A new study has found that snow can fall at warmer temperatures in Colorado than almost anywhere else in the world.
The study, by a group from the University of Colorado, discovered that snow could fall to the ground on Colorado's ski slopes at up to 4 degrees Celcius. Previously it had been thought that temperatures needed to be at freezing point or below for snow to survive down to ground level.
The researchers from the University's Institute of Arctic and Alpine Research based in Boulder, Colorado studied climate data from nearly 18 million precipitation, temperature, and humidity observations spanning over 100 countries and four continents across the Northern Hemisphere.
It does need to be at freezing point or below in the clouds for snow to form, the study really looked at why snow formed at freezing point was still snow when it landed in areas where air temperatures near the ground temperatures could be above 3C.
They discovered that what makes the difference is not just the temperature, but also the air's moisture content. If the air is very dry, as it usually is in Colorado, snowflakes can survive in warmer air; if it has a high moisture content, the opposite is true, and snow can turn to liquid (freezing rain) even at sub-zero temperatures as it falls. The process is known as evaporative cooling.
Most Colorado ski resorts are reported to already be aware of the process from experience and successfully make snow in positive temperatures early in the autumn.
Now the researchers hope the study will be used to make climate modelling and weather forecasting more accurate. Existing systems assume it has to be freezing for snow to fall in Colorado.