Extreme heat

Extreme heat

Aug 23, 2023

Extreme heat events have become more frequent in California since 1950, especially at night.

Heat causes the most weather-related deaths in the United States. “Extreme heat events” are days or nights that are unusually hot compared to historical data (from 1960 to 1990) for a given location. For more information, download the Extreme heat chapter.

What does the indicator show?

Number of extreme heat events: days

For this indicator, a daytime extreme heat event occurs between April and October when the temperature is above the highest 5 percent of historical daily high temperatures, and a nighttime heat event when the temperature is above the highest 5 percent of historical daily low temperatures. “Heat maps” show the number of extreme heat events each year since 1950 during the day (top) and at night (bottom) at 14 locations across California.

Heatmap of daytime heat magnitude for each year and each weather station. In general heat magnitude is increasing at several stations over time.

Number of extreme heat events: nights

Heatmap of nighttime extreme heat events for each year and each weather station. Some stations show an increasing number of events over time.

The 14 locations studied are shown in the map below.
Outlines show the eleven climate regions in California (see report chapter for more information).

Map shows locations of weather stations throughout California

Source: WRCC

Extreme heat events have also become notably hotter in the last decade and are happening more often across the state.
Over the past decade, four of the 14 locations shown have had 30 or more daytime heat events. Nighttime heat events occurred 30 or more times at 10 locations in at least one year; as many as 85 nighttime heat events were recorded in San Francisco Airport, and 73 in San Diego.
Since 1950, the number of daytime heat events increased the fastest in Southern California, at Edwards Air Force Base and in San Diego; the number of nighttime heat events increased the fastest at Blue Canyon and at San Francisco Airport in Northern California.

Why is this indicator important?

Periods of extreme heat have significant public health, ecological and economic impacts.
Long periods of high temperatures are particularly dangerous when nights remain warm and humid: warm nights do not allow the body to recover after a hot day, and humidity prevents sweat from evaporating to cool down the body.
Extreme heat affects agriculture, water, power, and transportation systems.

What factors influence this indicator?

The increased frequency and intensity of extreme heat events are attributable to greenhouse gas emissions.
Differences across regions are influenced by the same factors that affect temperature.

The sun sets on a hot summer day in Sacramento (August 2019). A dark orange sky at sunset