Energy Budget: Heat Index and Wind Chillagron- · PDF fileAgronomy 541 : Lesson 6a Energy...

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 thelesson.

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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 thetemperature of the forehead is usually near 30°C. It is possible that there is a physiological control mechanismto maintain a nearly constant forehead temperature. Under some conditions, the effect of heat loss by the headcan be seen. Maybe you have seen this effect when you have looked at the shadow of an automobile against abuilding 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 willtypically happen when light reflected from a car windshield shines in your window, and then you look at yourshadow on the wall. Often you can see the heat rising from your face or from your hand. It always seems towork 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 cansee wavy lines that may look like water on the road at some distance in front of you. The image of the road isdistorted 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 seeheat 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 ofdifferences 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

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light travels in a straight line, we see what looks like water on the surface.



Heat Index

Heat loss and heat gain are important to human comfort. The concepts of "Heat Index" and "Wind Chill" provideindexes of two important aspects of heat balance. Heat or "energy" must be balanced because a person cannotlong remain comfortable nor even long endure an out-of-balance environment. In other words, if you arebecoming 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 ofthe atmospheric condition that influences our comfort, our well-being, and our happiness. Temperature is notthe only factor that determines our comfort. In some cases, temperature is not even the primary environmentalfactor 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 termsare 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 ofhumidity 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 combinedeffects of heat and humidity. It was designed in 1979 by Dr. R. G. Steadman. The Heat Index existed prior to1960, 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. Apparenttemperatures and level of danger categories are indicated along the chart. If, for example, the temperature isactually 100°F (37.8°C) and the relative humidity is 60%, it will feel like 130°F (54.4°C). The heat index tablegives 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 theheat index is 88. In other words, it feels considerably warmer than the reported actual temperature. This isbased on a neutral situation, assuming that at 80°F (26.7°C), 40% humidity would be typical. Conditions of80°F (26.7°C) and 40% humidity have no impact on apparent temperature. If the humidity is very dry, perhaps10%, the apparent temperature is cooler than the actual temperature of the air. In Idaho, Nevada, Utah, or mostwestern 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 theday the mid-day humidity is usually near 40%. The exception is during extremely high temperatures orextremely 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



Heat Index

Historical Extreme Heat Events

In the summer of 1995 a disastrous heat wave developed in the Midwest. Glenwood, Iowa, experienced theworst stress in recent history. In 1936 on the 13th of August the air temperature was 107°F (41.7°C), and thedew point was 62°F. The heat index charts use either air temperature or dew point numbers. The dew pointwas between 60°and 65°F (15.6-18.3°C) with air temperatures was around 105°F (40.5°C) creating a heatindex near 111°F (43.9°C).

The most extreme condition in recorded history for Iowa was on 25 July, 1936 in Glenwood, IA, when thetemperature 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)! Thatis the hottest we have had in this century prior to 1995. The temperature in 1995 did not reach 115°F in theplace where the humidity was high, but were close enough to create a dangerous situation. These conditionswere 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, 1995was the highest apparent temperature since 1894. The heat of 1995 essentially wiped out the poultry industryin the state. Everywhere that the heat index was near 145°F had high mortality of the poultry, both chickensand turkeys. Interestingly, swine, being more sensitive to temperatures, had the least problems, because sinceswine are so sensitive, everyone has taken good care of them and made sure that they were sheltered fromhigh temperatures. But the cattle, being less sensitive than swine, had been neglected. Poultry is sensitive, andthese extreme conditions overcame heat control devices causing extensive poultry loss.

Heat indices are also calculated for animals.

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IN DETAIL : Animal Heat Indices

Fig. 6.5 Poultry Heat Stress



Heat Index

Heat Stress Impacts

This is a tragic example of heat stress and its agricultural significance. Curiously, this event wasn't highlysignificant 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 givecrops direct temperature stress, so there was little water stress and temperature stress. Temperature stress onthe 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 temperaturesdid not get that high, so the plants did not cook. Nor did they wilt, since humidity was high . The disastrouslyhigh heat for cattle was almost gentle on crops. If swine had not been protected, it would have been a disasterto them. It was a disaster to poultry, and disaster to human life, particularly in Chicago. All of these were relatedto the heat index and heat stress. The heat wave of the summer of 1999 was compared to that of 1995. Whilethe 1995 one was more extreme, it had a time span of a few days. The heat wave of 1999 was more consistentand persistent.



