Geoscience Reference
In-Depth Information
Human comfort and bioclimatology
APPLICATIONS
Climatology can be viewed as an abstract concept based on the synthesis of day-to-day values of meteorological
elements that affect that location. We can also examine the factors that influence this climate, as we do in this topic,
in order to explain the patterns of climate that Earth experiences. However, we can also look at the relationship
between climate and humans in an area known as bioclimatology. In this instance we consider the impact of climate
directly on the human body so that we can comment on the sensations that may be experienced by the average
individual from extremes of heat stroke and hypothermia.
The human body normally operates with a core temperature of 37
C. It tries to maintain this value through a balance
between heat gain and heat loss. We can think of these energy exchanges as a human energy budget in which
inputs and outputs to the body are evaluated. For an individual there is a metabolic heat input, dependent on level
of energy exertion, created chemically within the body. We can add to this any solar radiational inputs, which are
especially noticeable when the sun is shining. There will also be a net radiational exchange of long-wave radiation,
though this is usually negative from a warm body. Convection may give either a gain or a loss of energy to the body,
depending on external conditions, whilst evaporation normally leads to cooling through perspiration or loss of moisture
through breathing. For the core body temperature to remain stable there must be a balance between the gain and
loss of heat. In practice this is achieved most easily by the use of clothes, which can modify many of the heat exchange
processes mentioned.
There are two individual and four atmospheric factors which are most important in determining the balance of heat
for the body:
•
Level of activity of the individual (the metabolic rate), which can vary from less than 50 W m
-2
during sleep to
over about 1,000 W m
-2
for hard running.
•
Thickness and nature of clothing. This is often indicated by the clo unit or a measure of the insulational value of
the clothing.
•
Air temperature.
•
Radiant temperature of the surroundings.
•
Rate of air movement.
•
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
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.
accompanied by mean temperatures of about -17
