May 31: ORD

Today is May 31st. It's the 152nd day of the year. 214 days remain. It's week 22 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 75 and the normal low is 53. I've calculated the 33-year averages at 77 and 55.

On the hot side, in 1991 (O'Hare's warmest May 31st) the high was 89 with a low of 67. Last year, in 2011 (O'Hare's second warmest May 31st), the high was 85 with a low of 69 (O'Hare's warmest low of the date). In 1988 the high was 92 (O'Hare's only 90s high and thus warmest high of May 31st) with a low of 61, a 31 degree spread. In 2002 the high was 86 with a low of 67. In 1987 the high was 88 with a low of 64. Two years later, in 1989, the high was 84 with a low of 67. The other hot high was 87 in 1986. The other warm lows were 66 two years ago, in 2010 and 65 in both 2006 and 2007. The record high of 98 was set back in 1934 at the University of Chicago. The record warmest low of 72 occurred back in 1919, at a time when the official readings were taken adjacent to Lake Michigan. 26 of the past 33 highs (79%) were in the: 70s (15) and 80s (11). O'Hare recorded seven consecutive 70s & 80s highs from 2005 through last year, 2011. One high (3%) was in the hot 90s. Six highs (18%) were in the cool: 50s (2) and 60s (4).

In looking back through the years to the first May at O'Hare in 1959, the following instances were notable: In 1978 the high was a hot 88 with a low of 56, a 32 degree spread. In 1972 the high was a cool 63 degrees. In 1970 the low was a warm 70 degrees. In 1969 the high was 85 with a low of 53, a 32 degree spread. In 1966 the high was 66 with a cold low of 37, a cool day. In 1965 the high was 80 with a low of 44, a 36 degree spread. In 1963 the high was 80 with a low of 46, a 34 degree spread. In 1961 the high was 85 with a low of 53, a 32 degree spread.

More recently, there were additional large spreads of 36 degrees in 1990 (76/40), 33 degrees in 1992 (77/44), 33 degrees in 1996 (74/41), 31 degrees in 1981 (72/41) and 30 degrees in 1995 (84/54).
 
On the cool side, in 2003 (O'Hare's coolest May 31st) the high was 58 (O'Hare's coolest high of the date) with a low of 38. A decade earlier, in 1993, the high was 65 with a low of 39. The other cool lows were 40 in 1990, 41 in 1981 and 41 in 1996. The other cool highs were 59 in 1983 and 62 in 2001. Both records were set way back in the 19th century, at a time when the official readings were taken adjacent to Lake Michigan. The record low of 35 was set way back in 1873. The record coolest high of 43 occurred way back in 1889. 31 of the last 33 lows (94%) were in the: 40s (9), 50s (9) and 60s (13). Two lows (from 1979-2011) were in the cold 30s. O'Hare's third and only other 30s low was 37 in 1966 (O'Hare's coldest low of the date). The 50s highs (58 in 2003 and 59 in 1983) were O'Hare's only 50s highs for the date in its history. However, today's high (in 2012) is predicted to be in the 50s and may become O'Hare's coolest high ever for the date!

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 2002 the high was 86 with a low of 67,  overall 12.5 degrees ABOVE normal. Then in 2003 the high was 58 with a low of 38, overall 16 degrees BELOW normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:46 and sunrise is at 5:20. Sunset is at 20:23 and twilight ends at 20:57. There's a total of 15 hours, 3 minutes of daylight today and 16 hours, 11 minutes between twilights. We gained a minute of daylight from yesterday. Tomorrow twilight begins at 4:45 and sunrise is at 5:19. The moon is waxing gibbous. Illumination was 76% at midnight, will be 81% at noon and 85% at midnight tonight. Moonset was at 2:22 this morning. Moonrise is at 16:08 this afternoon. Moonset is at 2:57 tomorrow morning. Moonlight time is 10 hours, 14 minutes which is 42 minutes shorter than yesterday. 

Spring began on March 12th and runs for 84 days through June 3rd. Warm Spring began on April 13th and runs for 52 days through June 3rd. High temperatures in this period are usually in the 50s, 60s and 70s. Highs in the 80s and 90s occur infrequently. Flag Day is in 14 days.