7/30/2019 Wind Effect on Structural 1
1/14
2. Wind characteristics
7/30/2019 Wind Effect on Structural 1
2/14
Nature of wind from meteorology
Meteorology: the science of the atmosphere and its phenomena;
the study of weather and climate
Micrometeorology: the study of detailed structure of atmospheric flow near
the ground, ex: the mean wind speed profile, the
atmospheric turbulence, etc. (some studies in
micrometeorology can be directly used by structural engineers)
Climatology: the prediction of wind conditions at a given geographical
location, ex: extreme wind climatology
7/30/2019 Wind Effect on Structural 1
3/14
Temperature of atmosphere
The earths heat balance
[Source: http://clivebest.com/blog/?p=4432]
Sun6000K
original energy source
short wave
atmosphere istransparent
Earth
long wave: absorbed by atmosphere
Earth 250K
The earth is the direct energy source of wind
7/30/2019 Wind Effect on Structural 1
4/14
A B
1
2
A B
1
2
A B
heating
1
2
A B
heating
1
2
A B
heating
1
2
A B
heating
1
2
A B
heating
1
2
A B
heating
1
2
A B
heating
1
2
A B
heating
Tube 1 & 2 are closed
tA: raised
tB
: maintained
Fluid in A expands
Tube 1 is closed, tube 2 is opened no flow
Humphreys experiment
Tube 1 is opened: flow A B
& tube 2 is opened: flow B A
7/30/2019 Wind Effect on Structural 1
5/14
Simple model of atmospheric circulation
AirHeating
Solar / terrestialWind = motion of air with respect to the earths surface,
fundamentally caused by variable solar heating of the earths atmosphere
[Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter10/single_cell.html]
A = column of air above equator
B = column of air above pole
Cold
Hot
Equator
PoleGeneral circulation
7/30/2019 Wind Effect on Structural 1
6/14
Atmospheric hydrodynamics
The motion of an elementary mass of air is determined by the Newtons second
law:
The forces F include:
(1) The horizontal pressure gradient
(2) The Coriolis force
(3) The centrifugal force
(4) The friction force
aF m
7/30/2019 Wind Effect on Structural 1
7/14
1. The horizontal pressure gradient:
Vertical: main pressure, decrease with height
Changes in pressure gradient
Horizontal: changes are small wrt vertical
Wind direction: high pressure low pressure (isobar)
2. The Coriolis force (apparent force)
v : the velocity vector of the mass
: the angular velocity vector of the earth
Pressure gradient & Coriolis
vF mc 2
7/30/2019 Wind Effect on Structural 1
8/14
3. The centrifugal force:
r : radius of curvature
4. The friction force
The surface of the earth retards the
flow of air by the effects of friction
Boundary layer of the atmostphere
r
vmF
2
Centrifugal & Friction
v
r
F
d: boundary layer depth
free atmosphere (frictionless wind)
vgr : gradient wind speed
earth surface
7/30/2019 Wind Effect on Structural 1
9/14
Horse
latitude
Tricellular model of atmospheric circulation
Considering: earth rotation/friction, seasons, geographic Tricellular meridional circulations
7/30/2019 Wind Effect on Structural 1
10/14
Secondary circulations
Traveling High and Low
pressure areas,
by heating or cooling of
the lower atmosphere
7/30/2019 Wind Effect on Structural 1
11/14
Wind directions
Mean pressure systems and representative wind vectors at the surface for (a) January and (b) July.
The red dashed lines represent the equatorial trough (and monsoon trough over Asia).[Source: http://www.goes-r.gov]
The motion of air
relative to the earth
surface is determined,
in unaccelerated flow,
by the balance among
these forces
7/30/2019 Wind Effect on Structural 1
12/14
Moonsoons & Hurricanes
Tropical cyclones with surface wind speed >120km/h
Diameter is in the order of 100km
Depth is in the order of 10km
Hurricane Gladys as seen by the Apollo crew
[Source: http://www.astronautix.com/flights/apollo7.htm]
Moonsoons: seasonal reversing winds associated
with asymmetric heating of land and sea
(summer: ocean to land, with heavy rains)
[Source: http://www.taringa.net]
7/30/2019 Wind Effect on Structural 1
13/14
Tall convective clouds,
produced by upward motion of warm, moist air
Local wind
Tornado
[Source: http://commons.wikimedia.org/wiki/File:F5_tornado_Elie_Manitoba_2007.jpg]
Dry, warm, down-slope wind / leeward of a mountain range
Foehn wind
[Source: http://fineartamerica.com/featured/foehn-wind-artwork-gary-hincks.html]
Most powerful wind
A vortex of air: ~300m diameter, 30-100 km/h
Thunderstorm [Source: Wind effects on structures, Simiu, p.29]
Small-scale / negligible influence on general circulation
7/30/2019 Wind Effect on Structural 1
14/14
The Beaufort scale
Degree Wind speed
(m/s)
Description of wind Wind effects
0 0 0.5 Calm
1 0.5 1.7 Light airs Smoke rises nearly vertically
2 1.7 3.3 Light breeze Wind felt on face
3 3.3 5.2 Gentle breeze Leaves move
4 5.2 7.4 Moderate breeze Small branches move, wind extends flags
5 7.4 9.8 Fresh breeze Large branches also move
6 9.8 12.4 Strong breeze Saplings move
7 12.4 15.2 Light gale Strong branches bend, wind is heard in buildings
8 15.2 18.2 Moderate gale Impeded walking, branches break, tree-trunks sway
9 18.2 21.5 Fresh gale Wind displaces lighter objects, strong branches and
saplings break, damage to chimneys
10 21.5 25.1 Strong gale Trees are uprooted, telegraph poles break
11 25.1 29 Destructive gale Severe damage
12 29 and over Hurricane Disastrous damage