Understanding Heatwave Thresholds: Heat Advisories, Warnings, & Safety Tips
We often hear terms like Heat Advisory and Excessive Heat Warning during summer. However, it can be confusing to know exactly what temperature triggers these alerts. Here is a full breakdown of heatwave criteria, alert conditions, the heat index vs. air temperature, and essential heat safety tips.
1. At What Temperature Is a Heatwave Declared?
Meteorological agencies do not base heat alerts solely on high daily air temperatures. Instead, official alerts are issued when the daily maximum heat index (feels-like temperature) is expected to cross a specific threshold for a prolonged period.
2. Heat Advisory Thresholds
A Heat Advisory is typically issued when the following conditions are forecast:
- Daily maximum heat index reaches 33°C (91.4°F) or higher
- Expected to persist for 2 or more consecutive days
The feels-like temperature factors in relative humidity and wind alongside actual air temperature. Therefore, even if the thermometer reads 31–32°C, high humidity can cause the heat index to exceed 33°C.
3. Excessive Heat Warning Thresholds
An Excessive Heat Warning is a higher tier of danger than a Heat Advisory.
- Daily maximum heat index reaches 35°C (95°F) or higher
- Expected to persist for 2 or more consecutive days
When a warning is issued, avoid outdoor activity as much as possible. Extra precautions should be taken for vulnerable populations, including the elderly, children, and people with chronic health conditions.
4. Actual Temperature vs. Feels-Like Temperature (Heat Index)
Many wonder why a Heat Advisory is issued when the thermometer only shows 32°C.
This happens because heat alerts are based on the feels-like temperature, not just air temperature. When humidity is high, sweat cannot evaporate efficiently, making the air feel significantly hotter to the human body.
| Actual Temp | Humidity | Feels-Like Temp (Heat Index) |
|---|---|---|
| 31°C (87.8°F) | 80% | Can exceed 33°C (91.4°F) |
| 32°C (89.6°F) | 85% | 34–35°C (93.2–95.0°F) |
| 33°C (91.4°F) | 90% | Can exceed 35°C (95.0°F) |
5. Essential Heat Safety Tips
- Avoid non-essential outdoor activities between 12:00 PM and 5:00 PM
- Drink water frequently to stay hydrated
- Make active use of air conditioning or fans
- Limit caffeine and alcohol consumption
- Wear lightweight, loose-fitting clothing
- Ventilate indoor spaces periodically
- Check in regularly on children and elderly family members
6. Why Extreme Heat Is Dangerous
Prolonged exposure to heatwaves significantly increases the risk of heat-related illnesses such as heat stroke, heat exhaustion, and severe dehydration. Children and older adults are especially vulnerable because their bodies are less efficient at regulating body temperature.
7. Frequently Asked Questions (FAQ)
At what temperature is a heatwave declared?
A Heat Advisory is typically issued when the daily maximum feels-like temperature is expected to reach 33°C (91.4°F) or higher for two or more consecutive days. Official heat alerts are based on the heat index rather than actual air temperature.
What temperature triggers an Excessive Heat Warning?
An Excessive Heat Warning is issued when the daily maximum heat index is expected to reach 35°C (95°F) or higher for two or more consecutive days, indicating extreme heat conditions that pose severe health risks.
Is the feels-like temperature different from the actual air temperature?
Yes. The feels-like temperature (heat index) factors in humidity and wind alongside actual air temperature to show how hot it really feels to the human body. Higher humidity prevents sweat evaporation, making it feel hotter than the actual temperature.
What is the most important heat safety rule?
Drink plenty of water to stay hydrated, avoid outdoor activities during peak heat hours (2:00 PM – 5:00 PM), and rest in air-conditioned or shaded areas whenever possible. Always check in frequently on young children and the elderly.
Heatwaves are evaluated by the feels-like temperature rather than simple air temperature alone. Since advisories start around 33°C and warnings around 35°C heat index, it is always a good idea to monitor both actual temperatures and heat alerts during the hot summer months.