December 18: ORD

Today is December 18th. It's the 352nd day of the year. 13 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 (O'Hare's warmest high and our most recent 50s high) with a low of 37 (tied with 1970 as O'Hare's warmest low). In 1998 the high was 53 with a low of 32. A year earlier, in 1997, the high was 46 with a low of 32. Four years later, in 2001, the high was 48 with a low of 29. Note that the four warm highs occurred in a six year span from 1997 through 2002! The other warm lows were 35 in 1993, 33 in 1995 and 32 in 1982. The record high of 62 and record warmest low of 43 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 (6) and 50s (2). Prior to 1978, O'Hare had a high of 53 in 1962. 11 highs (33%) were in the cold: 0s (a.k.a. single digits) (1), 10s (a.k.a. teens) (6) and 20s (4). Prior to 1978, O'Hare had a cold high of 9 in 1963.

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 42 with a warm low of 36, a six degree spread and warm day. In 1976 the high was a warm 47 degrees. In 1975 the high was 17 with a low of 4, a cold day. In 1973 the high was 26 with a low of 20, a six degree spread. In 1970 the high was 46 with a low of 37, a warm day. In 1968 the high was 37 with a warm low of 32, a five degree spread. In 1967 the high was 48 with a low of 33, a warm day. In 1964 the high was 18 with a low of 0, a cold day. In 1963 the high was 9 with a low of -9, a cold day. In 1962 the high was 53 with a low of 32, a warm day. In 1961 the high was 35 with a low of 29, a six degree spread.

More recently, there were no large spreads of 30 degrees or more. There were small spreads of four degrees in 1995 (37/33), five degrees in 1986 (34/29), five degrees two years ago, in 2009 (36/31) and seven degrees in 1990 (37/30).

On the cold side, in 1983 the high was 3 (O'Hare's coldest high) with the record low of -11. Two years later, in 1985, the high was 10 with a low of -9. In 2000 the high was 16 with a low of -1. In 1989 the high was 16 with a low of 2. In 2005 the high was 15 with a low of 4. In 1996 the high was 18 with a low of 7. Three years ago, in 2008, the high was 23 with a low of 3. Last year, in 2010, the high was 19 with a low of 10. In 1991 the high was 22 with a low of 7. A decade earlier, in 1981, the high was 24 with a low of 7. The other cold low was 6 in 1999. The record coldest high of 1 occurred way back in 1884, at a time when the official readings were taken adjacent to Lake Michigan. 30 of the last 33 lows (91%) were in the: 0s (a.k.a. single digits) (7), 10s (a.k.a. teens) (8), 20s (6) and 30s (9). Three lows were in the cold: negative 10s (1) and negative 0s (a.k.a. negative single digits) (2). O'Hare had a pre-1978 negative low of -9 in 1963. O'Hare has never had a low in the warm 40s.

There were three instances of back-to-back years in which temperatures were just about as opposite as they can be: 1. In 1981 the high was 24 with a low of 7, overall 11.5 degrees BELOW normal. Then in 1982 the high was 43 with a low of 32, overall 10.5 degrees ABOVE normal. Then in 1983 the high was 3 with the record low of -11, overall 31 degrees BELOW normal. 2. In 1996 the high was 18 with a low of 7, overall 14.5 degrees BELOW normal. Then in 1997 the high was 46 with a low of 32, overall 12 degrees ABOVE normal. As reinforcement, in 1998 the high was 53 with a low of 32, overall 15.5 degrees ABOVE normal. 3. In 2000 the high was 16 with a low of -1, overall 19.5 degrees BELOW normal. Then in 2001 the high was 48 with a low of 29, overall 11.5 degrees ABOVE normal. As reinforcement, in 2002 the high was 55 with a low of 37, overall 19 degrees ABOVE normal.

In 1973, O'Hare had 0.6 inches of snow today. This was the sixth day of a string of eight days through the 20th 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 6:45 and sunrise is at 7:17. Sunset is at 16:23 and twilight ends at 16:55. There's a total of 9 hours, 6 minutes of daylight today and 10 hours, 10 minutes between twilights. We lost no minutes of daylight from yesterday. Tomorrow twilight begins at 6:46 and sunrise is at 7:18. The moon is waning crescent, having reached last quarter yesterday at 18:48. Illumination was 48% at midnight, will be 42% at noon and 36% at midnight tonight. Moonrise is at 0:11 this morning. Moonset is at 11:53 this morning. Moonrise is at 1:23 tomorrow morning. Moonlight time is 11 hours, 42 minutes which is 18 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 Eve is in six days. Christmas is in seven days.