Precipitation includes rain, snow, sleet, and hail, resulting from the condensation of atmospheric water vapor. Measuring the total amount is fundamental for climate science and water management. Determining which state gets the most rain is complex, as “most rain” can mean annual accumulation, frequency, or intensity. The standard method for comparison relies on a long-term total of all forms of water equivalent.
The State with the Highest Average Annual Precipitation
Hawaii consistently records the highest average annual precipitation across its entire area. The state receives a statewide average of approximately 63.7 inches of precipitation per year, which includes both rainfall and the water equivalent of frozen forms. This high total is significantly influenced by extreme localized measurements rather than uniform distribution across the islands.
Mount Waialeale on Kauai is the most dramatic example, cited as the wettest location in the United States. This single peak averages 450 inches of precipitation annually, making it one of the rainiest spots in the world. While the state average is calculated from thousands of weather stations, the sheer volume recorded in these select locations skews the total far above any other state.
Geographic Factors Driving Extreme Rainfall
The extraordinary rainfall totals in Hawaii are caused by constant moisture supply and unique topography. The primary mechanism is orographic lift, where warm, moist air is forced upward by mountain barriers. The islands sit directly in the path of persistent northeasterly trade winds that carry moisture from the Pacific Ocean.
When these air masses encounter the steep volcanic slopes, they are forced to rise rapidly. As the air ascends, it cools and the water vapor condenses, leading to nearly continuous precipitation on the windward sides of the mountains. This creates a sharp contrast, as the leeward sides, sheltered from the trade winds, often experience desert-like conditions known as a rain shadow. The modest height of many Hawaiian mountains is enough to wring the moisture out of the atmosphere.
Understanding Precipitation Metrics and Variability
Precipitation totals for statewide comparisons use 30-year climate normals, which are long-term averages of weather variables. This 30-year period is the standard established by the World Meteorological Organization, providing a stable baseline that smooths out extreme year-to-year variability caused by temporary climate cycles like El Niño or La Niña. These normals are updated every ten years to reflect gradual shifts in climate patterns.
Data for these normals are collected using instruments, primarily rain gauges and weather radar. Rain gauges, such as the tipping bucket type, directly measure the accumulated depth of water at a specific point. Weather radar systems offer a broader view by sending out microwave pulses that reflect off precipitation particles. This radar data allows scientists to estimate the intensity, type, and total amount of precipitation over a wider region.
Other Notably Wet Regions and States
While Hawaii leads in total annual precipitation, other states are perceived as the wettest due to different metrics, such as the frequency of rainy days. States ranking immediately after Hawaii are typically situated along the Gulf Coast, benefiting from warm, moist air from the Gulf of Mexico. Louisiana, Mississippi, and Alabama consistently rank high, with statewide averages often exceeding 58 to 60 inches annually.
Rainfall in the Gulf Coast states is often linked to intense, short-duration summer thunderstorms and tropical systems like hurricanes. In contrast, states in the Pacific Northwest, such as Washington and Oregon, are known for frequent but lighter, long-duration rainfall. While mountainous areas in the Pacific Northwest receive high totals, their annual average precipitation is lower than that of the Gulf Coast states and significantly less than Hawaii’s.