A winter storm is an event in which varieties of precipitation are formed that only occur at low temperatures, such as snow or sleet, or a rainstorm where ground temperatures are low enough to allow ice to form (i.e. freezing rain). In temperate continental climates, these storms are not necessarily restricted to the winter season, but may occur in the late autumn and early spring as well. Very rarely, they may form in summer, though it would have to be an abnormally cold summer, such as the summer of 1816 in the Northeast United States of America.
Snowstorms are storms where large amounts of snow fall. Two inches (5 cm) of snow is enough to create serious disruptions to traffic and school transport (because of the difficulty to drive and maneuver the school buses on slick roads). This is particularly true in places where snowfall is not typical but heavy accumulating snowfalls can occur. In places where snowfall is typical, such small snowfalls are rarely disruptive, because of effective snow and ice removal by municipalities, increased use of four-wheel drive and snow tires, and drivers being more used to winter conditions. Snowfalls in excess of 6 inches (15 cm) are usually universally disruptive.
A massive snowstorm with strong winds and other conditions meeting certain criteria is known as a blizzard. A large number of heavy snowstorms, some of which were blizzards, occurred in the United States during 1888 and 1947 as well as the early and mid-1990s. The snowfall of 1947 exceeded 2 feet (61 cm) with drifts and snow piles from plowing that reached 12 feet (3.7 m) and for months, temperatures did not rise high enough to melt the snow. The 1993 "Superstorm" manifested as a blizzard in most of the affected areas.
Large snowstorms could be quite dangerous: a 6 in (15 cm) snowstorm will make some unplowed roads impassable, and it is possible for automobiles to get stuck in the snow. Snowstorms exceeding 12 in (30 cm) especially in southern or generally warm climates will cave the roofs of some homes and cause the loss of electricity. Standing dead trees can also be brought down by the weight of the snow, especially if it is wet or very dense. Even a few inches of dry snow can form drifts many feet high under windy conditions.
Dangers of snow
Snowstorms are usually considered less dangerous than ice storms. However, the snow can bring secondary dangers. Mountain snowstorms can produce cornices and avalanches. An additional danger, following a snowy winter, is spring flooding if the snow melts suddenly because of a dramatic rise in air temperature. Deaths can occur from hypothermia, infections brought on by frostbite or car accidents due to slippery roads. Fires and carbon monoxide poisoning can occur after a storm causes a power outage. Large amounts of snow can also significantly reduce visibility in the area, a phenomenon known as a whiteout; this can be very dangerous to those who are in densely populated areas, since the whiteout can cause major accidents on the road or while flying. There are also several cases of heart attacks caused by overexertion while shoveling heavy wet snow. It is difficult to predict what form this precipitation will take, and it may alternate between rain and snow. Therefore, weather forecasters just predict a "wintry mix". Usually, this type of precipitation occurs at temperatures between −2 and 2 °C (28.4 and 35.6 °F). Snowstorms generally occur when different types of air masses in the mid-latitudes interact. These storms feed on differences in temperature and moisture. Initially, a wave is typically formed in the mid levels of the atmosphere as a result of a variety of things, be it a mountain range, injection of vorticity (energy), or several other reasons. Assuming certain conditions are in place for this wave to amplify, it will do so and begin to rotate, which effectively moves warm and moist air from one air mass to the north, and much colder and dryer air behind it to the south and east. The boundaries between the air masses constitute the warm and cold fronts of the new cyclone/storm.
Snowstorms that produce a lot of snow require an outside source of moisture, such as the Gulf of Mexico or the Atlantic Ocean in the United States. This requires yet more conditions to be just right, namely for the flow (the general wind flow at certain levels of the atmosphere) in the low-mid levels of the atmosphere to be aligned such that moisture can readily be transported from these regions directly into the storm. Another condition that must be met for a healthy storm is the air that is converging and rising in the center of this low pressure system, the snowstorm, to have an outlet as it rises up in the atmosphere. In other words, this air must be able to readily diverge at the mid levels of the atmosphere, effectively removing it from the storm and allowing this cycle to continue.
Heavy showers of freezing rain are one of the most dangerous types of winter storm. They typically occur when a layer of warm air hovers over a region, but the ambient temperature a few meters above the ground is near or below 0 °C (32 °F), and the ground temperature is sub-freezing.
While a 10 cm (3.9 in) snowstorm is somewhat manageable by the standards of the northern United States and Canada, a comparable 10 mm (0.39 in) ice storm can paralyze a region: driving becomes extremely hazardous, telephone and power lines are damaged, and crops may be ruined. Because they do not require extreme cold, ice storms often occur in warm temperature climates (such as the southern United States) and cooler ones. Ice storms in Florida will often destroy entire orange crops.
Notable ice storms include an El Niño-related North American ice storm of 1998 that affected much of eastern Canada, including Montreal and Ottawa, as well as upstate New York and part of New England. Three million people lost power, some for as long as six weeks. One-third of the trees in Montreal's Mount Royal park were damaged, as well as a large proportion of the sugar-producing maple trees. The amount of economic damage caused by the storm has been estimated at $3 billion Canadian.
The Christmas Day Ice Storm of 2000 caused devastating electrical issues in parts of Arkansas, Oklahoma, and Texas. The city of Texarkana, Arkansas experienced the worst damage, at one point losing the ability to use telephones, power and running water. In some areas in Arkansas, Oklahoma, Texas and eventually Louisiana, over an inch of ice accumulated from the freezing rain.
The Ice Storm of December 2002 in North Carolina resulted in massive power loss throughout much of the state, and property damage due to falling trees. Except in the mountainous western part of the state, heavy snow and icy conditions are rare in North Carolina.
The Ice Storm of December 2005 was another severe winter storm producing extensive ice damage across a large portion of the Southern United States on December 14 to 16. It led to power outages and at least 7 deaths.
In January 2005, Kansas had been declared a major disaster zone by President George W. Bush after an ice storm caused nearly $39 million in damages to thirty-two counties. Federal funds were provided to the counties during January 4–6, 2005 to aid the recovery process.
The January 2009 Central Plains and Midwest ice storm was a crippling and historic ice storm. Most places struck by the storm, saw 2 inches (51 mm) or more of ice accumulation, and a few inches of snow on top of it. This brought down power lines, causing some people to go without power for a few days, to a few weeks. In some cases, some didn't see power for a month or more. At the height of the storm, more than 2 million people were without power.
Ice crystals fall through a cloud of super-cooled droplets—minute cloud droplets that have fallen below freezing temperature but have not frozen. The ice crystal plows into the super-cooled droplets and they immediately freeze to it. This process forms graupel, or snow pellets, as the droplet continues to accumulate on the crystal. The pellets bounce when they hit the ground.
Out ahead of the passage of a warm front, falling snow may partially melt and refreeze into a frozen rain drop before it reaches the ground. These ice pellets are called sleet. Because it is easily seen and does not accumulate ice, it is not as dangerous as freezing rain.
Rime is a milky white accumulation of super-cooled cloud or fog droplets that freeze when they strike an object that has a temperature of 32 °F (0 °C), the freezing point of water. The process is called riming when super-cooled cloud droplets attach to ice crystals in the formation of graupel. Rime ice can pose a hazard to an airliner when it forms on a wing as an aircraft flies through a cloud of super-cooled droplets.