Courtesy: Minnesota State Climatology Office
Frost depth varies across the state in January 2018, but is generally deeper than it was last two years.
The map on the right depicts the depth that the ground is frozen under sod across the state. The frost depth is measured by a simple instrument called a frost tube. The liquid freezes in the tube as the ground becomes colder. Frost tubes were installed by the National Weather Service and the US Army Corps of Engineers (USACOE) at various places around the region. Frost depth readings are updated by the USACOE on a weekly basis.
There are also temperature sensors that can be placed in the ground and measure the temperature and this way one can approximate the depth of the frost. The North Dakota Agricultural Weather Network (NDAWN) has sensors along the border with Minnesota. This information can be seen here
Snow can insulate the ground from the air above it and studies have shown that about four inches of fluffy snow will form an effective layer of insulation. When there is a deep snow pack, there can be little to no frost in the ground. Where the landscape is free of snow, the frost can penetrate much deeper.
Below is a history of Frost Depth Information from a volunteer site in Maplewood, MN just north of St. Paul.
Frost under sod at Maplewood MN 55109 Year Frost In Frost Out Deepest frost --------------------------------------- 2000-2001 Nov 22 April 4 29.3 inches 2001-2002 Dec 21 April 12 17.0 inches 2002-2003 Nov 13 April 11 32.3 inches 2003-2004 Dec 2 March 28 29.3 inches 2004-2005 Nov 24 April 10 31.2 inches 2005-2006 Nov 30 April 2 19.5 inches 2006-2007 Nov 29 April 9 30.0 inches 2007-2008 Nov 23 April 18 35.2 inches 2008-2009 Nov 10 April 7 33.8 inches 2009-2010 Dec 4 March 16 10.0 inches 2010-2011 Nov 23 April 2 5.0 inches 2011-2012 Dec 1 March 16 26.0 inches 2012-2013 Nov 24 April 18 32.5 inches* thawed from Early Dec to Dec 24 2013-2014 Nov 23 April 12 19.3 inches 2014-2015 Nov 28 April 4 30.5 inches 2015-2016 Dec 19 March 9 9.5 inches (shortest duration) 2016-2017 Dec 8 March 29 16.0 inches 2017-2018 Dec 19 17.5 inches (as of March 5, 2018) ---------------------------------------- Median Nov 23 April 8 26.5 inches 2000-2001 to 2014-15