Averaged series

An air-quality standard does not speak of instants: it speaks of averages — one hour, eight moving hours — because that is how air is measured and regulated. The demo’s forecast, by contrast, has a value every 15 minutes. Averaged series are the bridge between the two.

What is computed

For every species with an entry in the demo’s standard, the platform builds, when the forecast is published:

• The hourly average, clock-aligned — one figure per hour, at the top of the hour.
• The 8-hour moving average — at each step, the mean of the preceding eight hours.

No 24-hour average is computed, even for particulate matter, whose standard is written that way. A 24-hour mean of a forecast averages hours that have not happened yet, flattens the very peak an operator decides against, and is only complete once the day is over — by which time it is no longer a forecast. It is the right statistic looking back over hours that did happen, which is a consulting question rather than a forecasting one.

All are computed for the receptors and for the whole grid, in every scenario. From them, also at publication, come each species’ margin — the averaged maximum against the standard’s value, as a percentage — and the grid point where that maximum occurs.

The annual and percentile forms some standards use are not computed; the evaluation is limited to what the registered standards require today.

Instant and average, side by side

The map and the markers are instantaneous: the value of the step shown, and the alert levels compared step by step — the demo’s own thresholds, chosen so the forecast reads. The averages are where the standard is read:

• The Regulatory panel (hidden for now; the rail carries no button for it) puts the receptor’s instantaneous value, its current hour’s mean and the norm on three lines, with the margin as a percentage of the norm. It is an hourly reading: a species whose norm is a 24-hour mean reads as not applicable there.
• The Series panel’s builder offers each curve as the instant value, the 1-hour mean, the 8-hour moving mean, and draws the norm as a line where the standard has one for that species in one of those forms.
• The Controls panel can step the forecast by the hour instead of by 15 minutes, so the regulatory reading is walked the way the standard is written.
• An Alerts row that comes from the standard is tagged Regulatory.

The two readings can disagree on purpose: a receptor can show an alert on an instant and sit well inside the norm over the hour, or the reverse. Both are true; they answer different questions.

→ Air-quality standards · Units and species