The Terrestrial Climate and its classification  

The system of classification of climates normally adopted, based on the differences in temperature and precipitations, is that formulated by the German climatologist Wladimir Köppen at the beginning of the XX century. Its base consists of the observation that the most evident and direct effect of the climate is the type of vegetation associated with it. As a result of this, the Earth is subdivided into five great climatic areas, each corresponding to the area of distribution of a particular category of plants.  

These plants are in fact divided into five classes, according to the environmental conditions which they need: the megathermic grow in presence of  medium temperatures above  20 °C; the mesothermic are typical of the temperatures between 15 and  20 °C; the microthermic are characteristic of the temperatures between 0 and 15 °C; the echistothermic grow in the presence of very low temperatures, beyond the limit of the arboreal vegetation; finally the xerophytes are the plants which have adapted to arid environments, characterized by long periods of drought.  

According to this classification of  vegetation five great climatic areas can be distinguished: that of the humid tropical climates, corresponding to the area in which megathermic plants are diffused; that of the arid climates, in which  xerophyte plants grow; that of the moderate-warm climates, in which the mesothermic plants are found; that of the northern climates, corresponding to the area in which microthermic plants are distributed, and finally, the polar zone, in which echistothermic plants grow.   

Humid tropical climates     

Typical of the warm equatorial area between the two tropics, the humid tropical climates entirely lack a winter season. The medium temperature is constantly above 18 °C and the temperature range is very small. Within this climatic zone is found the climate of the rain forest, or the equatorial climate and the climate of the savannah. The first is characterized by frequent precipitations, whose more evident effect is the extremely luxuriant vegetation of the rain forest; it is found in the Amazonian region, in central Africa and in the coastal regions of the Indian Ocean. The second, that of the savannah, is the climate of transition between equatorial humidity and the aridness of the desert area. It is characterised by a dry season and its vegetation is dominated by the grassy meadowland, interrupted by some  xerophyte-type trees

Arid climates  

The arid climates, characteristic of the low and middle latitudes between the tropics, are those in which the quantity of precipitations does not succeed in compensating for the water lost due to  evaporation. Within this category are found the  semiarid climates, to which the pre-desert steppes are associated, and the real desert climates. Depending on the geographical position, further hot, desert regions are found,  with average temperatures above 18 °C (among these are the Sahara, the Libyan  and the Iranian deserts to north, and the Kalahari and the Great Australian sandy desert to south), as well as cold, arid regions, with average temperatures lower than 18 °C, situated inside the continental areas (like the Gobi desert  and the arid zones of  southern Patagonia). The vegetation, which is very scarce, is limited to few grassy or shrubby species of the xerophyte type.  

Warm moderate climates  

Inside this ample category are found different types of climates: humid subtropical, characteristic of the eastern regions of the continents (such as the eastern coasts of China)  between  25° and  40° of latitude; these have hot, sultry summers with abundant precipitations and winters which are also quite rainy, but relatively cold; the vegetation that results is the humid subtropical forest, in which   plants with caducous leaves, such as the beech tree and the oak, live side-by-side with conifers and tropical plants such as bamboo. The moderate-cool maritime climates, characteristic of the western coasts of the continents  between  40° and  60° of latitude, have cool summers and mild winters; the average temperature does not go below  0 °C and does not climb above 15 °C; it is the climate typical of the European Atlantic regions, and produces a vegetation of mixed forests of evergreens and caduceus leaves. Instead, the Mediterranean climates, typical of the western zones between 30° and 45° of latitude, and in particular of the regions of the Mediterranean basin, have the greatest precipitations during the winter. The annual temperature range is mitigated by the presence of the sea and the typical vegetation  is the shrub. 

Northern climates   

These are climates characterized by cold winters, present in the inner continental zones, at middle latitudes. They  are therefore characteristic of the northern hemisphere (hence the name), given that in the southern one the continental masses are far less extended. We can make a second distinction  within this climatic area between the humid continental climate and sub-arctic one. The first , found in the central-oriental regions of  North America and of  Eurasia between  40 and  50° of latitude, has a cold season which lasts 8 months, during which the temperatures are lower  than 0 °C, and a warm season with temperatures of around 20 °C. The precipitations are more abundant in the summer; those in winter are partly snowy. The vegetation associated with it consists of forests of deciduous plants (oaks, beech trees, chestnut trees, birch trees, maples, limes) and of  steppes, the latter found especially in the lowlands of Russia and some areas of  North America. The climate defined to be sub arctic is  found between  50° and  70° of latitude, or rather, where the taiga forests, dominated by  evergreen conifers, stretch out.. It is characterized by long, cold winters and by summers that, even though brief, reach relatively mild temperatures.

Polar climates   

Polar climates are those in which the temperature of the warmest season always remains below 10 °C. In this climatic class  are found the  sub polar climate and the perennial ice climate. The first one, with very rigorous winters, cool summers and scarce precipitations during the whole year,  produces vegetation devoid of trees which is essentially constituted by moss and lichens (tundra). The perennial ice climate has temperatures which are constantly below 0 °C and practically no vegetation. In the area where this  climate is present,  Antarctica, at the meteorological station of Vostok, has been registered the lowest measured temperature  ever recorded on the planet: -91,5 °C.

Af = tropical climate without arid season  

Aw = tropical climate with dry winter  

BS = arid climate of the steppe  

BW = arid climate of the desert  

Cf = warm moderate climate without dry season  

Cs = warm moderate climate with dry summer  

Cw = warm moderate climate with dry winter  

Df = cold moderate climate without dry season  

Dw = cold moderate climate with dry winter  

ET = cold climate of the tundra

EF = cold climate of  perennial ice