Fresh Snow Cover and Clear Skies: Why so Cold at Night?

It is common for us to experience particularly frigid temperatures the night after a snow event. It turns out that there are a number of factors that make cloudless post-winter storm nights anomalously cold:

The earth receives energy from the sun during the day as shortwave radiation (ultraviolet, visible, and shortwave infrared radiation), and emits energy at night in the form of longwave radiation (infrared radiation).

Earth's Energy Budget|Source:NASA

However, it is important to note that not all of the energy that the earth receives during the day is lost at night.

When the earth is emitting longwave radiation at night, clouds, buildings, and other man-made features can limit how much energy is emitted back into space. This is why we often experience warmer temperatures in urban areas at night in comparison to rural areas where there are a lack of structures; buildings reflect this outgoing longwave radiation back to the surface. Additionally, the lack of vegetation in cities greatly reduces the amount of evapotranspiration given off by plants (plants transpire moisture, and evaporation is a cooling process). Lastly, aerosols (pollutants), increase the number of cloud condensation nuclei over cities, which may lead to increased cloud cover over urban environments. This cloud cover acts as a blanket, which reflects the outgoing longwave radiation back towards the earth's surface. When skies are clear, this radiation is able to escape, resulting in cooler temperatures at the surface.

Fresh snow cover is also effective at cooling the surface. Snow has a high albedo, meaning that it is highly reflective. During the day, snow is very effective (80-90% effective) at reflecting incoming shortwave radiation back into space, while bare ground and soil has a lower albedo, meaning that it absorbs more of this incoming solar radiation, thus reflecting less radiation back into space.

Additionally, more of the sun's energy is being used up to melt the snow instead of heating the ground. Melting of snow is also a cooling process, which further limits warming at the surface.

Albedo of some common surface characteristics|Source: Penn State

At night, snow is also very effective at emitting longwave radiation back to space, which in turn decreases the temperature at the surface. When the air is dry, which is often the case here in Colorado, snow will sublimate, which is the phase change from a solid to a gas. Sublimation is a cooling process due to conservation of energy in the atmosphere.

So the next time that we have a fresh snowfall with clear skies and calm winds in the wake of a winter storm, consider nudging your forecast low temperatures down a few degrees!

Noah Brauer

Noah Brauer is a meteorologist at MeteoStar in Denver, CO. He holds a degree in geography and minors in math and atmospheric science. Noah is from Vancouver, Canada, and now calls Boulder, CO home.

Boulder, CO
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