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Agronomy 541 : Lesson 6a
Energy Budget: Heat Index and Wind Chill
Introduction
It is suggested that you watch Video 6A and complete the exercise in the video before continuing with the
lesson.
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Developed by E. Taylor and D. Todey
There is an old saying: "to keep your feet warm, wear a hat; 40 percent of your heat is lost from your head."
The rate of heat loss from your head can be substantial. The blood flow near the surface is significant, and the
temperature of the forehead is usually near 30°C. It is possible that there is a physiological control mechanism
to maintain a nearly constant forehead temperature. Under some conditions, the effect of heat loss by the head
can be seen. Maybe you have seen this effect when you have looked at the shadow of an automobile against a
building or on the road, and you have seen the heat rising from the hood (Fig. 6.1).
Fig. 6.1 Heat loss out of your head can be seen on your shadow in
reflected sunlight.
Or maybe you have actually seen it rising from your hand or your head when the light is just right. This will
typically happen when light reflected from a car windshield shines in your window, and then you look at your
shadow on the wall. Often you can see the heat rising from your face or from your hand. It always seems to
work best on light reflected from a car windshield and then into the darkened room so the shadow shows up.
The windshield is typically somewhat polarized, and that may allow the heat effects to begin to show up.
This may also be seen as a mirage. Sometimes driving down the road, when the surface is quite hot, you can
see wavy lines that may look like water on the road at some distance in front of you. The image of the road is
distorted by the warm air near the road. Warm air has an index of refraction different from air that is cooler.
Sometimes this makes a visible disruption in the atmosphere, but it always causes some change of index of
refraction in the atmosphere, and occasionally it becomes visible. It is not an uncommon thing to be able to see
heat rising from yourself or from some other object.
This lesson will discuss the energy balance and its effect on heat of crops and animals.
What You Will Learn in This Lesson:
What an energy balance is.
How the energy balance affects crops and humans.
How these can be put in to a scale of "feels like" indexes.
IN DETAIL : Mirage Conditions
Fig. 6.2
Fig. 6.3
Very warm temperatures near the surface with cold air aloft can occur on a very warm sunny day. Because of
differences in how light passes through air of different temperatures, light can be bent as seen below.
Fig. 6.4
What the driver sees looks like water but what he actually sees is light bent from the sky. Because our brain assumes
light travels in a straight line, we see what looks like water on the surface.
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Agronomy 541 : Lesson 6a
Energy Budget: Heat Index and Wind Chill
Heat Index
Heat loss and heat gain are important to human comfort. The concepts of "Heat Index" and "Wind Chill" provide
indexes of two important aspects of heat balance. Heat or "energy" must be balanced because a person cannot
long remain comfortable nor even long endure an out-of-balance environment. In other words, if you are
becoming hotter by exercising or sitting in the sun, you must balance the heat gain by giving off more heat.
Heat index is a term that you may or may not have heard. Air temperature is the normal or usual measure of
the atmospheric condition that influences our comfort, our well-being, and our happiness. Temperature is not
the only factor that determines our comfort. In some cases, temperature is not even the primary environmental
factor determining our comfort.
The heat index has various names. Sometimes it is called "apparent temperature." Sometimes it is called
"humidity index." Occasionally it is referred to as the "temperature humidity index." Although all of these terms
are apparently interchangeable, there are some intended differences by those who originated the terms.
Apparent temperature normally refers to the measure of human comfort or discomfort due to the combination of
humidity and temperature (Table 6.1).
Table 6.1 Heat stress index
The apparent temperature index (Table 6.1) is a measure of human comfort or discomfort due to the combined
effects of heat and humidity. It was designed in 1979 by Dr. R. G. Steadman. The Heat Index existed prior to
1960, however, the 1979 version is the one announced regularly over the radio or TV during hot weather.
Often it appears as a chart (Table 6.2).
Table 6.2 Apparent temperature index
The chart has the actual air temperature on the left with relative humidity across the top. Apparent
temperatures and level of danger categories are indicated along the chart. If, for example, the temperature is
actually 100°F (37.8°C) and the relative humidity is 60%, it will feel like 130°F (54.4°C). The heat index table
gives values based on humidities from 10% to 90% and air temperatures from 65°F (18.3°C) to 120°F
(48.9°C).
If the relative humidity is at 90% and the air temperature is at 80°F (26.7°C), the apparent temperature or the
heat index is 88. In other words, it feels considerably warmer than the reported actual temperature. This is
based on a neutral situation, assuming that at 80°F (26.7°C), 40% humidity would be typical. Conditions of
80°F (26.7°C) and 40% humidity have no impact on apparent temperature. If the humidity is very dry, perhaps
10%, the apparent temperature is cooler than the actual temperature of the air. In Idaho, Nevada, Utah, or most
western states the apparent temperature will usually be cooler than the actual air temperature. In the Midwest,
on average the true temperature and the apparent temperature are the same because during the heat of the
day the mid-day humidity is usually near 40%. The exception is during extremely high temperatures or
extremely humid periods.
