This radiational cooling is especially strong when there is snow cover. This is because the snow insulates the atmosphere from the grounds warmth, effectively reflects back what little sunshine is received during the day, and is a very efficient emitter of radiation to space during the night. The result is lower daytime and nighttime readings. Sub-zero cold has been found as far south as Atlanta, Georgia and El Paso, Texas in December. High pressure systems too can be massive in December.
Some polar highs can move into the nation from the north Pacific or north Atlantic.
Tropical air masses are rare and usually confined to the southernmost areas. In other areas tropical air appears only in narrow streamers in the warm sector of the stronger storm systems. In the north, this warm air is usually found mostly aloft where it is forced to rise over the colder air in low levels. The main tracks for polar high pressure centers are from northwest Canada southeast to Texas or east to southeastern Canada. Polar Pacific or sometimes Polar Canadian air masses will drop into the Great Basin and build as ridging takes place aloft.