December 13: ORD

Today is December 13th. It's the 347th day of the year. 18 days remain. It's week 50 of the year.

Unless noted, the following data is based upon observations collected by the National Weather Service, Chicago, at O'Hare International Airport (ORD) between November 1978 and last year. ORD has been the official site of record for Chicago since Thursday, January 17, 1980. Before this, the official site was on the south side of Chicago at Midway International Airport (MDW), beginning on Wednesday, July 1, 1942. Before that, the official site was at various locations in Chicago going back to Monday, October 16, 1871. Observations taken from October 15, 1870 to October 8, 1871 were lost in the Great Chicago Fire.

According to the National Climatic Data Center (NCDC), the normal high is 35 and the normal low is 21. I've calculated the 33-year averages at 35 and 22.

On the warm side, in 1993 the high was 46 with a low of 35. In 2006 the high was 45 with a low of 33. The other warm high was 49 in 1998. The other warm low was 33 in 2002. The record high of 67 was set in 1975 at Midway. The record warmest low of 46 occurred back in 1920, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the past 33 highs (55%) were in the 30s. There were eight consecutive 30s highs from 1977 through 1984. Eight highs (24%) were in the warm 40s. Seven highs (21%) were in the cold: 10s (a.k.a. teens) (2) and 20s (5). Our most recent 20s high was 28 in 2005. The pre-1978 cold highs were 12 in 1958, 12 in 1963 and 16 in 1961. The last time it hit 60 or higher was 36 years ago in 1975. That year it was 64 at O'Hare (O'Hare's warmest), 57 at Rockford, 67 at Midway and 60 at Marengo. O'Hare has never had a high in the 50s or low in the 40s in it's history on this date!

In looking back through the years to the first December at O'Hare in 1958, the following instances were notable: in 1977 the high was 39 with a warm low of 36, a mere three degree spread. In 1976 the high was 23 with a low of 2, a cold day. In 1975 the high was 64 with a low of 38, a warm day and year of the record high of 67. In 1972 the high was 22 with a low of 8, a cold day. In 1970 the high was 32 with a low of 25, a seven degree spread. In 1968 the high was a warm 49 with a low of 18, a 31 degree spread. In 1966 the high was 29 with a low of 23, a six degree spread. In 1965 the low was a warm 33 degrees. In 1964 the high was 35 with a low of 28, a seven degree spread. In 1963 the high was 12 with a low of -2, a cold day. In 1962 the high was 20 with a low of 2, a cold day. In 1961 the high was 16 with a low of 1, a cold day. In 1958 the high was 12 with a low of -8, a cold day.

More recently, there were no other large spreads of 30 degrees or more. There were small spreads of four degrees in 1983 (35/31), five degrees in 1984 (37/32), five degrees in 2005 (28/23), five degrees two years ago, in 2009 (37/32), six degrees in 2002 (39/33), six degrees in 2007 (34/28) and seven degrees in 1981 (36/29).

On the cold side, in 1985 the high was 12 with a low of -3. In 2000 the high was 24 with a low of -8 (our most recent negative low), a 32 degree spread. Last year, in 2010, the high was 16 with a low of 7. In 1989 the high was 20 with a low of 6. Three years earlier, in 1986, the high was 25 with a low of 1. The record low of -13 and record coldest high of 3 were set on the same cold day in 1903, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the last 33 lows (70%) were in the: 20s (15) and 30s (8). There were ten consecutive 20s & 30s lows from 1990 through 1999. Ten lows (30%) were in the cold: negative 0s (2), 0s (a.k.a. single digits) (3) and 10s (a.k.a. teens) (5). The pre-1978 negative lows were -8 in 1958 and -2 in 1963. The lows of -8 (in 1958 & 2000) are O'Hare's coldest of the date.

In 1973, O'Hare had 4 inches of snow today. This was the first day of a string of days through the 20th which produced 17.6 inches, or 94% of the month's total of 18.8 inches.

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 2000 the high was 24 with a low of -8, overall 20 degrees BELOW normal. Then in 2001 the high was 46 with a low of 31, overall 10.5 degrees ABOVE normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:42 and sunrise is at 7:13. Sunset is at 16:21 and twilight ends at 16:53. There's a total of 9 hours, 8 minutes of daylight today and 10 hours, 11 minutes between twilights. We lost no minutes of daylight from yesterday. Tomorrow twilight begins at 6:42 and sunrise is at 7:14. The moon is waning gibbous, having reached the full Cold or Long Nights moon three days ago at 8:36. Illumination was 93% at midnight, will be 90% at noon and 87% at midnight tonight. Moonset is at 9:22 this morning. Moonrise is at 19:38 tonight. Moonset is at 9:56 tomorrow morning. Moonlight time is 13 hours, 44 minutes which is 26 minutes shorter than yesterday.

Winter began December 1st and runs for 102 days through March 11th. High temperatures in this period are usually in the 20s and 30s. Highs in the 40s and 50s occur infrequently. Bill of Rights Day is in two days.