Hi sydpilot,

I can see where the confusion arises there since according to the isobars, the gradient wind is a South-Westerly so which one should you choose...???? (b) Westerly or (d) Southerly????

The question is asking for the wind at YMML which would infer they want the surface wind. Since surface wind tends to flow across the isobars towards the region of low pressure, the actual surface wind at YMML will be more of a Westerly. Even without inferring that, you'd have to go with (b) since the wind will either be along the isobars (SW) or deflected towards the Low, namely Westerly. It could never be deflected to a more Southerly direction.

A question asking for the [b]gradient[/b] wind at YMML from that synoptic chart would have to give South-Westerly as one of the answer options.

Cheers,

Rich

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Thanks Rich. One more question for you.

I was doing a practice exam online got the below wrong. I selected answer [c].

As height is decreased during a descent to land in a strong crosswind from the right, the crosswind component is likely to -

(a) remain the same
(b) increase
(c) decrease
(d) reverse to come from the left

My reasoning was, closer to the ground, due to surface friction, the wind would slow down. Can you please let me know why the correct answer is (b).

Cheers,

Hi sydpilot,

Indeed the wind will reduce in speed closer to the ground but what this question wants you to think about is what is happening to the crosswind component of the wind in this case. Don't think about numbers, think more about what proportion of the wind is crosswind.

As an example, let's say we are tracking 360[sup]o[/sup] and have a strong right quartering crosswind of 045/30 at 5000ft. We are experiencing gradient wind and it is flowing along the isobars and so there there must be a region of low pressure somewhere ahead of us (wind flows clockwise around a low).

As we descend below about 3000ft, the surface friction will start to slow the wind down, Coriolis force is directly proportional to the wind speed so if you reduce the speed, you reduce the effect of the Coriolis force. It starts to lose the battle against the pressure gradient force and so the wind will start to flow across the isobars and more directly towards the Low.

Let's assume we are over land so we use the rule of thumb that the surface wind will veer by about 30 degrees and slow by 2/3rds when compared to the gradient wind.

As we continue the descent, the wind will start to veer round towards about 075[sup]o[/sup] and slow down to about 10kt by the time we get close to the ground.

The wind was 45[sup]o[/sup] off the nose at altitude but now it is 75[sup]o[/sup] off the nose, nearly a complete crosswind. Even though the wind speed has reduced the [b]proportion[/b] of the wind which is coming from the right will be more.

Don't worry too much about this question, it is there to make you think about the effects of surface friction on the wind direction and speed. You were already on the right track!

Cheers,

Rich

Rich makes the important distinction here. The [b][u]proportion[/u][/b] of the crosswind will increase. It would be so nice if the questions that are designed to test our understanding actually reflected those intentions. There is no way to reliably determine the answer to this question without that important word, proportion (and disregarding local effects).

The terrain and surroundings have far more effect on the surface wind than the gradient v's surface wind component ever will in many situations (for example it is not unusual for the windsocks at each end of the runway at Lord Howe Island to indicate a downwind regardless of the gradient wind).