Map from Google - Hawaii

In Hawaii, the climate is tropical, with a hot season from June to October (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.
The temperatures vary little throughout the year; they tend to be a bit lower in the windward slopes, and a little higher, but with lower humidity, in the leeward slopes. The temperatures vary little even compared with the averages, due to the location at tropical latitudes and far 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).
Of course, the temperatures drops in mountainous areas.
At Mauna Kea Observatory, at 4,200 metres (13,800 ft) above sea level, the lowest record is -11 °C (12 °F), but since the minimum average 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 Mauna Kea Observatory.
Average temperatures - Mauna Kea
Mauna KeaJanFebMarAprMayJunJulAugSepOctNovDec
Mauna KeaJFMAMJJASOND
Min (°C)-3-3-4-3-2-1-1-10-1-2-2
Max (°C)6655910111010776
Min (°F)272725272830303032302828
Max (°F)434341414850525050454543

Above 4,000 metres (13,000 ft), in the volcanoes Mauna Kea and Mauna Loa, during some winter you can see the snow.
At 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 milder, but still a bit cold, especially in winter.
Average temperatures - Mauna Loa
Mauna LoaJanFebMarAprMayJunJulAugSepOctNovDec
Mauna LoaJFMAMJJASOND
Min (°C)222346555432
Max (°C)111011121314141413131110
Min (°F)363636373943414141393736
Max (°F)525052545557575755555250

Mauna Loa Observatory
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 in the plateau between the two volcaones, at 1,900 metres (6,200 ft) above sea level.
Average temperatures - Pohakuloa
PohakuloaJanFebMarAprMayJunJulAugSepOctNovDec
PohakuloaJFMAMJJASOND
Min (°C)435666887775
Max (°C)191819192021222222212119
Min (°F)393741434343464645454541
Max (°F)666466666870727272707066

The rains in Hawaii 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 landascape in a few kilometres (or miles), just by climbing over a hill.
In the north facing slopes, where it rains throughout the year, we can speak of equatorial climate. For example, in Hilo, at the foot of the windward side of the island of Hawai'i ("The Big Island"), 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
HiloJanFebMarAprMayJunJulAugSepOctNovDecYear
HiloJFMAMJJASONDY
Prec. (mm)2352453402902051902752502502503952953220
Prec.(in)9.39.613.411.48.17.510.89.89.89.815.611.6126.8
Days161621252325272724242321272

In the leeward sides, generally less than 1,000 mm (40 in) of rain per year fall, 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 the clashes of air masses can create disturbances which can cause wet currents from all directions, while in summer the rare downpours occur only when the most severe thunderstorms manage to climb over the mountain ridges. In the windward slopes, at low altitudes it rains often, but at intermediate altitudes it rains almost every day. For example, in the top of Mount Wai'ale'ale, at 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. Even in the windward slopes of Haleakala, the rainfall exceeds 6,000 mm (235 in) per year.
Here are the average temperatures of Hilo; as we can see, they are lower than those of Honolulu (see below) which is located on the leeward side.
Average temperatures - Hilo
HiloJanFebMarAprMayJunJulAugSepOctNovDec
HiloJFMAMJJASOND
Min (°C)181818191920212121202018
Max (°C)262626262728282829282726
Min (°F)646464666668707070686864
Max (°F)797979798182828284828179

The trade winds blow at sea level, and 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).
The trade winds also concern the surfers, which flock to the beaches facing north, such as Sprecksville and Ho'okipa Beach, in the island of Maui.

Hookipa Cove

In the capital Honolulu, on the southern side of the island of O'ahu, the daytime temperature goes from 27 °C (81 °F) in January and February to 32 °C (90 °F) in August and September, which are the warmest months because of the thermal inertia of the ocean.
Here are the average temperatures.
Average temperatures - Honolulu
HonoluluJanFebMarAprMayJunJulAugSepOctNovDec
HonoluluJFMAMJJASOND
Min (°C)191920212223242424232220
Max (°C)272727282931313231302927
Min (°F)666668707273757575737268
Max (°F)818181828488889088868481

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, therefore it's also dry: in the airport, located on the coast, only 430 mm (17 in) of rain per year fall, of which 80 mm (3.2 in) in December, and less than 20 mm (0.8 in) per month from April to September. However, the rains vary according to district: for instance in Pearl Harbor the rainfall amounts to 550 mm (21.5 in) per year, and at Black Point it's 690 mm (27 in), while at the university, located near the green hills above the town, rainfall reaches 1,000 mm (40 in) per year, with more abundant rains from November to March, and even some more showers in summer.
Here is the average precipitation at the international airport.
Average precipitation - Honolulu
HonoluluJanFebMarAprMayJunJulAugSepOctNovDecYear
HonoluluJFMAMJJASONDY
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.
For swimming, the sea 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
HawaiiJanFebMarAprMayJunJulAugSepOctNovDec
HawaiiJFMAMJJASOND
Sea (°C)252424252525262627272625
Sea (°F)777575777777797981817977

In theory the Hawaiis are in the path of tropical cyclones, although they are rarely affected by them. The hurricane season runs from June to November, although historically they have not been affected before July; as a matter of fact, 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, bringing not too serious effects, such as waves and wind, it has been assumed that their very high volcanoes protect these islands, by disturbing the spiral structure of the cyclones.

When to go

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 remember that in the windward slopes it often rains even in summer, and even the cloudiness is more frequent. In the second part of the season, hurricanes may occur, from which, however, as we have said, Hawaii is almost always sheltered; however if you want to play it safe, you can choose May and June.
In theory, Hawaii can be visited all year round, and also in winter, from December to March, but in this period the temperatures are a bit lower, nights can be cool, rains are more abundant in the northern slopes, and fairly frequent even on the southern slopes, and even some brief storms may occur, but for the rest it is good and the sun shines.

What to pack

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-conditioned places, 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.