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On average, about 175 Americans
succumb to the taxing demands of heat every year. Our bodies
dissipate heat by varying the rate and depth of blood circulation,
by losing water through the skin and sweat glands, and as a last
resort, by panting, when blood is heated above 98.6°F. Sweating
cools the body through evaporation. However, high relative humidity
retards evaporation, robbing the body of its ability to cool itself.
When heat gain exceeds the level
the body can remove, body temperature begins to rise, and heat
related illnesses and disorders may develop.
The Heat Index
(HI) is the temperature the body feels when heat and humidity are
combined. The chart below shows the HI that corresponds to the
actual air temperature and relative humidity. (This chart is based
upon shady, light wind conditions. Exposure to direct sunlight
can increase the HI by up to 15°F.)
Temperature (F) versus
Relative Humidity (%)
|
°F |
90% |
80% |
70% |
60% |
50% |
40% |
30% |
20% |
10% |
|
65 |
65.6 |
64.7 |
63.8 |
62.8 |
61.9 |
60.9 |
60. |
59.1 |
58.1 |
|
70 |
71.6 |
70.7 |
69.8 |
68.8 |
67.9 |
66.9 |
66. |
65.1 |
64.1 |
|
75 |
79.7 |
76.7 |
75.8 |
74.8 |
73.9 |
72.9 |
72. |
71.1 |
70.1 |
|
80 |
88.2 |
85.9 |
84.2 |
82.8 |
81.6 |
80.4 |
79. |
77.4 |
76.1 |
|
85 |
101.4 |
97. |
93.3 |
90.3 |
87.7 |
85.5 |
83.5 |
81.6 |
79.6 |
|
90 |
119.3 |
112 |
105.8 |
100.5 |
96.1 |
92.3 |
89.2 |
86.5 |
84.2 |
|
95 |
141.8 |
131.1 |
121.7 |
113.6 |
106.7 |
100.9 |
96.1 |
92.2 |
89.2 |
|
100 |
168.7 |
154. |
140.9 |
129.5 |
119.6 |
111.2 |
104.2 |
98.7 |
94.4 |
|
105 |
200 |
180.7 |
163.4 |
148.1 |
134.7 |
123.2 |
113.6 |
105.8 |
100. |
|
110 |
235. |
211.2 |
189.1 |
169.4 |
151.9 |
136.8 |
124.1 |
113.7 |
105.8 |
|
115 |
275.3 |
245.4 |
218 |
193.3 |
171.3 |
152.1 |
135.8 |
122.3 |
111.9 |
|
120 |
319.1 |
283.1 |
250. |
219.9 |
192.9 |
169.1 |
148.7 |
131.6 |
118.2 |
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| HI |
Possible Heat
Disorder: |
| 80°F -
90°F |
Fatigue
possible with prolonged exposure and physical activity. |
| 90°F -
105°F |
Sunstroke,
heat cramps and heat exhaustion possible. |
| 105°F -
130°F |
Sunstroke,
heat cramps, and heat exhaustion likely, and heat stroke
possible. |
| 130°F or
greater |
Heat
stroke highly likely with continued exposure. |
Below is a table comparing Temperature and
Dewpoint, with the same disorders possible:
Temperature (Down) versus
Dewpoint (across)
| °F |
50 |
55 |
60 |
65 |
70 |
75 |
80 |
85 |
| 65 |
62.7 |
63.8 |
65.0 |
66.6 |
|
|
|
|
| 70 |
67.8 |
68.7 |
69.8 |
71.1 |
72.6 |
|
|
|
| 75 |
73.1 |
73.9 |
74.8 |
75.9 |
79.2 |
80.7 |
|
|
| 80 |
79.8 |
80.6 |
81.6 |
82.8 |
84.4 |
86.9 |
90.9 |
|
| 85 |
83.5 |
84.7 |
86.1 |
88.0 |
90.5 |
94 |
99 |
106.6 |
| 90 |
87.9 |
89.4 |
91.2 |
93.6 |
96.9 |
101.2 |
107.2 |
115.6 |
| 95 |
92.9 |
94.5 |
96.7 |
99.6 |
103.4 |
108.4 |
115.2 |
124.3 |
| 100 |
98.1 |
99.9 |
102.4 |
105.6 |
109.8 |
115.3 |
122.7 |
132.3 |
| 105 |
103.4 |
105.4 |
108.1 |
111.6 |
116.1 |
122.0 |
129.7 |
139.7 |
| 110 |
108.7 |
110.9 |
113.8 |
117.5 |
122.3 |
128.4 |
136.3 |
146.5 |
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How cold is it outside? Simply knowing the
temperature doesn't tell you enough about the conditions to enable
you to dress sensibly for all winter weather. Other factors
including wind speed, relative humidity and sunshine play important
roles in determining how cold you feel outside. A description of the
character of weather known as "coldness" was proposed about 1940 by
scientists working in the Antarctic. The "wind chill index"
was developed to describe the relative discomfort/danger resulting
from the combination of wind and temperature.
On November 1, 2001, the National Weather Service
began using a new wind chill index. The reason for the change is to
improve upon the current index, which is based on the 1945 Siple and
Passel Index. During the Fall of 2000, the Office of the Federal
Coordinator for Meteorological Services and Supporting Research (OFCM)
formed a special group consisting of several Federal agencies, MSC,
the academic research community (Indiana University-Purdue
University in Indianapolis (IUPUI), University of Delaware, and
University of Missouri), and the International Society of
Biometeorology. Their job was to evaluate the existing wind chill
formula and make necessary changes to improve upon it. The group is
called the Joint Action Group for temperature Indices (JAG/TI) and
is chaired by the NWS. Weird name, but very important work. The goal
of JAG/TI is to internationally upgrade and standardize the index
for temperature extremes (a.k.a. Wind Chill Index). They ultimately
reached an agreement on a new wind chill formula. It will make use
of the advances in science, technology, and computer modeling to
provide a more accurate, understandable, and useful formula for
calculating wind chill.
Lots of time and energy was put into coming up
with the new formula and what it does differently. Specifically, the
new wind chill index will use wind speed calculated at the average
height (5 feet) of the human body's face instead of 33 feet (the
standard anemometer height); be based on a human face model;
incorporate modern heat transfer theory (heat loss from the body to
its surroundings, during cold and breezy/windy days); lower the calm
wind threshold to 3 mph; use a consistent standard for skin tissue
resistance; and assume the worst case scenario for solar radiation
(clear night sky). In 2002, adjustments for solar radiation (i.e.,
the impact of sun) for a variety of sky conditions (sunny, partly
sunny and cloudy) will be added to the calculation model.
Wind chill does not affect your car's antifreeze
protection. It will have an impact on how quickly your home's
exposed water pipes freeze, but has little impact on whether they
would freeze or not. The importance of the wind chill index is as an
indicator of how to dress properly for winter weather. In dressing
for cold weather an important factor to remember is that entrapped
insulating air warmed by body heat is the best protection against
the cold. Consequently, wear loose-fitting, lightweight, warm
clothing in several layers. Outer garments should be tightly- woven,
water-repellant and hooded. Mittens snug at the wrist are better
protection than fingered gloves.
To use the chart, find the approximate
temperature on the top of the chart. Read down until you are
opposite the appropriate wind speed. The number which appears at the
intersection of the temperature and wind speed is the wind chill
index.
Note: Wind Chill Temperature is only
defined for temperatures at or
below 50 degrees F and wind speeds above 3 mph.
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