[sara]

sara asked the Naked Scientists:
   
Why does high pressure air move down?

What do you think?

[Andrew K Fletcher]

sara asked the Naked Scientists:
   
Why does high pressure air move down?

What do you think?

Hi Sara

Air pressure and air density make for an interesting discussion. Where the air becomes denser due to moisture content and temperature changes, gravity must act on density and dictates that denser air moves towards the ground and to do this less dense air must move back up causing the air to circulate.

On good example of this has been published regarding temperatures of ocean water in Florida. It was determined that the water was warmer at the ocean bed than it was at the mid range water.

Another analogy is urine in a toilet pan. We have all observed warm urine sinking rapidly to the bottom of the basin, again the salt content in excreted urine overcomes the thermally reduced density of the urine. if a thermometer was placed in the toilet, we would see that the water and urine at the bottom of the pan is warmer than the surface water.

The higher we go in the atmosphere, the less dense the air becomes and much colder than the air containing moisture at and below cloud level.

Hope this helps

Andrew

[Bored chemist]

I wonder how, in general, high pressure air can move down. Surely the ground is in the way?

[Andrew K Fletcher]

I wonder how, in general, high pressure air can move down. Surely the ground is in the way?

The ocean is below the atmosphere because it is denser and the ocean bed is below that because it is denser than the ocean. You are right to point out that air cannot continue to move either down or up without lateral movement. The air being unable to move down due to the ground so must move outwards and air must replace the downward flowing air creating air circulation. An example of this is a cold floor surface, particularly in our conservatory during the winter months. We have double glazing which pretty much excludes external drafts. Yet we can feel the cold air current moving into the lounge as the warmer air moves out. This is amplified when the log burner is warming the home and the chimney provides the upward flowing air current venting cold air in from outdoors.

The desert surface, heated by the sun provides the same air circulation as the log burner. At the coast the heated arid land generates less dense air which rises high into the atmosphere, this draws more heated air from the desert land to be drawn towards the coast and it also draws moisture laden air from the ocean towards the coast. As this air rises the moisture laden air cools more quickly and becomes denser, falling back to the ocean, causing the air to roll like waves breaking on a beach. Here in Torbay we have observed mist rolling around the coast like washing in a tumble dryer, channelled along the coast when the weather is very warm and appears to be unable to cross onto the land, forming a wall of mist on one side of the coastal road and a blazing hot dry atmosphere on the other side of the solar heated coastal road.

Andrew

[frethack]

Desert air is generally dry, high pressure air, but does have localized thermal updrafts.  Air laden with water vapor is less dense than dry air, more buoyant, and creates low pressure, rising air columns.  Rising air masses from surface low pressure systems create a high pressure bulge in the upper troposphere.  As air moves from surface high pressure to surface low pressure to restore mass balance, upper level high pressure (located over the surface low) moves to upper level low pressure (located over the surface high).  This creates convection, and is the reason that high pressure systems have descending air columns.

This, of course, is a simplified explanation.

[Candydoll]

high pressure air is heavier and coll in temperature so it moves down

[Andrew K Fletcher]

http://books.google.co.uk/books?id=Lb5I0tcnYNkC&pg=PA44&lpg=PA44&dq=thermals+coast+desert&source=bl&ots=Z5QhElgg3r&sig=Xc18nMJw8G5buczD-gknoe2S5qw&hl=en&ei=ja5sTb28BYW2hQeM6rWWBA&sa=X&oi=book_result&ct=result&resnum=8&sqi=2&ved=0CFcQ6AEwBw#v=onepage&q=thermals%20coast%20desert&f=false

The link above is to an interesting page in a book published on Google Books, which explains the relationship between the desert surface and thermals. The book is titled: A natural history of the Sonoran Desert
 By Steven J. Phillips, Patricia Wentworth Comus