Notes on Land-sea moisture exchange and Meridional moisture exchange

There are two important aspects of the transfer of humidity: (1) Land-sea moisture exchange, and (2) meridional moisture exchange.

(1) Land-sea moisture exchange:

There is a continuous exchange of moisture taking place between the continents and oceans. The amount of moisture gained through precipitation on the continents is more than that lost through evaporation.

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Oceans, on the contrary, lose more moisture through evaporation than what they receive through precipitation. Winds carry a part of atmos­pheric moisture from over the oceans to the continents where water vapour condenses and falls as rain or snow.

However, part of this precipitation returns to the oceans through surface run-off or ground water seepage. The remaining part also is brought back to the oceans by off-shore winds laden with land-evaporated moisture.

As a matter of fact, direct precipitation over oceans brings back a larger part of the moisture evaporated to the atmosphere.

(2) Meridional moisture exchange:


Meridional moisture exchange involves the transfer of moisture between different latitude belts on the earth. This also involves a transfer of energy. In certain latitude zones there is a marked excess of precipitation over evaporation.

On the contrary, there are certain zones where evaporation exceeds precipitation. The latitudinal imbalance thus produced requires a meridional exchange of moisture as well as energy between the dry and wet- zones.

In each hemisphere the surplus of precipitation over evaporation is found in equatorial as well as middle-latitude zones extending from 40 to 70 degrees north and south latitude. On the other hand, in latitudinal zones extending from 10 to 40 degrees latitude in each hemisphere evaporation exceeds precipitation.

It may be noted that in such zones where evaporation is in excess of precipitation, huge amount of energy is trans­formed into the latent heat of evaporation. Condensation, Contrary to it, in such zones where precipitation exceeds evaporation, there is a tremendous gain of energy through the latent heat of condensation.


This naturally upsets the latitudinal heat balance. However, planetary winds, cyclones, anti-cyclones, and other atmospheric disturbances play a very significant role in removing the latitudinal heat imbalances.

In fact, direct precipitation over the oceans brings back a larger part of moisture evaporated to the atmosphere. Thus, it is obvious that winds play a large part in completing the hydrologic cycle.

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