Snowstorms are storms where large amounts of snow fall. Snow is less dense than liquid water, by a factor of approximately 10 at temperatures slightly below freezing, and even more at much colder temperatures.Therefore, an amount of water that would produce 0.8 in (20 mm) of rain could produce at least 8 in (20 cm) of snow. 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" was manifest as a blizzard in most of the affected area.
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 is 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. Snow storms 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 snow storm, 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.
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