Precipitation type: From snow to rain and everything in between

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Weekend Meteorologist Joseph Dames

PORTLAND, Ore. (KOIN) – Living in Oregon, we get to experience fall, winter, spring and summer. Of course, with each season, we may experience different types of precipitation.

In the winter, we may see snow in Portland. That is not the case in the summer, when temperatures are much warmer. During the fall, winter and spring, we may see all types.

How does that work? It can be difficult forecasting snow in the winter in Portland because it’s tough to see the Willamette Valley cool all the way down to the surface. Let’s take a look at what we call a temperature profile and what it means for precipitation types.

First, let’s start with a definition of each common precipitation type:

SNOW: A solid form of precipitation composed of ice crystals.

SLEET: Precipitation in the form of an ice pellet.

FREEZING RAIN: When liquid water drops freeze on contact because of a freezing surface.

RAIN: Precipitation in the form of liquid water drops.

I’m sure you’ve experienced each in the winter. Write some of your questions down before you take a look at the slideshow below. You may be asking yourself, “does rain start as snow?” How about, “what is the cause of freezing rain versus regular rain”? These are great questions!


These graphics represent a temperature profile, which is the path of the measurement and its relationship to temperature. For example, if we send a balloon up into the sky, as it is climbing altitude, what is the temperature at that point. That is represented by the black line that says “temperature profile.” We know that the temperature typically cools as you get higher, based on previous lessons about the layers of the atmosphere, which you can find here. That is generally true, but in many cases, there can be warmer layers above and sometimes colder air at the surface than 3,000 feet above your head. This can cause a ruckus when it’s time to forecast the type of precipitation.

Freezing rain came down over Portland, December 19 2016. (NWS)_383032

When forecasting snow, if we know that it’s going to be below freezing from the ground all the way up, then there will be no question that the water that reaches the surface will be snow. If you look at the first profile, that is what it looks like if you take a measurement of the temperature from the ground up.

Sleet can be a tricky forecast because it means that there is a warmer layer above the surface that will melt that snow to rain before it freezes again. What makes it difficult is the need to know how thick the warm layer is and how thick the cold layer is at the surface to help aid in freezing that rain enough to turn to an ice pellet. That can be difficult during the ever-changing moments of our day.

When forecasting rain, we know that a large slice of the atmosphere is going to be warmer than freezing. Once that snow hits the warm layer of air, it will melt. The water droplet will then carry all the way to the surface without dealing with any more cold air.

Unless it’s freezing rain, which means a shallow layer of cold air at the surface is present. Cold enough for that rain to freeze and glaze the surface with ice. That is very dangerous and happens frequently in the Gorge. Next time we start seeing snow in the forecast, you can imagine what is going on above your head.

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