December 20: ORD

Today is December 20th. It's the 354th day of the year. 11 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 33 and the normal low is 19. I've calculated the 33-year averages at 32 and 17.

On the warm side, in 1988 the high was 56 (O'Hare's warmest of the date) with a low of 32. Two years later, in 1990, the high was 49 with a low of 30. Four years after that, in 1994, the high was 42 with a low of 34 (our most recent 30s low). The record high of 67 and record warmest low of 55 were set on the same warm day way back in 1877, at a time when the official readings were taken adjacent to Lake Michigan. 14 of the past 33 highs (43%) were in the 30s. Eight highs (24%) were in the warm: 40s (7) and 50s (1). Prior to 1978, O'Hare's had a warm high of 55 in 1967. 11 highs (33%) were in the cold: 0s (a.k.a. single digits) (1), 10s (a.k.a. teens) (4) and 20s (6). Our most recent 10s high was 17 in 2000. Prior to 1978, O'Hare had a cold high of 6 in 1963.

In looking back through the years to the first December at O'Hare in 1958, the following instances were notable: in 1976 the low was a cold 6 degrees. In 1972 the high was 37 with a warm low of 34, a mere three degree spread. In 1968 the high was 33 with a low of 26, a seven degree spread. In 1967 the high was 55 with a low of 36, a warm day. In 1966 the high was 36 with a warm low of 32, a four degree spread. In 1965 the high was 35 with a low of 29, a six degree spread. In 1964 the low was a cold 7 degrees. In 1963 the high was 6 with a low of -16, a cold day. That low of -16 was colder than the record low of -9, set at Midway. The high of 6 was near the record coldest high of 4, set way back in 1901. In 1960 the high was 22 with a cold low of 6, a cold day. In 1959 the high was 31 with a low of 28, a mere three degree spread. In 1958 the high was a cold 21 with a low of 8, a cold day.

More recently, there was a large spread of 32 degrees in 1999 (37/5). There were small spreads of four degrees two years ago, in 2009 (30/26), five degrees in 1991 (35/30) and six degrees in 2002 (35/29).

On the cold side, in 1989 the high was 5 (O'Hare's coldest high of the date) with a low of -6. In 1980 the high was 10 with a low of -1. In 2000 the high was 17 with a low of -5 (our most recent negative single digit low). In 1983 the high was 20 with a low of -1. In 1996 the high was 17 with a low of 3. In 1981 the high was 22 with a low of -1. In 1985 the high was 19 with a low of 3. In 2005 the high was 21 with a low of 5. The other cold lows were 5 in 1999 and 6 degrees last year, in 2010. The record low of -9 was set in 1963 at Midway. The record coldest high of 4 occurred way back in 1901, at a time when the official readings were taken adjacent to Lake Michigan. 14 of the last 33 lows (42%) were in the 20s. 15 lows (46%) were in the cold: negative single digits (5), single digits (5) and 10s (a.k.a. teens) (5). O'Hare's coldest temperature for the date was -16 in 1963. Four lows (12%) were in the warm 30s. Those 30s lows happened in a seven year period from 1988 through 1994.

There was an instance of back-to-back-to-back years in which temperatures were just about as opposite as they can be. In 1988 the high was 56 with a low of 32, overall 18 degrees ABOVE normal. Then in 1989 the high was 5 with a low of -6, overall 26.5 degrees BELOW normal. Then in 1990 the high was 49 with a low of 30, overall 13.5 degrees ABOVE normal.

In 1973, O'Hare had 0.1 inches of snow today. This was the eighth day of a string of eight days which produced 17.6 inches, or 94% of the month's total of 18.8 inches. In 1960, Midway had 11 inches while O'Hare had just 3.1 inches. O'Hare also had 1.1 on the 19th.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:46 and sunrise is at 7:18. Sunset is at 16:24 and twilight ends at 16:56. There's a total of 9 hours, 6 minutes of daylight today and 10 hours, 10 minutes between twilights. We gained a minute of daylight from yesterday. Tomorrow twilight begins at 6:47 and sunrise is at 7:19. The moon is waning crescent. Illumination was 26% at midnight, will be 21% at noon and 16% at midnight tonight. Moonrise is at 2:36 this morning. Moonset is at 13:01 this afternoon. Moonrise is at 3:50 tomorrow morning. Moonlight time is 10 hours, 25 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. Christmas Eve is in four days. Christmas is in five days.