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Weather extra: forecasting freezing rain vs. sleet

A representation of weather balloon data taken Feb. 6, 2019, in Detroit. (WWMT/Courtesy National Weather Service)

Freezing rain and sleet present two of the more problematic types of precipitation that winter could throw down in West Michigan, but the two are separate in terms of weather phenomena.

The difference between the two has to do with where the precipitation freezes. If the rain freezes when just before or on contact with the ground it is considered freezing rain. If rain freezes as it falls and has a longer time to solidify, creating a partially frozen raindrop, it is considered sleet. Sleet can also form when a falling snowflake encounters warm air, melts and refreezes.

THE SCIENCE OF FREEZING RAIN VERSUS SLEET

When forecasting, meteorologists not only look at what's happening at the surface of the earth, but also what's happening tens of thousands of feet above our heads. You may hear references to things like the jet stream, which is a river of air flowing around the same height as cruising altitude for many airlines. The jet stream is just one of many things looked at when forecasting the weather, because what's at the top of our atmosphere has big implications for what could happen at the surface.

The National Weather Service is responsible for the launch of just over 90 weather balloons across U.S. territories and the Caribbean twice per day. The launches record atmospheric data such as temperature, dew point, wind speed, and wind direction among other details, from the surface of the earth all the way up to elevations of 100,000 feet or higher. Governmental weather agencies across the globe also follow the same practice, totaling around 900 balloons launch locations across the globe.

Data the balloons collect is plotted on a graph, like the one below, called a skew-T log-P diagram.

The vertical y-axis represents atmospheric pressure, while the temperature, in Celsius, is plotted along the x-axis. Meteorologists use different pressure layers of the atmosphere to analyze conditions because it is a variable that always decreases with height.

The red and green lines represent both temperature and dew point. The red lines in the skew-T log-P diagram represent temperature and the green lines represent dew point.

The data taken directly from a weather balloon is called a sounding. The closer the two line are together, the more moisture is available at that particular height in the atmosphere.

The slanted-dashed line highlighted in blue the photo represents the freezing line. When the temperature and dew point are to the right of that slanted dashed line it means the air in that layer is above freezing. When it's to the left of the line, this means it's below freezing.

In this particular example, there is an area where the temperature and dew point line both protrude to the right of the freezing line, which means there is a layer of air that is warmer than freezing temperature. Notice how closer to the bottom of the plot, and therefore the surface of the earth, the lines fall back below freezing.

At the time of this particular launch, freezing drizzle and mist were being observed at the Detroit Metropolitan Wayne County Airport, which makes sense due to very shallow (less than 1,500 feet) layer of below freezing air, immediately at the surface and the ample moisture in play.

If this sounding were to represent sleet, there would be a deeper layer of below freezing air close to the surface. This would allow for raindrops to freeze on their way from the cloud to the ground. Sleet also often forms from a brief layer of warm air partially melts snowflakes, which then refreeze once they hit the below freezing layer close to the ground.

As you might already have noticed, forecasting whether sleet or freezing rain will occur can be a very complex forecasting challenge for meteorologists. When it comes to temperatures both at the surface and a few thousand feet up, every degree counts.


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