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Nowcast (Air Quality Index) : ウィキペディア英語版
Nowcast (Air Quality Index)

The NowCast is a weighted average of hourly air monitoring data used by the United States Environmental Protection Agency for real-time reporting of the Air Quality Index (AQI) for PM (PM10 or PM2.5) data.
The NowCast is computed from the most recent 12 hours of PM monitoring data, but the NowCast weights the most recent hours of data more heavily than an ordinary 12-hour average when pollutant levels are changing. The NowCast is used in lieu of a 24-hour average PM concentration in the calculation of the AQI until an entire calendar day of hourly concentrations has been monitored.〔(【引用サイトリンク】title=Air Quality 101 )〕〔(【引用サイトリンク】title=How Does AirNow Make the “Current PM2.5” Air Quality Index (AQI) maps? )〕〔
〕〔(【引用サイトリンク】publisher=US Environmental Protection Agency )
==Calculation of the NowCast==
Let c1,c2,...c12 represent the hourly PM concentrations for the most recent 12-hour period, with c1 the most recent hourly value, and let cmin and cmax represent the minimum and maximum hourly concentration for the 12-hour period.


Define:
:w^
*=\frac}

and let
:w =
\begin
w^
*& \mbox w^
* > \frac, \\
\frac & \mbox w^
*\le \frac. \\
\end

With these definitions the NowCast is given by:
:NowCast = \frac w^ c_}w^}.

For the special case where there is no variability in the hourly values, cmin = cmax, w = 1, and the NowCast reduces to the twelve-hour average:
:NowCast = \frac c_i}.

For the special case where w=1/2:
:NowCast = \frac})c_2+...+})^
+(})^+...+})^}

But 1/(1-x)=1 + x + x2+ ... , x < 1, so to a good approximation, when w = 1/2:
:NowCast = \fracc_1+(})^2 c_2+...+})^{c_{12}}

Because the most recent hours of data are weighted so heavily in the NowCast when PM levels are changing, EPA does not report the NowCast when data is missing for c1 or c2.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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