Wai‘anae Ecological Characterization

Prevailing Conditions

Temperature

Four factors generally impact the mild, relatively constant temperatures found on O‘ahu and the other Hawaiian Islands: the ocean, day length, exposure to winds, and elevation.

The Pacific Ocean. Large bodies of water, such as the ocean, act as heat sinks, absorbing solar radiation when the weather is hot and transferring heat to the air when it is cold. Under identical temperature variation, large bodies of water increase and decrease in temperature much less than land. As a result, land masses near large bodies of water remain milder than land farther away from the water. This moderation of temperature is called the oceanic effect.

Day Length. Due to the island's proximity to the equator, there is not a large fluctuation in the length of daylight hours. The longest day, in the summer (kau), is approximately 13.4 hours. The shortest day, in the winter (ho‘oilo), is approximately 11 hours. This relatively constant duration of sunlight results in small seasonal variation in temperatures because the incoming solar energy does not fluctuate. In locations farther away from the equator, the duration of sunlight varies more throughout the year. As shown in the graph, the farther north in latitude a location is, the greater annual variability in the length of daylight.

Exposure to Winds. The trade winds, from the northeast, provide a relatively constant, temperate air source across the Hawaiian island chain. In areas where the trade winds are blocked by local topography (mountains), the temperature tends to increase. The relief map cross section of the island of O‘ahu shows how the Ko‘olau and Wai‘anae mountains (Mount Ka‘ala is shown) block the interior valley and leeward coast from the trade winds (see map). For this reason, Wai‘anae is one of the hotter locations on O‘ahu, with average high temperatures a couple of degrees higher than those of Honolulu.

Elevation. The elevations on O‘ahu range from sea level to a high of 4,003 feet (1,220 meters) at the peak of Mount Ka‘ala. As elevation increases, the ambient air temperature decreases. This is best illustrated by the existence of snow at the higher elevations on the island of Hawai‘i.

The Hawaiian Islands have a very limited annual temperature range. In Honolulu the average monthly temperatures range from a high of 81.4 degrees Fahrenheit (August) down to 72.9 degrees Fahrenheit (January), a difference of only 8.5 degrees Fahrenheit. The chart shows the small variability in the average daily high and low temperatures for Honolulu.

The mean maximum temperature map illustrates the impacts of the elevation and exposure to wind on high temperature (see map). Using recorded temperature data from across the island of O‘ahu, the map shows how the windward coast and mountain ranges are, on average, cooler than the interior valley and leeward coast.

Precipitation

The ocean surface surrounding the Hawaiian Islands receives approximately 25 inches (63 centimeters) of precipitation annually, whereas the average annual precipitation for the island of O‘ahu is approximately 45 inches (115 centimeters). The additional 20 inches (50 centimeters) of precipitation, averaged across the island, is caused by the existence of the island itself (Western Regional Climate Center 2004). The mountains and other topographic features draw moisture out of clouds. The amount of precipitation an area on the island of O‘ahu receives per year depends largely on its location with respect to mountains.

Storms are generally driven by two wind sources: the trade winds and the Kona winds. The trade winds are the dominant winds and come from the northeast. The Kona (leeward) winds approach from the southwest and appear generally in the winter. The rainfall patterns presented in the mean annual precipitation map demonstrate the two natural phenomena that affect the amount of precipitation an area receives: orographic rainfall and rain shadow (see map).

Orographic Rainfall. This type of precipitation is caused by moist air being forced over the Ko‘olau and Wai‘anae Mountain ranges by winds. As the moist air is forced up the steep slopes of the mountains, it cools, causing the moisture to condense and form rain. The majority of rainfall occurs near the peaks of the mountains, but once the air is past the peaks it does not rise any higher, so condensation and precipitation decrease.

