Heat Waves in Southern California: Are They Becoming More Frequent and Longer Lasting?

Los Angeles is experiencing more heat waves and also more extreme heat days. These numbers have increased by 3.09°F (1.72°C) per century and 22.8 per century occurrences, respectively. Both have more than tripled over the past 100 years as a consequence of the steady warming of Los Angeles. Our research explores the daily maximum and minimum temperatures from 1906 to 2006 recorded by the Department of Water and Power (DWP) downtown station and Pierce College, a suburban valley location. The average annual maximum temperature in Los Angeles has warmed by 5.0°F (2.8°C), while the average annual minimum temperature has warmed by 4.2°F (2.3°C). The greatest rate of change was during the summer months for both maximum and minimum temperature, with late fall and early winter having the least rates of change. There was also an increase in heat wave duration. Heat waves lasting longer than six days occurred regularly after the 1970s but were nonexistent from the start of 1906 until 1956, when the first six-day heat wave was recorded. While heat days have increased dramatically in the past century, cold days, where minimum temperature is below 45°F (7.2°C), show a slight decreasing trend.