July 27: ORD

Today is July 27th. It's the 209th day of the year. 157 days remain. It's week 30 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 that, 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 84 and the normal low is 64. I've calculated the 33-year averages at 84 and 64.

On the hot side, in 1989 (O'Hare's warmest July 27th) the high was 93 (one of O'Hare's two hottest highs of the date) with a low of 73, O'Hare's warmest low of the date. In 1997 the high was 92 with a low of 72. The second of O'Hare's two hottest highs of the date was 93 in 1988. Note that those hottest highs were in the back-to-back years of 1988 and 1989! The record high of 100 was most recently tied in 1955 at Midway. It was originally set back in 1916 adjacent to Lake Michigan. The record warmest low of 82 occurred back in 1916, at a time when the official readings were taken adjacent to Lake Michigan. 19 of the past 33 highs (58%) were in the 80s. Six highs (18%) were in the hot 90s. Eight highs (24%) were in the cool: 60s (1) and 70s (7). Our most recent 70s high was 74 in 2005.

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1977 the low was a cold 52 degrees. This is tied with the official record low. In 1972 the high was a cool 71 degrees. In 1964 the high was 89 with a low of 59, a 30 degree spread. In 1962 the low was a cold 51 degrees, a low colder than the official record low.

More recently, there was a small spread of six degrees in 1981 (69/63). The large spreads were 35 degrees in 1985 (87/52), 33 degrees in 1979 (89/56) and 33 degrees in 1988 (93/60).

On the cool side, in 1984 (O'Hare's coolest July 27th) the high was 74 with a low of 55. In 2005 (just half a degree warmer than 1984) the high was 74 with a low of 56. O'Hare's only 60s high and record coolest high of the date was 69 degrees in 1981. The record low (but not O'Hare's coldest low) of 52 was set in 1985. 18 of the last 33 lows (55%) were in the 60s. Nine lows (27%) were in the cold 50s. Our most recent 50s lows were 59 in 2004 and 56 in 2005. Six lows (18%) were in the warm 70s. O'Hare's coldest low was 51 degrees in 1962, colder than the official record low. Another cold low was 52 degrees in 1977, tied with the official record low.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:09 and sunrise is at 5:42. Sunset is at 20:17 and twilight ends at 20:49. There's a total of 14 hours, 35 minutes of daylight today and 15 hours, 40 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:11 and sunrise is at 5:43. The moon is waxing gibbous, having reached first quarter yesterday at 3:56. Illumination was 60% at midnight, will be 66% at noon and 71% at midnight tonight. Moonset was at 0:14 this morning. Moonrise is at 15:15 this afternoon. Moonset is at 1:01 tomorrow morning. Moonlight time is 8 hours, 59 minutes which is 57 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 90s and 100s occur infrequently. Labor Day is in 38 days.