Dew is often much smaller in quantity that the rainfall. In humid areas its contribution to moisture balance in the area is usually ignored. In arid areas and desert environment however it is more important than is generally recognized. In this chapter we give some evidence of its significance from investigations carried out at many locations overseas. It is followed by presentation of data of dewfall measurements in India and in the coastal arid region of Kutch in Gujarat. Quality of dew water from Kutch is examined. Finally, it is argued that given the good quality of water, appreciable quantity and high frequency of occurrence in arid coastal Kutch, dew harvest systems should be developed for the use of people in the region.

Dew, Fog and Frost

Dew is atmospheric water vapour condensing on a surface that has been cooled by radiative cooling at night. Dew deposit can be seen on the windshield of cars parked in the open on winter nights. In the course of the night, glass looses heat by radiative exchange more rapidly than the surrounding air. If it cools down to the dew-point temperature of the air, condensation will occur on its surface. That is dew.

Fog is closely related. It occurs when the air near the ground is saturated and condensation occurs (not on cold surface) but on tiny particles suspended in the air. It is difficult to see through fog because of those tiny particles of water suspended in the air, which reduce visibility.