Heat Index

Radiation

There are factors other than temperature and humidity that influence our comfort. These include exposure tothe sun, type of clothing, and the wind. These factors are not accounted for by the heat index chart. All of thesethings are significant factors in considering comfort. But the heat index contains just two factors: temperatureand humidity.

We will now consider several examples of comfort and the environment. We should look at the index in a moredetailed and accurate manner that shows all of the influences on a person. The scantily clad jogger is receivingenergy 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 bereflected from the ground. Snow, particularly, has this effect, in that it reflects a great deal of the sun's energyand can have a major influence on comfort. Remember from Lesson 3b that snow has a high albedo.



Heat Index

Radiation

Direct sun is the primary radiation factor. On a cold day if we are in the sun, we are bit more comfortable thanin the shade. Thermal radiation is significant from warm items around us. An item that is much warmer than abody will radiate heat directly to us. Correction(wind) is second to radiation in the transfer of heat. If wind isblowing past our person, it may be carrying heat to or away from the individual.



Heat Index

Wind

Wind is second to radiation in the transfer of heat. If wind is blowing past our body, it may be carrying heat to oraway from the individual. Air moving past the body is usually a cooling factor. Wind, though, is not always acooling factor.

Fig. 6.8a Little air movement limits heat transfer from air to your hand

because a temperature gradient exists between your hand and the

hottest air. Click on the image above to hear an explanation.

Fig. 6.8b Wind moves the cooler air away from the skin and replaces it

with much warmer air. Click on the image above to hear an

explanation.

Transcription of the audio on heat transfer.

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FYI : Transcription of Audio

Another experience is told by a co-worker who visited a sauna in Finland. A Finnish sauna is a good place tounderstand the heat load at the personal level. In a sauna the air temperature is often above the boiling point ofwater. In a classic Finnish sauna, the air is heated to something around 215° to 220° F (102-105° C). Holding verystill, being very wet and with low humidity, you are not too uncomfortable for several minutes. But you soon begin tonotice the heat. Blowing on your hand, you will blow away the unmixed layer of air that envelopes your hand, andallow the heat of this scalding hot air (over the boiling point of water) to come against your hand. You can literallyscald yourself by just blowing across your hand. A person without a moustache is at a disadvantage in a sauna. Themovement of air from the nostril over the upper lip will reduce the boundary layer of unstirred air and let the hot air ofthe room come next to the skin (Fig. 6.8b).

My friend discovered quite quickly that it is a tradition in Finland, if you are a "real man," to be able to stay in thesauna longer than the "semi-real men." The men will go into the public sauna and see who can stay there the longest.My friend was well aware of the exchanges of energy that affect humans, including that of wind. He also discernedthat, in the finest tradition of the Finnish sauna, they always have some branches there. They used these for sort of askin massage. They flail themselves with them, supposedly in the finest tradition. When there were just one or twopeople sitting beside him, to show the finest tradition of the sauna, he would pick up his oak branches and not beginhitting himself, but fanning the person next to him. This blew away the boundary layer of air next to that person. Asthat person scalded, he would get up and leave. Then he would fan the other side until that person left. Our Americanwas the "hero" in his sauna because he understood the nature of the movement of air carrying heat to or away fromthe body or from any object.



Heat Index

Evaporation

The other major exchange of energy is evaporation. People usually perspire under warm conditions.Perspiration has a cooling effect assuming that the air temperature or humidity are such that evaporation cantake place. If evaporation is taking place, there may be considerable evaporative cooling (Fig. 6.9), which caninfluence the heat endurance of an individual. You have already heard the Finnish sauna story when a drysauna with very low humidity allowed people to perspire freely, helped people maintain a skin temperaturemuch cooler than the air in the sauna. The unstirred air next to the skin, of course, is at skin temperature and afew centimeters away from your skin, the air is at the scalding temperature of the room's air.

Fig. 6.9 Heat needed to evaporate water from the skin is drawn from

the body, thus cooling the area.

You can experience the same thing in a bathtub. If you fill a bathtub with water hotter than it ought to be, youwill discover that you can sit very still in the tub and endure it. But if you slosh the water around stirring it up, itwill scald you. That is because the water next to the individual takes on the temperature of the individual's skin,or the temperature of the adjacent surface. While wind and evaporation usually cool a body, they cansometimes have counteracting effects on plants, which will be discussed in Lesson 12a.



