St. Thomas
Charlotte Amalie
, , °F WBGT
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U.S. Virgin Islands · live heat-safety instrument
The heat index stops at humidity. WBGT sees the sun, the wind, and the sky, the number safety officials actually use to decide who works, practices, and plays outside.
Hover anywhere on the islands for live conditions. Click to pin a spot and unlock the full analysis, personal heat risk, surface effects, hydration and work-rest guidance.
WBGT: Liljegren 2008 (0.7·Tnwb + 0.2·Tg + 0.1·Ta)Weather: Open-Meteo, 59-point gridLand cover: ESA WorldCover 2021Cloud layer is a stylized visualization of real cloud-cover %
On a Caribbean afternoon, the difference between shade and sun is the difference between “warm” and “medical emergency.”
Wet Bulb Globe Temperature was built for exactly this environment. It began as a U.S. military standard after heat casualties in training, and it is now the metric behind OSHA heat guidance andNFHS athletic policy because it measures what a body actually experiences: evaporation fighting humidity, breeze carrying heat away, and the radiant load of direct tropical sun.
The USVI sits at 18°N. In July the sun passes nearly overhead, andfull sun can add the equivalent of up to 15°F to how heat stresses your body compared to shade, load the heat index simply cannot see.
Solar radiation
Direct + diffuse sun and its angle in the sky. Invisible to the heat index.
Wind
Moving air strips heat from skin and the instrument alike. Also invisible to the heat index.
Humidity
Governs whether sweat can evaporate at all, the body’s only real cooling system.
Air temperature
The familiar number, necessary, but on its own it badly under-warns in the tropics.
The short answer: the heat index estimates how hot it feels in theshade from temperature and humidity alone. WBGT measures total outdoor heat stress, adding direct sun, wind, and cloud cover, which is why safety organizations use WBGT to regulate work and sport.
| Factor | Heat Index | WBGT |
|---|---|---|
| Air temperature | Included | Included |
| Humidity | Included | Included |
| Wind speed | Ignored | Included |
| Direct sun & radiant heat | Ignored, assumes shade | Included |
| Sun angle & cloud cover | Ignored | Included |
| Designed for | General public forecasts | Outdoor work, sport & military safety |
| Used by | Weather media | OSHA guidance · NFHS athletics · U.S. military |
The same WBGT reading demands different decisions from a foreman, a coach, and a visitor on their second day of vacation. Pin any location on the map for guidance tuned to age, activity, and surface.
Outdoor workers
Construction, marine, agriculture. OSHA recommends WBGT-based work/rest cycles; radiant load on job sites makes shade-only metrics under-read.
Caution ≥ 84.7°FCoaches & athletics
NFHS calls WBGT the gold standard for practice modification. Schedule conditioning by the forecast band, not the air temperature.
Modify ≥ 87.8°FSchools
Recess, PE, and field-day decisions need one defensible number. WBGT bands translate directly into activity policy.
Shade ≥ 84.7°FTourists & hikers
Reef Bay Trail at 2pm is a different planet from the beach at 9am. Unacclimatized visitors should treat every band one level more seriously.
Plan ≥ 80.0°FEvent organizers
Carnival, races, festivals: go/no-go timing, water stations, shade, and medical staffing all key off the WBGT forecast.
Prepare ≥ 80.0°FCaregivers & health
Infants, elders, and people on certain medications overheat far sooner. Use the personal risk profile in the map’s analysis panel.
Watch all bandsLive WBGT right now at each island’s main town, computed with the same physics as the map.
Charlotte Amalie
, , °F WBGT
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Cruz Bay
, , °F WBGT
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Christiansted
, , °F WBGT
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Fetching live conditions…
Full island forecasts & local heat context:St. Thomas ·St. John ·St. Croix
A safety number is only as good as its provenance. Ours is a three-link chain, and every link is independently verifiable.
Liljegren et al. (2008)
The peer-reviewed physical model for outdoor WBGT: the globe and natural wet-bulb sensors solved from their steady-state energy balances.
doi:10.1080/15459620802310770
ECMWF reference library
thermofeel, Europe’s operational implementation, itself validated bit-for-bit against Liljegren’s original code.
oracle for 840 test conditions
ClimaSafe’s engine
The TypeScript port running in your browser, checked against that oracle across the full tropical envelope, and re-audited after every change.
max deviation 0.0000 °C
840/840engine cases bit-exact
±0.11 °Fworst-case solar-geometry impact (vs NREL SPA)
1,440zenith samples validated
15 mindata refresh cadence
Sources & further reading:NWS, WBGT ·OSHA, heat hazards ·Liljegren et al. 2008 ·Kong & Huber 2022. The full audit record (every check, method, and honest limitation) ships with the project and lives in full on the methodology page.