a. Air Temperature Reading. The air temperature is read from an ordinary-dry
bulb-thermometer. The thermometer should be in the shade so that the reading is
affected only by the air temperature.
b. Relative Humidity + Air Movement. The combined effect of relative
humidity and air movement is measured by the wet bulb temperature.
The wet bulb temperature is the reading of a thermometer when the bulb
is covered with a wet wick and a current of air (wind) is passed over the wick.
The amount of heat lost by the bulb under these conditions, and thus the
reading of the thermometer, is affected by both temperature and humidity.
The wet bulb temperature is always below the dry bulb temperature
except when the relative humidity is 100 percent, at which point both temperatures are
c. The Radiant Heat Factor.
(1) Radiant heat defined. Remember that radiation is the transfer of heat as
infrared heat rays from one object to another without physical contact.
The human body loses heat by the radiation of heat waves from the
body to cooler objects nearby such as ceilings, floors, and walls.
If these objects are at a higher temperature, an individual's body
absorbs the heat--also by the process of radiation.
The temperature of the air has no relation to the radiation of heat to
and from objects.
-- That is why skiers can remove their shirts and be warm in bright
sunshine even though the air temperature is low. The radiant heat from the sun warms
(2) The radiant heat factor. The radiant heat factor can be determined using
a "black globe" thermometer or by a radiometer.
The globe thermometer is the simpler instrument.
It consists of a 6-inch hollow copper sphere painted flat black with an
ordinary thermometer inserted so that the temperature at the center of the sphere can