What Does AQI Mean in a Weather Forecast or Weather App?
The number next to the temperature
Most phone weather apps and many TV forecasts now display an air quality reading — usually a number and a color — alongside temperature and rain chances. That number is the Air Quality Index (AQI), the same 0–500 scale used by U.S. government agencies to describe outdoor air pollution. Lower is cleaner; higher means more pollution. The scale and its color categories are explained on AirNow, the federal government's air quality site, and in plain-language form by the American Lung Association.
The AQI is not itself a weather measurement. It is calculated from pollutant concentrations — typically ground-level ozone and fine particles (PM2.5), the two pollutants that most often drive the daily number. If you want the full walkthrough of the categories and colors, see our guide on how to read the AQI.
Why air quality appears in a weather forecast
Air quality ends up in weather forecasts because weather largely controls it. The pollution emitted in a city can be roughly similar from one weekday to the next, yet the AQI can swing dramatically, because the atmosphere decides what happens to those emissions:
- Sunlight and heat drive the chemical reactions that create ground-level ozone, which is why ozone problems cluster on hot, sunny afternoons.
- Wind disperses pollution when it blows and lets it accumulate when it doesn't. Calm, stagnant days concentrate whatever is being emitted.
- Temperature inversions — a layer of warm air sitting on top of cooler surface air — act like a lid, trapping pollutants near the ground. They are common on clear nights and in valley towns in winter.
- Smoke transport: wildfire smoke can ride upper-level winds for hundreds of miles, which is how a fire in one state raises PM2.5 readings in another.
The National Weather Service discusses the weather–air quality connection, and the roles of ozone and particle pollution, on its air quality safety page. NWS forecasters also issue air quality alerts in partnership with state and local environmental agencies, which is why those alerts often arrive through the same channels as storm warnings.
Forecast AQI vs. current AQI
One detail worth noticing in your app: some AQI readings are current conditions (based on the latest monitor or sensor data) and some are forecasts (a prediction of today's or tomorrow's peak). They can legitimately disagree. A forecast of 120 with a current reading of 55 usually means the bad hours are expected later in the day — ozone, in particular, tends to peak in the afternoon and fall off in the evening.
Apps also differ in which pollutant they display and how often they refresh, and some non-government apps use their own proprietary indexes that resemble, but don't exactly match, the official U.S. AQI. When two apps show different numbers for the same city, that is usually why. The official U.S. figures are the ones published through AirNow.
What the colors are telling you
The official index divides air quality into six named categories, from "Good" (green) through "Hazardous" (maroon), with the descriptions for each level published by AirNow. The category names are standardized, which is the point: "Unhealthy for Sensitive Groups" means the same thing in Phoenix as it does in Pittsburgh. Health questions about what a given level means for you personally are best directed to those official category descriptions and to your own clinician — the index is a reporting tool, and the American Lung Association's guide is a good plain-English companion to it.
If you're curious what pollutants the number is actually built from, our overview of the six criteria air pollutants covers each one.