 The hottest months in the city are August and September, during which temperatures average .Summers are by far the driest time of year since influences from the California Current and the North Pacific High suppress the formation of rainfall caused by the North American Monsoon.As in coastal Southern California, air pollution sometimes occurs during periods of temperature inversion, especially during summer and fall, but (unlike Mexico City) is seldom severe and in recent years has lessened due to cleaner car engines.