Abstract: Because of the rain was a relative humidity of 100% at a temperature of about 6 ° C available. Over the snow-covered fields cooled the moist air, exceeding the 100%, ie the condensed moisture contained in small droplets, and it was the local fog.
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| a) A snow-covered meadow with fog. b) photo of a field without snow cover in the immediate vicinity - there is no visible smoke. | |
ago three days I noticed at lunch time to just above the meadows and fields which were still covered with snow lying fog while on the grass beside it was not a fog without snow. It was raining quite heavily and the temperature was 6 ° C.
One, two weeks before it snowed heavily and there was snow. The heat of the last days and the rain for two days, most of the snow is already melted. Only a few meadows and fields of snow still lies fairly consistently on other any more. Such large differences are interesting, because the meadows alternate with and without snow and shows no external differences such as more or less shade, altitude etc. . Let I suspect the difference comes from different high or thick grass under the snow about, which acts as an insulating layer. But for that (if it proceeds it) later, now back to the fog and snow.
Since it rained a long time at a constant temperature, relative humidity at 6 ° C has increased to 100%. Most of the meadows was no snow and it was formed from a balance between water and humidity in these 6 ° C. Just over the snow-covered fields, the air must be cooler, but because the snow may even have even more than 0 ° C. Since the very moist air moves from the vicinity of 6 ° C colder in these areas (it was absolutely no wind, it was more a diffusion, a very slow flow due to temperature differences), they cooled off from there and could not keep the 100% humidity, the higher temperature in the gaseous state. The humidity was too high for the colder zones and condensed into droplets that were visible in the form of fog.
The whole "spitting" has only held about one hour. I had to first get a camera and after an hour the fog had already been partially resolved. The temperature had probably changed something and it was a light air movement, because we saw that the fog in south moved and broke up. I could barely make a halfway decent shot of a slightly larger area, the other fog had already been resolved.
Back to my assumption, why does the snow kept on some fields considerably longer. If the earth is covered by a thick layer of grass, so this has several implications. When the first snow falls, the earth is sometimes still warm and the snow does not lie still. If a dense grass layer is present, this acts as insulation. It cools off quickly at the top through the falling snow and cold air, the snow is still, though the earth is under even warmer. It is formed on the dense grass layer a thick blanket of snow. But this is not the only effect that the heat can in certain circumstances, to "come back" below.
It was this winter in December, on average, long very cold. Then the ground freezes up a little way down, ie, the temperature in the soil below 0 ° C. But that's still not very deep. Half a meter below the surface of the ground is not frozen more often, including the temperature is above 0 ° C and it saved a lot of heat there.
Now, if the air temperature is a few days at about 0 ° C, as we actually below a dense high fog was the case, then melts the snow is not from above, but the heat from the deeper soil layers is due to the thermal conductivity of the soil from the bottom up. Only if the air would remain rather cold, the temperature gradient could be maintained in the depths of the earth into it. If this is not exactly the case, then the earth beneath the snow layer "of itself" getting warmer and the snow may melt from the bottom - but only if no good insulating layer in between. Thus, if a thick layer of grass is present, the well insulated, so the snow can be melted from below, barely, a little ground but already protected. Under certain weather conditions
, an insulating layer of grass lead to the earth therefore doubly a thicker blanket of snow. And over the snow-covered meadows may occur local fog.
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