A cup of boiling water turns into an impressive, dense cloud of snow as Chicago suffers the coldest day in its history.

"I threw boiling water straight out of the pot and it froze in mid air!" said the thrower, Melody.

Temperatures dropped past -21 degrees Celsius and have been projected to plummet further.

The city is in the throes of the polar vortex, which is causing the Chicago River to "steam". States of emergency have been declared in Midwestern Wisconsin, Michigan and Illinois.