The Big Island is dominated by a pattern of east-northeast trade winds with an average wind speed of 18 miles an hour for the majority of the year. The terrain of the Big Island, with its high mountain peaks, causes the trade winds to flow around the mountains. This means there is less rainfall on the summits of these mountains. It also means that the leeward side of the island (and the other Hawaiian islands) is typically drier than the windward side of the island – hence Kona is generally dry with low rainfall while Hilo experiences a high annual rainfall.
Wind and cloud patterns on the Big Island
The heating and cooling processes of the islands in conjunction with patterns of trade winds contribute to causing these puffy cloud trails. Scientists have determined that as the sun heats the islands, clouds begin to form over the leeside of the island, with the trade winds carrying the warmed air downstream. The surface pressure downstream drops and the convergence increases. The warm air rises, condenses and the cloud trail extends in length. At night the islands cool and the cold air hinders cloud formation as the air moves downstream.
On the Big Island the Kona Coast has its own unique cloud processes. Typically clouds begin to form along the coast before noon, while the ocean remains free of clouds. By early afternoon cloud along the coast has extended out to sea. By nightfall the temperature has dropped in the coastal area covered with cloud – indicating that a cloud deck (a bank of cloud that has formed a layer at a specific altitude) has formed.
The topography of the Big Island greatly influences the wind and cloud patterns. The strong easterly winds around both the northern and southern ends of the island form a ‘westerly reverse flow’. This flow reaches up to 2000 meters in altitude, just below the dominant easterly trade winds. During the day, as the island heats up, the westerly reverse flow grows stronger and moist air is carried to the Kona Coast. Clouds begin to form on the slopes along the coast, but the reverse flow stops the warmth created by the island from forming the cloud trail typically seen on the other islands.
The Hawaiian names for wind, clouds and rain
Hawaiians have many names for sky and cloud formations. These names demonstrate the Hawaiian culture’s profound connection to the physical environment.
- ao puaʻa – these are cumulus clouds of different sizes massed together. These types of patterns are common on the Kona coast, and indicate that good weather is ahead and not a storm.
- ao pehupehu – common in summer, these refer to cumulus clouds increasing in size. Often present with trade winds, these formations grow darker (especially at their base) causing rain on the windward slopes.
- hoʻomalumalu – sheltering cloud
- hoʻoweliweli – threatening cloud
- ānuenue – rainbow, considered to be a favorable omen
- ua loa – an extended rainstorm
- ua poko – a short spell of rain
- Kūkalahale – the name of a type of wind and rain famous in Honolulu.
- kili hau – an ice-cold shower, or a cold drizzle.
- makani – a general term for wind. The prevailing northeast trade winds of Hawaiʻi are called moaʻe, aʻe, aʻe loa, Moaʻe Lehua, or moaʻe pehu. A leeward wind is a Kona wind.
- Kaiāulu – the name of a gentle trade wind famous in song at Waiʻanae, Oʻahu.
- ʻōlauniu – the name of a wind on Hawaiʻi. The figurative translation means promiscuous, and a literal translation means coconut-leaf piercing.
The Hawaiian Islands’ cloud patterns are influenced by winds and mountain height as well as the heating and cooling processes of the island itself. These factors affect the cloud formation on the leeside of the islands helping to create this puffy cloud trail phenomenon. On the Big Island though, clouds are formed by more complex processes that create a typically sunny morning, followed by a cloudy afternoon with an increased chance of rain.
International Pacific Research Center. (2008). The Cloud Trails of the Hawaiian Isles. IPRC Climate, 8(2). http://iprc.soest.hawaii.edu/newsletters/newsletter_sections/iprc_climate_vol8_2/cloud_trails_hawaii.pdf