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 Scale
0
1 Light Air
Or just sufficient to give steerage way.
2 Light Breeze
Or that in which a man-of-war 1 to 2 knots
3 Gentle Breeze with
3 to 4 knots
all sail set, and clean full
Moderate
would go
4
5 to 6 knots
Breeze
in smooth water from.
5 Fresh Breeze
Royals, &c.
6 Strong Breeze Or that to which a wellconditioned
7 Moderate Gale man-of-war could just carry in
chase,
8 Fresh Gale
full and by.
9 Strong Gale
Single-reefed topsails and
top-gal. sail
Double reefed topsails, jib, &c.
Treble-reefed topsails &c.
Close-reefed topsails and
courses.
10 Whole Gale
Or that with which she could scarcely bear close-reefed maintopsail 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 Scale
Denomination of the wind
Wind speed (V nn)
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
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
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
Low
P
1000
1004
Co
1008
High
Balanced Geostrophic flow
•
•
•
•
•
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
1000
Low
P
Vg
1004
1008
High
Co
Surface wind flow
Low
P
•
•
•
•
•
•
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
V
F
1004
Vg
Co
1008
1012
High
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)
L
15
0
H
L
40 0
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
P
Ce
Co
High
Vgr
Vg
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
Low
P
Ce
Co
High
Vg
Vgr
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
1000
•
•
•
•
•
•
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
1001
-1
1002
-2
Low
1003
1004
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 charts4hPa 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
High
X 1005
X 1002
Low
X 997
X 999
X 998
x1002
X 1008
X 1008
X 1013
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 air
At 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 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
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
Channelling
Gaps in barrier strengthen wind flow
e.g. Mistral (between Alps & Massif Centrale)
Katabatic wind
• Down-slope wind, usually nocturnal
Cooling
• Speed: a few knots
• Depth: typically ~100 m
• Best on even, gentle slopes
Anabatic wind
• Day-time up-slope wind
• Speed: 5–10 knots
• Depth: up to 200 m
• Best on smooth, hot slopes
Heating
Föhn / Chinook winds
Condensation &
release of latent
heat
Warm
Cool
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
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
Light
Moderate
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
Heavy
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 Showers
Fronts -give widespread rain
•Warm
•Cold
•Occlusion
Showers - small Scale
•20km
•last 10-20mins
•Convective •develop over warm sea in winter
Forced Ascent
•Air forced to rise
•Stratus cloud forms on
higher ground
•Drizzle or rain likely
Convection - creates instability
Cooler
Warm air rising
Hot
Warm
Air in contact with high
ground is warmer than
free air at the same
height.
Cooler
Warm air rising
Hot
Convection
Showers and
thunderstorms
Warm
Cooler
Hot
Cooler
Hot
Orographic Rainfall
• 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
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
2002
Cherrapunji
Rainfall (mm)
12,262
Mawsynram
Rainfall (mm)
11,300
2001
9,071
10,765
2000
11,221
13,561
1999
12,503
13,444
1998
14,536
16,090