The Solomon Islands, lying within 12 degrees latitude of the equator and more than 1500km from the nearest continent, has a climate typical of many tropical areas, being characterised by high and rather uniform temperature and humidity and, in most areas, abundant rainfall in all months. Rainfall is the least uniform of these climatic elements, as topographical effects cause significant variations between locations. The Islands, because of their low latitude, are less subject to the damaging effects of tropical cyclones than elsewhere in the Southwest Pacific, though cyclones still pose a serious threat each year.
The weather and climate of the region can be explained largely by the seasonal movement and development of the equatorial trough; a belt of low pressure that migrates between hemispheres following the apparent movement of the sun, and the subtropical ridge of the southern hemisphere (a belt of high pressure typically located at about latitude 30 to 35 degrees south).
From about January to March the equatorial trough is usually found close to, or south of the Solomons, and this is a period of West to North-westerly monsoonal winds. The heaviest rainfall at most places also occurs at this time. The equatorial trough is in the Northern hemisphere from May to October and the Islands then lie within the region of the Southeast trade winds; the trades being the stronger and more persistent winds blowing out from the subtropical ridge towards the equatorial trough. These winds are moisture bearing, having had a long path over the ocean and heavy rainfall can also occur during the South-easterly season, especially on the windward side of the Islands. The transition months between the two seasons are marked by a greater frequency of calm winds.
Because of the low latitude of the Solomons, atmospheric pressure has only a small variation from month to month and, unlike places in temperate latitudes, records little change from day to day except when a tropical cyclone is in the area. The lowest mean monthly pressure (for Honiara for example) at 9 a.m is 1007.6 hpa in January, when the equatorial trough is in the vicinity, and the highest 1010.9 hpa in August. The decrease of pressure from 9 a.m to 3 p.m is part of an atmospheric tidal effect caused largely by the alternate heating and cooling every 24 hours.
The average annual rainfall is mostly within the range 3000 to 5000 millimetres with the majority of monthly rainfall amounts in excess of 200 millimetres. In most of the Solomons, the wettest months are during the Northwest monsoon season, with a tendency for reduced amounts during February when the equatorial trough is normally furthest south. Places on the southern sides of the larger islands also tend to have a rainfall maximum between June and September.
As there are no elevated rainfalls stations (with long‑term averages) the effect of increasing rainfall with height above mean sea level is unrecorded. Depending on the local topography, rainfall could be expected to increase with elevation with a maximum at about 600 to 1000 metres level on windward slopes. It is possible that the heaviest average yearly rainfall could reach 9000mm at some elevated sites. The extreme falls seem to be confined to the transition months of December and April when the equatorial trough is migrating across the islands. Between these months, the Northwest monsoon tends to give frequent rain but with lesser daily amounts. Very heavy daily falls can also occur during the South-easterly season at places well exposed to the prevailing wind. For example, the heaviest daily fall recorded, 380mm at Auki in April 1970, accounted for more than 40% of the station's rainfall for that month.
The main feature of temperature in the Solomon Islands is its uniformity, with seasonal variation extremely small, and little variation with latitude evident.
The range of average maximum temperature is approximately 2 degrees Celsius throughout the year. The range of minimum temperature is almost the same. The mean daily range of temperature (or diurnal variation) is about 7 degrees Celsius. The differing exposure of each station to the prevailing wind and the effect of local topography in causing a downhill flow of cooled air at night at some stations are the main reasons for the variation between stations.
Although no temperature data for elevated stations are given here, a decrease of mean monthly temperature (calculated as the average of monthly maximum and minimum temperatures) of about 2 degrees Celsius for each 300 meters of elevation has been found in many tropical areas of the Solomon Islands. The diurnal range of temperature tends to be greater at an elevated station than at a place near the coast.
This is a climatological element, which, like temperature, shows little seasonal variation in the Solomon Islands but has a marked diurnal fluctuation. The lowest values occur near the time of maximum temperature and would be slightly lower than the typical afternoon values.
The seasonal nature of the prevailing winds in the Solomon Islands has already been mentioned. East to Southeast winds are usual from May to October and, although not usually as strong as in other Pacific regions further South or East, still have a large degree of constancy. The typical speed of the winds over the sea, free from the influence of the mountainous Islands of the region, would be about 30km/hr. Stronger Southeast winds occur at times, possibly blowing at more than 40km/hr for several days, when the subtropical high pressure belt is stronger than usual in the South.
West to Northwest winds from about November to April is usually lighter than the Southeast trades and much less persistent.
In addition to the seasonal winds, there is also a strong diurnal wind pattern caused by the islands themselves and several effects contribute to this. Over land areas the wind speed tends to increase during the morning, reaching a maximum during the afternoon at about the time of the maximum temperature, and then dying away at night to become light and variable or calm. In coastal areas, the greater heating of the land during the day allows a flow of air from over slightly cooler sea, the sea breeze strength being typically 20 ‑ 30km/hr. Conversely, at night a land breeze can occur because of the more rapid cooling of the land. The offshore breeze is much weaker than the sea breeze. Finally, where there is hilly or mountainous terrain, cool and relatively dense air can flow downhill at night as a katabatic wind. If this reaches the coast, it can combine with the land breeze effect to give an offshore wind as strong as 20km/hr in the early morning. All of these effects are important in the Solomons in determining the daily wind pattern at any particular location.
Thunderstorms are a relatively frequent occurrence over the large and more mountainous islands, building up inland on many afternoons and, if winds are favourable, drifting later towards coastal areas. Over the ocean, storms are more likely to occur in the night or early morning. There is a minimum of thunderstorm activity during the south-easterly season and a maximum from about December to March.
A number of tropical low pressure systems occur each year over the Solomon Islands at times when the equatorial trough is in the vicinity, but few of these develop into tropical cyclones associated, by definition, with winds of at least gale force ‑ 34kts). The average frequency of cyclone occurrence is between one and two per year, tending to increase southward. Because the cyclones that do affect the Solomon Islands are usually in the early stage of their life cycle, they are relatively small. Nevertheless, they can cause serious damage to structures, crops, forests and local water supplies and have caused loss of life in the past.
These may be caused by the occasional tropical cyclone between November and April season, or thunderstorm squalls at any time of the year. A very intense cyclone would be rare but have winds of 200km/hr near its centre.
The frequency of strong winds at places for which measured wind speeds are available is quite low. Strong winds (the speed averaging at least 39km/hr are likely on less than six days each year.
The occurrence of fog in coastal plain areas is quite unusual. Fog is likely to occur more often inland. In elevated valleys where it would be a common event overnight and in the early morning.