Clouds are surrounded by an invisible haze that expands several kilometers.
Reading aloud Clouds are bigger than they look: According to research by Israeli and American scientists, a haze that is invisible to the naked eye surrounds the visible white parts of the cloud. This diffuse transitional area consists of small particles of water and dust and stretches over several kilometers. Previous climate models may have misinterpreted the role of clouds in the global climate, as they were insufficiently aware of the effect of the veil, the researchers suggest. Clouds consist of water molecules that precipitate on floating dust particles and reflect most of the visible light. For this reason, clouds have two very opposite effects on the climate: on the one hand they promote warming because they absorb the heat radiated from the earth's surface, on the other hand they reflect sunlight back into space, so that only part of the radiation emitted by the sun actually arriving on earth.

Forecasts of global warming therefore always consider the degree of cloud cover and distinguish between cloud-free and cloudy areas in the atmosphere. The studies of Ilan Koren and his colleagues now show that these areas are less clearly distinguishable from each other than previously thought, since the clouds are surrounded by an invisible haze. The cloud fog is already hinted at on digital photos by simple image processing steps, explain the researchers. With the help of satellite imagery and exposure meters, meteorologists examined the diffuse transition area more closely. They found that the veil spreads around the cloud for at least twenty to thirty kilometers and has a significant effect on the brightness.

The extensive haze does not occur in all cloud types. In normal cloud cover, however, the scattered invisible particles comprise about two-thirds of the sky areas, which were previously classified as cloud-free in the models, the scientists estimate. How much the veil actually affects the climate is still unclear. However, climate models have probably underestimated the effect so far, which is why model results and satellite measurements often did not agree.

Nature, online service Original text: Ilan Koren (Weizman Institute Rehovot) et al .: Geophysical Research Letters, vol. 34, L08805 ddp / science.de? Claudia Hilbert ad

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