December 15: ORD

Today is December 15th. It's the 349th day of the year. 16 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 37 and 22.

On the warm side, in 1998 the high was 51 (our most recent 50s high) with a low of 37. In 1992 the high was 56 with a low of 31. The next year, 1993, the high was 46 with a low of 38. In 1984 the high was 50 with a low of 33. In 1990 the high was 45 with a low of 34. The other warm high was 47 in 1986. The other warm lows were 33 in both 1982 and 1999. The record high of 64 was set in 1971 at Midway. The record warmest low of 44 was set back in 1928, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the past 33 highs (70%) were in the: 30s (13) and 40s (10). There were ten consecutive highs in the 30s & 40s from 1999 through 2008. Three highs were in the warm 50s. Pre-1978, the warm highs were 52 in 1959 and O'Hare's warmest for the date, a high of 62 in 1971. This was also O'Hare's only 60s high for the date. Seven highs were in the cold: 10s (a.k.a. teens) (1) and 20s (6). O'Hare's coldest high was 11 in 1963.

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 low of 32, a seven degree spread. In 1974 the high was 43 with a warm low of 36, a seven degree spread. In 1973 the high was 28 with a low of 26, a mere two degree spread. In 1972 the high was 23 with a low of 9, a cold day. In 1971 the high was 62 with a low of 39, a warm day and year of the record high of 64 at Midway. In 1964 the high was 30 with a cold low of 1, a cold day. In 1963 the high was 11 with a low of -6, a cold day. In 1959 the high was 52 with a low of 34, a warm day. In 1958 the high was 14 with a low of -9, a cold day.

More recently, there were no large spreads of 30 degrees or more. There were small spreads of five degrees in 1982 (38/33), five degrees in 1999 (38/33) and six degrees in 2007 (31/25).

On the cold side, in 1989 the high was 16 with a low of -8. Last year, in 2010, the high was 25 with a low of 2. The other cold lows were 3 in 2000, 5 three years ago (in 2008), 7 in 1988, 7 in 1991 and 8 two years ago, in 2009. The other cold high was 22 in 1981. The record low of -12 and record coldest high of -2 were set on the same cold day way back in 1901, at a time when the official readings were taken adjacent to Lake Michigan. 21 of the last 33 lows (64%) were in the: 20s (11) and 30s (10). There were eight consecutive 20s & 30s lows from 1992 through 1999. 12 lows (36%) were in the cold: negative 0s (1), 0s (a.k.a. single digits) (6) and 10s (a.k.a. teens) (5). Our most recent 10s low was 18 in 2004. The pre-1978 negative lows were -6 in 1963 and O'Hare's coldest for the date, a low of -9 in 1958. O'Hare has never had a warm low of 40 or higher for the date.

In 1973, O'Hare had 3.2 inches of snow today. This was the third 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. In 1987, O'Hare had 8 inches, day two of a two-day storm of 9.1 inches.

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be: in 1989 the high was 16 with a low of -8, overall 24 degrees BELOW normal. Then in 1990 the high was 45 with a low of 34, overall 11.5 degrees ABOVE normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:43 and sunrise is at 7:15. Sunset is at 16:22 and twilight ends at 16:54. There's a total of 9 hours, 7 minutes of daylight today and 10 hours, 11 minutes between twilights. We lost a minute of daylight from yesterday. Tomorrow twilight begins at 6:44 and sunrise is at 7:16. The moon is waning gibbous. Illumination was 79% at midnight, will be 74% at noon and 69% at midnight tonight. Moonset is at 10:27 this morning. Moonrise is at 21:53 tonight. Moonset is at 10:55 tomorrow morning. Moonlight time is 12 hours, 34 minutes which is 37 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 today.