December 25: ORD

Today is December 25th. It's the 359th day of the year. Six days remain. It's week 52 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. Observations have been official at O'Hare 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 32 and the normal low is 18. I've calculated the 33-year averages at 29 and 13.

On the warm side, in 1982 the high was the record high of 64 (O'Hare's only 60s high of the date) with a low of 36. In 1994 the high was 52 (O'Hare's only 50s high of the date) with a low of 24. The other warm lows were 33 in 2005 and 30 in 2006. The record warmest low of 46 occurred in 1936, at a time when the official readings were taken adjacent to Lake Michigan. 20 of the past 33 highs (61%) were in the: 20s (7) and 30s (13). On the warm side, five highs (15%) were in the: 40s (3), 50s (1) and 60s (1). On the cold side, eight highs (24%) were in the: negative single digits (1), single digits (2) and 10s (a.k.a. teens) (5). O'Hare's most recent 10s high was 19 in 2004.

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 14 with a low of -2, a cold day. In 1975 the high was 36 with a warm low of 31, a five degree spread. In 1973 the high was 40 with a warm low of 37, a three degree spread and warm day. In 1972 the high was 35 with a warm low of 31, a four degree spread. In 1971 the high was 45 with a low of 30, a warm day. In 1969 the high was 28 with a low of 21, a seven degree spread. In 1968 the high was 18 with a low of 0, a cold day. In 1964 the high was 27 with a low of 23, a four degree spread. In 1961 the low was a cold 6 degrees.

More recently, we had small spreads of three degrees in 2005 (36/33), six degrees in 1986 (32/26), six degrees in 2002 (29/23), seven degrees in 1987 (36/29) and seven degrees in 2006 (37/30).

On the cold side, in 1983 the high was the record coldest high of -5 with the record low of -17. Two years later, in 1985, the high was 2 with a low of -11. Five years earlier, in 1980, the high was 17 with a low of -8. Two decades later, in 2000, the high was 17 with a low of -7. Four years before that, in 1996, the high was 9 with a low of 1. In 1984 the high was 19 with a low of -3. In 1993 the high was 18 with a low of 5. Three years ago, in 2008, the high was 24 with a low of 0. In 1990 the high was 28 with a low of -1. The other cold high was 19 in 2004. 20 of the last 33 lows (61%) were in the: 10s (11) and 20s (9). O'Hare's most recent 10s low was 11 in 2004. On the cold side, ten lows (30%) were in the: negative 10s (a.k.a. negative teens) (2), negative single digits (4) and single digits (4). Prior to 1980, O'Hare's coldest low was -2 in 1977. On the warm side, three lows (9%) were in the 30s. Through Winter 2010-11, this is the average sixth coldest day of the year.

There were two instances of back-to-back years in which temperatures were just about as opposite as they can be. In the first instance, in 1982 the high was 64 with a low of 36, overall 25 degrees ABOVE normal. Then in 1983 the high was -5 with a low of -17, overall 36 degrees BELOW normal. In the second instance, in 1993 the high was 18 with a low of 5, overall 13.5 degrees BELOW normal. Then in 1994 the high was 52 with a low of 24, overall 13 degrees ABOVE normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:49 and sunrise is at 7:20. Sunset is at 16:26 and twilight ends at 16:58. There's a total of 9 hours, 6 minutes of daylight today and 10 hours, 9 minutes between twilights. Daylight time is the same as yesterday. Tomorrow twilight begins at 6:49 and sunrise is at 7:21. The moon is waxing crescent, having reached new yesterday at 12:06. Illumination was 0% at midnight, will be 1% at noon and 3% at midnight tonight. Moonrise is at 7:57 this morning. Moonset is at 17:48 this afternoon. Moonrise is at 8:38 tomorrow morning. Moonlight time is 9 hours, 51 minutes which is 20 minutes longer 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 is today.