In Hawaii, the climate is tropical, with a hot season from June to October (called kau in the Hawaiian language) and a relatively cool season (hooilo) from December to March. The trade winds, constant winds blowing from the north-east, strongly influence the climate of these islands, creating microclimates, wet or dry, depending on slope exposure.

Index


The coasts



Temperatures in Hawaii vary little throughout the year; they tend to be a bit lower on the windward slopes, and a little higher, but with lower humidity, on the leeward slopes. The temperatures vary little also when compared with the averages. This happens both because of the location at tropical latitudes and because of the enormous distance from the continents, from which cold or hot air masses could arrive. At sea level, the highest recorded temperatures are about 35 °C (95 °F), while the lowest are about 11 °C (52 °F).
Rainfall is influenced by the trade winds, so it varies according to slope orientation: you can move from a forest to a desert in a few kilometers (or miles), just by climbing over a hill.
In the following map, we can see a scheme of the situation, relative to the island of O'ahu: the trade winds bring rainfall on the exposed slopes and hot and dry weather on the leeward slopes. This situation also occurs on other islands where elevations are found in the interior, while flat islands such as Ni'ihau and Kaho'olawe are entirely arid.

Trade winds on the island of O'ahu

Honolulu

In the capital, Honolulu, located on the southern side of the island of O'ahu, the daytime temperature ranges from 27 °C (81 °F) between December and March to 31/32 °C (88/90 °F) between June and September. Being on the leeward side, where the trade winds create a sort of slight down-slope, foehn-type effect, Honolulu is one of the warmest places in Hawaii. Here are the average temperatures.
Honolulu - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)191920212223242424232220
Max (°C)272727282931313231302927
Min (°F)666668707273757575737268
Max (°F)818181828488889088868481

Rainfall on the leeward side is generally lower than 1,000 mm (40 in) per year, with a minimum from May to September and a maximum from December to March: in practice, it rains almost only in winter, when the clash between air masses can produce wet currents from the southwest, while in summer, the rare downpours only occur when the strongest thunderstorms produced by the trade winds on the other side manage to pass over the mountain ridges.
At the Honolulu airport, located on the coast, only 430 mm (17 in) of rain fall per year, of which 80 mm (3.1 in) fall in December, while less than 20 mm (0.8 in) per month fall from April to September. However, the amount varies according to district: for instance, in Pearl Harbor, the rainfall amounts to 550 mm (21.5 in) per year, and at Black Point, it reaches 690 mm (27 in), while at the university, located near the green hills above the city, it reaches 1,000 mm (40 in) per year, with more abundant rains from November to March, and some extra showers also in summer.
Here is the average precipitation at the international airport.
Honolulu - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)60505015158131520456080430
Prec.(in)2.4220.60.60.30.50.60.81.82.43.116.9
Days879866767891091

The amount of sunshine in Honolulu is never poor: it is acceptable from November to January and good or very good in the rest of the year, when the sun usually shines. Here are the average sunshine hours per day.
Honolulu - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours788891010109876

Hilo

On the coasts exposed to the north-east, which directly receive the wind from the sea, the temperatures are a bit lower. Here are the average temperatures of Hilo, located at the foot of the windward side of the island of Hawai'i ("The Big Island"): as we can see, they are lower than those of Honolulu, which, as previously mentioned, is located on the leeward side.
Hilo - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)181818191920212121202018
Max (°C)262626262728282829282726
Min (°F)646464666668707070686864
Max (°F)797979798182828284828179

In addition, on the northeast-facing slopes, it rains all year round, and the climate can be defined as equatorial. In Hilo, 3,200 millimeters (125 inches) of rain fall per year, without there being a dry month: June is the "driest" month with 190 mm (7.5 in) of rain, while the rainiest is November, with 395 mm (15.5 in). Here is the average precipitation in Hilo.
Hilo - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)2352453402902051902752502502503952953220
Prec.(in)9.39.613.411.48.17.510.89.89.89.815.611.6126.8
Days161621252325272724242321272

Because of frequent clouds and rains, the sun does not shine very often on the windward side, as can be seen in the following table.
Hilo - Sunshine
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Hours555556565444

On the coasts exposed to the wind, higher waves are also formed, so surfers flock to beaches facing north, such as Sprecksville and Ho'okipa Beach on Maui Island.

Hookipa Cove

The sea in Hawaii is warm enough to swim in all year round, even though it drops to 24 °C (75 °F) in February and March, as can be seen in the following table, concerning the water temperature on the southern island of Hawai'i.
Hawaii - Sea temperature
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Temp (°C)252424252525262627272625
Temp (°F)777575777777797981817977

The mountains



Hawaii are volcanic islands. The highest volcanoes are found on the island of Hawai'i, and are Mauna Kea, 4,205 meters (13,796 feet) and Mauna Loa, 4,169 meters (13,679 ft). On the island of Maui, we find Haleakala (or East Maui Volcano), 3,055 meters (10,023 ft) high. On the other main islands, there are lower volcanoes.
Obviously, the temperature in the mountains decreases with altitude. Here too, precipitation is more abundant on the slopes exposed to the trade winds than on the leeward ones; moreover, they are more abundant at intermediate levels, since the trade winds only blow from sea level to about 1,800 meters (5,900 ft). So on the highest mountains, above a certain altitude, a mountain desert is found, which becomes cold above 3,000 meters (9,800 ft).

