Northern Virginia sunlight in 2018 compared to previous years

Sean Heath wrote with this question:

“do you or anyone else know of data being tracked for sunlight or cloud cover? Specifically how much sunlight did Northern VA receive this year with all the rain and cloud cover? Curious how it compares to previous years”

Easily accessible (usually) and accurate data of this type are available from the NOAA, but those data weren’t available for some time because of the USA’s government shutdown. This week those data were available again.

What I did

I downloaded daily data for the past decade from USCRN stations at Avondale, PA; Cape Charles, VA; and Charlottesville, VA. I then took the average daily global solar radiation, converted it to photosynthetically active radiation (PAR), and took the average of those stations to get estimates for northern Virginia. One can expect the data shown in the charts below to be the average PAR at somewhere south of Avondale, northeast of Charlottesville, and northwest of Cape Charles.

For some background info on this, see Estimating daily light integral (DLI) in 4 Tennessee cities.

What I found

I had a decade of data, and to smooth it out I calculated the DLI—that’s the effective light for photosynthesis as plants experience it—averages on a monthly basis.

This shows lines for the previous 9 years, and then 2018 highlighted in red.

Then I looked specifically at 2018 monthly averages, and compared those to the average from the previous 9 years.

During the growing season—I’ll call that April through October—the average decrease in photosynthetically active radiation (PAR) seen in 2018 works out to about half an hour of full sunshine per day, compared to the normal amounts seen in previous years.

Expressed another way, that’s about 107 hours of light—by light in this case I mean midday sunshine uninterrupted by clouds—that didn’t fall on the turf during the 2018 growing season, but in a normal year, would have.

Looking at average DLI anywhere in the USA

This is a reminder about the high resolution DLI maps of the entire United States prepared by Faust and Logan to accompany their article about DLI in HortScience.

Average daily light integral data (based on 1998 to 2012 measurements) for anywhere in the USA are available from the map prepared by Joanne Logan and James Faust.
Average daily light integral data (based on 1998 to 2012 measurements) for anywhere in the USA are available from the map prepared by Joanne Logan and James Faust.

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