 Rainfall is strongly concentrated in the cooler half of the year, particularly the months December through March, although precipitation is lower than any other part of the U.S. west coast.The summer months are virtually rainless, although subtropical moisture from the North American Monsoon usually results in increased humidity and thunderstorms for at least a few days each summer.Rainfall is highly variable from year to year and from month to month, and San Diego is subject to both droughts and floods.