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The climatic conditions of present century have been established on the basis of meteorological data collected from worldwide network of climatic stations. The data shows that elements of meteorological regime change perceptibly over time. These changes are both periodic (daily or yearly) oscillations as well as non-periodic oscillations of different time intervals. Short interval non-periodic changes (of days or months) occurring in meteorological regime determine the oscillations in weather. These changes are not spatially homogeneous and are largely explained by the instability of atmospheric circulation. For longer time intervals (of several years), there occur irregular oscillations of individual elements of meteorological regime as well as long-term changes similar over large territories. Such changes characterize fluctuations in climate. Since climatic fluctuations in the present times are relatively modest, average values of meteorological elements over a period of several decades could be used in order to describe the climatic features of the present age. The use of such average values makes it possible to exclude the influence of unstable atmospheric circulation pattern. Following Table-6 shows average temperature in January and July and also average yearly atmospheric precipitation at various latitudes.
Data show that difference between average temperatures at Earth’s surface at various latitudes is almost 700 C. The temperature is maximum at Equator and lowest at South Pole. Earth’s spherical shape exerts a substantial influence on the distribution of these temperatures by producing variations in the total radiation reaching the upper boundary of atmosphere. Further, permanent ice covers are found at high latitudes where air temperature does not rise above freezing point almost throughout the year. Apart from substantial changes in meridonial direction, average air temperature at Earth’s surface also changes substantially in most latitudinal zones at various longitudes. This is largely explained by the distribution of continents and oceans.
The influence of ocean’s thermal regime extends to a large part of the surface of continents on which maritime climate exists. This is characterized by a relatively modest yearly oscillations in air temperature at middle and high latitudes. In those extratropical continental regions where influence of oceanic thermal regime is less pronounced, the amplitude of yearly temperature fluctuations increase sharply characterizing the continental climate.
The distribution of average latitudinal precipitation values produces a pattern in which the principal maximum value occurs in the equatorial zone, total precipitation declines at subtropical latitudes, two secondary maxima lie at middle latitudes and precipitation declines in polar latitudes. Changes in average precipitation at different latitudes are explained by the distribution of average air temperatures and by specific characteristics of atmospheric circulation. Other conditions being equal, total precipitation increases with temperature because it increases the volume of atmospheric water vapour. Vertical air currents that carry water vapour through condensation level producing clouds also have important role in precipitation.
The atmosphere’s overall circulation is closely associated with geographical distribution of stable pressure systems. Particularly important such systems are low-pressure belt near the equator, high-pressure region at high tropical and subtropical latitudes and the region of frequent cyclonic formations at middle latitudes. Downward movements of air within high-pressure zones substantially reduce the precipitation while pronounced upward air movements at equatorial latitudes and in several regions at middle latitudes increase the precipitation.
Largest desert areas on Earth having negligible precipitation are found in the subtropical high-pressure zone. Total precipitation also declines in continental regions at middle latitudes which are distant from oceans because very small quantity of water vapour carried by air currents from oceans reaches these regions.
Thus in the continents, zones of humid climates are largely located at equatorial latitudes and in regions of maritime climate, at middle and high latitudes. Similarly, at high tropical and subtropical latitudes and in regions of continental climates, conditions of insufficient moisture prevail.