 
Rain is Sent Down in due Measure
 In
the eleventh verse of Surat az-Zukhruf, rain is defined as
water sent down in "due measure". "It
is He (God) Who sends down water in due measure from the sky"
(Qur’an, 43:11)
Indeed, rain falls on the earth in an unerring measure.
The first of the measures related to rain is its speed of
descent. When dropped from a height of 3937 feet, an object
having the same weight and size as a rain-drop would continuously
accelerate and fall on the ground at a speed of 347 miles/h.
The average speed of raindrops, however, is only 9.3-12.4
miles/h.
The reason for this is that the raindrop has a special form that increases the frictional effect of the atmosphere and helps it fall on the ground more slowly. A glance at the figures below is sufficient to understand the disaster the earth would face every time it rained if rain raindrops were in a different form, or the atmosphere did not have the quality of friction.
The minimum altitude of rain clouds is 3937 feet. The effect
caused by a single drop falling from that height is equal
to a 1 kg object dropped from 5.9 inches. There are also rain
clouds at altitudes of 32808 feet. In this case, a single
drop would have an effect equal to a 1 kg object dropped from
43.3 inches.
It is estimated that in one second, approximately 16 million
tons of water evaporate from the earth. This number is equal
to the amount of water that drops on the earth in one second.
In one year, this figure amounts to 505x1012 tons. Water continuously
circulates in a balanced cycle according to a "measure".
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