Rain Shadow. This phenomenon is caused by the orographic effect mentioned above. The "shadowed" area on the leeward side of the mountain range is blocked from receiving precipitation by the mountains because the clouds are no longer rising and creating rainfall. There is a rain shadow on the Wai‘anae Mountains due to their position leeward of the Ko‘olau Mountains. There is also a second rain shadow for the areas leeward of the Wai‘anae range. This explains why this region of O‘ahu receives so little precipitation. Mount Ka‘ala, roughly 1,000 feet (300 meters) higher than the peaks of the Ko‘olau range, escapes the impacts of the rain shadow.

Wai‘anae Rainfall

Due to the impact of the rain shadow on storms driven by the trade winds, Wai‘anae is one of the driest areas on the island. The average annual rainfall in Wai‘anae is 21.3 inches (55 centimeters), less than half of the average for O‘ahu as a whole. The rainfall table contains the monthly precipitation averages for the Wai‘anae area and other locations on O‘ahu. Rainfall values from other areas of O‘ahu are provided for comparison. Kahana and Wahiawa are generally located along the trade wind flow path that leads to Wai‘anae. Kahana is on the windward side of the Ko‘olau Mountains, and it has an average annual rainfall of 233.2 inches (almost 600 centimeters). The Wahiawa location is in the rain shadow of the Ko‘olau Mountains and has an average annual rainfall of 21.2 inches (54 centimeters). These precipitation values illustrate the impacts the mountains have on rainfall.

Average Monthly and Annual Rainfall Totals for Locations on the island of O‘ahu

Month	Wai‘anae	Wahiawa	Kahana	Honolulu Airport	Kailua	Makapuu Point	Ka‘ena Point
January	3.97	3.87	15.4	3.29	9.65	5.34	3.51
February	2.53	2.22	18.06	2.35	10.54	3.43	2.63
March	2.23	2.02	23.12	2.66	12.06	4.57	2.61
April	1.04	1.11	23.42	1.23	12.27	2.3	1.95
May	0.87	0.94	21.49	0.95	8.55	1.31	1.91
June	0.48	0.27	13.93	0.39	6.62	0.59	1.06
July	0.5	0.55	22.49	0.54	10.11	1.12	0.92
August	0.56	0.68	21.39	0.58	10.11	1.09	0.53
September	0.84	0.84	14.26	0.66	6.62	0.63	1.24
October	2.32	2.09	18.02	1.93	9.71	2.56	1.76
November	2.73	2.69	22.2	2.75	12.63	4.01	2.27
December	3.23	3.96	19.41	3.22	10.71	4.57	2.85
Annual	21.3	21.24	233.19	20.55	119.58	31.52	23.24

Source: Created from data provided by the Western Regional Climate Center (2004).

The bar chart provides average monthly precipitation values for three rainfall gages located within the Wai‘anae area. The three gages illustrate the effect of elevation on the amount of precipitation an area receives. The higher elevations receive the greater precipitation amounts due to the orographic effect, while the lower elevations are affected by the rain shadow of the Wai‘anae Mountains.

For the values shown in the rainfall table for various locations on O‘ahu, it is important to notice when different areas around the island receive precipitation. At locations on the windward coast such as Kahana and Kailua, rainfall is fairly evenly distributed throughout the year. But for locations on the leeward side of the Ko‘olau Mountains, there is a well-defined wet season of October through March.

During this wet season, or ho‘oilo, the trade winds weaken and are periodically replaced by Kona winds. The storms carried by the Kona winds approach the island from the southwest. Because the majority of O‘ahu's land mass is southwest of the Ko‘olau Mountains, precipitation generated from the Kona wind storms is evenly distributed across the island, with the exception of the area east of the Ko‘olau Mountains. During Kona storms, the windward side of the island is within the rain shadow of the Ko‘olau Mountains.

Kona storms are a very important source of water supply in the Wai‘anae area. About 65 percent of the annual recorded precipitation for the area comes from these wet season storms. For the island of O‘ahu as a whole, the annual rainfall is about 45 inches (114 centimeters). Approximately 30 inches (76 centimeters) of the annual total are generated from Kona storms, with the remaining 15 inches (38 centimeters) from storms driven by the trade winds.

