Winter storms can leave behind a thick layer of snow.
Are you hoping to wake up tomorrow morning and run outside to play in the snow rather than sit in a classroom or office? Winter storms sometimes result in snow days, where the weather conditions make travel and exposure to cold air too risky for regular daily activities. A winter storm is a weather event where the precipitation is mainly snow, sleet, or freezing rain. Often coupled with strong winds and below-freezing temperatures, winter storms can be dangerous. But how does a winter storm form?
Winter Storm Formation
Winter storms begin with moist air rising up into the atmosphere, just like other types of storms. Rising air is common at a cold front, where warm air is lifted above cold air and is necessary for cloud formation and precipitation. Rising air can also happen as air moves up a large hill or mountain. A source of moisture, such as air blowing across a large lake or ocean, is necessary for clouds and precipitation to form. The final ingredient, and the one that makes a winter storm different from other storms, is cold air. Below freezing air temperatures near the ground and up to the clouds will cause precipitation to fall as either snow or ice. However, extremely cold air is not able to hold as much moisture and thus will not make much snow. This explains why some of the coldest places on Earth, like Antarctica, receive very little precipitation throughout the year.
Types of Winter Storms
Snowstorms are one type of winter storm. Blizzards are snowstorms with high winds, and lake effect storms are snowstorms that form near the Great Lakes. Ice storms can bring freezing rain or sleet as well as snow. Read below to learn more about the different kinds of winter storms.
A storm where precipitation falls as snow is called a snowstorm. In the winter, most precipitation forms as snow within the clouds because temperatures at the top of the storm are cold enough to make snowflakes. Snowflakes are collections of frozen ice crystals that form as water vapor condenses into water droplets and freezes. These ice crystals stick together as they fall towards the ground, forming snowflakes. If the air temperature remains at or below 0℃ between the cloud and the ground, the precipitation will fall as snow. If the air near the ground is above freezing temperature, the precipitation will melt to form rain or freezing rain.
A snowstorm where there is no accumulation beyond a light dusting of snow is called a snow flurry. A brief snowstorm with snow falling at varying intensity and some accumulation is called a snow shower. If the snow showers are accompanied by strong gusts of wind and accumulation of lots of snow, they are called snow squalls. A blizzard is considered a severe kind of snowstorm and is described in more detail below.
A blizzard is a severe snowstorm defined by the strength of the winds rather than the amount of snow it brings. With wind speeds at or above 35mph, blizzards create blowing snow conditions, where snow on the ground is picked up by the wind, causing reduced visibility and the accumulation of snowdrifts. A blizzard lasts for three or more hours and often leads to the accumulation of lots of snow, either as new snowfall or as redistribution of previously fallen snow as blowing snow.
Lake Effect Storms
Most snowstorms form due to low-pressure systems that lift moist air into the atmosphere, but lake effect storms form due to the abundance of moisture from the Great Lakes. When cold, dry air from the north passes over the Great Lakes area, it picks up large amounts of water, which falls back to the ground as heavy snowstorms in the areas south and east of the lakes.
An ice storm is a winter storm that has an accumulation of at least 0.25 inch (6.35mm) of ice on all outdoor surfaces. The ice forms a slick layer on the ground that can make driving and walking conditions dangerous and can cause branches and powerlines to snap due to the weight of the ice. There are different types of icy winter weather, which are determined by the temperature of air masses in the storm. Sleet forms when the snowflakes falling towards the surface first pass through a layer of air that is above freezing temperature, which causes the snowflakes to partially melt, and then pass through a layer of air that is below freezing temperature, which causes the snowflakes to re-freeze into pellets of ice. Similar to sleet formation, freezing rain forms when precipitation (either rain or snow) passes through a layer of warmer air, allowing it to become rain, and then through a layer of much colder air. But this time, the rain isn't able to re-freeze as is falls through the shallow layer of freezing air near the surface. The rain is supercooled through this process and freezes instantly upon contact with cold surfaces.
Winter Weather Safety
Winter storms often create conditions where exposure to the outdoors and travel become dangerous due to cold temperatures, wind, snow, or ice. Check the forecast for information about winter storms in your area and watch the weather before heading outdoors during the winter months. The National Weather Service issues weather alerts for areas that should prepare for snowstorms, blizzards, wind chill, lake effect storms, and ice storms based on three-tiered levels:
- Winter Weather Warning: take action!
- It is highly likely that a winter storm will impact your area. Be prepared for heavy snow or ice, strong winds, and freezing temperatures that will make travel and outdoor exposure dangerous.
- Winter Weather Watch: be prepared!
- Conditions are favorable for a winter storm in your area, and there is the potential for severe winter weather. If a winter storm does hit your area, be prepared for heavy snow or ice, strong winds, and freezing temperatures that may make travel and outdoor exposure dangerous.
- Winter Weather Advisory: be aware!
- Winter weather conditions should be expected but will not be severe enough to meet warning levels. Exercise caution when traveling and avoid prolonged exposure to the outdoors.
© 2019 UCAR