December 17: ORD

Today is December 17th. It's the 351st day of the year. 14 days remain. It's week 51 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 34 and the normal low is 20. I've calculated the 33-year averages at 31 and 19.

On the warm side, in 2006 the high was 54 (O'Hare's most recent 50s high) with a low of 41 (O'Hare's most recent 40s low). Five years prior, in 2001, the high was 46 (our most recent 40s high) with a low of 33. In 1993 the high was 41 with a low of 36, a five degree spread. The other warm high was 48 in 1997. The other warm lows were 34 in 1990, 33 in 1986 and 32 in 1994. The record high of 60 and record warmest low of 44 were set on the same warm day in 1977 at Midway. 14 of the past 33 highs (43%) were in the 30s. Seven highs (21%) were in the warm: 40s (6) and 50s (1). The 50s highs, pre-1978 were 50 in 1962 and 56 in 1977, O'Hare's warmest. 12 highs (36%) were in the cold: 0s (a.k.a. single digits) (1), 10s (a.k.a. teens) (4) and 20s (7).

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 56 with a low of 41, a warm day and year of both the record high of 60 and record warmest low of 44 at Midway. In 1975 the high was 20 with a low of 8, a cold day. In 1973 the low was a cold 7 degrees. In 1972 the high was 23 with a cold low of -5, a cold day. In 1970 the high was 38 with a warm low of 36, a mere two degree spread. In 1966 the low was a warm 32 degrees. In 1964 the high was 24 with a cold low of 2, a cold day. In 1963 the high was 15 with a low of -9, a cold day. In 1962 the high was a warm 50 degrees. In 1961 the high was 34 with a low of 31, a three degree spread.

More recently, there were no large spreads of 30 degrees or more. There were small spreads of five degrees in 1990 (39/34), six degrees in 1992 (33/27) and seven degrees in four years: 1982 (32/25), 1986 (40/33), 2002 (37/30) and 2003 (26/19).

On the cold side, in 1985 the high was 13 with a low of -6. Two years prior, in 1983, the high was 9 (O'Hare's coldest high) with a low of 0. In 2000 the high was 16 (our most recent 10s high) with a low of -1. In 1989 the high was 16 with a low of 3. In 1979 the high was 19 with a low of 1. Three years ago, in 2008, the high was 20 with a low of 6. Three years before that, in 2005, the high was 20 with a low of 11. In 1988 the high was 21 with a low of 10. The other cold high was 22 last year, in 2010. The record low of -7 was set in 1951. The record coldest high of 8 was set in 1945. Both records were set at Midway. 13 of the last 33 lows (39%) were in the 10s. Six lows (18%) were in the cold: negative 0s (2) and 0s (a.k.a. single digits) (4). O'Hare's two pre-1978 negative lows were -5 in 1972 and -9 in 1963 (O'Hare's coldest). 14 lows (43%) were in the warm: 20s (7), 30s (6) and 40s (1). Our most recent: 20s low was 26 in 1998 and 30s low was 30 in 2002. O'Hare also had a low of 41 in 1977. This ties the low of 41 in 2006 as the warmest lows.

There were two instances of back-to-back years in which temperatures were just about as opposite as they can be: 1. In 2000 the high was 16 with a low of -1, overall 19.5 degrees BELOW normal. Then in 2001 the high was 46 with a low of 33, overall 12.5 degrees ABOVE normal. 2. In 2005 the high was 20 with a low of 11, overall 11.5 degrees BELOW normal. Then in 2006 the high was 54 with a low of 41, overall 20.5 degrees ABOVE normal.

In 1973, O'Hare had 1.4 inches of snow today. This was the fifth day of a string of eight days through the 20th which produced 17.6 inches, or 94% of the month's total of 18.8 inches.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:44 and sunrise is at 7:16. Sunset is at 16:22 and twilight ends at 16:54. There's a total of 9 hours, 6 minutes of daylight today and 10 hours, 10 minutes between twilights. We lost no minutes of daylight from yesterday. Tomorrow twilight begins at 6:45 and sunrise is at 7:17. The moon reaches last quarter today at 18:48. Illumination was 59% at midnight, will be 53% at noon and 48% at midnight tonight. Moonset is at 11:24 this morning. Moonrise is at 0:11 tomorrow morning. Moonset is at 11:53 tomorrow morning. Moonlight time is 11 hours, 24 minutes which is 30 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. Christmas Eve is in seven days. Christmas is in eight days.