DES MOINES, Iowa — When it comes to winter in Iowa, it often seems like anything goes!
Although we most often experience rain or snow, there are occasions when freezing rain and/or sleet occurs, too.
Understanding why and how these different wintry precipitation types unfold and develop can be a bit confusing.
Believe it or not, the precipitation type seen at ground level is almost always dependent on the vertical temperature profile in the atmosphere high above the surface.
Here is a quick breakdown of how each of these precipitation types forms:
- RAIN occurs when water droplets falling from the sky completely melt before reaching the Earth's surface. Yes, these droplets most often begin as snow, because temperatures are normally much colder higher in the atmosphere. However, as the air warms closer to the surface during the spring, summer, and fall months, these droplets totally melt and simply fall as plain rain.
- FREEZING RAIN occurs in a very similar process, with snow melting to fall completely as rain at the surface. The difference arises in the temperatures AT the surface. If temperatures are at 32 or below, the rain freezes on contact, leading to icy conditions on surfaces like roads, bridges and overpasses. If temperatures at the surface are below 30, expect treacherous conditions to arise, as the rain will freeze almost instantaneously when it makes contact with the ground.
- SLEET is a bit different in its formation and development. Like rain and freezing rain, sleet falls as snow initially. However, if a shallow layer of warmer air in the atmosphere develops, this snow may partially melt, at least briefly. The air closer to the surface is still pretty cold, so these partially melted snowflakes don't have time to melt completely, and fall to the ground as sleet.
- SNOW is probably the most easy-to-understand type of wintry precipitation. The air above the surface is all sub-freezing, so that the water droplets begin as snow high in the atmosphere, and stay that way, even when they fall at the surface.
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As you might imagine, this can be tricky to forecast when a complex system moves into the region, as temperatures vary throughout all different levels of the atmosphere.
This also explains why rain, freezing rain, sleet and snow can often occur within the same system!
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