Lecture
Atmospheric circulation, along with heat exchange and moisture exchange, is the third important climate-forming factor. Atmospheric circulation is associated with the transfer of air masses with different physical properties, which shape the weather over large territories. Air currents prevailing in various parts of the globe are not isolated, but are part of the system of general atmospheric circulation.
The general circulation of the atmosphere refers to the circulation of air over the globe, leading to its transfer from low latitudes to high latitudes and back, as well as to its movement along parallels over considerable distances. The most important cause of the emergence of air currents in the atmosphere is the uneven distribution of heat received from the Sun, which leads to unequal heating of the soil and air in different belts of the globe. Additional causes arise from the heterogeneity of the surface – land, seas, mountains, etc.
Uneven heating of the air leads to differences in atmospheric pressure values, which is the direct cause of the emergence of air currents. These currents do not flow exactly from areas of high pressure to areas of low pressure, but, owing to the rotation of the Earth, are deflected from the pressure gradient to the right in the Northern Hemisphere and to the left – in the Southern Hemisphere. This deflecting force of the Earth is an extremely important factor of atmospheric circulation.
Wind – the movement of air relative to the Earth's surface, caused by an uneven distribution of pressure and directed from an area of high pressure toward an area of low pressure. The wind regime is associated with the horizontal distribution of pressure and its change over time. The distribution of atmospheric pressure is called the pressure field.
Atmospheric pressure is a scalar quantity, and at each point it is characterized by its own numerical value, expressed in hectopascals (hPa). The pressure field is graphically represented by means of surfaces at all points of which the pressure is the same. Such surfaces are called isobaric surfaces. Lines connecting points with equal pressure are called isobars.
Areas of closed isobars with a pressure decrease at the center are called a cyclone.
Areas of closed isobars with a pressure increase at the center are called an anticyclone.
Pressure fluctuations are mostly non-periodic in nature and are caused by both thermal and dynamic reasons. Uneven heating or cooling of the Earth's surface leads to a change in pressure over various parts of it. The invasion of cold air masses into a given territory also causes a change in pressure. If warm, i.e., less dense air masses are replaced by cold, denser ones, the pressure rises; otherwise, it falls. Pressure fluctuations can be comparatively large and sharp (pressure can change by 20 – 30 hPa in the course of a day).
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