Dew Deposition over India

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Dew is an important source of moisture for plant growth. This is useful particularly in the arid and semi–arid climatic zones which receive low quantum of rainfall. In this publication monthly dew data of 79 stations in the country were provided.
Dew is defined as the deposition of water drops by direct condensation of water vapour from the adjacent clear air, upon surfaces cooled by nocturnal radiation.
Dew is an important secondary source of moisture for crops during the non-rainy season and plays vital role inplant growth. Dew occurances benefit the plants in many ways, for instance

(1) it is directly used by plants from absorption by leaf surface
(2) it reduces transpiration and helps conserve moisture
(3) it helps acceleration of photosynthesis by plants in forenoon hours due to water saturation of leaves during night.

These benefits are significant particularly in arid and semi-arid areas.
In India Met. Department dew is recorded at 4 heights above the ground surface, namely 5, 25, 50 and 100 cm.

Network of stations
Dew measurements commenced in India since 1968 and presently dew is measured at 75 stations. The distribution of these locations is shown in Fig. 1. A minimum of 5 yearswas chose as a period in preparing the dew deposit normals. The period 1969-88 has been utilised in preparing the normal.

Presentation of data
Dew is observed mosly from Sept. to April in different parts of India. Mean monthly values of dew deposits (in mm) at each of the four heights mentioned above have been computed from the daily values and given in this publication. Number of dewy nights have been indicated. The seasonal totals (i.e. from Sept. to April) have also been computed and shown.