Like much of coastal British Columbia, Nanaimo experiences a temperate climate with mild, rainy winters and warm, dry summers. Due to its relatively dry summers, the Köppen climate classification places it at the northernmost limits of the Csb or warm-summer Mediterranean zone. Other climate classification systems, such as Trewartha, place it firmly in the Oceanic zone (Do).
Nanaimo is usually shielded from the Aleutian Low’s influence by the mountains of central Vancouver Island, so that summers are unusually dry for its latitude and location—though summer drying as a trend is found in the immediate lee of the coastal ranges as far north as Skagway, Alaska.
Heavy snowfall does occasionally occur during winter, with a record daily total of 0.74 metres (29.13 in) on 12 February 1975, but the mean maximum cover is only 0.2 metres (7.9 in).
The highest temperature ever recorded in Nanaimo was 40.6 °C (105 °F) on 16 July 1941. The coldest temperature ever recorded was −20.0 °C (−4 °F) on 30 December 1968.