Wind

Question 1

Define standard surface wind.

Answer 1

A 10 min average of both speed and direction measured at 10 m above the surface.

Question 2

How is wind direction expressed in NZ and how does this affect use of information given in meteorological products?

Answer 2

Wind direction is expressed in degrees magnetic.

Throughout most of NZ, to convert from the published degrees true values, subtract 20 degrees.

Question 3

Define anemometer.

Answer 3

An instrument to measure surface wind. Most common type in NZ is the turning cup. Readings are recorded in graphical form on an anemograph.

Question 4

Define pilot balloon.

Answer 4

Freely moving hydrogen filled balloons that measure upper winds. Tracking of the balloons is usually done by radar, or by GPS. Results in a vertical profile of wind speed and direction.

Question 5

Define gust.

Answer 5

A short-term increase in the wind speed which lasts for only seconds. For a gust to be reported in a METAR, METAR AUTO etc, the gust speed must exceed the mean wind speed by at least 10 kn.

Question 6

What causes a gust?

Answer 6

Caused by turbulence dragging the stronger wind down to the surface from the top of the friction layer.

Question 7

Define lull.

Answer 7

Brief decrease in the wind speed. Lulls are of little consequence to aviation.

Question 8

Define squall.

Answer 8

A squall is a sudden increase in wind speed which:
Reaches a speed of at least 22 kn.
Increases by at least 16 kn.
Lasts for at least one min.

Question 9

What weather phenomenon are associated with squalls?

Answer 9

Passing cumulonimbus showers, marked changes in wind direction, thunderstorms and/or very heavy bursts of precipitation.

Question 10

Define gale.

Answer 10

A 10 min mean wind speed of between 34 and 47 kn.

Question 11

Define veering.

Answer 11

A clockwise change in the wind direction.

Question 12

Define backing.

Answer 12

An anti-clockwise change in the wind direction.

Question 13

What is the general rule of thumb for calculating the day-time surface wind (in relatively flat country)?

Answer 13

2/3 of the 2000 ft mean wind speed.
Gusting to the mean 2000 ft mean wind speed.
Veered by 30 degrees from the mean 2000 ft wind direction.

Question 14

If the 2000 ft wind at Ohakea is forecast to be 27030KT, what will the surface wind likely be?

Answer 14

30020G30KT.

Question 15

Describe the changes in wind conditions that are experienced as an aircraft climbs through the friction layer.

Answer 15

The wind will back by about 30 degrees, and light to moderate turbulence may be experienced until the aircraft climbs through the top of the friction layer. Winds will increase steadily and may continue to increase past the top of the layer.

Question 16

Do the wind conditions in a friction layer remain consistent?

Answer 16

The general pattern through the friction layer may change. Local effects in mountainous regions may cause significant differences.

Question 17

List the six descriptors of wind strength.

Answer 17

Light.
Moderate.
Fresh.
Strong.
Gale.
Strong gale.

Question 18

List the seven visual methods for estimating wind direction and speed.

Answer 18

Beaufort scale.
Ripples on water.
Wind lanes on water.
Cloud type or shape.
Cloud shadows.
Aircraft drift and apparent ground speed.
Cows.

Question 19

What are the three forces which generate wind at low level?

Answer 19

Pressure gradient.
Coriolis force.
Friction.

Question 20

Define coriolis force.

Answer 20

An inertial force which acts on objects that are in motion relative to a rotating frame of reference. It effects all objects moving across the surface of the rotating earth, including the wind.

Question 21

State and explain the simplified equation for the horizontal component of the coriolis force.

Answer 21

CF = 2Ωvsinφ

CF = coriolis force
Ω = the earth's angular momentum
v = the speed of the wind
sinφ = sine of the latitude (zero at the equator and 1 at the poles)

Question 22

List the three properties of the coriolis force.

Answer 22

It acts at right angles, and to the left of motion in the southern hemisphere.
Its strength is proportional to the wind speed - the stronger the wind, the stronger the coriolis force.
Its strength is also proportional to the sine of the latitude; therefore, it is zero at the equator and at its maximum strength at the poles.

Question 23

At what minimum distance can coriolis force be observed in wind?

Answer 23

30 km.

Question 24

Define geostrophic wind.

Answer 24

Wind that results from an exact balance of pressure gradient force (PGF) and CF when the isobars are straight.

Question 25

Define gradient wind.

Answer 25

A horizontal wind having the same direction as the geostrophic wind but with a magnitude consistent with a balance of three forces, being: the PGF, the CF and the centripetal force arising from the curvature of a parcel’s trajectory.
Essentially, it is the balance of forces where the isobars are curved.

Question 26

Does wind flow faster around cyclonically or anti-cyclonically curved isobars?

Answer 26

“For equal isobar spacings, the wind flies around a high and goes slow around a low”

Question 27

Describe how friction affects the surface wind velocity.

Answer 27

Frictions is proportional to surface roughness and acts opposite to the wind direction, thus slowing the wind down.

Question 28

What forces (in relation to wind velocity) are affected by friction?