Heat Index

Metabolism

Humans, as with most mammals and many other animals, generate heat. Metabolic heat can be significant. It isprobably not significant in most lizards or most insects, and in most plants metabolic heat is not significant.There are conditions where metabolic heat is a significant contribution to the energy of some plants and forcold-blooded animals.

In the same volume of Journal of Applied Meteorology where the apparent temperature was published(Steadman, 1979), another factor was added to the heat index, the metabolic activity of the person. Oneexample has the metabolic activity of a person running at 10 mph (a competitive running speed) (16 kph).Olympic champions in the marathon run about 5 minutes per mile, close to 13 mph; 10 mph would beconsidered a competitive speed for someone college age usually running a long distance. A jogger mighttypically run at a speed of 7 - 8 mph (8 or 9 minutes a mile). At 10 mph, a person generates about 10 to 12times their resting metabolic heat. Olympic champions generate 15 times their basal metabolism at their optimalperformance. The amount of heat created in exercise can be significant.

Fig. 6.10 Heat created by humans

Sitting in the room, each person represents the equivalent of a 100-watt light bulb. This assumes a metabolismof 2,000 kcal per day (96.9 watts).

Activity has some impact on thermal comfort. If you have been out walking on a cold day, you know that afterthe first 20 or 30 minutes you probably need fewer clothes to be comfortable than you needed at thebeginning.

The 10 mph runner, if the air temperature is 80°F and the relative humidity is 80%, has 30 minutes until heoverheats and cannot run any more. If the air temperature is 100°F, he has less than 10 minutes until his bodywill have heated up to the point of heat distress. Usually the distress point is reached when core temperature(rectal temperature) reaches 104-105°F. The person could rapidly become non-conversant with the world and

would soon be unable to help himself.

Transcription of the audio on effects of heat on health and comfort

The thermal limit charts indicate very real effects of metabolism on health and comfort (Fig. 6.11). Even for the"average" jogger running at 6 mph (1 mile in 10 minutes) metabolic heat is a consideration. These charts addmetabolic heat to the temperature and humidity, leaving out the wind. Most joggers would use the 6 mph chart.At 60% humidity and 100°F one hour would be the limit of human endurance. Half of that would be about aslong as anyone ought to perform under duress. Zero performance would be for sensible people. This is theheat index plus the metabolic component. Dehydration times are also included in the figures. As a rule-of-thumb, a person running loses 1 pint (pound) of water every 20 minutes. Bright sun will place additionallimitations on performance.

Fig. 6.11 Limit of human endurance with metabolism

added to the heat index chart.

In review, thermal energy depends upon the temperature of our surroundings, the air temperature of theatmosphere around us, the movement of air, the humidity of the air, evaporation, and the interception of solarradiation. All of these factors determine our comfort.

When we consider an apparent temperature that includes only two of the factors, it can be misleading underextraordinary conditions.

Study Question 6.3

If the relative humidity is 30% and the temperature is 90°F (it is night), what is the maximum recommendedduration for a person that runs at 10 mph?

minutes Check Answer

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During the Pan American games a few years ago, the temperatures during the marathon were such that peopleshould not run over about 90 minutes. The marathon takes more than 2 hours. Two runners died in that event. Oneof our American runners just quit. When asked, "When did you realize you should quit?" his comment was, "I wasrunning as hard as I could, figured I was doing fine, and there was a nun pushing a baby carriage going my samedirection and passed me." That brought him to the realization that he was literally dying. So he went into the shade,sat down, and quit the race, still spending more than a day in the hospital. The nun saved his life by walking the babycarriage faster than this Olympic runner could run.



Wind Chill

At the opposite end of the scale from heat stress is cold stress, often expressed as a Wind Chill Relationship.The wind chill relationship contains two factors: temperature and wind. It does not consider humidity, sunshine,clothing, or others. Originally the wind chill chart was created as a series of charts at the NATIC/ArmyLaboratories near Boston. They were created by putting soldiers in wind tunnels at different air temperaturesand wind speeds. Different temperatures of the sides of the tunnels and the air, some different humidities, andifferent types of clothing were tested. Some of these soldiers were placed in the wind tunnels in arctic clothing,some in street clothing, and some in essentially no clothing. Then the wind speeds would be adjusted at a fixedtemperature. The 20-30 soldiers were asked to write down how they felt, comfortable, cold, really cold, orfreezing. They found that there is a significant factor of preconditioning for a human. If a person is used tobeing in cold conditions, they are much less uncomfortable with the wind chill than the chart would indicate.

