The ocean surrounding the Hawaiian Islands supplies plenty of moisture and acts as a giant thermostat, moderating the temperatures over the island chain.  The seasonal range of sea surface temperature near the island chain is only about six degrees, from a low of 74 degrees between late February and March to a high near 80 degrees in late September or early October.  The variation from day to night is only one or two degrees. 

Because the island chain is more than 2,000 miles from the nearest continental land mass, air that reaches it, regardless of its source, has been greatly modified by the equable ocean.  Thus the air temperature range varies only eight degrees between the warmest months (August and September) and the coolest months (January and February), and about 12 degrees between day and night. Windward coasts exposed to the cool tradewind (winds from the northeast) air off the ocean have the least variation in temperature between day and night (8-12 degrees Fahrenheit).  Farther inland, on leeward coasts and at elevations above 6,000 feet, the daily temperature range increases to 16-20 degrees Fahrenheit.  Daily maximums run from the high 70's in the Winter to the mid to upper 80's in the Summer, except near 90 in urban areas.  Daily minimums run from the mid 60's to low 70's.  The extreme highs for all of the islands are in the mid 90's and the extreme lows are in the low 50's. It should be pointed out that these ranges are true for near sea level locations unless otherwise stated.  Hawaii's mountains profoundly influence its weather and climate. The endless variety of peaks, valleys, ridges, and broad slopes cause marked variations in temperature, wind speed and direction, cloudiness, and rainfall.  Mountains reach heights of nearly 14,000 feet on the Island of Hawaii, 10,000 feet on Maui, 5,000 feet on Molokai, 4,000 feet on Oahu, and 5,000 feet on Kauai.  Since temperature decreases with elevation by up to five degrees per thousand feet in dry air, Hawaii's mountains encompass a climate range from tropical to sub-arctic.

The northeasterly tradewinds prevail throughout the year (although they may be absent for days or even weeks at a time), and result from the semipermanent Eastern Pacific High which is usually centered to the north and east of the Hawaiian Islands.  The average frequency of the trades varies from more than 90% during the Summer to only 50% in January.  These trades also tend to be stronger in the afternoon than at night.  Winds from the southerly quadrant often occur when extratropical low pressure systems are located north of the islands.  In places sheltered by terrain, local air movements are quite at variance with the prevailing winds.

Annual rainfall in the Honolulu area averages 20-25 inches along the coast, but increases to 35 inches about a mile inland, and to 60 to 70 inches about 2 miles inland.  In the mountain areas where warm moist air is forced to rise, cloudiness and rainfall are much greater than over the nearby open sea.  Leeward areas, where the air descends, tend to be sunny and dry.  Parts of the Koolau Range average 300 inches or more of rain a year.  East (windward) of the Koolaus, coastal areas receive 30 to 40 inches.  Oahu is driest along the coast west of the Waianaes, where rainfall drops to about 20 inches a year.  However, variations from month to month and year to year are considerable.  This is true more so during the cooler season when occasional major storms provide much of the rain.  In Summer, rain occurs primarily as showers that form within the moist tradewinds as they override the mountains.  Thus March rainfall at Honolulu International Airport has ranged from more than 20 inches to as little as 1/100th of an inch.  About half of the airport's mean annual total occurs during its three wettest months, November through January.  Tradewind rainfall is more frequent at night.  Daytime showers, usually light, often occur while the sun continues to shine a phenomenon referred to locally as "liquid sunshine".

Hawaii is not without its share of hazardous weather. Being a small island chain in the middle of the Pacific puts us at risk for tsunamis. Hurricanes threaten our islands from May to December. Our wet season usually results in some level of flooding. An active volcano on the Big Island is responsible for earthquake activity throughout the year. These natural hazards can result in widespread devastation. The University of Hawaii in conjunction with the National Oceanic and Atmospheric Administration published a handbook to help Hawaii homeowners prepare for tsunami, hurricane, flood, and earthquake disasters. Click here to learn how to reduce this devastation to your family and property.