May 15: ORD

Today is May 15th. It's the 136th day of the year. 230 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 48. I've calculated the 33-year averages at 68 and 47.

On the warm side, in 1998 (O'Hare's second warmest May 15th) the high was 86 with a low of 64 (O'Hare's second warmest low of the date). In 1991 the high was 87 with a low of 57, a 30 degree spread. In 1988 the high was 80 with a low of 63. In 2001 the high was 88 with a low of 55, a 33 degree spread. In 1982 the high was 85 with a low of 56. The record high of 91 and record warmest low of 71 were set on the same hot day in 1962 at Midway. 11 of the past 33 highs (33%) were in the 60s. On the cold side, eight highs (24%) were in the: 40s (1) and 50s (7). On the warm side, 14 highs (43%) were in the: 70s (8) and 80s (6). Our most recent 70s high was 75 in 2002.  O'Hare's warmest May 15th (really, it was hot) was in 1962 as the high was 89 with a low of 71. Both the high and low were the warmest O'Hare has seen for the date.
 
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 cool 58 degrees. In 1977 the high was a warm 82 degrees. In 1971 the high was a warm 83 with a low of 58, a warm day. In 1968 the high was 82 with a low of 60, a warm day. In 1967 the high was a cool 56 degrees. In 1965 the high was 81 with a warm low of 60, a warm day. In 1964 the high was 81 with a low of 45, a 36 degree spread. In 1963 the high was a cool 55 degrees. In 1962 the high was 89 with a low of 71, a hot day. 1962 was also the year of the record high of 91 and record warmest low of 71 at Midway. In 1960 the high was a warm 82 degrees. In 1959 the high was a cool 55 with a low of 39, a cool day.

More recently, we had additional large spreads of 35 degrees in 1992 (77/42) and 35 degrees in 2002 (75/40).
 
On the cool side, last year, in 2011 (O'Hare's coldest May 15th), the high was 46 (O'Hare's only 40s high of the date) with a low of 41. In 1997 the high was 51 with a low of 38. In 1983 the high was 53 with a low of 37 (O'Hare's coldest low of the date). In 2004 the high was 55 with a low of 39. In 2005, the very next year, the high was 54 with a low of 41. The other cool highs were 57 in 2003 and 57 in 2008. The record low of 35 was set way back in 1895. The record coolest high of 44 occurred way back in 1904. Both records were set at a time when the official readings were taken adjacent to Lake Michigan. 19 of the last 33 lows (58%) were in the 40s. Four lows (12%) were in the cold 30s. Ten lows (30%) were in the warm: 50s (8) and 60s (2). There were no significantly cooler than normal highs, lows or days from 1959 through 1982.

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 85 with a low of 56, overall 11.5 degrees ABOVE normal. Then in 1983 the high was 53 with a low of 37, overall 14 degrees BELOW normal. In the second instance, in 1997 the high was 51 with a low of 38, overall 14.5 degrees BELOW normal. Then in 1998 the high was 86 with a low of 64, overall 16 degrees ABOVE normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:00 and sunrise is at 5:32. Sunset is at 20:08 and twilight ends at 20:40. There's a total of 14 hours, 36 minutes of daylight today and 15 hours, 40 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:59 and sunrise is at 5:31. The moon is waning crescent. Illumination was 27% at midnight, will be 23% at noon and 19% at midnight tonight. Moonrise was at 2:39 this morning. Moonset is at 15:22 this afternoon. Moonrise is at 3:05 tomorrow morning. Moonlight time is 12 hours, 43 minutes which is 35 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 four days. National Maritime Day is in seven days.