The 12 fields and concepts you'll meet in the response — defined in plain English, each with a real example value.
A measure of the warmth or coldness of the atmosphere, typically expressed in Celsius (°C) or Fahrenheit (°F).
Weather APIs usually provide both actual temperature and "feels like" temperature. Actual temperature is measured by thermometers in shaded, ventilated enclosures. Feels-like temperature (apparent temperature) adjusts for humidity and wind effects on human perception.
The apparent temperature that accounts for humidity and wind, reflecting how the temperature actually feels to humans.
In hot conditions, high humidity makes it feel hotter because sweat evaporates slower. In cold conditions, wind makes it feel colder due to increased heat loss (wind chill). Feels-like is often more useful than actual temperature for user-facing features.
ExampleAt 30°C with 80% humidity, feels-like might be 38°C.
The amount of water vapor in the air, typically expressed as relative humidity (percentage of maximum moisture the air can hold).
100% relative humidity means the air is saturated. The same amount of moisture produces different relative humidity at different temperatures because warm air can hold more water vapor. For comfort, 30-50% relative humidity is ideal indoors.
Examplehumidity: 65 (meaning 65% relative humidity)
The temperature at which air becomes saturated with moisture and dew forms.
Dew point is a better comfort indicator than relative humidity. Below 50°F (10°C) feels dry and comfortable. 50-60°F (10-15°C) is comfortable. 60-65°F (15-18°C) feels slightly humid. Above 65°F (18°C) feels muggy. Above 70°F (21°C) is oppressively humid.
ExampledewPointC: 18 indicates moderately humid conditions.
The pressure exerted by the atmosphere, measured in millibars (mb) or inches of mercury (inHg).
Standard sea-level pressure is 1013.25 mb (29.92 inHg). Rising pressure generally indicates improving weather. Falling pressure suggests deteriorating conditions or approaching storms. Rapid pressure drops (3+ mb in 3 hours) often precede significant weather events.
The perceived decrease in temperature felt due to wind, calculated at temperatures below 50°F (10°C).
Wind increases heat loss from skin, making cold temperatures feel colder. Wind chill is only calculated below 50°F (10°C) and at wind speeds above 3 mph. At 20°F (-6°C) with 20 mph wind, wind chill is about 4°F (-15°C).
ExampleAt tempF: 25 with wind 15 mph, wind chill is approximately 13°F.
The maximum distance at which objects can be clearly seen, measured in kilometers or miles.
Visibility is reduced by fog, rain, snow, dust, smoke, and haze. Good visibility is 10+ km (6+ miles). Poor visibility below 1 km (0.6 miles) affects driving safety. Fog is defined as visibility below 1 km. Aviation uses more granular visibility categories.
A standardized scale (0-500) indicating air pollution levels and associated health effects.
AQI is calculated from multiple pollutants (PM2.5, PM10, O3, NO2, SO2, CO), with the highest individual pollutant value determining the overall AQI. Different countries use different scales, but the US EPA scale (0-500) is most common in APIs.
Exampleaqi: 42 indicates "Good" air quality with minimal health risk.
Fine particulate matter with diameter less than 2.5 micrometers, small enough to penetrate deep into lungs.
PM2.5 is typically the most health-relevant air quality metric. Sources include vehicle exhaust, industrial emissions, wildfires, and cooking. Long-term exposure is linked to respiratory and cardiovascular disease. WHO guidelines recommend annual average below 5 µg/m³.
Examplepm25: 12.5 (measured in micrograms per cubic meter)
A gas pollutant formed when vehicle and industrial emissions react with sunlight.
Ground-level ozone is harmful to breathe (unlike stratospheric ozone which protects from UV). Ozone levels typically peak in afternoon on hot, sunny days. It irritates respiratory systems and worsens asthma. High ozone alerts recommend limiting outdoor exertion.
Exampleo3: 45 (measured in parts per billion or µg/m³)
A scale measuring the intensity of ultraviolet radiation from the sun that causes sunburn.
The scale is open-ended but typically ranges 0-11+. UV 0-2 is low risk, 3-5 moderate, 6-7 high, 8-10 very high, 11+ extreme. UV varies by time of day, season, latitude, altitude, and cloud cover. Peak UV occurs around solar noon.
ExampleuvIndex: 7 indicates high UV requiring sun protection.
The likelihood (as a percentage) that precipitation will occur at a location during a forecast period.
A 30% chance of rain means roughly 30 out of 100 identical forecast situations would produce rain. It does NOT mean 30% of the area will see rain, or that it will rain 30% of the time. Higher probability doesn't indicate heavier rain—use accumulation forecasts for amount.
ExampleprecipProbability: 40 (40% chance of precipitation)