Answer 28

PGF is not, however, the CF. One of the properties of CF that it is proportional to the wind speed. Therefore, as friction causes the wind speed to reduce, the CF must also decrease in magnitude.

Question 29

How does friction affect wind direction?

Answer 29

As the CF decreases, the excess PGF turns the wind across the isobars towards low pressure. The size of this direction change is dependant on the roughness of the underlying surface. At sea this angle between wind and isobars is 10 - 20 degrees but can increase to as much as 90 degrees in mountainous areas.

Question 30

Describe the venturi effect which is created by a strong northwest flow over the Southern Alps.

Answer 30

Wind piles up against the mountain range, causing a rise in sea level pressure, thus creating a strong windward ridge. This then causes a deficit of air on the leeward side so a strong lee trough is created. Between these two features, the isobars are very close together, creating an extremely strong pressure gradient and therefore very strong winds. Although the isobars would suggest a southwesterly flow, it is still northwest due to the effect of friction.
Almost overtime a strong northwest blows, a stable layer or inversion is created above the ridge line. The air is squeezed through this gap and accelerates.

Question 31

Define friction layer.

Answer 31

The surface of the earth exerts a frictional drag on the wind flowing over it. This drag acts to slow the wind down and causes the air to tumble within the portion of the atmosphere. This tumbling, turbulent air is confined within the friction layer.

Question 32

What two factors influence the depth of the friction layer?

Answer 32

The roughness of the underlying surface and the speed of the wind blowing over it.

Question 33

Why is the friction layer generally deeper across land than it is across the sea?

Answer 33

The ocean surface exerts little frictional drag as even the biggest waves are only as big as small hills on land and also because waves tend to yield to the wind and thus offer less resistance. Over land, however, there is no give in the surface features, so features such as trees, hills, buildings etc have a much greater slowing effect.

Question 34

What atmospheric feature will enhance turbulence in a friction layer?

Answer 34

A stable layer.

Question 35

What is Buys Ballot’s law?

Answer 35

If you stand with your back to the wind in the southern hemisphere, the low pressure is on your right.

Question 36

What is a significant limitation on the accuracy of Buys Ballot’s law?

Answer 36

This law does not consider the effect of friction on wind direction. The differences over open country and at sea can be accounted for but not in mountainous regions.

Question 37

With a southerly wind on your back, suing BB’s law explain where the centre of the low pressure is.

Answer 37

The centre of the low must be out to your right i.e. to the east. By implication, the high must be out to the west.

Question 38

What is the general trend for the movement of weather across NZ?

Answer 38

Most of the weather in NZ moves from west to east across the country.

Question 39

With a southerly wind, what will the general weather be doing in NZ?

Answer 39

When the low is to the east, it will almost always continue to move away from the country to the east or southeast, taking the poor weather with it. The high, which must be out to the west, will be moving onto NZ, with the promise of improving weather over the next day or two at least.

Question 40

Describe the diurnal variation of the surface wind over the land.

Answer 40

Wind strength near the earth’s surface tends to reach a maximum in the afternoon. Maximum gustiness is likely to occur then as the stronger wind at the top of the friction layer is dragged briefly down to the surface by frictional tumbling.
At night, the air in contact with the cooling ground cools through conduction, often resulting in a surface inversion or isothermal layer. This stable layer decouples the surface winds from the stronger winds at the top of the friction layer, decreasing surface wind strength.

Question 41

What are the two main atmospheric factors that contribute to diurnal wind variation?

Answer 41

Changes in atmospheric stability within the friction layer, which in turn is caused by changes in surface temps.

Question 42

Why is diurnal wind variation less pronounced over the ocean?

Answer 42

The effect is slight because the sea temp, unlike land, does not change much diurnally.

Question 43

Explain why the doldrums are characterised by light winds.

Answer 43

North of 10 ºS there is an area known as the doldrums. There are pressure gradients in this area but they tend to by small and therefore winds are generally light. Additionally, coriolis force is barely noticeable, and therefore what little wind there is tends to blow directly from high to low.

Question 44

Why isn’t the coriolis force apparent when you throw a ball?

Answer 44

The scale of motion is too small. For the effects of coriolis force to be noticed, a scale of approximately 30 km is required. For this reason, the effects of coriolis is seen on sea breeze circulations, but is not observed when water swirls down a plug hole.

Question 45

Unlike winds aloft, which blow nearly parallel to the isobars, surface winds nearly always cross the isobars. Explain why this difference exists.

Answer 45

The winds are not affected by friction. Near the earth’s surface however, within the friction layer, friction generated by the air flow over the surface, which may vary from smooth over the ocean to a major mountain range, causes the wind to slow down, which reduces the coriolis force, meaning the PGF now dominates and drags the air flow across the isobars towards low pressure.

Question 46

What are the general direction differences between geostrophic and surface winds over:
sea.
open land.
mountainous regions.

Answer 46

sea - 10 - 20 º
open land - 30 º
mountainous regions - up to 90 º but this is highly inaccurate.