From October through April, the heart of the trade winds moves south of Hawaii; thus there average wind speeds are lower across the islands. Due to Hawaii being at the northern edge of the tropics (mostly above 20 latitude), there are only weak wet and dry seasons unlike many tropical climates. Island wind patterns are very complex. Though the trade winds are fairly constant, their relatively uniform air flow is distorted and disrupted by mountains, hills, and valleys. Usually winds blow upslope by day and downslope by night. Local conditions that produce occasional violent winds are not well understood. These are very localized, sometimes reaching speeds of and are best known in the settled areas of Kula and Lahaina on Maui. The Kula winds are strong downslope winds on the lower slopes of the west side of Haleakala. These winds tend to be strongest from above mean sea level. The Lahaina winds are also downslope winds, but are somewhat different. They are also called "lehua winds" after the "ōhia lehua" ("Metrosideros polymorpha"), whose red blossoms fill the air when these strong winds blow. They issue from canyons at the base of the western Maui mountains, where steeper canyon slopes meet the more gentle piedmont slope below. These winds only occur every 8 to 12 years. They are extremely violent, with wind speeds of or more. Under trade wind conditions, there is very often a pronounced moisture discontinuity between . Below these heights, the air is moist; above, it is dry. The break (a large-scale feature of the Pacific High) is caused by a temperature inversion embedded in the moving trade wind air. The inversion tends to suppress the vertical movement of air and so restricts cloud development to the zone just below the inversion. The inversion is present 50 to 70 percent of the time; its height fluctuates from day to day, but it is usually between . On trade wind days when the inversion is well defined, the clouds develop below these heights with only an occasional cloud top breaking through the inversion. These towering clouds form along the mountains where the incoming trade wind air converges as it moves up a valley and is forced up and over the mountains to heights of several thousand feet. On days without an inversion, the sky is almost cloudless (completely cloudless skies are extremely rare). In leeward areas well screened from the trade winds (such as the west coast of Maui), skies are clear 30 to 60 percent of the time.