A cold spell in Minnesota is a sustained period where air temperatures drop far below freezing, often remaining below zero degrees Fahrenheit (-18 degrees Celsius) for several days. This persistent drop transforms the northern landscape and is caused by a large-scale shift in global air patterns. Understanding these accompanying conditions is important for managing the unique hazards of deep winter.
The Origin of Arctic Air Masses
The meteorological event that delivers Minnesota’s most severe cold is the southward shift of the polar jet stream, a fast-moving ribbon of air high in the atmosphere. This shift allows a vast dome of frigid air from the Arctic region to plunge directly into the Upper Midwest. The air travels across the flat expanse of central North America, which acts as an open channel for the air mass to move south without being blocked or warmed.
The arrival of this cold air is accompanied by a large, stationary high-pressure system settling over the region. High-pressure systems feature sinking air, which prevents cloud formation and keeps warmer air from mixing in, locking the cold air mass in place for an extended time.
Typical Atmospheric Conditions
The most significant atmospheric condition accompanying a cold spell is the intensified wind chill factor. Wind chill measures the rate of heat loss from the body based on the combination of air temperature and wind speed felt on exposed skin. During a Minnesota cold spell, an air temperature of -10°F can translate to a wind chill of -35°F or -40°F with moderate winds. These dangerously low values trigger official Extreme Cold Warnings from the National Weather Service.
At a wind chill of -32°F, exposed skin can develop frostbite in as little as ten minutes. When wind chill values drop to -48°F, the time required for frostbite onset is reduced to less than five minutes, making outdoor exposure hazardous.
The air during these cold spells is also extremely dry, characterized by very low humidity. This low moisture content results from the high-pressure system’s sinking air, which compresses and dries out the atmosphere. The dry air inhibits heavy precipitation, meaning the peak of a cold spell often features clear or mostly sunny skies rather than active snowfall. This dense, cold air also causes a noticeable rise in barometric pressure readings.
Associated Optical and Ice Phenomena
The combination of clear skies and extremely cold, dry air often results in striking optical phenomena known as halos. Sun dogs, officially called parhelia, are a common sighting when the sun is low on the horizon. These bright, sometimes rainbow-colored spots appear on either side of the sun, formed by sunlight refracting through tiny, hexagonal ice crystals suspended in the atmosphere.
Another visual effect is the formation of hoar frost, which occurs on still, clear nights when water vapor turns directly into ice crystals on cold surfaces. This covers trees, fences, and utility lines in delicate, feathery white crystals. This is distinct from rime ice, which forms when super-cooled liquid water droplets in fog freeze on contact with surfaces.
In areas near open water, such as Lake Superior or smaller rivers, extremely cold air moving over warmer water creates a phenomenon known as steam fog or sea smoke. The rapid evaporation of water followed by immediate condensation in the sub-zero air forms a dense, shallow layer of fog that clings close to the water’s surface.