By Vince Condella Published Feb 05, 2003 at 5:11 AM

It may not require wearing a special pair of glasses, just a good imagination. When trying to understanding the workings of the atmosphere above your head, always think in terms of three dimensions. That may be difficult because when we see weather maps they are always in two dimensions. But that extra dimension, height, is the key to knowing why freezing rain is falling on you rather than snow.

Meteorologists have been trained to see 3-D whenever we look at a weather situation. The structure of the lower atmosphere, especially the lowest 3,000 feet, holds the key when it comes to sorting out a tricky forecast situation. The weather can occur anywhere in the lowest 55,000 or 60,000 feet of the atmosphere. The upper reaches of that thickness contain the tops of the tallest thunderstorms in summer. The core of the jet stream can be found between 25,000 and 35,000 feet.

But the good stuff usually hangs out below 5,000 feet. That is the zone where an innocent looking light rain shower can suddenly lead to the ground being covered with a sheet of ice. Or a light snow event can quickly turn to a snow burst of blizzard proportions. It all depends upon the temperature and moisture structure in the low levels.

Getting a good handle on the structure of these low levels of the atmosphere is not as easy as it seems. There is a lot of filling-in-the-blanks. The surface weather conditions are easy. There are thousands of surface observation sites around the country. But what about the weather conditions 3,000 feet above southeast Wisconsin? Weather balloons are sent up twice a day from about 95 locations in the United States.

These balloons carry a radiosonde, a small box carrying various computerized weather sensors that relay data back down to the ground as the balloon travels upward. But radiosonde launch sites are spread apart so we don't get the data density that we do with surface observations. For example, the nearest upper air data sites for Milwaukee are Green Bay, Minneapolis, Danville (Illinois), and Detroit. There is a lot of real estate between these data points. So the atmospheric structure of temperature, dew point, pressure, and wind over Milwaukee must be estimated based on the data from balloon launches at the four nearest radiosonde locations.

So what do we look for when forecasting tricky weather conditions over our area? One of the biggest red flags is the shallow layer of cold air located at the Earth's surface with warmer air above it. By shallow layer I mean a depth of only 300 to 500 feet. If this air is below freezing but the air above it is above freezing, any precipitation that falls from the clouds will be in liquid form. But when that liquid (raindrop) reaches the ground, it will hit surfaces that are below freezing and turn to ice.


This is called freezing rain and it can turn a casual journey to the local grocery store into an ice skating rink. The key to forecasting freezing rain is noting the depth of the freezing layer. Shallow layers occur when a warm front approaches, causing warm air from the south to lift up and over the colder, and heavier air at the surface. If the sub-freezing depth is deep enough, then the liquid precipitation falling from the clouds may have a chance to freeze before it reaches the Earth's surface. The result is sleet, which can be thought of as a frozen raindrop.

Here is another tricky forecast situation. Let's say the lowest 1,000 feet of the atmosphere is quite dry. The air temperature is 34 or 35 degrees but the dew point is only 15 or 20 degrees. Light rain or drizzle begins to fall from the clouds into the dry layer below the clouds. As the liquid precipitation drops into the dry air it evaporates. But evaporation is a cooling process. It takes heat energy from the surrounding air in order to change the liquid water into water vapor.

So now the air below the cloud begins to cool due to evaporation. If this evaporative cooling process continues for a few hours, the air below the cloud deck gets colder and colder. Suddenly the air is now cold enough to support snow, and ice crystals start falling from the clouds. The light rain can turn to light snow. If the lift in the atmosphere is great enough, the precipitation may fall as moderate to heavy snow.

If that isn't enough to give a forecaster some extra gray hairs, how about the formation of graupel! Now there is a word you don't see very often. Graupel are also called snow pellets. It is a small, soft white ball that looks like a miniature hail stone. When they fall from the sky, they hit the windshield of the car and gently bounce off. They are not as noisy as sleet hitting the window. These snow pellets form when ice crystals and liquid water co-exist within the same cloud.

Yes, under certain conditions "super-cooled" water can be found in clouds. This is water that remains liquid even when the temperature drops below freezing. The super-cooled water attaches to the ice crystals to form a rime of ice and snow producing the graupel.

The next time you look at the sky, think in terms of three dimensions. The things that take place in the lowest 3,000 feet above our heads can be very interesting but they can also make our heads spin. It's just another reason why our atmosphere is so amazing.