Heat index while hiking: what the number means and where it stops

The heat index is the temperature a person feels in shade and light wind when air temperature and relative humidity are combined. In the NWS wording, it is "a measure of how hot it really feels when relative humidity is factored in with the actual air temperature". It is a single number built from two inputs, so it answers the question a thermometer alone cannot: how hot a humid day feels.

What the heat index describes

The NWS defines the heat index as how hot it feels when relative humidity is factored in with the actual air temperature. That makes it a value for the body, not for the thermometer. It does not include the sun or the wind. The NWS notes that heat index values were devised for shady, light wind conditions, and that exposure to full sunshine can increase them by up to 15 °F.

The NWS also warns that strong winds, particularly with very hot, dry air, can be extremely hazardous. Because the heat index takes no wind input, that warning sits outside its numbers. The index is therefore one input to a day's plan, not a complete assessment of heat.

The formula, with °C input

The NWS computes the heat index in °F from air temperature T in °F and relative humidity RH in %. The simple equation comes first. It is itself an average of the air temperature and Steadman's simpler estimate, which is what the NWS means by averaging the result with the temperature:

simple HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}

If that simple result is 80 °F or higher, the Rothfusz regression replaces it, with its adjustments:

HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541T·RH − 0.00683783T²
     − 0.05481717RH² + 0.00122874T²RH + 0.00085282T·RH² − 0.00000199T²RH²

if RH < 13% and 80 ≤ T ≤ 112 °F:  subtract [(13 − RH) ÷ 4] × √{[17 − |T − 95|] ÷ 17}
if RH > 85% and 80 ≤ T ≤ 87 °F:  add [(RH − 85) ÷ 10] × [(87 − T) ÷ 5]

Trailwright takes air temperature in °C. Convert first with T °F = T °C × 1.8 + 32. The result is in °F, and converting back with °C = (°F − 32) × 5 ÷ 9 gives the figure in °C. The NWS states that the regression equation has an error of ±1.3 °F, so a result near a category boundary should be read as approximate.

NWS categories in °C

The NWS publishes its categories in °F. The table converts each boundary to °C using the same formula. The effect column is the NWS wording, which describes what each band can bring with prolonged exposure or activity.

The arithmetic behind the °C column is straightforward: 80 °F is (80 − 32) × 5 ÷ 9 = 26.7 °C, 103 °F is (103 − 32) × 5 ÷ 9 = 39.4 °C, and 124 °F is (124 − 32) × 5 ÷ 9 = 51.1 °C.

−42.38 + 183.59 + 608.60 − 1,208.29 − 54.90 − 197.34 + 591.87 + 275.09 − 57.51
= 98.7 °F = (98.7 − 32) × 5 ÷ 9 = 37.1 °C

The result, 98.7 °F, sits in the NWS Extreme Caution band. The NWS reading assumes shade and light wind. If the same air were in full sun, the NWS allowance of up to 15 °F would take the figure to at most 113.7 °F (98.7 + 15). That is an upper bound on the NWS statement, not a computed sun value.

Where the regression does not apply

The Rothfusz regression is only used when the simple result reaches 80 °F. Below that, the NOAA page says the regression "is not appropriate" and that a simpler formula is applied, "consistent with Steadman's results". The same page adds that the regression "is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman".

Take 24 °C and 50% humidity. In °F that is 75.2 °F. The simple equation gives 74.8 °F (23.8 °C), below 80, so the regression is not used. Under the site's rules, neither the heat index nor the wind chill applies at that reading, and the air temperature is the reference.

The adjustments matter at the edges. The dry-air adjustment reduces the regression value when humidity is below 13% between 80 °F and 112 °F. The humid adjustment adds to it when humidity is above 85% between 80 °F and 87 °F. Neither applies at 32 °C and 60%, which is why the worked example has no adjustment.

Sun, wind and WBGT

The heat index is a shade-and-still-air number, so it understates heat on an open slope in the sun. The NWS heat safety page also describes Wet Bulb Globe Temperature (WBGT), a measure that accounts for sun and wind as well as temperature and humidity. It states that WBGT uses temperature, humidity, wind, solar radiation and other weather parameters, and that it is an effective indicator of heat stress for active people such as outdoor workers and athletes. The NWS also notes that WBGT can be used to inform activity changes during exercise or outdoor work.

Trailwright does not compute WBGT, because the site's tools do not take solar radiation as an input. If your route has long sun exposure, check the NWS WBGT forecast for your area rather than reading the heat index as the full picture.

Using the category in a plan

The categories are most useful for timing. A Caution-band afternoon argues for an earlier start and a shorter day. For an Extreme Caution or Danger forecast, the reasonable choice is to change the plan before you leave, not on the trail. A turnaround time set in advance gives a fixed point to stop, whatever the heat feels like at that moment.

Check the ten essentials before a hot day. If the route climbs, the air is cooler higher up, and the altitude and temperature guide shows by how much. Use the air temperature and humidity forecast for the elevation you will be at, because the formula takes whatever values you enter.

The cold-side counterpart of the same kind of reading is in the wind chill guide, which uses wind rather than humidity. Carry enough water for the day's temperature, and choose a treatment method using the filter and purifier comparison.

For heat illness, follow the NWS heat safety guidance and local emergency services. The heat index does not tell you what to do for a person who is unwell.