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Posted at 10:00 AM ET, 11/30/2010

2010 finishes #2 in D.C. for freeze-less category

By Ian Livingston

Graph representing days between the last freeze of the previous cold season and first freeze of the following cold season. Measurements were recorded in D.C. from 1872-2010 and at National Airport since the 1940s.

Despite initial reporting (erroneous ... oops!) yesterday, National Airport finally hit 32! Based on the historical record dating back to the 1800s, the 274 days without a freeze in a row this year was pretty extraordinary.

Only one year finished with a longer consecutive streak -- 1980. When it comes to warmth in D.C., 2010 and 1980 have shared several battles (warmest summer? | tied 90+!). In many cases, they came out pretty close.

National's first official freeze on 11/29 ranks as 11th latest on record and the second latest in the month. For comparison, Dulles saw its first freeze on Nov. 1 and has accumulated 16 total while Baltimore-Washington saw its first on Nov. 2 and has amassed 8 in total.

By Ian Livingston  | November 30, 2010; 10:00 AM ET
Categories:  Latest, Local Climate  
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According to Accuweather data, the DCA winter of 1979-80 had 20.1" of snow, followed by 4.5" in 1980-81. Hard to determine how much snow fell in CY80 but certainly it was nowhere near the 37-plus inches so far this year.


If anyone predicted this year would have played out the way it has (including the Dallas Cowboys at 3-8), they would have been hauled away in a banana boat.

Posted by: JerryFloyd1 | November 30, 2010 10:27 AM | Report abuse

This plot really is quite interesting and it came up in a Thanksgiving discussion. While DC is one of the warmer locations in the area do you know if the slope of the change in freeze free days would be similar at a colder spot like Frederick?

And for fun with the current slope it will be a few hundred years until there is a freeze free winter.

Posted by: dqreads | November 30, 2010 11:02 AM | Report abuse

"Measurements were recorded in D.C. from 1872-2010 and at National Airport since the 1940s." What are we seeing in 1940-2010 then?

If you look only at years since the 1940s, when weather was measured at DCA, it looks as if any trend line would be much flatter. (It also looks as if there's a cyclical component.) It was discussed on the blog extensively last winter that measurements were once taken at 24th & M NW, a colder place. Therefore, the trendline displayed may result primarily from a change in measurement site.

Posted by: kperl | November 30, 2010 11:21 AM | Report abuse

Wow, really interesting trend.

Posted by: Ann-CapitalWeatherGang | November 30, 2010 11:26 AM | Report abuse

i get what you're saying about if you moved the sensor to a warmer place it would make the slope steeper, but surprisingly it looks to me like the slope would be pretty darn similar to what's shown here.

could one of you show us a graph/slope beginning in the 40s?

"interesting", indeed. a rational person would call it just one more indicator of a warming earth. i guess from this one graph, i can't really say "earth". i'd love to see similar plots for a variety of other locations.

Posted by: walter-in-fallschurch | November 30, 2010 12:07 PM | Report abuse

disappointing about the 32 at dca. i thought we were going to add another record to our crazy weather year... oh well. let's start setting some cold/snow records now.

Posted by: walter-in-fallschurch | November 30, 2010 12:09 PM | Report abuse

I'm with Walter; I don't think the change in official measuring stations would have materially impacted the first/last freeze days slope. Snow measurements, however, may be a different story.

Posted by: JerryFloyd1 | November 30, 2010 12:18 PM | Report abuse

I can post a comparison before and after here this evening. I believe the trend is similar in both periods, but there are undoubtedly at least minor differences between the locations.

Posted by: Ian-CapitalWeatherGang | November 30, 2010 12:33 PM | Report abuse

Thanks, Ian.

Given the complexity of weather analysis and the way the board heats up (pun intended) whenever there is a climate discussion, it is imperative to be extra careful with the #s.

Posted by: kperl | November 30, 2010 1:07 PM | Report abuse

Well, I had considered that issue, but in the end it's all considered on climate record and this was meant to be a short post. We can run into these same questions in every category of D.C. numbers. I do know that other trends, like snowfall, remained pretty consistent after the change in location. But, I will post comparos by like 7 or 8 tonight in the comments here.

Posted by: Ian-CapitalWeatherGang | November 30, 2010 1:25 PM | Report abuse

Since the DC snow guage moved from the Imperial Valley to the Salton Sea, the slope might not change much. ; ))

Posted by: JerryFloyd1 | November 30, 2010 2:45 PM | Report abuse

thanks so much. that's mighty nice of ya. i'll check back later.

