We are dealing with another round of freezing drizzle and freezing rain this week. Here is a look what is happening in the atmosphere for this to happen.
The three images below show what the vertical profile of the atmosphere should look like for freezing rain to form. The temperatures are below zero above a warm layer. The snowflake is in this cold layer and as it falls through the cold layer it remains a snowflake.
The snowflake continues to fall, reaches a warm layer and melts into a raindrop. The layer close to the ground is below freezing. There is not enough time for the raindrop to change back to ice before it reaches the ground in this scenario. The ground and air temperature at the ground are below freezing so the raindrop freezes on impact. The difference between freezing rain and sleet is how thick this warm layer is and how high it is above the ground.
Now let’s take a look at real data from Wednesday evening. Two times a day the National Weather Service in Davenport sends up a balloon with weather instruments attached to it. It sends back all kinds of data. In this post we will focus on the temperature (red line). The bottom (x axis) is the temperature scale and side (y axis) is height. (See image below)
The image below is the data plotted from the weather balloon. The blue line is 0C/32F. To the left of the line the temperature is below freezing and to the right it is above freezing. The snowflake in the upper levels of the atmosphere falls through the cold air and remains a snowflake until it reaches the warm layer. The top of the warm layer this evening is around 2,700 feet above ground and the bottom of it is around 1,200 feet above the ground. That means the warm layer is about 1,500 feet thick.The warmest part of this layer is 45F. The snowflake falls through this warm layer and melts into a raindrop.
Below 1,200 feet, the temperature drops below freezing again. So now the raindrop has about 1,200 feet to fall and either refreeze or remain a raindrop. Tonight the raindrop cools off (becomes supercooled) as if falls through the cold air.
The temperature of the ground and objects on the ground are below freezing. The raindrop makes contact with the ground or the objects and freezes. The more this process continues the thicker the ice becomes.
Then the next question is how do you measure how thick the ice is? The video below helps explain how you should measure ice produced by freezing rain.