May 27: ORD

Today is May 27th. It's the 148th day of the year. 218 days remain. It's week 21 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 74 and the normal low is 52. I've calculated the 33-year averages at 72 and 50.

On the warm side, in 1987 (until 2012, O'Hare's warmest May 27th) the high was 88 with a low of 68 (until 2012, O'Hare's warmest low of the date). Four years later, in 1991 (until 2012, O'Hare's second warmest May 27th and tied with 1978), the high was 88 with a low of 66. The other warm high was 87 in 2006. The record high of 94 and record warmest low of 75 were set on the same hot day way back in 1911, at a time when the official readings were taken adjacent to Lake Michigan. That record high of 94 was broken in 2012 as the high reached 97 degrees. 23 of the past 33 highs (70%) were in the: 60s (12) and 70s (11). O'Hare recorded six consecutive 60s & 70s highs from 1981 through 1986. There were another six consecutive 60s & 70s highs from 2000 through 2005. Our most recent 60s highs were 66 in 2000 and 61 in 2001. Seven highs (21%) were in the warm 80s. Three highs (9%) were in the cool 50s. Until 2012, 1978 was tied with 1991 as the second warmest May 27th. In 1978 the high was 88 with a low of 66. The other pre-1979 hot days occurred in 1969 (high of 87, low of 63) and 1959 (high of 83, low of 66). Until 2012, O'Hare's warmest May 27th highs were 88 degrees, occurring in 1978, 1987 and 1991. Until 2012, O'Hare's second warmest lows were 66 degrees, in 1959, 1978 and 1991. 

As of 4:00 PM on March 27, 2012, the National Weather Service had noted the following highs: 99 at Rockford, 97 at: O'Hare, Aurora, Midway, the Chicago lakefront and Kankakee. It had reached 96 at West Chicago, Romeoville (Nat'l Weather Svc) and DeKalb. Joliet had reached 95 degrees.  

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 88 with a low of 66, a hot day. In 1976 the high was 77 with a low of 46, a 31 degree spread. In 1971 the low was a cold 38 degrees. In 1970 the high was a cool 61 degrees. In 1969 the high was 87 with a low of 63, a hot day. In 1968 the high was a cool 61 with a low of 54, a seven degree spread. In 1966 the high was 85 with a low of 54, a 31 degree spread. In 1963 the high was a cool 60 degrees. In 1962 the high was a cool 59 degrees. In 1961 the high was 70 with a cold low of 34, a 36 degree spread. In 1959 the high was 83 with a warm low of 66, a warm day.

More recently, there was a small spread of six degrees in 1996 (54/48). There were additional large spreads of 35 degrees in 1999 (81/46), 33 degrees in 1980 (85/52), 33 degrees in 1998 (83/50) and 32 degrees in 1994 (67/35).
 
On the cool side, in 2008 (O'Hare's coolest May 27th) the high was 50 (O'Hare's coolest high of the date) with a low of 42. In 1992 the high was 63 with the record low of 33, a 30 degree spread. Last year, in 2011, the high was 58 with a low of 40. The other cold low was 35 in 1994. The other cool highs were 54 in 1996, 61 in 2001 and 62 in 1984. The record coolest high of 49 occurred way back in 1895, at a time when the official readings were taken adjacent to Lake Michigan. 28 of the last 33 lows (85%) were in the: 40s (16) and 50s (12). Two lows were in the cold 30s and three were in the warm 60s.

There was an instance of back-to-back years in which temperatures were just about as opposite as they can be. In 1991 the high was 88 with a low of 66,  overall 15 degrees ABOVE normal. Then in 1992 the high was 63 with a low of 33, overall 14 degrees BELOW normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:49 and sunrise is at 5:22. Sunset is at 20:20 and twilight ends at 20:53. There's a total of 14 hours, 58 minutes of daylight today and 16 hours, 4 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:48 and sunrise is at 5:21. The moon is waxing crescent, heading towards the first quarter tomorrow at 15:16. Illumination was 33% at midnight, will be 39% at noon and 44% at midnight tonight. Moonset was at 0:23 this morning. Moonrise is at 11:31 this morning. . Moonset is at 0:52 tomorrow morning. Moonlight time is 12 hours, 52 minutes which is 41 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. Memorial Day is tomorrow.