Many windblown drops can be forced together to form what Weather reporters call ‘sheeting rain’, but rain is always born as minuscule drops of condensed water vapour explains ‘Clouds and Weather’ by John A. Day and Vincent J. Schaefer (Houghton Miffin Company), U.S. The formation of these droplets depends on the right amount of water vapour at the right pressure and temperature, but it also requires the presence of tiny solid particles of matter in the air on which the water vapour can gather and condense.
These bits of dust and salt are called cloud condensation nuclei. Salt starts collecting vapour at about 80 percent relative humidity, while bits of clay begin to take on water molecules at 100 percent relative humidity.