| A ehthed ioeithetaediatedert abeaacbe wl eee *, ' + oF rok ® ao 4 —_- ‘ a te, ye aE ee tl ar Le: op thegedt cde, oe ae Ae | ee * ee ® - Mra ®t * f| | middle cloud and cirrus. Average cloud amounts at Eniwetok are given in Table Il. Precipitation and Tropical Storms. During the dry season, almost entirely the result of cumulus-produced showers. precipitation is These showers are nor- mally of short duration, but through their frequent occurrence may produce several inches of rainfall in a month. During the summer and early autumn months, periodic disturbances in the trade wind current, which are known as easterly waves, move across the Eniwetok area and produce greatly increased cloudiness and precipitation. These wavelike deformations of the general easter- ly flow are first observed in the trade wind current in the vicinity of 140° W longitude. They move westward and slowly deepen until in some cases cutoff cyclonic disturbances are produced. These cyclonic vortices or tropical storms continue their westerly movement in the basic current and under certain special circumstances may develop into typhoons. It is uncommon, however, for typhoons to become fully developed in the Eniwetok area; perhaps one every five years is typical, With the passage of an easterly wave over, or to the south of, Eniwetok a general increase in cloudiness at all levels is observed together with numerous moderate to heavy showera and in some cases with light to moderate continuous rainfall. As the wave passes on westward the cloud conditions slowly return (after a day or two) to a typical trade wind cumulus distribution and precipi- tation is again produced almost exclusively by individual cumulus activity. The intensity and frequency of easterly wave formation reaches its maximum in late Summer or early autumn, and a corresponding maximum in precipitation values is observed at that time. Mean precipitation amounts by months for Eniwetok are given in Table IL. 4, HYDROGRAPHY The four aspects of the hydrography of Eniwetok Atoll that are pertinent to the interpretation of the observations presented in this study are the bathymetry of the lagoon and immediately surrounding ocean waters, systems in the lagoon, tidal variations, current and mean water temperature relationships with special reference to geasonal variations in surface water temperature and changes in vertical temperature structure within the lagoon. Each of these topics is con- sidered below. Leenacae we wee » On the broadest scale, Eniwetok consists of a reef and superincumbent islets rent > a phen 4g ee ~ athe oom, . as . . Ne es ee ee eee PS ee ee ren ioa Raia