Small Wind Site Assessment
Learner Guide
Produced by the Institute for Sustainable Futures; UTS in partnership
with the Alternative Technology Association and TAFE NSW Northern Sydney Institute
Supported by the NSW Government as part of the
Energy Efficiency Training Program — visit savepower.nsw.gov.au
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GLOBAL WINDS
Wind can be divided into two main categories; global and local. While global winds are important in
determining the prevailing winds in a large area, local climatic conditions may wield an influence on the
most common wind directions.
Global scale winds are caused by the differential heating of different parts of the planet: put simply, the
equator gets much more solar radiation than the poles, heating the air which then rises, leaving low
pressure areas behind. Air then flows towards these areas of low pressure.
About thirty degrees north and south
of the equator, the warm air from the
equator begins to cool and sink.
Between thirty degrees latitude and
the equator, most of the cooling
sinking air moves back to the
equator. The rest of the air flows
toward the poles. The air movements
toward the equator are called trade
winds; warm, steady breezes that
blow almost continuously.
Figure 1 - Global Insolation
The Coriolis effect curves the trade winds to the west in both hemispheres. The trade winds coming from
the south and the north meet near the equator. These converging trade winds produce general upward
winds as they are heated, so there are no steady surface winds. This area of calm is called the doldrums.
Between thirty and sixty degrees latitude, the
winds that move toward the poles appear to
curve to the east. Because winds are named
from the direction in which they originate, these
winds are called prevailing westerlies.
At about sixty degrees latitude in both
hemispheres, the prevailing westerlies join with
polar easterlies to reduce upward motion. The
polar easterlies form when the atmosphere
over the poles cools. This cool air then sinks
and spreads over the surface. As the air flows
away from the poles, it is turned to the west by
the Coriolis effect. Again, because these winds
begin in the east, they are called easterlies.
Figure 2 - Global winds
Australia has some excellent wind resources as a result of global winds. A strong wind stream from the
Indian Ocean provides a significant winds to the western and northern shores of Tasmania, south western
West Australia, and south western and eastern Victoria.
LOCAL WINDS
The combination of local winds and varying topographical features means wind is a very localised
resource, to the extent that for wind farm development on-site monitoring is essential to determine the
suitability of a particular site for energy production.
Wind direction is effected at a local level by a combination of topography, insolation and (near the coast)
temperature differences between the land and the sea.
Local winds are always superimposed upon the larger scale wind systems, i.e. the wind direction is
influenced by the sum of global and local effects. When larger scale winds are light, local winds may
dominate the wind patterns.
Coastal land will heat up quicker than the sea will during the day. The cooler offshore air then moves into
the land, creating a sea breeze. At night the land cools to a similar temperature to the sea and so there is
a land breeze (literally the opposite to a sea breeze) which is rarely as strong as a sea breeze .
Sea breezes in the Sydney area
(for example) in late spring or early
summer extend 80 to 160km inland
during the afternoon, but may
penetrate as far as 200 to 300km
by midnight under favourable
conditions.
A sea breeze can be identified by a
lower temperature and higher
humidity than the existing wind.
The process is affected by time of
day, prevailing weather patterns,
seasonal changes in temperature
and sea currents, coastline shape
and geographical features on land.
Figure 3 - Sea breezes (left)
Figure 4 - Valley and mountain
breezes (below)
Valley breezes occur when a slope heats up from the sun. The air heats and becomes lighter causing a
breeze to flow up the slope of the mountain. Mountain breezes occur under the opposite conditions
Other idiosyncratic features of local terrain also affect the wind localised wind conditions.
EXAMPLES OF LOCAL WINDS IN THE MELBOURNE AREA
On hot afternoons Melbourne's
bayside suburbs often experience
sea-breezes that do not penetrate
to suburbs further inland. As a
result, bayside maximum
temperatures are often lower than
the maximum recorded in the
centre of the city.
The diagram to the left
demonstrates the effect of the
onset of a sea-breeze on the
afternoon of 22 January 2001.
Temperatures are indicated
numerically, and wind direction is
indicated by the shaft of the 'windarrows'. A long barb on the shaft of
the arrow denotes a speed of 10
knots (about 5 m/s); a short barb is
5 knots (2.5 m/s); a combination of
one long and one short barb
denotes 15 knots (7.5 m/s).
Courtesy BOM
The ranges north
and east of
Melbourne help
contain the
’Melbourne Eddy’ (or
Spillane Eddy), a
circulation that under
certain conditions
can carry airborne
pollutants in a loop
from Melbourne's
industrial western
suburbs over the city
and to residential
areas to the east
and south-east,
before circling back
over Port Phillip Bay.
The 'Melbourne
Eddy' to the right is
made visible by low
lying cloud, and was
recorded by a
weather satellite in
1985.
WIND TYPES BY SCALE AND DURATION
THERMAL LAYERING AND TEMPERATURE INVERSIONS
Usually during the day, the air nearer the ground is warmer than the air above it (within the lower
atmosphere at least). For the most part this is because the atmosphere is heated from below by solar
radiation that has been absorbed by the earth. The average lapse rate (the rate at which the temperature
decreases with increase in altitude) is about 3.5oF per 1,000 ft, or about 1oC per 150m.
In this state, instantaneous wind speeds at higher levels (for example, 70 metre hub-height for a wind
turbine) can generally be projected with reasonable accuracy from lower level wind data (such as a 10
metre Automatic Weather Station).
Sometimes however, what is known as an inversion layer occurs, and this most often happens at night:
when solar radiation ceases and the surface, and with it the immediately overlying atmosphere, begins to
cool. This can result in a layer of cool, damp, heavy air, ‘trapped’ below a layer of warmer, dryer, lighter
air. The principal characteristic of an inversion layer is its static stability (warmer air naturally rises), and
nocturnal temperature inversions usually remain until after sunrise when they are dissipated by the
warming of air near the ground.
Temperature inversions
prevent cooler air at lower
level air from mixing with
the surrounding atmosphere,
and can cause or exacerbate
fog and (along with the
requisite atmospheric
pollution) smog.
Warmer, moderate to strong
winds at higher levels.
Cooler, light winds nearer
ground level.
The occurrence of thermal layering is one important reason that hub-height, on-site data monitoring is
required when assessing the wind resource for a wind farm.
Surface level temperature inversion commonly effects wind up to about 20 metres and so using 10 metre
data is likely to result in an underestimation of the power output of a large wind turbine. As the inversion
has an inconsistent shear effect on the wind, meaning there is no consistent correlation between the two,
no factor can be reliably applied to the source data to account for this inaccuracy.
Inversion layering is a type of thermal layering. Thermal layering can also be caused (on a much larger
scale and usually for longer durations than surface level temperature inversions) by a warmer air mass
moving over a cooler air mass. When this occurs at higher altitudes it can cause thunderstorms.
A higher altitude temperature inversion plays a significant part in the dynamics of the large high-pressure
systems depicted on weather maps. Descending currents of air near the centre of the high-pressure
system cause air at middle altitudes to become warmer than the surface air. Rising currents of cool air
lose their buoyancy and are thereby prevented from rising further when they reach the warmer, lighter air
in the upper layers of a temperature inversion.
Due to the extremely low surface temperatures, much of Antarctica is a near-permanent state of
temperature inversion.