Troughs / Ridges?

[fukhar]

Hey guys,

I have been reading Aviation Weather by the FAA and I am a little confused about troughs and ridges.

They are defined as elongated areas of high / low pressure. but the interesting thing is that the FAA says that they are "an elongated area of low/high pressure with the lowest/highest pressure along the line marking maximum cyclonic/anticyclonic curvature.

why is this true? on the maps, the lines are indeed running between the areas of the highest curvature of the isobars, but why is this the case? for example, if a trough is an elongated area of low pressure, why doesn't a trough surround a low pressure area in a circle?

 

[kgruber]

Because of wind and surface friction.

 

[fukhar]

could you elaborate please?

 

[Dan Thomas]

Coriolis Effect causes the air mass to rotate counterclockwise around a low and clockwise around a high. The opposite is true in the Southern Hemisphere. Since various air masses from different sources are all pushing and shoving, we seldom see a very circular motion. Air masses get squeezed some.

In Canada we sometimes use the Air Command Weather manual. It was written for the Canadian Air Force quite some time ago but lays all this stuff out well. It does warn the reader that it's all still theory.

Dan

 

[Michele]

Wow, good stuff!

 

[gismo]

One more point. Typically these troughs are identified by a frontal system (warm, cold, stationary, occluded) when they separate air masses. You can see a good example of a trough in northern Nevada. See how the trough line (dashed) goes through the maximum area of curvature of the isobar in north-central Nevada.

I think that these effects would be a lot more visible/obvious with a 3D rendering of a mid level constant pressure surface. In such an image, ridges and troughs look like, well, ridges and troughs.

 

[fukhar]

thank you for all your responses, but i am still a little confused!
i understand that a trough, for example, is an elongated area of LOW PRESSURE.
the trough goes across the isobars at a right angle. each isobar represents a line of equal pressure. therefore, the pressure along the trough goes up as you go along it, so why is it defined as an area of low pressure?

 

[fukhar]

thank you scott. i now understand what the meanings of these systems are.
however, i am still confused as to why troughs and ridges are at the point of maximum curvature of the isobars. in your speed humps example, why does the point of maxumim curvature indicates lowest / highest relative pressure?

 

[airguy]

thank you scott. i now understand what the meanings of these systems are.
however, i am still confused as to why troughs and ridges are at the point of maximum curvature of the isobars. in your speed humps example, why does the point of maxumim curvature indicates lowest / highest relative pressure?

Actually, it's a chicken-and-egg kind of thing. The troughs and ridges are not IDENTIFIED by the location of maximum curvature, they CAUSE the location of maximum curvature. The high/low pressure causes air to flow from high to low, and the Coriolis force of the earths rotations causes that air to twist (as described earlier in this thread) away from a straight line, which is what creates the curvature. The curvature is strongest where the isobars are closest, that's what produces the most differential pressure, wind flow, and coriolis effect. Therefore you see the greatest curvature at the point of the ridge or trough.