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

Surfa

ce

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 daysurface 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 windDirection 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 WindGentle 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.