What is the difference between a Heat Advisory and an Extreme Heat Warning?

An advisory covers dangerous heat that mainly threatens vulnerable people. A warning means the heat is dangerous to a wide part of the population and is happening or imminent.

Is there a set temperature that triggers a heat warning?

No. Local National Weather Service offices set their own criteria, because the same temperature carries different risk depending on the climate people are acclimatised to and how common air conditioning is.

Why does humidity make heat more dangerous?

The body cools by evaporating sweat. Humid air slows evaporation, so the same temperature becomes harder to shed. The heat index combines temperature and humidity into what the air effectively feels like.

Heat Advisory or Extreme Heat Warning

Heat kills more people in an average American year than tornadoes, hurricanes and floods. The National Weather Service escalates its heat products in steps, and the steps are worth knowing before the first hot week arrives.

The ladder

A Heat Advisory is issued when a period of hot and often humid weather is expected that can be dangerous, particularly for people without air conditioning, people working outside, and anyone already vulnerable.

An Extreme Heat Watch means conditions are favourable for an episode of dangerous heat in the next few days, but the timing or the magnitude is still uncertain.

An Extreme Heat Warning means that dangerous heat is happening or is imminent. This is the level at which heat becomes a threat to life for a wide part of the population, not only the most vulnerable.

Why the numbers differ by region

There is no single national temperature that triggers a heat product. Local National Weather Service offices set criteria for their own area, because the same reading means different things in different places. A temperature that is routine in Phoenix is a genuine emergency in Seattle, where far fewer homes have air conditioning and bodies are not acclimatised.

Humidity matters as much as temperature. The heat index combines the two into what the air actually feels like, because humid air prevents sweat from evaporating, which is how the body sheds heat.

Who is actually at risk

Risk concentrates in people who cannot easily cool down: older adults, small children, people with chronic illness, outdoor workers, athletes, and anyone without reliable air conditioning. Overnight temperature matters enormously. A hot day is survivable if the night cools; consecutive hot nights are when heat becomes deadly, because the body never gets a chance to recover.

Never leave a person or an animal in a parked car. Interior temperatures rise far faster and far higher than most people expect, even with windows cracked and even on a day that does not feel extreme.

Common questions

What is the difference between a Heat Advisory and an Extreme Heat Warning?: An advisory covers dangerous heat that mainly threatens vulnerable people. A warning means the heat is dangerous to a wide part of the population and is happening or imminent.
Is there a set temperature that triggers a heat warning?: No. Local National Weather Service offices set their own criteria, because the same temperature carries different risk depending on the climate people are acclimatised to and how common air conditioning is.
Why does humidity make heat more dangerous?: The body cools by evaporating sweat. Humid air slows evaporation, so the same temperature becomes harder to shed. The heat index combines temperature and humidity into what the air effectively feels like.

The board shows the observed temperature alongside the heat index, and any heat product in effect.

See the current heat index where you are

Related guides

Categories and definitions on this page follow the National Weather Service and the Storm Prediction Center. For the official text of any product in effect, see weather.gov.