2. Measuring Wind A confusion of units! –Beaufort Forces –Knots (Nautical Miles per hour) –Miles per hour –Kilometres per hour –Metres per second

3. Windy or Calm? Admiral Francis Beaufort Born in Navan Hydrographer to the Royal Navy Devised one of the first wind scales, from Force 0 to Force 12

4. Original Beaufort Scale

5. Modern Beaufort Scale

6. Beaufort Scale on Land

7. Beaufort Cartoon

8. Velocity conversions 0.6210.540.278 km/hmphKtsm/sBeaufort 21.21.10.61 42.52.21.11 53.12.71.41 63.73.21.72 85.04.32.22 106532 159843 20121164 25161374 30191684 352219105 402522116 452824136 503127146 553430157 603732177 654035188 704338198 754740218 805043229 855346249 9056492510 9559512610 10062542810 10565572911 11068593111 11571623211 12075653312 12578673512 13081703612 13584733813 14087763913 14590784013 15093814214 15596844314 16099864414 Useful site: http://www.crh.noaa.gov/pub/metcon.shtml

9. Thomas Romney Robinson

10. Robinson Cup Anemometer

11. William Henry Dines

12. Dines Pressure Tube Anemometer

13. Fundamentals of Wind Measured at 10m above the ground (Always be aware that Malin Head is much higher. Treat wind readings from oil platforms, ships etc with caution). Mean Speed – average over a ten-minute period Gust Speed – highest instantaneous wind speed Gusts normally do the damage!!

15. Wind Speed and Gusts lWind speed mentioned in marine observations, forecasts, and warnings is the average speed over a 10 minute interval. lWind gusts may be up to 70% higher than the average wind speed. lFor example, if the average wind speed is 25 knots, occasional gusts up to 40 knots can be expected, depending on stability of the air-mass.

16. Surface wind Speed: 1knot = 0. 514 m/s = 1. 15 mph Direction: Direction from which wind blows measured clockwise from true North A veer is a clockwise change A back is an anticlockwise change Mean speed is average over 10 minute period Gusts and lulls are rapid fluctuations due to obstacles and instability which are called turbulence Speed Time

17. Surface Weather Systems lWeather systems in the northern hemisphere generally move from west to east due to the earth’s rotation. Movement of tropical systems such as hurricanes are more variable. lIn the northern hemisphere, winds blow anti-clockwise around lows such as depressions, and clockwise around highs. lWhen the isobars (lines of equal pressure) become more closely spaced, then winds increase. That is, the closer the isobars over a particular area, the higher the wind speed.

18. This is the chart for Monday A typical weather chart

19. 1000 1004 1008 Low High P Geostrophic wind Typically the wind speed at 2000 feet / 600m Assume air parcel moves from rest P is pressure gradient force Co is Coriolis = 2 Ω SinΦ Co acts at right angles Co

20. Balance when P=Co, ie equal and opposite Vg is the Geostrophic wind Blows parallel to isobars in free atmosphere Forecasters measure Vg from scale Vg=1/Co grad P Balanced Geostrophic flow Low High P Co Vg 1000 1004 1008

21. Surface wind flow Note Vg=Vgr Near ground friction(F) reduces wind speed Co must reduce Balance upset Vectors realign so that P+Co=F V-the real wind is reduced and blows towards low pressure P F Co Vg High    L ow V

22. Surface wind flow Over the Sea V =2/3 V gr, and is backed approximately 15 degrees to the isobars(depending on stability) Over the Land V =1/2 V gr and is backed as much as 40 degrees to the isobars(depending on roughness of ground and stability) H L 15 0 40 0 L H

23. Cyclonic curved flow C e is centrifugal force due to circular motion Co must reduce to maintain balance V g must reduce to V gr which is the gradient wind Forecasters make correction for curvature to get V gr Example eye of a storm Low High Co P Vg Ce V gr

24. Anticyclonic curved flow C e acts in unison with P C o must increase to maintain balance V g must increase to V gr Forecaster makes correction for radius of curvature to get V gr Example periphery of a winter High High Low P CoCo CeCe V gr VgVg

25. Another complication ! A difference between curvature of isobars and trajectories occurs when systems in motion Strongest winds on south flank of east’wards moving depression Strongest winds on north flank of westwards moving depression Similar for mobile anticyclones L L

26. Thermal wind effects Heating modifies isobars Trough near lee side of island Veering of wind on exposed side Backing on lee side Strengthening on high pressure side Slackening on low pressure side 1003 1002 1000 1001 1004 -2 Low High

27. Thermal wind effects Pre-existing wind Modifying or thermal wind Resultant wind

28. Pressure and drawing of Isobars Plotted values are reduced to MSL Isobars join areas of equal pressure and are drawn with low pressure on their left (Buy’s Ballots Law) On large Atlantic charts- 4hPa intervals On hourly charts –1 hPa intervals A pascal =1 Pa = 1N/m2 A hecto Pascal = 100 Pa = 10 N /m2 100Pa = 1mb = 1 hPa X 997 X 999 X 1002 X 1013 X 1008 X 998 High X 1005 Low X 1008 x1002

29. The Sea Breeze An onshore breeze which develops in coastal areas on a warm day. Differential heating between the land and sea.

