The NowCast formula shows the current air quality index for the main pollutant – fine particulate matter PM2.5. NowCast was developed by the United States Environmental Protection Agency (US EPA) in such a way as to respond to rapid changes in air quality, for example, during a forest fire.
The formula shows air quality for the last hour using a calculation that takes into account previously recorded data. NowCast uses average values over a longer period of time when air quality is stable and averages over shorter periods when air quality changes rapidly, for example, during a fire. NowCast also reflects changes in air quality throughout the day.
You can learn more about the NowCast calculation algorithm at this link.