It's a very fine line between rain, snow or sleet. Margin of error can be 1% or less. Skew-T BHM

National Weather Service Birmingham, AL

The Difference Between Rain and Freezing Rain

Weather Review - January 29, 2010 from the Birmingham, Alabama National Weather Service Office.
This could just as well apply and does to Atlanta, Georgia or any city in America.

See the slide show for graphics the first one is the Birmingham LAPS Sounding at Noon on Friday, January 29, 2010. The second is the Sounding for the same time for Marion County Alabama.

Many times during the winter months, central Alabama finds itself on the edge of disaster. There are many times when our counties are less than 100 miles away from having large amounts of winter precipitation, whether that is ice pellets, snow or even worse...freezing rain. Friday morning was no different. Portions of NW Alabama, including the Muscle Shoals area, received freezing rain accumulations Friday morning and afternoon of Jan 29th. The freezing rain was seen as far south as Winston and Marion Counties. The question is, why did these two counties see the freezing rain and the Birmingham area did not? We will attempt to illustrate why the Birmingham area was spared Friday.

The two graphs displayed here are known as Skew-T graphs or Thermodynamic diagrams. If you are having trouble seeing them visit the examiner.com/Atlanta page for bigger version in slide show. They are a depiction of what the atmospheric conditions are over a given location. Skew-T graphs are created from the release of a weather balloon with an attached instrument called a radiosonde. After release, the radiosonde sends a signal back to our office that gives us four parameters: environmental temperature, dew point, atmospheric pressure and wind speed and direction. These two graphs are actually computer generated in order to help us maintain a current picture of what's going on in central Alabama. The two green lines on each graph reflect the environmental or air temperature (green line furthest to the right) and the dew point temperature (furthest green line to the left). These two lines can tell meteorologists many things, such as, the closer they are together, the more moisture that is present in that layer of the atmosphere. The solid horizontal lines (white) are pressure levels in the atmosphere. The solid lines running from bottom left to top right indicated temperature in degrees celsius.

So what are these two Skew-Ts telling us about Friday's weather? In the winter months, forecasters use the Skew-T and a method called "The Top-Down Approach" in order to determine what type of precipitation is going to reach the ground. The first graph is a Skew-T over Birmingham and the second is over northern Marion County, both at noon Friday. In this method, a forecaster examines three different layers of the atmosphere.

For simplification, we have highlighted the three most important temperature lines:
The first layer to analyze is the one above -12 degrees Celsius. If the atmosphere is saturated (the green lines close together) above -12 degrees Celsius, then it is ripe for the growth of snow crystals. Atmospheric saturation has occurred above -12 degrees Celsius, so we already have snow forming.

The next layer to analyze is the one just above the surface. If there is a warm layer of air just off the surface and the temperature in this layer is warmer than 4 degrees Celsius, precipitation will usually fall in the form of rain. In the case of the atmosphere in Birmingham, the temperature in the warm layer is approximately 6 degrees Celsius. This means it's too warm to get anything but rain.

In Marion County, the warm layer temperature is slightly cooler than 4 degrees Celsius. This is warm enough for the snow crystals to melt, so at this point, the snow crystal has turned into a rain droplet, but depending on what's going on at the surface...there may still be time for some type of winter precipitation to occur.

Finally, we must examine the layer closest to the surface. If the temperature at the surface at or below freezing, as was the case in northern Marion County Friday, rain droplets will freeze upon contact with the ground. This is why freezing rain was seen in northern portions of the county. With temperatures at 31 to 32 degrees, the rain droplets froze upon contact with the ground (or the trees or the power lines).

In summary, over both areas, the upper atmosphere was ripe for snow crystal growth, but over Birmingham, there was a warm enough layer just above the surface to melt those snow crystals, allowing only rain to reach the ground. In Marion County, the layer above the surface was warm enough to melt the snow crystals, and with the temperature right at or just below freezing at the surface, when the precipitation reached the ground , it was cold enough to freeze on contact. But if the forecast for the surface temp was off by just a degree or two (well within the normal and expected margin of error) there could have been just rain for everyone or freezing rain, or if a tad colder ANYWHERE in the atmosphere then sleet.

Notice just how close a call it was! The lines just barely need to move for a different outcome. Research has shown that there is a margin of error of 1% or less in rain/snow/ice forecasts. This is why weather forecasting is such a difficult job. How would you like to have a margin of error of 1% or less in your job or personal life?

Another problem arises with these Skew-T diagrams that come from the weather balloon releases. The balloons are only released twice a day around sunrise and sunset. It then takes a couple hours before the results arrive. Then about 6 hours for the data to process in computers in time steps forward.

Any changes in the atmosphere between those times is unknown! On top of that, the balloons are released at only some 90 or so spots across the country. 90 tiny points. What the atmosphere is doing in between those points (most of the country) is unknown. There are zero balloons released over the oceans, none! The earth is more water than land so most of the atmosphere does not get a thorough measurement for forecasters to use! This is a major handicap to say the least. Meteorologists must make due with a tiny fraction of data compared to the whole.

And the balloons are not released over every major city nor across a whole city. In Georgia for example the closest Skew-T diagram generated from balloon release is way down in Peachtree City well South of the city and nothing for downtown, the North Suburbs, Athens or the Mountains! So all those unknowns must be extrapolated, estimated, approximated in time and space going forward in the future to make a forecast. Its as if the weatherman is blindfolded with one hand and leg tied behind them then asked to hit a bulls eye with a bow and arrow from 1000 paces in a strong wind. Its a small wonder forecasts are sometimes wrong, its amazing that against all odds they're often right.

Now that you've had instruction go ahead and try your hand at it for the next storm here or anywhere in the country. Have fun, enjoy.

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