Upward air currents undergo adiabatic expansion and cooling. The adiabatic lapse rate of air temperature which thus occurs is about 1K for every 100 m of increase in altitude. Comparing this with the normal fall of temperature, 0.65 K per 100 m (see equation (2.2)), it can be seen that in due course the rising air, which was originally, perhaps, at a higher temperature than ambient air, is eventually at a lower temperature than its environment. At this point, upward motion ceases.

If, during the adiabatic cooling process, the air temperature in the rising current fell below its dew point, condensation would occur in the presence of adequate condensation nuclei. The liquid droplets so formed would tend to fall under the influence of the force of gravity but the rate of fall would be countered by the frictional resistance between the droplet and the rising current of air. Whether the cloud, formed as the result of the condensation, starts to rain or not depends on the resultant of the force of gravity downwards and the frictional resistance upwards. The rate at which rain falls depends on the size of the drops formed.

Posted in Engineering Fifth Edition