If you live anywhere north of the Mason-Dixon line, the topic of weather in Hawaii may induce visions of cloudless skies, occasional rainbows and gentle breezes. It’s all true. Well, almost.
Severe weather conditions affecting aviation in Hawaii include high winds and turbulence, thunderstorms, heavy rain, volcanic emissions (VOG), and hurricanes. Even snow storms can occur in Hawaii, albeit at the altitude of the peaks of Mauna Kea and Mauna Loa.
The geographic location of Hawaii place the island chain between a semi-permanent high pressure system several hundred miles north-east, and the inner-tropical low pressure belt, creating a perpetual wind generator known as ‘trade winds’. Trade winds are fanning the islands for an average of 210 days per year at an approximate wind speed of 10-15 miles per hour from the north-east (All Hawaiian airports have runways oriented between 40 to 80 degrees as a result). Moisture traveling in the trade-wind flow predictably produces scattered clouds at 2500 ft. Compare the METAR at the bottom of this page with this generic prediction.
The descending air of high pressure systems leads to temperature inversions, typically between 4000 ft and 8000 ft, stopping extensive vertical development of clouds and associated convective activity. The trade wind temperature inversion is also accompanied by loss of moisture, causing excellent visibility above those altitudes. The temperature measurements from radiosonde data over Lihue (PHLI) below show that at higher altitudes the inversion is also accompanied by wind shear.
Whenever the trade winds encounter tall islands, they are both deflected around the mountains and are also forced to ascend. The ensuing cooling of the air mass results in condensation – and precipitation, which explains the stark differences in vegetation and temperature on the windward side of the islands versus the lee side. Once the air has lost most of its moisture it flows over to the lee side without much potential for precipitation, except on very strong trade wind days, when the rain sprays over to the leeward side of the island and causing the beautiful rainbows that Hawaii is famous for.
An exception are the small islands of Lanai and Kahoolawe, which are situated in the wind shadow of Haleakala and West-Maui, respectively, and receive less precipitation.
For aviators this general trade wind pattern results in a very simple rule-of-thumb, namely to expect broken clouds and light rain on the windward side of the islands, while the leeward side tends to be dry with scattered cloud cover. Even when conditions of the windward side turn marginal, a deviation to the south often provides a much safer and predictable flight path.
Hurricane season in the central-east Pacific is between May and November, with perceived peaks of activity in the middle of summer, as waters of the Pacific warm up. More than 100 low pressure systems develop every year which reach the strength of a tropical depression, or a hurricane, as defined by sustained wind speeds. Many of these storms traverse the wide open waters of the East-Pacific without causing any damage, but some come close enough to the Hawaiian islands to produce damage, sometimes of catastrophic proportions.
Warm waters off the Pacific coast of central America supply the energy and the humidity that fuels the birth of low pressure cells within the so-called Inter Tropical Convergence zone (ITCZ), aka the doldrums. This area of constant convection lies somewhere around 5-10 degrees to the North and South of the equator. The N-E and S-E trade winds, respectively, converge here and push these systems in a westerly direction. As these cells continue to move over warm waters they become more powerful, eventually reaching hurricane strength and packing powerful winds. For an exhaustive and well written article about the topic, see this article in Weatherwise Magazine.
The trajectory of these systems is generally north and west, in the general direction of the Hawaiian islands and depends on the location of high- and low pressure cells and jet streams. Once they drift over cooler waters they lose energy and decay. An alternative mechanism for hurricane demise is wind shear at high altitude, which may cause storm system to become disrupted, tilted, and dispersed. However, sometimes the remaining moisture gets caught in the trade wind flow and is carried to the islands. Powerful Hurricanes have struck Hawaii twice in the past 30 years, leaving the island of Kauai devastated and causing considerable damage on the other islands. Hurricane Iniki formed early in September 1992 during a strong El Niño year, hurricane Iwa ocurred late in the season in November of 1982. Both hurricanes appeared to pass by the islands to the south before they took a northern trajectory.
