While the winter solstice doesn't officially arrive in Minnesota and Iowa until December 21st at 5:38 P.M. CST, meteorological winter arrived at midnight this morning. Meteorological winter lasts from December 1st through February 28th.
Astronomical winter (the one that our calendars go by) is determined by the position of the earth and the sun. When the sun's direct rays pass over the tropic of Capricorn (At this moment, the sun's rays are as far as they will get from the northern hemisphere), winter begins in the northern hemisphere. This takes place on December 21st in the evening. Once we've hit December 21st, we receive a little bit more daylight each day.
Meteorologists call December 1st through February 28th "meteorological winter" because this is typically when three months of the coldest average temperatures in the northern hemisphere occur during the calendar year. Meteorologists divide the four seasons into four equals parts.