WIND

Question 1

Define standard surface wind

Answer 1

10 minute mean wind measured 10 metres above the surface.

Question 2

True or mag?

Answer 2

All wind products are in TRUE, unless spoken (ATIS)

Question 3

True to mag?

Answer 3

Subtract 20deg

Question 4

What is an anemometer

Answer 4

Instrument that measures surface wind

Question 5

How are upper winds measured?

Answer 5

Pilot balloons (hydrogen filled).
Tracked via GPS or satellitesD

Question 6

Define Gust

Answer 6

Short term increase in wind speed that lasts for a few seconds.
To be reported it must exceed mean wind speed by >10kts

Question 7

How/why to gusts occur?

Answer 7

When turbulence briefly drags the stronger wind near the top of the friction layer down to the surface

Question 8

Define a Squall
Criterea?

Answer 8

A sudden increase in wind speed which must meet the below criteria;
1. reaches a speed of >22kts
2. Must increase by at least 16kts
3. Must last for >1min

Question 9

When would you expect a squall?

Answer 9

Often associated with passing Cb showers & often accompanied by TS & heavy showers.
Marked wind change is also common.

Question 10

Define Gale

Answer 10

10 min mean wind speed of between 34-47kts

Question 11

Define Veering

Answer 11

CLOCKWISE change in wind direction around the compass

Question 12

Define Backing

Answer 12

ANTICLOCKWISE change in wind direction around the compass

Question 13

RoT for wind change through altitude?

Answer 13

Through day time flat-country, the SURFACE wind will be;
2/3 of 2000ft mean wind speed
Gusting to 2000ft mean speed
Veered by 30deg from mean 2000ft wind direction

Question 14

When do wind lanes form on water

Answer 14

Winds 6-25kts
wind speed & direction must be constant

Question 15

What can wind lanes on water tell us?

Answer 15

Wind direction within 20deg & that sfc wind must be 6-25kts.

Question 16

What does a layer of Sc indicate in terms of wind?

Answer 16

Almost certainly an inversion.
Sometimes this acts to decouple the stronger winds above the layer to the lighter winds below, especially around dawn & dusk

Question 17

What does a developing Cu leaning over suggest in terms of wind?

Answer 17

Air is unstable, so there is a decoupling effect between upper & lower winds.
Leaning suggest increasing windshear with height

Question 18

what are the 7 ways to estimate wind from the air?

Answer 18

1. Beaufort scale
2. ripples on water
3. Wind lanes of water
4. Cloud type/shape
5. Cloud shadows
6. Aircraft drift & ground speed
7. Cows (bums to wind >25kts)

Question 19

What are the 3 forces acting to generate wind at low levels?

Answer 19

1. Pressure gradient
2. Coriolis Force
3. Friction

Question 20

Define Coriolis force

Answer 20

CF is an inertial force that acts on objects that are in motion relative to a rotating frame of reference.

Question 21

State the equation for Coriolis force?

Answer 21

CF= 2x the earths angular momentum x speed of the wind x sin of the latitude

Question 22

What are the 3 properties of corilois force?

Answer 22

1. acts at right angles, and to the left of motion in the SH.
2. Its strength is proportional to the wind speed (stronger wind = stronger CF)
3. Its strength is also proportional to the sine of latitude. (zero at equator & max strength at poles).

Question 23

Define Geostrophic wind

Answer 23

The wind that would result from an exact balance of pressure gradient force (PGF) & Coriolis force (CF) when isobars are straight.

Question 24

Draw a diagram to illustrate Geostrophic wind balance

Answer 24

Pg 78 in Met book

Question 25

Define "gradient wind"

Answer 25

A horizontal wind having same direction as geostrophic wind but with a magnitude consistent with having a balance of THREE forces; - PGF - CF - Centripetal force (Balance of forces where the isobars are curved)

Question 26

What causes Centripetal force?

Answer 26

Arises from the curvature of a parcels trajectory. In terms of wind, it is the "bending" around H or L systems that creates this force.

