Weather forecasts describe conditions at an official observing site: a sensor at standard height, over short grass, in open ground, deliberately sited away from buildings, pavement and anything else that would bias it.
That description is useful, and it is close to the opposite of a jobsite. Your crew is on concrete, beside steel, in a trench, next to a running compressor, or on a dark roof. Every one of those differences pushes the real heat load up, and together they routinely add more than the uncertainty in any WBGT calculation.
How far off the forecast can be
Typical differences between an ambient WBGT estimate and the environment at the work face.
| Location | Typical difference | Why |
|---|---|---|
| Open ground, light surface, good exposure | At or near ambient | Close to the conditions a forecast describes |
| Over asphalt or dark concrete | Several degrees above | Surface absorbs solar energy and re-radiates it upward |
| Dark roof membrane | Substantially above | Surface temperature 30 to 50°F over air, radiating into the body all day |
| Trench or excavation | Well above | Wind cut off entirely, radiant heat reflected from the walls |
| Near a compressor, generator or torch | Well above, locally | Process and exhaust heat no weather model can see |
| Shaded, breezy, elevated | Below ambient | The one case where the forecast is conservative |
Taking a jobsite reading
- Place the instrument at the work area, at roughly chest height, on a tripod rather than on the ground or a truck bed.
- Put it in the sun if the work is in the sun. Shading the instrument to protect it produces a reading that describes nobody.
- Keep it clear of walls, vehicles, equipment exhaust and anything else radiating heat, unless that heat is part of the environment the crew is working in.
- Wet the wick with distilled water and let the device equilibrate, typically 10 to 20 minutes, before trusting the first reading.
- Read hourly as a baseline, more often near a trigger level or when conditions are moving.
- Log every reading with time, location and who took it.
Hot spots to read separately
A single site reading averages away exactly the places where people get hurt. Identify these in your plan and read them on their own.
Trenches and confined areas
No air movement and reflected radiant load. Often the hottest place on a site by a wide margin.
Roofs and decks
Full solar exposure with a surface radiating heat upward, and typically no shade within reach.
Near hot work or equipment
Welding, torch-applied roofing, asphalt, curing concrete, generators and compressors.
Enclosed or partially enclosed structures
Framed but unventilated spaces trap heat and lose the breeze the open site has.
Using a forecast properly
None of this makes forecasts useless. It makes them the right tool for a different question.
A forecast answers what is next week going to look like, and should the tear-off start at six instead of eight? That is a planning question, and it is answered days ahead when you can still do something about it. The site instrument answers what is happening here, right now, and what does the crew do about it?
The failure mode worth avoiding is using either one for the other's job: planning a month of heavy work with no forecast, or taking a compliance reading off a phone while standing in a trench.
What a heat illness prevention plan contains
Whether or not a specific standard applies to you, this is the structure regulators, insurers and plaintiffs' attorneys all expect to see.
- Measurement. Who takes the reading, where, how often, and how it is recorded.
- Trigger levels. What changes at each threshold, written down in advance rather than decided in the moment.
- Water. Cool potable water available at the work area, in quantity, with a target of roughly one cup every 15 to 20 minutes.
- Shade and rest. Shade available at or before the trigger, large enough for everyone on break at once, and rest that is actually taken.
- Acclimatization. A graded schedule for new workers and anyone returning after more than a week away, with closer supervision during it.
- Training. Symptoms, reporting, first aid and the emergency plan, for workers and supervisors, in a language they understand.
- Emergency response. Who calls, what the site address is, how responders reach the work area, and who starts cooling in the meantime.
Warning signs of heat illness
Heat stress shows up before it becomes an emergency. Anyone on site should speak up immediately on any of these:
When in doubt, cool first and call for help. Collapse or confusion in the heat should be treated as possible exertional heat stroke: begin cold-water immersion immediately and call 911. Cooling comes before transport.
Forecast the site before you mobilize
Search any location and see the WBGT forecast hour by hour and out to 14 days, so the schedule is built around the heat rather than rearranged on the day.
- Real-time WBGT for your location or any city
- Hourly and 14-day WBGT forecasts to plan ahead
- Color-coded flag levels with mode-specific guidance
- Humidity, wind, sun and cloud cover behind every reading
WBGT is estimated from weather data with the peer-reviewed Liljegren model. Where a policy requires on-site measurement, use the app for planning and as a cross-check alongside your instrument.
Common questions
Why is the jobsite hotter than the forecast?
Forecasts describe conditions over open grass at an official observing site. Jobsites add dark surfaces that re-radiate heat, structures that block wind, and equipment that generates heat. Trenches and roofs in particular can run well above ambient.
Where should the WBGT instrument be placed on a jobsite?
At the work area, chest height, on a tripod, in the sun if the work is in the sun, and clear of vehicles and equipment exhaust unless that heat is genuinely part of the crew's environment.
How often should a jobsite reading be taken?
Hourly as a baseline, more often when conditions are changing or the reading is near a trigger level. Log each one with time and location, since unrecorded readings cannot demonstrate anything afterwards.
Can I use a weather app instead of a jobsite instrument?
For planning and scheduling, yes. For a compliance reading, or anywhere with a known hot spot like a roof or trench, no. The app computes ambient WBGT and cannot see the site's own microclimate.
What is the hottest place on a typical construction site?
Usually a trench or a dark roof. Trenches eliminate wind and reflect radiant heat off the walls; roofs combine full solar exposure with a surface 30 to 50°F above air temperature radiating upward with no shade available.
Related WBGT guides
Know the real heat number
Real-time WBGT, flag levels and forecasts. Free on iPhone.