Posted by: walter-in-fallschurch | November 30, 2010 2:51 PM | Report abuse

One easy observation is that the increase from 1872 to the mid 1900's would have little or nothing to do with CO2. Possibilities are urbanization and the natural trend from the freeze-prone 1880's (colder on average, but even more prone to cold snaps). The CO2 theory also doesn't really explain why national struggled to reach 32 the night before last but Manassas dipped to 21.

OTOH, there are at least a few days of freeze free time increase that are explained by CO2 that meshes with the worldwide increase in average temperatures. Probably something on the order of 10 days of the increase shown in the graph (just a guess based on a few quick glances around the internet)

Posted by: eric654 | November 30, 2010 3:11 PM | Report abuse

Here's an 1991 paper for Alaska where AGW should be most prominent. For 1924 to 1989 (65 years) the lengthening was from 18 to 44 days. That's about the same steepness as the linear trend drawn in the graph above. So it can be said that DCA tracks pretty well with Alaska.

Posted by: eric654 | November 30, 2010 3:27 PM | Report abuse

Very interesting, but it is a bit frustrating that (justly so) these type posts always get distracted by a discussion about DC's change in reporting locations. Do any other major cities have this issue? On a similar topic I would like to see the Redskins return to RFK site to reverse the trend of the last two decades.

Posted by: caphillse | November 30, 2010 4:50 PM | Report abuse

@caphillse, I suspect as more cities started using airports for their measuring stations, quite a few locales changed, e.g., Chicago, Baltimore, etc. Ian or another CWG team member may be knowledgeable re: changes that took place in measuring station locales during the 20th century.

Posted by: JerryFloyd1 | November 30, 2010 5:37 PM | Report abuse

I've never been a fan of straight trend lines. Higher order polynomials, please! :)

Posted by: JTF- | November 30, 2010 6:20 PM | Report abuse

Ok, here are two charts comparing before and after the move to DCA. I used 66 years in each so it's missing some of the earliest -- starts at 1875 -- and has a gap in the 40s not shown.

For some reason I've not been able to get a definitive answer where exactly the switch to DCA in the data occurs... but there are 4 years ending in 1945 with some fields missing in the daily reports, so it seems safe to say by 1945 the numbers were coming from DCA.

I realize linear progressions might not be ideal, but it still seems to give a general idea. I am more a number watcher than a statistician/mathematician!

So, finally, if anyone else wants to play around with it, here is the spreadsheet covering all the years. I have IAD/BWI datasets as well, so someday I could do a comparison on those locations.. it's just sort of time consuming. ;)

Posted by: Ian-CapitalWeatherGang | November 30, 2010 7:39 PM | Report abuse

wow, ian, thanks much. u r da man!

surprisingly, to me, the slope is less steep from 1945-2010 (m=~.27). the freeze-to-freeze intervals are increasing all the while, but less so more recently... as if to indicate that the rate of washington (not necessarily global...) warming is slowing... still warming, but slower than 1875-1945(m=~.27).

the difference is probably insignificant, but counter-intuitive, to me. to me, it seems like if you moved the sensor to a warmer place, the very end of the graph would be "artificially" high.

but, now that i'm thinking about it, maybe the slope "should" stay the same (or increase if warming is increasing), but there "should" be a step-jump at the year of the location change. of course all my "shoulds" are probably overwhelmed by the vagaries and variability of weather...

ian, hate to ask this, but what's the slope of the original entire graph (1875-2010)?

also, i notice that in starting at 1875 instead of 1872, the first 2 large-interval years are left off the 1875-1945 graph - which would have the effect of lowering the start point ("b" in y=mx+b) thus "artificially" increasing the slope...

kperl? eric? other math-types?

Posted by: walter-in-fallschurch | November 30, 2010 8:23 PM | Report abuse

Ian, thanks, as always for this data. One thing that immediately jumps out that only one of the top 10 years was at the 14th & L location.

I wish there was similar data for a regional weather station that hasn't been urbanized as much the Washington DC area has been since the 1930s. This would enable a better determination as how much, if any, the urban "heat island" impacts the slope.

Going back to a previous post I made in this thread, i.e., the moving of selected weather locations during the 20th century: the topic would make for an interesting article in Weatherwise,

I'm assuming Chicago's data used to be taken somewhere near the lake, before being moved to ORD. As of a few years ago, ORD nighttime winter temps were still considerably colder than readings at Meigs, while summer daytime readings were usually higher at ORD. Chicago snowfall averages are higher a few miles further inland than along the lakefront.

