December 21: ORD

Today is December 21st. It's the 355th day of the year. 10 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 33 and 18.

On the warm side, in 2006 the high was 50 with a low of 38. In 1984 the high was 56 with a low of 30. A decade later, in 1994, the high was 51 with a low of 30. In 1979 the high was 48 with a low of 28. The other warm highs were 47 in 2003 (our most recent 40s high) and 45 in 1990. The other warm low was 33 in 2007. The record high of 62 was set in 1967 at Midway. The record warmest low of 49 occurred way back in 1918, at a time when the official readings were taken adjacent to Lake Michigan. 15 of the past 33 highs (46%) were in the 30s. Eight highs (24%) were in the warm: 40s (5) and 50s (3). Ten highs (30%) were in the cold: 0s (a.k.a. single digits) (1), 10s (a.k.a. teens) (4) and 20s (5).

In looking back through the years to the first December at O'Hare in 1958, the following instances were notable: in 1976 the high was 19 with a low of 4, a cold day. In 1973 the high was 20 with a low of 2, a cold day. In 1972 the high was 35 with a warm low of 33, a mere two degree spread. In 1967 the high was a warm 61 with a low of 19, a 42 degree spread, a warm day and also the year of the record high of 62 at Midway. Also in 1967, the temperature fell 53 degrees to a low of 8 on the 23rd. In 1965 the high was 35 with a low of 29, a six degree spread. In 1964 the high was 40 with a low of 10, a 30 degree spread. In 1963 the high was 13 with a low of -11, a cold day. In 1962 the high was 30 with a low of 24, a six degree spread. In 1960 the high was 9 with a low of -7, a cold day.

More recently, there were no large spreads of 30 degrees or more. There were small spreads of four degrees two years ago, in 2009 (29/25), five degrees in 1993 (29/24), five degrees in 2007 (38/33), five degrees last year, in 2010 (31/26), six degrees in 2002 (34/28) and seven degrees in 1983 (23/16).

On the cold side, in 1989 the high was the record coldest high of 0 with the record low of -14. Three years ago, in 2008, the high was 16 with a low of -6. In 2000 the high was 19 with a low of -7. A year earlier, in 1999, the high was 14 with a low of 1. In 1985 the high was 18 with a low of -2. In 2005 the high was 20 with a low of 3. In 1980 the high was 21 with a low of 5. 22 of the last 33 lows (67%) were in the: 10s (a.k.a. teens) (8) and 20s (14). Seven lows (21%) were in the cold: negative 10s (1), negative single digits (3) and single digits (3). Until 1989, O'Hare's coldest low was -11 in 1963. Four lows (12%) were in the warm 30s.

There were three instances of back-to-back years in which temperatures were just about as opposite as they can be. 1. In 1979 the high was 48 with a low of 28, overall 12 degrees ABOVE normal. Then in 1980 the high was 21 with a low of 5, overall 13 degrees BELOW normal. 2. In 1984 the high was 56 with a low of 30, overall 17 degrees ABOVE normal. Then in 1985 the high was 18 with a low of -2, overall 18 degrees BELOW normal. 3. In 2005 the high was 20 with a low of 3, overall 14.5 degrees BELOW normal. Then in 2006 the high was 50 with a low of 38, overall 18 degrees ABOVE normal.

In 1969, O'Hare had 0.3 inches of snow today. This was the first of four days of a total of 13.5 inches of snow.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:47 and sunrise is at 7:19. Sunset is at 16:24 and twilight ends at 16:56. There's a total of 9 hours, 5 minutes of daylight today and 10 hours, 9 minutes between twilights. We lost a minute of daylight from yesterday. Today is one of the shortest days of the year. The Winter Solstice is at 23:30 tonight. Tomorrow twilight begins at 6:47 and sunrise is at 7:19. The moon is waning crescent. Illumination was 16% at midnight, will be 12% at noon and 8% at midnight tonight. Moonrise is at 3:50 this morning. Moonset is at 13:44 this afternoon. Moonrise is at 5:02 tomorrow morning. Moonlight time is 9 hours, 54 minutes which is 31 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 three days. Christmas is in four days.