IA, NE, SD (KCAU) -- Winter weather is just around the corner for Siouxland, which means multiple types of precipitation is possible as we transition into the winter months.
Here in Siouxland, we experience all types of precipitation, but how exactly do these form?
The type of precipitation that we see is based on the temperatures of the atmosphere above us. At times, there is air that is above freezing above the surface, which melts water droplets as they fall toward the surface. This feature is commonly called a "warm nose."
The four most common types of precipitation are rain, freezing rain, sleet, and snow.
When forecasting, meteorologists look at Skew-T plots. A Skew-T plot is a diagram that meteorologists use to analyze the conditions of the atmosphere. This is how data from balloon launches are displayed.
Rain:
When it comes to rain, the air near and above the surface is above freezing (32°F or 0°C). Different conditions are needed for different types of rain events, including severe weather and tropical cyclones. Included below is the upper air map from a local severe weather event from June 2014.
Using the example Skew-T, we can see that the freezing line is between 4-5 km above the surface, allowing ample distance and time for water droplets to thaw as they reach the surface.
Freezing Rain:
Freezing rain and sleet are where the "warm nose" and how high it is in the atmosphere becomes an important factor in winter weather forecasting.
Freezing rain is seen when the "warm nose" is close to the surface and/or it is large in size.
This layer of above-freezing air allows water droplets to melt, but doesn't give the droplets enough time to refreeze before they hit the surface. This causes the droplet to become supercooled, or be below freezing but still be a liquid. The droplet then freezes on contact of "elevated" surfaces (cars, bridges, powerlines, trees, etc.)
Ice accumulation typically begins once the air temperature drops below 32°F (0°C) on these elevated surfaces. Roads typically begin to see accumulation when the air temperature drops below 28°F (-2°C). Soil temperatures need to drop below freezing for ice to accumulate in grassy areas on the ground.
Sleet & Graupel:
Sleet, similarly to freezing rain, needs a layer of warm air aloft in which water droplets are able to melt. This "warm nose" is less pronounced than that of the freezing rain profile and/or temperatures drop back to freezing higher in the atmosphere. This allows the melted water droplets to refreeze before they hit the surface.
In some cases, the warm nose is made up of supercooled water droplets, which freeze on contact with the snowflakes as they fall. This is called graupel.
Snow:
With snow, the atmosphere stays below freezing. Even if there is a temperature inversion (where temperature increases with height rather than decreases), like in the example Skew-T provided, the water droplets will remain frozen as they travel to the surface.
Light, powdery, dry snow happens when the entire temperature profile is well below freezing, whereas heavy and wet snow happens when temperatures at/near the surface are near freezing.
Moisture content also plays a role in the type of snow we see. Wet snow has a low snow-to-liquid ratio. In this case, (inches of snow):(inches of liquid) would have numbers super close to one another, dropping as low as 5:1.
"Normal" snow has an approximate 10:1 snow-to-liquid ratio. Temperatures near the surface are typically in the 20s for this range.
Dry snow happens when your air temperatures drop to the teens or lower near the surface. The snow-to-liquid ratio is typically around 15:1 or higher. This ratio is also highly dependent on the temperatures of the air aloft. The colder the air aloft, the higher your ratio can get. This is due to colder air not being able to hold as much moisture.
With winter weather around the corner, be sure to tune into KCAU 9 News for your forecasts and for any inclement weather updates.