Happy Monday and welcome to another edition of Beyond The Forecast!
We’re in a nice lull between winter weather at the start of this week. Thursday brought as much as a quarter inch of freezing rain to some spots, and more winter weather comes this Thursday.
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Let’s use that break as an opportunity to learn about the different types of precipitation we can see this season.
Most of our precipitation can be broken down into four categories:
- Rain: Liquid through most of the atmosphere and when it reaches the ground
- Freezing Rain: Temperatures aren’t cold enough in the atmosphere to freeze the water molecules, but once the rain hits an object below 32 degrees it freezes on contact
- Sleet: Ice crystals high in the sky fall through enough warmer air to melt the snowflake before passing through colder air that freezes it again
- Snow: Temperatures are well below 32 degrees from the sky to the ground and keep water frozen
Rain and snow are pretty straightforward/temperatures are consistent in the atmosphere, and that means the drops stay consistent as they fall. Rain usually forms as ice crystals in clouds, but once it reaches warmer air, it melts and stays liquid all the way down.
Freezing rain follows most of the same rules as rain, but surface temperatures play a big role. If the ground, trees, buildings, etc. are below the freezing point, it’s very easy for water to freeze on contact.
Freezing rain can be a big problem because of how heavy water is. Ice that builds up along the length of a tree branch can weigh enough to break the branch off and block a road. Another threat during freezing rain is a power outage. Power lines can accumulate ice in the same way as branches and cause plenty of harsh effects on their own.
To us on the ground, sleet tends to have more in common with snow. As it hits the ground, it’s solid and can accumulate much like snow.
Sleet forms when an ice crystal passes through warm air coming at a lower level of the atmosphere. The warmth is enough to at least partially melt the snow. Colder air closer to the surface refreezes that water into a less-neatly shaped snowflake. Often, a few separate snowflakes can join when they refreeze.
Snowflakes form as ice in the sky and fall that way all through the atmosphere. Depending on how cold temperatures are, water molecules can line up in a few different ways. If temperatures are just below freezing, the water might not have enough time to form a nice snowflake shape so instead stays as simple discs or thin needles.
The sweet spot for nice-looking snowflakes is between 3 and 10 degrees at the surface. At that temperature, water molecules can line up in repeating patterns and make all kinds of interesting designs. If temperatures are too cold, it gets difficult for the molecules to line up as nicely so more basic plates or columns become likely again.
We have all kinds of winter weather coming later this week. Keep up to date on what’s headed our way; you can download our weather app and get Meteorologist Chris Michaels’ latest updates online.
You can always get specific forecast details for your zone, whether it’s the Roanoke Valley, the Lynchburg area, the New River Valley or elsewhere around Southwest and Central Virginia, anytime at WSLS.com/weather. Know your zone!
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-- Marshall Downing