RodneyS

Weather Underground currently forecasts -6 at Dulles Airport on Thursday morning. If that holds -- or even if the minimum reaches -3 -- that would be the coldest there since -9 on February 6, 1996. The all-time low at Dulles was -18 on January 22, 1984. The all-time February low there was -14 on February 18, 1979 -- the Sunday of the first Presidents Day storm.

It looks as if IAD is headed for its coldest February since 1979. DCA could achieve that as well, but February 2007 featured a negative departure relative to the 1981-2010 normals of 8.1 degrees (15th coldest February in DC history), so that could be a close call.

January 2015 continued the trend of a big DCA/IAD average temperature differential. DCA averaged 35.6 degrees, while IAD averaged just 29.7. The 5.9 degree differential is the largest in January since 1984, when the differential was 6.4 degrees. The largest January differential was 6.9 degrees in 1975, while the smallest was 1.3 degrees in both 2005 and 2006.

I just ran a two-tailed T-Test for the periods 1872-1983 and 1984-2015 to determine the probability that the temperature differences between January 13th and January 22nd in DC for each period are attributable to random factors, or statistical noise. For the first period, the probability is 0.375%, or about one chance in 267. For the second period, the probability is 0.088%, or about one chance in 1132. The results for the second period are particularly interesting, as the 1981-2010 "normals" for DC indicate that temperatures bottom out in mid-January. Thus, if those normals are taken at face value, January 22nd should be slightly warmer than January 13th, not eight degrees colder.

Thanks for the response, Roger. However, DC temperature data show much more than a handful of anomalous warm days recently on January 13th. I've already compared that day to January 21st for the periods 1872-1983 and 1984-2015. Here now is the same comparison for January 13th and January 22nd: During the period 1872-1983, January 13th averaged 33.3 degrees (coldest January day, and third coldest day of the year), compared with 37.2 degrees on January 22nd (warmest January day). January 13th was the colder day 69 of those 112 years, or 61.6% of the time. During the period 1984-2015, January 13th averaged 40.3 degrees (warmest January day), compared with 32.1 degrees on January 22nd (coldest day of the year). January 13th was the colder day just 7 of those 32 years, or 21.9% of the time.

January 21st did manage to nose out January 13th in DC for a warmer average temperature this year, 36.5 to 35.5. So, the updated DC numbers for the two days are as follows: During the first 112 years of official DC temperature records, January 13th had a lower average daily temperature than January 21st 70 times (62.5%), with three ties (2.7%). During the most recent 32 years, January 13th had a lower average temperature than January 21st seven times (21.9%), with no ties. January 22nd has also grown much colder in DC since 1984, although this year it was a balmy 42.5. So, perhaps that's some support for your position. Still, it's striking how few cold waves have occurred in DC in mid-January for more than 30 years, while they have been common in the latter third of the month during that same period. Regarding your point about smoothing of temperature data, that's actually my point as well. In other words, the "normal" temperatures reflect underlying assumptions about temperature trends, which may not be valid in all instances.

December 1880 was only the fourth coldest December in DC history, with an average temperature of 29.0 degrees (1876: 27.1; 1917: 27.9; and, as some of us remember, 1989: 27.9), but it did end on a rather chilly note, with a memorable New Year's Day morning low: Dec 30, 1880: Max 9, Min -7 Dec 31, 1880: Max 13, Min -13 Jan 1, 1881: Max 19, Min -14 In contrast, December 1989 managed a low of only 5 above zero (on the 22nd), and was relatively balmy on New Year's Eve (Max 46, Min 33) and New Year's Day 1990 (Max 46, Min 35).

December 2014 averaged 43.7 degrees at DCA to bring the yearly average to 58.5 degrees, but only 38.3 degrees at IAD to bring the yearly average to 53.0 degrees. Thus, the 2014 differential was 5.5 degrees, tying 1975 for the second greatest differential ever, behind the 5.9 degree differential of 1976. The 58.5 degree average at DCA was the coldest only since 57.4 degrees in 2009, whereas the 53.0 degree average at IAD was the coldest since 51.9 degrees in 1976.

Same relative deal for November: November 2014 averaged 48.0 degrees at DCA (1.6 degrees below the 1981-2010 normal), but only 41.8 at IAD (4.6 degrees below the 1981-2010 normal). The 6.2 degree monthly differential brings the 11-month average differential to 5.45 degrees -- again the second largest ever. 1976 averaged a 6.05 degree differential through November.

October 2014 averaged 62.9 degrees at DCA (3.4 degrees above the 1981-2010 normal), but only 57.0 at IAD (1.0 degree above the 1981-2010 normal). The 5.9 degree monthly differential brings the 10-month average differential to 5.4 degrees -- the second largest ever. The largest differential was in 1976, which averaged 6.2 degrees through October and 5.9 degrees for the year as a whole.

I have put together a decade-by-decade comparison of the average daily difference in maximum and minimum temperatures at Reagan and Dulles Airports. Overall, since IAD opened in November 1962, DCA has averaged 1.4 degrees higher in maximum temperature and 5.8 degrees higher in minimum temperature. Decade by decade it looks like this, with the first number the degrees by which the DCA average maximum has exceeded IAD's, and the second number the degrees by which the DCA average minimum has exceeded IAD's: 11-17-62/12-31-70: 2.1/5.3 1-1-71/12-31-80: 2.1/6.9 1-1-81/12-31-90: 0.8/6.4 1-1-91/12-31-2000: 1.0/5.7 1-1-2001/12-31-2010: 0.7/4.6 1-1-2011/09-30-2014: 1.9/5.6

I don't have monthly minimum/maximum temperatures at Dulles at my fingertips, but I'll try and dig some out. I do know that some recent months have not featured such extreme minimum and maximum temperature differentials between the two airports. For example, in September 2011 the high temperature differential was 0.8 degrees and the low temperature differential was 3.6 degrees. Still 2.8 degrees greater on the low side, but that compares with 3.8 degrees greater on the low side this September.

