What is Wind?
Wind is simply moving air. The air moves due to a
combination of solar energy as well as earth’s topography and
rotation. It works like this: The sun heats the atmosphere causing
warm air to expand and rise up while cold air condenses and sinks
creating the flow of air. The result: wind.
However, if the earth was smooth like a billiard
ball and did not rotate, the wind would simply circulate from the
equator to the poles and back again. But wind flow patterns depend
on the geography of the earth. Everything on the surface from lakes
and forests to the terrain and rotation of the earth impact these
wind patterns.
Wind actually refers to the horizontal motion of
airflow including direction and speed. Wind direction indicates the
direction from which the wind is blowing and is either recorded as
one of the 16 points of the compass (N, NE, etc.) or 360° with
reference to true north. Wind speed indicates the rate of the
motion of the air per unit of time and is usually measured in meter
per second (m/s), kilometer per hour (km/h) or knots
How Does Wind Form?
Horizontally, at the Earth's surface wind flows
from areas of high pressure to areas of low pressure. Vertically,
winds flow from areas of low pressure to areas of high
pressure.
Air is composed of minute solid material and as a
result has mass. On the earth, gravity has a force of acceleration
of about 9.8 meters per second. As a result of this force, the
speed of any object (including air particles) falling towards the
surface of the Earth accelerates. On the other hand, the density
and pressure of air decrease exponentially as one moves away from
the surface of the earth. This gravitational force on air particles
results in air pressure.
Once the pressure gradient force causes air to
move, it deflects from its path. This is called the "Coriolis
force" and is a result of the earth's rotation. Since the earth is
rotating the air does not flow directly from high to low pressure.
It is deflected to the right in the Northern Hemisphere and to the
left in the Southern Hemisphere. The amount of deflection the air
makes is directly related to both wind speed its latitude.
Therefore, slowly blowing winds will be deflected only a small
amount, while stronger winds will be deflected more. In the same
manner, winds blowing closer to the poles will be deflected more
than winds at the same speed closer to the equator. The Coriolis
force is zero right at the equator.
Thus, wind results from a combination of gravity,
differences in air pressure, the rotation of the earth, and the
terrain on the surface.
A really neat activity to demonstrate how wind
forms can be found atThe Franklin
Institute.
How Does Terrain Effect Wind?
Local winds decrease by interaction with obstacles
in the terrain. This can be measured with wind shear which is the
change in wind speed as elevation changes. The wind shear is
effected by a variable called surface roughness length: the height
above ground level where the wind speed is theoretically zero. In
general, the more pronounced the roughness of the earth's surface,
the more the wind will be slowed down. For example, water surfaces
are smoother than concrete runways, and will have less influence on
the wind, while long grass and shrubs and bushes will slow the wind
down considerably.
Another variable in determining wind shear is the
power law exponent which is related to wind speed and surface
roughness length. It is generally accepted that as terrain
complexities increase, the wind shear also increases, and as wind
shear increases, friction between the wind and the ground becomes
greater. The influence from the terrain contours is referred to as
the orography of the area.
In short, the smoother the terrain, the less interference it has
on wind. However some elevation changes are needed to produce the
differences in air pressure to produce the wind to begin
with.
How Does Wind Effect Terrain?
Eolian processes refers to the activity of the winds
and their effect on landforms. Some of the mechanisms winds use to
modify our landscape are: erosion, abrasion, transportation, and
sediment deposition.
Wind erodes the Earth's surface by deflation.
This removes loose fine grained particles by blowing and grinding
surfaces much like a sand blaster. In deserts, the rock mantle in
the desert pavement, a sheetlike surface of rock fragments,
protects the underlying material from deflation.
The fine grained particles carried in the wind
creates grooves or small depressions by abrasion. Ventifacts are
rocks which have been cut, and sometimes polished, by the abrasive
action of wind.
Particles can be transported by winds through
suspension, saltation, and creep. The three methods differ in how
long the small particles are held in the air and how far above
ground they are lifted by the wind.
Wind can deposit small grained particles in sand
sheets, ripples, or dunes. Sand sheets are flat with gentle waves
of sand with by grains that are larger than 0.2 millimeters in
diameter. Ripples create a surface of crests and troughs whose long
axes are perpendicular to the direction of the wind. Finally, a
dune is an accumulation of sediment blown by the wind into a mound
or ridge.
Where Is The Best Wind in
Illinois?
In Illinois, there are five wind zones which are
identified as either a class 3+ (near good) or class 4 (good) for
harnessing wind energy. These are located near Quincy, Bloomington,
Peoria, Mattoon, and between Sterling and Aurora. The areas have
average wind speeds at 50 meters above ground of about 16 mph. In
particular, the McLean County area provides some of the best wind
in Illinois. (Of course we already know that!) Although it appears
so, this area is not completely flat. There are subtle hills and
slopes in the Bloomington Moraine, one of the four largest moraines
in the state. This area of glacial debris was formed between
14,0000 and 25,000 years ago and the elevation ranges between 650
to 900 feet above sea level. A combination of topography and the
lack of obstructions like tall buildings make this an ideal spot
for wind farming.
How Can We Use Wind?
Think of a windmill or wind turbine as a fan working
in reverse. Instead of using electricity to turn the fan, the “fan”
or wind turbine turns in order to generate electricity. The wind
turns turbine blades, which spins a shaft that is connected to a
generator. In the Bloomington Moraine 240 wind turbine generators
have been installed, each with a capacity of 1.65 megawatts that
yield about 1.3 billion kilowatt-hours annually. These turbines
have blades with a diameter of approximately 82 meters and are
attached to towers 60 - 80 meters above the ground.
How Much Wind?
The wind turbines can produce 1650 kW in winds of 13
m/s (about 29 mph) with the blades turning at 14.4 rpm. The wind
must have a speed of at least 3.5 m/s in order for the turbines to
turn and the brakes shut down the turbine at speeds of about 32m/s.
You can check the table above once you find the wind speed from
NOAA
to see what the actual energy output was during your
visit.
Your
Assignment:
Note: If any of the following
requirements are not completed your log will be deleted.
Email me the answers to these questions (do not post
these answers):
- How does the sun create wind?
- How does the pressure gradient force effect wind speed?
- What is the Coriolis Effect?
- What is surface roughness length?
- What would you estimate the surface roughness length to be at
the posted coords?
- Which Eolian Process would be at work at the posted
coords?
- How was the topography at the posted coords originally
formed?
- Based on your calculated rpm of the turbine, was it producing:
no energy, less than 1650kW or at 1650 kW during your
visit?
In your log post these items:
- Post a picture of yourself with your gps with the turbine in
the background
- List the wind condition nearest the time of your visit using
NOAA
sources:
US Dept of Energy Illinois
Wind Illinois Wind
Working Group
US Dept of Energy
American Wind Energy Association
Horizon Wind Energy Danish Wind Industry Association Vestas NOAA
weather
questions The
Encyclopedia of Earth
Analysis of wind shear models and trends in different terrains
USGS