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Chapter 06
Low Level Winds
Lessons 17,18,19,20,21
Definition of wind
Wind is basically the horizontal movement of air relative to the rotating earth surface.
Measurement of wind Wind direction
measured with reference to True North and is the direction from which it is blowing.
Velocity measured in- Knots (KT) Kilometers per hour (KPH) Metres per second (MPS)
The only instance where wind direction is referenced to Magnetic North is for take-off and landing.
Measurement of Wind Wind velocity is usually measured using an
anemometer.
Depiction of wind data
Surface Charts
Upper Level Charts
Primary Cause of Wind
Air moves from an area of HIGH pressure to an area of LOW pressure under the influence of a Pressure Gradient Force (PGF).
Buys Ballot’s Law Buys Ballot, Christoph H(endrick)
D(iederick) 1817 -- 1890 Meteorologist, born in Kloetinge, The Netherlands. He studied and taught at the University of Utrecht, and founded the Royal Netherlands Meteorological Institute in 1854. He was the inventor of the aeroklinoscope and of a system of weather signals. He stated the law of wind direction in relation to atmospheric pressure in 1857 (Buys Ballot's law).
Buys Ballot’s Law In the Northern Hemisphere if you stand
with your back to the wind Low pressure is to the left High pressure is to the right Opposite effect in the Southern Hemisphere
LOW
HIGH
NH LOW
HIGH
SH
Pressure Gradient Force
The net force directed from higher to lower pressure is called the: PRESSURE GRADIENT FORCE. It acts at 90° to the Isobars
PRESSURE GRADIENT FORCE
Pressure Gradient Force
The Pressure Gradient Force is expressed as pressure difference over a given distance:
i.e. mb/100nm.
dpPGF
Pressure Gradient Force
Directed from High pressure toward Low pressure, at right angles to the isobars.
Closely spaced isobars indicate steep pressure gradients, strong forces and high winds.
Conversely widely spaced isobars indicate shallow pressure gradients, weak forces and light winds
Geostrophic Force
Also known as Coriolis Force Named after a 19th C. French
scientist, Gaspard Coriolis. It is an apparent force due to the
rotation of the earth.
Coriolis Force
Causes all free-moving objects to deflect to the RIGHT in the Northern Hemisphere
And to the LEFT in the Southern Hemisphere.
Consider the following video demonstration.
Geostrophic Wind Is the wind blowing above the surface
friction layer under the influence of the Coriolis Force.
Geostrophic Wind
Geostrophic Force
Geostrophic Force is an apparent force due to the rotation of the earth
It’s magnitude depends on Wind speed (v). The sine of the latitude. 2 can be considered a constant
)(2 latVSinGF
Geostrophic Force
Geostrophic Force is directly proportional to Wind speed (v). The sine of the latitude.
)(latVSinGF
Conditions for a true Geostrophic Wind
Blows above the friction layer. Blows along parallel isobars. Constant Pressure Gradient Force. More than 10°N/S of the Equator
Geostrophic Wind Scale
Geostrophic Wind
Geostrophic wind blows along the isobars
Gradient Wind
Few Isobars are straight and parallel. Most winds will therefore blow along
curved isobars. When the wind blows along curved
isobars it is known as the GRADIENT WIND.
Properties of the Gradient Wind
Blows along curved isobars adjusted in speed.
Around a High the Gradient Wind is greater than the apparent Geostrophic Wind.
Around a Low the Gradient Wind is less than the apparent Geostrophic Wind.
Gradient Wind
LOW
(Outward acting)
Gradient Wind
HIGH
(Outward acting)
Gradient Wind
CF = Cyclostrophic Force
The Cyclostrophic Wind Small-scale sharply
curved pressure systems e.g. Tornadoes &
Dust Devils PGF > Coriolis Force PGF = Centrifugal
Force Resulting flow is the
cyclostrophic wind
Surface Friction
Causes surface winds to be reduced relative to the Geostrophic Wind. Relative to the Geostrophic Wind:
In the NH they are backed. In the SH they are veered.
Surface Friction
As the wind reduces due to surface friction, the speed is reduced, the Coriolis Force is reduced, causing the wind vector to move toward the
larger force, which is the PGF.
