What Is a Heat Index? The 'Feels Like' Temperature Explained for Indian Readers

What is a heat index?

The heat index is a measure of how hot the weather actually feels to the human body, calculated by combining the air temperature with the relative humidity. It is often called the "feels like" or apparent temperature. When a forecast says the temperature is 40°C but the heat index is 48°C, it means the conditions feel like 48°C because moisture in the air is stopping your body from cooling itself efficiently.

The term has been drawing fresh curiosity in India: Google Trends India recorded roughly 500 searches for "what is a heat index" in the 24 hours around 16 September 2026, with about 75% growth — a sign that more Indians are encountering the term in weather apps and heatwave coverage and want to know what it means.

Why humidity makes heat feel worse

Your body cools itself primarily by sweating. When sweat evaporates from the skin, it carries heat away. But when the air is already loaded with moisture — as it often is along India's coasts, in the Gangetic plains and during the pre-monsoon months — sweat evaporates slowly or not at all. The body's internal temperature then rises even though the thermometer reading has not changed.

This is why 38°C in humid Chennai or Kolkata can feel more punishing than 42°C in dry Jaipur. The heat index captures that difference in a single number. A recent example from US coverage illustrates the gap: during a North Texas heatwave in September 2026, FOX 4 Dallas-Fort Worth reported air temperatures near 100°F (about 38°C) while heat index values reached 105–106°F (around 41°C), because of how the conditions combined.

How the heat index is calculated

Meteorological agencies use formulas that take the air temperature and relative humidity as inputs and output an apparent temperature. The most widely referenced version comes from the US National Weather Service, which also publishes a colour-coded chart grouping values into risk bands such as "caution", "extreme caution", "danger" and "extreme danger". Exact formulas and thresholds vary between agencies, so the category labels in one country's chart may not match another's.

Two details are worth remembering when reading a heat index figure:

• It assumes shade. Standard heat index values are calculated for shaded conditions. Standing in direct sun can add several degrees to the perceived temperature.
• It assumes light wind and average clothing. Strenuous work, heavy clothing or still air all push the real stress on your body higher.

Heat index vs heatwave alerts in India

The India Meteorological Department (IMD) declares heatwaves mainly on the basis of actual maximum temperature thresholds and departures from normal — for example, around 40°C or more in the plains and 30°C or more in the hills, or a marked rise above the seasonal average. That is a different system from the heat index, which factors in humidity. Indian weather apps and media increasingly show "feels like" figures alongside IMD readings, which is why the two numbers on your phone can differ sharply on a muggy day.

For practical purposes during an Indian summer, treat the heat index as the better guide to personal risk: schedule outdoor work for early morning, hydrate even when not thirsty, and watch for signs of heat exhaustion — dizziness, nausea, cramps and a rapid pulse — especially when the feels-like figure runs well above the actual temperature.