September 21: ORD

Today is September 21st. It's the 264th day of the year. 101 days remain. It's week 38 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.

According to the National Climatic Data Center (NCDC), the normal high is 73 and the normal low is 52. I've calculated the 32-year averages at 71 and 52.

On the warm side, last year, in 2010, the high was 88 with a low of 68. Three years earlier, in 2007, the high was 88 with a low of 64. In 1980 the high was 79 with a low of 69. The other warm high was 87 in 2005. The record high of 92 was set at Midway in 1970. The record warmest low of 76 occurred in 1931, a time when the official readings were taken adjacent to Lake Michigan. 21 of the past 32 highs (66%) were in the: 60s (14) and 70s (7). Our most recent 70s high was 76 in 2003. Nine highs (28%) were in the warm 80s. Two highs (6%) were in the cool 50s.

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1976 the low was a cold 40 degrees. In 1975 the high was a cool 58 with a low of 51, a seven degree spread. In 1971 the high was 72 with a low of 42, a 30 degree spread. In 1970 the high was 88 with a low of 68, a hot day and year of the record high. In 1965 the high was 80 with a warm low of 68, a warm day. In 1964 the high was 85 with a low of 65, a hot day. In 1962 the high was 71 with a cold low of 33, a 38 degree spread. That low of 33 was colder than the record low. In 1959 the high was 84 with a warm low of 64, a warm day.

More recently, there were large spreads of 34 degrees in 2005 (87/53), 32 degrees in 1984 (80/48) and 31 degrees in 2004 (82/51). There was a small spread of seven degrees two years ago, in 2009 (67/60).

On the cool side, in 1995 the high was the record coolest high of 50 with a low of 41. In 1983 the high was 53, the only other 50s high, with the co-record low of 39. The other co-record low of 39 occurred in 1991. These were the only 30s lows. The other cool highs were 61 in both 1982 and 1999. The other cold low was 40 in 2006. 25 of the last 32 lows (78%) were in the: 40s (9) and 50s (16). Two lows were in the cold 30s. Five lows were in the warm 60s. Three of the last four years featured lows in the 60s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:11 and sunrise is at 6:39. Sunset is at 18:53 and twilight ends at 19:21. There's a total of 12 hours, 14 minutes of daylight today and 13 hours, 10 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:12 and sunrise is at 6:40. The moon reached last quarter yesterday at 8:39 and is waning crescent. Illumination was 43% at midnight, will be 38% at noon and 33% at midnight tonight. Moonset is at 15:08 this afternoon. Moonrise is at 0:56 tomorrow morning. Moonset is at 15:48 tomorrow afternoon. Moonlight time is 15 hours, 8 minutes which is 37 minutes longer than yesterday. Moonlight time is at it's peak.

Summer began June 4 and runs for 110 days through September 21 (today). High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Warm Fall begins tomorrow (September 22) and runs for 41 days through November 1. High temperatures in this period are usually in the 50s, 60s and 70s. Highs in the 30s and 40s occur infrequently. Columbus Day is in 19 days.