Ch 10 - Low Level Wind Flashcards
Measuring Wind
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
Wind Terms (in terms of speed)
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
Wind Direction
Northerly when coming from the north
Westerly when coming from the west
W->N (clockwise) = Veering
N -> W (anticlockwise) = Backing
Who Reports Wind Direction
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
Causes of Wind
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
Coriolis Effect (Geostrophic Effect / Force)
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
The Geostrophic Wind
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
Coriolis Force
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
The Geostrophic Wind Model
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
Buys Ballot’s Law
In the northern hemisphere with your back to the wind, the low pressure is on your left
The Gradient Wind Model
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)
The Surface Wind Model
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
Diurnal Variation of Surface Wind
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
Sea Breezes
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.
Land Breeze
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
Anabatic Wind
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
Katabatic Wind
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
The Bora
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
The Mistral
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
Foehn Winds / Föhn Winds
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
Harmattan Wind - W Africa
NE trade winds over N Africa which are dry and sand laden, can lead to massive impact on visibility and possibly engines
Dampero - S America
Cold polar air burst (Pampas, Brazil)
Winter months (May-Aug)
On a squall line - passage of a cold front
