What is extreme heat?

Extreme heat is a long period of high heat combined with humidity and temperatures above 90 degrees F.[1] Three or more days above 90 degrees F constitute a heat wave.


[1] https://ready.lacounty.gov/heat/

Which parts of LA County are affected by extreme heat?

The San Fernando and San Gabriel Valleys experience more extreme heat than areas near the coast due to geography. The air in valleys is warmer because surrounding mountains block the cooling breezes from the ocean and causes warm, dry air to remain within the valley bowl.

The Antelope Valley (AV), a triangular shaped plain with an elevation of 2000 ft and 3000 ft above sea level, is another community area in Los Angeles County that experiences extreme heat. The AV is surrounded by the Tehachapi Mountains to the north and the San Gabriel Mountains to the south. Moist air from the ocean releases its water content as it climbs up the mountain ranges before it crests the mountains and continues downslope into the valley as a warm, dry wind.

Urban areas where much of the landscape is largely covered by concrete and asphalt with few green spaces are often referred to as urban heat islands. Urban heat islands become hotter than other areas because surfaces covered by concrete and asphalt absorb more heat during the day than vegetated green spaces. Surfaces such as concrete and asphalt not only absorbs more heat, but also release heat more slowly, raising nighttime temperatures. Communities like East Los Angeles and Florence-Firestone can be considered urban heat islands as they are heavily built-up and are lacking in park space.  

How do I find out if I will be affected by extreme heat?

High desert areas, inland valleys, and urban areas are prone to extreme heat. See the urban heat island map.

How does extreme heat affect residents and communities?

Older adults, young children, outdoor workers, athletes, and people with a chronic medical condition are especially prone to be harmed by extreme heat. Residents can take measures to prepare themselves and their communities for extreme heat as temperatures and the number of extreme heat days are expected to rise in coming years due to climate change.

Individual actions to prepare for extreme heat include installing air conditioning and cool roofs, planting residential trees and improving insulation in homes. Community-level actions to prepare for extreme heat supporting urban forest and park space efforts, learning where Los Angeles County cooling centers are located and checking on vulnerable neighbors during heat waves.

This map provides information on where activated cooling centers in Los Angeles County are located during an extreme heat emergency.

How does climate change affect extreme heat?

When humans burn coal, gas and oil, this increases amount of heat-trapping greenhouse gases in the earth’s atmosphere, wrapping an extra “blanket” around the earth. When more greenhouse gases are emitted, more heat is trapped, resulting in higher average temperatures and more consecutive extreme heat days (heat waves). Communities located in inland valleys, high deserts, and park-poor urban areas within Los Angeles County are more likely to experience extreme heat that communities near the coast.

What is Los Angeles County doing to prepare for extreme heat?
Department Services
Public Health Urban forest management plan, emergency health services, health education
Parks and Recreation Cooling centers, swimming pools
Public Works Building permits (cool roofs, air conditioning), street tree management
Regional Planning Climate adaptation plan, residential tree permitting, development design evaluation
What can I do to prepare for extreme heat?

Residents can prepare for future extreme heat events by knowing how to protect themselves, their pets, neighbors, and loved ones by making behavioral changes, retrofitting their homes, and planting residential trees. Residents can also increase the resiliency of their community by educating others about climate change and extreme heat, informing others how to stay safe and where cooling centers are located, and supporting community-wide urban greening efforts.

Check out these resources:

First Aid and Heat-related Illnesses – Symptoms of extreme heat (Los Angeles County Chief Executive Office)

History of Extreme Heat in Los Angeles – Notable extreme heat days from 1877 to 1995 (Los Angeles County Chief Executive Office)


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