Hawai’i Weather and Climate
The Big Island has a balmy tropical climate, but this really only applies to the coastal regions of low elevation. The most distinctive feature of the climate is the small variance in seasonal temperatures. A common island weather joke says that “winter is at night.” This means that there is more daily variance in temperature than seasonal variance in temperature – it’s colder at night in July than during the day in December. Temperatures at sea level are usually in the 85 – 90°F range during the summer and in the 79 – 83°F range during the winter. It is extremely rare for the temperature to rise above 90 °F or fall below 60 °F. On the other hand, because the island reaches nearly 14,000 feet above sea level, it contains a majority of the climate zones that exist on earth. So, your particluar elevation above sea level will affect your temperature greatly. Generally, the Big Island receives most precipitation from October to April, and conditions are typically drier otherwise. The warmer temperatures of the summer months increases the risk of hurricane activity.
This next section is a bit technical, but knowing a bit of meteorology and climatology and how they apply to the Big Island can really help you choose what to do and where to go from day to day. With an almost constant temperature, the two big factors affecting island weather are wind and elevation. The trade winds almost always blow from the northeast, but mountains and valleys create very complex wind and weather patterns.
The most important weather phenomenon to understand here is a rain shadow. A rain shadow occurs when clouds must rise to pass over mountains. When this happens, the clouds have to lose some of their mass (in the form of precipitation) to be able to rise to increased altitude. This effect, called orographic lift, creates larger amounts of rain for the windward side (toward the wind) of the mountain than the leeward side (away from the wind) of the mountain. Applied to the Big Island, since the trade winds blow from the northeast and must pass over the mass of Mauna Kea and Mauna Loa, it means that the Hilo side is considerably rainier than the Kona side. The best place to view this rain shadow effect is to drive from Kohala to Hilo through the town of Waimea. There is a point on the road where the line of the rain shadow is abrupt: you can see cactus on one side and rainforest on the other.
Another wind factor is due to ground temperature, and changes the wind direction from uphill to downhill at different times of the day. For instance, while the sunny Kohala coast might have light morning winds coming downhill, as solar radiation heats up the lava along the coast, the hot air must rise and the cooler ocean air rushes in from offshore to replace the displaced warmer air. This creates strong ocean breezes in the afternoon.
Elevation is the other major factor for the Big Island. Because the island rises to 14,000 feet, there is great temperature difference in parts of the island. It actually snows commonly on the summits of Mauna Kea and Mauna Loa from December to March. A rough rule-of-thumb instructs that you should subtract about three degrees of temperature for every thousand feet of elevation gain.
Really In Depth:
These general guidelines don’t always help for a specific location. Kailua-Kona is usually sunny in the morning and cloudy and rainy in the afternoon and early evening (incidentally, these conditions are responsible for the superior taste of Kona coffee). The Kohala Coast gets the biggest effect of the island rain shadow – it’s almost always sunny, with around ten inches of rain per year (less than Nevada and Utah!). It can get windy in the afternoon because of the temperature-related wind effects described above. Upcountry towns like Waimea are often cooler and windy. The southern part of the island is always windy because there is little to block the trade winds as they curve around the southern point of the island. A look at the trees along the southern coast confirms this. It’s also fairly dry in the south compared to other parts of the island because of the rain shadow from Mauna Loa. The northern coast, from the rainy side of Kohala to Honoka’a, can also be breezy and is consistently wet and rainy. In places like Waipi’o Valley, ocean breezes often keep the beach sunny, but compete in a tug-of-war all day long with heavy clouds and rain in the back of the valley. Hilo is the wettest city in America. The opposite of Kona, rain is more common in the morning than the afternoon. A clear and sunny Hilo morning is a rare treat. Puna is similar to Hilo and is very wet. Curiously, despite the amazing amount of rainfall in Puna, there are no permanent watercourses because the new ground on this part of the island is so porous that all rainwater seeps in immediately. The National Park and the town of Volcano are similar to Hilo with rainforest conditions, but are much higher in elevation and thus much cooler. The area around the Saddle Road near the big volcanoes is unlike anywhere else on the island, where you are near or above a thermal inversion caused by the trade winds at about 4,000 – 8,000 feet that is present about 60% of the time and typically prevents cloud formation above the inversion. So, drier air sits on top of the inversion and the clouds stay below it. When you travel from, say, from Hilo and climb Mauna Kea, you will often begin the day in morning Hilo rain, drive up through the clouds to a dry and sunny day and spend your entire climb above the clouds below (this feeling of being thousands of feet above the clouds is one of the most special aspects of climbing Mauna Kea or Mauna Loa).
These weather patterns also explain the nature of beaches on the Big Island. It’s not a coincidence that most of the best beaches on the island are along the Kohala Coast, but not just because it’s nice to be on the beach during a sunny day. The lack of rain results in less freshwater runoff, which means clearer and warmer ocean water that’s better for recreating in the sea, especially for snorkeling and diving. The opposite is true for the Hilo side, where the ocean is often cloudier and colder due to increased freshwater runoff.
If you’re not entirely confused yet, hold on tight. The trade winds only prevail about 70% of the time on average (about 90% during the summer, they can be as low as only 40% during the winter). When the predominant winds blow from the southwest, they are called Kona winds. The typically leeward side becomes the windward side and vice-versa, and creates unusual weather all over the island. More dangerous vog conditions can occur in unusual places that rarely see it. Luckily, Kona winds don’t typically hang around for more than a day or so. Count on local forecasts to help you during these conditions.