14 DAY TRIAL // Until now, one of the largest mysteries associated with understanding how clouds form was how the atmospheric structures at times appear to form faster than the basic laws of physics allow for. Physicists now believe they may have uncovered the answer.
The mechanism through which “usual” clouds form is fairly straightforward and well-understood. Under specific atmospheric conditions, with certain levels of pressure and appropriate temperatures, water vapors in the air begin condensing.
They form small droplets, which then proceed to combine with each other until they form raindrops that have enough mass to start dropping towards the ground. Some studies indicate that the vapors first start coming together around aerosol particles.
But the intricacies of droplet aggregation are what skew most theories. There are reports of clouds appearing out of the blue within minutes, and of rain starting to fall from areas of the sky that were a short while before clear and blue.
In scientific measurements seeking to validate these claims, scientists have indeed determined that droplets measuring some 15 micrometers grow to 50 micrometers in less than half an hour.
The latter size is sufficiently large to allow for a rain shower to drench the ground below. The mystery physicists wanted to clear revolved around how this fast aggregation occurs.
Standard models of droplet formation cannot explain this phenomenon in the absence of ice formation. But Imperial College London (ICL) experts Vassilios Dallas and Christos Vassilicos have an answer.
The key to unlocking the puzzle lies in the work of 19th century Irish mathematician George Stokes, who developed an inertia-related number that bears his name.
This dimensionless quantity refers to how water droplets interact with each other in a flow of gas. The most important factor influencing this number is the scale of the medium in which these collisions take place, Technology Review reports.
When droplets have a small Stokes number, they don't have sufficient inertia, and therefore rarely collide with each other. Conversely, when the number is large enough, the droplets carry a lot of inertia, and simply can't avoid collisions, and subsequent aggregation.
The ICL experts say that turbulence is what causes the Stokes number to go up, and add that these phenomena can occur at a variety of scales, including the micrometer one.
When turbulences at this scale occur, they cause big variations in the Stokes number. As a consequence, the water droplets begin colliding more and more, and the cloud formation process is accelerated.
If the new model turns out to be accurate, then it could have significant implications for current climate models. Scientists will soon be able to calculate in more detail the total amount of sunlight clouds bounce back into space.