Weather calculations are only as reliable as their inputs and assumptions. Keep units explicit, use heat index and wind chill only within their documented ranges, and label calculated indices as estimates—not direct measurements. This guide uses National Weather Service (NWS) conventions and explains where the available documentation supports code and where it does not.
Keep temperature units explicit in code
Temperature conversion includes an offset, so Fahrenheit and Celsius cannot be converted by multiplying alone. The NWS meteorological calculator uses these equations:
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F = C * 9/5 + 32C = (F - 32) * 5/9
Use names or types that make the units clear, such as celsiusToFahrenheit(celsius) and fahrenheitToCelsius(fahrenheit). Avoid a generic function such as convertTemperature(value) when the input and output scales are not otherwise explicit. NWS conversion equations and help.
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How do I calculate dew point in code?
Choose and document a specific dew-point method before implementing it. The NWS calculator provides dew-point and related moisture calculations, but the cited calculator information does not establish a single general-purpose equation, set of constants, or phase convention that can safely be prescribed here.
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Pay particular attention to input assumptions: the NWS dew-point calculator identifies actual station pressure as an input for its associated moisture calculations; an altimeter setting is not interchangeable with actual station pressure for that purpose. Record the method, units, required inputs, and assumptions alongside any result so another developer can interpret or reproduce it. Consult the NWS dew-point calculator and the NWS weather-calculator index for the calculator context.
What does relative humidity tell you?
Relative humidity (RH) expresses atmospheric moisture relative to the amount of moisture air can hold at saturation. That saturation amount changes with temperature, so RH by itself is not a direct measure of the actual amount of water vapor in the air. It is also an input to heat-index calculations, where the NWS convention expresses RH as a percentage. NWS meteorological calculator help.
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What is the NWS heat-index formula?
The NWS technical document gives the Rothfusz regression below. In this convention, T is air temperature in degrees Fahrenheit and R is relative humidity in percent—not a fraction from 0 to 1.
HI = -42.379 + 2.04901523*T + 10.14333127*R - 0.22475541*T*R - 0.00683783*T^2 - 0.05481717*R^2 + 0.00122874*T^2*R + 0.00085282*T*R^2 - 0.00000199*T^2*R^2
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The technical document warns that this polynomial gives inappropriate results below 80°F. It also describes a low-humidity adjustment for RH below 13% when T is between 80°F and 112°F, and a high-humidity adjustment when RH exceeds 85% and T is between 80°F and 87°F. Those are method details in the cited document, not a substitute for checking current NWS operational recommendations before treating the adjustments as normative in an application. NWS heat-index technical document.
Heat index is a simplified estimate based on air temperature and RH for shady conditions. It is not a complete measure of heat stress: direct sunlight and other environmental factors matter. The NWS notes that direct sunlight can add as much as 15 degrees to heat index, but that is explanatory context, not a reliable correction to apply to an individual reading. NWS WBGT guidance.
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When does the wind-chill formula apply?
The NWS wind-chill equation uses air temperature T in degrees Fahrenheit and wind speed V in miles per hour:
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NWS defines wind-chill temperature only when air temperature is at or below 50°F and wind speed is above 3 mph. In code, validate those conditions and return an unavailable or out-of-domain result when they are not met; silently extrapolating the equation gives an unsupported result. The index estimates heat loss from exposed skin and assumes no impact from the sun. NWS guidance says bright sunshine may increase wind-chill temperature by 10°F to 18°F; do not apply that range as a guaranteed adjustment for a particular person or exposure. NWS Understanding Wind Chill.
Choose the calculation that matches the question
| Calculation | Inputs and units | Scope and interpretation |
|---|---|---|
| Temperature conversion | Celsius or Fahrenheit, as named by the input; output scale determined by the conversion | Unit conversion; use the offset equations above. |
| Dew point or related moisture calculation | Depends on the selected method; the NWS dew-point calculator identifies actual station pressure for associated moisture calculations | Describes atmospheric moisture; document the method and its assumptions. |
| Heat index | Temperature in °F and RH as a percentage | Shade-based estimate; the cited polynomial is inappropriate below 80°F. |
| Wind chill | Temperature in °F and wind speed in mph | Cold-weather estimate, defined by NWS only at or below 50°F with wind above 3 mph. |
These calculations answer different questions. Temperature conversion changes a unit scale; dew point and RH describe atmospheric moisture; heat index combines temperature and RH; wind chill combines temperature and wind. Do not treat a “feels like” index as an observed temperature or substitute one index for another. The NWS meteorological calculator documents input conventions for several of these calculations.
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