May 31: ORD

Today is May 31st. It's the 151st day of the year. 214 days remain. It's week 22 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.

According to the National Climatic Data Center (NCDC), the normal high is 75 and the normal low is 53. I've calculated the 32-year averages at 77 and 55.

On the hot side, in 1991 the high was 89 with a low of 67. Three years earlier, in 1988, the high was 92 (O'Hare's only 90s high on May 31) with a low of 61, a 31 degree spread. In 2002 the high was 86 with a low of 67. In 1987 the high was 88 with a low of 64. Two years later, in 1989, the high was 84 with a low of 67. Another hot 80s highs was 87 in 1986. The other warm lows were 66 last year (in 2010) and 65 in both 2006 and 2007. The record high of 98 was set back in 1934. The record warmest low of 72 occurred back in 1919. 25 of the past 32 highs (78%) were in the: 70s (15) and 80s (10). One high was in the hot 90s. Six highs were in the cool: 50s (2) and 60s (4).

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 hot 88 with a low of 56, a 32 degree spread. In 1972 the high was a cool 63 degrees. In 1970 the low was a warm 70 degrees. In 1969 the high was 85 with a low of 53, a 32 degree spread. In 1966 the high was 66 with a cold low of 37, a cool day. In 1965 the high was 80 with a low of 44, a 36 degree spread. In 1963 the high was 80 with a low of 46, a 34 degree spread. In 1961 the high was 85 with a low of 53, a 32 degree spread.

More recently, there were large spreads of 36 degrees in 1990 (76/40), 33 degrees in 1992 (77/44), 33 degrees in 1996 (74/41), 31 degrees in 1981 (72/41) and 30 degrees in 1995 (84/54).

On the cool side, in 2003 the high was 58 with a low of 38. A decade earlier, in 1993, the high was 65 with a low of 39. Other cool lows were 40 in 1990, 41 in 1981 and 41 in 1996. Other cool highs were 59 in 1983 and 62 in 2001. Both records were set way back in the 19th century. The record low of 35 was set way back in 1873. The record coolest high of 43 occurred way back in 1889. 30 of the last 32 lows (94%) were in the: 40s (9), 50s (9) and 60s (12). Two lows were in the cold 30s.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 4:46 and sunrise is at 5:20. Sunset is at 20:22 and twilight ends at 20:56. There's a total of 15 hours, 2 minutes of daylight today and 16 hours, 10 minutes between twilights. We gained two minutes of daylight from yesterday. Tomorrow twilight begins at 4:46 and sunrise is at 5:20. The moon is waning crescent and will be new tomorrow at 16:03. Illumination was 3% at midnight, will be 1% at noon and 0% at midnight tonight. Moonrise is at 4:15 this morning. Moonset is at 19:34 tonight. Moonrise is at 4:58 tomorrow morning. Moonlight time is 15 hours, 19 minutes which is 24 minutes longer than yesterday.

The warm part of Spring (Warm Spring) began on April 13. Warm Spring runs for 52 days, through June 3. High temperatures in this period are usually in the 50s, 60s and 70s. Warmer 80s and 90s occur infrequently. Flag Day is in 14 days.