September 7: ORD

Today is September 7th. It's the 251st day of the year. 115 days remain. It's week 36 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 78 and the normal low is 58. I've calculated the 33-year averages at 79 and 58.

On the hot side, in 1985 (with 1960, O'Hare's co-warmest September 7th) the high was 99 (with 1960, O'Hare's co-hottest high of the date) with a low of 73. In 2001 the high was 87 with a low of 68. Four years later, in 2005, the high was 88 with a low of 67. The other hot highs were 90 degrees in 1984 and 1991 (our most recent). The other warm low was 70 degrees in 2007. The record high of 100 degrees was originally set in 1939 and tied at Midway in 1960. The record warmest low of 78 occurred way back in 1922, a time when the official readings were taken adjacent to Lake Michigan. 26 of the past 33 highs (79%) were in the: 70s (15) and 80s (11). Three highs (9%) were in the hot 90s. Four highs (12%) were in the cool 60s, most recently 68 degrees in 1995. There's been an ongoing, 16-year stretch of highs in the 70s & 80s that began in 1996 and went through last year, 2011. In 1960 (tied with 1985 as O'Hare's warmest September 7th) the high was 99 with a low of 73. That high of 99 degrees is also tied with the same in 1985 as O'Hare's hottest of the date. O'Hare's second warmest September 7th occurred in 1959 as the high was 96 (O'Hare's second hottest high of the date) with a low of 74 (O'Hare's warmest low of the date). O'Hare's second warmest lows of the date of 73 degrees occurred in 1960, 1970 and 1985.

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1978 the high was a hot 92 degrees. In 1972 the high was 74 with a low of 68, a six degree spread. In 1971 the high was a hot 92 with a low of 68, a hot day. In 1970 the high was 88 with a warm low of 73, a hot day. In 1968 the high was 79 with a low of 47, a 32 degree spread. In 1967 the high was 80 with a cold low of 44, a 36 degree spread. In 1966 the high was 76 with a cold low of 45, a 31 degree spread. In 1964 the high was a hot 94 with a low of 65, a hot day. There's also another interesting thing about 1964: on the 5th the low was 46, so there was an incredible 48 degree swing to a high of 94 today. In 1962 the high was 78 with a cold low of 39, a 39 degree spread. That low of 39 was colder than the record low. In 1960 the high was 99 with a low of 73, a hot day. 1960 was the year of the record high of 100 degrees, occurring at Midway. In 1959 the high was 96 with a low of 74, a hot day.

More recently, there were large spreads of 33 degrees in 1988 (77/44), 30 degrees in 1999 (80/50) and 30 degrees in 2002 (89/59). There were no small spreads of up to seven degrees.

On the cool side, in 1986 (O'Hare's coolest September 7th) the high was 66 with the record low of 42. In 1979 the high was 66 with a low of 53. Those highs of 66 degrees are O'Hare's coolest of the date. The other cold low was 44 degrees in 1988. The record coolest high of 61 degrees occurred way back in 1924, a time when the official readings were taken adjacent to Lake Michigan. 28 of the last 33 lows (85%) were in the: 50s (16) and 60s (12). Three lows were in the cold 40s and two were in the warm 70s. For 13 consecutive years, from 1994 through 2006, lows were in the 50s and 60s. O'Hare's coldest low of 39 degrees occurred in 1962. This low was colder than the record low.

There's an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1985 the high was 99 with a low of 73, overall 18 degrees above normal. Then in 1986 the high was 66 with a low of 42, overall 14 degrees below normal. 

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:57 and sunrise is at 6:25. Sunset is at 19:16 and twilight ends at 19:44. There's a total of 12 hours, 51 minutes of daylight today and 13 hours, 47 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:58 and sunrise is at 6:26. The moon is waning gibbous and reaches last quarter tomorrow at 8:15. Illumination was 62% at midnight, will be 58% at noon and 53% at midnight tonight. Moonset is at 13:26 this afternoon. Moonrise is at 23:03 tonight. Moonset is at 14:16 tomorrow afternoon. Moonlight time is 14 hours, 23 minutes which is 12 minutes longer 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. Grandparents Day is in two days. 9/11 Remembrance Day is in four days.