[weather nerd alert: read on at your peril]
The climate on this side of the island is different from that of all the other Hawaiian Islands' leeward coasts. As in most parts of the world, the air temperature over the ocean in these latitudes drops as one rises in altitude. But at a certain elevation, the pattern of temperature's declining with height reverses briefly. For example, it may be 80F at sea level, 60F at 5000 feet above sea level, and then, surprisingly, 68F at the 7000-foot elevation. From there temperatures resume the normal pattern and start dropping again as you measure upward to the mountain summits. This reversal of the pattern is called a temperature inversion, because the expected gradient is turned upside-down. The same thing occurs on the California coast during the summer dry season but at a lower altitude, usually 800-3000 feet above sea level. Flying to Hawai'i from San Francisco in summer, I like to watch the flat coastal fog bank below slowly change character as the inversion-capped marine layer deepens, warms, and moistens along our southwesterly trajectory.
Here's why the South and North Kona Districts have a climate unique in Hawai'i: Because Mauna Loa, Mauna Kea, and Hualalai poke up above the moist marine layer and its capping temperature inversion at 1200m - 2400m (~4000ft to ~8000ft), they block the bulk of the prevailing northeast trade winds embedded in that layer that bring showers to windward and mountain areas. The trade wind flow impeded, day/night convection patterns are able to set up along these leeward slopes. The air over the island warms up under the sunshine and creates a localized low-pressure effect, drawing moist marine air upslope. The Big Island acts as a kind of mini-continent. Clouds form in late morning and showers often fall in the afternoon, as they do in Florida's summers. From our hotel in Kailua-Kona I was only able to photograph the full height of Hualalai (8271ft/2521m above sea level) at dawn, before clouds began to form.
Donald receives quite a bit of rain at his location. I believe he said 90 inches per year.
On the other islands, the trade winds blow all the way over the mountains, dropping their burden of moisture on the windward and mountain slopes and drying out as they descend the leeward slopes. That's why Honolulu, Lahaina, Wailea, and Po'ipu have nice, sunny weather and annual rainfall comparable to coastal California. The same is true of the Big Island's leeward Kohala coast, because the blocking mountain to its northeast, Kohala, is low enough, at 5480ft/1670m--and the island's narrow enough up there--to allow the trades to pass over, drop rain, and dry out on their descent. What about Maui, with its dormant volcano Haleakala at 10,000ft/3055m tall? While it also pierces the marine/trade wind layer and the inversion, it's not bulky enough to generate much of a lull in the leeward wind wake nor much convective action over its smaller land area.
