British weather is rarely extreme. Temperatures and cloud coverage change frequently, often daily, sometimes even hourly and there are few rules that dominate the weather system. Summers tend to be moderately warm with occasional hot days and less rain than other seasons. Winters are usually grey and wet with some wonderful cool, crisp clear days in between. Snow is infrequent. In London there is sometimes thick fog that rolls up the River Thames and hangs over the city making it very damp and cold. Visitors are advised to arm themselves against sudden downpours.