Lecture 8 Notes
Outline of this lecture
• Part I Measuring Wind
– Wind Chill
• Recording Wind
– Wind rose
• Sound of wind
• Part II Wind Strength Scales
• Part III Special Focus: Mach Speeds and Sonic Booms
Part 1 Measuring Wind
Anemometers are instruments used to measure wind speed. Weather vanes or
wind vanes are used to measure wind direction. There are at least seven types of
anemometers. These anemometers are primarily used to measure wind at or near
the surface of the earth. Higher level winds are measured using radiosonde
(weather balloons) or deduced from pressure distribution. Upper level winds are
geostrophic and thus the knowledge or the pressure gradient enables us to
calculate the wind speed and direction.
The first anemometer was invented by Leon Battista Alberti in Italy in 1450. It
was a deflection anemometer. He linked wind speed to the deflection of a
horizontal plate. He hooked the plate to a spring and measured the compression of
the spring. Leonardo da Vinci created a similar anemometer a few decades later.
A century later, Robert Hooke of England created the widely used Hooke
In 1774 James Lind developed a tube anemometer. It was a U tube in which wind
blows into and causes liquid to move up and down in a tube. It measures the force
of the wind relative to the force of gravity; the higher the wind speed the greater
amount of displaced liquid. It works on the same principle (hydrostatic balance) as the barometer. A chief advantage of this device was its low maintenance
A commonly used pressure anemometer was invented in 1892 by William Henry
Dines of England.
The cup anemometer was inoented in 1846 by Thomas R. Robinson of England. It
consisted of four cups (90 apart). The cup speed (rotation rate) was
approximately 1/3 of the wind speed. This fraction varied with cup shape and arm
length. An advantage of the cup anemometer is that it doesn’t need to be
reoriented as the wind direction changes.
The three cup anemometer was created in 1926 by Canadian John Patterson. It
proved to be more accurate, usually within 3% up to 100 km/h.
d) Wind mill
The windmill anemometer uses a propeller rather than a cup to gauge the wind
speed. An aerovane is used to insure the anemometer is pointing into the wind. It
is commonly used today.
In a thermoelectric anemometer a wire is heated to above the ambient
temperature. The rate of cooling of this wire is proportional to the wind speed.
Higher speed winds cool objects more quickly. The instrument has the advantage
of no moving parts.
f) Laser Radar
The laser radar anemometer uses the Doppler principle to measure wind speed.
The Doppler principle explains how sound waves from object moving towards us
have higher pitch (higher frequency) than sound waves from objects moving away
from us (lower frequency). This explains the apparent change in pitch of train
whistles and emergency vehicle sirens as the vehicles travel past.
Laser radar anemometers use the same physics is employed to measure wind
speed. A fraction of the laser light reflects off of nearby air molecules; some of
this is returned to the anemometer (backscatter). The frequency of the
backscattered light indicates the speed of the air molecules (and therefore the
Sonic anemometers were invented by Dr. Andreas Pflitsch, a geologist, in 1994.
They measure wind speed by how sound waves are modified by moving air.
1.1.2 Measuring Upper Level winds
Upper level winds are determined through measurements from radiosondes
(weather balloons). Radiosondes are launched from meteorological stations around
the world simultaneously twice each day, at 0:00 and 12:00 UTC (Universal
Coordinated Time). This provides what meteorologists call a synoptic observation
of the atmosphere (synaptic in this sense meaning simultaneous). Radiosonde
information can be supplemented by dropsondes, which are instrument packages
dropped from airplanes. Radiosondes travel up through the troposphere and much of the stratosphere,
generally bursting at around 30 km in altitude. They are often capable of
measuring the wind speed directly, but more importantly they record changes in
pressure at specific altitudes. Knowledge of the horizontal pressure gradients
between radiosonde locations allows meteorologists to calculate the strength and
direction of upper level winds.
Upper level winds are geostrophic (refer to Lecture 2); the effect of friction on
these winds is negligible. This means that knowledge of the pressure gradient
together with latitude (which is important for the Coriolis force) is sufficient
information to calculate the strength of the wind; and it always flows 90 to the
right of the pressure gradient force.
1.2 Wind Chill
How cold people feel depends on both temperature and wind speed. The air is
colder than a person’s body, and so it carries away head from conduction. In warm
weather it is common to get relief from the heat by using a fan. How cold we feel in
cold weather depends on the same principle and is called wind chill.
Wind chill is a measure of how cold it feels and is in essence a measure of heat
loss. It is a combination of temperature and wind speed and is often reported as an
equivalent temperature. The temperature does not decrease because of the wind,
but rather the wind increases the heat loss. Thus the equivalent temperature is the
temperature at which we would experience the sa