Post on 03-Jan-2016
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Measuring Wind
• A confusion of units!– Beaufort Forces
– Knots (Nautical Miles per hour)
– Miles per hour
– Kilometres per hour
– Metres per second
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
Original Beaufort Scale0
1 Light Air Or just sufficient to give steerage way.
2 Light Breeze 1 to 2 knots
3 Gentle Breeze 3 to 4 knots
4 Moderate Breeze
Or that in which a man-of-war with all sail set, and clean full would go in smooth water from.
5 to 6 knots
5 Fresh Breeze Royals, &c.
6 Strong Breeze Single-reefed topsails and top-gal. sail
7 Moderate Gale Double reefed topsails, jib, &c.
8 Fresh Gale Treble-reefed topsails &c.
9 Strong Gale
Or that to which a well-conditioned man-of-war could just carry in chase, full and by.
Close-reefed topsails and courses.
10 Whole Gale Or that with which she could scarcely bear close-reefed main-topsail and reefed fore-sail.
11 Storm Or that which would reduce her to storm staysails.
12 Hurricane Or that which no canvas could withstand.
Modern Beaufort ScaleDenomination of the wind Wind speed (V nn) Force
(n) English French (mph) (km/h)
0 Calm Calme 0 to 0.6 0 to 1
1 Light air Très légère brise 0.7 to 3 2 to 5
2 Light breeze Légère brise 4 to 7 6 to 11
3 Gentle breeze Petite brise 8 to 12 12 to 19
4 Moderate breeze Jolie brise 13 to 17 20 to 28
5 Fresh breeze Bonne brise 18 to 24 29 to 38
6 Strong breeze Vent frais 25 to 31 39 to 49
7 Near gale, moderate gale Grand frais 32 to 38 50 to 61
8 Gale, fresh gale Coup de vent 39 to 46 62 to 74
9 Strong gale Fort coup de vent 47 to 54 75 to 88
10 Storm, whole gale Tempête 55 to 63 89 to 102
11 (Violent) storm Violente tempête 64 to 72 103 to 117
12 Hurricane Ouragan over 73 over 118
Beaufort Scale on Land
Beaufort Cartoon
Velocity conversions
0.621 0.54 0.278
km/h mph Kts m/s Beaufort
2 1.2 1.1 0.6 1
4 2.5 2.2 1.1 1
5 3.1 2.7 1.4 1
6 3.7 3.2 1.7 2
8 5.0 4.3 2.2 2
10 6 5 3 2
15 9 8 4 3
20 12 11 6 4
25 16 13 7 4
30 19 16 8 4
35 22 19 10 5
40 25 22 11 6
45 28 24 13 6
50 31 27 14 6
55 34 30 15 7
60 37 32 17 7
65 40 35 18 8
70 43 38 19 8
75 47 40 21 8
80 50 43 22 9
85 53 46 24 9
90 56 49 25 10
95 59 51 26 10
100 62 54 28 10
105 65 57 29 11
110 68 59 31 11
115 71 62 32 11
120 75 65 33 12
125 78 67 35 12
130 81 70 36 12
135 84 73 38 13
140 87 76 39 13
145 90 78 40 13
150 93 81 42 14
155 96 84 43 14
160 99 86 44 14 Useful site: http://www.crh.noaa.gov/pub/metcon.shtml
Thomas Romney Robinson
Robinson Cup Anemometer
William Henry Dines
Dines Pressure Tube Anemometer
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!!
Wind Speed and Gusts Wind speed mentioned in marine observations,
forecasts, and warnings is the average speed over a 10 minute interval.
Wind gusts may be up to 70% higher than the average wind speed.
For 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.
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
Surface Weather Systems Weather 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.
In the northern hemisphere, winds blow anti-clockwise around lows such as depressions, and clockwise around highs.
When 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.
