Maui’s rugged, irregular topography produces marked variations in conditions. Air swept inland on the Trade winds is shunted one way or another by the mountains, valleys, and vast open slopes. This complex three-dimensional flow of air results in striking variations in wind speed, cloud formation, and rainfall.Maui displays a unique and diverse set of climatic conditions, each of which is specific to a loosely defined sub-region of the island. These sub-regions are defined by major physiographic features (such as mountains and valleys) and by location on the windward or leeward side of the island. These sub-regions (and their characteristic climates) are:
Windward Lowlands: Below 2,000 feet (610 m) on north- to northeast-sides of an island. Roughly perpendicular to direction of prevailing trade winds. Moderately rainy; frequent trade wind-induced showers. Skies are often cloudy to partly cloudy. Air temperatures are more uniform (and mild) than those of other regions.
Leeward Lowlands: Daytime temperatures are a little higher and nighttime temperatures are lower than in windward locations. Dry weather is prevalent, with the exception of sporadic showers that drift over the mountains to windward and during short-duration storms.
Interior Lowlands: Intermediate conditions, often sharing characteristics of other lowland sub-regions. Occasionally experience intense local afternoon showers from well-developed clouds that formed due to local daytime heating.
Leeward Side High-Altitude Mountain Slopes with High Rainfall: Extensive cloud cover and rainfall all year long. Mild temperatures are prevalent, but humidity is higher than any other sub-region.
Leeward Side-Lower Mountain Slopes: Rainfall is higher than on the adjacent leeward lowlands, but much less than at similar altitudes on the windward side; however, maximum rainfall usually occurs leeward of the crests of lower mountains. Temperatures are higher than on the rainy slopes of the windward sides of mountains; cloud cover is almost as extensive.
High Mountains: Above about 3,000 feet (910 m) on Haleakalā, rainfall decreases rapidly with elevation. Relative humidity may be ten percent or less. The lowest temperatures in the state are experienced in this region: air temperatures below freezing are common.
Rainfall: Showers are very common; yet while some of these are very heavy, the vast majority are light and brief – a sudden sprinkle of rain and it’s over. Even the heaviest rain showers are seldom accompanied by thunder and lightning. Throughout the lowlands, in summer an overwhelming dominance of trade winds produces a drier season. At one extreme, the annual rainfall averages 17 inches (430 mm) to 20 inches (510 mm) or less in leeward coastal areas, such as the shoreline from Maalaea Bay to Kaupo, and near the summit of Haleakalā. At the other extreme, the annual average rainfall exceeds 300 inches (7,600 mm) along the lower windward slopes of Haleakalā, particularly along the Hāna Highway. If the islands of the State of Hawaii did not exist, the average annual rainfall on the same patch of water would be about 25 inches (640 mm). Instead, the actual average is about 70 inches (1,800 mm). Thus, the islands extract from the air that passes over them about 45 inches (1,100 mm) of rainfall that otherwise would not fall. The mountainous topography of Maui and the other islands is responsible for this added water bonus.
Daily Variations in Rainfall: In the lowlands, throughout the year, rainfall is most likely to occur during the night or morning hours, and is least likely to occur mid-afternoon. The most pronounced daily variations in rainfall occur during the summer because most summer rainfall consists of trade winds showers that most often occur at night. Winter rainfall in the lowlands is the result of storm activity, which is as likely to occur in the daytime as at night. Rainfall variability is far greater during the winter, when occasional storms contribute appreciably to rainfall totals. With such wide swings in rainfall, it is inevitable that there are occasional droughts, sometimes causing economic losses. The real drought years are the ones where the winter rains fail, with too few significant rainstorms. Droughts hit hardest in the normally dry areas that depend on winter storms for their rainfall and receive little rain from the trade wind showers.