Heat advisories, explained
When the temperature and humidity climb, the National Weather Service issues heat alerts with specific words. This guide explains what they mean. It is general information, not medical advice.
The definitions below come from the National Weather Service and were checked on September 19, 2026.
The heat index
The heat index is how hot it feels when humidity is combined with the air temperature. The National Weather Service gives the example of a 96 degree day with 65 percent humidity, which has a heat index of 121 degrees. The heat index assumes shade and a light wind, and the service notes that full sun can add up to 15 degrees. See the heat index page.
Weather apps show the heat index as the feels-like temperature on hot days.
The three alerts
- Heat Advisory. Issued for dangerous heat conditions that are not expected to reach warning criteria. The service suggests considering postponing or rescheduling outdoor activities, especially during the heat of the day.
- Extreme Heat Watch. Issued when conditions are favorable for an extreme heat event but its occurrence and timing are still uncertain. The service suggests planning ahead, including finding cooling shelters or arranging to stay with someone who has air conditioning.
- Extreme Heat Warning. Issued when extremely dangerous heat conditions are expected or occurring. The service advises avoiding outdoor activities, especially during the heat of the day, and staying in an air-conditioned space as much as possible, including overnight.
In March 2025 the service replaced the older names Excessive Heat Watch and Excessive Heat Warning with Extreme Heat Watch and Extreme Heat Warning. See the heat watch, warning, and advisory page.
The thresholds for each alert are set locally, so a heat advisory in one region can begin at a different heat index than in another.
Who the service asks you to check on
The National Weather Service asks people to be aware of infants, older adults, sick or frail people, and pets during a heat wave, and lists age, certain illnesses, and some prescription drugs among the factors that make heat harder to tolerate. Its heat safety page has the full guidance.
In Canada
Environment and Climate Change Canada issues heat warnings using regional thresholds. See the Canada.ca weather alerts page.
What this means for Bold Weather
Bold Weather shows heat alerts at the top of the screen in large type with the agency named, keeps the official wording, and mentions the alert in the daily summary in plain words. The hourly list shows when the heat peaks. Read about alerts or learn about the TestFlight beta.