Atmospheric processes affect the heat flows coming from above, from space, and from below, from the earth's surface. The solar radiation that comes to Earth from outer space is the main source of energy of atmospheric processes. It is the radiant energy of the Sun that is converted into heat in the atmosphere and at the Earth's surface, kinetic energy, and other forms of energy. But the Sun's rays heat the earth's surface larger than the air directly, so between the earth's surface and the atmosphere, there is lively exchange of heat as well as moisture. The structure of the earth's surface and its relief are very important for these processes. The chapter presents the picture of heat and the moisture circulation in the atmosphere and gives physical basics of atmospheric general circulation including fundamentals of air mass circulation, local physiographic impact on the atmospheric air movement, in-mass atmospheric processes, and basic laws of pollution spreading in the atmosphere.
TopThe Heat And The Hydrologic Cycle In The Atmosphere
In addition to heat exchange by radiation, there is heat exchange by conduction between the Earth’s surface and the atmosphere. Mixing of air in the vertical direction is extremely important for the heat transfer inside the atmosphere. The air that contacts directly with the earth’s surface forms a thin laminar boundary sublayer and exchanges with the surface by heat due to molecular conduction. Above the laminar boundary sublayer inside surface atmospheric boundary layer, there acts another, more efficient heat transfer – by the turbulent thermal conductivity. Mixing air due to turbulence promotes the very rapid transfer of heat from one layer of the atmosphere to another. There are here the greatest vertical gradients of meteorological quantities that increase heat transfer from the ground into the air and vice versa. So, if there is a cooling (heating) air from the earth’s surface, the turbulent flow continuously deliver the warm (cold) air from the upper layers to the place of cold (warmed) air. This stream will maintain the temperature difference between the air and the surface and, therefore, support the process of heat transfer between the air and the surface. Changing the temperature in the upper layers of the atmospheric boundary layer is not so high (the vertical turbulent exchange reduces the vertical temperature gradient on the outer edge of the boundary layer), but this changing has spread to a more powerful layer of the atmosphere. Thus, the loss (acquisition) of heat by the Earth’s surface would be greater than it would be in the absence of turbulence.
Because of diurnal variation, air temperature above the sea surface usually reaches a minimum of 2-3 hours before sunrise, and the maximum at 15-16 o’clock. On land surface, this process has less inertia and is shifted to 1-1.5 hours. This diurnal variation is characteristic only for stable weather. It is violated by heat exchange processes in the atmosphere, for example, when the warm air mass is replaced by the cold one. The night time temperatures may be even higher in such cases than at daytime. In overcast, the temperature difference is considerably less than under clear skies. During precipitation and after, the temperature is usually lowered.
The daily amplitude of temperature changes depends on the time of year and on the cloudiness. In the latter case, it decreases. The daily amplitude in the open region of sea or ocean is about 1.0-1.5°C and in the closed sea, it can reach 10-15°C. In areas with sharply continental climate, for example, in the desert, the daily fluctuations of temperature are maximal and can reach 30°C.
Since the heating value of the earth’s surface decreases in an average from the tropics to the poles, the horizontal temperature gradient in the troposphere has the same direction. The dependence of pressure altitude field on the temperature field leads to the fact that the rotating globe holds west air transfer. Zonal movements of air masses over the underlying surface with dramatically different properties (continents or oceans) imply changes in their physical properties, i.e., the process of transformation. Therefore, the main features of the real circulation of the atmosphere in the zonal movement of air masses depend on the transformation caused by the distribution of continents and oceans, the surface of which are heated differently by the action of solar radiation.
Thus, the temperature in a restricted area may vary because of the continuous change of air at a given location, i.e. due to the entrance of air from the atmosphere of other places where it has a different temperature. These temperature changes associated with the influx into this place of new air masses from other parts of the globe are called advective. When air flows to this place at a higher temperature, then we talk about heat advection; if lower temperatures, then we talk about cold advection. Horizontal advective heat transfer between the southern and northern latitudes is carried by meridional transfer of air masses and is about 1019 Cal/day.
A significant part of the heat input to the Earth’s surface passes in a latent form by expenditure on the evaporation of water. Then, with condensation of steam at atmosphere, this heat is used for heating the air.