The Mariana Islands lie between the area of the Asiatic monsoon and the belt of the northeast trade winds. At Saipan from November until March or April, winds usually are easterly or northeasterly and are strong and steady since the northeast trades and the winter monsoon reinforce each other. In April and May the directions of the winds shift toward the southeast, and they become weaker and more variable. In this period there may be some easterly winds in addition to the predominating southeasterly winds. Detailed information is not available on the winds which occur in the Marianas north of Saipan, but at Pagan easterly winds probably prevail from May to July and westerly winds prevail in the remainder of the year. The Carolines lie in the belt of alternating northeast trade winds and southwest monsoons. The northeast trades begin in October and prevail until May or June. The southwest monsoon occurs from May to October and may be felt as far east as Truk. To the eastward, the winds of the summer are usually light and variable. In the Marshall Islands, the northeast trade winds predominate from about December to April, especially in the northern part of the Marshalls. In summer, winds are variable and weak; periods of calm may occur. Typhoons and squalls occur most frequently in the spring and summer in Micronesia. Some of the severe typhoons are known to engulf entire islands, as did the one at Woleai in 1907. SOILS The soils of the islands of Micronesia have been derived from volcanic materials or from depositions of coralline limestone. Volcanic soils occur on the "high" islands of Micronesia. In many places, especially on the islands of the northern Marianas there is little soil; there are large areas of bare igneous rock, because the islands are geologically of relatively recent origin and little erosion has occurred. On islands where volcanic rocks have decomposed, the resulting soil may have a top layer of humus. The richest soils of the islands are along drainage areas and in alluvial deposits. Coralline soils result from the decomposition of limestone, coral fragments, shells, and sand, and are overlain by some humus. Where the layer of humus is deep, the fertility is greatest. Coralline-volcanic soils occur on some "high" islands where coral rock and volcanic rock have become mixed in the decomposition process which forms soil. In parts of the Marianas and elsewhere, unwise practices of burning and overgrazing have allowed extensive erosion to occur, resulting in reduced fertility of the soil. On the island of Yap certain sedimentary rocks are exposed which are thought to have been elevated from the ocean bottom. Soils at Yap which have developed from this rock are considered more fertile than soils of coralline origin, although the fertility there also is dependent on the depth of the layer of humus. SURFACE WATER