Meteorology

[Sidious]

Hey everyone,
I have some weather questions Im trying to chase down the hole and could use some help....

1.) When air is heated the pressure should increase (hot air ballon pushing out) and density decreases.....

So why do we have the semi permeible low pressures on the equator where we get the most heat? (aka Doldrums)

From what I have gathered it is because as the air is heated on the surface it then heats and raises the pressure above it which in turn sets up a high to low pressure gradient with the cold air. (sea breeze, or global)

This then reduces the mass above and lowers the pressure

Is this correct?

I guess the reason I am confused is that I always thought the reason we had low pressures on the equator was that hot air was less dense and exerted lower pressure which I now think is incorrect...

 

[tgrayson]

When air is heated the pressure should increase

Depends on how much the volume is allowed to change. If the volume increases enough, there may be no pressure change.

When air is heated...density decreases.....

Again, depends on what volume does. At a constant volume, the density remains the same.

So why do we have the semi permanent low pressures on the equator where we get the most heat? (aka Doldrums)

1. The surface pressure depends on the quantity of air above the point where pressure is being measured.
2. Heating the air doesn't change this instantly, because the air column just expands and there is the same quantity of air above the ground.
3. However, the pressure levels expand with the column, so at every altitude, the hot air has greater pressure than the cooler air next to it, so it flows out of the heated column of air.
4. This reduces the quantity of air above the surface, lowering pressure.

 

[Sidious]

Thanks tgrayson

Okay so questions about lapse rates....

1.) Why do we have a "standard LR" of 2 degrees per 1000 ft as well as the DALR of 3 degrees per 1000 ft?

Does the "standard LR" factor in SALR AND DALR, Like an average between complelty dry and completly saturated?

2.) Why does dew point decrease by .5 degrees per 1000 ft? How do we explain it? Is this a function of SALR or something on its own?

 

[tgrayson]

Why do we have a "standard LR" of 2 degrees per 1000 ft as well as the DALR of 3 degrees per 1000 ft?

Those are two different things. The standard lapse rate is the rate that the air will cool if WE are the ones going up; the air just sits there.

The DALR is the rate at which a parcel of air will cool if IT rises in the atmosphere.

Why does dew point decrease by .5 degrees per 1000 ft? How do we explain it? Is this a function of SALR or something on its own?

I'll have to take a look in my meteorology books. Seems likely the SALR has a lot to do with it, but possibly there's more.

 

[Realms09]

Tgrayson, what are some good meteorology books that go beyond the FAA's Aviation Weather?

 

[tgrayson]

Tgrayson, what are some good meteorology books that go beyond the FAA's Aviation Weather?

Here's one that you would like:

http://www.amazon.com/Meteorology-S...bs_sr_3?ie=UTF8&s=books&qid=1205524951&sr=8-3

It's a mathematical companion to the standard introductory text, which is very good and more complete than the Aviation stuff:

http://www.amazon.com/Meteorology-T...orologyNOW/dp/0495011622/ref=pd_sim_b_title_2

 

[Sidious]

The standard lapse rate is the rate that the air will cool if WE are the ones going up; the air just sits there.

The DALR is the rate at which a parcel of air will cool if IT rises in the atmosphere.

Okay... but that air that WE are going up through had to cool somehow.... If it cooled by the DALR it should be 3 degrees cooler no?

Thats why I was thinking it was some average betwen the SALR and DALR. Thanks

 

[surreal1221]

I guess the reason I am confused is that I always thought the reason we had low pressures on the equator was that hot air was less dense and exerted lower pressure which I now think is incorrect...

No, this is correct.

SFC warming leads to air rising, this air as it raises is decreasing the amount of pressure exerted on the surface.

Try not to over think it.

Also, take a look at how coriolis works in the northern and southern hemisphere. That'll explain the "doldrums" a little better.

 

[tgrayson]

Okay... but that air that WE are going up through had to cool somehow.... If it cooled by the DALR it should be 3 degrees cooler no?

