Temperature and heat index are not the same number
Air temperature describes the thermal state of the air under standard measurement conditions. Heat index combines air temperature and relative humidity to estimate how hot conditions feel to the human body. The calculation reflects an important cooling process: sweat removes heat most effectively when it evaporates. High humidity slows evaporation, so the same air temperature can place more strain on the body.
The US National Weather Service describes heat index as an apparent temperature based on air temperature and relative humidity. If both rise, the calculated heat index generally rises as well. In dry air, the apparent value can be lower than the measured air temperature.
What the number leaves out
Heat index is a useful screening measure, not a complete description of personal heat exposure. Direct sun, warm surfaces, low air movement, clothing, physical effort, age, health and acclimatisation can all change the actual strain on a person.
A shaded weather instrument and a pedestrian crossing an unshaded square do not experience identical radiation. WHO identifies high temperature, high humidity, low wind and strong thermal radiation among the environmental contributors to heat stored by the body. Work or exercise adds internally generated heat. A city forecast therefore needs to be read with the setting in mind.
Wind can help sweat evaporate in some conditions, but hot moving air is not automatically safe. Buildings can trap heat after sunset, and warm nights reduce the opportunity for the body and home to cool. The daily maximum alone can miss that accumulated exposure.
Read the hourly sequence
Start with air temperature and humidity for the hours when you expect to be outside. Look for the time when temperature peaks, whether humidity remains high, and how quickly conditions ease after sunset. Then check wind, cloud and any official heat alert.
Do not assume the highest heat index occurs at the same hour as the highest temperature. Humidity often changes through the day. An afternoon temperature maximum can coincide with lower relative humidity, while an earlier or later hour may combine somewhat lower temperature with much more moisture.
Forecast values also change between model runs. For plans several days away, the pattern matters more than a precise apparent-temperature number. Confidence normally improves as the event approaches, but local shade, ventilation and building conditions remain outside a city-scale forecast.
Heatwave and heat index describe different ideas
Heat index is a calculated apparent temperature for a set of conditions. A heatwave is a prolonged spell of unusually hot days and nights relative to a place and season. Authorities define and warn for heat using local criteria, which may include duration, overnight temperature and health impacts rather than one universal heat-index threshold.
This is why copying a warning threshold from another country can be misleading. Berlin, Moscow, Tbilisi and New York have different climates, housing, public-health systems and official warning services. Follow the authority responsible for the location shown on the forecast page.
Use official warnings for health decisions
WHO says heat stress can worsen cardiovascular, respiratory, kidney, diabetes and other health conditions, and that heatstroke is a medical emergency. Its guidance advises people to stay informed about official warnings, reduce strenuous activity during the hottest time, seek cooler surroundings and check people who may be vulnerable.
A website forecast cannot assess an individual’s medical risk. If someone develops signs of serious heat illness, use local emergency and medical guidance. Do not wait for a forecast number to cross a particular value before responding to symptoms.
A sensible heat check
- Read air temperature and humidity together.
- Check several hours, including the evening and overnight low.
- Consider sun exposure, physical effort, ventilation and access to cooling.
- Open the local authority’s heat warning and public-health advice.
- Recheck the forecast close to the planned activity.
Heat index is most helpful as a prompt to look beyond the thermometer. It gives humidity a visible role while leaving room for the local and personal factors that a single number cannot contain.