20.07.2018 change 20.07.2018

#Askscientist: What determines the speed of moving clouds?

Photo: Fotolia Photo: Fotolia

What we perceive as movement of a cloud (or its lack of movement) is not always what it seems. That`s because a cloud is not an object, but a set of processes - cloud physicist Prof. Szymon Malinowski from the University of Warsaw explains in an interview with PAP.

During the Science Picnic, throughout the day we collected questions that visitors wanted to ask the scientists. Now we ask researchers some of these questions.

With the question " What determines the speed of moving clouds?" we turned to cloud physicist Prof. Szymon Malinowski from the University of Warsaw. He is a co-author of the website "Nauka o klimacie" ("Climate Science") and winner of the Science Populariser 2017 award.

"What determines the speed of clouds? - Prof. Malinowski wonders. - This is not a simple question because a cloud is not a thing, it`s a process. Or rather a whole set of processes that take place in the atmosphere".

Professor Malinowski reminds that clouds are places where water vapour has condensed. He points out that water vapour, water in the gaseous state of aggregation, is invisible - just like the air just above a kettle with boiling water. But when the water vapour condenses, it becomes visible, because light is dispersed by the droplets of water suspended in the air.

To put it simply, a cloud is the entire volume of air, in which water droplets or ice crystals have formed during the condensation process. It is carried by wind. "But that is not the only possibility" - the researcher says.

"We can imagine the atmosphere as layers of less and less dense liquid" - he explains. He compares it to pouring honey, water, oil and alcohol into one glass. These ingredients will not immediately mix. And at the contact of these layers, waves similar to those on water may form. It is similar in the air.

"If the wind carries a layer of air upward, water vapour in this area may condense. Then, if the air descends - the cloud evaporates." The droplets of water change into invisible water vapour" - he says.

And because cloud is a process, it can happen that a cloud appears in one place, and disappears in another. "And we perceive it from the ground as a cloud movement" - he points out.

Another situation may also happen: a stationary cloud can form in a mass of air that moves. "For example, altocumulus lenticularis clouds occurring in the mountains remain in one place, despite the fact that the air flows through them all the time" - describes the scientist. He explains that the flowing air masses are pushed upward over the mountain top and condensation occurs. Behind the summit, the air drops, warms up, droplets of water evaporate and the cloud ceases to exist. "The area in which condensation occurs is in one place, but what we see is not the same mass of air" - the scientist smiles.

The researcher adds that sometimes the wind blows in one direction near the surface of the earth, and higher up - in a different direction. Clouds at different altitudes can move in different directions and at different speeds.

PAP - Science in Poland, Ludwika Tomala

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