Chapter 06 Low Level Winds Lessons 17,18,19,20,21

1. Chapter 06Low Level WindsLessons 17,18,19,20,21

2. Definition of wind • Wind is basically the horizontal movement of air relative to the rotating earth surface.

3. 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.

4. Measurement of Wind • Wind velocity is usually measured using an anemometer.

5. Depiction of wind data • Surface Charts • Upper Level Charts

6. 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).

7. 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).

8. 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 NH SH LOW HIGH HIGH

9. 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

10. Pressure Gradient Force • The Pressure Gradient Force is expressed as pressure difference over a given distance: • i.e. mb/100nm.

11. 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

12. 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.

13. 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.

17. Geostrophic Wind • Is the wind blowing above the surface friction layer under the influence of the Coriolis Force.

18. Geostrophic Wind

19. 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

20. Geostrophic Force • Geostrophic Force is directly proportional to • Wind speed (v). • The sine of the latitude.

21. 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

22. Geostrophic Wind Scale

23. Geostrophic Wind • Geostrophic wind blows along the isobars

24. 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.

25. 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.

27. Gradient Wind LOW (Outward acting)

28. Gradient Wind HIGH (Outward acting)

29. Gradient Wind CF = Cyclostrophic Force

30. 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

31. 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.

32. 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.

33. 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.

34. Surface Friction Causes: H Highs to be Divergent at the surface L Lows to be Convergent at the surface

35. 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.

36. Surface wind compared to geostrophic wind Surface Top of friction layer

37. 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

38. 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.

39. Variation of wind across a front at the surface

40. Convergence and Divergence

41. Coastal Convergence/Divergence (NH)

42. 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°

43. Diurnal Variation of Surface Wind (NH) • Over oceans • Day/Night: • Speed about 70% of Geos Wind, (JAA exam) • Direction backed about 10°, (JAA exam)

44. Valley Wind H L E.g. The MISTRAL blowing down the R. Rhone Valley in S. France mainly during Winter and early Spring.

45. Katabatic Winds

46. Katabatic Wind e.g. The BORA blowing down the mountain valleys into the N. Adriatic during winter mainly and early spring.

47. Anabatic Wind Gentle up-slope wind blowing in the early morning on mountain slopes facing into the rising sun.

48. Sea Breeze SHORT WAVE RADIATION • 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. Warming FIRST MOVEMENT OF AIR PRESSURE LEVELS INCREASE IN HEIGHT SEA BREEZE CIRCULATION ESTABLISHED

49. 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.

50. 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.