Lecture
The water cycle is one of the three climate-forming processes. The water cycle includes:
1) evaporation of water from the earth's surface;
2) condensation of water in the atmosphere;
3) precipitation;
4) runoff (runoff and its characteristics are studied in hydrology).
Water vapor enters the atmosphere as a result of evaporation of water from the earth's surface, from the surface of plants, soil, snow and ice cover, etc.
Water molecules overcome molecular cohesion forces and break away from the surface, then rapidly spread through molecular diffusion (the actual motion of molecules) and turbulent diffusion (by means of air currents). Condensation processes continually occur alongside evaporation processes.
The state in which the amount of evaporated water equals the amount of condensed vapor is called equilibrium. The pressure of water vapor in the saturation state is called the saturated water vapor pressure (E, hPa).
Air humidity depends on how much water vapor enters the atmosphere through evaporation from the surface. Over a water surface it is naturally higher than over land. At the same time, humidity also depends on atmospheric circulation – air currents bring more humid or drier air into a given area.
To quantitatively express the water vapor content in the atmosphere, the concept of partial pressure of water vapor (e, hPa) is used.
Relative humidity (f, %) – the ratio of the actual partial pressure to the saturated water vapor pressure at a given temperature:
Absolute humidity (a, g/m3)– the mass of water vapor (g) in 1 m3 of air:
a = 220e/T
Dew point – the temperature at which the water vapor present in the air reaches the saturation state at constant pressure.
Humidity deficit - the difference between the saturated vapor pressure (E) and the partial pressure of water vapor (e).
Air humidity can be determined by the psychrometric method, i.e., from the readings of two thermometers, one of which has a wetted bulb. Hair hygrometers and other methods are also used.
Under natural conditions, the rate of evaporation depends on:
geographic latitude
on the humidity deficit
temperature and wind speed
During the day, maximum evaporation and maximum evaporation rate are observed in the first hours after noon and diminish as the sun's altitude decreases. Evaporation under natural conditions is characterized by the evaporation rate, defined as the thickness of the water layer evaporating per unit time from a unit area.
Evaporation occurs even when the air is saturated with vapor, if the evaporating surface is warmer than the surrounding air. Evaporation also depends on the depth of groundwater occurrence. Vegetation cover significantly reduces evaporation from the soil surface, but the plants themselves evaporate a great deal of water.
Condensation occurs when water vapor reaches saturation, most often as a result of a decrease in atmospheric air temperature.
As the temperature decreases to the dew point, water vapor reaches a state of equilibrium; a further decrease in temperature leads to condensation of the excess water vapor and its transition into the liquid state.
Droplet formation occurs on condensation nuclei. These can be salt crystals or solid particles that are combustion products (over the ocean, 1 cm3 contains 60 – 800 condensation nuclei, while over an industrial area it can be several million). As a result of water vapor condensation, accumulations of condensation products form – droplets and crystals. These accumulations are called clouds.
Cloud elements are so small that they are balanced by friction. The fall velocity of droplets in still air equals a few tenths of a centimeter per second.
In the presence of turbulent motion, droplets and crystals remain suspended for a long time, moving up and down. The sizes of cloud elements vary widely – from fractions to hundreds of micrometers. Under certain conditions, some cloud elements grow large enough to fall out of the clouds as precipitation. The cause of this growth is electrostatic interaction. Clouds are transported by air currents; if the relative humidity of the air decreases, the clouds evaporate.
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