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# Meteorology question

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# Meteorology question

3rd Mar 2019, 16:54

Join Date: Aug 2015
Location: In the air
Posts: 205
Meteorology question

I am struggling with some meteorology questions from a certain well known air pilot's manual (vol 2!). I am pretty sure I understand the principles, but seem to be having problems with definitions. Here's one:

In the N hemisphere, in a particular point of a weather system where there is a geostrophic wind direction of 020T, the centre of the low pressure is:

a. In the direction of 020 true from that point
b. In the direction of 290 true from that point <---book's answer
c. In the direction of 110 true from that point <---my answer
d. In the direction of 200 true from that point

I thought this was easy with the simple application of Buy Ballot's law. But the only way I get this right with B-B's law is to take the wind direction as towards 020.

What am I doing wrong here

Last edited by double_barrel; 4th Mar 2019 at 07:44.
3rd Mar 2019, 17:07

Join Date: May 2005
Location: Milton Keynes
Posts: 858
Stand with the back to your wind the low pressure is on the left. Cyclonic flow is anticlockwise.

Has that helped? What do you think is "right".
3rd Mar 2019, 17:09

Join Date: Aug 2015
Location: In the air
Posts: 205
I think the answer is c. The book says b
3rd Mar 2019, 18:21

Join Date: Mar 2006
Location: Vance, Belgium
Age: 56
Posts: 134
I also have c as the right answer.
Wind flows from 020T to 200T, thus low pressure center is towards 200T-90°=110T
4th Mar 2019, 04:11

Join Date: Aug 2015
Location: In the air
Posts: 205
Thanks.

Of the perhaps 200 questions in the meteorology section, there are maybe half a dozen where I cannot understand the answers they show as correct. This is a very commonly used book (Air Pilot's Manual: Air Law & Meteorology, 2015 edition) and I would be surprised if errors persist.

How about a couple more (hope I am not infringing anyone's copyright here?!)

A parcel of air's total water content (vapour +liquid + ice) is important for airframe and engine icing potential. When it cools this potential:

a. increases
b. decreases
c. remains the same <---book's answer
d. evaporates

The wind that blows around curved isobars is called the:

a. curved wind
b. geostrophic wind
d. isobaric wind
For that 2nd one, I wonder if I am confused about terminology. I know the pressure gradient drives the wind flow, and I understand that the Coriolis force then turns it to the right. I understood that the net result of these 2 forces was called the geostrophic wind and it blows more-or-less parallel to the isobars. So I think the answer is b. But I suppose the actual initial airflow is gradient wind??
4th Mar 2019, 09:38

Join Date: Jun 2002
Location: CYYC (Calgary)
Posts: 4,583
In response to the query about the first question, it could be that ‘b’ is correct, but there is a typo in the question. For ‘N’ read ‘S’.

I’ve done a fair bit of flying and sailing in NZ, but I still have difficulty with the wind flowing clockwise around depressions!
4th Mar 2019, 10:21

Join Date: Mar 2006
Location: Vance, Belgium
Age: 56
Posts: 134
Double_barrel,

- for the first question, the right answer is c "remains the same".
Generally speaking, the total water content of a parcel of air remains constant.
With varying temperature, the vapour content diminishes and transforms or water or ice (cooling), or the water/ice content transforms into vapour (warming) ; but the total water content remains the same.
This of course becomes invalid when the water droplets coalesce and fall as rain (water content reduction). It is also invalid for the parcels in immediate contact with a patch of water or a some ground with moisture (water content increase). So these are exceptions to the general rule "remains constant".

- the second question is badly worded ; there are 2 correct answers.
Both the "geostrophic wind" and the "gradient wind" are models for wind and not real wind and both are modeled so that they follow the curves of the isobars.
The difference between the 2 is the computed speed of the wind.
In the geostrophic model, the speed is such that the Coriolis force compensates the pressure gradient force and no other force is considered.
In the gradient model, the speed is such that the Coriolis force compensates the pressure gradient force plus or minus the centrifugal pseudo-force.
As a result, the gradient wind blows parallel to the isobars at a less than geostrophic speed when the isobars curve around a low pressure (a lower Coriolis force is needed for compensating the gradiant force reduced by centrifugal pseudo-force) and the gradient wind blows parallel to the isobars faster than geostrophic speed around a high pressure (a higher Coriolis force is needed for compensating the gradiant force increased by centrifugal pseudo-force).
So both answers b and c are correct but the book answers "c" as if the question was
"Which wind blowing around curved isobars has a speed closer to real wind speed ?"

