Isobars

[martint235]

Ok I'm a bit thick on this but it has puzzled me for years. I know that isobars on a weather map connect points of equal pressure, that much I understand. What I don't understand is why the wind follows the line of the isobar rather than (as I would logically expect) running at right angles across the isobar from high pressure to low pressure.

An analogy would be contour lines but if you had a perfectly smooth hill, water would tend to run at right angles across them from high to low.

Can anyone explain?

 

[nappadang]

Really difficult to explain on my phone but think interconnecting cogs. High pressure (big cogs) revolve clockwise, thus, the wind direction around rising warm air is clockwise.
This high pressure is drawing air in, causing smaller cogs (low pressure) to form around it moving in an anti clockwise direction.
I've not explained this vert well but I'm on my phone and off to work. I'm sure cleverer people that me will answer your query much better than I have but if not I'll have another go tonight.

 

[asterix]

From vague memories of Yachtmaster theory I believe it is linked to the rotation of the earth, Buys Ballots Law and all that. In the Southern Hemisphere the wind goes in the opposite direction around pressure zones.

Edit: look at Coriolis effect.

http://en.wikipedia.org/wiki/Coriolis_effect

 

[Pro Tour Punditry]

Ok I'm a bit thick on this but it has puzzled me for years. I know that isobars on a weather map connect points of equal pressure, that much I understand.

More than I did, I have no need to know these things. No matter how much people on TV think I do. Is it going to rain? Is it going to be windy? Guess.

 

[Globalti]

Isobars are a consequence of air movement not a cause of it.

The best model I ever saw for explaining the weather was in a museum somewhere, where somebody had constructed a solid and perfectly spherical ball, inside a clear perfectly spherical container with a layer of liquid in between, mixed with a glitter like you get in some shampoos. The ball rotated smoothly in its bath of liquid and as it did so the liquid, representing the atmosphere, formed vortices and swirled in a mirror image north and south of the equator. This showed me clearly how weather systems form and why they rotate in opposing directins in the two hemispheres. In a low pressure system air is rotating as a consequence of the earth's movement within the atmosphere and as a mass of air spins it is flung outwards, creating a reducing pressure gradient as you move towards the centre of the swirl (shown by your isobars). As the air tumbles outwards it meets other air of differing temperature, density and humidity and this interface between air masses is what creates fronts, which bring the gentle warm rain that blesses our little island in the Atlantic ocean.

This is the best illustration I can find; it shows that the weather is of course a good deal more complex than my explanation!

http://www.ux1.eiu.edu/~cfjps/1400/circulation.html

 

[Beebo]

@nappadang explaination of moving cogs is a good one.
Weather systems move in circles, imagine a large hurricane, but smaller.
If the isobars are very tightly packed then the differential between two masses of air is greater and the wind speeds will be higher. But as with a cog, the outside will move quicker than the inside, so the wind flows along the line of the bars.

 

[ASC1951]

..... What I don't understand is why the wind follows the line of the isobar rather than (as I would logically expect) running at right angles across the isobar from high pressure to low pressure....

Wind is gradually spilled 'downhill' from highs into lows in exactly the same way as water, to use your analogy, which is why individual highs and lows eventually dissipate. But it's such a complex interlocking system, and the variation in speeds can be so large, that the spilling can be quite slow - the Great Red Spot, for instance, has been there for as long as we have been able to observe it.

 

[rich p]

@Marmion thinks that Isobars are a southern poncey variation of a deep-fried Mars bar

 

[martint235]

Ta all (well with the possible exception of Rich ) it does actually make more sense now!