What Is a Heat Warning?
A heat warning is an official alert issued by the National Weather Service (NWS) that signals an upcoming period of dangerously high temperatures and humidity. The warning is designed to give people time to take precautions, reduce heat exposure, and protect vulnerable populations such as the elderly, children, and those with chronic illnesses.
Types of Heat Alerts Issued by the NWS
The NWS uses a tiered system to communicate heat risks. Each level reflects the severity, expected duration, and potential impact on public health.
- Heat Advisory – A short‑term alert for temperatures that may cause heat‑related illness for those who are not acclimated to the heat.
- Heat Watch – A notice that conditions could become hazardous within the next 24–48 hours.
- Heat Warning – A more urgent alert indicating that temperatures will reach extreme levels, often accompanied by high humidity.
- Extreme Heat Warning – The highest level of alert, used when temperatures exceed 110°F (43°C) or when heat index values become life‑threatening.
Heat Advisories vs. Extreme Heat Warnings
Heat advisories are far more common than extreme heat warnings. An advisory is issued when temperatures are high enough to cause discomfort and potential health risks, but not so severe that they threaten widespread health problems. In contrast, an extreme heat warning is reserved for situations where the heat could cause immediate danger, such as heat stroke or cardiovascular events.
How the NWS Issues Heat Warnings
The NWS monitors a combination of temperature, humidity, wind speed, and atmospheric pressure. Using these data points, forecasters calculate the heat index—a measure of how hot it actually feels. When the heat index exceeds certain thresholds for a sustained period, a warning is issued.
- Data collection from weather stations and satellite feeds.
- Analysis of projected temperatures and humidity levels.
- Determination of heat index thresholds based on regional climate norms.
- Issuance of the appropriate alert level.
- Distribution of the alert through local media, mobile apps, and the NWS website.
Ranking Some of the Most Intense High Temperatures
Throughout history, certain heat waves have set records for intensity. The NWS keeps a database of extreme events, allowing scientists and planners to compare current conditions with
