At a glance
What matters most
- 01
A forecast estimates future pollutant concentrations; it is different from an observation or final daily AQI.
- 02
Weather, emissions and atmospheric chemistry are major forecast inputs.
- 03
AirNow current ozone and particle readings use NowCast algorithms based on multiple recent hours.
- 04
Forecast uncertainty generally increases farther into the future and during rapidly changing events.
Observation, NowCast, forecast and final AQI
| Reading type | What it represents | Main limitation |
|---|---|---|
| Observation | A measured concentration for a location and time | May be provisional, delayed or geographically sparse |
| Current NowCast | Recent hours weighted to communicate current ozone or particle conditions | It is an algorithmic index, not one raw hourly measurement |
| Forecast | Modelled future pollutant concentration or AQI category | Depends on uncertain future weather and emissions |
| Final daily AQI | A completed daily value using the required pollutant method | Available only after the reporting period and validation process |
What goes into an air-quality forecast
- Emissions from transport, industry, fires, dust and other sources
- Wind, temperature, sunlight, rainfall, humidity and boundary-layer depth
- Chemical reactions that create or remove ozone and secondary particles
- Starting conditions from observations, satellite products or earlier model runs
- Terrain, land use and the model’s horizontal and vertical resolution
Why the forecast can change
A small change in wind direction can move a smoke plume or urban pollution away from the location originally forecast. Unexpected fire activity, dust, cloud cover, rain or emissions can also alter the outcome.
Forecasts are rerun as new weather predictions and observations arrive. An updated forecast replacing an older one is evidence that the system incorporated newer information, not necessarily that the earlier model was carelessly produced.
Model resolution is not street-level certainty
A model grid represents an area, while roadside traffic, construction, terrain and buildings can produce smaller local differences. Downscaling can add detail but cannot create observations where none exist.
For near-term decisions, compare the forecast with the latest local observation, dominant pollutant, wind and any smoke or dust advisories.
How to use a forecast responsibly
- Use the forecast to plan, then recheck current conditions before acting.
- Pay attention to the pollutant and category, not only the headline number.
- Expect wider uncertainty farther into the future and near sharp pollution boundaries.
- Use official emergency and public-health guidance during wildfire, dust or industrial events.
- Do not treat a forecast as evidence of regulatory compliance.
Frequently asked questions
Why is today’s AQI different from yesterday’s forecast?
Actual weather, emissions and pollution transport may have differed from the assumptions available when the forecast was produced.
Is a NowCast the same as a forecast?
No. A NowCast uses multiple recent measured hours to communicate current ozone or particle conditions, while a forecast estimates future conditions.
Are longer-range AQI forecasts less certain?
Generally yes. Uncertainty in weather, emissions and atmospheric transport grows as the forecast horizon extends.
Which forecast does AQI.news use?
AQI.news accesses atmospheric composition forecast data through the Open-Meteo Air Quality API, which provides fields based on Copernicus Atmosphere Monitoring Service products.
Primary sources
This reference was prepared from the following primary public-health and regulatory sources: