About air temperature,
relative humidity and rain
Measurements
The Energy Studies Weather Station measures temperature and relative
humidity using a Vaisala HMP45A Temperature and Humidity Probe (pictured
below). Temperature measurement is based on the changing electrical
resistance of a platinum sensor. Humidity measurement is based on
the electrical capacitance of a thin polymer film. The sensors are
contained in an enclosure that allows air to freely pass through
while sheltering it from direct sunlight and high winds that might
affect the reading.
Measurements from these sensors are taken every twenty seconds
and averaged to give a single reading for each sensor every five
minutes.
The amount of rainfall is measured with a ‘tipping-bucket’
rain gauge (pictured bottom of page). A funnel collects rain and
drains it out a small hole. The trickle of rain fills a small cup
beneath, which tips and drains when it receives a certain volume
of water, moving an identical cup under the stream. These two cups
tip back and forth, each movement closing a circuit and indicating
0.1 mm of rain has fallen.
Interpretation - Temperature
Although air temperature is intuitively simple to understand, there
are a number of factors to bear in mind.
The influence of solar radiation may make the perceived temperature
much greater than the actual ambient air temperature. Energy from
the sun is transmitted as electromagnetic radiation at a wide range
of wavelengths at various intensities. Although the atmosphere absorbs
some sunlight resulting in its direct heating, most of it reaches
the Earth’s surface. A person outside on a clear day will
absorb this solar energy directly, and hence may mistake the sun’s
radiative heat for an elevated air temperature. (See
the solar energy page for more information)
The influence of wind flow - the movement of air increases the
effect of convection - a mechanism of heat transfer. Wind flow over
a person’s body will increase convective heat loss (provided
the air temperature is cooler than their surface temperature) and
this increased heat loss can be mistaken for a lower air temperature.
The air temperature required to create the same heat loss in still
air compared to a real situation with moving air is known as ‘wind-chill
factor’. For instance, if the air temperature is 10 °C
and the wind speed is 30kph, the wind chill factor is approximately
1 °C.
Interpretation - Humidity
Humidity is a measurement of the amount of water vapour in the
air. Readings on the display are given as ‘relative humidity’
(RH). This gives humidity as a percentage of the maximum amount
of water vapour the air can carry at its current temperature. Air’s
water vapour carrying capacity increases with increasing temperature,
and decreases with decreasing temperature. This is why on the display
graph temperature and RH mirror each other’s trends. If the
air temperature drops, its carrying capacity is reduced. So while
the actual amount of moisture in the air may have remained constant,
it is a larger percentage of the maximum. Hence an increase in RH
is observed.
Things to look for
- Temperature decreasing overnight, and sharply increasing in
the morning when the sun rises.
- Sharp decreases in temperature coinciding with sharp decreases
in pressure and changes in wind-direction – this indicates
a cold frontal system has arrived.
- 100% relative humidity – if the air is still, this indicates
the formation of fog.
- Sub-zero temperatures and still air will normally cause a frost
Interpretation – Rain
Five minute total rainfall readings are given on the graph. It
should be easy to observe if it has been raining steadily (similar
sized bars, evenly distributed) or if it has been raining in violent
squalls (sharp spikes).
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