Winds

The winds around the island of O‘ahu are nearly constant (see table). These winds maintain the moderate temperatures on the island and carry the moisture that provides rain. There are generally two winds that dominate the Hawaiian Islands: the trade winds and Kona winds. Other, less frequent winds from various directions are caused by large storms. The average daily wind speeds recorded at Honolulu Airport are shown in the graph.

During the kau, the trade winds are fairly constant in speed and direction. The trade winds blow from the northeast quadrant 80 to 95 percent of the time, with speeds measured over the ocean exceeding 12 miles per hours 50 percent of the time.

The Kona winds usually occur during ho‘oilo. During this time the trade winds are not dominant, which allows the Kona winds to develop. The Kona winds blow from the southwest direction, impacting the leeward side of the island first. Since the winds come from the south, they are generally warmer breezes with the capacity of carrying a lot of moisture.

Although there are two dominant winds impacting the island, the mountainous topography of O‘ahu creates exceedingly complex wind patterns throughout the island. Because of the multiple hills and valleys throughout the island, there is uneven warming and cooling of air mass over the land. This difference in air temperature creates local winds. The hills and valleys of the Wai‘anae area provide an example of this localized wind occurrence.

The usual regime of local breezes in Wai‘anae consists of onshore breezes during the late day and off-shore breezes at night. The onshore breeze (kai a ulu) is created when the air mass over the inner valley heats up and rises, and the mass of air is replaced by the more dense, cooler air over the ocean. At night, the wind pattern reverses. The cooler air from the peaks of the Wai‘anae Mountains cools to a temperature less than the air over the ocean. This causes an off-shore breeze as the cooler air moves down the slopes of the mountains and back out over the ocean.

References Cited

Climate Source, Inc. 2002. Prism 1961-1990 Mean Annual Precipitation Oahu, Hawaii. http://www.climatesource.com/hi/fact_sheets/oahuppt_xl.jpg

National Oceanic and Atmospheric Administration (NOAA). 2004d. Hawaii Climate Information. National Weather Service Forecast Office Honolulu, Hawaii. http://www.prh.noaa.gov/hnl/pages/climate.php

National Oceanic and Atmospheric Administration (NOAA). 2004e. Hawaiian Islands Recent Marine Data. National Data Buoy Center. http://www.ndbc.noaa.gov/Maps/Hawaii.shtml

National Oceanic and Atmospheric Administration (NOAA). 2004f. National Climatic Data Center Home Page. http://www.ncdc.noaa.gov/oa/ncdc.html

U.S. Naval Observatory. 2004. Astronomical Applications Data Services. http://aa.usno.navy.mil/data/

Western Regional Climate Center. 2004. Climate of Hawaii. http://www.wrcc.dri.edu/narratives/HAWAII.htm

Related References

Fletcher, C.H., E.E. Grossman, B.M. Richmond, and A.E. Gibbs. 2002. Atlas of Natural Hazards in the Hawaiian Coastal Zone. United States Geological Survey (USGS), University of Hawaii, State of Hawaii Office of Planning, and National Oceanic and Atmospheric Administration. http://pubs.usgs.gov/imap/i2761/

Leeward Community College Botany Department. 1998. Plants in the Hawaiian Environment: Learning Objective 19. University of Hawaii Distance Education. http://emedia.leeward.hawaii.edu/millen/bot130/learning_objectives/lo19/19.html

Maly, K. 1998b. Wai‘anae Excerpts of He Mo‘olelo Ka‘ao no Hi‘iaka-i-ka-poli-o-Pele. Originally published in the Hawaiian language newspaper Ka Hoku O Hawaii, 1924-1927. Honolulu, HI.

Nakuina, M. 1992. Traditions of Oahu: Stories of an Ancient Land. Winds of Oahu. In The Wind Gourd of Laamaomao 2nd Edition. Kalamaku Press. http://apdl.kcc.hawaii.edu/~oahu/stories/winds.htm

Pukui, M.K. 1983. ‘Olelo No‘eau: Hawaiian Proverbs and Poetical Sayings. Bishop Museum Press. Honolulu, HI.

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