UTCI-based outdoor heat stress forecaster for cities worldwide. Know when it is safe to go outside, exercise, or open windows based on real thermal stress, not just temperature.

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HeatLens — Outdoor Heat Stress Forecast Worldwide

Know before you go out. Search any city worldwide to see current and next-24-hour heat stress for walking, running, outdoor work, children, or hiking.

What HeatLens measures

HeatLens uses the Universal Thermal Climate Index (UTCI), a peer-reviewed metric that combines air temperature, humidity, wind speed, and solar radiation into a single "feels-like" value reflecting actual physiological heat stress. It is the same index used by the World Meteorological Organization and European meteorological services for official heat warnings.

Plain temperature is misleading because humidity blocks sweat evaporation: 35 °C in dry Jaipur or Phoenix feels very different from 35 °C in humid Chennai or Houston. UTCI captures that gap directly. Categories run from "no thermal stress" (9–26 °C) through moderate, strong, and very strong, up to extreme heat stress (above 46 °C).

Heat Stress Exposure (HSE)

Alongside the live UTCI reading, HeatLens shows a daily Heat Stress Exposure score: the number of hours each day with UTCI above 32 °C (the threshold for strong heat stress). HSE is a better planning tool than peak temperature because it captures duration — a city with seven hours above 32 °C is more dangerous than one that only briefly peaks higher. The 7-day HSE chart helps you spot heatwave build-up days in advance.

Personalised advice for vulnerable groups

Heat tolerance is not uniform. HeatLens adjusts its safe-exposure guidance for six vulnerable groups based on published medical evidence: people over 65 (reduced sweat response and thirst sensation), children under 5 (higher surface-area-to-mass ratio), pregnant women (elevated baseline core temperature), people with cardiovascular or respiratory conditions, outdoor workers (cumulative exposure), and anyone taking medications that impair thermoregulation such as diuretics, beta-blockers, anticholinergics, and some psychiatric medications.

When to use HeatLens

Heat risk follows the calendar of wherever you are, and that calendar is shifting. Most regions have a defined high-risk season — pre-monsoon and post-monsoon in South Asia, summer heat domes in Europe and North America, an extended hot season across the Gulf — but heatwaves are increasingly arriving earlier and lasting longer than historical averages suggest. Check the live reading rather than relying on the season alone.

Use the next-24-hour forecast to pick safer windows, typically before 09:00 or after 18:00 during peak heat. Use the 7-day HSE chart to plan exercise, school events, weddings, outdoor work shifts, or travel itineraries around heatwave peaks.

New to UTCI? Read why temperature alone misleads you about heat safety →

Runner? How to use heat stress data to train smarter this summer →

UTCI (Universal Thermal Climate Index) is a "feels-like" temperature in °C that combines air temperature, humidity, wind speed and solar radiation into a single value representing the heat (or cold) stress the human body actually experiences outdoors. It was developed by ISB Commission 6 and is widely used by meteorological services worldwide.

HSE (Heat Stress Exposure) is the number of hours in a day where UTCI exceeds 32 °C — the threshold for "strong heat stress". It summarises the daily burden of heat exposure: a day with HSE = 6 means six hours of conditions that are unsafe for prolonged outdoor activity.

UTCI is computed in your browser using the 6th-order polynomial approximation from Bröde et al. 2012, the same formulation used by the official UTCI Fortran source and the Python pythermalcomfort library. Hourly weather inputs (air temperature, relative humidity, 10 m wind speed, shortwave radiation) come from Open-Meteo.

Following the prototype: wind is reduced from 10 m to ~1.1 m by v = max(wind_10m, 0.5) × 0.5, and mean radiant temperature is approximated as tr = tdb + shortwave × 0.20.

Stress thresholds follow ISB Commission 6 (Bröde 2012): ≤26 °C no stress, ≤32 °C moderate, ≤38 °C strong, ≤46 °C very strong, >46 °C extreme.

Heat Stress Exposure (HSE) is the count of hours per day where UTCI exceeds 32 °C — the "strong heat stress" boundary. It is a daily-burden indicator, not a clinical limit.

Selecting a vulnerable group does not change the UTCI value or its stress category. It only modifies the suggested safe duration and the accompanying tip, drawing on published heat-health guidance.

Most published vulnerable-group thresholds are expressed in WBGT or air temperature, not UTCI. Converting between these is humidity- and radiation-dependent, so we deliberately avoid claiming a unit conversion and instead adjust the response (duration, breaks, warnings).

Live weather inputs come from Open-Meteo's ECMWF-backed forecast API. UTCI is calculated on the client using the standard Bröde et al. (2012) polynomial approximation. Activity-level guidance draws on WHO heat-health action plan guidelines, American Academy of Pediatrics youth-sport advisories, Bonell et al. (2024) on outdoor worker heat strain in South Asia, Bunker et al. (2016) on elderly heat mortality, the National Institute of Occupational Health (NIOH) India, and the National Disaster Management Authority (NDMA) India heat action framework. European and North American guidance aligns with WHO Europe heat-health action plans and the US National Weather Service HeatRisk framework, applied to the same Open-Meteo forecast feed.

All references collated on the Reference Papers page.