December 27: ORD

Today is December 27th. It's the 361st day of the year. Four 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 34 and 19.

On the warm side, in 1982 the high was the record high of 61 with a low of 40. Three years ago, in 2008, the record high of 61 was tied with a low of 32. Also three years ago, I wrote "As I write this, we are experiencing a thunderstorm (heavy rain and lots of wind)." In 1994 the high was 51 with a low of 29. A decade earlier, in 1984, the high was 49 with a low of 30. In 2005 the high was 47 with a low of 32. Two years prior, in 2003, the high was 49 with a low of 28. The other warm high was 45 in 1998. The other warm lows were 34 in 1991 and 30 in 2007. The record warmest low of 51 was set in 1959 at Midway. 22 of the past 33 highs (67%) were in the: 20s (11) and 30s (11). Seven consecutive 20s & 30s highs occurred from 1985 through 1991. On the warm side, eight highs (24%) were in the: 40s (5), 50s (1) and 60s (2). Prior to 1982, O'Hare's warmest high was 57 in 1959. On the cold side, three highs (9%) were in the 10s (a.k.a. teens). O'Hare's coldest high was 9 in 1977.

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 9 with a low of -2, a cold day. In 1975 the high was 26 with a low of 22, a four degree spread. In 1974 the high was 34 with a low of 29, a five degree spread. In 1973 the high was 35 with a warm low of 30, a five degree spread. In 1971 the high was a warm 47 degrees. In 1968 the high was 43 with a warm low of 34, a warm day. In 1967 the high was 20 with a low of 6, a cold day. In 1963 the high was 24 with a cold low of 3 degrees, a cold day. In 1962 the high was 25 with a cold low of -1, a cold day. In 1959 the high was 57 with a low of 50, a seven degree spread and warm day. In 1958 the high was 44 with a low of 32, a warm day.

More recently, we had small spreads of three degrees in 1991 (37/34) and seven degrees two years ago, in 2009 (21/14). There were no large spreads of at least 30 degrees or more.

On the cold side, in 2000 the high was 17 with a low of 6. In 1985 the high was 22 with a low of 3. In 1978 the high was 22 with a low of 3. In 1993 the high was 18 with a low of 8. A decade earlier, in 1983, the high was 19 with a low of 8. The record low of -10 was first set in 1886 and tied in 1950 at Midway. The record coldest high of 3 was set in 1925. Both the 1886 and 1925 occurrences happened at a time when the official readings were taken adjacent to Lake Michigan. 20 of the last 33 lows (61%) were in the: 10s (a.k.a. teens) (12) and 20s (8). O'Hare had six consecutive lows in the 10s & 20s from 1994 through 1999. On the cold side, seven lows (21%) were in the single digits, most recently 8 degrees in 2004. O'Hare's only negative lows were -2 in 1977 and -1 in 1962. On the warm side, six lows (18%) were in the: 30s (5) and 40s (1). O'Hare's warmest low was 50 in 1959.

There were three 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 61 with a low of 40, overall 25.5 degrees ABOVE normal. Then in 1983 the high was 19 with a low of 8, overall 11.5 degrees BELOW normal. In the second instance, in 1984 the high was 49 with a low of 30, overall 14.5 degrees ABOVE normal. Then in 1985 the high was 22 with a low of 3, overall 12.5 degrees BELOW normal. In the third instance, in 1993 the high was 18 with a low of 8, overall 12 degrees BELOW normal. Then in 1994 the high was 51 with a low of 29, overall 15 degrees ABOVE normal.

Today is a major snow day in O'Hare's history. Back in 1894 in Chicago at the Auditorium Tower, which was the official reporting site for Chicago, 10.1 inches of snow fell. Back in 1978 tomorrow a major four-day snow of 16.8 inches began. But more on that tomorrow!

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:49 and sunrise is at 7:21. Sunset is at 16:28 and twilight ends at 17:00. There's a total of 9 hours, 7 minutes of daylight today and 10 hours, 11 minutes between twilights. We gained a minute of daylight from yesterday. Tomorrow twilight begins at 6:50 and sunrise is at 7:21. The moon is waxing crescent, having reached new three days ago at 12:06. Illumination was 8% at midnight, will be 11% at noon and 14% at midnight tonight. Moonrise is at 9:13 this morning. Moonset is at 20:05 tonight. Moonrise is at 9:42 tomorrow morning. Moonlight time is 10 hours, 52 minutes which is 33 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. New Year's Eve is in four days. New Year's Day is in five days.