Weather Calculator
EverydayCalculate wind chill, heat index, or dew point temperature from air temperature, wind speed, and humidity. Includes comfort and danger zone interpretation.
Mode
35 °C
15 km/h
70 %
Feels Like
—
°C
Scale
Comfortable
Caution
Extreme Caution
Danger
Extreme Danger
Heat index formula is most accurate for temperatures ≥ 27 °C and humidity ≥ 40%.
What is a Weather?
A Weather Calculator computes three meteorological quantities that describe how weather actually feels on the human body rather than what a thermometer reads: the Heat Index (apparent temperature in hot and humid conditions), Wind Chill (apparent temperature in cold and windy conditions), and Dew Point (a measure of atmospheric moisture and comfort). These "feels like" figures are the standard tools of meteorology for communicating real thermal comfort and public health risk.
Air temperature alone is an incomplete description of thermal experience. At 40°C with 75% humidity — common in Mumbai in May — the heat index is approximately 61°C, a level that can cause heat stroke in minutes of outdoor exposure. At −5°C with a 40 km/h wind — winter conditions in Jammu or Himachal Pradesh mountain passes — the wind chill is approximately −17°C, cold enough to cause frostbite on exposed skin within 30 minutes. These are the numbers that matter for public health advisories, outdoor activity planning, and emergency management.
The Weather Calculator provides all three calculations in a single interface. Switch between Heat Index, Wind Chill, and Dew Point modes using the three toggle buttons. The result is displayed with a colour-coded danger/comfort category that contextualises the numerical output — a critical addition that makes the number actionable rather than abstract.
How to use this Weather calculator
- Select the Mode — choose Heat Index (temperature + humidity), Wind Chill (temperature + wind speed), or Dew Point (temperature + humidity) using the three mode buttons.
- Enter Air Temperature — the actual measured air temperature in °C. Note that wind chill is only meaningful below 10°C; heat index is only meaningful above 27°C with humidity ≥ 40%.
- Enter Wind Speed (Wind Chill mode) — the wind speed in km/h. For calm conditions, enter 0.
- Enter Relative Humidity (Heat Index and Dew Point modes) — the humidity percentage from a weather report or hygrometer.
- Read the result — the coloured result box shows the computed temperature and the danger/comfort category. The legend shows where current conditions sit on the full scale.
Formula & Methodology
Heat Index (Rothfuss-Steadman polynomial, NWS standard): Valid for T ≥ 27°C, RH ≥ 40%. All computations in °F, result converted to °C. HI (°F) = −42.379 + 2.04901523·T + 10.14333127·RH − 0.22475541·T·RH − 6.83783×10⁻³·T² − 5.481717×10⁻²·RH² + 1.22874×10⁻³·T²·RH + 8.5282×10⁻⁴·T·RH² − 1.99×10⁻⁶·T²·RH² Wind Chill (NWS 2001 revision): Valid for T ≤ 10°C, V > 4.8 km/h. WC (°C) = 13.12 + 0.6215·T − 11.37·V^0.16 + 0.3965·T·V^0.16 Dew Point (Magnus approximation): γ = ln(RH/100) + (17.625·T) / (243.04 + T) Td = 243.04·γ / (17.625 − γ) Worked example (Heat Index): A construction site safety officer in Bhubaneswar checks conditions for the morning shift on a June day: air temperature 38°C (100.4°F), relative humidity 72%. Converting: T = 100.4°F, RH = 72% Applying the Rothfuss polynomial: HI ≈ 143°F ≈ 61.7°C This falls squarely in the Extreme Danger category (above 54°C). Per the site safety protocol, mandatory 30-minute rest breaks every hour with shade and 500 mL water are required, and work above 3 metres is suspended until the heat index drops below 40°C (typically 5–6 PM).
Frequently Asked Questions
What is the heat index and how is it different from air temperature?
The heat index (also called the 'feels like' temperature) combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity reduces the effectiveness of sweat evaporation, which is the body's primary cooling mechanism, making the same air temperature feel significantly hotter. At 38°C with 80% humidity, the heat index can reach 55°C or higher.
