Is it ever too warm to snow? That's the question in this Moment of Science.
Snowflakes are ice crystals; water turns to ice at a temperature no higher than 32 degrees Fahrenheit, or zero degrees Celsius. So at first it might seem that if the temperature outside is above 32 degrees Fahrenheit, it won't snow.
Remember, though, that the atmosphere generally gets colder as you go higher. Even on a warm summer day, the air temperature above about 15,000 feet is below 32 degrees. Even on a warm day, snowflakes can form at high altitudes. In fact, meteorologists have concluded that most of the raindrops that fall in our part of the world originate as ice crystals which then melt as they pass through warm air on their way to the ground.
So snowflakes can form high above the ground even if the temperature near the ground is above freezing. Whether we on the ground see rain or snow depends on whether the snowflakes melt completely before they get to us.
A snowflake that has partially melted may be saved by evaporation. A snowflake in warm air picks up heat from that air. But water evaporating from the surface of a melting snowflake carries heat away from the snowflake. Energy in the form of heat is consumed in driving water molecules from the wet snowflake into the air.
Evaporation of sweat from our skin cools our bodies in exactly the same way.
Snowfall is possible at ground temperatures above 32 degrees. If evaporation can carry heat away from a falling snowflake fast enough to balance the heat coming in from warm surrounding air, that snowflake may survive long enough for us on the ground to see it.
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This episode is an adaptation of a broadcast featured in the book Why You Can Never Get to the End of the Rainbow and Other Moments of Science, published by Indiana University press, and edited by producer Don Glass.
