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Posted at 11:00 AM ET, 05/26/2010

When a drought is not a drought

By Don Lipman

* It's hot: Full Forecast | UnitedCast | TV Weather revolving door *

Drought-wh.jpg
Image courtesy National Weather Service.

Despite recent rains, overall precipitation has been somewhat below normal here since February. Most of us, however, would not construe the current weather pattern as anything close to being a "drought." We tend to reserve that term, along with the expected heat and humidity, for use during the summer.

But although we in the D.C. area usually experience at least one or two rainless stretches each summer, does the long-term average really substantiate the perception held by some that summertime drought is common here? The answer may surprise you.

Keeping reading for more on what a drought really is...

Most dictionaries define drought as something like "an extended period of dry weather, relative to normal, especially when injurious to crops." But average summer rainfall in the D.C. area is actually quite plentiful. June, with 3.13 inches, July, with 3.66 inches, and August with 3.44 inches, constitute the fourth rainiest three-month period of the year here (as measured at Reagan National Airport).

Obviously, to really define the word properly, we need to consider such factors as soil moisture, river levels, and local weather history. (The 39 inches of annual DC rainfall would be an extreme drought for southern Florida, which usually gets about 60 inches, but a bonanza for much of the West, where 10 to 30 inches is more common.)

Locally, even with the above-stated average amount of rainfall, drought-like conditions can still develop quickly under a strong summer sun, depending on rainfall distribution and intensity. Unfortunately, much of our summer precipitation is erratic and comes in the form of sudden, violent storms, which unleash very heavy rains over a short period of time. Some communities get deluged (the exception), while others remain parched. If you happen to live in one of those parched areas and a thunderstorm skirts by, well, you're still in a drought.

Sometimes a week, or even two or three, will pass before it rains again. Since vegetation needs about one inch of rain per week during the growing season, plants can quickly deteriorate without watering.

Though some may complain about the streaky summer rainfall pattern here, it does mean there are many fine, sunny days (roughly 2 out of 3) that are custom-made for the beach or other outdoor activities. Residents of other areas, such as the Pacific Northwest coastal strip from San Francisco northward, are not nearly so lucky (assuming they like beach weather). There, although rain is not the summer spoiler, fog is. And even though the sun usually breaks through for at least a few hours each day, summer weather can be a cool and invigorating experience, with the heat usually held at bay in the inland valleys. Mark Twain once said, "The worst winter I ever experienced, was summer in San Francisco."

Local residents are often surprised to learn that Portland, OR, among other West Coast locations, actually receives slightly less annual rainfall than Washington, DC. And in terms of summer rain, despite Portland's many foggy (and soggy) days, total rainfall is scarcely 3 inches, compared with Washington's 10.

The comparison with San Francisco is even more striking, as it receives little more than half our annual rainfall and only 1-2 percent of our summer moisture. Does such meager seasonal and annual rainfall constitute a drought? Probably not, since that's normal for those areas during those time periods.

As to whether drought (which rhymes with pout, not mouth) is really the weather villain that we seem to make it out to be, I suppose it all depends on your point of view. If you're a farmer or a water supply official, it's a worrisome issue. If, on the other hand, you're a sun-bather or vacationer, a drought presents no problem at all, at least in the near-term, you believe.

Personally, I agree with John Ruskin, the English writer, who said, "There is no such thing as bad weather, only different kinds of good weather." How about you? Do you agree?

[Officially, the driest calendar month on record in D.C. was October 1963, when only a trace of rain fell. The driest year was the dust bowl year of 1930, which could only muster 21.66 inches of precipitation (45% below normal). For more information on drought measurement, refer to the National Drought Monitor]

By Don Lipman  | May 26, 2010; 11:00 AM ET
Categories:  Droughts, Lipman, Local Climate  
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Comments

I like this run-down of DC metrics.. and comparisons to other cities - thanks Don!

Posted by: Camden-CapitalWeatherGang | May 26, 2010 12:04 PM | Report abuse

Sometimes it depends on what message you are trying to convey. For example: not much drought right now: http://www.drought.unl.edu/dm/monitor.html But a report emphasizing the negative in 2008 might choose a drought map from 2007: http://www.usgcrp.gov/usgcrp/Library/ocp2009/OCP09-Fig-9.htm

The bottom line is that rainfall varies all the time, always has and always will: http://www.ncdc.noaa.gov/sotc/get-file.php?report=national&file=timeseries01&id=044-00&byear=2010&bmonth=4&year=2010&month=4&ext=gif

Posted by: eric654 | May 26, 2010 12:06 PM | Report abuse

How much rain has DC received this month? Normally, April and May are pretty wet, but they don't appear to have been so far. This is pretty evident in the garden, where the earth dries up and cracks.

Posted by: 20016DC | May 26, 2010 3:21 PM | Report abuse

To me the most important aspect of drought is whether it impacts water supply, agriculture, forest fire potential, etc. The single best overall source for this sort of info I know of is :

http://droughtreporter.unl.edu/map.jsp

Posted by: SteveT-CapitalWeatherGang | May 26, 2010 4:28 PM | Report abuse

As one who gets highly anxious over drought, thank you for this insightful analysis.

20016DC: April and May have been very dry, with only 1.50" at National Airport in April and 1.81" so far in May. In the District, precipitation has been more varied. The WeatherBug station at Children's Hospital recorded 1.99" in April, with 2.83" so far in May. The WeatherBug station at the Lowell School has recorded 2.95" thus far in May.

Let's hope for more rain! If not, Bombo and ThinkSpring are more than welcome to water my beloved trees and gardens!

Posted by: TominMichiganParkDC | May 26, 2010 4:57 PM | Report abuse

We are not currently in a drought.

Despite the supposed precipitation deficit, and the mental gymnastics of the "much-needed-rain" crowd, who are now trying to rain on my dance outings by asserting that our 56 inches of snow this winter "weren't that high in water content", everything is nice and green...and we seem to be getting ample rainfall.

First: That snow melted slowly and nearly all the melt sank into the ground to maintain and replenish the water table.

Second, as pointed out by ThinkSpring [with whom I don't always agree during meteorological winter] some strategically situated rainfall just about wiped out the purported "precipitation deficit", at least in the Chantilly/Centreville/Dulles/Herndon corridor Saturday night. Other areas, mine included, got considerable rain in the wee hours of Sunday morning, when it wasn't interfering with hiking plans and other activities while it was largely dry during the day in my area.

Third: I doubt any of us want the kind of rain that has been hitting Poland, the Czech Republic and neighboring areas of Eastern Europe, as well as Middle Tennessee earlier in the past few weeks. Farmers do need sufficient rain to enable their crops to grow, but do not wish that the same crops get waterlogged or that rainy weather and floods hit at harvest time. Waterlogged crops die when their roots are deprived of oxygen.

In sum, too much rain can be as devastating for agriculture as too little rain.

Posted by: Bombo47jea | May 26, 2010 7:46 PM | Report abuse

The comments to this entry are closed.

 
 
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