30. Sea breeze formation Two columns of air At dawn:

31. Sea breeze formation As land heats up a circulation develops

32. How… and When? Land temperatures need to be at least 3.5 o C warmer than sea temperatures … They are very common and strong in tropical regions In Ireland generally from March to late September.

33. Land breeze Another thermally driven circulation. Sea warmer than land at night. Usually weaker than the sea breeze. Very rarely exceeds 10 kt.

34. It’s not just a coastal thing Sea breezes can occasionally penetrate over 50km inland Sea breezes can enhance convection due to convergence, particularly on peninsulas

35. Sea breeze front Offshore wind opposes sea breeze Enhanced convergence Tightening temperature and humidity gradients

36. Sea Breeze Summary Nice cooling breeze on the coast. Can bring in offshore stratus to spoil a sunny day right on the coast Useful for yachtsmen and inshore fishermen Enhanced convection can lead to some severe weather.

37. A good sea breeze day

38. Mountain Airflow Modification of broadscale winds Deflection, channelling and shelter Effect on depressions and fronts Lee waves Locally induced winds Katabatic and anabatic winds Valley wind circulations Downslope winds Föhn and Chinook winds Bora wind

39. Deflection Factors favouring deflection over mountain barrier: Long barrier Perpendicular wind flow Concave barrier Unstable air Factors favouring deflection around mountain barrier: Short barrier Oblique wind flow Convex barrier Stable air

40. Channelling Gaps in barrier strengthen wind flow e.g. Mistral (between Alps & Massif Centrale)

41. Katabatic wind Down-slope wind, usually nocturnal Speed: a few knots Depth: typically ~100 m Best on even, gentle slopes Cooling

42. Day-time up-slope wind Speed: 5–10 knots Depth: up to 200 m Best on smooth, hot slopes Anabatic wind Heating

43. Föhn / Chinook winds Cool Warm Condensation & release of latent heat

44. Fog, Rain, Drizzle and Showers Fog, Drizzle and Rain distinguished by DROP SIZE If droplets are suspended in the air (not falling) then we have FOG or MIST (drop size up to 0.2mm diameter) Falling droplets from 0.2mm to 0.5mm are termed DRIZZLE Drops of greater size constitute RAIN

45. Rain and Drizzle Rates RAINLightModerateHeavy Intermittent< 2.0 mm/hr2.0-6.0 mm/hr>6.0 mm/hr Continuous< 2.0 mm/hr2.0-6.0 mm/hr>6.0 mm/hr DRIZZLELightModerateHeavy Intermittent< 0.3 mm/hr0.3-0.5 mm/hr>0.5 mm/hr Continuous< 0.3 mm/hr0.3-0.5 mm/hr>0.5 mm/hr

46. Rain and Showers Rain –Primarily large geographical scale –Origin in dynamical processes Showers –Small spatial scale (500m – 20Km) –Convective in origin –Much higher rates of rainfall –Can be embedded in larger scale rain bands

47. Fronts Versus Showers Showers - small Scale 20km last 10-20mins Convective - develop over warm sea in winter Fronts -give widespread rain Warm Cold Occlusion

48. Air forced to rise Stratus cloud forms on higher ground Drizzle or rain likely Forced Ascent

49. Convection - creates instability Warm Cooler Air in contact with high ground is warmer than free air at the same height. Warm air rising Hot

50. Convection Hot Warm Cooler Showers and thunderstorms

51. There is a clear statistical link between average rainfall and altitude. The higher the site, the heavier the rainfall. Mechanisms leading to the increase. –Forced Ascent –Enhanced Convection Orographic Rainfall

52. Irish Rainfall Rates Range from about 800mm/yr (Dublin) to about 3000mm/yr (Kerry Mountains) Very variable in nature Greatest rainfall totals: –Hourly 97mm Co. Antrim 1887 –Daily 243.5mm Co. Kerry 1993 –Monthly 790mm Co. Waterford 1996 Hourly totals of > 10mm are uncommon in Ireland

53. Worlds Highest Rainfall YearCherrapunji Rainfall (mm) Mawsynram Rainfall (mm) 200212,26211,300 20019,07110,765 200011,22113,561 199912,50313,444 199814,53616,090