Ch 10 - Low Level Wind

Question 1

Measuring Wind

Answer 1

Anemometer - measures surface wind
- Located 30ft (10m) above the surface to avoid surface friction, has a wind vain on it

1kt = 1.85km/h

1kt = 0.5m/s

1kt = 1.15mph

Question 2

Wind Terms (in terms of speed)

Answer 2

Calm <1kt
Gale 34-47kt
Storm 48-63kt
Hurricane >64kts

Gusts 10kts or more over the average wind speed and last <1min

Squall - Gust that lasts >1min

Lull - Sudden drop in windspeed

Question 3

Wind Direction

Answer 3

Northerly when coming from the north
Westerly when coming from the west

W->N (clockwise) = Veering

N -> W (anticlockwise) = Backing

Question 4

Who Reports Wind Direction

Answer 4

MET Office -> TRUE direction

ATC/ATIS - Magnetic direction

(Automated terminal information service)

World Area Forecast Centre; Only 2 - London, Washington and they are responsible for upper wind and temperature charts

Question 5

Causes of Wind

Answer 5

Movement of air from high pressure to low pressure due to the pressure gradient force (PGF) - directly proportional to wind speed

Closer isobars = faster wind speed

Further apart = Slower Wind speed

Question 6

Coriolis Effect (Geostrophic Effect / Force)

Answer 6

Effect caused by the rotation of the earth.

Causes objects which are moving independent of the Earth’s Surface to deflect to the right when view from the earth in the northern hemisphere

Southern Hemisphere - Causes objects to deflect left when viewed from the earth

Question 7

The Geostrophic Wind

Answer 7

The PGF is the force that acts from high pressure to low pressure and is directly proportional to the wind speed. We can gauge the strength of this force by measuring the spacing between isobars.

The relationship between isobar spacing, PGF and the wind speed is set out on a Geostrophic Wind scale

Measured; Measure the perpendicular distance between two isobars (noting latitude), read from left to right at the correct latitude to find the geostrophic wind speed

Question 8

Coriolis Force

Answer 8

Causes wind to turn to the right in the northern hemisphere - A force acting upon the wind

CF = 2x(angular rotation of the earth-omega)xdensity (q)x windspeed (V)x sin(latitude)

At equator and up to 15N, coriolis force = 0, at the poles 90 north CF=Max

Question 9

The Geostrophic Wind Model

Answer 9

Explains Wind behaviour on terms of simple vector forces.

Assumptions; Geostrophic wind only blows above friction layer, blows parallel to isobars,
And only assumes two forces; The PGF and the Coriolis Force (CF)

Geostrophic windspeed increases as latitude decreases

To find speed, refer to Geostrophic wind scale in corner of surface pressure charts

Question 10

Buys Ballot’s Law

Answer 10

In the northern hemisphere with your back to the wind, the low pressure is on your left

Question 11

The Gradient Wind Model

Answer 11

Essentially the same as geostrophic wind except it takes account of turning air flow

Assumptions; gradient wind only blows above the friction layer, it blows along curved isobars,

3 Forces; PGF, CF and the centrifugal force (CfG)

For a particular gradient force, the gradient wind speed increases as latitude decreases

In low pressure, CfG force helps windspeed (higher windspeed around low), around a high, CfG works against windspeed (lower speed around high)

Question 12

The Surface Wind Model

Answer 12

Assumptions; the surface is below the friction layer, the wind blows across the isobars towards the low pressure

3 forces considered; PGF, CF and the Friction Force (FF)

The surface wind increases as latitude decreases

Over land, the surface wind is roughly 50% of the geostrophic wind speed and it backs (anticlockwise) by 30 degrees

Over sea, the surface wind is roughly 70% of the geostrophic wind speed and backs (anti-clockwise) by 10 degrees.

Veers as altitude increases away from the surface

Question 13

Diurnal Variation of Surface Wind

Answer 13

In the northern hemisphere the surface wind increases and veers during the day, and then decreases and backs during the night.

Coriolis force is more prominent during the day as the windspeed is higher

Most prominent change is around 1500hr local time (maximum wind speed) and lowest when at the thermal minimum just after dawn

Question 14

Sea Breezes

Answer 14

Sea breezes are onshore winds that blow cooler air from the sea onto the land, only occurs during the day.

Can lead to vertical cloud formations over the coast (cumulus) which can develop into TS and are especially common around Florida in summer months

Localised, 10-15 miles inland, less than 1000ft and roughly 15kts.

Question 15

Land Breeze

Answer 15

Occurs at night under clear skies as the land mass cools rapidly, the air moves from on shore to off shore (diverges) and then begins to rise and warm over the sea.

Land breezes are offshore winds than only occur at night

Roughly 15 kts
Land to the sea
5/10 miles inland

Question 16

Anabatic Wind

Answer 16

Only effects mountainous or hilly areas

An uphill wind during the day, as the sun heats the slopes, surface air warms and rises which draws air from lower down the slope to replace it as it created a temporary low pressure. In the afternoon it can be very strong and can draw air from way down in the valley (valley wind)

Can’t lead to cloud formation

Question 17

Katabatic Wind

Answer 17

Only effects hilly or mountainous areas

Occurs down hill at night as the warmer air cools and condenses, it falls down the mountainside towards the valley, creates a cold wind that moves down the valley as the air gets colder and denser

Question 18

The Bora

Answer 18

An example of Katabatic wind

South Easterly

Cold wind descends down onto the Adriatic Sea
Can happen day and night but is strongest at night in winter when it forms a high pressure over cold Central Europe landmass whilst a low pressure develops over the warmer Adriatic Sea

Happens all year and can impact Italy

Question 19

The Mistral

Answer 19

South Eastern France

Very strong, relatively cold wind that is driven by a high pressure over north-western Europe.

Channeled down the Rhone Valley between Massif Central and the alps. A natural Venturi that amplifies the wind speed. Can extend to the tropopause

Can last for days and may even effect weather in N. Africa, Sicily

Question 20

Foehn Winds / Föhn Winds

Answer 20

A local warm, dry wind which blows on the downwind (leeward) side of a mountain.

Occurs in the alps, Rocky Mountains (Chinook), Andes and California (Santa Anna)

Expect cloud and rain on the upwind side and warm, dry, clear skies on the downwind side

Quickly melts snow and dries out vegetation, can lead to forest fires

Question 21

Harmattan Wind - W Africa

Answer 21

NE trade winds over N Africa which are dry and sand laden, can lead to massive impact on visibility and possibly engines

Question 22

Dampero - S America

Answer 22

Cold polar air burst (Pampas, Brazil)

Winter months (May-Aug)

On a squall line - passage of a cold front