December 19: ORD

Today is December 19th. It's the 353rd day of the year. 12 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 32 and 18.

On the warm side, in 2002 the high was 55 (our most recent 50s high) with a low of 34. Five years earlier, in 1997, the high was 50 with a low of 31. In 1988 the high was 56 (O'Hare's warmest high for the date) with a low of 23, a 33 degree spread. The other warm low was 33 in 1993. The record high of 64 and record warmest low of 57 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. There were six consecutive 30s highs from 1990 through 1995. Nine highs (27%) were in the warm: 40s (6) and 50s (3). Ten highs (30%) were in the cold: 0s (a.k.a. single digits) (1), 10s (a.k.a. teens) (7) and 20s (2). Prior to 1978, O'Hare had it's coldest high of 2 in 1963. All highs prior to 1978 were below 50 degrees.

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 39 with a warm low of 35, a four degree spread. In 1976 the high was 49 with a low of 35, a warm day. In 1973 the high was 30 with a low of 24, a six degree spread. In 1972 the high was 37 with a warm low of 35, a mere two degree spread. In 1970 the high was a warm 46 degrees. In 1968 the high was a warm 47 with a low of 31, a warm day. In 1966 the high was 37 with a low of 31, a six degree spread. In 1963 the high was 2 with a low of -14, a cold day. That high of 2 was colder than the record coldest high of 5 at Midway this same year and the low of -14 equals the record low at O'Hare in 1983. In 1962 the high was 45 with a warm low of 35, a warm day. In 1961 the high was 36 with a low of 30, a six degree spread. In 1960 the high was 24 with a cold low of 5, a cold day.

More recently, there were no other large spreads of 30 degrees or more. There were small spreads of three degrees in 1993 (36/33), four degrees two years ago, in 2009 (33/29), six degrees in 1978 (37/31), six degrees in 1982 (35/29) and six degrees in 1987 (37/31).

On the cold side, in 1983 the high was 6 with the record low of -14. O'Hare also had it's only pre-1978 negative low of -14 in 1963. But in 1963 the official station of record was Midway, so this is not considered a co-record low. In 2005 the high was 11 with a low of -1. In 1985 the high was 15 with a low of -5. In 1996 the high was 13 with a low of 3. In 1989 the high was 15 with a low of 1. In 1980 the high was 16 with a low of 2. A year later, in 1981, the high was 19 with a low of 1. In 2004 the high was 18 with a low of 4. In 2000 the high was 20 with a low of 7. Last year, in 2010, the high was 21 with a low of 8. The other cold low was 6 in 1991. The record coldest high of 5 occurred in 1963 at Midway. 15 of the last 33 lows (46%) were in the 20s. 12 lows (36%) were in the cold: negative 10s (1), negative single digits (2), single digits (8) and 10s (a.k.a. teens) (1). Six lows (18%) were in the warm 30s.

There were two instances of back-to-back years in which temperatures were just about as opposite as they can be: 1. In 1988 the high was 56 with a low of 23, overall 13.5 degrees ABOVE normal. Then in 1989 the high was 15 with a low of 1, overall 18 degrees BELOW normal. 2. In 1996 the high was 13 with a low of 3, overall 18 degrees BELOW normal. Then in 1997 the high was 50 with a low of 31, overall 14.5 degrees ABOVE normal.

In 1973, O'Hare had 3.5 inches of snow today. This was the seventh day of a string of eight days (through tomorrow) 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 6:46 and sunrise is at 7:18. Sunset is at 16:23 and twilight ends at 16:55. 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 our shortest days. Tomorrow twilight begins at 6:46 and sunrise is at 7:18. The moon is waning crescent, having reached last quarter two days ago at 18:48. Illumination was 36% at midnight, will be 31% at noon and 26% at midnight tonight. Moonrise is at 1:23 this morning. Moonset is at 12:25 this afternoon. Moonrise is at 2:26 tomorrow morning. Moonlight time is 11 hours, 2 minutes which is 40 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 five days. Christmas is in six days.