Weather Calculators

Weather calculators on CalcMountain handle the standard derived meteorological quantities — heat index, wind chill, dew point — that surface in National Weather Service forecasts and that affect personal decisions about outdoor activity, comfort, and safety. The formulas we use are the same ones the NWS uses operationally.

The heat index calculator uses the Rothfusz regression equation, which is the NWS operational formula for the "feels like" temperature in hot, humid conditions. Heat index becomes meaningful above about 80°F; below that it tracks the actual temperature. Above 105°F, heat-related illness risk becomes significant, and above 130°F, immediate danger.

The dew point calculator uses the Magnus formula (a well-validated approximation to the August-Roche-Magnus equation) to compute dew point from temperature and relative humidity. Dew point is a more intuitive measure of how humid the air feels than relative humidity — a 70°F dew point feels muggy to most people regardless of temperature, while 50°F is comfortable and 40°F dry.

The sunrise/sunset calculator uses the standard algorithms from the U.S. Naval Observatory's Astronomical Almanac to compute civil sunrise, sunset, and solar noon for any latitude/longitude and date. Civil twilight (Sun 6° below horizon) is included as the most useful "still light enough to read outside" boundary.

Where appropriate, we cross-reference the NOAA National Weather Service operational formulas. For aviation-specific weather products (METAR, TAF parsing, density altitude), specialized pilot tools are more appropriate than our general-purpose calculators. For severe weather products (storm tracking, lightning detection, precipitation forecasts), NWS Weather Prediction Center and Storm Prediction Center are the authoritative sources.

These calculators are educational and recreational tools. For weather-sensitive decisions (severe weather safety, agricultural irrigation timing, aviation flight planning), use authoritative meteorological products from NOAA, your country's national weather service, or a certified meteorologist.