Rainfall tends to be associated with Atlantic depressions or with convection. In summer, convection caused by solar surface heating sometimes forms shower clouds and a large proportion of rainfall falls from showers and thunderstorms at this time of year. The wettest months are November and December and on the highest parts of the moor the average annual total rainfall is over . This compares with less than in the lower land to the east around the Exe Estuary, which is in the rain shadow of the moor. Due to the influence of the Gulf Stream snowfall is not common, though due to its high altitude it is more vulnerable to snowfall than surrounding regions.