Friday, August 8, 2014
What do all these numbers and colors mean?
There is a lot of information about air quality available on the internet and it can be confusing to understand what it all means.
How we’re calculating air quality
The amount of smoke at any one place can change continuously during a day. At times smoke may be highly concentrated and visible and other times not.
We can measure the smoke levels in an area hour by hour and even down to minute by minute if we choose to. However, the EPA sets many health standards that are based on 24-hour averages. So in order to compare our measurements to the EPA standard we need to use 24 hours of data. We take that data and use a color ramp that is based on how close the measured values are to the EPA standard.
Because we use 24-hour averages there may be times when smoke may be heavy and visible in an area but the air quality map displays good.
How Washington and others calculate air quality
It is possible to use an hourly measurement with the previous 23 hours to calculate a 24-hour average but that may not be a good predictor of the future. And there are other more complex calculation schemes that use data from the previous hours to predict the future 24-hour average. The Washington air monitoring site uses this type of method when it colors the dots on the map on their website: Washington Air Monitoring
On Washington’s map the value for the current hour can be found if you click on a colored dot and look at the bottom left of the window that opens.
Please keep in mind that we are not making a judgment whether one approach is better than the other. We simply want to help you understand what you are looking at and why maps may show different levels of air quality.
For the past two weeks an Air Resource Advisor has been assigned to forecast the 24-hour average each day. These forecasts are prepared for a number of sites and posted daily to this blog. The ARA looks at forecast meteorology, forecast fire activity (including planned burn-outs) and past monitoring data and uses their experience to produce forecasts. The daily outlooks, and other good information on smoke and health, can be found on this blog.
Feel free to post any questions you have on these or other smoke issues to the blog.