When does snow form?
Anytime the temperature is 32 degrees (that’s freezing) or less where our clouds are, snow falls from those clouds. Even in summertime this is true. Here in the northwest, most days those snowflakes fall into warmer air on their way to earth so they melt into raindrops. We never even knew they were snowflakes when they started out!

However, some days snow will land at your house even when your air temperature is 38 or 39 degrees and the flakes will melt at that point. This only happens when most of the air above us freezing. Snowflakes fall through that cold air, only starting to melt in the last seconds of their journey as they enter that warm air in your neighborhood.

Why are some snowflakes small and others very large?
If small snowflakes are falling at your school, then you will instantly know the air above is very cold and well below freezing. So small snowflakes mean a very cold sky. If you see big snowflakes it’s just the opposite. There must be some warm air somewhere up above you. Big snowflakes pass through that warm air on their way down and partially melt. They collide together at this crucial point and start sticking together to make very big flakes.

Why is it so hard to get snow forecasts right?
Wait a minute…who told you to ask that question? I’m only kidding! Snow forecasts are better than they used to be and they continue to improve, but snow forecasting remains one of the more difficult challenges for meteorologists. One reason is that for many of the more intense snows, the heaviest snow amounts fall in surprisingly narrow bands that are on a smaller scale than observing networks and forecast zones. Also, extremely small temperature differences that define the boundary line between rain and snow make night-and-day differences in snow forecasts. This is part of the fun and frustration that makes snow forecasting so interesting. And around our part of the northwest, we’ve got winds from the Pacific, winds from the Gorge and microclimates caused by hills, mountains and valleys everywhere you look. It all adds to the challenge of forecasting snow.

I’ve heard it can be too cold to snow. Is this true?
No, it can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground—typically -9 degrees Celsius (15 degrees Fahrenheit) or warmer—since air can hold more water vapor at warmer temperatures. And around here – our snows days are almost always warmer than 15 degrees. Often, our temperatures are close to twice as warm while the snow flies.

Where is the snowiest place in the United States?
Based on National Weather Service records for 1961 through 1990, Rochester, New York averages 94 inches of snow annually and is the snowiest large city in the United States. Rochester has a population more than 200,000 and annual municipal snow-removal budget of $3.7 million (1995 figures).

How much snow does Portland receive each year?
Looking back at all the years to 1871 in Portland, we average about 6.6” per year of snowfall in Portland. But that “average” takes into account some monster years when we had really big snows, like in December 2008. Portland had almost two feet of snow during the winter of 2008-2009. Wow. So when people ask how much snow we get here I usually just say, “we get a few inches a year…and most of it melts in a few hours anyway”. And that’s about right! Except the higher up in elevation you are, the more snow you’ll see any given year.

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