The temperature is below freezing and precipitation is falling from the sky.
It ought to be snow. But it’s not. It’s rain that is freezing on everything making life dangerous and miserable for everyone. Why ice?
Much of the rain which is falling when the temperature at the ground is at or slightly below freezing was probably snow at one time.
High up in the cloud where the temperature is very cold, the snowflakes form. But, as the flakes drop, they go through a layer of air which is above freezing. The graph below is actually a profile of the atmosphere in Topeka, Kansas from early in the morning on December 11, 2007 — the day of the big ice storm.
Notice how the temperature of the atmosphere is actually warmer a few thousand feet above the ground than it is at the ground? There is a very thin layer of air below freezing (32 degrees Fahrenheit) at the surface of the Earth. But as you go up, the temperature gets warmer for a time. Then, at about 12,000 feet, the temperature drops back below freezing.
On this particular day, the atmosphere was saturated up into sky well beyond 12,000 feet. So, any precipitation which forms up there is likely to be snow. As the snowflakes fall, they go through that warm stuff, and melt. Now, you have raindrops falling.
Once those raindrops get close to the ground where the temperature is again below freezing, they might re-freeze. If that happens, you get sleet. If the layer of sub-freezing air is too shallow, the raindrops don’t have time to freeze until they reach the ground and strike a surface which is at freezing or below. That’s freezing rain as the picture below indicates.
By the way, we get atmospheric profiles like this one twice each day from many, many places around the world. Weather balloons are launched simultaneously which feed information back to meteorologists on the ground – helping us to predict whether the precipitation will fall as rain, snow, sleet or even hail.
I'm a TV weatherman in south Texas. I get blamed for the bad weather, but I also get credit for the beautiful days. I absolutely love my job!