Septembeer 18: ORD

Today is September 18th. It's the 262nd day of the year. 104 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 74 and the normal low is 53. I've calculated the 33-year averages at 75 and 53.

On the warm side, in 1985 (O'Hare's warmest September 18th) the high was 87 with a low of 67. Three years later, in 1988 (tied with 1965 as O'Hare's second warmest September 18th), the high was 86 with a low of 67. In 2007 (O'Hare's third warmest September 18th) the high was 88 with a low of 63. In 1983 (O'Hare's fifth warmest September 18th) the high was 84 with a low of 65. The other post-1978 warm high was 86 degrees in 2000. The record high of 94 degrees was set in 1955. The record warmest low of 71 degrees occurred in 1965. Both records were set at Midway. Over the past 33 years, the tally of highs has been equal. 11 highs (33%) were in the cool 60s. 11 highs (33%) were in the 70s. 11 highs (33%) were in the warm 80s. In fact, O'Hare has never had a high in the 50s or 90s on September 18th. As mentioned above,  1965 (high of 85 and second warmest low of the date of 68) is tied with 1988 as O'Hare's second warmest September 18th. There's a tie for O'Hare's fourth warmest 9/18: in 1963 (high of 89, low of 61) and in 1978 (high of 84, low of 66). O'Hare's sixth warmest 9/18 was in 1977 as the high was 79 with the warmest low of the date of 69. 

In looking back through the years to the first September at O'Hare in 1959, the following instances were notable: in 1978 the low was a warm 66 degrees. In 1977 the low was a warm 69 degrees. In 1976 the high was 78 with a low of 48, a 30 degree spread. In 1965 the high was 85 with a warm low of 68, a hot day and year of the record warmest low of 71 (at Midway). In 1963 the high was a hot 89 degrees. In 1962 the high was 75 with a low of 45, a 30 degree spread. In 1959 the low was a cold 40 degrees, which is equal to the record low.

More recently, there were large spreads of 35 degrees in 2000 (86/51), 31 degrees in 1984 (74/43), 30 degrees in 1980 (75/45) and 30 degrees in 1997 (83/53). There were small spreads of four degrees in 1986 (65/61) and seven degrees last year, in 2010 (65/58).

On the cool side, in 1981 (O'Hare's coolest September 18th)  the high was 60 (O'Hare's coolest high of the date) with a low of 42. A decade later, in 1991, the high was 69 with a low of 42. The year prior, in 1990, the high was 63 with a low of 49. The record low of 40 degrees was set way back in 1889 and tied in 1901 and again in 1903. The record coolest high of 54 degrees was set way back in 1901. All of these records were set when the official readings were taken adjacent to Lake Michigan. The last 40s low of 48 degrees occurred 13 years ago in 1999. 18 of the last 33 lows (55%) were in the 50s. Nine lows (27%) were in the cold 40s. Six lows (18%) were in the warm 60s. O'Hare's coldest low of 40 degrees (a low equal to the record low) occurred in 1959. It is not the record low because Midway was the official station of record in 1959.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 6:09 and sunrise is at 6:37. Sunset is at 18:57 and twilight ends at 19:25. There's a total of 12 hours, 20 minutes of daylight today and 13 hours, 16 minutes between twilights. We lost three minutes of daylight from yesterday. Tomorrow twilight begins at 6:10 and sunrise is at 6:38. The moon is waxing crescent. Illumination was 6% at midnight, will be 9% at noon and 13% at midnight tonight. Moonrise is at 9:42 this morning. Moonset is at 20:15 tonight. Moonrise is at 10:54 tomorrow morning. Moonlight time is 10 hours, 33 minutes which is 34 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. Columbus Day is in 20 days.