Recent snowfall across the Rockies has me jonesing for ski season. Unfortunately, August in the midwest spells 90-degree temps and high humidity. If we’re extremely lucky, the earliest snowfall won’t be till late October, and ski season won’t start until the middle of November. So we’re still a ways off. But snow can fall even if it’s above 32 degrees, and I decided to find out how much above freezing it can snow to give me a glimmer of hope for some early season snowfall.
Essentially, the limiting factor is the wet-bulb temperature, the temperature at which air can be cooled by the evaporation of water at a constant pressure. This is opposed to dry-bulb temperature, which is the temperature of the ambient air. All precipitation as it falls begins as a solid; then, as it falls, it will turn into a liquid if it’s warm enough or stay a solid if it’s cool enough. As the precipitation falls, it evaporates and will cool the air immediately surrounding the precipitation. By how much it will cool the surrounding air depends on the amount of humidity in the atmosphere. If the solid precipitation can cool the surrounding air enough, the snow will stay snow as it reaches the ground.
The wet-bulb temperature is measured by wrapping a damp cloth around the bulb of a mercury thermometer. The cloth will have a higher percentage of moisture than the surrounding air. The water will then evaporate from the cloth and cool the surrounding air. Once the cloth and surrounding air have reached equilibrium in terms of water vapor, the thermometer will read the wet-bulb temperature.
What does this have anything to do with snow? The drier the air is, the more the precipitation can evaporate into the surrounding air. This is why the dry climate of Utah has some of the lightest powder in the world, and the rainforests of the Pacific Northwest have much denser snow. If the atmosphere has 100% humidity, then the precipitation will not evaporate to cool off the surrounding air. If the atmosphere has 100% humidity, it will not snow unless it is 32 degrees Fahrenheit. And if it’s 32 degrees, the snow will have a much higher moisture content. Often, a storm will bring snow if it’s few degrees above freezing, but the air temperature will be below freezing within an hour. This cooling is partly due to the evaporation cooling the surrounding air.
Arapahoe Basin and Loveland ski areas are often the first mountains to open in the country due to the right combination of the dry climate and cold temps. Having the right combination correct is crucial for snowmaking operations, and yes, it is possible to make snow above freezing if the wet-bulb temperature is low enough. Just a handful of weeks, and we could see ski areas in the Rockies blowing snow. Cross your finger for a low wet-bulb temperature!