Wind is likely the major meteorological factor affecting small aircraft in Hawaii. A regular trade wind day features 13-16 mph winds which help during take-off and landing but pilots need to also be able to competently correct for any cross-wind component and gusts due to the windward mountain ranges. Airmet ‘Tango’ (for turbulence) is a regular feature of Hawaii NOTAMS as a result. The main airport on the island of Maui (PHOG, Kahului) is situated in a valley between the eastern and western parts of the island, which are ~10,000 and 5,000 ft tall, respectively. The resulting Venturi effect accelerates the winds by ~30-50%, which makes just taxiing to the runway in a cross-wind a challenge. Kahului reputedly has the highest average wind speed of any US airport (Wind surfing and kite-surfing championships are held just off the departure end of runways 2 and 5) . The image below shows wind speeds for PHOG 5-10 mph higher than for any other station For pilots unfamiliar with strong, turbulent winds some practice in techniques to maintain directional stability on the ground and on approach is highly advisable. Even more so, the southerly cross-winds experienced during approaching cold fronts require the ability to control the aircraft alignment with the runway through final approach and roll-out since they can exceed the performance limitations of aircraft or pilot capability. Finally, strong winds impact the required fuel for long cross-country flights, which demands careful planning. Avgas cannot be obtained at all Hawaiian airports or at all times.
The 10-14,000 ft mountains of Maui and Hawaii, respectively, also deflect wind directions on the leeside of the island, and to some extent on the windward side, also. Kona airport regularly features southerly and westerly winds under trade wind conditions, caused by wind vortexes forming in the lee of the island.
Cold Fronts and Shear Lines
The fall season brings low pressure cells that develop in the northern hemisphere, and which move eastward with associated frontal systems. These low pressure systems can temporarily displace the high pressure cells that generate the trade winds, replacing them with inland sea breezes, afternoon convection and localized showers. During the night, cooling of land masses reverses the process. Clouds disappear and sunrise reveals cloudless skies promising another perfect day in paradise, only to repeat the pattern during the day. As low pressure systems move closer to the islands, the wind swings to the south and south-west, parallel to the approaching shear line. Frontal passage is marked by a clockwise change in wind direction to the west, and eventually to the north.
Approaching cold fronts or shear lines are preceded by exceptionally good visibilities, which makes for spectacular flying experiences in Hawaii. It is common that the peaks of the big island mountains can be seen after take-off from Honolulu under these conditions – more than 150 Miles away. Like anywhere else, frontal systems can cause rapid deterioration of visibility, lowered ceilings and heavy precipitation with mountain obscuration. Hawaii is considered ‘mountainous terrain’ for the purposes of IFR minimum terrain clearance rules (part 95.19). CFIT accidents have occurred after pilots attempted to remain VFR by following island coastlines into deteriorating conditions. In Honolulu in particular, a complicating factor is that wind changes associated with frontal passage require runway changes that severely limit the available instrument approaches. During westerly winds only the LDA Rwy 26 approach is used, while VFR traffic is restricted to the “Kona Approach” over the freeway and close to the mountains, which prohibits this option in marginal weather. Pilots of small aircraft asking for the LDA approach have to expect significant delays in the holding pattern due to spacing issues with airliners. While the closest airport to PHNL with a precision approach is Lanai (PHLY), and an alternate airport with a precision approach (backcourse ILS in southerly winds) is Kahului (PHOG).
More detailed discussions on Hawaiian climate and weather pattern can be found at these sources:
Climate of Hawaii (Western Region Climate Center)
Climate of Hawaii (Wikipedia, probably authored by the Western Region Climate Center based on similar content and wording, a lighter lighter read)
Rainfall Atlas of Hawaii (Geography Department, Univ. of Hawaii)
Atlas of Hawai’i. University of Hawaii at Hilo. Dept. of Geography, Sonia P. Juvik, University of Hawaii Press, Jan 1, 1998 – Reference – 333 pages