Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
Chapter 3. Physical Environment
3.1 Climate
Located approximately 2,400 miles (mi) southwest of the nearest continental landmass, the Hawaiian
Islands are the most isolated archipelago in the world. The climate of Hawai‘i is generally constant
throughout the year, with only minor periods of diurnal and seasonal variability. In general,
temperatures during the summer season (May-September) are warm, conditions are dry, and trade
winds originate from the northeast direction. The winter season (October-April) is characterized by
cooler temperatures, higher precipitation, and less equable winds (Juvik and Juvik 1998).
The trade winds also produce differences within the two physiographic provinces, windward and
leeward zones, as a result of orographic rainfall. Moisture is carried from the ocean to the island by
the northeasterly trade winds. Orographic rainfall is rain generated when moist air rises against steep
slopes, cools, and forms rain producing clouds. As the air descends on the opposite side of the slope,
it becomes warmer and less moist, resulting in less rain. Orographic rainfall patterns strongly
influence the climate of the two zones. On the windward side, climatic conditions are relatively wet,
while the leeward areas experience decreased winds, less rain, and are subject to southerly Kona
(leeward) storms (Juvik and Juvik 1998).
Episodic oceanic and atmospheric events also influence climate in the islands during specific
intervals. The El Niño Southern Oscillation (El Niño) usually results in light trade winds in the
western Pacific and drier conditions (Duffy 1993). During El Niño years in Hawai‘i, average rainfall
has dropped below historical records (USFWS 2002a). Hurricanes result in intense rain and wind.
The two major hurricanes that most affected Hawai‘i Island, Hurricanes Fico (July 18-20, 1978) and
Estelle (July 22, 1986), had peak gusts of 58 and 55 miles per hour (mph), respectively (HDBEDT
2007). These climate differences determine vegetation patterns, which in turn can affect local
hydrological movements of surface and especially ground water (Sack and Frole 2006).
Climatic conditions on the Island of Hawai‘i vary dramatically due to its large size and elevation
range. Clouds form against the windward mountain slopes creating drier conditions around the high
mountains of Mauna Kea and Mauna Loa. Annual rainfall fluctuates from 10 in on the leeward coast
to 270 in in the windward forests (Mitchell et al. 2005).
Other moisture-producing mechanisms, besides the orographic effect, include convection, Kona
storms, and fog drip. Kona storms are low pressure areas which bring southerly winds and rain.
These storms typically occur during the winter months when trade winds are light (Juvik and Juvik
1998). Fog drip occurs when clouds gather along mountain slopes and condensation causes the
moisture on vegetation to drip to the ground. Fog drip usually develops in late winter and early
spring during the afternoon (USFWS 1996a, 2008a). In addition to rainfall, the summits of Mauna
Kea and Mauna Loa on Hawai‘i Island also receive snow in the winter months (Juvik and Juvik
1998).
A particularly unique aspect of the climate in Hawai‘i is the trade wind temperature inversion. This
layer occurs from 5,000 - 10,000 ft where rising air meets sinking air and warmer air exists above
cooler air. The temperature inversion layer prevents warm, moist surface air from rising to form rain
clouds. Because humid moist air cannot reach high altitudes, a cloud ceiling is formed, causing the
Chapter 3. Physical Environment 3-1