4
Precipitation :
Water vapour forms due to evaporation and transpiration rise above the surface
and joins the atmosphere. Vapour thus formed behaves like a gas and follows
the general laws of gas like Charle's and Boyle's law. As the vapour goes up
and up, the temperature goes down. The fall in temperature is not due to heat
transfer or in other words there is no loss of heat to the surrounding air and the
process is known as adiabetic. The drop in temperature is entirely due to
expansion and it is estimated that for every Km of ascent of air, the temperature
drop is about 10
0
C. As the evaporation continues, the vapour that rises joins the
already evaporated vapour and a stage is reached when the space cannot
accommodate further vapour and any addition of vapour will get condensed on
the surfaces. The condensed vapour may take any of the forms like mist, rain,
hail, snow, sleet etc. The evaporated water, thus, comes back to the earth's
surface in the form of mist, rain, hail etc. Rainfall is the most important
component that falls on the earth as precipitation, while the other forms like
mist, snow, hail etc. are negligible.
It is well known that some amount of water or moisture is always present in the
air and its presence in the air is expressed in terms of humidity. Thus -
Absolute humidity
at a given temperature = Mass of moisture present in unit
volume of air at the given temperature.
Relative humidity is defined as the ratio of the actual vapour pressure to the
saturation vapour pressure at the same temperature. Thus -
Relative humidity (R.H.) =
Actual vapour press at a given temperature (ea)
Saturation vapour pressure at the same temperature (es)
Relative Humidity thus gives an idea of the extent to which the air is saturated.
