Kim Deneris, age 13, of Midvale, Utah, for his question:

How do they measure sea level?

Sea level is the surface of the sea as compared with the land that pokes up above the oceans. A flat sandy beach is at sea level just where it meets the watery waves. However, the problem is far more complicated than this. In some places, the surface of the global ocean is higher than average. Gale winds, ocean currents and seaquakes cause other variations. And every day, the picture changes as high and law tides chase each other around the planet.

Geographers and scientists in many other fields need exact data on sea level. So do sailors who depart and return from ocean voyages. The job of determining the level of the sea in comparison with the land is very important    and very difficult. Since the sea changes with the tides, it seemed sensible to estimate standard sea level as being the exact half way point between high and low tides.

This seems a simple sort of chore    but it is not. Any person., you might suppose, can measure a high and low tide on a beach and divide the difference. However, the next high tide may be slightly higher or lower. Tidal height varies through the lunar month and other changes occur throughout the year. Our tides are pulled mostly by the moon's gravity. But at the time of the new moon, the gravity of the sun adds a tug to our oceans and the tides are somewhat higher. In September, the rather wobbly moon comes a trifle closer and the highest tides of the month are the highest tides of the year.

We start with the idea that sea level marks the surface of the sea compared with the land. But this simple idea must be adjusted to the very changeable tides. Finding a standard sea level, mid way between high and low tides calls for patient measurements taken twice a day, through many months and many years. Phen computed together, these details give a mean or average mid way point of the tidal variations    in just one spot on the globe. But the rise and fall of tidal waters varies enormously from place to place. Fortunately, for many generations a lot of interested people have been patiently measuring their local tides. When computed together, all this global data should give us a pretty good idea of standard sea level.

But    other factors can confuse the very uneven seas. Near the equator, the pushing trade winds pile up mountains of water and the mean sea level is higher than the global average. Storms, barometric pressure and changes in wind direction also  cause variations in the level of the sea's surface. Even gravity upsets the picture. The oceans, of course, are hugged to the earth by the force of gravity. Regardless of tides, winds and other factors, gravity holds the surface fairly level. But its pull causes distortions along very steep mountainous coastlines. Off the west coast of South America, sea level is slightly higher than the world average. I Then measuring standard sea level, scientists must allow for these factors, plus a few others, and average out masses of measurements from around the globe.

The changing details involved in sea level are bewildering. But what is going on now is nothing compared to immense changes that occured in the past. Through eons of time, land and sea areas have changed places. As ocean floors rise or sink, sea level may be changed thousands of miles away. During the ice ages, countless tons of water that normally returned to the ocean as rain were frozen in massive glaciers on the land. Sea level sank 200 feet or more.