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Do clouds move?

TUESDAY MAY 01, 2012

We have all looked through the window of a 747 and noticed way below, banks of clouds stationary around hills or valleys especially on a winter's morning when heat from the morning sun meets colder air coming off ice-covered ground. We call it fog if it is low enough.  In evenings clouds form when colder air meets rising heat from sun-warmed ground. Cloud is mostly over land in the morning and over sea in the evening. In either case, it is warm air meeting cooler air. Because heat rise and colder heavier air fall tends to be vertical, clouds form around imaginary columns. When you look down from above you see stationary cloud. But looking up from the ground we see clouds racing across the heavens. 


We can draw an analogy from the sea. The water does not move or all fish would end up on the beach. Instead, a wave current moves through water. Each bit of water is trapped by the bit ahead, behind and adjacent and so it cannot get out. But pressure gradients cause wave movement from high to low pressure to restore upset equilibrium just as in a dish when more water is added to one end, and the water level stabilises because of the pressure difference.


Unequal heating or cooling makes air masses move because of the pressure gradient. Cooler air descends then warmer air rises. Heavier cooler air exerts higher pressure on whatever is beneath. Lighter warmer air carries steam to higher altitudes, which as the distance from the sun-warmed ground increases become cool areas that form clouds because air cools as it rises. So air is always on the move as colder air falls, warmer air rises and wind-air is transported along by the pressure/temperature differences.


Whether or not clouds move is a question of scale. Looking down from a plane the sea looks motionless bar the odd whitecap here and there because you are at least 20 times further away than when you look from the ground at a cloud a half mile above. But right down on the sea the waves may be in a huge rolling motion viewed from a row boat. Looking down from a helicopter at 1,000 feet hovering above a stadium of football fans the crowd looks like a stationary mass, but on the ground there are people scurrying, carrying refreshments, visiting toilets and returning to seats. 


Clouds are not separate entities from the rest of the vapour that covers the whole sky. They are vapour that has become visible because of condensation due to colder air. The clouds are along for the ride, as long as the colder air made visible by condensation remains a colder part of the air. But it is the pressures and temperatures that move. Clouds can be seen to quickly disintegrate arriving at certain parts of the sky because air may be warmer there. Some clouds from the ground can appear still when really moving or moving when there is no wind. New cloud-drops will condense from the water vapour in the same place where cold air may be constant, as next to a hill where air is deflected upwards. The illusion of clouds moving is the replacement of visible vapour as the colder air passes through new air arriving at that spot, blown in by the wind.


Clouds are tricks of light on cold air, and because they cannot be contained are not structures. If illusory at the outset, then our perception of an illusion moving will also be an illusion. The equivalent might be to ask what moves during an earthquake, the earthquake or the land. If the answer is the land then it is not the earthquake. If you reply that the earthquake moves the land, then that implies the earthquake is disconnected with the land, but somehow visits and leaves. If it leaves where does it go?  And where does sound go after it is uttered? The people in a shaking zone will always say they felt a movement and saw land move. They could not say they saw an earthquake, but that they felt it and saw what it did. Earthquake is the name we give to the phenomenon of a land pressure gradient sending a wave through an area of subsoil, and clouds are a name we give to the ways our eyes perceive temperatures and their effect on water vapour. We perceive that something has formed. Having given a result a name, the tendency is to then treat that named as a tangible object. That is the point of confusion because it may be different for every viewer.


But it is how we try to make sense of the world. It is hard to sort perception, interpretation and appearance from realities about movement and from the trappings of careless language. For instance we might on occasion say ‘I can see nothing’ which is impossible because in this sentence nothing is something – something we say we can see!  We cannot get out of being part of the picture. When we witness a rainbow it is as much about where we are standing with respect to the sun's position as to what makes it. Each person therefore sees a slightly different rainbow even if two people stood side by side. It is the same with clouds. The less you move, the more you will see the clouds move. Looking at clouds from a moving car or plane they appear still. The answer to the question about whether clouds move is also about what we are doing, what our brain tells us we are seeing and the way we wish to interpret it.


It's cloud illusions I recall.

I really don't know clouds at all.

-Joni Mitchell

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