During the winter months, children wait anxiously for news of the first snow. In some parts of the country this only happens rarely, while others are bombarded for months. As winter descends meteorologists deliver news of many different types of freezing weather conditions. Ice builds up and schools close down. What causes the productions of sleet one day and snow the next? Subtle differences in the atmosphere determine the type of frozen material that greets you on the ground.
The upper levels of the atmosphere responsible for many changes in moisture forms. The temperatures vary at different levels in the upper atmosphere. The sleet, rain, or snow that originates in a top layer may not be the same form that presents at ground level. The common ingredient necessary for all of these items to form is moisture. A dry climate with freezing temperatures may resist rain, sleet, and snow. There is quite a bit of action in the atmosphere before results are visible to us.
Freezing rain depends on a warm upper atmosphere and a freezing ground level temperature. When the air in the upper atmosphere remains above freezing, moisture stays in liquid form. The liquid changes, however, when it is met with freezing temperatures at ground level. Rain continues to fall in liquid form and turns to ice as it makes contact with the ground. Low temperatures often cause this ice to remain on the ground for an extended amount of time.
Sleet is simply small pieces of frozen rain. The key word here is frozen. Sleet falls in a solid state, unlike freezing rain. Sleet is solid and hard enough to be heard as it lands on windows and other surfaces. Sleet often starts as snow in the freezing upper atmosphere. When snow crystals pass through a warmer level of air, they melt. As they continue to descend, another cold level is often encountered. This causes the melted snow to refreeze.
Crystals of snow first form in the upper atmosphere. On its way down to the earth the crystals accumulate increased moisture from the air. By the time we see these crystals floating in the air, they are much bigger than they first started out. This is all contingent on steady levels of freezing temperatures in the upper atmosphere. Otherwise the snow changes form. There also must be sufficient moisture in the air. Snow often falls when temperatures are above freezing. In this case the snow melts as soon as it makes contact with the warm ground.
Dangers and Damage
Winter brings many hazards for travelling and homes. Freezing temperatures often bring ice to the roads. Bridges are especially concerning because of their open access to cold air. Water can freeze on a bridge earlier than it freezes on main streets. Elevated highways are also at risk. Ice that is visibly undetectable on roads is called black ice. The name represents the color of the street below it. This unexpected slippery surface often catches drivers by surprise, increasing the risk of an accident. Slippery sidewalks are also high risk areas for slipping while walking. Be aware of all surfaces during harsh winter weather.
Accumulated ice and snow weigh more than you might imagine. The weight can disrupt buildings that are not well supported. Roofs cave in and endanger residents. Water damage also occurs when snow is not removed from the roof in time or is allowed to melt on the roof. Melted snow often leaks through small spaces and creates water damage inside the home. Maintain the surfaces of your home during long periods of snow.
The many versions of winter weather start far above the earth’s surface. The upper atmospheric levels vary in temperature and moisture saturation. They all depend on moisture and freezing temperatures at some level of exposure. Variations at the upper and lower levels of the atmosphere determine if you see sleet, snow, or freezing rain. Dry climates are less likely to experience large quantities, if any, of these conditions. Danger on the road often increases during times of winter weather. Pay attention to your weather forecast so you know what to expect.