Cloud Seeding

This is news? …

Originally posted by The Hindu:

Through a ‘seeding' process, the aircraft leave behind odd-shaped holes or channels in the clouds that have long fascinated the public.

As turboprop and jet aircraft climb or descend under certain atmospheric conditions, they can inadvertently ‘seed' mid-level clouds, causing narrow bands of snow or rain to develop and fall to the ground, new research finds.

The key ingredient for developing these holes in the clouds: water droplets at subfreezing temperatures, below about -15 degrees Celsius.

As air is cooled behind aircraft propellers or over jet wings, the water droplets freeze and drop toward Earth.

The cause

“Anytime aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby,” says Andrew Heymsfield.

Dr. Heymsfield is a scientist with the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, and lead author of a paper reporting results of research into the phenomenon.

Seeding effect

A similar process can occur with jet aircraft, which need colder temperatures (below about -20 to -25 degrees C) to generate the seeding effect.

Air is forced to expand over the wings as the aircraft moves forward, cools, and freezes the cloud droplets, according to a National Science Foundation (NSF) press release.

© Copyright 2000 – 2009 The Hindu

What happens is that as air rises, dirt sediments out and the upper atmosphere tends to be cleaner than stuff near ther surface. Dirt gets into the air from smoke, dust-storms and agricultural and forest pollen. It's all quite natural.

When air rises it cools at a regular pace called the Adiabatic Lapse Rate.
It is held in a system called a Low. The air is a fluid in a reservoir -as near as damn it.
And the heat exchange processis virtually contained in the system.

When the clean air raches the tropopause it may leave the train and become part of a larger reservoir full of very clean air. Then the spell changes and the acoustic wave is holding a huge supply of wet air that is supercool.

When nature takes its course, this layer will condense its water as ice. If it forms slowly enough, it will form snow. But a rapid transfer will allow seeds to form and then the stuff will drop.

As it drops it can pick up more water, becoming large hail or rain and it can sublimate. It can return to the air or even rise back in a rotaing stream. All sorts of things are possible.

And all this research has uncovered is that aircraft can influence things.

If you observe enough condensation trails (con-trails) you will notice this condensate forming occasionally. But you will only see it on clear days. And clear days are the days when the situation is most likely to be just right for it to happen. On cloudy days, the phenomenon has already occurred.

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2 thoughts on “Cloud Seeding

  1. Originally posted by Weatherlawyer:

    Another way they influence the atmosphere comes from all the paraffin they burn. Although very highly refined compared to the stuff used on ships, the paraffin does not burn completely at all times.

    Let me go back to the article:

    The key ingredient for developing these holes in the clouds: water droplets at subfreezing temperatures, below about -15 degrees Celsius.“Anytime aircraft fly through these specific conditions, they are altering the clouds in a way that can result in enhanced precipitation nearby,” says Andrew Heymsfield.

    When air is full of water it forms clouds. Or not as the case may be. It has to have a seeding agent if the air is so clean the water can remain as water to temperatures as low as minus 40 degrees (at surface pressures.) (At higher elevations the pressure is lower and presumably, the water less likely to freeze -it may even bypass the liquid phase change in a process called sublimation)So on a clear fresah day the air above may be full of water and unable to shed it. Down below we may see huge columns of water flying upwards in cauliflowering clouds of white steam.If they form thunder clouds the air is dirty enough to shed water. If they dsappear the air is too clean.If an aircraft passed from dry air into cloud, the ratio of air to fuel changes slightly, doe it not?I don't know if they are engineered to cater for this change. It wouldn't surprise me. But if not, then the air fule mix is now richer than normal. And this is also true if the air is still clear of cloud but as full of moisture.A slight increase in the paraffin will be left in the trail. A lot of it must remain airborne for a while. Nobody can say how long.Nobody can say how long volcanic ash remains aloft. The problem is called aerosols. They have become a pet of global warmers.As soon as the news broke that there are such things, it was propounded as a bad thiong for us.We don't know that. How can we know something like that about a phenomenon that has only been recently observed?A phenomenon that has only recently become its own branch of science?But I have to agree that is must be doing something. Consensus has it that it produces haze.

  2. Things to notice about con trails:They form as two streams, no matter how many engines the aircraft has. You can see (occasionally) that a four engined craft has 4 white streams leaving it. These rapidly form into two streams because of something called vortex shedding.Large craft weigh many tons, all of which is kept up by air pressure. A 400 ton airliner is pushing down 400 tons and more of air at any given moment it is in the sky.Another way they influence the atmosphere comes from all the paraffin they burn. Although very highly refined compared to the stuff used on ships, the paraffin does not burn completely at all times.Aircraft exhaust must occasionally produce carbon along with its carbon dioxide and water. And it leave some unburnt or chemically altered fuel behind the plane.At 5 gallons or more per mile this may seem like a lot but at 4 or 500 miles an hour it is a thin stream, for, to go forwards at 500 mph, the plane must push behind it an equal and opposite stream at 500 mph.Or something equivalent at much higher speeds through smaller channels. (Presumably, all the air in contact with the surface of the plane has to have an equal and equivalent volume passing through the engines?)

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