Bob Tait's Aviation Theory School

Gday,

As you might have read, I sat the Met exam again today but I came across the same question as on my first attempt.

It was something along these lines. "If a pressure system remains relatively stationary over an area of vast flat land, what type of wind variation and at what levels would you expect to notice changes?" Answers were like, "No variation at any levels", "Variation only in lower levels" etc etc.. - Obviously not verbatim..

I assume I got it wrong, because I can never guess right. Now on my KDR it cited A.2.9.6.4 - Diurnal variation of wind (again, assuming this is the syllabus reference the question relates to?)

I don't understand the question?!!!? Does it refer to Page 2.18 of the Met study guide? If so, how would I have gotten this question right.. arrgh! I guess I'm asking, what is the answer

Hi Michael,

We're not going to be dealing with diurnal variations such as land or sea breezes since it is over a "vast stretch of land" and katabatic or föhn winds can be ignored since we're told the land is flat.

The conditions described seem stable so there would be little variation in general. However, the surface layer would still have to be affected by the diurnal changes in temperature.

How strong that effect would be is dependent on whether there are clear skies or not (hot day, cold night) and also on the presence of strong pressure gradients since these can overwhelm any diurnal effects.

So, I'd go for "Variation only in Lower Levels". What answer did you give?

Cheers

Rich


For the benefit of others reading, here's that Syllabus part 9.6.4 reference in the KDR:
List the "factors" that effect the diurnal variation of wind and describe typical "variations" in surface wind strength during a 24-hour period.

Pretty sure I took the safe route (I thought) and said no variation at any level, considering it said everything that would be a factor was doing relatively nothing.. But your reasoning makes a lot of sense, maybe I can sound like I know what I'm talking about when I do my KDR after a little more reading up.

Thanks for looking at it,

Cheers

You're welcome. The KDR is a great tool to help polish up those areas where you might have gone wrong.

... maybe I can sound like I know what I'm talking about...
You had a good question and I'm glad you asked it! We're all learning here mate so don't sweat asking about stuff. That's what we're here for

Have fun with Nav!

Congratulations Michael, a good solid pass mark especially for home study. Concerning the question, an almost stationary high pressure system usually features weak pressure gradients and a stable air mass. The most likely wind variations would be at the surface and be due to diurnal effects. At dawn the surface wind would be calm because of a surface inversion which formed over night. As the sun climbed higher convection would start [high pressure systems usually have little cloud]. This would produce variable light gusty surface winds, which would become calm in the late afternoon. You would not expect much change in the gradient wind at higher levels.

So I would go for variations but only in the lower levels. Richard and I often agree; especially after a few glasses of fine red!

Spoken like a true aviator, thanks guys, appreciate you both taking time out to help me understand things a little better!