Atmospheric sciences are full of counterintuitive phenomena, and clouds are no exception. It seems to us, for example, that cumulus clouds in good weather drift in the blue sky, carried by the wind like the egrets of a dandelion. In this article, however, we will see that things work quite distinctly.
A cloud is not blown by the wind
This challenges common sense quite head-on. And yet! Even though observing the sky on a daily basis gives us the impression that the wind carries the clouds, the relationship between the two is actually far more complex than one might first think. And for good reason, a cloud is not a material object as can be a balloon or a boat but a region of the atmosphere occupied by a set of condensates. In other words, a cluster of water droplets or ice crystals that are formed and broken down according to the temperature and humidity conditions that surround them.
What we call a “cloud” is neither more nor less than the trace left by the condensed water. It materializes the saturated parts of atmospheric circulation: the air continuously enters and leaves these regions!
Thus, what appears to us as the displacement of a cloud is quite simply that of the shape drawn by the set of condensates – not the movement of the condensates themselves. In the end, the displacement of saturated zones will depend on that of the circulation system to which they belong. Indeed, it alone controls the regimes of ascending and subsidences, therefore the zones of condensation and evaporation, which influence the clouds. It may for example be a convective cell in an unstable atmosphere.
A concept illuminated by the observation of lenticular clouds
Observation of a cloud lenticular – linked to the passage of air over a mountain – allows us to better appreciate these not very trivial concepts. As shown in the video above, the cloud pattern on the left remains almost stationary. However, the air circulates at high speed! Why then does the cloud element stay in place? The answer lies in the fact that, constantly, droplets form on one side and evaporate on the other, thereby following the vertical movements imposed by the relief. With this example, we realize that the displacement of the cloud and that of the condensates which constitute it can in no case be considered as equivalent phenomena.
The displacement of saturated zones will depend on that of the circulation system to which they belong. Indeed, it alone controls the ascending and subsidiarity regimes, therefore the condensation and evaporation zones, which influence the clouds.
An astonishing behavior which is reminiscent of that of atmospheric depressions and anticyclones. In this regard, the interested reader can consult our dedicated article.
Source : Physics and Dynamics of Clouds and Precipitation, Pao K. Wang (2013).