April 11: ORD

Today is April 11th. It's the 102nd day of the year. 264 days remain. It's week 15 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 57 and the normal low is 37. I've calculated the 33-year averages at 57 and 38.

On the warm side, in 2005 the high was 76 with a low of 53. Four years before, in 2001, the high was 77 with a low of 51. Five years before, in 1996, the high was 82 with a low of 45, a 37 degree spread. In 2002 the high was 75 with a low of 50. Four years later, in 2006, the high was 75 with a low of 48. The other warm highs were 70 in 2003 and 69 in 1981. The record high of 90 was set back in 1930 at the University of Chicago. The record warmest low of 60 was set in 1977 at Midway. 25 of the past 33 highs (76%) were in the: 40s (11), 50s (6) and 60s (8). Our most recent 50s high was 51 in 1999. On the warm side, six highs (18%) were in the: 70s (5) and 80s (1). All of the post-1978 70s highs occurred in a six year span from 2001 to 2006. O'Hare's warmest April 11th was in 1977 as the high was 84 with a low of 59. The other pre-1979 warm highs were 80 in 1971, 76 in 1964, 75 in 1965 and 74 in 1968. On the cold side, two highs (6%) were in the 30s. The 30s highs happened fairly close in years, in 1997 and 2000. 

In looking back through the years to the first April at O'Hare in 1959, the following instances were notable: In 1977 the high was 84 with a low of 59, a warm day and year of the record warmest low of 60 at Midway. In 1976 the high was a cold 41 degrees with a low of 30, a cold day. In 1975 the high was a cold 45 degrees. In 1974 the low was a warm 50 degrees. In 1973 the high was 45 with a low of 25, a cold day. In 1971 the high was a warm 80 with a low of 47, a 33 degree spread and warm day. In 1968 the high was a warm 74 with a low of 39, a 35 degree spread. In 1965 the high was 75 with a low of 49, a warm day. In 1964 the high was a warm 76 with a low of 41, a 35 degree spread and warm day. In 1959 the high was a cold 44 degrees.

More recently, the other large spreads (aside from 1996, see "warm side") were 38 degrees in 2003 (70/32), 35 degrees in 1986 (66/31) and 32 degrees in 1998 (66/34). There were no small spreads other than in 1997 (see the next section).

On the cold side, in 1997 (O'Hare's coldest April 11th) the high was 35 with a low of 32, a three degree spread. In 2000 the high was 39 with a low of 29. The other cold low was the record low of 25, set in 1989. Two other years share the record low of 25: originally way back in 1882 (near the lakefront) and then in 1973 at Midway. O'Hare also had cold lows of 25 in 1973 and 26 in 1959. The other cold highs were 40 in 1990, 43 in 2004, 43 in 2007, 44 in 1991, 45 in 1979, 45 in 1980 and 45 in 1983. The record coldest high of 33 was set way back in 1882, at a time when the official readings were taken adjacent to Lake Michigan. So since 1882 has both the record low and record coldest high, it's the coldest April 11th for Chicago. 25 of the last 33 lows (78%) were in the: 30s (16) and 40s (9). Four lows were in the cold 20s. Our most recent 20s low was 29 in 2000. Three lows were in the warm 50s.

There were two instances of back-to-back years in which temperatures were just about as opposite as they can be. In 1996 the high was 82 with a low of 45, overall 16.5 degrees ABOVE normal. Then in 1997 the high was 35 with a low of 32, overall 13.5 degrees BELOW normal. In the second instance, in 2000 the high was 39 with a low of 29, overall 13 degrees BELOW normal. Then in 2001 the high was 77 with a low of 51, overall 17 degrees ABOVE normal.

The following astronomical data is provided by the United States Naval Observatory. In Crystal Lake, twilight begins at 5:49 and sunrise is at 6:18. Sunset is at 19:31 and twilight ends at 20:00. There's a total of 13 hours, 13 minutes of daylight today and 14 hours, 11 minutes between twilights. We gained three minutes of daylight from yesterday. Tomorrow twilight begins at 5:48 and sunrise is at 6:17. The moon is waning gibbous. Illumination was 74% at midnight, will be 69% at noon and 63% at midnight tonight. Moonrise was at 0:31 this morning. Moonset was at 10:03 this morning. Moonrise is at 1:23 tomorrow morning. Moonlight time is 9 hours, 32 minutes which is 31 minutes longer than yesterday. 

Spring began on March 12th and runs for 84 days through June 3rd. Cool Spring began on March 12th and runs for 32 days through April 12th (tomorrow). High temperatures in this period are usually in the 30s, 40s and 50s. Highs in the 60s and 70s usually occur infrequently. Earth Day is in 11 days.