September 17: ORD

Today is September 17th. It's the 260th day of the year. 105 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 75 and the normal low is 54. I've calculated the 32-year averages at 73 and 52.

On the hot side, in 1988 the high was 90 with a low of 67. In 1992 the high was 77 with a low of 66. The record high of 93 was set in 1955. The record warmest low of 71 occurred back in 1906, a time when the official thermometer was located adjacent to Lake Michigan. 24 of the past 32 highs (75%) were in the: 60s (9) and 70s (15). Seven highs were in the warm: 80s (6) and 90s (1). One high was in the cool 50s. Recently we had three consecutive years of 80s highs: 2006 (82), 2007 (80) and 2008 (82). The extremes of our highs (50s & 90s) happened back in the 1980s.

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1974 the high was 83 with a low of 50, a 33 degree spread. In 1973 the high was a cool 60 degrees. In 1968 the high was 73 with a warm low of 67, a six degree spread. In 1965 the high was a hot 88 degrees. In 1964 the high was 84 with a low of 49, a 35 degree spread. In 1963 the high was a hot 87 degrees. In 1961 the high was 75 with a low of 44, a 31 degree spread. In 1959 the high was a cool 61 with a low of 44, a cool day.

More recently, there were no other large spreads of at least 30 degrees or small spreads of up to seven degrees.

On the cool side, in 1981 the high was the record coolest high of 51 with a low of 46, a five degree spread. Three years later, in 1984, the high was 69 with the record low of 37. Four years earlier, in 1980, the high was 62 with a low of 45. 21 of the last 32 lows (66%) were in the 50s. Eight lows (25%) were in the cool: 30s (1) and 40s (7). Three lows (9%) were in the warm 60s. For the last 18 years (1993-2010), lows were in the 40s & 50s, including ten straight 50s lows from 2000 through 2009.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:07 and sunrise is at 6:35. Sunset is at 19:00 and twilight ends at 19:28. There's a total of 12 hours, 25 minutes of daylight today and 13 hours, 21 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:08 and sunrise is at 6:36. The moon is waning gibbous. Illumination was 80% at midnight, will be 76% at noon and 72% at midnight tonight. Moonset is at 11:44 this morning. Moonrise is at 21:22 tonight. Moonset is at 12:41 tomorrow afternoon. Moonlight time is 14 hours, 22 minutes which is 21 minutes longer than yesterday.

Summer began June 4 and runs for 110 days through September 21. High temperatures in this period are usually in the 70s and 80s. Highs in the 60s, 90s and 100s occur infrequently. Citizenship Day is today. Columbus Day is in 23 days.