Advection Defined

[kontiki]

All,

I finally got off my butt and looked up advection in a decent Technical Encyclopedia, McGraw Hill, 1984. I just wanted to share.

Advection - Transport of an atmospheric property soley by the mass motion of the atmomsphere

Convection - Atmospheric motion, predominatly vertical, resulting in the vertical transport and mixing of atmospheric properties.

In addition to flying, I love all this sciency stuff. A decent definition makes it all come together.

 

[MAKG1]

Yup, advection is like throwing wood chips into a stream and watching them flow. Convection always requires a temperature gradient of one sort or another. You can also drive winds by pressure gradients. There are other processes in weather -- clouds can advect, or they can form from condensation. Often both.

Lots of good stuff about weather is available, and it's a very interesting subject.

 

[Sac Arrow]

Use "Advection" in a sentence:

"Ma'am, I would like to do my report on how to install cameras in the girls' locker room."

"Little Johnny, I have extreme advection to that topic, please choose another."

 

[AggieMike88]

In addition to flying, I love all this sciency stuff. A decent definition makes it all come together.

You need to check out ScottD's weather training site, www.AvWxWorkshops.com, and one day attend his live events.

It will definitely awaken your inner aviation weather geek.

 

[eman1200]

Use "Advection" in a sentence:

"Ma'am, I would like to do my report on how to install cameras in the girls' locker room."

"Little Johnny, I have extreme advection to that topic, please choose another."

↑ eh, a little weird....

 

[MAKG1]

↑ eh, a little weird....

Not for Sac Arrow, it isn't.....

 

[MAKG1]

In fact, all motion of the atmosphere (horizontal or vertical) is largely enabled by the pressure gradient force. Fortunately for us pilots, the atmosphere is typically in hydrostatic equilibrium.

Not necessarily on the small scales. When the wind blows over the tops of mountain peaks and tries to roll you over, that's the advection term. Wind shear turbulence is the advection term. The shear itself may be caused by thermal and/or pressure gradients, but the rolls you get in strong shear are due to the advection term (v dot grad v). Geometry doesn't work if you try to do that with a pressure gradient; velocity is strictly parallel to the pressure gradient in the absence of advection (well, more correctly, it's the rate of change of momentum density, but density is essentially constant).

There are multiple quantities that advect. If you talk about the air density advecting, that's due to flow (and it's real close to constant in strongly subsonic flow like weather systems). Water vapor can advect as well, and this is most obvious in the motion of cloud formations. The real interesting advection is of the flow field itself. That's what gives rise to turbulence and all kinds of interesting nonlinear (i.e., nonsuperposing) phenomena.

 

[comanchepilot]

Two types basically

Cold Air Advection (CAA) and Warm Air Advection (WAA). . .

WAA is the easiest to get cause you warmer and lighter air being transported up and over cooler and drier air - which forms stratus and nimbostratus most times - usually some distance from the surface trace of the front - the mass of the air moving causes the air to ride up and over the dissimilar airmass. . .


CAA happens when a cold front of denser and colder and/or drier air moves into an area of warmer and moister air - this is generally accompanied by rapidly rising air pressure which ushers in winds - which are dependent on the differential in pressure between the cold and warmer air.

Pretty simply concepts to understand . . and you will see WAA and CAA abbreviated in the Forecast discussions . . .

 

[MAKG1]

Those are specific instances; advection is generic. And you're describing the results of the advection, not the advection itself. The OP basically got the more generic case right.

You can advect smoke particles long distances from a fire, for instance. Oil slicks advect along the surface of the ocean.

Weather forecasters occasionally talk about things like "advection fog," which is what happens when saturated air advects over something cooler than the air. The interesting weather phenomenon is due to the temperature change.

 

[kontiki]

Use "Advection" in a sentence:

"Ma'am, I would like to do my report on how to install cameras in the girls' locker room."

"Little Johnny, I have extreme advection to that topic, please choose another."

Funny you said this, I noticed when I was studying for the GRE (awhile back) that after you looked a word up in a Webster's, you generally couldn't use it in a sentence.

So I sprung for an early digital copy of Oxfords unabridged dictionary (about $200), a paper set is about 21 volumes and ran about $2k.

In the Oxford dictionary every definition of a word is demonstrated in the a sentence and mapped to a date on a word entomology time line. After you look a word up, you can usually use it in a sentence.

I really like it. You can probably tell I'm single with disclosures like this.

 

[BillTIZ]

Advection, slight breeze moves airmass up a slope, rising airmass cools, cooler airmass cannot contain water vapor, condenses, upslope fog.

Would that be correct?

 

[MAKG1]

Advection, slight breeze moves airmass up a slope, rising airmass cools, cooler airmass cannot contain water vapor, condenses, upslope fog.

Would that be correct?

The slight breeze is advection (but it doesn't have to be slight). The rest is fog formation.

Advection doesn't require a slope, rising air, or water vapor. It's just transport.

 

[kontiki]

Advection, slight breeze moves airmass up a slope, rising airmass cools, cooler airmass cannot contain water vapor, condenses, upslope fog.

Would that be correct?

I originally looked advection up to get a handle on advection fog.

Thanks for point out how it falls into the context of upslope fog.

Going back to premise that fog has two basic causes:

1- narrowing temp & dew point spread
2- increasing moisture in air

I guess each principle is responsible for fog in a couple different situations.