A bird flied over frozen Lake Michigan in Chicago, the United States on Jan. 18, 2016. It is the coldest morning of this winter season so far with sub-zero temperatures and wind chills across the Chicago area on Monday. The National Weather Service warmed Chicagoans of dangerously cold temperatures that can cause frostbite to exposed skin in as little as 30 minutes. (Xinhua/He Xianfeng)