Western Samoa has a warm humid climate marked by a distinct wet season (November-April) and dry season (May-October). Average annual rainfall varies from 2500 mm on the western side of both islands and in northern and part of eastern Savai’i, to about 6000 mm in upland country. During the wet season, this varies from about 1500 in the drier parts of the islands, to 4000 mm in the uplands and during the dry season it varies from 750 mm to about 2000 mm. The easterly and south-easterly trade winds create this high rainfall in eastern Upolu and a rainshadow effect on western Upolu, eastern, northern and western Savai'i.
In terms of Soil Taxonomy the broad division of udic and perudic moisture regimes differentiate soils. An udic moisture regime implies dry periods of less than 90 days (cumulative) can occur (i.e., W and NW sides of Upolu and Savai'i and the N and E parts of Savai'i.
A perudic moisture regime implies the soil is moist throughout the year.
An aquic moisture regime occurs in wet or swampy areas where the soil is saturated by groundwater for long periods (reducing conditions).
In tidal marshes the moisture regime is called peraquic.
Mean monthly temperature at all elevations vary little during the year, 25.5 to 26.5°C at sea level and 21 to 22°C in the mountains. The diurnal range is much larger, varying between 6.0°C and 8.9°C. The ’Fohn' effect of the prevailing south-easterly trade winds causes slightly higher temperatures in the north-west parts of the islands.
