Avalanche Weather Forecasting

Cloud cover affects both the amount of radiational cooling that can occur at night and the amount of direct solar heating that can occur during the day.

Clear, cold nights are accompanied by rapid radiative cooling and can create weak layers in the snowpack, especially when very cold temperatures persist for several nights. Even thin layers of high cloudiness at night will inhibit radiative cooling.

Daytime direct solar radiation warms the snow surface and can cause melting or sublimation, especially as the sun gets higher in the sky in late winter and spring. During this time, higher temperatures combined with increased solar radiation can lead to rapid snowmelt, especially near rock outcroppings and cliff bands and on south-facing slopes (in the Northern Hemisphere). All but a thick overcast will allow some solar radiation to reach the surface and potentially cause melting.

Late in spring, once the free water has percolated all the way down through the snowpack, climax avalanches can occur, removing what remains of the season’s snowpack. (A climax avalanche involves all or most of a snowpack, exposing the ground.)

The duration of cloud cover determines how much it reduces nighttime radiative cooling and daytime direct solar radiation.

Summary table:

Select Operational Information at the top.

Data: Observed cloud cover information for a specific location can be extrapolated from manual and automated surface observations at mountain airports, ski areas, and mountaintops that have instrumentation (a rare situation).

Due to the scarcity of these observations, you can use data from weather satellites and/or radar to estimate cloud cover over a particular mountainous location. Satellite data have many advantages:

• Wide availability
• High resolution worldwide
• Can detect and differentiate multiple levels of cloudiness
• Can be used to detect atmospheric water vapor in the absence of cloudiness

Amount and duration: Gather as much cloud data as possible for the preceding 24 hours. Note periods of clear skies, dense overcast, and any cloudiness whatsoever at night.

Summary table: