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.
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
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. |
Wet- and
dry-bulb thermometers |
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