September 17: ORD

Today is September 17th. It's the 261st 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. Before this, the official site of record was on the south side of Chicago at Midway International Airport (MDW), beginning on Wednesday, July 1, 1942. From Friday, January 1, 1926 through Tuesday, June 30, 1942, the official site was at the University of Chicago. Before this, 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 75 and the normal low is 54. I've calculated the 33-year averages at 73 and 52.

On the hot side, in 1988 (O'Hare's warmest September 17th) the high was 90 (O'Hare's hottest high of the date) with a low of 67 (with 1968, O'Hare's warmest low of the date). In 1992 the high was 77 with a low of 66 (O'Hare's second warmest low of the date). The record high of 93 degrees was set in 1955 at Midway. The record warmest low of 71 degrees occurred back in 1906, at a time when the official readings were taken adjacent to Lake Michigan. 25 of the past 33 highs (76%) were in the: 60s (10) and 70s (15). O'Hare recorded consecutive 60s & 70s highs as follows: six from 1982 through 1987 and eight from 1989 through 1996. Seven highs (21%) were in the warm: 80s (6) and 90s (1). One high (3%) was in the cool 50s. The extremes of our highs (50s & 90s) happened back in the 1980s. O'Hare's second through fifth warmest 9/17 days occurred prior to 1979: the 2nd is 1965 (second hottest high of 88 with a low of 61), the 3rd is 1963 (third hottest high of 87 with a low of 61), the 4th is 1972 (high of 83, low of 64) and the 5th is 1967 (high of 82, low of 64).  

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 additional large spreads of at least 30 degrees or small spreads of up to seven degrees. 

On the cool side, in 1981 (O'Hare's coolest September 17th) the high was the record coolest high of 51 degrees 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. The other post-1978 cool high is 63 degrees in 1990. 22 of the last 33 lows (67%) were in the 50s. O'Hare recorded ten consecutive 50s lows from 2000 through 2009. Eight lows (24%) were in the cold: 30s (1) and 40s (7). Three lows (9%) were in the warm 60s. O'Hare's second coolest September 17th was in 1959 as the high was 61 with a low of 44.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:08 and sunrise is at 6:36. Sunset is at 18:59 and twilight ends at 19:27. There's a total of 12 hours, 23 minutes of daylight today and 13 hours, 19 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 6:09 and sunrise is at 6:37. The moon is waxing crescent, having passed new two nights ago at 21:11. Illumination was 2% at midnight, will be 4% at noon and 6% at midnight tonight. Moonrise is at 8:29 this morning. Moonset is at 19:36 tonight. Moonrise is at 9:42 tomorrow morning. Moonlight time is 11 hours, 7 minutes which is 39 minutes shorter than yesterday.

Summer began on June 4th and runs for 110 days through September 21st. 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 21 days.