Relative humidity: Summer vs. winter

Dear Tom,
The humidity is often very high in the winter here in Chicago, yet we always hear how dry the air is and that it is desirable to add moisture to the air in our houses. Can you explain this contradiction?

– Billy Kleiman

Dear Billy,
Averaged through the year, Chicago’s relative humidity is 71 percent but, surprisingly, it runs a little higher during the winter (73 percent) than during the summer (68 percent).
The explanation for higher winter humidity levels is that very cold air contains minimal moisture, even when saturated (100 percent relative humidity). It therefore takes little moisture to elevate the humidity of frigid air. Far more moisture is required to bring hot air to saturation. It takes only 0.001 ounce of water to saturate one cubic foot of air at 0 degrees, but 0.022 ounce (22 times as much) to saturate air at 80 degrees.

Tsunami wave reports

This is a list of measured Tsunami activity in the Pacific Ocean on Saturday and Sunday.  The listing is in chronological order using U.S Central Standard Time (Chicago). 

7.7′   Talcahuano, Chile                12:53 AM  2/27   

1.1′   Easter Island, Chile               6:05 AM  2/27

5.9′   Hiva Oa, Marquesas Is.        11:41 AM  2/27    (French Polynesia)

0.5′   Papeete, Tahiti                   12:11PM  2/27

1.2′   Cabo San Lucas, Mexico       12:33 PM  2/27

2.0′   Acalpulco, Mexico                 1:31 PM  2/27

0.5′   Lottin Point, New Zealand      1:34 PM  2/27

3.2′   Pago Pago (US)                    2:27PM  2/27    (American Samoa)

0.4′   San Diego, CA                      2:36 PM  2/27

1.4′   Santa Monica, CA                  2:36 PM  2/27

3.2′   Kahului, HI (Maui)                  3:47 PM  2/27 

0.8′   Honolulu, HI                          4:00PM  2/27

0.3′   Sitka, Alaska                          7:11PM  2/27

0.8′   Midway Attol (US)                  7:37PM  2/27

1.4′   Kodiak, Alaska                       8:21 PM  2/27

1.2′   Adak, Alaska                        10:37PM  2/27  

2.7′   Hanasaki, Hokkaido, Japan      12:37AM  2/28

4.0′   Kuji, Japan                           12:49AM  2/28

1.3′   Omaezaki, Honshu, Japan       12:59 AM  2/28 

Snowy February to end on a dreary, chilly note.

February 2010 will soon be history, and if measurable snow does not fall Sunday this month will go into the record books, tied with February, 1967  for the fourth snowiest on record, with a total snowfall of 22.5 inches. In addition to the heavy snow, the month featured virtually no warmth—only one day broke 40 degrees. Since the start of the year, Chicago has recorded only  four days in the 40s, and the highest temperature so far in 2010 has been only 46 degrees. The chilly trend will continue through March’s opening days but signs of a modest warm-up are visible by the end of the week as readings are expected to climb into the 40. The next storm is likely to bring rain instead of snow.
Cold, snowy February suppresses severe weather

One positive aspect to this nation’s cold and snowy February has been a nearly total lack of severe weather. With only one day left in February, not a single tornado has been reported in a month that has averaged 37 twisters since 2000.


December-February snow cover in Chicago

Dear Tom,
Has snow ever covered the ground for the entire December-February meteorological winter period?

—William Ooms Jr., Alsip, and Greg Wolf, Barrington Hills

Dear William and Greg,
Snow cover has been quite persistent in the Chicago area since the first snowfall on Dec. 7-8, but it has not been continuous. Phil Rider, the Mundelein observer, reports 74 days with 1 inch or more of snow cover through the end of February but noted that during January’s last week just traces of snow remained. Midway Airport observer Frank Wachowski logged 66 days with similar periods of bare ground. In a typical Chicago winter there is 1 inch or more of snow cover on 43 days, so this season is well above average. Only once has Chicago been snow covered the entire winter: in 1978-79 with a record 100 days of continuous snow cover from Nov. 26 to March 5.