Hilo, situated on the windward coast of the Big Island of Hawaiʻi, is the wettest city in the United States. Its geographical position subjects the area to a massive amount of moisture, establishing its reputation as a tropical rain capital. This constant, heavy precipitation defines the region’s climate, creating an environment unlike any other in the country. The city’s weather results from atmospheric and geological forces providing a constant supply of water to the eastern slopes of the island.
Annual Rainfall Averages
Hilo receives an extraordinary amount of annual rainfall, averaging approximately 130 inches (3,300 millimeters) in the downtown area. This figure is highly dependent on elevation, with some upslope regions receiving over 200 inches per year. For perspective, the annual rainfall in Seattle, Washington, is typically less than 40 inches. Even a subtropical city like Miami, Florida, generally records less than 60 inches of rain annually. Rain accumulation occurs on roughly 275 days each year.
Meteorological Causes of High Precipitation
The high rainfall is caused by orographic lift, driven by the prevailing Northeast Trade Winds. These winds push warm, moisture-laden air across the Pacific Ocean toward the island chain. As this humid air mass encounters the Big Island, it is forced upward by the slopes of Mauna Kea and Mauna Loa. Rising air expands and cools, which lowers its capacity to hold water vapor. This cooling leads to condensation, forming dense clouds that release water as rain on the windward side of the mountains. Hilo, positioned directly in the path of this forced ascent, is the primary recipient of this continuous precipitation.
Seasonal Patterns and Daily Rhythm
Hilo’s precipitation follows a distinct rhythm that residents and visitors anticipate. The wettest part of the year occurs from November through March, when the city records its highest monthly totals. The summer months, from June through September, are considered the driest, though this designation is relative due to the area’s high overall averages. The rain often follows a predictable daily pattern, frequently occurring as heavy showers during the late afternoon, evening, and overnight hours. This cycle allows for many mornings to be relatively clear and sunny before the clouds build up again.
The Resulting Lush Tropical Climate
The continuous, abundant rainfall is responsible for the region’s lush tropical rainforest environment. This consistent moisture sustains biodiversity, resulting in a landscape dominated by deep, vibrant greens. The high precipitation feeds numerous waterfalls and creates ideal conditions for exotic flora, including an abundance of orchids and anthuriums. This moisture also contributes to the area’s high relative humidity, which remains muggy throughout most of the year. Tropical temperatures combined with the heavy rain ensure the Hilo area maintains its status as a verdant destination.