What are the characteristics of storms?

No two storms are similar. Maybe the dark clouds slowly covered the sky, and then it began to rain, or maybe the sunny day suddenly turned into a thunderstorm. Many organizations in history have tried to explain this behavior of weather, from human religious rituals to the interference of gods. Now, physicists from Britain and Sweden have explained a strange weather phenomenon: Cumulus clouds often bring sudden rainfall.

Physicists Michael Wilkinson, Bernhard Mehlig and Vlad Bezuglyy explained that when the turbulence intensity exceeds a threshold, sudden rainfall will occur due to the sharp increase of the collision rate of tiny water droplets. Their work was published in Physical Review Letters. In this paper, they propose that when the function of velocity versus position of tiny water droplets becomes a multi-valued function, their collision rate will suddenly increase. When particles in the same position move at different speeds, the possibility of collision will greatly increase.

Michael Wilkinson said, "Our theory explains how turbulence causes rainfall. This theory is related to any cumulus-related rainfall. For example, clouds like' cauliflower' indicate that there is convection in the air. "

Vertical cumulus clouds produced by air convection can produce turbulence because of heat conduction by convection. This turbulence can make tiny water droplets get enough initial energy to combine into complete raindrops. On the contrary, horizontally developing stratiform clouds generated by a stable atmosphere will not cause such rapid rainfall.

In the past, scientists thought that an important reason for the increase of water droplet collision was the aggregation of particles in local areas. Wilkinson and his colleagues found that although particle aggregation exists in turbulent atmosphere, it has little effect on collision rate. They found that even under high turbulence intensity, the particle aggregation effect is weak and the collision rate is high. Moreover, when cumulus clouds are formed, aggregated particles need higher density than individual particles.

Based on their theory, scientists have studied the previous experimental data and found that when the air turbulence intensity exceeds a threshold, there will be "erosion" and the collision speed of water droplets will increase sharply.

Wilkinson said that at first, the word "erosion" came from the burning of focused sunlight, and later it refers to the bright lines produced by partially focused light, such as the bright lines produced by the water ripples at the bottom of the swimming pool. Now, this term is used to study the motion of particles or light, which means that the function develops and folds into a multi-valued function. In Wilkinson's model, "erosion" refers to the point where the velocity of water droplets folds as a function of position.

At these multi-value points, the collision rate jumps sharply, so it will rain within a few minutes when dark clouds appear.

Wilkinson said: "Although this theory is not helpful for weather forecast, we believe that the collision of particles in turbulence will play an important role in the formation of planets. Our next step is to consider the application of our theory in the study of planetary formation. "