Climate and Rainfall
The climate in the Pouhala area is subtropical and warm with two seasons, summer (May through September) and winter (October through April). The climate is controlled by four general factors. 1) The subtropical latitude whereby both tropical and temperate weather patterns affect Hawai‘i. 2) Atmospheric circulation patterns which contribute to persistent northeasterly winds (trades). 3) The varied terrain of the Ko‘olau Mountains which causes orographic rainfall. 4) The moderating effect of the Pacific Ocean (Juvik and Juvik eds. 1998).
Average temperatures vary from 72.9 degrees Fahrenheit (°F) in January to 81.6°F in August at Honolulu Airport (National Weather Service 2007). Mean annual rainfall in the project area is about 24 inches as shown in Figure 2-9. Although tradewind rainfall contributes a proportion of average rainfall, storm events are the most important climatic factor for this study area.
In general, there are four types of storms in Hawai‘i. Cold fronts may come in from the north and produce heavy showers. Low pressure systems may bring southerly winds and widespread heavy rains (Kona Storms). Tropical storms or hurricanes cause very heavy rainfall. Low pressure systems in the upper atmosphere also cause heavy rainfall (Western Regional Climate Center 2007).
Figure 2-9. Mean Annual Rainfall for Island of O'ahu