December 23: ORD

Today is December 23rd. It's the 357th day of the year. Eight 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 34 and 20.

On the warm side, in 1979 the high was 62 with a low of 46 (O'Hare's warmest low of the date). That high of 62 ties the record, but O'Hare was not the official station of record in 1979. Just three years later, in 1982, the high was the record high of 62 with a low of 42 (our most recent 40s low). In 1996 the high was 52 with a low of 32. In 1988 the high was 48 with a low of 33. In 2005 the high was 44 with a low of 37, a seven degree spread. In 1994 the high was 42 with a low of 35, a seven degree spread. The other warm high was 52 in 2007, our most recent 50s high. The other warm lows were 33 in 2006, 32 in 1997, 31 in 1980 and 31 two years ago, in 2009. The record warmest low of 49 occurred way back in 1877, at a time when the official readings were taken adjacent to Lake Michigan. 20 of the past 33 highs (61%) were in the: 30s (13) and 40s (7). Four highs (12%) were in the warm: 50s (2) and 60s (2). Prior to 1979, O'Hare's warmest high was 55 in 1965. Nine highs (27%) were in the cold: negative single digits (1), single digits (1), 10s (a.k.a. teens) (3) and 20s (4).

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 1975 the high was 34 with a low of 27, a seven degree spread. In 1972 the high was 32 with a low of 30, a mere two degree spread. In 1971 the high was 46 with a low of 33, a warm day. In 1969 the high was 28 with a low of 24, a four degree spread. In 1965 the high was 55 with a low of 41, a warm day. In 1964 the high was 48 with a low of 37, a warm day. In 1963 the high was 22 with a cold low of -8, a 30 degree spread and cold day. In 1962 the high was 24 with a cold low of 0, a cold day. In 1961 the high was 31 with a low of 24, a seven degree spread. In 1960 the high was 11 with a low of -17, a very cold day. In 1959 the high was 29 with a low of 25, a four degree spread.

More recently, there was a large spread of 38 degrees in 2007 (52/14). There were small spreads of one degree two years ago, in 2009 (32/31), five degrees in 1986 (34/29), five degrees in 1997 (37/32), seven degrees in 1980 (38/31), seven degrees in 1995 (30/23) and seven degrees in 2006 (40/33).

On the cold side, in 1983 the high was the record coldest high of -6 with the record low of -21. In 1989 the high was 8 with a low of -11. A year later, in 1990, the high was 11 with a low of 0. A decade later, in 2000, the high was 20 with a low of -4. A year earlier, in 1999, the high was 16 with a low of 3. Five years later, in 2004, the high was 18 with a low of 5. The other cold lows were 7 degrees, in both 1992 and three years ago in 2008. 28 of the last 33 lows (85%) were in the: single digits (6), 10s (a.k.a. teens) (5), 20s (8) and 30s (9). Our most recent 20s low was 26 in 2003. Three lows were in the cold: negative 20s (1), negative 10s (1) and negative single digits (1). Prior to 1983, O'Hare had below zero lows of -17 in 1960 and -8 in 1963. Two lows were in the warm 40s. Prior to 1979, O'Hare's warmest low was 41 in 1965.

There were three instances of back-to-back years in which temperatures were just about as opposite as they can be. 1. In 1982 the high was 62 with a low of 42, overall 26 degrees ABOVE normal. Then in 1983 the high was -6 with a low of -21, overall 39.5 degrees BELOW normal. 2. In 1988 the high was 48 with a low of 33, overall 14.5 degrees ABOVE normal. Then in 1989 the high was 8 with a low of -11, overall 27.5 degrees BELOW normal. As reinforcement, in 1990 the high was 11 with a low of 0, 20.5 degrees BELOW normal. 3. In 2004 the high was 18 with a low of 5, overall 14.5 degrees BELOW normal. Then in 2005 the high was 44 with a low of 37, overall 14.5 degrees ABOVE normal

In 1969, O'Hare had 9.9 inches of snow today. This was the third of four days of a total of 13.5 inches of snow. In 1961 Midway had 10.2 inches of snow. O'Hare had 7.4 inches in 1961, part of a 3-day event of 8.2 inches (including half an inch on the 21st and three tenths on the 22nd).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:48 and sunrise is at 7:20. Sunset is at 16:25 and twilight ends at 16:57. 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. Tomorrow twilight begins at 6:48 and sunrise is at 7:20. The moon is waning crescent and will reach new tomorrow at 12:06. Illumination was 3% at midnight, will be 1% at noon and 0% at midnight tonight. Moonrise is at 6:09 this morning. Moonset is at 15:33 this afternoon. Moonrise is at 7:08 tomorrow morning. Moonlight time is 9 hours, 24 minutes which is 8 minutes shorter than yesterday. Moonlight time is at it's least today.

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 tomorrow. Christmas is in two days.