Series editors' preface - Wood Tools

climate. An important aspect of the control of
temperature is to ensure that the internal space
of the building is properly insulated from
external conditions. Excessive radiant heat
must be avoided and to this end both
spotlights and control gear carefully sited. The
temperature rise which can occur through
lighting depends on the illuminance, on the
spectral quality of the light (notably the
proportion of infra red) and on the colour of
the object (more for a black object than for a
white one). Halving the distance between the
light source and the object will result in four
times the amount of light falling on the surface
of the object (an example of the inverse
square law). Tungsten spotlights can be
obtained with a special ‘dichroic’ reflector
which allows infra red radiation to pass
through the back of the lamp so that only
visible radiation is directed at the object. A
standing person emits about the same amount
of heat as a 100 W light bulb. The effect of
different heat sources on the internal temperature
of buildings and the control of temperature
are further discussed by Thomson (1986)
and Cassar (1995). The effect of temperature
changes on relative humidity is of very great
importance.
6.2.4 Absolute humidity and relative
humidity
Absolute humidity is a measure of the actual
amount of moisture in the air and is expressed
as grams of water per cubic metre of air (g
water/m3 air). The higher the amount of water
vapour in the air, the higher the absolute
humidity. The amount of moisture (water
vapour) the air can hold before becoming
saturated varies with temperature (Figure
6.3a). In other words, warm air can hold more
moisture than cold air. Relative humidity (RH)
measures the amount of moisture in the air,
relative to the amount of moisture the air
could hold at a given temperature and is
expressed as a percentage.
Relative humidity may be defined as:
Amount of water in a given amount
of air at a given temperature
100%
Maximum amount of water that
amount of air can hold at that
temperature
General review of environment and deterioration 253
(a)
(b)
Figure 6.3 Relationship between humidity and
temperature
(a) Graphical representation of the relationship between
absolute humidity and temperature.
The amount of moisture the air can hold before
becoming saturated is dependent on temperature. This
graph illustrates that the warmer the air, the more
moisture it can hold. The line plotted on this graph
represents the maximum amount of moisture air can
hold at a range of temperatures. Above the line, the air
is saturated, whilst below the line it is not
(b) Representation of the relationship between relative
humidity and temperature. Whilst the amount of
moisture in the air may remain constant, changes in
temperature will affect the relative humidity. Thus, in
this example, air with an absolute humidity of 5 g/m 3
will have a relative humidity of about 40% at 10 °C. If
temperature increases to 20 °C, the same absolute
humidity will produce an RH of about 20%. Similarly,
air containing 10 g/m 3 of water at 22 °C will have an
RH of about 45%, but if the temperature falls below
12 °C the RH will rise to 100% and moisture will begin
condensing out of the air. Note that the figures in this
graph are approximate and should not be used for RH
calculations