Forecasting Precipitation Types

Using Weather Balloon Soundings to Determine Freezing Rain or Sleet

Dec 5, 2007

Freezing Rain Covers Everything, Eona Martin

Meteorologists use weather balloon sounding data from the National Weather Service to forecast precipitation such as freezing rain, sleet, snow, hail and rain.

What is Precipitation?

The National Weather Service defines precipitation as the “process where water vapor condenses in the atmosphere to form water droplets that fall to the Earth as rain, sleet, snow, hail, etc.” Any moisture that falls from the clouds above is considered precipitation.

Types of Precipitation

When most people think of precipitation, they think of rain and snow. Yet, precipitation comes in all shapes and sizes. Rain, hail, freezing rain, sleet and snow are all types of precipitation.

When moisture in clouds grows too heavy, it falls as rain. Rain can begin as a group of ice crystals forming snowflakes. As the snowflake falls, it melts as it passed into warmer air, turning into a raindrop. When it rains, the temperature near the ground is above freezing.
Hail is made up of large frozen drops of moisture, usually produced by thunderstorms. Starting as a snowflake, it grows into an ice pellet as it continues to cycle around the inside of the cloud. As the ice pellet is lifted by the updraft, circling around the cloud, it grows larger as layers of ice form around its exterior. Hailstones are usually irregularly shaped.
Freezing rain is considered super-cooled rain that freezes on impact with the frozen ground. Freezing rain happens less frequently than any other type of precipitation. Freezing rain falls like snow but passes through a layer of warm air, which melts the snowflake turning it to rain. Then this droplet of moisture, passed through a layer of cold air just above the ground, where it instantly freezes on contact. Freezing rain happens most frequently to the east of the Rocky Mountains in the United States, because of the existence of shallow arctic air. Freezing rain usually only falls in narrow bands, not widespread like rain or snow. Ice storms, containing freezing rain, bring damaging winter weather.
Sleet is similar to freezing rain and hail but forms through a slightly different process. As the snowflake falls, it passed through a shallow layer of warm air but only partially melts. As it passes through the lower layer of cold air, it freezes again, forming ice pellets. These ice pellets bounce when they hit the ground.
As the primary form of all moisture falling from clouds, snowflakes are the most predominant form of precipitation in the atmosphere. However, in order for snow to fall near the Earth’s surface, hitting the ground, the air temperature must be below freezing.

Using Weather Balloons to Predict and Forecast Precipitation

As some types of precipitation, especially freezing rain, can be difficult to forecast, meteorologists use weather balloon soundings to determine precipitation types. Because slight variations in temperature make all the difference, weather balloons record temperatures at various heights within the atmosphere.

Weather balloons, filled with helium gas, are released at various locations. The National Weather Service uses weather balloons to measure air pressure, temperature, wind speed and direction and relative humidity. A small device called a radiosonde attached to the balloon, records data at difference heights.

Meteorologists receive the data as a set of numbers, which are then computed into diagrams, called Skew-T diagrams. Meteorologists use these diagrams to analyze vertical temperatures, to determine what type of precipitation will form and the extent.

High Tech Weather Forecasting

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