Low Level Local Winds

Question 1

How is surface wind measured?

Answer 1

Using a cup anemometer for speed.
weather vane for wind direction.

Usually measured at 10m AGL too minimize ground interference.

Usually measured in knots.
Some use ms^-1 (Beijing)

Question 2

Reporting wind direction.

Answer 2

If you listen to it: Magnetic
If you read it: Meteorological reports.

Question 3

Definitions for wind strength.

Answer 3

Calm: <1kt
Gale force: 34-47kt
Storm force: 48-63kt
Hurricane force: >64kt
(Beaufort scale)

Question 4

Gust

Answer 4

Sudden increase of 10kts or above.
Lasts less than 1 minute.

Question 5

Squall

Answer 5

similar to gust, lasts more than 1 minute.

Question 6

Lull

Answer 6

Sudden drop in wind speed.

Question 7

Wind direction being reported

Answer 7

Direction of wind is where it is blowing from.

Question 8

Changes in wind direction

Answer 8

Clockwise - veering
Anticlockwise - backing

Question 9

Isotach

Answer 9

Isotach line are also drawn to show areas of equal wind strength.

Shown in 5km/h increments.

Question 10

Low level chart

Answer 10

Lat and Long
1000s ft
Wind direction - T
Wind speed - kts
Temperature - Celsius

Question 11

Forces involved

Answer 11

Pressure gradient
Coriolis effect/force
Centrifugal force
friction force (1000-3000ft)

Question 12

Pressure gradient

Answer 12

High pressure to low pressure
This would mean air flow across the isobars, only occurs if earth stopped spinning.
Air flows parallel to isobars, at right angles to pressure gradient.

Question 13

Coriolis effect

Answer 13

Due to rotational speed at different latitudes.
When a parcel of air leaves a latitude it retains its original rotational speed.
-moves towards the poles, parcels rotational speed faster than earths.
- Moves towards equator, parcel rotational speed slow than earths.
Parcel of air deflects to the right compared to its movement over the surface in northern hemisphere.

In the southern hemisphere, same process occurs:
However, air deflects to the left compared to its movement over the earths surface.
Because the coriolis effect appears to cause the wind to turn, we will use the term ‘coriolis force’

Question 14

Coriolis Force increases with latitude

Answer 14

Coriolis force (Ce) = 2 Omega Rho V Sin0

omega - angular rotation
Rho - density
V - wind speed
0 - latitude

Question 15

Geostrophic wind - Northern hemisphere

Answer 15

Wind moves from high pressure to low pressure as PGF > then CF, when PGF=CF wind speed is ‘straight’, in the same pressure, called geostrophic wind.
(parallel to isobars).
Travels clockwise.

Question 16

Geostrophic wind - Southern Hemisphere

Answer 16

Same process just anti-clockwise.

Question 17

Wind speed, geostrophic wind

Answer 17

V = PGF / 2 omega rho sin0

Question 18

Geostrophic wind scale

Answer 18

For the same isobar spacing, wind speed at lower latitudes is faster.

Question 19

Geostrophic wind and gradient wind

Answer 19

Straight isobars are rare.
Curved isobars are common.

Question 20

Gradient wind

Answer 20

Formula only applies to geostrophic wind (straight isobars).
Coriolis force = PGF

When air follows a circular path, wind speed is influenced bt centrifugal force.

Centripetal force acts inwards.
Centrifugal force acts outwards.

When a cyclone (low) has formed, PGF acts towards centre.
Centrifugal force opposes the pressure gradient.
Therefore, resulting wind speed is lower than the geostrophic wind for the same isobar spacing (sub-geostrophic).

When an anti-cyclone (high) has formed, PGF force acts outwards (always towards low pressure).
Centrifugal force supports the PGF.
Therefore, resulting windspeed is higher for same isobard spacing (super-geostrophic)

Question 21

Buys ballot law

Answer 21

Observer stands with their back to the wind, the low pressure is on their left in the northern hemisphere. Southern hemisphere right side.

Question 22

Friction layer

Answer 22

Average vertical extent of 2000-3000ft.
Affected mainly by terrain, wind speed and stability.
At night, layer may reduce to 1000-1500ft.

Question 23

Wind variation at surface.

Answer 23

Surface friction reduces the speed of the wind.
Coriolis force reduces as wind speed reduces.

Question 24

Surface winds - pressure systems

Answer 24

Surface wind will flow ‘somewhat’ across the isobars towards an area of lower pressure due to friction layer. (never towards an area of high pressure).

Question 25

Diurnal variation of surface wind

Answer 25

daytime thermal turbulences mixes skiw moving the surface air with the faster free stream air above.
Little to no thermal activity at night time so no mixing, just slow moving surface air.

Question 26

Friction layer

Answer 26

Over the sea - 10 degree - 70%
Over land day - 30 degree - 50%
Over land night - 45 degree - 25%
North of east.

Question 27

Friction layer

Answer 27

Easy way to remember wind direction chnages is draw vectors:
Northern hemisphere, Coriolis to the right:
Climb veer (after sunrise)
Descend Back (After sunset)

Southern hemisphere, Coriolis to the left:
Climb back
Descent veer

Question 28

Sea breeze

Answer 28

Timing - usually begins around 10am and peaks 2-4pm.
strength - Around 10-15kts
extent - Around 25-40km inland, 2000-3000ft high.
Cloud - Cumulus development over the land.
Temperature - Air over the land is replaced by cold sea air
Synoptic - Usually during anticyclones due to weak PGF.

Question 29

Sea breeze

Answer 29

Warmer land heats up, climbs to form a cloud, moves out to sea and cools falls to surface, moves inland and repeat.

Question 30

Land breeze

Answer 30

Warm sea, air climb and forms cloud, moves inland cools down and falls to surface, moves back out to sea and repeat.

Question 31

Anabatic and valley winds (day)

Answer 31

Caused by daytime surface heating
Can develop mid to late afternoon.

Question 32

Katabatic and mountain winds (night)

Answer 32

Caused by night time cooling.
Can develop during night.

Question 33

The bora

Answer 33

Katabatic wind - cold.
Wind speeds can exceed 100kt in winter,
Blows into the Adriatic from the Balkans and the alps of Slovenia, Italy and Austria.

Question 34

The mistral

Answer 34

cold and dry north, northeast wind.
Forms between two systems: the anticyclone in the near Atlantic, and the low over the gulf of genoa.
The northerly wind will accelerate passing between the Alps and the Massif centra.
Wind speeds can reach 70kt.
venturi effect.

Question 35

Fohn wind

Answer 35

There must be a substantial mountain range.
Wind must be blowing close to right angles to the range.
Approaching air must have high moisture content.

Moist stable air flows up a mountain cooling at the SALR, condenses and forms cloud.
Descending air on the leeside warms at the DALR. Now warmer than same air on other side and drier.

Question 36

Fohn wind and fohn effect (Alps)

Answer 36

Westerley wind.
Warm and dry blowing off the swiss alps.
Also chinook, canadian rockies.

Question 37

Mountain waves

Answer 37

Light winds, less than 15kt, predictable.
Stronger wind, 15kt+, unpredictable.
Disturbance of transverse airflow by high ground can create an organized flow pattern - mountain waves, lee waves or standing waves.

Needs a degree of stability.
15-20kt wind at mountain height, increasing at altitude
Within 30 degrees of being perpendicular to the mountain.

Can spread from low to high levels.

Common by product and hazard to flight is the formation of mountain waves and rotor zones.
MTW can be as high as 60,000ft affecting the strartosphere.
Can be found 100-300nm from mountain range, lee side (dowrange).
600nm.

Question 38

Rotor zones

Answer 38

Strong rotating current of air on lee of a mountain, often near or slightly above mountain top height. Turbulence may be servere.

Rotor clouds can give a clue to their presence if they are formed and will show as ragged and rapidly rotating streaks.

Question 39

TEM: MTW

Answer 39

Overfly at 90 degrees to mountain range
FLy as high as possible
Avoid flying leeward side and rotor zone.

Can be reported in sigmet.