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Difference between Gradient and Actual wind

  • 172_Freighter
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172_Freighter created the topic: Difference between Gradient and Actual wind

Completed AGK with 95% and now onto the study for Met...

I understand that gradient flow is the flow that occurs when the wind flows exactly along (with) the isobars, However in the book description it states "when the Press grad is stronger than coriolis the isobars (and gradient flow) curve to the right..."

It makes sense that with a decreased coriolis (left tendency) the flow would swing to the right. However is this the ACTUAL flow or the GRADIENT flow that is swinging? I write with reference to question 9 in Excercise GM2.


Also, another slight matter. At the end of chapter 2 it states the definitions for Gusts and Squalls. Ive searched gen 3.5 for these definitions, just so I dont need to commit the definitions to memory but couldnt find it, do you have a reference?

Much Thanks,
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  • benharris10
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benharris10 replied the topic: Re: Difference between Gradient and Actual wind

G'day 172_Freighter

I understand that gradient flow is the flow that occurs when the wind flows exactly along (with) the isobars, However in the book description it states "when the Press grad is stronger than coriolis the isobars (and gradient flow) curve to the right..."


Firstly it pays to remember that if the earth didn't rotate, the wind would blow straight from an area of high pressure, into an area of low pressure, however because of the earth's rotation, the coriolis force acts on this airflow as it attempts to move from high to low pressure.

In a high pressure system, we have sinking air, which as it hits the surface has nowhere to go but away from the centre of the system. The coriolis force then acts on this air to produce a deflection to the left as seen in the diagram below (in the southern hemisphere).


The opposite occurs in a low pressure system. Air is rising therefore air is being sucked into the system at the surface. However once again due to coriolis force it takes a deflection to the left as it attempts to move over the surface of the earth. This is why we get clockwise or cyclonic flow (cyclonic simply meaning "as the earth rotates") around a low pressure system, and anti-clockwise or anti-cyclonic flow around a high pressure system as you can see from this diagram(once again in the southern hemisphere).




The other thing to be aware of is that the coriolis force is the strongest when the wind speed is highest, and zero when there is no wind.

Gradient wind is accepted by the met man to be above 3000 feet. From this height up the wind flow follows parallel to the isobars as you said. Below 3000 feet the wind speed is slowed down due to friction created by the surface of the earth, and we call this the surface wind. Now When we slow the wind speed down we reduce the coriolis force, and this no longer counters the pressure gradient force, so now the surface wind now deflects across the isobars in favour of the pressure gradient force. The amount by which this deflects depends upon the nature of the surface. So to answer your question, it is the surface wind (below 3000 feet) which is deflected to the right of the gradient wind (above 3000 feet) no matter whether in a high or low pressure system.





This degree of deflection is accepted to 30 degree's over land ( due to more friction over land therefore reducing the coriolis force) and 10 degrees over the sea due to less friction over ocean. The point to remember is that the surface wind is always deflected to the right of the gradient wind, or if you remember your backing and veering, the surface wind has veered with respect to the gradient wind. Bob might jump on later and tell a story about how hot air ballooning pilots can use this theory to somewhere control where their balloon will end up by adjusting their altitude to pick up the wind that they would prefer.

It makes sense that with a decreased coriolis (left tendency) the flow would swing to the right. However is this the ACTUAL flow or the GRADIENT flow that is swinging? I write with reference to question 9 in Excercise GM2.



That would depend on if your ACTUALLY in the gradient wind or the surface wind :)

The surface wind is veering when compared to the gradient wind, or if you prefer, the gradient wind has backed compared to the surface wind. Does your head hurt yet? Mine does :)


The definitions for Gusts and Squalls unfortunately are not in the AIP, but as long as you have Bob's book you will have the definitions.

Good Luck.
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  • 172_Freighter
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172_Freighter replied the topic: Re: Difference between Gradient and Actual wind

Brilliant reply,

Many thanks benharris
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Andrewnielsen@me.com replied the topic: Re: Difference between Gradient and Actual wind

Great reply. So, gradient wind is due to the Coriolis effect, and surface wind is due to the gradient effect? Sounds about right.
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bobtait replied the topic: Re: Difference between Gradient and Actual wind

Thanks for your detailed reply Ben. For everybody's information, the definition of gusts and squalls can be found the the Bureau of Meteorology's Aviation Meteorology publication Page 51 para 9.8.

Gusts are momentary increases in wind speed that last no more than a few seconds and are typically 30% to 40% above the mean wind speed, but may be up to 100% increase above the mean wind speed.

Technically, the term 'squall' applies to an increase of at least 16 knots above the mean wind speed to reach a minimum of 22 knots that lasts for at least one minute. (sorry you asked?)
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