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On September 17th, 1998 the Ithaca Journal ran its first "Ask A Scientist!" article in which Professor Neil Ashcroft , who was then the director of CCMR, answered the question "What is Jupiter made of?" Since then, we have received over 1,000 questions from students and adults from all over the world. Select questions are answered weekly and published in the Ithaca Journal and on our web site. "Ask A Scientist!" reaches more than 21,000 Central New York residents through the Ithaca Journal and countless others around the world throught the "Ask a Scientist!" web site.

Across disciplines and across the state, from Nobel Prize winning scientist David Lee to notable science education advocate Bill Nye, researchers and scientists have been called on to respond to these questions. For more than seven years, kids - and a few adults - have been submitting their queries to find out the answer to life's everyday questions.

Previous Week's Question Published: 28 October, 2004 Next Week's Question
Cold can play role in snowfall amount
Question
Is it possible to snow at any temperature below 32 degrees Fahrenheit? Is it ever possible for it to be too cold to snow?

Question
Too cold to snow? Never! However, this piece of folklore would be true if it were worded, "it's too cold to snow a lot". All precipitation, rain and snow, starts off as water vapor in the atmosphere. In order for clouds, and eventually precipitation, to form, the amount of water vapor in the atmosphere must be at or near the saturation vapor pressure. You can think of this term as the maximum amount of water vapor that the air can hold. Above this point the vapor condenses and forms clouds and eventually rain or snow.

So what does temperature have to do with this? The saturation vapor pressure changes with temperature. At warm temperatures it is a larger value than at colder temperatures. Simply put, the warmer the air is the more water vapor it can hold. This means that higher precipitation amounts are more likely with warmer air. This physical principle holds whether the temperature is above or below 0°C. So all other things being equal, a winter day with a temperature of -1°C is likely to produce more snow than a day when the temperature is -10°C because the colder air holds less water vapor. You may think that it snows a lot in a cold place like Antarctica, but this part of the world is essentially a desert, receiving less than 16 cm of precipitation per year. Phoenix, Arizona receives more rain than this! Since the temperature never rises above freezing, the ice and snow that we see represents years and years of accumulation.

Temperature also affects the shape of the snowflakes that form in clouds. Between -10 and -20°C (and also around 0°C), snowflakes are the classic six-sided figure that are often depicted in books or pictures. These are called dendrites. At other temperatures, snowflakes typically resemble pencils, needles or plates. Snow composed of lacy dendrites is fluffy when it accumulates on the ground. It doesn't take a lot of water vapor to condense and form a foot of snow. Needles and plates are not fluffy, so even if the same amount of vapor condenses and falls, the more compact accumulation will be less than a foot.