaching the ground; on the other hand, it retains the heat radiation emitted by the soil, preventing a strong cooling at night and thus reducing the diurnal temperature range.

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