First we want to point out there is considerable confusion about there being a "wet/dry" season and a "high/low" travel season in Hawai'i. The two assumptions are both true and not true; from a certain point of view. Let's take a look.
Spring in Hawaii
Less rain starts to fall on the islands as the trades die down to their summer levels making this is one of the most ideal times to visit. Some call this the start of the "dry" season. The weather is almost always perfect and in general this is the best time to find good travel bargain activities. After mid-April most airfare and lodging prices drop as a result of "high season" ending. The last week of April and first week of May are often a time when the most Japanese visitors will arrive due to the "Golden Week" holiday in Japan.
Water temperatures measure near the high 70-degree mark this time of year, and the surf begins to die down as the trade winds let up. The best part of the spring is the fresh flowers, fruit, and vegetation that can be seen around the island. This is our favorite time to visit. We should note that the spring does hold one large event on the Big Island of Hawai'i that can cause crowding on the Hilo side of the Island. The Merrie Monarch festival is held annually starting on Easter Sunday.
Summer in Hawaii
Typically summer is classified on Hawai'i as the "low season." This results in lower rates for both accommodations and/or rental vehicles. However, this "low season" classification can often be misleading, as June - August are very popular with travelers. Airline prices will often increase into late June through July as demand is higher for seats nationwide. Thus, if you are flying from the east coast, the most expensive flight you pay for might ironically be the continental flight to the west coast. Book your flight with Hawaiian Airlines
According to the tourism authority in the islands, July continues to be the month with the most visitor arrivals. To us, this makes sense because this is when the most families visit the island, mainly due to their children being out of school. The same rule applies for local children on the island. This is their summer and they often frequent the same beaches, trails, and places you'll be visiting too.
The summer months are always the warmest on the island, and temperatures can reach as high as the 90's under the right conditions. Combined with the high humidity, the heat can occasionally be unpleasant for some travelers not accustomed to it. Surf is at its lowest during the summer, and the rain clouds are few and far between except on the windward coast. Note the difference in flow rates at Rainbow Falls (below), located on the Big Island, between summer (left) and winter (right).
Seasonal Waterfall Differences
Water temperatures usually rise above the 80-degree mark. While a rare event, the summer months are also hurricane season in Hawai'i, and the last major storm to hit Hawai'i was Iniki in September 1992, when it came ashore on Kaua'i as a strong category 3 (border-line category 4) storm. In 2007, another storm grazed the southern portion of the Big Island, giving some folks quite an initial scare when it appeared to be locked on the island chain. These large storm systems are, as noted, rare - just be aware they do occur from time to time.
In our opinion, the summer is simply too warm for a lot of outdoor hiking and adventure, especially on locations like the Big Island's black lava fields or any other island's coastal trails (Kalalau on Kauai for example). Select lower prices are nice on the wallet, but the sun combined with the humidity can make going anywhere but the beach a chore. We'd say come a few months earlier in the spring or a few months later in the fall if given a choice.
Fall in Hawaii
More rain begins to fall in November as the trades increase to their winter levels, and there are days where rain showers will dominate on the windward coast and mauka areas. While Hawai'i had been going through a fairly dry spell for several years, 2006 was a year of almost extreme precipitation amounts, causing flooding on many islands in the early winter months. 2007-2008 have also been considerably wetter when compared to recent years; and many say Hawai'i is returning to its more "true" weather cycle. A Kona storm in late 2007 dumped a tremendous amount of rain on the islands and caused wind damage in several areas. Kona storms, in general, can be quite nasty in Hawai' when they do hit. So what exactly IS a Kona storm?
When a Kona storm develops or moves into the area, the prevailing wind pattern changes - south and southwesterly winds replace the trades. Since the winds typically affect the Kona coast of the Big Island, the storms have thus taken on that name.
Although destructive Kona storms are relatively rare, they occur often enough to warrant a mention here. Kona storms bring high winds, heavy rain, snow atop higher mountains (like Mauna Kea and Mauna Loa on the Big Island), lightning and thunder can occur (a rarity in Hawai'i). Due to extreme terrain differences in the islands weather conditions can be highly variable across a relatively small area. Gusts of winds up to 60 miles per hour have been known to happen on the Big Island, while other portions of the Hawaiian chain were simply dealing with increasing breezes.
"High season" for traveling begins in mid-December which typically causes rates for flights, tours, and accommodations to the islands to rise. In general the fall months and the spring months are very similar. Like in the spring, the Big Island of Hawai'i can become quite busy during October as a major event is held on the island. Typically in October, the Ironman Triathlon event is held on the Kona side of the Big Island of Hawaii. Check online calendars for the exact dates each year. Here are some other great Big Island tours, activities and adventures to check out:
Winter in Hawaii
By the time winter is in full swing, the "high season" has begun on the islands. This term of course brings back up the debate about defining "low" and "high" season. While the tourism authority in the islands, using 2005 data, will show the second highest number of visitors arriving in December of each year, it's important to note that most of these visitors are showing up for about a week - Christmas through New Years. This is always a very busy time in the islands and can almost single-handedly make this the "high season" for accommodations. Thus, on average, the prices are higher and occupancy rates are higher. Bottom-line: expect to pay more in winter months. As noted in the introduction, Christmas is a time that anyone who dislikes crowds should avoid coming to the islands.
Hawaii - a diverse land of contrasts
The high temperature remains a comfortable average of near 78 degrees. Water temperatures are often around 74 degrees during this period, and swimming can be rough as a result of the higher surf. If you're coming to Hawai'i to see any surfing events, then this is definitely the time to come - especially on Maui and Oahu. During the winter months, this is one of the only places in the world you can come play in the snow (on the Big Island and occasionally Maui) and swim in the (warm) ocean in the same day.
Winter is also whale watching season in Hawaii. Humpback Whales travel all the way from Alaska every winter to birth their young (usually in the shallow water off the coast of Maui).
Unless you are coming specifically for big surf or to see whales, in our opinion, winter is usually the least desirable time to visit the islands. Despite the waterfalls usually being more active due to the rain, the potential to see huge surf and the amazing shows put on by whales breaching off the coast; the combination of higher prices, wetter weather in general, and cooler temperatures has never sat well with us when you can easily visit during the spring or fall for much cheaper prices and better weather.
Before we end this section we want to stress again that this "low" or "high" season classification changes on a year to year basis, so that predicting the visitation rates season to season becomes more and more difficult. Several factors, including the economy and weather, can affect travel to and from Hawai'i. The same is true of natural events (like flooding, earthquakes, etc). During the span of one week in August 2007, the Big Island was faced with a 5.4 earthquake, a Category 3 Hurricane, and a Tsunami warming after a earthquake off the coast of Peru. On a personal note, despite all of that, we still enjoy the Big Island the most. As we say in the islands, "There's never a dull moment living here."