Winter is here! And with it comes snow, freezing rain and sleet in many parts of the country. These storms can mean school-closing snow days. Or they can mean nothing but rain. How do meteorologists make their forecasts?
Almost all of our weather, including winter weather, occurs in the lowest several miles of the atmosphere. Generally how much warm/cold air is in the atmosphere determines whether we’re going to get snow, freezing rain or sleet.
The main types of winter precipitation are snow, freezing rain and sleet (also known as ice pellets). Critical ingredients are moisture and very cold (sub-freezing) temperatures.
• Snow: Precipitation in the shape of ice crystals, mainly of intricately branched, hexagonal form and often gathered into snowflakes, made directly from the freezing of the water vapor in the air.
• Freezing Rain: Rain that falls as a liquid but freezes when it hits the ground, creating a coating of ice on roads, walkways, trees and power lines.
• Sleet (also known as ice pellets): Pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces.
How do snow, freezing rain and sleet form? See the illustration below. A wedge of warm air is depicted by the red area. This air is warmer than freezing (32°F or 0°C). The thicker the wedge of warm air the less frozen/freezing precipitation would be expected. Meanwhile in the picture cold air is represented by the blue area. This air is cooler than freezing.
Look at the above illustration again and imagine that you’re a meteorologist. Your forecast decides whether the schools are open or closed in the morning. Do you give the kids a snow day? Or is it just going to rain? To make an accurate prediction, you need an understanding of the amount of warm and cold air and where it is in the atmosphere (along with the necessary moisture). Challenging, isn’t it? A few degrees of temperature can turn your snow forecast into rain.
After seeing how snow, freezing rain and sleet form you can understand how difficult they can be to predict. Now you know what it’s like to be a meteorologist facing a tricky winter forecast.
