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Posted at 10:00 AM ET, 08/08/2011

What is Washington, D.C.’s weather like in August? Breaking down norms and extremes


August 7, 2010 - Dusk on the Potomac River. Photo by author. See a larger version of this image.
It’s the dog days of summer, the songs of cicadas and crickets are plentiful. Despite its reputation for unrelenting heat, August actually begins the cooling of monthly averages in Washington, D.C. By the time the month arrives, the region has already experienced 64% of its 90-degree days in a “normal” year. Still, it is often anything but cool with 10 90+ days in an average month, second to only July.

What was once one of the wetter months on average has dried up over recent times. August’s average monthly precipitation of 2.93” puts it well below the other summer month totals in June and July, as well as those of September to follow. At the same time, when it comes to extreme rainfall events, August has delivered.

Keep reading for more in this fifth installment of a year-long series of posts detailing specific statistics on climate norms, extremes, and averages for the each month in Washington, D.C…

PRECIPITATION

As noted above, August’s average* precipitation is a relatively low (2nd lowest only to Feb.) 2.93”. This is a change of -0.51” from the previous 1971-2000 climate normal and is the largest negative departure (June was positive 0.65”) among the months from the old normal to the new 1981-2010 normal.


Monthly rainfall (in inches) in Washington, D.C. from 1871-2010. See sources for data.
The monthly maximum rain is 14.41”, and it fell during 1928. The least rain any August has brought was the 0.55” in 1962. August can perform on the big rain metrics, with four separate Augusts coming in above the 12.00” mark.

The month of August holds the all-time record for most rain in one calendar day with 6.39” on the 23rd in 1933. Four of the top-five daily rainfall records are in August (one is in June). In the top-10 daily records, the month holds six spots, and each of them equals significantly more than the average monthly rainfall of the rainiest month in D.C.

Despite these heavy rainfall event, it has been noted that August seems to be drying up in Washington, D.C. to the point of having an impact on local plant life. Those types of trends are clearly visible in year-to-year precipitation, rolling averages and regional changes in climate normals.


Trends in average number of days with .1”, .5”, 1” and 2” of rainfall over 30-year climate periods throughout recorded history at Washington, D.C. See sources for data.

As with July, August is often home to a persistent area of high pressure. This is occasionally, and more frequently as the month progresses, interrupted by cold fronts or other rain events. The first four of six August entries into the “top-10 rainiest days on record” were set by tropical cyclones (tropical storms, hurricanes or their remnants). In all four cases -- 1933, 1944, 1928, 1955 – the system tracked directly over Washington dumping ~5.50” or more in just one day with some higher multi-day totals. Outliers in history, even if “expected” here and there.

Still, declining trends are evident – if maybe less extreme – in counts of the average number of days with conditions such as a “nuisance” 0.10” of rain, in addition to those that we might term a washout with 2” or more. Tropical systems account for a significant amount of the variability in August extremes, but more robust — or longer-lasting — summer high pressure ridging may also be playing a role in the slow decline of rainy days in the more common lesser amounts.

TEMPERATURES


August temperature averages throughout recorded history (in degrees F) at Washington, D.C. See sources for data.

After July, I’d like to say the worst is behind us and call it a day, but August is still pretty hot in Washington. On the positive side, signs of autumn to come may start to become more apparent by months end as high temperature averages dip below 85 and cold fronts begin to appear with more regularity.

The average August temperature of 78.1 is 1.7 degrees lower than July — the warmest month of the year — but still 2.9 degrees above the other summer month of June. August’s temperature average increased 0.7 degrees for the new climate normal compared to the 1971-2000 period. The record monthly temperature for August came in 1980 with 82.8 degrees, and the coolest August on record was in 1927 when the average was 70.0.

While the August high temperature average is in slow descent heading out of peak warmth, a majority of August days are mid-80s or higher. A breakdown of ranges during the 1981-2010 climate period featured highs in the 80s 58% of the time, the 90s 30% of the time and the 70s 11% — 100s and 60s make up the last 1%+. When it comes to lows, they tend to fall in the 70s 54% of the time, the 60s at 42% and everything else making up about 4%.


