A forecast that says 96°F tells you less than it seems to. The National Weather Service’s heat forecast tools page explains why, and one line on it changes how to plan an afternoon outdoors: the heat index was devised for shady, light wind conditions, so standing in full sunshine can add up to 15°F. This sheet turns the page into five steps. We read it on 31 October 2026. Statements marked as the NWS’s are from its page; anything marked as ours is our judgement, not the NWS’s.
What the page says
- Heat index: how hot it really feels once humidity is added to the air temperature. Its example is 96°F at 65% humidity, which feels like 121°F.
- Full sun: can raise heat index values by up to 15°F, because the values assume shade and light wind.
- Wind: strong winds, particularly with very hot, dry air, can be extremely hazardous.
- WBGT: combines temperature, humidity, wind and solar radiation, and suits active people such as outdoor workers and athletes.
- OSHA’s measures for outdoor work: acclimatise workers, provide shade, schedule work earlier or later in the day, use work and rest schedules, limit strenuous work and use relief workers.
- HeatRisk: a seven-day forecast that rates heat concern by colour and number; the NWS flags it as experimental.
The five-step sheet
| Step | Read | What the NWS says | Our decision note |
|---|---|---|---|
| 1. Feels-like | The heat index, not the air temperature | 96°F and 65% humidity feels like 121°F | Plan around the feels-like number |
| 2. Sun or shade | Where you will stand | Full sun can add up to 15°F | Add the sun to your number before you decide |
| 3. Effort | How long and how hard | WBGT suits active people; OSHA lists shade and earlier or later scheduling | The more you move, the more shade and timing matter |
| 4. Wind | Gusts and how dry the air is | Strong wind in very hot, dry air can be extremely hazardous | Do not count a hot gust as relief |
| 5. Days ahead | HeatRisk, if your area has it | Seven-day colour and number, experimental | Use it to choose the cooler day |
Where an umbrella fits
This part is ours. The NWS page does not mention umbrellas, and an umbrella does not change the heat index. What it does is make shade you can carry, which is the thing OSHA’s list asks sites to provide and the thing step 2 says you are missing in open sun. It suits standing, queueing and walking. It does not replace water, rest or an earlier start, and it is the wrong tool in a strong hot gust, so check our wind speed chart before you open one in the wind.
For the sun itself, our UV index chart is the companion sheet, and sun hat, sun umbrella or UV clothing compares the three ways to carry shade.
One option
Disclosure: xzvzx and Calm Eden have the same owner. Calm Eden’s guide to an umbrella for Florida and Texas summers applies the same sun-and-heat question to a hot, humid climate. Its Haven Umbrella has an SPF 50+ canopy that Calm Eden says blocks 99.99% of UV and keeps it up to 8°C / 15°F cooler underneath, opens and closes with one press and folds to 40 cm. Those are the maker’s statements, not NWS figures.
In short
- Read the feels-like number, then add the sun.
- Plan for shade, an earlier start and rest, as OSHA’s list does.
- Count a hot, dry gust as a hazard, not relief.
Source: National Weather Service, Heat Forecast Tools, read on 31 October 2026. The sheet’s decision notes are our own.
Common questions
What is the heat index?
The National Weather Service describes it as a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. Its example: an air temperature of 96°F with 65% relative humidity gives a heat index of 121°F.
Does standing in the sun change the heat index?
The NWS says heat index values were devised for shady, light wind conditions, so exposure to full sunshine can increase them by up to 15°F. That is the reason to ask where you will be standing, not only what the forecast says.
Does the NWS say anything about umbrellas?
No. The page does not mention umbrellas. It does say OSHA recommends providing shade at outdoor work sites. Treating an umbrella as portable shade is our reading, and we do not put a number on how much it helps.
What is WBGT?
Wet Bulb Globe Temperature uses temperature, humidity, wind, solar radiation and other weather parameters. The NWS calls it a particularly effective indicator of heat stress for active people such as outdoor workers and athletes.
Is wind a relief in the heat?
Not always. The NWS says strong winds, particularly with very hot, dry air, can be extremely hazardous. A breeze that feels welcome on a mild day is not the same thing in very hot, dry air.