Weather Guide Help

[RDW]

I need help with 500mb charts. I assume the "L" on a 500 mb chart is a low altitude. If that is true I would think that the air is colder and denser. If that is true I would think it would be associated with surface high pressure but in reality it is not it seems.
In a similiar confusing line of thinking, a book I read says "warm air aloft is associated with a surface high pressure" Again I think of warm air as less dense and therefore associated with a low pressure not high.
WHERE HAVE I GOTTEN MYSELF CONFUSED?
RDW

[franksingleton]

Originally Posted by RDW

I need help with 500mb charts. I assume the "L" on a 500 mb chart is a low altitude. If that is true I would think that the air is colder and denser. If that is true I would think it would be associated with surface high pressure but in reality it is not it seems.
In a similiar confusing line of thinking, a book I read says "warm air aloft is associated with a surface high pressure" Again I think of warm air as less dense and therefore associated with a low pressure not high.
WHERE HAVE I GOTTEN MYSELF CONFUSED?
RDW

First, remember that the pressure is a measure of the total weight of the atmosphere above the barometer. Therefore a high pressure area has more air above it than a low pressure area. Think of a bowl of syrup with a hole scooped out of the middle. Imagine lines of equal syrup height.

At a height where the pressure is 500 hPa, a low value at a point X might be, say, 5200 metres and a high value at point Y about 5700 m. Now think of what happens at between X and Y, at a height of 5200 metres. The pressure at X will be 500 hPa, ie the weight of the air above that point. Now go to point Y, at the same height of 5200 metres. There is more air above that height than at point X; you would have to go up another 500 metres to get to a pressure of 500 hPa. Therefore air will try to move from point Y at 5100 metres towards point . Coriolis makes it go round cyclonically.

At X, The air below the 500 hPa level is more dens than at point Y. That is, the air from the surface to a height of 500 hPa occupies a smaller volume than the air below 500 hPa at Y. Some charts are called thickness charts (or relative topography.) Literally, this is the depth of air between the two levels.

See How Pressure Differences are formed / Franks-Weather | The Weather Window and associated pages.

As a frontal wave starts to form, the air aloft will be warm – having come from the south of the polar front. Buts, when the low is mature, the supply of warm air from the south will be cut off- occluded. By that time, the vortex will be right up from the surface up to a height of 300 hPa – ie about 10 km. The occlusion will still be an area of warm air throughout its depth but with a trough of low pressure at the surface. A TROWAL

A subtropical (warm) high – say the Azores high - will have warm air over it and a ridge at high level. Look at 500 hPa charts. A cold high, such as that over Siberia will have a lot of dense air low down but lower heights of the 500 hPa level. A typical Azores high will have a central pressure of around 1030 hPa. A cold Siberian high may have a pressure of 1050. That latter has more air over it than the former. The air over the Azores high is less dense than over the Siberian high.

The atmosphere is 3-dimensional and has to be thought of in that way. You are taking a two dimensional view.

I hope that helps.

[RDW]

Thanks for the kind and good reply.
On a 500 mb chart is "H" in any way related to the high or low of a surface chart?
Sorry if I am a pest. Just stop answering when you are tired of me.
RDW

[franksingleton]

Originally Posted by RDW

Thanks for the kind and good reply.
On a 500 mb chart is "H" in any way related to the high or low of a surface chart?
Sorry if I am a pest. Just stop answering when you are tired of me.
RDW

A high or a ridge of high pressure at the 500 hPa level will often be associated with a surface high. But, a surface high may not have a ridge above it. This will be more marked if you look at higher levels, eg at 300 hPa.

It is a question of whether it is a warm high, usually in low latitudes, such as the Azores high or a cold high in northern latitudes, such as the winter Siberian high. The warm high will always have a 500 hPa level ridge or high centre over it. The air in the lower half of the atmosphere will be warm and not dense.
In a cold high, the cold dense air below 500 hPa creates the surface high pressure. But, higher up at, say, 8 or 9 km (around 300 hPa) the pressure will be lower than to the south.

As I said, you have to think about the atmosphere in three dimensions. The relationship between surface pressure patterns and those higher up depends upon the temperature of the air and the vertical temperature structure.

Does that make sense?

[RDW]

I am beginning to get a little 3 d image in my head. Thanks for your help. I will study more.
RDW

[franksingleton]

A good example of the 3-dimensional working of the atmosphere is when you look at the inter-actions between a developing frontal low and the upper level flow.

In this highly idealized diagram, I show 4 stages of the development of a low and, the yellow arrows, the jet stream.

[franksingleton]

Further to my post above, if you look at Numerical Model Charts - Environment Canada, you can see charts of surface isobars and upper level contour lines over much of the N Hemisphere. They will give a good idea of the relationships between different levels of the atmosphere.

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