This is the chart for Monday
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
• 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
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
FCo
Vg
High
Low
V
Surface wind flow
• Over the Sea V=2/3 Vgr, and is backed approximately 15 degrees to the isobars(depending on stability)
• Over the Land V =1/2 Vgr 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
Cyclonic curved flow
• Ce is centrifugal force due to circular motion
• Co must reduce to maintain balance
• Vg must reduce to Vgr which is the gradient wind
• Forecasters make correction for curvature to get Vgr
• Example eye of a storm
Low
HighCo
PVg
Ce
Vgr
Anticyclonic curved flow• Ce acts in unison with P• Co must increase to
maintain balance
• Vg must increase to Vgr
• Forecaster makes correction for radius of curvature to get Vgr
• Example periphery of a winter High
High
Low
P
Co
Ce
Vgr
Vg
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
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
-1 -2Low
High
Thermal wind effects
Pre-existing wind
Modifying or thermal wind
Resultant wind
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 1008x1002
The Sea Breeze
• An onshore breeze which develops in coastal areas on a warm day.
• Differential heating between the land and sea.
Sea breeze formation
Two columns of airAt dawn:
Sea breeze formation
As land heats up a circulation develops
How… and When?
• Land temperatures need to be at least 3.5 oC warmer than sea temperatures …
• They are very common and strong in tropical regions
• In Ireland generally from March to late September.
Land breeze
• Another thermally driven circulation.
• Sea warmer than land at night.
• Usually weaker than the sea breeze.
• Very rarely exceeds 10 kt.
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
Sea breeze front
• Offshore wind opposes sea breeze
• Enhanced convergence
• Tightening temperature and humidity gradients
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.
A good sea breeze day
Mountain Airflow
• Modification of broadscale windsDeflection, channelling and shelter Effect on depressions and fronts
• Lee waves
• Locally induced winds Katabatic and anabatic windsValley wind circulations
• Downslope winds Föhn and Chinook windsBora wind
Deflection• Factors favouring deflection
over mountain barrier: Long barrierPerpendicular wind flow Concave barrierUnstable air
• Factors favouring deflection around mountain barrier: Short barrierOblique wind flowConvex barrier Stable air
Channelling
Gaps in barrier strengthen wind flowe.g. Mistral (between Alps & Massif Centrale)
Katabatic wind• Down-slope wind, usually nocturnal
• Speed: a few knots
• Depth: typically ~100 m
• Best on even, gentle slopes
Cooling
• Day-time up-slope wind
• Speed: 5–10 knots
• Depth: up to 200 m
• Best on smooth, hot slopes
Anabatic wind
Heating
Föhn / Chinook winds
CoolWarm
Condensation & release of latent
heat
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
Rain and Drizzle Rates
RAIN Light Moderate Heavy
Intermittent < 2.0 mm/hr 2.0-6.0 mm/hr >6.0 mm/hr
Continuous < 2.0 mm/hr 2.0-6.0 mm/hr >6.0 mm/hr
DRIZZLE Light Moderate Heavy
Intermittent < 0.3 mm/hr 0.3-0.5 mm/hr >0.5 mm/hr
Continuous < 0.3 mm/hr 0.3-0.5 mm/hr >0.5 mm/hr
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
Fronts Versus ShowersShowers - small Scale
•20km•last 10-20mins•Convective -
•develop over warm sea in winter
Fronts -give widespread rain
•Warm
•Cold
•Occlusion
•Air forced to rise•Stratus cloud forms on higher ground•Drizzle or rain likely
Forced Ascent
Convection - creates instability
WarmCooler
Cooler
Air in contact with high ground is warmer than
free air at the same
height.
Warm air rising Warm air rising
Hot Hot
Convection
Hot Hot
Warm
Cooler Cooler
Showers and thunderstorms
• 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
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
Worlds Highest Rainfall
Year Cherrapunji Rainfall (mm)
Mawsynram Rainfall (mm)
2002 12,262 11,300
2001 9,071 10,765
2000 11,221 13,561
1999 12,503 13,444
1998 14,536 16,090