May 1: ORD

Today is May 1st. It's the 122nd day of the year. 244 days remain. It's week 18 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 65 and the normal low is 44. I've calculated the 33-year averages at 63 and 43.

On the warm side, in 2001 (O'Hare's warmest May 1st) the high was 83 with a low of 62 (O'Hare's warmest low of the date). Six years later, in 2007, the high was 83 with a low of 50, a 33 degree spread. The other warm high was 88 degrees in 1992 (O'Hare's warmest high of the date). The other warm low was 58 two years ago, in 2010. The record high of 90 was set in 1951 at Midway. The record warmest low of 68 occurred way back in 1899, at a time when the official readings were taken adjacent to Lake Michigan. 23 of the past 33 highs (70%) were in the: 50s (10) and 60s (13). O'Hare recorded eight consecutive 50s & 60s highs from 1979 through 1986. On the warm side, eight highs (24%) were in the: 70s (5) and 80s (3). A pre-1979 hot high was 87 in 1965. The warmest low prior to 2001 was 60 in 1973. On the cold side, two highs (6%) were in the 40s. Both 1966 and 2004 had highs of 48. These are O'Hare's coldest highs of 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 49 with a low of 32, a cold day. In 1977 the high was a warm 78 degrees. In 1973 the high was 75 with a warm low of 60, a warm day. In 1966 the high was a cold 48 with a low of 31, a cold day. In 1965 the high was a hot 87 with a low of 44, a huge 43 degree spread and warm day. In 1963 the low was a cold 29 degrees, a temperature that is colder than the official record low of 30. In 1959 the high was 75 with a low of 41, a 34 degree spread.

More recently, we had small spreads of five degrees in both 2004 (48/43) and 2006 (59/54). The additional large spreads were 47 degrees in 1992 (88/41), 33 degrees in 1999 (72/39), 32 degrees in 1987 (70/38) and 31 degrees in 1988 (68/37).
 
On the cool side, in 2005 the high was 49 with a low of 35. In 1979 the high was 57 with a low of 30. Since O'Hare was not the station of record in 1979, the official record low of 30 was set in 1943 at Midway. The other cool highs were 48 in 2004 and 53 in 1995. The record coldest high of 38 occurred back in 1909, at a time when the official readings were taken adjacent to Lake Michigan. 28 of the last 33 lows (85%) were in the: 30s (12) and 40s (16). Five lows were in the warm: 50s (4) and 60s (1). O'Hare's coldest low of the date was 29 degrees in 1963. This low is colder than the record low (30 in 1943 at Midway). O'Hare's coldest May 1st was in 1966 as the high was 48 with a low of 31. The second coldest was in 1978 as the high was 49 with a low of 32.

Today is a major day in Chicago's snow history. 1940 holds the record for the latest two inches or more. According to the record, 2.2 inches of snow fell in Chicago between the 1st, 2nd and 3rd. There is no indication of how much fell on what day. Tom Skilling, WGN-TV, says that 3.7 inches fell at the University of Chicago on the 1st and 2nd. Rockford had a total of 2 inches of snow (1.5 on the 1st and 0.5 on the 2nd).

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:18 and sunrise is at 5:48. Sunset is at 19:53 and twilight ends at 20:24. There's a total of 14 hours, 5 minutes of daylight today and 15 hours, 6 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 5:16 and sunrise is at 5:47. The moon is waxing gibbous, having reached first quarter two days ago at 4:57. Illumination was 69% at midnight, will be 74% at noon and 79% at midnight tonight. Moonset was at 2:51 this morning. Moonrise was at 14:51 this afternoon.  Moonset is at 3:21 tomorrow morning. Moonlight time is 12 hours which is 39 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. May Day is today. Mother's Day is in 12 days.