July 29: ORD

Today is July 29th. It's the 211th day of the year. 155 days remain. It's week 31 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 85 and 63.

On the hot side, in 1983 the high was 96 (O'Hare's co-second hottest high of the date) with a low of 72. In 2006 the high was 94 (our most recent 90s high) with a low of 74 (O'Hare's warmest low of the date). In 1988 the high was 97 (O'Hare's hottest high of the date) with a low of 70. 1983 and 2006 are O'Hare's warmest days by half a degree over 1988. In 1999 the high was 96 (O'Hare's other co-second hottest high of the date). The record high of 99 was set back in 1913. The record warmest low of 85 occurred back in 1916. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. We had very high dew points in 1999. The top was 82 degrees, making 1999 perhaps the most humid July 29th on record. 18 of the past 33 highs (55%) were in the 80s. Seven highs (21%) were in the hot 90s. Eight highs (24%) were in the cool 70s. Our most recent 70s high was 79 in 2000. There is an interesting phenomenon. In the year following a high of at least 95 degrees, the high is 16 or 17 degrees cooler. The four instances so far: 1983 (96) followed by 1984 (79), 17 degrees cooler; 1988 (97) followed by 1989 (81), 16 degrees cooler; 1995 (95) followed by 1996 (78), 17 degrees cooler; 1999 (96) followed by 2000 (79), 17 degrees cooler. O'Hare's second warmest lows were 73 degrees in 1970 and two decades later in 1990.

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1971 the low was a cold 52 degrees. In 1965 the high was 80 with a cold low of 50, a 30 degree spread. That low of 50 equals the record low. In 1962 the high was a cool 71 with a low of 54, a cool day.

More recently, there were large spreads of 30 degrees in both 1979 (88/58) and 1994 (82/52).

On the cool side, in 1981 (O'Hare's co-coolest July 29th) the high was 74 with a low of 51. Three years later, in 1984, the high was 79 with the record low of 50. Another cold low was 52 in 1994. The record coolest high of 63 occurred way back in 1892, at a time when the official readings were taken adjacent to  Lake Michigan. 19 of the last 33 lows (58%) were in the 60s. Eight lows (24%) were in the cool 50s. Our most recent 50s low was 54 in 2003. Six lows (18%) were in the warm 70s. O'Hare's other co-coolest July 29th was in 1962 as the high was 71 (O'Hare's coolest high of the date) with a low of 54. Tied with 1984 as O'Hare's coldest low of the date is 1965's 50 degrees.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:12 and sunrise is at 5:44. Sunset is at 20:15 and twilight ends at 20:47. There's a total of 14 hours, 31 minutes of daylight today and 15 hours, 35 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:13 and sunrise is at 5:45. The moon is waxing gibbous. Illumination was 81% at midnight, will be 85% at noon and 89% at midnight tonight. Moonset was at 1:56 this morning. Moonrise is at 17:23 this afternoon. Moonset is at 2:58 tomorrow morning. Moonlight time is 8 hours, 33 minutes which is six minutes shorter than yesterday. Moonlight time is at its shortest today for the next several days. 

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 36 days.