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| | Rain is an atmospheric precipitation which falls out from the clouds in the form of water drops with a diameter ranging from 0.5 to 6 -7mm.
Liquefied precipitates with a smaller diameter of drops are called drizzle. Drops with a diameter as big as 6 -7mm while falling disintegrate into smaller drops. The intensity of rain fluctuates from 0.25 mm/hr (drizzle) to 100 millimeters an hour (downpour).
Rain falls, as a rule, from mixed clouds (predominantly nimbostratus clouds and altostratus cloud), containing super cooled droplets and ice crystals at a temperature of less than 0 C. The saturation tension of water vapor over the drops is more than over the ice crystals at this temperature; hence clouds, which are not even saturated with water vapor with regard to water drops, shall be super saturated with respect to crystals. It leads to the growth of crystals during the simultaneous evaporation of the drops. By becoming larger and heavier, the crystals fall out from the cloud by freezing-on the super cooled drops to them. By entering into the lower part of the cloud or into the layers below the clouds with positive air temperature, the crystals thaw turning into rain drops. The merging of cloudy drops amongst them play an insignificant role in the formation of rain.
Rainfall, during the presence of sun, which is not covered by clouds, is called blind rain (sometimes — mushroom rain).
The absence of rain for longer periods leads to drought. In many cultures, a special ceremony is performed during droughts with an aim to call for rain.
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* The acidity of normal rain is рН 5.6. Acidity is much less in acid rains. When the acidity of water is рН 5.5, the useful ground bacteria of the reservoir perish and at рН 4.5 all fish and the majority of amphibians and insects perish.
Rainfall on other celestial bodies
Details mentioned about rainfall concerns only Earth with its water-air atmosphere. Water rainfall (possibly rainfall occurs even now) apparently occurred on Mars or at least, traces of rain erosion on rocks are found. Methane rains occur regularly on Titan, the moon of Saturn and this was confirmed during the mission of “Cassini –Huygens”. Apart from this, specific water rains, caused by anomalous high geological activity, occur periodically on Enceladus, the other moon of Saturn.
Day is a unit of time, approximately equaling to the rotation period of the Earth about its axis.
Days are distinguished by solar days (24 hours) and sidereal days (23 hours 56 minutes 4 seconds). They are not equal thanks to the rotation of the Earth around the Sun. The Sun displaces on the background of stationary stars for the observer who is on the Earth.
A day is divided into 24 hours (1440 minutes, 86400 seconds) and consists of an afternoon, evening, morning and night.
Calendar days make up weeks and months.
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