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Posted at 01:20 PM ET, 07/07/2011

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


July 1, 2005 — Lightning strikes the Washington Monument as seen from the Jefferson Memorial. Photo by CWG photographer Kevin Ambrose.
July is Washington’s hottest month of the year. Though D.C.’s average high falls just short of 90 degrees, this is the month that averages the most such days, accounting for about 40% of the yearly total.

Usually for big heat, D.C. also needs a fair amount of sunshine, and July delivers plenty of that as the storm track lifts north. But, with the high heat, there’s also that seemingly constant risk of isolated to widely scattered afternoon and evening thunderstorms — it’s one of the most active months on that scale as well.

Keep reading for more in this fourth 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

July’s average monthly rainfall is 3.73” (up from 3.66 during the 1971-2000 climate normal period)*. Compared to some of the other months (like June), this change was fairly minimal.


July rainfall (in inches) by year throughout the historical period. See sources for data.
The monthly maximum rainfall is the 11.06” that fell during 1945. The least rain during July is the 0.82” that fell in 1872. Like June, 2 Julys have featured more than 10” of rain. 23 years have brought rainfall totals greater than 6”.

The most rain that has fallen on one day was the 4.69” accumulating on the 9th in 1970. 37 days have piled up daily totals past 2”. This is considerably more than the months prior and attests to the slow-moving nature of storms that fire during the month as well as the atmospheric moisture available.


Average number of days with thunder reported at D.C., calculated using 30-year records from 1981-2010. See sources for data.
This is part of the heart of thunderstorm season. July is king of thunderstorm days in D.C., averaging just a hair over 7 days with rumbles in the air. Keep in mind that tally includes a few days with thunder reported, but no rain at the observation location -- a testament to the hit-or-miss nature of the events.

By July, the jet stream has typically retreated so far north that most storms which form simply limp along and dump copious rains over a small area. This can both leave some lawns parched and create localized flash flooding in not too far flung locations.

While the storms that form in July are often not as widespread a severe weather risk as earlier months that have better dynamics and cold air available, they tend to have plenty of juice thanks to the warming ocean sending in higher humidity. One of the biggest threats during the month is collapsing “pulse” severe storms that send out microburst or macroburst straight line winds.

TEMPERATURES


July temperature averages (highs and lows combined), by year in degrees F. Red line is the NWS 1981-2010 monthly average of 79.8 degrees. See sources for data.
However you examine it, July is a sweaty month. For some readers, that might be enough for this section!

July’s average monthly temperature at D.C. is 79.8 degrees per new NWS 1981-2010 norms (it was 79.2 in the 1971-2000 norms). This is about five degrees higher than June and as noted, the peak of heat here in Washington. It’s been as hot as 84.5 degrees in 2011, followed by the toasty years 1993 and 2010 when the average was 83.1 degrees. It’s been as “cool” as 72.0 in 1891. The coolest recent July was 2000’s 74.7

Daily high temperature averages, while never touching 90, do hit 89 for quite a stretch, running from the 7th to the 22nd. This 16-day period is an increase of four days compared to the old normal, when the 16th to the 27th featured D.C.’s typical 89-degree heat.

Perhaps more exceptional than the additional (and earlier) warmth is the increase in our “highest lows”. The new normal gives us a stretch of 71-degree lows from the 4th of July through the 10th of August. The old was a much shorter period, from the 19th to the 27th of July. As comparison, the old 70+ lows period fell between July 7 and August 10.

With an average daily high 88-degrees or above for the entire month, we don’t see too much relief from the heat. A breakdown of highs during the 1981-2010 period showed 47% in the 80s (mostly high) and 44% in the 90s. Lows reach their peak as well, with 66% of them in the 70s and 32% in the 60s.


Number of July days at or above 90-degrees F in Washington, D.C. since 1872. See sources for data.
As to be expected with the hottest month of the year, days above 90 reach their peak in July with an average of about two weeks of such days. This average came down slightly over the 1980-2010 period, but not enough to be affected by rounding. There have been 58 days in July 100+ and 23 days 102+. The most 100+ days in July in one year? Six in 1930.

On the extremes for highs, it’s been as warm as 106 (the all-time high) on the 20th in 1930 and as cool as 62 in 1891 (more recently 68 in 1972). The highest July temperature at the current observation location was 104 in on the 16th in 1988 and again on the 29th in 2011. Many of the other 103-degree plus temperatures for July occurred during the 1930s or prior.

The lowest July temperature was 52 in 1895, 1907 and 1933. At the current observation location, 54 is July’s coldest and it came on the 1st in 1988. There were 17 days in D.C. 54-degrees or less prior to records moving to National. The highest low temperature in July (and ever) is 84 degrees, and it was first recorded on the 16th in 1983, and then again on both the 23rd and 24th in 2011.

DATA

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

Highs 85 or above: 23.4 / 21.3
Highs 90 or above: 14.2 / 11.9
Highs 95 or above: 5.0 / 3.5
Highs below 80: 3.0 / 4.3
Lows below 70: 10.1 / 14.9
Lows 75 or above: 6.9 / 3.8
Days with a trace of rain or more: 14.2 / 14.8
Days with .1” of rain or more: 6.3 / 6.6
Days with .5” of rain or more: 3.0 / 2.8
Days with 1” of rain or more: 1.0 / 1.2

Days with thunder: 7.1 / NA
Days with hail: .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 (2011) / 8 (1891)
Highs 90 or above: 25 (2011) / 1 (1906, 2000)
Highs 95 or above: 14 (1999, 2011) / 0 (31 years)
Highs below 80: 14 (1891, 1895) / 0 (10 years)
Lows below 70: 28 (1891) / 2 (1955)
Lows 75 or above: 17 (2010, 2011) / 0 (26 years)
Days with a trace of rain or more: 22 (1897, 1904) / 8 (1997)
Days with .1” of rain or more: 14 (1945) / 2 (1957, 1974, 2002)
Days with .5” of rain or more: 8 (1905, 1945) / 0 (10 years)
Days with 1” of rain or more: 5 (1886, 1945) / 0 (41 years)

Days with thunder: 11 (4 years) / 2 (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 July daily norms (1971-2000) 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 fourth 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. (viewing)
-August weather in Washington, D.C.

Article last updated: August 8, 2011.

By  |  01:20 PM ET, 07/07/2011

Categories:  Local Climate, Latest

 
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