Effects of Surface Friction
Surface friction reduces the wind speed
Geostrophic force reduces
Reduced wind is backed towards the greater PGF
Surface friction opposes surface wind
Friction force and coriolis force combine to balance PGF.
Surface Friction
H
L
Causes:
Highs to be Divergent at the surface
Lows to be Convergent at the surface
Backing Factors
The speed reduction and degree of backing (NH) or veering (SH) depends upon a number of factors: Surface roughness. Windspeed. Stability of the atmosphere. Day/night.
Surface wind compared to geostrophic wind
Surf
ace
Top of friction layer
Diurnal Variation of Wind in the Boundary Layer
Comparing surface wind, 1000 feet and 2000 feet winds respectively (Northern Hemisphere) the transition is:
day to night
• surface wind decreases and turns counter clockwise (backs)
• 1000 feet wind increases and turns towards the geostrophic direction(veers)
• 2000 feet shows little variation
Diurnal Variation of Wind in the Boundary Layer, cont’d
night to day
surface wind increases and turns clockwise (veers)
1000 feet wind decreases and turns counter clockwise(backs)
2000 feet shows little variation
Southern Hemisphere -opposite changes in wind direction.
Variation of wind across a front at the surface
Convergence and Divergence
Coastal Convergence/Divergence (NH)
Diurnal Variation of Surface Wind (NH)
• Over land:
Day:
Speed about 50% of Geos Wind, (JAA exam)
Direction backed by about 30°, (JAA exam)
Night:(Not stated for JAR exam)
Speed of Geostrophic wind
Direction backed by 40°
Diurnal Variation of Surface Wind (NH)
Over oceans Day/Night:
Speed about 70% of Geos Wind, (JAA exam) Direction backed about 10°, (JAA exam)
Valley Wind
E.g. The MISTRAL blowing down the R. Rhone Valley in S. France mainly during Winter and early Spring.
L
H
Katabatic Winds
Katabatic Winde.g. The BORA blowing down the mountain valleys into the N. Adriatic during winter mainly and early spring.
Anabatic Wind
Gentle up-slope wind blowing in the early morning on mountain slopes facing into the rising sun.
Sea Breeze
Warming
FIRST MOVEMENT OF AIR
PRESSURE LEVELS INCREASE IN HEIGHT
SEA BREEZE CIRCULATION ESTABLISHED
Differential heating causes pressure gradient aloft.
First movement of air is always aloft from land to sea for the sea breeze.
Air moving away aloft causes pressure to drop over the land.
Air arriving aloft over the sea causes pressure to rise at the surface.
Pressure gradient established from sea to land resulting in
The sea breeze.
SHORT WAVE RADIATION
Sea Breeze (cont.) On a good day may
extend approx. 20nm inland in UK.
Rarely exceeds 10 KT. Above 500 ft speed
decreases rapidly by about 1000 ft.
as a rule extends about 10 - 15 NM (15-25km) either side of coast.
Sea Breeze (cont.) Initially sea breeze at 90
degrees to coast. Coriolis effect causes wind to
veer approx 45deg by late afternoon and may eventually align with coast.
In lower (i.e. tropical) latitudes sea breeze may exceed 20 KT and penetrate further inland up to 50 nm.
In equatorial latitudes air flows directly from high to low pressure areas due to absence of Coriolis Force.
COOLING
PRESSURE LEVELS DECREASE IN HEIGHT
FIRST MOVEMENT OF AIR
LAND BREEZE CIRCULATION ESTABLISHED
Differential cooling causes pressure gradient aloft.
First movement of air is always aloft from sea to land for the land breeze.
Air moving away aloft causes pressure to drop over the sea.
Air arriving aloft over the land causes pressure to rise at the surface.
Pressure gradient established from land to sea resulting in the land breeze.
Land Breeze (Night)LONG WAVE IR RADIATION
Land Breeze (cont.) Night Land Breeze rarely more than a
few kt at most. Land Breeze occurs only in stable
conditions. Influenced by surface contours. Shallow in depth not exceeding a few
100 feet.
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