Weather Unit Conversions
Weather forecasts use different units globally. The US uses °F and mph; most of the world uses °C and km/h; aviation uses knots and hPa. Knowing the conversions helps you interpret any forecast.
Temperature
°F to °C: (°F - 32) × 5/9
°C to °F: (°C × 9/5) + 32
Key references:
0°C = 32°F (freezing)
10°C = 50°F (cold)
20°C = 68°F (comfortable)
30°C = 86°F (warm)
37°C = 98.6°F (body temperature)
100°C = 212°F (boiling)Wind Speed
1 mph = 1.609 km/h = 0.447 m/s = 0.869 knots
1 knot = 1.852 km/h = 1.151 mph
Beaufort scale:
3 (gentle breeze): 12-19 km/h, leaves move
6 (strong breeze): 39-49 km/h, large branches
10 (storm): 89-102 km/h, structural damage
12 (hurricane): ≥ 118 km/hAtmospheric Pressure
Standard sea-level pressure: 1013.25 hPa = 29.92 inHg
1 hPa = 1 mbar = 100 Pa
High pressure (fair weather): > 1020 hPa
Low pressure (storms): < 990 hPaConvert weather units: Free Weather Unit Converter
Weather Unit Conversion Quick-Reference Table
| Measurement | Unit A | Unit B | Conversion |
|---|---|---|---|
| Temperature | 0°C | 32°F | °F = (°C × 9/5) + 32 |
| Temperature | 100°C | 212°F | °C = (°F − 32) × 5/9 |
| Wind speed | 1 m/s | 3.6 km/h = 2.237 mph | m/s × 3.6 = km/h |
| Wind speed | 1 knot | 1.852 km/h = 1.151 mph | knots × 1.852 = km/h |
| Pressure | 1 atm | 1,013.25 hPa = 29.92 inHg | hPa × 0.02953 = inHg |
| Precipitation | 1 inch | 25.4 mm | inches × 25.4 = mm |
| Visibility | 1 mile | 1.609 km | miles × 1.609 = km |
How Weather Unit Conversions Work
Weather reports use different unit systems depending on the country. The US and its territories primarily use Fahrenheit, mph (wind), inches of mercury (pressure), and inches (precipitation). Most of the world uses Celsius, km/h or m/s (wind), hPa/mbar (pressure), and millimetres (precipitation). Aviation globally uses knots for wind speed and feet for altitude regardless of country. Marine forecasts worldwide use knots.
The Beaufort scale provides a descriptive wind classification: Beaufort 0 = calm (< 1 km/h); 6 = strong breeze (39–49 km/h); 9 = severe gale (75–88 km/h); 12 = hurricane (≥ 118 km/h). Wind chill (apparent temperature) combines air temperature and wind speed: the Wind Chill Index (US/Canada) uses the formula T_wc = 13.12 + 0.6215T − 11.37V^0.16 + 0.3965T × V^0.16, where T is °C and V is wind speed in km/h.
Common Mistakes
- Confusing dew point and relative humidity: Relative humidity (%) changes with temperature even if moisture content is constant — warm air can hold more water vapour. Dew point temperature (°C/°F) directly measures moisture content and is more useful for assessing how humid a day actually feels. A 25°C day with 60% RH and a 15°C day with 100% RH have the same dew point (~17°C).
- Misreading barometric pressure trends: Absolute pressure values matter less than the trend. Rapidly falling pressure (>3 hPa in 3 hours) indicates approaching low-pressure systems and deteriorating weather regardless of the absolute value.
- Confusing mph (miles per hour) with mph (knots): Knots (nautical miles per hour) are 1.151 mph, not mph. A 30-knot wind is 34.5 mph (55.6 km/h) — a meaningful difference for storm intensity assessments.
Frequently Asked Questions
Weather is the short-term atmospheric state at a specific location — temperature, precipitation, cloud cover, wind — measured in hours to days. Climate is the long-term average of weather patterns over 30+ years (the standard climate period defined by WMO is 30 years). "Climate is what you expect; weather is what you get." Weather is chaotic and hard to predict beyond ~10 days; climate trends are statistically stable and measurable over decades.
The heat index (apparent temperature) combines air temperature and relative humidity to estimate how hot it actually feels. High humidity prevents sweat evaporation — the body's main cooling mechanism — making it feel hotter. At 35°C with 90% humidity, the heat index is approximately 52°C — potentially dangerous. The formula is the Rothfusz regression equation used by NOAA; the simplified version is only valid above 27°C and 40% RH.
The Saffir-Simpson Hurricane Wind Scale classifies Atlantic hurricanes by sustained wind speed: Category 1 = 119–153 km/h (74–95 mph); Category 3 = 178–208 km/h (111–129 mph, major hurricane); Category 5 ≥ 252 km/h (157 mph). The Enhanced Fujita (EF) Scale rates tornadoes from EF0 (minor damage, 105–137 km/h) to EF5 (catastrophic, > 322 km/h). These scales inform evacuation decisions and building code requirements in hurricane- and tornado-prone regions.