No doubt my meteorologist friends have often heard the “joke” that it must be nice to have a job where you can be wrong half the time and still get paid. This is especially the case in wintertime. And there is no doubt that many of you reading this have been disappointed when your 1-3 inches of snow turned out to be rain, or aggravated when it turned into six inches of heavy snow that made travel almost impossible. Take it from a meteorologist who has lived in Michigan, Oklahoma, Illinois, and Colorado: winter precipitation forecasting is one of the hardest things for a meteorologist to do.
To know what kind of precipitation (and how much) is going to fall on you, it is important to understand the conditions of the air between you and the top of the cloud above you. Did you know that most precipitation starts as a snowflake or ice crystal? But it’s what happens between the cloud and the ground that impacts whether you’ll see snow, sleet, or rain, as the image below illustrates.
Figure 1: Impacts of the air temperature profile on precipitation type.
Source: National Weather Service
The amount of snow, sleet, or rain that falls is also dependent on the temperature and humidity in the atmosphere. You may have heard that an inch of rain is equal to 10 inches of snow. This is just a rough estimate, but the amount of snow for each inch of water can range from 3 to 30 inches or more. For weather forecasters to better understand how much of what type of precipitation is going to fall in the winter, they rely on weather balloons.
How is winter weather predicted?
The Vaisala RS41 radiosonde gathers extremely accurate information about temperature, humidity, air pressure, wind speed and direction as it rises from the surface to the stratosphere. This accuracy is especially important for weather forecasters to correctly predict the type and amount of winter precipitation that is going to fall.
Figure 2: Launching a Vaisala Radiosonde RS41 to get the atmospheric profile.
Having accurate measurements isn’t just important before snow, ice, or rain falls. For the road crews who have to clean up after a winter precipitation event, knowing the road and air temperatures is important to get the roads ready for traffic.
Not all ice melts are created equal. Traditional rock salt (sodium chloride) is effective only until 20°F (-7°C). If it gets colder than that, crews are going to have a hard time keeping the road snow and ice free, unless they switch to a different kind of ice melt. Ice melt including magnesium chloride works until the temperature is around 0°F (-18°C), and calcium chloride works until the temperature reaches -25°F (-32°C).
For road crews, air and road temperatures can be monitored both remotely prior to going on patrol and from the snow plows while road clean-up is underway with a variety of in-road and truck sensors that Vaisala has developed. These ensure that the correct de-icer is loaded into plows and that changes can be made as conditions change.
So the next time the weather turns snowy or icy, take it easy on the meteorologist. Predicting the type and amount of winter weather is really hard to do, but we’re always trying to do our best!