Environmental Engineering Reference
In-Depth Information
5 Rate of air movement.
6 Atmospheric humidity.
When air temperatures are very low, thick clothing and/or a considerable degree of
activity is required to maintain the body in thermal equilibrium. If we have the added
factor of strong winds, the rate of heat loss from an exposed skin surface is even greater
and so the environment will 'feel' colder even though in terms of its temperature it may
not be different. To allow for the factor of wind speed, many attempts have been made to
produce an index which gives a measure of what the combination of temperature and
wind speed will feel like to the human body. Effective temperature, equivalent
temperature and subjective temperature have all been used to give an indication of the
combined effect of wind and temperature, though they all have limitations.
At high temperatures the main problem for the body is to lose heat rather than retain it.
If air temperatures are above 37° C the main mechanism for losing heat is through the
evaporation of sweat. Air movement will assist this process and so help to make it feel
cooler. This is why humidity becomes more significant at higher temperatures, as
evaporation will be reduced in humid air and so the capacity of the body to cool itself is
lower. Hard physical work, by increasing the metabolic rate, can lead to hyperthermia (or
heat stroke) unless the individual is acclimatized.
These elements of bioclimatology help to explain why certain climates feel better in
terms of human comfort than others. In tropical areas, low humidity and breezes will help
to counteract the heat input of high temperatures and so make the climate more
comfortable. Conversely, high humidity and no wind can be unpleasant when
accompanied by high temperatures, such as in the tropical rain forest. In cold climates,
strong winds will lead to greater chilling at the same temperature and so make the climate
feel more unpleasant. One of the worst locations for a combination of strong winds and
low temperatures is Cape Denison in Antarctica, where sustained katabatic winds in
winter give a monthly mean of 24·9 m s
−1
accompanied by mean temperatures of about
−17° C. Attempts have been made to map such 'physiological climates' so that there is a
more meaningful indication of what the climate may feel like rather than the basic
climatic figures alone.
