May 17: ORD

Today is May 17th. It's the 138th day of the year. 228 days remain. It's week 20 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 70 and the normal low is 49. I've calculated the 33-year averages at 70 and 48.

On the warm side, in 2001 the high was 85 with a low of 62. In 1982 the high was 87 with a low of 56, a 31 degree spread. The other warm highs were 89 in 1987, 88 in 1996, 86 in 1998, 85 in 1984 and 82 in 1992. The other warm low was 62 in 1999. The record high of 94 was set in 1962 and the record warmest low of 70 occurred in 1977. Both records were set at Midway. 17 of the past 33 highs (52%) were in the: 40s (1), 50s (5) and 60s (11). 16 highs (48%) were in the warm: 70s (6) and 80s (10). Our most recent 80s high was 80 in 2004. O'Hare's hottest May 17th was in 1962 as the high was 92 with a low of 71. 

In looking back through the years to the first May at O'Hare in 1959, the following instances were notable: In 1977 the high was 86 with a low of 63, a hot day and year of the record warmest low of 70 at Midway. In 1975 the high was 74 with a low of 43, a 31 degree spread. In 1973 the low was a cold 36 degrees. In 1972 the high was 80 with a low of 50, a 30 degree spread. In 1971 the high was a hot 88 with a low of 48, a 40 degree spread. In 1967 the high was 74 with a low of 44, a 30 degree spread. In 1964 the high was 80 with a low of 49, a 31 degree spread. In 1962 the high was 92 with a low of 71, a hot day. 1962 was also the year of the record high of 94 at Midway.

More recently, we had a small spread of seven degrees in 1980 (56/49). The large spreads were: 40 degrees in 1984 (85/45), 39 degrees in 1996 (88/49), 38 degrees in 1987 (89/51), 36 degrees in 1998 (86/50), 35 degrees in 1989 (80/45) and 30 degrees in 1992 (82/52).
 
On the cool side, in 2002 (O'Hare's coldest May 17th) the high was the record coldest high of 45 (broke the former record of 46 in 1945). The low was the record low of 36. This ties the original record low of 36, set in 1973. In 1983 the high was 58 with a low of 37. The other cool highs were 55 in 1981, 56 in 1980 and 58 two years ago in 2010. 27 of the last 33 lows (82%) were in the: 40s (16) and 50s (11). Four lows were in the cold 30s. Two lows were in the warm 60s.

There were two instances of back-to-back years in which temperatures were just about as opposite as they can be. In 1982 the high was 87 with a low of 56, overall 12.5 degrees ABOVE normal. Then in 1983 the high was 58 with a low of 37, overall 11.5 degrees BELOW normal. In the second instance, in 2001 the high was 85 with a low of 62, overall 14.5 degrees ABOVE normal. Then in 2002 the high was 45 with a low of 36, overall 18.5 degrees BELOW normal.  

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:57 and sunrise is at 5:30. Sunset is at 20:10 and twilight ends at 20:43. There's a total of 14 hours, 40 minutes of daylight today and 15 hours, 46 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:56 and sunrise is at 5:29. The moon is waning crescent. Illumination was 12% at midnight, will be 9% at noon and 7% at midnight tonight. Moonrise was at 3:32 this morning. Moonset is at 17:20 this afternoon. Moonrise is at 4:00 tomorrow morning. Moonlight time is 13 hours, 48 minutes which is 32 minutes longer 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. Armed Forces Day is in two days. National Maritime Day is in five days.