Measuring the weather

Measuring with tools

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How can we measure the weather?
The science of studying weather is called meteorology. Weather scientists
or meteorologists measure temperature, rainfall, air pressure, humidity (the
amount of moisture in the air), sunshine and cloudiness, and they make
predictions and forecasts about what the weather will do in the future. This
is important for giving people advance notice of severe weather such as
floods and hurricanes.
Anemometer ­ Wind Speed
An anemometer is a device for measuring wind speed. Anemometers can be
divided into two classes: those that measure the velocity of the wind, and
those that measure the pressure of the wind. The simplest type of
anemometer is the cup anemometer. It consisted of four hemispherical cups
each mounted on one end of four horizontal arms, which in turn were
mounted at equal angles to each other on a vertical shaft. The air flow past
the cups in any horizontal direction turned the cups in a manner that was
proportional to the wind speed. Therefore counting the turns of the cups
over a set time period produced the average wind speed for a wide range of
speeds. On an anemometer with four cups it is easy to see that since the
cups are arranged symmetrically on the end of the arms, the wind always
has the hollow of one cup presented to it and is blowing on the back of the
cup on the opposite end of the cross. Studies of the forces involved show
that the force is greater on the cup side of the device and the net force
causes the cups to spin, but in this case the balance of forces is not obvious.
Becky McCartney L67

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Sonic anemometer ­ Wind Speed
Sonic anemometers, first developed in the 1970s,
use ultrasonic sound waves to measure wind speed and direction. They
measure wind velocity based on the time of flight of sonic pulses between
pairs of transducers. Measurements from two or three pairs of transducers
can be combined to yield a measurement of 2- or 3-dimensional flow. The
spatial resolution is given by the path length between transducers, which is
typically 10 to 20 cm.…read more

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Daily rainfall amounts in the UK range from zero to (very rarely) 100 mm,
but are most frequently in the range 0 to 10 mm. If it is warm with bright
sunshine, dry air and a strong wind, it is possible to evaporate away 10 mm
of water in a day.…read more

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A normal diurnal range may be from 95%
around dawn to about 60% in the afternoon. Values below 40% are unusual,
and only rarely do they fall below 10% in this country. In fact, values of
10% are generally common over deserts during the day.
Hygrometer ­ Relative Humidity
A device that uses the property of human hair to change its length as it
becomes drier or wetter.…read more

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Hair hygrographs are meteorologists use a method that involves reading a
pair of thermometers known as a 'wet- and dry-bulb psychrometer'. The
dry bulb is a glass thermometer that registers the actual air temperature.
The wet bulb is a similar thermometer but with the bulb enclosed in a
muslin bag and secured with a string that dips into a bottle of distilled
water to keep the muslin bag damp.…read more

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Water-based barometers
This concept of "decreasing atmospheric pressure predicts stormy
weather" was invented by Lucien Vidie and is the basis for a basic weather
prediction device called a weather glass or thunder glass. It can also be
called a "storm glass" or a "Goethe barometer" (the writer Goethe
popularized it in Germany). It consists of a glass container with a sealed
body, half filled with water.…read more


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