There are probably other cities/locales worth studying. Since my area is the performing arts, not weather, I hope you and/or other CWG members will consider looking into the impact changing weather measurement stations in the 20th century has had on temps, precip, etc., starting with Washington and including a few other representative locations. Puncture the misconceptions, etc.

Posted by: JerryFloyd1 | November 30, 2010 8:27 PM | Report abuse

dag nabbit...

first paragraph should read:
surprisingly, to me, the slope is less steep from 1945-2010 (m=~.27). the freeze-to-freeze intervals are increasing all the while, but less so more recently... as if to indicate that the rate of washington (not necessarily global...) warming is slowing... still warming, but slower than 1875-1945(m=~.29).


Posted by: walter-in-fallschurch | November 30, 2010 8:41 PM | Report abuse

The original from 1872 was: y = 0.3025x + 197.38. I see your point about removing those first two which had higher numbers. I probably should have left out 1872 to start with because there are some discrepencies between the dailies and the records on the public LWX site. But it did not seem outlandish so I left it.

Posted by: Ian-CapitalWeatherGang | November 30, 2010 8:57 PM | Report abuse

Thanks so much Ian. There is a difference in trends between the two periods, but it is small. BTW, it's not my intent to make you miserable, but stats must be handled with care and linear trends may not tell the whole story.

@Walter-in-falls-church, the trend equation for the entire period is
y = 0.3025x + 197.38
-- an even higher slope than the two separate graphs and a lower intercept. (I did this by turning on the trend line display option in Ian's spreadsheet.)

Posted by: kperl | November 30, 2010 9:24 PM | Report abuse

wow... those two years changed the slope a lot! ...probably showing that changing the location isn't "significant".

i would like to see dulles' graph someday. might detect urban heat island effect?

a dulles/dca comparison that always depresses me is snowfall... so much more out there.

Posted by: walter-in-fallschurch | November 30, 2010 9:26 PM | Report abuse


I would bet a lot more of the first period increase was due to urban heat island effect than the second period. Not an expert on this but imagine more urbanization was occurring around the respective observation stations in the early 20th century compared to the late 20th century.

Posted by: Jason-CapitalWeatherGang | November 30, 2010 9:29 PM | Report abuse

No worries, kperl. I was hesitant to turn that on but I felt it was not totally inaccurate in a simple sort of way... if that's incorrect I'd rather know! Though my day job involves mining numbers (in foreign policy arenas), I'm limited on my true stats knowledge... so that transfers to climate data I'm sure.

When I have time I will do this up on the other local stations.

Posted by: Ian-CapitalWeatherGang | November 30, 2010 9:32 PM | Report abuse

Jason, when urbanization really took off depends on which metro area you're talking about. Certainly Chicago, New York and other northern cities were heavily urbanized by the early 20th century and the addition of concrete highways, and later parking lots, shopping malls, etc. added to the heat island effect.

But many sun belt cities were still relative tank towns even at mid century. (I grew up in one of those, Tampa, and the urban sprawl has been incredible since I was a kid).

Even Washington was a relatively small potatoes burg until at least until WWI. And this region only really took off during the Depression, when the New Dealers came to town.

Even as late as the mid-1950s, Shirley Highway was pretty much country 'til you reached Fairlington. And we all know what happened to the former cowpasture known as the Dulles Airport Corridor.

Posted by: JerryFloyd1 | November 30, 2010 9:52 PM | Report abuse

(nitpick alert) regression, not progression, and the starting point is smack dab in the American LIA adding natural slope to the early years. Walter, rather than calling it "warming", I would call it early spring, and to a lesser extent a decrease in early fall cold outbreaks. Obviously the main source is warmth, but also other weather factors that make it less likely to get those particular cold pushes (e.g. less blocking). That might help explain some of the smaller cycles.

As for location, you can't tell anything from comparing slopes. Essentially there's no way to know if the data would have had a lesser or greater slope in the old location. My guess from considering the actual locations (both essentially urban) is that it doesn't affect the data much.

Posted by: eric654 | November 30, 2010 10:01 PM | Report abuse

(good-natured ribbing alert) uh...ok, eric, henceforth: not "AGW", but "AGESADIEFCO" (anthropogenic global early spring and decrease in early fall cold outbreaks).

Posted by: walter-in-fallschurch | December 1, 2010 7:11 AM | Report abuse

Walter, I like it! The long acronym reminds me of Mogambo Guru who I read for years before the bottom fell out of the economy. Then since he was right, I stopped reading him. Someday I will be right too!

Posted by: eric654 | December 1, 2010 7:28 AM | Report abuse

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