CHAPTER 12
Dew Point Temperature
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ew point temperature,
which is a measure of the amount of moisture in the air, plays an important
role in discussing leisure thermal com–fort because it is one of several independent
variables that define PMVSHADE.
We feel uncomfortable when ambient and dew
point temperatures are high because high dew point temperature inhibits the
evapora–tion of moisture from our skin and thereby diminishes the evaporative
cooling effect.
However, at sufficiently low ambient
temperature, dew point temperature does not affect comfort. For example, studies
by Koch et al.*
in 1960 demonstrated that dew point temperatures in still air had a negligible
effect on comfort until the dew point temperature reached 50°F and the
ambient temperature reached 65°F.
Be–low these temperatures, ambient temperature alone was the governing factor.
This chapter discusses the following two
topics:
• The relationship between relative humidity
and dew point temperature, and
• The relative stability of dew point
temperature.
Dew Point Temperature and
Relative Humidity
Air moisture content
can be expressed either in terms of dew point temperature or relative humidity.
Table 1-1, pages 11-12, displays the relationship between these two measures of
air moisture. To illustrate the difference between the two measures, consider
an arbitrary location for example, Columbus, Georgia, in July, with:
• MINTMP equals 70ºF,
• Dew point temperature equals 70ºF, and
• MAXTMP equals 91ºF.
Dew point temperature is the
temperature at which moisture in the air begins to condense;
accordingly, that temperature measures the amount of moisture in the air. In
other words, the higher the dew point temperature, the higher the moisture
content of the air. Dew point temperature remains relatively constant
throughout the day.
Relative humidity is the amount of saturation
of the air at a specified temperature. For a given amount of air moisture (that
is, dew point temperature), the amount of moisture saturation increases with a
decrease in temperature, and decreases with an increase in temperature.
Humidity commonly varies a great deal throughout the day – higher in the
morning and lower in the afternoon.
Thus,
for the above example (Columbus, GA, in July), when MINTMP (occurring at about
sunrise) equals dew point temperature (that is, 70ºF), the relative
humidity is 100 percent – and this corresponds to the condition for incipient
condensation. Now consider an afternoon MAXTMP of 91ºF for Columbus, Georgia,
in July; at this temperature, the relative humidity (that is, saturation) is
only about 50 percent.
So, with relative humidity as a measure of
air moisture, we have the somewhat perplexing situation that at high relative
humidity, we may feel relatively cool (that is, at 70ºF) while at low
relative humidity, we can feel uncomfortably warm (that is, at 91ºF).
As suggested by the Center for Mathematics
and Science Education at the University of North Carolina*,
instead of saying “It’s not the heat, it’s the humidity,” it’s more precise to
say, “It’s not the heat, it’s the dew point.”
Throughout the remainder of this book, we
express air moisture only in terms of dew point temperature.
Hourly Variation of Dew Point Temperature is Negligible
A practical feature of
dew point temperature is that it generally varies little throughout the day as
illustrated by the two following examples.
A Typical Example
Table 1-2, page 13, shows
the monthly variation of dew point temperature at three-hour intervals for
Birmingham, Alabama. Review of this table shows that for July, the average dew
point temperature varies throughout the day by less than 1°F. For example,
refer to any column within Table 1-2.
This amount of variation is typical for many
regions of the United States.
A More Variable Example
Table 1-3, page 14,
illustrates monthly dew point temperature variation at three-hour intervals for
Ely, Nevada. The July dew point temperature varies just slightly more than 3°F throughout
the day.
Ely, Nevada, has
unusually high variability of recorded dew point temperatures.
It’s Easy to Measure Dew Point Temperature
Add warm water into a soup
can (without the label) until about two-thirds full. Stir with a thermometer
while slowly adding one ice cube at a time. As soon as the outer surface of the
can begins to fog, read the thermometer – this is the approximate dew point tem–perature.
The average monthly dew point temperature
often equals the average monthly minimum temperature (MINTMP). The implications
of this potentially predictive feature are discussed in Chapter 11, and an
application is described in Chapter 10. ■