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WINTER WEATHER FAQs

It rarely snows when the temperature drops below zero degrees Fahrenheit because the atmosphere is too stable. One of the ingredients for snow is enough lifting of saturated air that snow can develop aloft and fall to reach the surface. When it is said, "it is too cold to snow, "in reality it means there is not enough lifting of air to cause snow to reach the surface. Even at very cold surface temperatures, significant snowfall can occur. Intense lifting can produce significant precipitation, even at very low temperatures. The temperature higher in the atmosphere can be much warmer than the air temperature at the surface, and that warm air aloft can hold more moist air than the colder air at the surface. And, moisture can be transferred into the area where lifting is occurring through advection (the horizontal movement of an air mass).

Although thunderstorms are less common in the winter, sometimes lightning can occur within snowstorms and is called thundersnow. Thundersnow can be found where there is relatively strong instability and abundant moisture above the surface, such as above a warm front. Thundersnow is sometimes observed downstream of the Great Salt Lake and the Great Lakes during lake-effect snowstorms too.

On average, thirteen inches of snow equals one inch of rain in the US, although this ratio can vary from two inches for sleet to nearly fifty inches for very dry, powdery snow under certain conditions.

Yes, it's true. It is important to keep the head and neck covered and as warm as possible to delay or prevent hypothermia.

The official definition of a "Nor'easter" is a strong low pressure system that affects the Mid-Atlantic and New England states. It can form over the land of the eastern U.S. or over the Atlantic coastal waters. These winter events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas. What makes them so strong is the warm and moist air from the Atlantic that feeds the storm, causing it to grow explosively.

White lightning is a colloquial term for thunderstorms occurring with snow.

We have not heard of this happening. Evaporation of snow in such storms typically doesn't produce enough cooling to drive a downdraft circulation in the squall.

Winter Weather Basics

How do winter storms form?

Just like any other storm at other times of the year, just the right combination of ingredients is necessary for a winter storm to develop.

Three basic ingredients are necessary to make a winter storm.

Cold air – below freezing temperatures in the clouds and near the ground are necessary to make snow and/or ice.
Lift – something to raise the moist air to form the clouds and cause precipitation. An example of lift is warm air colliding with cold air and being forced to rise over the cold dome. The boundary between the warm and cold air masses is called a front. Another example of lift is air flowing up a mountainside.
Moisture – to form clouds and precipitation. Air blowing across a body of water, such as a large lake or the ocean, is an excellent source of moisture.

Snow – Most precipitation that forms in wintertime clouds starts out as snow because the top layer of the storm is usually cold enough to create snowflakes. Snowflakes are just collections of ice crystals that cling to each other as they fall toward the ground. Precipitation continues to fall as snow when the temperature remains at or below 0 degrees Celsius from the cloud base to the ground.

Snow atmospheric layers
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Snow Flurries – Light snow falling for short durations. No accumulation or light dusting is all that is expected.
Snow Showers – Snow falling at varying intensities for brief periods of time. Some accumulation is possible.
Snow Squalls – Brief, intense snow showers accompanied by strong, gusty winds. Accumulation may be significant. Snow squalls are best known in the Great Lakes Region.
Blowing Snow – Wind-driven snow that reduces visibility and causes significant drifting. Blowing snow may be snow that is falling and/or loose snow on the ground picked up by the wind.
Blizzard – Winds over 35mph with snow and blowing snow, reducing visibility to 1/4 mile or less for at least 3 hours.

Sleet occurs when snowflakes only partially melt when they fall through a shallow layer of warm air. These slushy drops refreeze as they next fall through a deep layer of freezing air above the surface, and eventually reach the ground as frozen rain drops that bounce on impact.

Sleet atmospheric layers
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Freezing Rain occurs when snowflakes descend into a warmer layer of air and melt completely. When these liquid water drops fall through another thin layer of freezing air just above the surface, they don't have enough time to refreeze before reaching the ground. Because they are "supercooled," they instantly refreeze upon contact with anything that that is at or below O degrees C, creating a glaze of ice on the ground, trees, power lines, or other objects. A significant accumulation of freezing rain lasting several hours or more is called an ice storm.

Freezing rain atmospheric layers
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» More About ICE CRYSTALS

HOW DOES NSSL CONTRIBUTE?

Specialized atmospheric conditions and small variations in temperature – even tenths of a degree – determine whether precipitation will fall as sleet, snow, or freezing rain, and make forecasting these events very difficult. Scientists at NSSL are looking to improve our understanding of hazardous winter weather. This is accomplished through an examination of:

the climatologies of winter weather patterns
the structure and evolution of winter fronts and cyclones
improved forecasting techniques