Oceanography

Now we get a hint of the importance of having good “communication” between the Atlantic, the Pacific, and the other oceans. The freer the circulation, the smaller the differences in climate from equator to pole; the more restricted the circulation, the greater the climatic contrasts. A planet with landlocked seas, even though they had just as much water as earth’s oceans, might have greater polar ice caps and hot seas at the equator.

When sea water evaporates, the salts are almost entirely left behind, and the fresh water vapor moves into the atmosphere. As the vapor rises it cools, condenses, and finally rains back on the sea. If there were no continents the system would be simple enough. For the oceans as a whole, the surface would be lowered about three feet each year by evaporation, but the depth would be restored by condensation and resultant rain. Total evaporation would equal total precipitation.

The Water Cycle

There is a time lag between evaporation and rain return; the evaporated moisture travels long distances in the atmosphere before it comes back to the oceans. Part of the water evaporated from the oceans moves over the land before it condenses, then it falls as rain or snow on the land, where it may evaporate again, or collect as ice, or sink into the ground, or run off across the surface in rills and brooks to coalesce into rivers that flow back to the sea. Given enough time, all of the water in the oceans will pass down the rivers to the sea. Cleopatra’s bath water has run to the sea and mixed throughout the oceans. About 5 percent has already been evaporated and returned to the continents as rain. A few molecules of her bath are present in every tub full of water drawn today.

Yearly precipitation on the continents averages a little less than the three feet that falls on the sea, enough to cover the land to a depth of two and a half feet. Two thirds evaporates into the atmosphere and comes back to the sea as rain, and about one third returns to the oceans via streams. When it first falls and begins to run across the land surface, rain is satisfactory as a water supply for almost every purpose. It can be drunk; it has just enough salts inherited from the sea so that it doesn’t have the bad effects of perfectly pure water, yet there is too little salt to have toxic effects. A generation or two ago many homes had a barrel or tank in which rainwater was collected from the roof for drinking and washing, as is still done in Bermuda today.

If we neglect distribution problems engendered by the fact that rain refuses to fall where we want it to, and assume that all that falls could somehow be collected before it evaporated, we come up with a maximum figure for the naturally available water supply. By multiplying the total area of the earth by the average feet of rainfall, we arrive at about forty million gallons of water per year for every person on earth, a total of one hundred and forty million billion gallons.

The sun is very kind in the share of its energy it gives to the water cycle. The amount of energy required to evaporate the one hundred and forty million billion gallons of rainwater is half of the total radiation of the sun received at the earth’s surface.