isobars and wind
Guest
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isobars and wind
Hello,
I know that when the isobars are 'close together', the winds will be strong.
The question is, what does 'close' mean ? ... i.e. XXX pressure change over what YYYY distance ?
... and if you have XXX pressure change over YYY distance, can you calculate the wind speed ... or approximate it ?
What other factors do you need to consider when calculating wind ?
Mike
I know that when the isobars are 'close together', the winds will be strong.
The question is, what does 'close' mean ? ... i.e. XXX pressure change over what YYYY distance ?
... and if you have XXX pressure change over YYY distance, can you calculate the wind speed ... or approximate it ?
What other factors do you need to consider when calculating wind ?
Mike
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Isobar spacing is really just a graphical way of showing rate of change of pressure over a given distance. More accurately, rate of change of distance for a given change in pressure.
The relationship between this rate and windspeed is dependent on the latitude at which it occurs.
Scales are available that are calibrated for the isobar spacing eg 2 HPa or 4 HPa etc, and the latitude.
Laying the scale across the isobars allows one to read the wind speed.
This windspeed presumes that no other effects are present eg surface friction, venturi effect through valleys, jets etc.
Some met. charts have the scale printed on them & you can use dividers or a ruler to measure the isobar spacing & then measure it against the wind speed scale.
[This message has been edited by Tinstaafl (edited 14 April 2001).]
The relationship between this rate and windspeed is dependent on the latitude at which it occurs.
Scales are available that are calibrated for the isobar spacing eg 2 HPa or 4 HPa etc, and the latitude.
Laying the scale across the isobars allows one to read the wind speed.
This windspeed presumes that no other effects are present eg surface friction, venturi effect through valleys, jets etc.
Some met. charts have the scale printed on them & you can use dividers or a ruler to measure the isobar spacing & then measure it against the wind speed scale.
[This message has been edited by Tinstaafl (edited 14 April 2001).]
Guest
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I agree, but looking at a synoptic chart will, as you say give you an indication of how the surface winds may be. If you want to know more detailed information, then only a METAR or TAF etc will give you the precise wind velocity for a particular airfield. Lines on a chart are subject to all sorts of things, including the quality of the toner cartridge in the photocopier, so its ok for a rough guide for a wide area, but certainly not to be used for flight planning.
Guest
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Depends aswell on which latitude you're on. Don't forget the coriolis effect that turns everything to the right on the northern hemisphere and opposite on the southern. this makes the geostrophic wind blow parallel to the isobars with the high pressure to the right. And the greater the pressure diff on a given distance, the stronger the wind. I don't know how to calculate this though. I could be wrong on this though, so you better check it up in some book.
Guest
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As Tinstaafl said, on sig Wx charts and the like there is usually a 'geostrophic wind scale' for equating the isobar spacing and latitude with the windspeed. Doesn't account for Jets though, but they should be on the chart anyway.
If yoyu've got access to Met charts anyway, it's far easier just to get the upper wind charts as well as a synopsis!
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...proceeding below Decision Height with CAUTION...
If yoyu've got access to Met charts anyway, it's far easier just to get the upper wind charts as well as a synopsis!
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...proceeding below Decision Height with CAUTION...
Guest
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I should add that the coriolis effect is really only valid at some distance away from the equator. It's a function of the change in distance from the earth's axis of rotation as the wind moves across the earth's surface.
N or S hemisphere isn't really relevent to wind speed (apart from determining wind direction around the pressure system!)
In the tropics it all falls apart due lack of coriolis effect so 'streamlines' (wind direction) & 'isotachs' (wind speed) are used on tropical met. charts to denote wind velocity.
[This message has been edited by Tinstaafl (edited 16 April 2001).]
N or S hemisphere isn't really relevent to wind speed (apart from determining wind direction around the pressure system!)
In the tropics it all falls apart due lack of coriolis effect so 'streamlines' (wind direction) & 'isotachs' (wind speed) are used on tropical met. charts to denote wind velocity.
[This message has been edited by Tinstaafl (edited 16 April 2001).]