These geostrophic vs gradient models are well explained on this internet page:
Gradient Wind: non-geostrophic winds which blow parallel to isobars
4th Mar 2019, 11:13

Join Date: Aug 2015
Location: In the air
Posts: 205
Originally Posted by Luc Lion
Double_barrel,

- for the first question, the right answer is c "remains the same".
Generally speaking, the total water content of a parcel of air remains constant.
With varying temperature, the vapour content diminishes and transforms or water or ice (cooling), or the water/ice content transforms into vapour (warming) ; but the total water content remains the same.
This of course becomes invalid when the water droplets coalesce and fall as rain (water content reduction). It is also invalid for the parcels in immediate contact with a patch of water or a some ground with moisture (water content increase). So these are exceptions to the general rule "remains constant".
Thanks. I would understand that the total water content remains unchanged as a parcel of air cools. But I cannot understand how its potential for airframe icing does not increase as the water gets colder. Maybe I am misreading the question!

Originally Posted by Luc Lion
- the second question is badly worded ; there are 2 correct answers.
Both the "geostrophic wind" and the "gradient wind" are models for wind and not real wind and both are modeled so that they follow the curves of the isobars.
The difference between the 2 is the computed speed of the wind.
In the geostrophic model, the speed is such that the Coriolis force compensates the pressure gradient force and no other force is considered.
In the gradient model, the speed is such that the Coriolis force compensates the pressure gradient force plus or minus the centrifugal pseudo-force.
As a result, the gradient wind blows parallel to the isobars at a less than geostrophic speed when the isobars curve around a low pressure (a lower Coriolis force is needed for compensating the gradiant force reduced by centrifugal pseudo-force) and the gradient wind blows parallel to the isobars faster than geostrophic speed around a high pressure (a higher Coriolis force is needed for compensating the gradiant force increased by centrifugal pseudo-force).
So both answers b and c are correct but the book answers "c" as if the question was
"Which wind blowing around curved isobars has a speed closer to real wind speed ?"

These geostrophic vs gradient models are well explained on this internet page:
Gradient Wind: non-geostrophic winds which blow parallel to isobars
4th Mar 2019, 12:01

Join Date: Mar 2006
Location: Vance, Belgium
Age: 56
Posts: 134
The first question is, again, badly written.
The icing potential depends on both total water content AND temperature.
Further, this potential vary differently if one is speaking of engine icing or airframe icing.
I understand that the question ought to have been written:
"A parcel of air's total water content (vapour +liquid + ice) is important for airframe and engine icing potential. When it cools this total water content:"

When you take the actual exam, you have to quickly go through each question reading it at face value then, as time permits, go through a second sweep figuring which are the badly written questions and what might have the author's intentions been.
4th Mar 2019, 12:19

Join Date: Aug 2015
Location: In the air
Posts: 205
Originally Posted by Luc Lion
I understand that the question ought to have been written:
"A parcel of air's total water content (vapour +liquid + ice) is important for airframe and engine icing potential. When it cools this total water content:"

When you take the actual exam, you have to quickly go through each question reading it at face value then, as time permits, go through a second sweep figuring which are the badly written questions and what might have the author's intentions been.
Yep. Makes sense. I was reading it as meaning, "when it cools this icing potential:"

Thanks
5th Mar 2019, 07:36

Join Date: Jul 2008
Location: cheshire
Posts: 4
Re original question: answer b is correct based upon:
1) In Northern hemisphere
2) Geostrophic (so winds follow the isobars)
3) Low pressure (so flow anti-clockwise)

So, imagine you are 'flying' an anti-clockwise circle around the centre of the low. You are currently flying 020T. Therefore, the centre of the low will be 90 degrees to your left. 020-90 (360+20-90) = 290.
5th Mar 2019, 10:12

Join Date: Mar 2006
Location: Vance, Belgium
Age: 56
Posts: 134
Skyboy, the wind direction is the direction it comes from.
Therefore a wind direction 020T is actually flowing towards 200T
6th Mar 2019, 17:38

Join Date: Jul 2008
Location: cheshire
Posts: 4
Not necessarily so. You are absolutely right that a 'wind vector' states direction from (and strength) but that's not what the question is giving. Rather it states 'geostrophic wind direction of 020T' so it is not a 'wind vector'. Whether the way the question is worded is deliberate, or accidental, is moot but often questions are worded like this in order to test the candidate is thinking about what is happening rather than lamely following a rule without, necessarily, understanding the application. I stand by my earlier post.
7th Mar 2019, 08:56

Join Date: Oct 2004
Location: Europe
Posts: 5,765
RTFQ
there is a geostrophic wind direction of 020T
Not the same as the wind is 020/xxx
The purpose of the question is to see if you understand Geostrophic winds and Buys Ballot's law.

7th Mar 2019, 13:30

Join Date: Nov 2005
Location: Zulu Time Zone
Posts: 563
Now let me think.....is it more likely that the question bank simply contains an error, or that the question arbitrarily and discreetly turns the otherwise universal convention of reporting wind direction as 'from' on its head? Intentionally.

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