CHAPTER 4
A Bird’s-Eye View of
MAXTMP Leisure
Thermal Comfort
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picture is worth a
thousand words. Figures 4-1 through 4-12 (pages 45 through 56) display outdoors
monthly PMVSHADE isoline contour maps at MAXTMP thermal comfort. In
most areas of the United States, MAXTMP occurs at about 3:00 PM local standard
time.
By looking at each of these coarsely spaced,
monthly contour maps, we can pinpoint ideal outdoors leisure thermal comfort regions
in the continental United States for any month.
Looking at Figure 4-3 (March, page 47), for
example, the tip of Florida has a comfort index of about zero (ideal) – but everything
to the north is progressively negative (colder) for this month.
On the other hand, as Figure 4-7 (July, page
51) shows, northernmost states, like Washington, Minnesota and Maine, take
their turn in July at presenting more desirable values of PMVSHADE
when compared with states to the south during this typically hot and uncomfortable
month.
It is worthwhile to review the implied assumptions
we make when considering outdoors leisure thermal comfort.
Valid Range of the Comfort Indices
Valid indices lie only
within the range of “-3” through
“+3.” Any values outside this
range should be regarded only as trend indicators.
MRTSHADE is Assumed
Equal to the
Ambient Air Temperature
We assume throughout
this book that mean radiant temperature (MRTSHADE) is equal to the
ambient air temperature – whether for MAXTMP, MINTMP or AVETMP.
We designate this
condition by appending the word “shade” to the MRT abbreviation. This implies
that our outdoors environment is equivalent to a fictional enclosure
whose surfaces are at the same temperature as the air*.
MRTSHADE is equivalent to a
well-shaded environment. Example: the shaded interior of a large
screened porch with an overhanging roof or the shaded interior of a park
shelter surrounded by lawn and trees.
Related PMVSHADE
Assumptions
Like mean radiant
temperature, Predicted Mean Vote (PMV)†,
is also subscripted by “shade” (PMVSHADE). PMV is really a “majority
vote” scale, and as such, is not perceived the same by all individuals (see
Chapter 8 for more discussion). The following assumptions are applied to PMV in
this book:
• Air velocity is zero.
• Metabolic rate is sedentary (that is,
seated, quiet).
• Clothing is lightweight summer apparel.
• A three-hour exposure to the fictional
isothermal enclosure.
Predicted Percentage Dissatisfied
(PPD)
The Fanger PPD
function (see chart on right-side of this page) illustrates how the Predicted
Percentage Dissatisfied increases as PMV departs from neutral comfort.
Even when Predicted Mean Vote is zero,
Predicted Percentage Dissatisfied is 5 percent. This means that five percent of
people are dissatisfied; 2.5 percent are uncomfortably cold and 2.5 percent are
uncomfortably warm. We see that it is not possible to satisfy everyone, even
within a perfectly controlled climate environment.
Consider a larger range in PMV. When PMV is
-1.5, then about 50 percent of people feel too cold. When PMV is +1.5, then
about 50 percent of people feel too warm. Or, from the opposite viewpoint,
about 50 percent of people are still comfortable at PMV = ±1.5.
When PMV is ±2.0, then about 75 percent of
people are dissatisfied. From the opposite viewpoint, this means that about 25
percent of people are still comfortable. ■
Predicted Percentage Dissatisfied vs. Predicted Mean Vote