Air, left to its own devices, will cool towards the temperature of the universe, close to absolute zero. It would be quite solid ice. The fact that it remains a gas and at human tolerable temperatures means that it's being continually heated. The sun is the primary culprit, but solar rays pass through transparent air without raising the temperature much. However, the sun heats the opaque earth and the earth, in contact with the air, heats it through conduction and convection.

[Note: The following is speculation, based on a thin knowledge of meteorology, but there are corollaries in other branches of physics. Perhaps Realms09 can ofter some fluid dynamics insight.]

Since the air is heated from the bottom up, that alone would product a lapse rate that doesn't depend on the adiabatic lapse rate. However, the lapse rate that we actually have no doubt is affected by the adiabatic lapse rate, dry or wet, since it comes into play when air convects. I would expect that without convection, our standard lapse rate would be more severe, since there would be reduced mixing of the warmer air would occur with the colder.

 

[tgrayson]

this air as it raises is decreasing the amount of pressure exerted on the surface.

And why do you think that?

The surface pressure is determined by the weight of the air above it, basic physics and meteorology. Movement up or down is irrelevant.

 

[tgrayson]

Here are two pages from Donald Ahrens "Meteorology Today":

Page1.jpg
Page2.jpg

 

[surreal1221]

And why do you think that?

The surface pressure is determined by the weight of the air above it, basic physics and meteorology. Movement up or down is irrelevant.

Oh I don't know tgrayson, because I spent the past few years of my life as a meteorologist.

You tell me which exerts more pressure upon the earth below it. The warm air is rising because that is what warm(er) air does. Cold air sinks, warm air raises. Not difficult processes to understand. Surface is warmed through long wave radiational energy from the sun, this energy eventually raises due to convection. . .it's a fundamental tenant for "weather". This convection develops lower pressure (developing instability, etc).

It is a basic meteorology concept that pressure decreases as altitude increases, but the pressure at the surface is determined on a number of factors. . .the key being temperature differential. Without a large-scale temperature differential we would not have the weather systems we are so thankful for.

It should also be noted that the vertical dynamics of a semi-permanent pressure system (tropical, poler, high or low) differ from a migratory pressure system. But I really won't get into that as I surely don't want to be challenged again.

I'll also just keep my mouth shut. You seem to be the authoritative figure on the matter so I'll reserve any further comment.

To the OP: Might I recommend Tim Vasquez's Weather Forecasting Handbook. While it is more geared to the operational forecaster, it has been of extremely valuable use to all my fellow pilots I have given to.

http://www.weathergraphics.com/fcstbook/

 

[tgrayson]

I spent the past few years of my life as a meteorologist.

Which highlights my observation that those involved in most trades don't have a strong theoretical understanding of their fields.

Ahren's book, as all books on meteorology, I suspect, says this:

"The pressure at any level in the atmosphere may be measured in terms of the total mass of air above any point."
​

Are you saying that these books are wrong on this? It says nothing about air being hot, or cold, or moving up or down.

What you don't seem to understand is that the temperature differentials only serve to increase or decrease the mass in a column of air. If you were the heat or cool the air, while somehow preventing mass transfer, you would see no pressure change at the surface.

 

[Sidious]

Oh I don't know tgrayson, because I spent the past few years of my life as a meteorologist.

Thanks for the input, since you are a meteorologist what is your take on the lapse rate discussion above?

 

[surreal1221]

Which highlights my observation that those involved in most trades don't have a strong theoretical understanding of their fields.

Not me friend. I'm a fairly decorated weather forecaster. But I realize credentials don't really matter.

What you don't seem to understand is that the temperature differentials only serve to increase or decrease the mass in a column of air. If you were the heat or cool the air, while somehow preventing mass transfer, you would see no pressure change at the surface.

You tell me how mass is going to be moved without temperature influence.

Moreover, just read what you wrote. You mention temperature and mass and how they relate, then you say something about preventing mass transfer after changing the temperature structure .

