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The amount of water vapour in our atmosphere is less than
4%, however, it is very important because clouds, precipitation
and fog are all different forms of water. This being so, we
need a way to measure the amount of water vapour in the atmosphere.
There are various ways of expressing the humidity, depending
on the particular use. The dew point of a sample of air is
the temperature to which the sample must be cooled to make
it saturated. Hence, the closer the air temperature is to
the dew point, the nearer the air is to saturation. Dew point
does not vary a great deal within an air mass and is therefore
useful in weather forecasting for locating warm and cold fronts,
which are boundaries between different air masses.
Relative humidity (expressed as a percentage) is a measure
of the amount of water vapour in the air compared to the maximum
that could be contained by the air at the same temperature.
If the amount of water vapour in the air remains constant,
the relative humidity decreases (or increases) as the temperature
rises (or falls). 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.
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| How are relative humidity and dew-point temperature
measured? |
Relative humidity can be estimated using a hygrometer — a
device that uses the property of human hair to change its length
as it becomes drier or wetter. A continuous record of the relative
humidity can be obtained using a hair hygrograph — one of
these is usually found in the thermometer screen that meteorologists
use.
The length of human hair is stripped of all its natural oils and
is clamped between two rigid supports and kept taut by a loaded
hook at the middle. The movement of the hook is transmitted through
a lever and two quadrants to the pen arm — the quadrants acting
as a lever of varying length. The pen arm is fitted with a gate
suspension and the pen records the humidity on a chart wound on
a clock-driven drum — this may be done on a daily or weekly
basis.
As the relative humidity increases, so does the length of the
hair. However, the rate of increase in length is not uniform throughout
the range. For example, a change from 90 to 95% gives a much smaller
change in length than a change from 40 to 45%.
Hair hygrograph
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Hair hygrographs are not very accurate and should be used
only as a guide. For this reason 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. This thermometer measures
the wet-bulb temperature, which, unless the relative humidity
is very close to 100%, is generally lower than the dry-bulb
temperature. This is because the temperature is lowered by
the evaporation of water from the muslin bag.
The difference between the dry-bulb and wet-bulb temperatures
is called the wet-bulb depression. It is this, along with
the dry-bulb temperature, that enables meteorologists to calculate
the dew-point temperature and relative humidity using suitable
tables, a special slide-rule or a computer program.
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Wet- and dry-bulb thermometers
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