Number of 90-degree or higher days in Washington, D.C., by year since 1872. See sources for data.

Coming in with 10 such days on average, August usually sees about four fewer 90-degree or higher days than July, but its peak months in that category (22 in 1980) do not fall too far behind.

There have been 29 days in August 100-degrees or higher and 11 days 102-degrees plus. Compared to July, both extremes are a bit less than half as frequent historically.

August holds a tie with July for the highest temperature on record at Washington with the 106 recorded on August 6, 1918. The highest temperature at the current observation location in August was the 105 on the 17th in 1997. Those oh-so-rare cool August days have to beat the lowest maximum temperature of 61 on the August 26, 1908 to find themselves in the record books. At National? 67 on the 19th in 1957. Good luck, I say!

The coolest overnight August temperature is a chilly 49, as far back as the 24th in 1890 and as recently as the 29th in 1986. The warmest overnight low in August is 81 on the 3rd in 2006. That’s three degrees off the record warmest low temperature ever, which happened in July 1983 and two times in July 2011.

DATA

30-year (1981-2010) Averages / Overall (all history) Averages...

Highs 85 or above: 20.4 / 17.5
Highs 90 or above: 9.7 / 8.0
Highs 95 or above: 5.1 / 3.7
Highs below 80: 3.9 / 5.9
Lows below 70: 13.9 / 18.7
Lows 75 or above: 5.8 / 4.0
Days with a trace of rain or more: 12.4 / 13.9
Days with .1” of rain or more: 5.2 / 6.1
Days with .5” of rain or more: 2.3 / 2.7
Days with 1” of rain or more: 1.5 / 1.9

Days with thunder: 4.6 / NA
Days with hail: 0.1 / NA
-This count is done through records on Weather Underground, and does not include the entire historical period.

All-Time Records (High / Low)...

Highs 85 or above: 30 (1978) / 4 (1927)
Highs 90 or above: 22 (1980) / 1 (7 years)
Highs 95 or above: 16 (1980) / 0 (65 years)
Highs below 80: 19 (1903) / 0 (4 years)
Lows below 70: 30 (1927) / 3 (1978)
Lows 75 or above: 16 (1980) / 0 (55 years)
Days with a trace of rain or more: 24 (1974) / 5 (1995)
Days with .1” of rain or more: 15 (1875) / 2 (16 years)
Days with .5” of rain or more: 8 (1906, 1955) / 0 (14 years)
Days with 1” of rain or more: 7 (1906) / 0 (49 years)

Days with thunder: 11 (2001) / 1 (3 years)
-Same as above. (30-year high / low listed)

SOURCES

Daily climate records for Washington, D.C., 1871-present
- See selected and additional statistics compiled for this post (Excel)

D.C. climate norms 1981-2010

DC August daily norms and records

DC monthly temperatures

DC monthly precipitation

NWS Baltimore/Washington climate information

NOTES

*All averages, unless otherwise noted, refer to the 30-year period of 1981-2010. Unlike prior months, a full set of new climate norms is now available and will be used throughout. Past months will be edited to reflect the new norms.

ABOUT THIS SERIES

While numerous local climatology statistics including daily, monthly, and seasonal normals and records are available from National Weather Service Baltimore/Washington, there are myriad other pieces of information which help frame the story. Rather than attempt to recreate what is already available, this series of posts will act as a companion to most information presented there.

This is the fifth installment of a series of living documents. Through early 2012, each month will be broken down in a similar or evolving way. As input is taken from the community, some items may be added to or changed in older postings. These postings will be updated as necessary to reflect changes when needed. Other articles may also become part of this series.

Feel free to share thoughts or additional data you may be interested in seeing.

Learn more about each month:

-April weather in Washington, D.C.
-May weather in Washington, D.C.
-June weather in Washington, D.C.
-July weather in Washington, D.C.
-August weather in Washington, D.C. (viewing)

Article last updated: August 8, 2011.

By  |  10:00 AM ET, 08/08/2011

Categories:  Local Climate, Latest

 
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