How would YOU prevent mass transfer after introducing temperature change?

What exactly is your line of work, so that I can ignore any input from you in that subject area.

 

[SteveC]

SFC warming leads to air rising, this air as it raises is decreasing the amount of pressure exerted on the surface.

Surreal, I'm not a meteorologist, nor a physicist, but I believe that this is the sentence that tgrayson is debating, no?

It seems to me that this is an incomplete idea. My laymen's understanding is that the fact that the air is being heated and is rising does not, by itself, decrease the air pressure at the surface. The heated air and rising causes other things to happen that reduces the pressure. Am I misunderstanding this or not?

 

[surreal1221]

A website with pretty pictures to hopefully explain the original poster's question in regards to the equator, and the permanent pressure centers that call the equator their home.

http://www.atmosphere.mpg.de/enid/5057ce0c8397e666f5a77ee2a0017790,55a304092d09/18z.html

The following is in regards to SEMI-PERMANENT Pressure centers, NOT migratory

Now, this graphic is representative of the three-cell theory.

Notice at the equator, Low pressure, due to the equator being the location where the most long wave radiational energy from the sun (heat) is received. This then, if you look on the outer sides of the graphic, provide a lifting motion (green arrows). Which is eventually capped, and in the northern hemisphere the airflow then flows Northward toward the poles. Also take note that the #1 cell (The Hadley Cell) and the cell directly beneath it in the southern hemisphere are indicating air flow towards each other - convergence.

Also note, that due to Coriolis force, the flow toward the equator at the surface is deflected to the right in the northern hemisphere, and deflected to the left in the southern hemisphere, hence creating the doldrums. Also take note of the surface convergence. Convergence at the surface leads to a lifting action, movement upward.

Warm air + Convergent airflow at the surface = Low pressure.

So yes, there is another element that can cause a reduction or increase in surface pressure, that being the amount of convergence or divergence of air at the surface. Convergent air at the surface causes the mass of air to move upward, and out of the column, where divergent air at the surface indicates convergence aloft which is pushing the mass of air downward through the column and eventually outward and away from the center of the column at the surface.

chimney effect (in atmospheric chemistry):
A vertical movement of a localized mass of air or other gases which occurs
due to local temperature differences.

Temperature is the driving factor. Other circumstances, the strength of surface convergence or divergence will drive the amplitude of any pressure change - low or high.

 

[SteveC]

surreal, I presume you were composing the post above at the same time that I posted a question for you above that...

 

[surreal1221]

No, it was partially as a reply to you. . .and a reply to both the OP in regards to his question regarding warm air, low pressure, and semi-permanent pressure centers (band of Low pressure around the equator), which he said he thought should be high pressure.

The warming at the surface due to radiational heating in turn produces convection - warm air raising. This warm air, as it raises, is a mass that is climbing and moving upward through the column. This movement of mass upward decreases the amount of surface pressure being exerted on the earth.

 

[mhcasey]

Warm air + Convergent airflow at the surface = Low pressure.

Convergent air at the surface causes the mass of air to move upward, and out of the column, where divergent air at the surface indicates convergence aloft which is pushing the mass of air downward through the column and eventually outward and away from the center of the column at the surface.

chimney effect (in atmospheric chemistry):
A vertical movement of a localized mass of air or other gases which occurs
due to local temperature differences.

Temperature is the driving factor. Other circumstances, the strength of surface convergence or divergence will drive the amplitude of any pressure change - low or high.

It seems to me that you all are debating whether the entire air mass shifts upward to reduce the surface pressure (Surreal) or the heat differential results in some other process(es) that actually reduce the surface pressure (Tgrayson).

My understanding is that most heating at the surface is caused by conduction, which results in very little heating of the air itself. Most of the heat transfer in the air occurs through convection when little pockets of the heated surface air move around (thermals - not the entire air column). This seems to lend some credibility to Tgrayson, but I'll let you guys duke it out.