The winds which caused widespread damage across California on Wednesday and Thursday were the result of what is known as the Santa Ana winds.
These winds are dry and gusty, most frequent in the autumn and spring seasons.
They occur when there is a strong airflow from the east, usually the result of a ‘squeeze’ in the isobars brought about by low pressure across the southwestern states and high pressure developing towards the Canadian border.
The Santa Anas are usually very dry and warm as they descend from the high ground over the Mojave Desert or Great Plains to the east.
The geography of the region tends to accelerate these winds as they funnel through the ravines.
Winds are likely to have reached as high as 140 kph in gusts, strong enough to uproot trees and bring down power lines. Los Angeles seems to have particularly badly affected.