What is wind chill and when does it matter?
Wind chill is the perceived decrease in air temperature felt on exposed skin due to wind. Moving air accelerates heat loss from the skin surface, making cold temperatures feel even colder. The NWS wind chill formula applies when air temperature is at or below 10°C and wind speed is above 4.8 km/h. Below those thresholds, wind chill equals the actual temperature.
What is dew point, and why is it a better measure of humidity comfort?
Dew point is the temperature at which air must be cooled (at constant pressure) for water vapour to condense into dew. Unlike relative humidity (which changes with temperature even when moisture content is constant), dew point directly reflects the actual moisture content of the air. Dew points below 10°C feel dry and comfortable; above 20°C feels humid; above 24°C is oppressive by most standards.
What is the heat index formula?
The calculator uses the Rothfuss-Steadman polynomial regression formula, which is the NWS standard for heat index calculation. It requires air temperature ≥ 27°C and relative humidity ≥ 40% for accurate results. The formula is: HI = −42.379 + 2.04901523T + 10.14333127RH − 0.22475541TR − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² − 0.00000199T²RH², where T is in °F and RH is in percent.
What is the formula for wind chill?
The NWS wind chill formula (2001 revision) is: WC = 13.12 + 0.6215T − 11.37 × V^0.16 + 0.3965T × V^0.16, where T is air temperature in °C and V is wind speed in km/h. This formula is valid for T ≤ 10°C and V > 4.8 km/h. For higher temperatures or calm conditions, wind chill equals the air temperature.
What is the dew point formula?
The calculator uses the Magnus approximation: γ = ln(RH/100) + (17.625 × T) / (243.04 + T), then Td = 243.04 × γ / (17.625 − γ), where T is air temperature in °C, RH is relative humidity in percent, and Td is the dew point in °C. This approximation is accurate to within ±0.35°C for most atmospheric conditions.
What are dangerous heat index levels?
The NWS danger scale for heat index (in °C): below 27°C — comfortable; 27–32°C — Caution (fatigue possible); 32–41°C — Extreme Caution (heat cramps/exhaustion possible); 41–54°C — Danger (heat cramps/exhaustion likely, heat stroke possible); above 54°C — Extreme Danger (heat stroke highly likely). During India's summer (April–June), heat index values regularly reach the Danger category in regions like Rajasthan, Vidarbha, and the Gangetic Plain.
How does humidity affect comfort in Indian cities?
Mumbai, Chennai, Kolkata, and coastal cities experience high humidity (70–90% in monsoon season), which makes 35°C feel like 45–50°C on the heat index scale. Inland cities like Jaipur and Ahmedabad experience extreme dry heat — 45°C with 10% humidity — which feels slightly lower than the actual temperature due to effective sweat evaporation. The Weather Calculator shows this distinction clearly by comparing heat index (humid conditions) against actual temperature.
What dew point is considered comfortable in India?
Dew points below 15°C are comfortable and dry (typical of Delhi winters and hill stations year-round). Dew points between 15–20°C are comfortable for most people. Above 20°C begins to feel muggy — typical of pre-monsoon conditions in many Indian cities. Dew points above 24°C are oppressive and common in coastal cities during monsoon; above 27°C is extreme and can be dangerous for outdoor exertion.
Can the Weather Calculator be used for planning outdoor activities?
Yes — use the heat index mode to assess whether an outdoor cricket match, trekking, or construction work at a given temperature and humidity level presents a heat stress risk. Use the wind chill mode for cold weather planning (treks in Himachal, Uttarakhand, Ladakh). Use the dew point mode to understand whether humidity will feel oppressive during monsoon festivals or outdoor events.
Why does my 'feels like' temperature on a weather app sometimes differ from this calculator?
Weather apps use proprietary algorithms that may switch between heat index and wind chill based on the season, or blend multiple formulas. Some apps also account for solar radiation (sunshine on clear days adds heat) and shade, which the standard NWS formulas do not include. The Weather Calculator uses the official NWS formulas without solar radiation adjustment, giving a standard baseline feels-like temperature.