May 18: ORD

Today is May 18th. It's the 139th day of the year. 227 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 71 and the normal low is 49. I've calculated the 33-year averages at 69 and 49.

On the warm side, in 1996 (tied with 1962 as O'Hare's warmest May 18th) the high was 90 with the record-tying warmest low of 72 (tied with 1911). In 1984 the high was 91 with a low of 61, a 30 degree spread. In 1998 the high was 90 with a low of 62. In 1979 the high was 85 with a low of 60. In 1982 the high was 82 with a low of 61. The record high of 93 was set in 1962. 27 of the past 33 highs (82%) were in the: 50s (8), 60s (10) and 70s (9). Six highs were in the warm: 80s (3) and 90s (3). O'Hare's highest temperature was in 1977 as the high was 92 degrees. 1962 is tied with 1996 as O'Hare's warmest May 18th. In 1962 the high was 91 with a low of 71. O'Hare also had a very warm low of 69 in 1971. O'Hare's coldest high was 47 degrees in 1969.

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 92 with a low of 64, a hot day. In 1975 the high was a hot 86 with a low of 50, a 36 degree spread. In 1974 the high was a cool 58 degrees. In 1972 the high was a warm 85 degrees. In 1971 the high was 85 with a low of 69, a warm day. In 1970 the high was 78 with a low of 45, a 33 degree spread. In 1969 the high was a cold 47 with a low of 45, a two degree spread and cold day. In 1967 the high was a warm 85 with a low of 55, a 30 degree spread. In 1965 the high was 82 with a low of 46, a 36 degree spread. In 1964 the high was a hot 89 with a low of 56, a 33 degree spread and warm day. In 1962 the high was 91 with a low of 71, a hot day and year of the record high of 93 at Midway. In 1961 the high was a cool 55 with a low of 41, a cool day.

More recently, we had large spreads of 36 degrees in 1997 (77/41), 33 degrees in 1993 (70/37), 30 degrees in 1987 (81/51) and 30 degrees in 1988 (70/40).
 
On the cool side, in 2002 (O'Hare's coolest May 18th) the high was 54 with the record low of 34 degrees (broke the former record of 35 set way back in 1894). In 2008 the high was 55 with a low of 44. In 1991 the high was 54 with a low of 45. The other cool highs were 55 in 1986, 57 in 1981, 57 last year (in 2011), 58 in 1995 and 59 in 1992. The other cold low was 37 in 1993. The record coldest high of 45 occurred way back in 1900, at a time when the official readings were taken adjacent to Lake Michigan. 19 of the last 33 lows (58%) were in the 40s. Three lows (9%) were in the cold 30s. 11 lows (33%) were in the warm: 50s (6), 60s (4) and 70s (1). Our most recent 50s low was 57 in 2005. O'Hare recorded six consecutive 40s lows from 2006 through last year, 2011.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:56 and sunrise is at 5:29. Sunset is at 20:11 and twilight ends at 20:44. There's a total of 14 hours, 42 minutes of daylight today and 15 hours, 48 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:55 and sunrise is at 5:28. The moon is waning crescent. Illumination was 7% at midnight, will be 4% at noon and 3% at midnight tonight. Moonrise was at 4:00 this morning. Moonset is at 18:19 this afternoon. Moonrise is at 4:32 tomorrow morning. Moonlight time is 14 hours, 19 minutes which is 31 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 tomorrow. National Maritime Day is in four days.