Hawaii’s climate is often characterized as perpetually tropical, but this description hides the high variability in precipitation across the island chain. The archipelago contains some of the wettest locations in the United States mere miles away from areas verging on desert conditions. This rapid fluctuation of rainfall over short distances makes finding a single, representative “average” for the entire state impractical for measuring local weather. The diverse topography of the islands, created by shield volcanoes, interacts uniquely with prevailing weather systems to produce an array of microclimates. The actual rainfall experienced depends entirely on the exact location on an island.
Defining the State Average and its Limitations
The estimated mean annual precipitation for the state of Hawaii is approximately 63.7 inches. This figure, calculated by aggregating data from various weather stations, statistically places Hawaii as the wettest state in the nation. However, this single number is misleading because it averages regions with vastly different rainfall totals, making it irrelevant for understanding local conditions. The true annual range extends from 5.7 inches in the driest spots to over 460 inches in the wettest.
Precipitation data is collected through a network of instruments, including manually read rain gauges and automated weather stations. Historical data from programs like the National Weather Service Cooperative Observer Program (NWS COOP) and community-based systems are combined with modern techniques. Scientists also employ methods such as radar estimates and computer modeling to interpolate rainfall across remote, mountainous areas where physical gauges are sparse. This data collection acknowledges that the state’s official average represents a mathematical center point, not the reality experienced by the population living in drier coastal areas.
Rainfall Distribution Across the Major Islands
The precipitation patterns across the major Hawaiian Islands—Kauai, Oahu, Maui, and the Island of Hawaii (Big Island)—show a consistent contrast between the windward and leeward sides of the mountains. On Kauai, Mount Waialeale receives an average of about 460 inches of rain annually, contrasting sharply with the drier coastal areas of the leeward side. For example, the Lihue airport receives an average closer to 36 inches per year.
On Oahu, the difference between the windward Ko’olau Range and the leeward coast is pronounced. The central Honolulu area records an annual average of around 18 inches, while the windward slopes just a short distance away can receive over 100 inches. Certain leeward areas on Oahu, particularly those shielded by the Wai‘anae mountain range, receive so little rainfall that they are considered near desert-like.
Maui features another illustration of this orographic effect, with the Big Bog area on the windward slope of Haleakalā volcano logging annual totals exceeding 400 inches. In contrast, the Kahului airport area on the leeward side averages only about 16 inches of precipitation annually. The Island of Hawaii (Big Island) hosts the most extreme gradients, with the city of Hilo on the windward side receiving an average of over 120 inches of rain.
The Puako area on the leeward South Kohala coast of the Big Island is one of the driest spots in the state, averaging under six inches. The leeward Kona coast has a unique pattern where afternoon heating and local wind circulation cause clouds and rain to form on the slopes, rather than the coast. This gives the area a distinct summer rainfall maximum, which is uncommon in the rest of the islands.
Geographical Drivers of Precipitation Variation
The cause of Hawaii’s rainfall variation is the interaction between the persistent Northeast Trade Winds and the high-elevation volcanic mountains. The trade winds consistently blow moist, warm air from the northeast across the Pacific Ocean toward the islands. As this moisture-laden air encounters the steep mountain slopes, it is forcibly lifted upward in a process called orographic lifting.
This rapid ascent causes the air to cool quickly, leading to the condensation of water vapor and the formation of clouds and rain. The majority of the precipitation is consequently dumped onto the windward slopes, creating lush, wet conditions. Once the air crests the mountain, it descends on the leeward side, warming and drying out as it falls. This process suppresses cloud formation and results in the characteristic “rain shadow” effect.
A secondary factor influencing this pattern is the Trade Wind Inversion (TWI), a layer of warm, dry air that sits between 5,000 and 8,000 feet above sea level. This inversion acts as a lid, capping the vertical development of rain clouds. On the tallest mountains, such as Mauna Kea and Mauna Loa, the summits extend above this inversion layer, meaning the air at the very top is dry. This phenomenon explains why the maximum rainfall zone is often found on the mid-slopes of the mountains, rather than at the highest peaks.