Question 27

Note with regards to centripetal force & wind

Answer 27

Centripetal force is not an additional external force but rather results from an imbalance between CF and PGF.

Question 28

Describe how Wind flows around a High pressure system with reference to CF & PGF.

Answer 28

Straight isobars = geostrophic wind balance As isobars curve left, the PGF changes (to continuously reflect a direct path from High to low pressure). Due to this, imbalance is created. To resolve this, the wind speed will increase, and therefore so does CF (proportional). CF is now stronger than PGF. The excess CF provides centripetal acceleration required to "bend" the wind around the isobars.

Question 29

Describe how Wind flows around a Low pressure system with reference to CF & PGF.

Answer 29

As isobars curve cyclonically, the PGF reacts to the change in isobars by rotating clockwise. (To continuously reflect a direct path from High to low pressure). To now balance PGF & CF, there is a resulting component acting against the wind which slows it down. This also decreases CF. The resulting PGF is stronger than the CF. Excess PGF = centripetal acceleration that "bends" the wind around the isobars.

Question 30

RoT/poem for winds in highs vs. lows

Answer 30

Wind flies around a high, and goes slow around a low. (FOR EQUAL ISOBAR SPACINGS)

Question 31

How does friction affect PGF vs. CF?

Answer 31

Friction acts opposite to surface wind direction. PGF is not affected by friction. CF is however proportional to wind speed, so therefore as friction causes wind to slow down, CF will also decrease.

Question 32

How does friction affect surface wind & velocity (wrt CF & PGF)

Answer 32

< CF and excess PGF turns the wind across the isobars towards low pressure. Resulting balance of new new CF & frictional force balances with PGF.

Question 33

RoT: Friction and angle between wind direction & isobars for sea vs. flat land vs. mountains

Answer 33

Sea: 10-20deg Flat land: 30deg Mountains: 90deg

Question 34

Why, if friction caused by mountains cause wind angle to be 90deg across the isobars, does it not then cancel out & make wind speed zero?

Answer 34

1. Wind 'piling up' against west coast = rise in pressure. a ridge is created on the windward side & a resulting leeward trough is created. Isobars here would be very close together = strong pressure gradient & therfore high winds. 2. Inversion sitting atop the ridgeline forces air to 'squeeze' between inversion & top of mountains = venturi effect.

Question 35

Define Friction layer

Answer 35

Surface of earth produces friction drag as wind blows across it. Causes the air below the friction layer to 'tumble'. This turbulent air is contained below the friction layer.

Question 36

What affects the depth of the friction layer? Ocean vs. land?

Answer 36

1. Wind speed 2. Roughness of terrain Ocean: exerts little friction on the wind, as waves are proportionally very small. Water also has "give" and tend to travel with the wind. Friction layer <1000ft Land: Nil 'give', therefore objects/obstacles have a massive slowing effect on wind. Friction layer 2000ft - 15,000ft.

Question 37

Buys ballots law?

Answer 37

if you stand with your back to the wind in the SH, the LOW PRESSURE is on your right.

Question 38

What does Buys ballots law not take into accoount?

Answer 38

Friction. For example, the law does not work in Mountainous terrain.

Question 39

Describe diurnal variation of surface wind over land: Day

Answer 39

As land heats up, stable layer breaks down, and vertical flows develop & deepens the friction layer. Upper level winds mix with lower levels & surface winds therefore increase. Wind strength reaches maximum in the afternoon & is when max gustiness is likely to occur.

Question 40

Describe diurnal variation of surface wind over land: Night Reference CF?

Answer 40

At night, air in contact with the ground cools, usually resulting in surface inversion which decouples surface winds from stronger winds at the top of the friction layer = calmer winds. Slower winds = less CF, therefore, night surface winds are more strongly affected by PGF & turn towards PGF direction.

Question 41

Describe diurnal variation of surface wind over sea

Answer 41

Effect is slight because sea temps, unlike land, do not change much diurnally.