A Few Meteorology Questions..

[Easy226]

Hi everyone, i have been practicing a few exam questions and a few questions i was not so sure with. I would be grateful if an explanation could be given:

1. What meteorological situation would create an energy gain shortly after take-off.
a. A sudden gust directly towards the aircraft.
b. An encounter with a gust front ahead of a thunderstorm
c. Flight through a low level wind shear line

( i was thinking the answer was a for this, but maybe b., even thought you should not be flying within the vicinity of the thundercloud)

2. Is the diurnal variation of convective cloud over the sea:
a. nil
b. Very little
C. 'Over the Sea'
(again i thought the answer would be b. due to the specific heat of the water)

3. If you are asked to find the 8000ft wind velocity on metform 214, how do you average the direction and the speed, as well as finding the velocity at an altitude that isnt even specified on the chart?

Many Thanks
Dan

[Dick Whittingham]

On the energy gain, answers b. and c. might give an energy gain, depending on the direction and strength of the gusts, but answer a. will definitely give an energy gain, a transient rise in IAS and an increase in lift. A. is the most correct answer.

Sea temp hardly changes night and day, so convective cloud remains the same. Answer b.

The 214 question implies that you are on a PPL exam. The national low level charts are not tested in the ATPL exam. You ought not to arithmetically average vector quantities like wind direction and strength, but on the 214 the spots are so close together that the adjacent wind directions only vary by a few degrees. In this case you can add the figures and divide by 2, either sideways or up and down. Do not, however, add wind directions of, say, 350 and 010 and find a mean direction of 180!!

Dick W

[FlyingForFun]

Dick,You ought not to arithmetically average vector quantities like wind direction and strengthI don't have my Trevor Thom books with me, so I can't swear to this, but I'm sure the technique that the books teach for PPL is to use a weighted average of adjacent wind directions and speed from form 214 - it's certainly a technique which I use quite regularly when I'm flight planning. Why do you say you should not do this?

Have to admit that I don't remember being asked to average wind vectors for the ATPL exams.

FFF
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[Dick Whittingham]

I only said you ought not to arithmetically mean vector quantities, not that it is impossible!

Vectors have both size and direction, and 25kt tailwind then 25kt headwind does not mean to 25 + 25 = 50, divided by 2 equals 25. You solve vectors graphically all the time on the nav plot on the wizzwheel, and if you wanted an accurate mean wind you could stick them all on the wizz. Another way would be to first factor the winds to a component along a chosen track for each spot then add up and divide by the number of spots to give a wind vector along the track. Next repeat for across track. Finally, sum the cross track and along track vectors to find the average, or mean, wind.

This is not worth the effort, so there are two easy ways out. One is to gash it, and guess the mean wind. However, if the wind directions to be meaned are nearly the same you can just add up all the speeds on the spots and divide by the number of spots to get a mean speed that is good enough for government work. The mean direction, by definition if we are using this method, is roughly the same as the arithmetic mean of all the directions.

Because, as I said, on the 214 the spots are close together geographically, the adjacent winds are similar, the arithmetic mean works. On the big high level charts the winds can vary in direction 180deg, spot to spot, so it doesn't work. We used to teach the gash method for the ATPL, but I agree, mean winds did not often come up.

However, back to the begining, if you need an absolutely accurate mean wind there is no substitute for a full vector calculation.

Dick W

[FlyingForFun]

Thanks Dick - excellent reply. Your "gashing it" methods are pretty much exactly the way I do it, but I agree that it's no substitute for doing it properly.

FFF
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