Study Question 6.1
What is the apparent temperature if the air temperature is 80°F and relative humidity 20%?
°F Check Answer
Study Question 6.2
What is the apparent temperature if the air temperature is 90°F and relative humidity 65%?
°F Check Answer
Agronomy 541 : Lesson 6a
Energy Budget: Heat Index and Wind Chill
Heat Index
Historical Extreme Heat Events
In the summer of 1995 a disastrous heat wave developed in the Midwest. Glenwood, Iowa, experienced the
worst stress in recent history. In 1936 on the 13th of August the air temperature was 107°F (41.7°C), and the
dew point was 62°F. The heat index charts use either air temperature or dew point numbers. The dew point
was between 60°and 65°F (15.6-18.3°C) with air temperatures was around 105°F (40.5°C) creating a heat
index near 111°F (43.9°C).
The most extreme condition in recorded history for Iowa was on 25 July, 1936 in Glenwood, IA, when the
temperature hit 115°F (46.1°C) with a dew point of 89°F (31.7°C), giving a heat index of 145°F (62.8°C)! That
is the hottest we have had in this century prior to 1995. The temperature in 1995 did not reach 115°F in the
place where the humidity was high, but were close enough to create a dangerous situation. These conditions
were a disaster to cattle. Animals cannot endure 145°F apparent temperatures.
Before the turn of the century one day did make it to 150°F apparent temperature. For most of the state, 1995
was the highest apparent temperature since 1894. The heat of 1995 essentially wiped out the poultry industry
in the state. Everywhere that the heat index was near 145°F had high mortality of the poultry, both chickens
and turkeys. Interestingly, swine, being more sensitive to temperatures, had the least problems, because since
swine are so sensitive, everyone has taken good care of them and made sure that they were sheltered from
high temperatures. But the cattle, being less sensitive than swine, had been neglected. Poultry is sensitive, and
these extreme conditions overcame heat control devices causing extensive poultry loss.
Heat indices are also calculated for animals.
IN DETAIL : Animal Heat Indices
Fig. 6.5 Poultry Heat Stress
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Agronomy 541 : Lesson 6a
Energy Budget: Heat Index and Wind Chill
Heat Index
Heat Stress Impacts
This is a tragic example of heat stress and its agricultural significance. Curiously, this event wasn't highly
significant to the crops. The humidity was high (dewpoints in the upper 70s and lowers 80s°F). Consequently,
the crops used little water and did not come under water stress. The temperature was not high enough to give
crops direct temperature stress, so there was little water stress and temperature stress. Temperature stress on
the crop begins when temperatures surpass over 110°F (43.3°C) (Fig. 6.6).
Fig 6.6 Plants die (or are cooked) when leaf temperatures are greater
than 110°F (43.3°C). Cooler temperatures could be survived with high
moisture conditions similar to 1995.
When leaf temperatures reach 117°F, the plants may die, but this was not the case in 1995. Leaf temperatures
did not get that high, so the plants did not cook. Nor did they wilt, since humidity was high . The disastrously
high heat for cattle was almost gentle on crops. If swine had not been protected, it would have been a disaster
to them. It was a disaster to poultry, and disaster to human life, particularly in Chicago. All of these were related
to the heat index and heat stress. The heat wave of the summer of 1999 was compared to that of 1995. While
the 1995 one was more extreme, it had a time span of a few days. The heat wave of 1999 was more consistent
and persistent.
Agronomy 541 : Lesson 6a
Energy Budget: Heat Index and Wind Chill
Heat Index
Radiation
There are factors other than temperature and humidity that influence our comfort. These include exposure to
the sun, type of clothing, and the wind. These factors are not accounted for by the heat index chart. All of these
things are significant factors in considering comfort. But the heat index contains just two factors: temperature
and humidity.
We will now consider several examples of comfort and the environment. We should look at the index in a more
detailed and accurate manner that shows all of the influences on a person. The scantily clad jogger is receiving
energy directly from the sun and energy that is reflected from bright clouds (Fig. 6.7).
Fig. 6.7 Interaction of radiation with the human body.
Short wave radiation comes directly from the sun, and
reflected off, particles in the atmosphere, and the
ground. Some of this radiation is reflected from the
body back to the atmosphere. Longwave (thermal)
radiation is given off and absorbed, also.
The sun is heating the ground with the heat from the ground influencing the person. The sun's energy may be
reflected from the ground. Snow, particularly, has this effect, in that it reflects a great deal of the sun's energy
and can have a major influence on comfort. Remember from Lesson 3b that snow has a high albedo.