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The strength of the late April sun in Washington, D.C.

Monthly variation in maximum UV index values in D.C. based on data from NOAA and TEMIS. (Justin Grieser) Monthly variation in maximum UV index values in D.C. based on data from NOAA and TEMIS. (Justin Grieser)

Clear skies and comfortable temperatures have graced the D.C. area this week, making it ideal to spend time outside in the sun. And with the sun above the horizon for over 13½ hours now, there’s plenty of light to bask in.

Just don’t underestimate the intensity of the late-April sun – even on some of the cooler days we’ve had recently. While it’s not quite beach weather, the sun’s rays are now just as strong as they are in mid-August.

High pressure and a very dry air mass yesterday allowed the UV Index to reach some of its highest levels of the year so far. During the latter part of April, a sunny day in the nation’s capital often yields UV index values in the 7 to 9 range (high to very high). In fact, the sun in late April appears slightly higher in the sky at 9 a.m. than it does at noon on the winter solstice!

This means if you have fair skin and burn easily, limit your sun exposure during midday and be sure to apply sunscreen regularly.

How to read the UV Index scale (EPA)

National UV Index Forecast for April 26, 2013. The mid-Atlantic will see UV levels in the 6-7 range, though higher levels are possible at high elevations and along bodies of water. (EPA)

How can the sun already be so strong when it’s only April? Consider that the sun’s path through the sky on April 26 is about identical to that of August 15.

While average high temperatures are “only” 70 degrees now (compared to 87ºF in August), the sun follows the same steep trajectory in the sky. In fact, sometimes the sun’s ultraviolet rays are stronger in late spring than in August because we still get cool, dry air masses this time of year. This means less water vapor (humidity) is present for filtering the sun’s rays.

(Justin Grieser, data from NWS and

In the chart above, we see that the time of sunrise, sunset, and the sun’s height above the horizon at solar noon are about equal on April 26 and August 15. Outside air temperatures vary considerably, but have no bearing on how quickly you’ll get a sunburn.

As we turn the calendar to May, the sun will continue its steeper path in the sky and UV levels will reach very high levels (8-10 range) more frequently. By June, a clear day with low humidity can bring UV levels into the extreme range (11+). Fortunately, D.C. only gets these Florida-like levels of UV exposure right around the summer solstice.

Until then, we have another hour and 14 minutes of daylight to gain. Between April 30 and August 17, sunset in the D.C. area won’t be until 8 p.m. or later, so there’s still plenty of evening daylight to enjoy when the sun’s rays are less intense. But if you do head outside midday for a baseball game or lunch al fresco, just remember that the sun’s rays are strong – no matter what the thermometer reads.


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Kevin Ambrose · April 26, 2013

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