Wind

B. Backs and decreases

Friction slows the surface wind; a slower wind has less Coriolis force, so the pressure gradient turns it towards low pressure. In the NH that is a backing. Climbing out of the friction layer (to about 2000 ft), the wind veers and strengthens.

B. Pressure gradient force and the Coriolis force

Above the friction layer, with straight isobars, the pressure gradient force pushing towards low pressure is exactly balanced by the Coriolis force, and the wind blows parallel to the isobars.

C. Poles

Coriolis force = 2 x omega x V x sin(latitude). Sin(latitude) is largest at 90 degrees, so it is greatest at the poles and zero at the equator.

B. Right, giving port drift

In the SH, with your back to the wind the low is on your right. Heading towards the low, the wind therefore blows across your track from the right and you drift to port. It is the mirror image of the Northern Hemisphere.

B. Anticlockwise

NH: anticlockwise and inwards around a low, clockwise and outwards around a high. The SH is the reverse.

C. 34 kt or more

Gale force starts at 34 kt (Beaufort force 8). In Indian cyclone classification 34-47 kt is a cyclonic storm.

A. A change of wind speed and/or direction over a short distance

Wind shear is any change in wind speed or direction over a short distance, vertically or horizontally. Near the ground it changes your airspeed suddenly - the danger on approach and take-off.

A. Same

A surge is a pulse of stronger wind inside one air mass (common in a vigorous monsoon). A change of air mass would be a front.

B. Right

Buys Ballot: back to the wind, low on the LEFT in the Northern Hemisphere and on the RIGHT in the Southern Hemisphere.

A. Port drift

NH: back to the wind, low on the left. Flying from low towards high, the wind comes across from the right and you drift to port - and the altimeter under-reads. From high to low it is starboard drift.

A. Stronger

Around curved isobars the centrifugal effect is added. Around a high it adds to the pressure gradient, so the gradient wind is stronger than the geostrophic value; around a low it is weaker.

A. Gust

A gust is a short, sudden rise in wind speed that dies away again within seconds, like 10 kt to 30 kt and back to 15 kt. A squall lasts minutes.

Our independent check could not confirm this answer from the official sources we hold; treat it with care and confirm with your instructor.

B. Velocity Convergence

A surge is a strengthening of the wind within the same air mass, and the faster air running into the slower air ahead produces velocity convergence - which lifts the air and enhances rain.

A. Converge

Friction slows the surface wind so Coriolis turns it less, and it blows across the isobars in towards the low. The air converges at the centre and rises - which is why lows bring cloud and rain.

B. Easterlies

The correct answer is: Easterlies.

A. Wind shear while climbing through it

Under a night inversion the surface wind is light while the wind above is much stronger and from a different direction, so an aircraft climbing through the inversion meets a sudden change of wind - wind shear.

A. A difference of pressure between two places

Air moves because of a pressure gradient - that is the only force that starts it. Temperature differences matter because they CREATE the pressure differences, and the earth rotation (Coriolis) only deflects air that is already moving. No pressure gradient, no wind.

A. Equator

With sin(0) = 0, the Coriolis force vanishes at the equator. That is why the geostrophic idea fails near the equator and why tropical cyclones cannot form within about 5 degrees of it.

A. Clockwise

The Coriolis force acts to the left in the SH, so the circulation reverses: clockwise around lows, anticlockwise around highs.

C. Stability, wind speed and surface roughness

The rougher the ground and the stronger the wind, the higher friction effects reach; stable air resists vertical mixing and keeps the layer shallow. So the depth depends on roughness, wind speed and stability - deeper by day, shallower at night.

B. 230/15 kt

Thermal wind = upper wind minus lower wind (as vectors). Subtracting 050/10 is the same as adding 230/10, and 230/05 + 230/10 = 230/15 kt.

C. 240/15 kt

Upper minus lower: minus 060/10 is plus 240/10, so 240/05 + 240/10 = 240/15 kt. When the two winds are opposite, add the speeds and take the direction of the upper wind.

C. The vector to add to the lower geostrophic wind to obtain the upper geostrophic wind

The thermal wind is not a real wind. It is the vector difference between the geostrophic winds at two levels: add it to the lower wind to get the upper wind. It blows parallel to the isotherms with cold air on its left in the northern hemisphere.

A. Knots per 100 ft

Vertical wind shear - the change of wind with height - is quoted in knots per 100 ft. Horizontal wind shear uses knots per 1000 ft.

B. Isotachs

Isotachs are lines joining places of equal wind speed. Isobars join equal pressure; isogonals join equal magnetic variation.

A. 49 kt

One knot is 1.852 km/h, so 90 ÷ 1.852 ≈ 48.6 kt. A quick check: knots are roughly half of km/h.

B. Isotherms

The thermal wind is the change of wind with height caused by the horizontal temperature difference, and it blows along the isotherms with the colder air on its left in the northern hemisphere.

A. Isotachs

Isotachs join points of equal wind speed and are drawn on upper-air charts to show where the jet streams are. Isotherms join equal temperature and isallobars equal pressure tendency.

A. The north-east and south-east trade winds converge

The ITCZ lies between the air masses either side of the heat equator. There the NE trades of the northern hemisphere meet the SE trades of the southern hemisphere, and the converging air rises in towering cloud.

A. Weaker

Around a low, part of the inward pressure force is used to curve the flow, so less Coriolis force - and therefore less speed - is needed to balance it: the gradient wind is weaker than geostrophic. Around a high it is the reverse.

B. Right

The earth turning beneath moving air makes the air appear to turn - to the right in the northern hemisphere and to the left in the southern. This apparent force is the Coriolis force.

A. Differences in pressure

Air moves from high to low pressure under the pressure gradient force. Coriolis only turns moving air and friction only slows it - neither can start it moving.

A. Wind shear at the top of the inversion

Surface cooling stops the vertical mixing, so below the inversion the wind is light and from a different direction, while above it the wind is much stronger. An aircraft climbing or descending through that boundary meets a sudden change of wind - wind shear.

A. Longer duration

The difference is duration. A gust is a rapid fluctuation lasting a few seconds; a squall is a sudden increase of at least 16 kt that reaches 22 kt or more and lasts at least one minute, usually with a temperature drop and precipitation.

A. Coriolis force

The pressure gradient force is what starts the air moving from high to low. As soon as it moves, the Coriolis force deflects it - to the right in the northern hemisphere - until the flow ends up running along the isobars rather than across them. Friction only modifies this near the surface.

A. The wind that must be added to the geostrophic wind at the bottom of the layer to give the geostrophic wind at the top

Thermal wind is a difference, not a wind you can feel. It is the vector that turns the lower-level geostrophic wind into the upper-level one, and its size depends on the horizontal TEMPERATURE gradient through the layer. That is why the subtropical westerly jet exists: a standing equator-to-pole temperature gradient makes a westerly thermal wind that builds with height.