Mauna Kea

At the Mauna Kea Observatory, at 4,200 meters (13,800 feet) above sea level, the average temperature in the coldest months is around freezing; however, at night, it can drop below freezing all year round. The lowest record is -11 °C (12 °F), but since the average minimum of January and February is -3 °C (27 °F), even in this case, the record does not differ too much from the average.
Here are the average temperatures of the Mauna Kea Observatory.
Mauna Kea - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)-3-3-4-3-2-1-1-10-1-2-2
Max (°C)6655910111010776
Min (°F)272725272830303032302828
Max (°F)434341414850525050454543

At this altitude, precipitation is very scarce: just 190 mm (7.4 inches) per year, with a maximum of 25 mm (1 inch) in March and November. However, above 4,000 meters (13,000 ft), during some winters, you can see the snow on both Mauna Kea and Mauna Loa.

Mauna Loa

At the Mauna Loa Observatory, located not in the summit but on the northern slope of the volcano of the same name, at 3,400 meters (11,100 ft) above sea level, the temperature is a bit higher, but still quite cold, especially in winter.
Mauna Loa - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)222346555432
Max (°C)111011121314141413131110
Min (°F)363636373943414141393736
Max (°F)525052545557575755555250

At the Mauna Loa Observatory, precipitation amounts to 430 mm (17 in) per year, so it is quite scarce, but not at desert levels as on the summit.
Mauna Loa - Average precipitation
MonthJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec.(mm)604045352515304035304550430
Prec.(in)2.41.61.81.410.61.21.61.41.21.8216.9
Days45654345555455

Mauna Loa Observatory

At lower elevations, around 2,000 meters (6,500 feet), in winter, the night temperature can sometimes drop below freezing (0 °C or 32 °F), while the air becomes mild during the day, as we can see from the averages of the Pohakuloa Training Area, located on the plateau between the two volcaones, at 1,900 meters (6,200 ft) above sea level.
Pohakuloa - Average temperatures
MonthJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)435666887775
Max (°C)191819192021222222212119
Min (°F)393741434343464645454541
Max (°F)666466666870727272707066

As we said before, on the windward slopes, it rains often at low altitudes, but at intermediate altitudes, it rains almost every day. For example, at the top of Mount Wai'ale'ale, 1,500 meters (5,000 feet) above sea level, on the island of Kaua'i, 9,500 mm (374 in) of rain fall per year (according to other sources, the amount is as high as 11,500 mm or 453 in), which makes it one of the rainiest places in the world. The windward slopes of Mount Haleakala are very rainy as well, so much so that rainfall exceeds 6,000 mm (235 in) per year.

Tropical cyclones



In theory, the Hawaiis are in the path of tropical cyclones, but as a matter of fact, they are rarely affected by them. The cyclone season runs from June to November, although the islands have historically almost never been affected before July; however, they are most likely in August and September. The most intense hurricanes in the islands' history have been Dot in August 1959, Iwa in November 1982, and Iniki in September 1992. Since hurricanes often weaken as they approach Hawaii, to the point that the effects they bring are usually not too serious, such as waves and wind, it has been assumed that they are protected by their very high volcanoes, which could disturb the spiral structure of cyclones.

Best Time



The best time to visit Hawaii is from May to October: the temperature is high, but the heat is usually bearable because of the trade winds. Tourist resorts are located on the leeward sides, however, it can be useful to recall that on the windward slopes, it often rains even in summer, and even cloudiness is more frequent. In the second part of the season, hurricanes may occur, although, as we have said, Hawaii is almost always spared. However, if you want to play it safe, you can choose May and June.
In theory, you can go to Hawaii all year round, and therefore even in winter, ie from December to March. However, in this period, temperatures are a bit lower, nights can be cool, rainfall is more abundant on the northern slopes and is fairly frequent also on the southern slopes, and even some short storms (i.e. intense waves of bad weather) may occur, but for the rest, the weather is fine and the sun shines in winter as well.

What to pack



In winter (December to March): bring light clothes for the day, a scarf for the breeze, a sweatshirt and possibly a light jacket for the evening; a light raincoat or umbrella. When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.
In mountainous areas, around 1,000 meters (3,300 feet): bring spring/autumn clothes, a sweater or jacket for the evening; around 2,000 meters (6,500 feet), a warm jacket and a hat for the evening; for the highest peaks (Mauna Kea and Mauna Loa), very warm clothes, such as a down jacket, gloves, a scarf, a hat, sunglasses, sunscreen, and hiking shoes.

In summer (June to October): bring light clothes of natural fibers, a sun hat, a scarf for the breeze, a light sweatshirt for the evening and for air conditioning, and a light raincoat or umbrella for the windward slopes.
In mountainous areas, around 1,000 meters (3,300 feet), bring spring/autumn clothes (light for the day), a sweater and a light jacket for the evening; around 2,000 meters (6,500 feet), a sweater and a jacket for the evening. For the highest peaks (Mauna Kea and Mauna Loa), warm clothes, such as a down jacket, gloves, a scarf, a hat, sunglasses, sunscreen, and hiking shoes.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.

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