The beat goes on -- September 2014 averaged 73.9 degrees at DCA (2.9 degrees above the 1981-2010 normal), but only 67.5 at IAD (0.4 degrees below the 1981-2010 normal). The 6.4 degree monthly differential is the greatest since that same differential in January 1984.

With respect to overall temperatures, the 2014 astronomical summer at DCA was a virtual carbon copy of 2013, averaging a minimum of 69.5 degrees and a maximum of 85.8 degrees. And, believe it or not, the overall average was identical no matter how many decimal points you carry it out -- 77.66 degrees to two decimal points, which is the 26th warmest in DC history.

I believe that was just three years ago -- July 19-25, 2011 averaged 89.2 degrees at DCA, making it also the hottest week ever in DC history. July 2-8, 2012 gave that week a good run for its money, averaging 89.1 degrees. July 7, 2012 featured DC's hottest average daily temperature ever at 93.5 degrees, with a high of 105 and a low of 82.

Also, the week of August 31-September 6 was the hottest seven consecutive day period of the year at DCA, averaging 84.7 degrees. During the 2014 meteorological summer, the hottest week was June 27-July 3, which averaged just 81.8 degrees. Eyeballing historical DC temperature records to determine which year had the latest hottest week, 1985 may be the champion. September 4-10, 1985 averaged 84.3 degrees, which was hotter than the hottest meteorological summer week of August 10-16, 1985, at 82.6 degrees. Also, in 1881, which is not only the hottest September in DC history, but also the only year in which September was the hottest month of the year in DC, September 3-9, 1881 averaged 82.8 degrees, which was slightly hotter than the hottest meteorological summer week of July 10-16, 1881, at 82.6 degrees.

Remarkable August 2014 temperature differential between DCA and IAD. DCA averaged 77.7 degrees -- warmer than last August and only the 22nd coldest August there since 1963, when records began at Dulles. IAD, on the other hand, averaged just 71.8 degrees -- the coldest since the record of 70.4 in August 1992, and the third coldest August ever there. The 5.9 degree August 2014 differential between the airports was the greatest since that same differential in October 1997. The greatest monthly differential between the airports was December 1975, at 7.3 degrees.

I have data for DC only -- that's where the aspiring Ph.D meteorology student or meteorology professor comes in. But you're right -- if meteor dust is involved, one would think it would be a worldwide phenomenon. And if there is a bona fide connection, maybe there actually could be some merit in those weather planning services that advise folks six months or a year in advance as to the best day to hold an outdoor wedding: "Stay away from August 12th in the mid-Atlantic unless you have an ample supply of rafts for the reception."

This 1956 article -- http://www.atmos.was...eteors_1956.pdf -- states: "The rainfall peaks occur approximately 30 days after meteor showers, and it is suggested that they are due to the nucleating effect of meteoric dust falling into cloud systems in the lower atmosphere, the time difference being accounted for by the rate of fall of the material through the atmosphere." The 30-day time frame does not work for the Perseid meteor shower and August 12th and 15th rainfall in DC, but perhaps some aspiring Ph.D meteorology student or meteorology professor can look into this general theory with 21st Century technology at his/her disposal. The effect could be more complex than the author of the above article envisioned; for example, perhaps a series of events has to occur, and then a meteor shower can trigger a significant rain event.

Time for an update. With yesterday's 1.60 inches of rain at DCA, August 12th rainfall there during 1991-2014 totals 7.20 inches, vs just 0.63 inches on August 15th during 1991-2013 (and the strong likelihood of a dry day this Friday, the 15th). So, the historical rainfall totals for these two days, 1871-2014 in DC, are: August 12th: 42.55 inches vs August 15th: 11.12 inches. The rainiest August 15th in DC history would rank as just the 11th rainiest on August 12th. Coincidence, or does the annual Perseid meteor shower have something to do with it?

This temperature differential to this point in the year is the 4th greatest between the two airports. The greatest was 6.6 in 1976, followed by 5.33 in 1981, and 5.26 in 1975. The smallest was 1.4 in 2009. For the year as a whole, the greatest was 5.9 in 1976 and the smallest was 1.7 in 2009.

DCA is an outlier for the mid-Atlantic, with a 2014 average monthly departure from the 1981-2010 January-July normals of only -0.6 degrees. IAD's average monthly departure is -2.9 degrees.

The first seven months at DCA this year averaged 56.5 degrees -- 5.1 degrees higher than IAD. That makes January-July 2014 at DCA only the 20th coldest during the 52 years of IAD's existence. In fact, the coldest first seven months at DCA during that period was just 11 years ago -- 2003 averaged 54.0 degrees there.

Pretty ho-hum historical average July 2014 temperature at DCA at just 0.4 degrees below the 1981-2010 normal. IAD, however, was 2.5 degrees below that normal, and for the first seven months of the year its average temperature is 51.4 degrees -- fifth coldest of 52 years, and the coldest since 1984. 1970 was the coldest first seven months at IAD, averaging 50.5 degrees.

BWI got down to 55 this morning (July 30th) to beat the old record low by a degree. Dulles made it all the way down to 48 this morning to beat the old record low by 3 degrees.