Ice Storms -- Winter's Destructive Side
The beauty of a landscape with freshly fallen snow can make even the die-hard winter weather-phobes appreciate the season. The problem is that sometimes the snow comes with sleet and freezing rain; something most everybody would like to do without. To understand why some places will see a nice white blanket while others get turned into an ice rink, you have to look at temperatures from the ground to the cloud tops.
In the winter months, temperatures from the base of a cloud, all the way down to the surface, can be below freezing - - 32 degrees Fahrenheit. If this is the case, the moisture falls through this cold air and stays all snow. If there is a layer of above-freezing temperatures that sneaks its way in, the snowflakes will melt into raindrops as they fall through that layer.
With a thin layer of warmer air, the raindrop will quickly fall back into below-freezing air. From here the raindrop freezes, and the frozen drop hits the ground as a small ball of ice, called sleet, or ice pellets.
If more of the warm air takes over, the melted snowflakes have less and less cold air to fall through. In time, only a very shallow layer of cold air remains close to the surface. The drop remains a liquid as it hits the surface, but instantly freezes on contact. This is called freezing rain and it doesn't take much to glaze objects near the surface with a layer of ice.
The concern for icing vanishes as surface temperatures finally rise above the critical freezing mark. This transition process can take a few hours, several hours, or even a day or two, making the overall forecast process even more challenging. In many cases, sleet poses the same hazards as snow. It can accumulate and create very slick and slippery travel conditions.
Freezing rain, on the other hand, can turn highways into skating rinks, and send ice-weighted tree limbs and powerlines crashing down, leaving hundreds of thousands of people in the dark for days.
To know which is expected, meteorologists look at something called an "atmospheric profile." This allows for a look at a vertical section of the atmosphere from the surface, all the way up to several thousands of feet in the air. By determining such things as temperature and relative humidity, meteorologists can predict if you`ll be shoveling a few inches of the fluffy white stuff... or dealing with a potentially destructive ice storm.
