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.


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



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

Honolulu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Honolulu	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	19	19	20	21	22	23	24	24	24	23	22	20
Max (°C)	27	27	27	28	29	31	31	32	31	30	29	27
Min (°F)	66	66	68	70	72	73	75	75	75	73	72	68
Max (°F)	81	81	81	82	84	88	88	90	88	86	84	81

Rainfall in 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 the leeward slopes it rains almost only in winter, when clashes of air masses can create wet currents from the southwest, while in summer the rare downpours occur only when the most severe thunderstorms produced by the trade winds manage to climb over the mountain ridges.
At the Honolulu airport, located on the coast, only 430 mm (17 in) of rain per year fall, of which 80 mm (3.1 in) in December, and less than 20 mm (0.8 in) per month 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 more showers also in summer.
Here is the average precipitation at the international airport.
Average precipitation - Honolulu

Honolulu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Honolulu	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	60	50	50	15	15	8	13	15	20	45	60	80	430
Prec.(in)	2.4	2	2	0.6	0.6	0.3	0.5	0.6	0.8	1.8	2.4	3.1	16.9
Days	8	7	9	8	6	6	7	6	7	8	9	10	91

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.
Sunshine - Honolulu

Honolulu	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Honolulu	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	7	8	8	8	9	10	10	10	9	8	7	6

On the coasts exposed to the north-east, the temperatures are a bit lower, since they receive the wind directly from the sea. 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 we have seen is located on the leeward side.
Average temperatures - Hilo

Hilo	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Hilo	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	18	18	18	19	19	20	21	21	21	20	20	18
Max (°C)	26	26	26	26	27	28	28	28	29	28	27	26
Min (°F)	64	64	64	66	66	68	70	70	70	68	68	64
Max (°F)	79	79	79	79	81	82	82	82	84	82	81	79

In addition, on the north-east facing slopes it rains all year round, therefore the climate can be defined as equatorial. In Hilo, 3,200 millimetres (125 inches) of rain per year fall, 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.
Average precipitation - Hilo

Hilo	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Hilo	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	235	245	340	290	205	190	275	250	250	250	395	295	3220
Prec.(in)	9.3	9.6	13.4	11.4	8.1	7.5	10.8	9.8	9.8	9.8	15.6	11.6	126.8
Days	16	16	21	25	23	25	27	27	24	24	23	21	272

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

Hilo	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Hilo	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	5	5	5	5	5	6	5	6	5	4	4	4

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, in the island of Maui.



For swimming, the sea in Hawaii is warm enough 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 in the southern island of Hawai'i.
Sea temperature - Hawaii

Hawaii	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Hawaii	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	25	24	24	25	25	25	26	26	27	27	26	25
Sea (°F)	77	75	75	77	77	77	79	79	81	81	79	77

Hawaii are volcanic islands. The highest volcanoes are found on the island of Hawai'i, and are Mauna Kea, 4,205 metres (13,796 feet), and Mauna Loa, 4,169 metres (13,679 ft). On the island of Maui we find Haleakala (or East Maui Volcano), 3,055 metres (10,023 ft) high. In the other islands there are less elevated volcanoes.
Obviously, in the mountains the temperature 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, because the trade winds blow up to about 1,800 metres (5,900 ft), so on higher mountains, above a certain altitude a mountain desert is found, which becomes cold above 3,000 metres (9,800 ft).
At the Mauna Kea Observatory, at 4,200 metres (13,800 feet) above sea level, the average temperature in the colder 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.
Average temperatures - Mauna Kea

Mauna Kea	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Mauna Kea	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	-3	-3	-4	-3	-2	-1	-1	-1	0	-1	-2	-2
Max (°C)	6	6	5	5	9	10	11	10	10	7	7	6
Min (°F)	27	27	25	27	28	30	30	30	32	30	28	28
Max (°F)	43	43	41	41	48	50	52	50	50	45	45	43

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 metres (13,000 ft), in the volcanoes Mauna Kea and Mauna Loa, during some winters you can see the snow.
At the Mauna Loa Observatory, located not in the summit but in the northern slope of the volcano of the same name, at 3,400 metres (11,100 ft) above sea level, the temperature is a bit higher, but still quite cold, especially in winter.
Average temperatures - Mauna Loa

Mauna Loa	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Mauna Loa	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	2	2	2	3	4	6	5	5	5	4	3	2
Max (°C)	11	10	11	12	13	14	14	14	13	13	11	10
Min (°F)	36	36	36	37	39	43	41	41	41	39	37	36
Max (°F)	52	50	52	54	55	57	57	57	55	55	52	50

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

Mauna Loa	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Mauna Loa	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	60	40	45	35	25	15	30	40	35	30	45	50	430
Prec.(in)	2.4	1.6	1.8	1.4	1	0.6	1.2	1.6	1.4	1.2	1.8	2	16.9
Days	4	5	6	5	4	3	4	5	5	5	5	4	55

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

Pohakuloa	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Pohakuloa	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	4	3	5	6	6	6	8	8	7	7	7	5
Max (°C)	19	18	19	19	20	21	22	22	22	21	21	19
Min (°F)	39	37	41	43	43	43	46	46	45	45	45	41
Max (°F)	66	64	66	66	68	70	72	72	72	70	70	66

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


In theory the Hawaiis are in the path of tropical cyclones, but in reality they are rarely affected by them. The cyclone season runs from June to November, although historically the islands have not been affected before July; moreover, they are more 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, so 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.


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 in the leeward sides, however, it can be useful to recall that in 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, Hawaii can be visited all year round, therefore, also in winter, from December to March, but in this period the temperatures are a bit lower, nights can be cool, rainfall is more abundant in the northern slopes, and it's fairly frequent even on the southern slopes, and even some brief storms (intense waves of bad weather) may occur, but for the rest the weather is fine and the sun shines even in winter.


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

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