Danger from the Tides

If there are severe storms in either the Atlantic or Pacific oceans around Jan. 8, Americans living in coastal areas may well be hit by bad floods. This unusual warning was sounded last week by federal scientists. Why Jan. 8? Because of a relatively rare combination of circumstances, tides will be abnormally high around that time. Although the tides alone will not cause flooding, strong, persistent onshore winds accompanying a coastal storm would pile the water even higher, spilling it into low-lying areas.

Tides are caused largely by the gravitational tug of the moon, which daily forces great upward and downward movement in the oceans. The pull of the distant sun also influences the tides, and when the orbits of the moon around the earth and the earth around the sun bring all three bodies roughly into line, the tidal changes are much larger than usual. These "spring" tides (named for the verb rather than the season) occur twice a month: when the moon is full and when it is new. Spring tides themselves may be driven to further extremes when the elliptical path of the moon brings it closest to the earth's surface, increasing the effect of lunar gravity.

The year 1974 will bring several such outsize tides. On Jan. 8, and again on Feb. 6-7, the moon will be particularly close to earth. The earth will also be close to the sun, and all three bodies, sun, moon and earth, will have moved almost into a straight line.* Thus spring tides along the Atlantic and Pacific coasts in early January and February --and again around July 19 and Aug. 17, when similar conditions will occur --should be particularly extreme.

Scientists know from experience what could happen if a coastal storm should blow up on these dates. Research Scientist Fergus J. Wood, of the National Ocean Survey, recalled last week that a spring tide of 5.2 ft. at Atlantic City in March 1962 was whipped by gusts of up to 70 knots and rose 9.5 ft. above the average low-water mark. Huge waves battered the Atlantic coast. The accompanying floods cost 40 lives and some $500 million in damage.

* If they were directly in line, the result would be an eclipse of the moon.

This file is automatically generated by a robot program, so viewer discretion is required.