July 30: ORD

Today is July 30th. It's the 212th day of the year. 154 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 83 and the normal low is 64. I've calculated the 33-year averages at 83 and 64.

On the hot side, in 1999 (O'Hare's warmest July 30th) the high was 101 (a degree short of the record and O'Hare's hottest high of the date) with a low of 78 (O'Hare's warmest low of the date). In 2006 the high was 90 with a low of 77. (O'Hare's second warmest low of the date). In 1995 the high was 96 (O'Hare's second hottest high of the date) with a low of 70. In 1988 the high was 95 (O'Hare's third hottest high of the date). The record high of 102 and record warmest low of 84 were set on the same hot day back in 1916, at a time when the official readings were taken adjacent to Lake Michigan. We had very high dew points in 1999. The top was 83 degrees, making 1999 perhaps the most humid July 30th on record. The dew point was at or above 80 degrees for 18 hours in 1999. 30 of the past 33 highs (91%) were in the: 70s (10), 80s (12) and 90s (8). One high was in the hot 100s. Two highs were in the cool 60s. An interesting phenomenon is the drop in high temperature in the year following a minimum 95 degree high. The three examples so far: 1988 (95) followed by 1989 (75), 20 degrees cooler; 1995 (96) followed by 1996 (75), 21 degrees cooler; 1999 (101) followed by 2000 (74), 27 degrees cooler.

In looking back through the years to the first July at O'Hare in 1959, the following instances were notable: In 1971 the high was 72 with a low of 53, a cool day. In 1965 the high was 82 with a cold low of 50, a 32 degree spread. That low of 50 is equal to the record low.

More recently, there were large spreads of 32 degrees in 1984 (82/50) and 31 degrees in 2003 (84/53). There were small spreads of five degrees in 1985 (69/64) and six degrees in 2000 (74/68).

On the cool side, in 1992 (O'Hare's coolest July 30th) the high was 65 (O'Hare's coolest high of the date) with a low of 58, a seven degree spread. In 1981 the high was 76 with a low of 53. The record low of 50 was set in 1984. The low in 1965 was also 50 degrees, making it and 1984 the coldest lows in O'Hare's history for the date. The other cool high was 69 degrees in 1985. The record coolest high of 62 occurred way back in 1895, at a time when the official readings were taken adjacent to Lake Michigan. 27 of the last 33 lows (82%) were in the: 50s (11) and 60s (16). Six lows were in the warm 70s. O'Hare's second coolest July 30th was in 1971 as the high was 72 with a low of 53.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:13 and sunrise is at 5:45. Sunset is at 20:14 and twilight ends at 20:46. There's a total of 14 hours, 29 minutes of daylight today and 15 hours, 33 minutes between twilights. We lost two minutes of daylight from yesterday. Tomorrow twilight begins at 5:14 and sunrise is at 5:46. The moon is waxing gibbous. Illumination was 89% at midnight, will be 93% at noon and 95% at midnight tonight. Moonset was at 2:58 this morning. Moonrise is at 18:16 tonight. Moonset is at 4:06 tomorrow morning. Moonlight time is 8 hours, 42 minutes which is nine 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 90s and 100s occur infrequently. Labor Day is in 35 days.