With an intense low pressure system located over far southern Lake Michigan this morning, the city's official barometer at O'Hare Airport dropped to extremely low levels with a reading of 28.91inches (979 millibars) at precisely 7:39 am CST. By 9 a.m. the pressure had risen to 28.93 inches, a sign that the low was beginning to depart.
According to records provided by Chicago climatologist Frank Wachowski this is the lowest air pressure logged here in nearly 20 years since October 17, 1990 when readings fell to 28.81 inches (976 millibars). Readings this low here are quite rare and are recorded only in the most intense storms. To put this low barometer reading in perspective it is the typical pressure observed for hurricanes straddling the Category 1-2 threshold with highest sustained winds just under 100 mph.
The city's all-time lowest barometer reading on record is 28.70 inches (972 millibars) observed on March 12 1923. In comparison the world's lowest barometer reading was 25.69 inches (870 millibars) recorded on Oct. 12, 1979 520 miles northwest of Guam in the eye of Typhoon Tip.
