The WeatherBlog was recently on a ski tour. Due to a lack of competence in choosing a destination, he found himself in a cloud. Not under the cloud, but actually in the cloud, which from the point of view of those affected becomes fog. Above the fog, the sun was shining, while below it you could at least enjoy ski-length visibility. There wasn't much to see in the cloud apart from fog. It was damp and somehow warm. Warmer than outside the cloud. The snow was also noticeably heavier. The WeatherBlog didn't get greased up, as it was cloudy, and got sunburnt.
Why, wherefore, why?
When you go uphill, you usually sweat, especially when it's already April. Sweat has a cooling effect on the body when it evaporates. The energy that the sweat needs to become gaseous is obtained from the environment, for example from our skin, which cools down accordingly. For the whole thing to work, the water vapor pressure in the air must be lower than on the skin. If we are standing in fog and the air is already saturated with water, the sweat cannot evaporate and the cooling system does not work. When we are finally above the cloud, we immediately feel cooler as the air is drier and evaporation can take place, even more so if there is still a light wind blowing. Better to put on a jacket straight away. The sweat remains between the fabric and the skin and we don't cool down so quickly as evaporation is slowed down. The snow in the cloud doesn't sweat, but gets heavy and wet. When sunlight hits a beautiful, white snow surface directly, most of the short-wave solar radiation is simply reflected and we are glad that we have dark ski goggles. Clouds, like everything else, have a certain radiation temperature. The water droplets in the cloud absorb radiation in a wide range of the long-wave spectrum. They absorb radiant energy and emit it, again in the long-wave range. Snow cannot reflect long-wave radiation, but absorbs it. The energy supplied increases the water content and the snow becomes heavy.
And the sunburn? The responsible UV rays are partially absorbed by the cloud, but the rest is reflected in the cloud and around the snow until it reaches us. In certain constellations with broken clouds, even more UV radiation can reach us than if there were no clouds at all. However, the main factor in the problem of "sunburn in clouds" is that we are obviously simply too stupid to apply sunscreen when we can't see the sun.
Weather outlook
We will see the sun from time to time over the next few days. We will only see fresh snow in the west tomorrow (Thursday 11.4.13), but with very strong winds. In the east, the snowfall will be more cosmetic and windy, especially in the foehn corridors. On Friday, the disturbance will move eastwards from the western Alps and cause cloudy weather there. The weekend should be mostly sunny and warm, with the possibility of spring clouds and isolated showers in the afternoon.
