A cool day in July might seem peculiar, but it happens more often than you might think. Such days are often forgettable because they’re typically cloudy and wet. But a cool, rain-free day in July with ample sunshine is very rare – making Tuesday’s weather all the more remarkable.

Here are the stats: 78 for a high, low humidity (dew points in the 40s), sunshine, and no rain. Mid-September, just two months early.

Running a 1-2 percent chance over the historical record (more on that later), a sun-filled July day like the one we just experienced is unquestionably an oddball.


Visible satellite at 4:30 p.m. July 29, 2014. As with most of the day, clouds were minimal. A haze is seen over the area from fires in Canada. (NASA)

Since the observation location has been at National Airport (DCA), 80 percent of July days with highs in the 70s or lower featured rain. More often than not extensive cloud cover as well. But even these cloudy and/or rainy days in the 70s are quite atypical, making up an average of 6 percent (DCA, 1946-2013) to 8 percent (All D.C., 1872-2013) of all July days.

Still, we don’t have to go back far for our last instance of a high in the 70s in July. 2013 and 2012 both had them.


Highs of 78 degrees or less by year at D.C. since observations began at National Airport. 78 was used as a comparison as it was the high on July 29, 2014. An additional 24 days throughout the period had highs of 79 degrees, with 7 recording no rain. (Author analysis of NCDC data)

But both of these recent July days in the 70s had rain. In 2013 it was heavy at times through mid-afternoon. In 2012 it was drizzle with showers throughout. Maybe a win in July, but hardly worth bragging about.

As you might suspect, given its 1 percent occurrence since the first full year of observations at DCA  and 2 percent occurrence overall from 1872-2013, it has been a while since our last day with no rain and temperatures so low in July.

It has been considerably longer than even the piddly average might suggest.


Model analyzed temperatures at 4 p.m., July 29, 2014 across the mid-Atlantic. These are not exact, but generally within a degree or three. (Weatherbell.com)

In fact, to find the last comparable day — generally scattered clouds or better, low or no humidity, northwest winds — we need to go back to July 6, 2001. That’s 425 July days since matchable summer bliss.

Throughout the 2001-yesterday period, there were 20 other July days with temperatures in the 70s or lower at DCA, but they all featured mostly cloudy to cloudy conditions, and all but one had at least a trace of rain. Many with considerably more.

An at times milky sky — thanks to wildfires in Canada — aside, we hope you took Tuesday in. It could be a while till we see a day like that that again in the heart of D.C. summer!