Hawaii has comfortable weather year-round. Near the coast, the average daytime high temperature in winter is about 82°F, with the average daytime high in summer raising the thermometer only a few degrees to 87.5°F. Nighttime temperatures drop about 10 degrees. Elevation, however, does reduce temperatures: about three degrees for every 1,000 feet you climb.
Temperatures are both constant and moderate because of the trade winds, prevailing northeast breezes that blow at about 10-20 miles per hour. You can count on the trades to be blowing on an average of 300 days per year, hardly missing a day during summer and occurring half the time in winter. Although usually calm in the morning, they pick up during the heat of the afternoon, then weaken at night.
The Tropic of Cancer runs through the center of Hawaii, yet the latitude’s usually oppressively hot and muggy weather is most often absent in the islands. Honolulu, on the same latitude as sweaty Hong Kong and Havana, has an acceptable 60-75 percent daily humidity factor.
Winds blowing from the south and southwest are known as kona winds. Kona means leeward in Hawaiian. Kona winds bring hot, humid air and unstable weather. If they persist for more than a couple of days, they also bring vog, a thick haze caused by the Kilauea Volcano on the Big Island. Kona winds are most common October-April.
Precipitation is the biggest differentiating factor in Hawaii’s climate. Rainfall is most often localized and comes in waves of passing showers. Precipitation also occurs mostly at and below the 3,000-foot level with the mountains acting as rain magnets. As the trade winds push warm, moist air up against the mountains, the air rises, cools, and drops a payload of rain to the ground. The heaviest rainfall occurs on the windward side of the islands and over mountain ranges, and dry conditions prevail on the leeward sides and south shores. For example, the Honolulu International Airport and Waikiki average only 20-25 inches of rain per year, while the Nu‘uanu Reservoir in the mountains above Honolulu gets a whopping 120-130 inches yearly.
Localized weather means that weather patterns are very specific. If it’s raining where you are, simply relocate to another part of the island or just wait a few minutes for the precipitation to pass. You can usually depend on south shore beaches and the leeward side to be sunny and bright. Ocean temperatures run 75-80°F year-round.
Tsunami is the Japanese word for tidal wave. Hawaii’s location in the middle of the Pacific Ocean puts it in the path of tsunamis as they travel from their point of origin and spread across the Pacific. A Hawaiian tsunami is actually a seismic sea wave that has been generated by an earthquake or landslide that could easily have originated thousands of miles away in Japan, South America, or Alaska. Not actual waves that break like the ones seen along the reefs, tsunamis show up as a series of three to five tidal surges that affect coastal waters and shorelines over several hours, with about 30 minutes between each surge. Tsunamis can range from simply a larger than usual fluctuation in sea level over the duration of the event to a devastating tidal surge that can flood and damage shoreline property, especially the harbors, and cause loss of life. For visitors staying in shoreline hotels with six or more stories, the safest place to be during a tsunami event is on the third floor or higher. Otherwise, shoreline areas should be evacuated.
Hurricanes are also a threat in Hawaii. They are rare, but destructive. Most hurricanes originate far to the southeast off the Pacific coast of Mexico and Latin America; some, particularly later in the season, start in the midst of the Pacific Ocean near the equator south of Hawaii. Hurricane season is considered June through November. Most hurricanes pass harmlessly south of Hawaii, but some, swept along by kona winds, do strike the islands. The most recent and destructive was Hurricane ‘Iniki, which battered the islands in 1992, killing eight people and causing an estimated $2 billion in damage. It had its greatest effect on Ni‘ihau, the Po‘ipu Beach area of Kaua‘i, and the leeward coast of O‘ahu.
Kona storms are another matter. These subtropical low-pressure storms develop west of the Hawaiian Islands, and as they move east, they draw winds up from the south. Common only in winter, they can cause considerable damage to crops and real estate. There is no real pattern to kona storms. Some years they come every few weeks, whereas in other years they don’t appear at all.