September 5: ORD

Today is September 5th. It's the 249th day of the year. 117 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 79 and the normal low is 59. I've calculated the 33-year averages at 80 and 57.

On the hot side, in 1983 the high was 95 (O'Hare's hottest high of the date) with a low of 68. Two years later, in 1985, the high was 92 (O'Hare's co-second hottest high of the date) with a low of 71 (O'Hare's co-second warmest low of the date). 1983 and 1985 are O'Hare's co-warmest September 5ths. Another co-second hottest high of 92 degrees occurred in 1999. The other co-warmest low of 71 degrees occurred in 1990. The record high of 98 was set way back in 1899. The record warmest low of 77 occurred way back in 1912. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. 23 of the past 33 highs (70%) were in the: 70s (10) and 80s (13). O'Hare recorded consecutive 70s & 80s highs as follows: seven from 1976 through 1982 and six from 2001 through 2006. Our most recent 80s high is 87 degrees in 2005. Five highs (15%) were in the hot 90s. Five highs (15%) were in the cool 60s. O'Hare's second warmest September 5th was in 1971 as the high was 88 with a low of 70. The third and final co-second hottest high of 92 degrees occurred in 1959. O'Hare's warmest low of the date is 72 degrees in 1969.   

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 88 with a low of 57, a 31 degree spread. In 1975 the high was a cool 67 degrees. In 1974 the high was 72 with a cold low of 42, a 30 degree spread. In 1969 the low was a warm 72 degrees. In 1967 the high was 78 with a low of 48, a 30 degree spread. In 1964 the high was 79 with a cold low of 46, a 33 degree spread. In 1962 the high was 67 with a low of 42, a cool day. In 1959 the high was a hot 92 degrees. In 1959, today was the start of five straight days of highs in the 90s.

More recently, there were large spreads of 35 degrees in 1998 (87/52), 30 degrees in 1987 (88/58) and 30 degrees in 1999 (92/62). There was a small spread of seven degrees in 1994 (65/58).
 
On the cool side, in 1988 (O'Hare's coolest September 5th) the high was 64  (O'Hare's coolest high of the date) with the co-record low of 44. Four years earlier, in 1984, the high was 69 with the other co-record low of 44. O'Hare's second coolest highs of the date are 65 degrees in 1994 and last year (in 2011). O'Hare's third coolest highs of the date are 67 degrees in 1962 and 2000. The record coolest high of 62 degrees occurred way back in 1909, at a time when the official readings were taken adjacent to Lake Michigan. 18 of the last 33 lows (55%) were in the 50s. Five lows (15%) were in the cold 40s. Ten lows (30%) were in the warm: 60s (8) and 70s (2). O'Hare's second coolest September 5th was in 1962 as the high was 67 with a low of 42 (O'Hare's coldest low of the date and a low colder than the official record low).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:55 and sunrise is at 6:23. Sunset is at 19:19 and twilight ends at 19:48. There's a total of 12 hours, 56 minutes of daylight today and 13 hours, 53 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 5:56 and sunrise is at 6:24. The moon is waning gibbous. Illumination was 79% at midnight, will be 75% at noon and 71% at midnight tonight. Moonset is at 11:34 this morning. Moonrise is at 21:42 tonight. Moonset is at 12:31 tomorrow afternoon. Moonlight time is 13 hours, 52 minutes which is 24 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 four days. 9/11 Remembrance Day is in six days.