Weather calculators
A thermometer only tells part of the story. Humidity decides whether sweat can cool you, wind decides how fast heat leaves exposed skin, and dew point decides whether the air feels sticky and whether condensation will form on a cold surface. These calculators convert raw readings into the apparent conditions that actually govern comfort, safety and building performance.
3 calculators in this category
Most used weather calculators
Why apparent temperature differs from the reading
The body loses heat by evaporation, convection, radiation and conduction, and weather changes the balance between them. Humid air blocks evaporation, so 90°F at 85% humidity feels close to 110°F. Wind strips away the insulating boundary layer of warm air against the skin, so 5°F with a 25 mph wind cools exposed skin as fast as −18°F still air would. Both effects are large enough to change what is safe to do outdoors.
These indices apply to people, not objects. Wind chill never cools a water pipe or a parked car below the actual air temperature — it only makes them reach it faster. Only a body generating its own heat experiences the difference, which is why frostbite guidance and freeze-protection guidance use different numbers.
Standard models and their limits
The heat index uses the Rothfusz regression adopted by the US National Weather Service, valid at or above 80°F. Wind chill uses the 2001 NWS and Environment Canada index, valid at or below 50°F with winds above 3 mph. Dew point uses the Magnus-Tetens approximation with the Alduchov-Eskridge coefficients, accurate to about 0.1°C across all terrestrial conditions. Each calculator reports the plain temperature outside its model's valid range rather than producing a confident nonsense figure.
Every model carries assumptions. Heat index assumes shade and light wind, so direct sunlight can add up to 15°F. Wind chill assumes a face at standard measurement height. Neither adjusts for age, medication, acclimatisation or fitness, all of which shift individual risk substantially. For regulated outdoor work, wet bulb globe temperature is the measure occupational standards actually specify.
Dew point does double duty
As a comfort measure, dew point is more informative than relative humidity because it is absolute — 20°C dew point feels muggy whatever the air temperature, whereas 70% humidity feels entirely different at 5°C and at 30°C. Meteorologists watch the dew point, not the humidity percentage, for exactly this reason.
As a building measure, it sets the condensation threshold. Any surface colder than the dew point will collect water, which is the mechanism behind misted windows, sweating pipes and hidden damp inside wall cavities. Holding indoor humidity low enough that the dew point stays below the coldest interior surface is the whole of condensation control.
Every weather calculator
Weather calculator questions
Why does the heat index calculator use Fahrenheit?+
The Rothfusz regression is defined with Fahrenheit coefficients, so the calculation is performed in Fahrenheit and the Celsius equivalent is converted for display.
Does wind chill affect pipes and cars?+
No. Wind makes objects reach the air temperature faster but never colder than it. Only living tissue producing its own heat experiences wind chill.
Is heat index the same as wet bulb globe temperature?+
No. WBGT also accounts for solar radiation and air movement and is the measure occupational safety standards specify. Heat index assumes shade and is a general-public guide.
What indoor humidity prevents window condensation?+
Keep the dew point below your coldest surface. In winter with single glazing that usually means holding indoor relative humidity at 40% or below.