WIND Flashcards
Define standard surface wind
10 minute mean wind measured 10 metres above the surface.
True or mag?
All wind products are in TRUE, unless spoken (ATIS)
True to mag?
Subtract 20deg
What is an anemometer
Instrument that measures surface wind
How are upper winds measured?
Pilot balloons (hydrogen filled).
Tracked via GPS or satellitesD
Define Gust
Short term increase in wind speed that lasts for a few seconds.
To be reported it must exceed mean wind speed by >10kts
How/why to gusts occur?
When turbulence briefly drags the stronger wind near the top of the friction layer down to the surface
Define a Squall
Criterea?
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
When would you expect a squall?
Often associated with passing Cb showers & often accompanied by TS & heavy showers.
Marked wind change is also common.
Define Gale
10 min mean wind speed of between 34-47kts
Define Veering
CLOCKWISE change in wind direction around the compass
Define Backing
ANTICLOCKWISE change in wind direction around the compass
RoT for wind change through altitude?
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
When do wind lanes form on water
Winds 6-25kts
wind speed & direction must be constant
What can wind lanes on water tell us?
Wind direction within 20deg & that sfc wind must be 6-25kts.
What does a layer of Sc indicate in terms of wind?
Almost certainly an inversion.
Sometimes this acts to decouple the stronger winds above the layer to the lighter winds below, especially around dawn & dusk
What does a developing Cu leaning over suggest in terms of wind?
Air is unstable, so there is a decoupling effect between upper & lower winds.
Leaning suggest increasing windshear with height
what are the 7 ways to estimate wind from the air?
- Beaufort scale
- ripples on water
- Wind lanes of water
- Cloud type/shape
- Cloud shadows
- Aircraft drift & ground speed
- Cows (bums to wind >25kts)
What are the 3 forces acting to generate wind at low levels?
- Pressure gradient
- Coriolis Force
- Friction
Define Coriolis force
CF is an inertial force that acts on objects that are in motion relative to a rotating frame of reference.
State the equation for Coriolis force?
CF= 2x the earths angular momentum x speed of the wind x sin of the latitude
What are the 3 properties of corilois force?
- acts at right angles, and to the left of motion in the SH.
- Its strength is proportional to the wind speed (stronger wind = stronger CF)
- Its strength is also proportional to the sine of latitude. (zero at equator & max strength at poles).
Define Geostrophic wind
The wind that would result from an exact balance of pressure gradient force (PGF) & Coriolis force (CF) when isobars are straight.
Draw a diagram to illustrate Geostrophic wind balance
Pg 78 in Met book
Define “gradient wind”
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)
What causes Centripetal force?
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.
Note with regards to centripetal force & wind
Centripetal force is not an additional external force but rather results from an imbalance between CF and PGF.
Describe how Wind flows around a High pressure system with reference to CF & PGF.
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.
Describe how Wind flows around a Low pressure system with reference to CF & PGF.
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.
RoT/poem for winds in highs vs. lows
Wind flies around a high, and goes slow around a low.
(FOR EQUAL ISOBAR SPACINGS)
How does friction affect PGF vs. CF?
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.
How does friction affect surface wind & velocity (wrt CF & PGF)
< CF and excess PGF turns the wind across the isobars towards low pressure.
Resulting balance of new new CF & frictional force balances with PGF.
RoT: Friction and angle between wind direction &
isobars for sea vs. flat land vs. mountains
Sea: 10-20deg
Flat land: 30deg
Mountains: 90deg
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?
- 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.
- Inversion sitting atop the ridgeline forces air to ‘squeeze’ between inversion & top of mountains = venturi effect.
Define Friction layer
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.
What affects the depth of the friction layer?
Ocean vs. land?
- Wind speed
- 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.
Buys ballots law?
if you stand with your back to the wind in the SH, the LOW PRESSURE is on your right.
What does Buys ballots law not take into accoount?
Friction. For example, the law does not work in Mountainous terrain.
Describe diurnal variation of surface wind over land: Day
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.
Describe diurnal variation of surface wind over land:
Night
Reference CF?
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.
Describe diurnal variation of surface wind over sea
Effect is slight because sea temps, unlike land, do not change much diurnally.
