Sandra Scott, age l0, of Belfair, Wash., for her question:

What makes a rip tide dangerous?

Most of our planet is covered with seas, and the color of the ocean is basically blue. It may mirror the glowering gray of the clouds, or sandy shoals below may tinge it muddy brown. It may be tinted brown or green with floating algae, and sometimes a patch of the sea turns red.

So called red tides have been known since ancient times, and sometimes people thought the sea had turned red with blood. The water was slimy to touch and almost always thousands of fish were washed up dead and dying on the beaches. Some months ago such a red tide occurred along the shores of California, and in l947 a disastrous red tide occurred off the west coast of Florida.

These grim events are rare and arrive without warning. It has taken the scientists a long time to find what causes them. They now know what ocean conditions make them happen and what causes fish to die in untold numbers, but they still are unsure of how the fish are killed.

The ocean is a watery world of teeming populations, and its surface waters swarm with floating plankton. Plankton is a vast assortment of drifting plants and animals, many of them too small to be seen. Some varieties like warm and others prefer chilly water, some thrive in the saltiest water and some do better in fresher sea water. All of them must drift at the msrcy of the sea, which is often as changeable as the weather.

In average waters, the most plentiful plankton population is diatoms. These one celied plants have delicate cells of silica. Among them are a few one celled dinoflagellates. These plant animals have shells of plant cellulose and whip tails for swimming. As a rule, they are far outnumbered by the harmless diatoms. But a sudden change may bring hardship to the diatoms and favor the dinoflagellates. The diatoms perish or fail to multiply, and the thriving dinoflagellates dye the blue sea red. But no one knows just how the slimy red tide destroys such hosts of fish that depend directly or indirectly upon the ocean's meadows of floating plankton.

Several varieties of dinoflagellates that are normally present in plankton can multiply and cause a red tide. One of these destroyers is named gymnodinium brevis. This single celled plant animal measures one thousandth of an inch, and under the microscope it looks somewhat like a four leafed clover with a thin swimming tail instead of a stem.