Hawaii is globally recognized as a tropical paradise, but its climate is complex. The moisture content of the air is measured as relative humidity (RH). RH is the amount of water vapor currently in the air expressed as a percentage of the maximum amount the air can hold at that specific temperature. Because warm air holds more moisture than cool air, the RH value constantly fluctuates throughout the day. Hawaii’s unique geography ensures that humidity varies dramatically across localities.
Statewide Numerical Averages
Across Hawaii’s main populated coastal areas, the average annual relative humidity (RH) generally falls between 60% and 75%. For instance, the capital of Honolulu averages around 64% annually, while the windward city of Hilo averages a higher 74%. These figures are daily averages, blending the typically higher values of the morning with the lower percentages of the afternoon. RH is often highest in the late night and early morning hours, frequently spiking into the 75% to 85% range. Conversely, the mid-afternoon, when temperatures are highest, often sees the RH drop to between 50% and 68% in drier, leeward areas. This consistent moderation is largely due to the persistent northeasterly trade winds that sweep across the islands.
Geographic Factors and Microclimates
The “average” humidity is misleading because the islands’ volcanic topography creates distinct microclimates within short distances. Local humidity is primarily determined by positioning relative to the prevailing trade winds, which establish a clear distinction between windward and leeward sides. Windward sides, typically the north and east coasts, are directly exposed to moist trade winds blowing off the Pacific. As this air encounters steep mountain slopes, it is forced upward in a process called orographic lifting. The rising air cools, causing condensation and frequent rainfall, resulting in constantly higher humidity, often sustained between 70% and 80%.
In contrast, the leeward sides, generally the south and west coasts, lie in the mountain’s rain shadow. By the time the air descends on this side, it has already shed most of its moisture, making these regions significantly drier. Leeward lowlands experience much lower average humidity, sometimes dipping below 60% in the afternoon, and the landscape reflects this with sparser, arid vegetation. Elevation also plays a role in moisture distribution. Above the persistent trade wind inversion layer, which typically sits between 5,000 and 8,000 feet, the air becomes notably dry. At these high elevations, the relative humidity can drop below 40% and sometimes as low as 5% to 10%.
Seasonal Changes in Humidity
Hawaii experiences two main seasons: a slightly warmer, drier summer (May to October) and a cooler, wetter winter (November to April). Despite being the warmer period, the summer often feels muggier because the trade winds, while present 80% to 95% of the time, can be weaker or less consistent. This reduction in air movement allows moisture to linger, leading to a greater number of days where humidity is noticeable. The winter months are characterized by stronger, more frequent storm systems and the occasional Kona wind event. These storms can bring significant rainfall and temporary spikes in humidity, particularly in leeward areas that rely on these systems for their annual precipitation. Generally, the annual fluctuation in relative humidity percentage is small, but the perception of moisture increases noticeably during the summer months.
Understanding the Comfort Level
While relative humidity indicates the air’s saturation level, the dew point is a more accurate measure of human comfort. The dew point is the temperature at which the air must be cooled to become saturated, and it directly correlates with the air’s actual water vapor content. A dew point below 60°F is considered comfortable, but once it rises above 65°F, most people begin to perceive the air as “muggy” or “sticky”. In Hawaii, the muggy season typically runs from May to November, with dew points frequently exceeding this 65°F threshold. When the dew point climbs above 70°F, the air is considered oppressive, making it difficult for sweat to evaporate and cool the body. Controlling indoor humidity is necessary to prevent the growth of mold and mildew, which thrive in environments where RH exceeds 60%. Air conditioning and dehumidification are often utilized, especially on the warmer leeward sides, to manage the interior climate and maintain comfortable conditions.