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Dew Point, Heat Index, and Wind Chill on Weather Sites

Dew point, heat index, and wind chill on weather sites: required inputs, NWS definitions, validity limits, and why an index is not a thermometer reading.

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Personal weather sites often print dew point, heat index, and wind chill beside the thermometer. Only one of those is a state variable of the air. The others are indexes: scenario calculations with stated inputs and validity limits. This notebook defines each quantity, lists required inputs, and shows where a site should refuse to print a number. Compass encoding and mph/knot conversion are a different notebook.

The historical URL returned a 404. The notebooks index still pointed here for “PHP calculate” helpers. The honest modern article is the psychrometric and apparent-temperature layer those helpers were meant to feed—not a restored error page and not a dump of copyrighted coefficient tables.

Historical context

Station PHP in the 2000s often converted units, then applied NWS-style apparent-temperature formulas. Weather Display also emits humidex and an “apparent temperature” slot in ClientRaw on many builds (field-family reference). Those packet fields are already derived. Re-deriving them without matching the vendor’s equation and limits will disagree with the dashboard.

Three different kinds of number

| Quantity | Kind | What it answers | |---|---|---| | Dew point | Thermodynamic moisture variable (derived from T and humidity, or measured) | How much cooling is needed to reach saturation at constant pressure | | Heat index | Apparent-temperature index (NWS / Rothfusz implementation of Steadman) | How hot it feels in shade, given temperature and humidity, inside a stated range | | Wind chill | Apparent-temperature index (NWS / Meteorological Service of Canada, 2001 implementation) | How cold exposed skin feels given temperature and wind, inside a stated range |

A thermometer does not read heat index. A cup anemometer does not read wind chill. Printing them in the same font as air temperature is acceptable only if the caption says “index.”

Dew point

Definition. Dew point is the temperature to which air must be cooled, at constant pressure, for water vapor to reach saturation. Relative humidity is the ratio of actual vapor pressure to saturation vapor pressure at the current temperature; dew point holds the vapor-pressure information in degrees.

Required inputs. Air temperature and one moisture variable: relative humidity, measured dew point, or vapor pressure. Hobby sites usually skip full pressure corrections; a Magnus-type saturation formula is then the dominant method error.

Process (conceptual). Compute saturation vapor pressure at T, scale by RH to get actual vapor pressure, invert to dew-point temperature. Pick one documented Magnus–Tetens coefficient set and use it in both directions so RH and dew point cannot drift.

Output. Dew point in the same unit as T, plus the formula family. At RH 100%, dew point equals T within rounding. Do not report 0.01° from a 3% RH sensor. Dew point above T usually means RH was stored as a fraction (0.45 vs 45) or the sensor is uncalibrated.

Heat index

Definition. NWS heat index is an apparent temperature intended to express the combined effect of air temperature and humidity on a human in a shaded, light-wind setting. It is based on Steadman’s apparent-temperature work as implemented for operations by Rothfusz (NWS Technical Attachment SR 90-23) and documented by NWS/WPC.

Required inputs. Air temperature and relative humidity. The operational U.S. implementation is in degrees Fahrenheit and percent. Convert SI inputs first; do not mix units inside the regression.

Validity (the part websites skip). The full Rothfusz regression is intended when the result is about 80°F or higher. NWS documents a simpler Steadman-consistent formula below that threshold, and additional adjustments at very low or very high humidity inside specified temperature bands. The regression is not valid for extreme T/RH combinations beyond the data Steadman considered. If you are outside the stated range, do not print a heat index. Print air temperature and RH, or an explicit n/a.

Process. Follow the official NWS/WPC procedure: evaluate the simple formula first; only if the blended result reaches the threshold apply the full regression and humidity adjustments. Source of record: WPC heat index equation and NWS heat-index explainer. This notebook does not reprint the coefficient table. If you cannot implement the official steps, link the NWS calculator and stop.

Output. An apparent temperature labeled “heat index,” with the NWS method named, or a missing value. Direct sun, strong wind, and heavy work are outside the scenario. Heat index can be lower than air temperature when humidity is very low. If you print 50°F heat index on a 50°F morning, the summer regression ran out of range. If the index is wildly above T at RH 10%, RH was probably stored as 0.1 versus 10.

Wind chill

Definition. NWS wind-chill temperature is an apparent-temperature index for how cold exposed skin feels given air temperature and wind. The United States and Canada implemented a joint index in 2001–2002 based on heat-transfer modeling of a face, with wind reduced from standard anemometer height to a typical face height, and with no solar radiation in the scenario.

Required inputs. Air temperature and wind speed. The NWS formula in common U.S. documentation uses °F and miles per hour. ClientRaw wind is typically knots (family reference); convert with the units notebook before applying the index.

Validity. NWS states that wind chill is defined for temperatures at or below 50°F and wind speeds above 3 mph. Below the wind threshold, report air temperature (calm). Above 50°F, do not report wind chill. Sunshine can make the outdoor sensation warmer than the index; the formula assumes a clear night-like radiative setting.

Official references:

Use those pages for the operational equation. Do not treat a copied coefficient line in a 2004 PHP file as the standard.

Output. Wind-chill temperature labeled as an index, or n/a when inputs are out of range.

Limitations. Wind chill is not air temperature and not a frostbite timer by itself; NWS pairs the chart with frostbite guidance on the official page. A 10-metre anemometer used as face-height wind makes the index too severe unless you apply the NWS height adjustment. Most PWS sites skip that; if you skip it, say so. Do not print wind chill above 50°F. If wind chill equals T at 20 mph, you likely passed knots into an mph formula. If calm nights explode to extreme values, a zero speed or missing-data zero reached the formula (wind notebook).

Related indexes you may see in station software

Humidex (Canadian; temperature and dew point) is not NWS heat index. Logger “apparent temperature” may include wind or radiation and will not match NWS heat index. Wet-bulb is a thermodynamic quantity, not heat index. Label the family if you print “feels like.”

Process: a site-wide calculation policy

  1. Store native observations: T, RH, wind speed, unit, height, time.
  2. Compute dew point if RH is the moisture input; otherwise store measured dew point.
  3. Gate heat index and wind chill on official validity ranges. Fail closed.
  4. Convert units once, before the index, with documented factors.
  5. Caption every index. Never replace T with “feels like” in the only temperature field.
  6. If the logger already computed an index, either display the logger value or your value, and say which. Do not silently average them.
  7. For live safety (heat wave, freeze), send readers to weather.gov. An index on a PWS is not a Heat Advisory.

Output

A clean conditions object looks like:

  • t_air, rh, tdew (observed/derived state)
  • heat_index or null + heat_index_method
  • wind_chill or null + wind_chill_method
  • wind_speed + unit + height
  • valid_flags

That object is suitable for a dashboard and for later comparison with NWS.

Modern relevance

Derived indexes are a distinct evidence type: useful, bounded, easy to mislabel as observations. TNET’s public pages separate observed, modelled, and derived information; see how the service works. That page does not publish TNET formulas.

Related: wind direction and units, bibliography methods, notebooks hub.

Sources