July 28: ORD

Today is July 28th. It's the 210th day of the year. 156 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 1983 (O'Hare's warmest July 28th) the high was the record high of 100 with a low of 78 (O'Hare's warmest low of the date). In 1988 the high was 98 with a low of 70. The record warmest low of 84 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. Five highs (15%) were in the hot: 90s (4) and 100s (1). Nine highs (27%) were in the cool: 60s (1) and 70s (8). There was a 10 year sub-90 degree period that started in 1989 and went through 1998. Another interesting phenomenon is that the two instances of very hot highs were followed the next year by cool highs. In 1983 the high was 100, followed by a high of only 75 in 1984, 25 degrees cooler. In 1988 the high was 98 followed by a high of only 74 in 1989, 24 degrees cooler. There was such a contrast in between 1983 and 1984 that in 1983 the record high occurred followed the following year by the record low!

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1973 the high was a cool 70 degrees. In 1962 the high was a cool 67 with a low of 63, a four degree spread.

More recently, there were no small spreads (diurnals of seven degrees or less) or large spreads (diurnals of 30 degrees or more).

On the cool side, in 1981 (O'Hare's coolest July 28th) the high was the record coolest high of 66 with a low of 52. This (along with 1962's high of 67) were the only 60s highs at O'Hare on July 28th. In 1984 the high was 75 with the record low of 51. 25 of the last 33 lows (76%) were in the: 50s (11) and 60s (14). There were seven consecutive 50s & 60s lows from 2003 through 2009. Eight lows (24%) were in the warm 70s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:11 and sunrise is at 5:43. Sunset is at 20:16 and twilight ends at 20:48. There's a total of 14 hours, 33 minutes of daylight today and 15 hours, 37 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:12 and sunrise is at 5:44. The moon is waxing gibbous, having reached first quarter two days ago at 3:56. Illumination was 71% at midnight, will be 76% at noon and 81% at midnight tonight. Moonset was at 1:01 this morning. Moonrise is at 16:22 this afternoon. Moonset is at 1:56 tomorrow morning. Moonlight time is 8 hours, 39 minutes which is 20 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 37 days.