The charts are usually broken down into uncomfortable, dangerous, very dangerous, and exposed fleshfreezes. Even the exposed flesh freezing portion of the chart that you often see is somewhat subject toadaptation.

The commonly used wind chill chart assumes appropriate clothing for the conditions for an average person.The wind chill chart has wind in knots (1 kt= 1.1 mph) (Table 6.3). In a 5 knot wind, a temperature of 30°F feelslike 25°F; that is, feels like at 25°F wind-less environment. A temperature of 25°F feels like 19°F. If the windspeed goes up to 15 knots, that temperature of 25°F feels like it is -1°F. The wind effected temperature chartsare the most common charts and assume appropriate clothing for the air temperature. The table indicates the"bitterness" of a cold windy day.

Table 6.3 Wind chill chart

Wind(knt)

Temperature (F)

40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30

5 36 30 25 19 14 8 3 -2 -8 -13 -19 -24 -30 -35 -40

10 26 20 13 7 1 -6 -12 -18 -25 -31 -37 -44 -50 -56 -63

15 20 13 6 -1 -7 -14 -21 -28 -35 -42 -49 -56 -63 -70 -77

20 16 9 2 -6 -13 -20 -28 -35 -42 -50 -57 -64 -72 -79 -86

25 13 6 -2 -9 -17 -25 -32 -40 -47 -55 -63 -70 -78 -85 -93

30 11 4 -4 -12 -20 -28 -35 -43 -51 -59 -66 -74 -82 -90 -98

35 10 2 -6 -14 -22 -30 -37 -45 -53 -61 -69 -77 -85 -93 -101

40 9 1 -7 -15 -23 -31 -39 -47 -55 -63 -71 -79 -87 -95 -103

Study Question 6.4

If there is a 30 knt wind and the temperature is 5°F, what is the equivalent temperature if a person was in a

wind free location?

°F Check Answer

The Canadian "wind chill" report is often given in terms of heat loss (expressed in watts). Some find it easier tocomprehend heat loss than visualizing equivalent temperatures.

Wind chill is an apparent (feels like) temperature only. Objects cannot get below the actual temperature, sincewind accentuates the heat loss, an object is brought to the actual temperature faster, but does not get belowthe actual temperature.

Wind chill also covers just a few of the conditions where actual comfort depends on type of clothing, sun,shade, moisture in the air, physical conditioning, several other factors. These factors are discussed laterincluding the interaction of humidity with clothing and the influence of solar and thermal radiation.

Discussion Topic 6.1

How might a heat loss method be better than an equivalent temperature method given thediscussion regarding solar radiation, metabolism, etc.?



Wind Chill

Adaptation

When hands get cold and are close to the point of being numb, they become very sensitive to pain. A sharpblow that has no effect other than to make a noise at temperatures near 70°F (21°C) will, when your hands arealmost cold enough to be numb, be painful enough to make you think your fingers are going to break. Onceyou have become accustomed to working in cold conditions, your hands do not become numb. They stay warm.Even quicker in adapting are your nose, forehead, and ears. Go out in cold conditions and your ears becomecold quite quickly. Your body immediately begins to send more blood to your ears and your forehead. It tries tomaintain your forehead temperature at close to body temperature or a few degrees below it. Foreheadtemperature is usually at 86°F (30°C) and is fairly constant. This takes us back to the point of "if you are cold,put on your hat," because your body will expend a terrific amount of your core heat by circulating blood to keepyour forehead and ears warm. This is futile. Your blood flow does usually succeed at keeping your foreheadwarm, at great expense to the body (Fig. 6.12).

Figure 6.12 Adaptive heat loss from person.

Figure 6.12a. During cold weather, blood flow is concentrated in the

main body cavity to protect major organs

Figure 6.12b The body can adapt and adjust blood flow. Heat loss is

increased to keep hands and head warm

Transcription of the audio on environment conditioning

There may be some adaptation to being cold in general. The Indians at Terra del Fuego, the extreme southernisland at the top of South America, would sleep outside, essentially uncovered, or with one light animal skin, onalmost freezing temperature nights. It was probably a case of adaptation, that any human can do.

Anyone could soon adapt to leaving the window open at night in the winter. Measuring the temperature in theroom, you find out that you could endure colder and colder temperatures in the room with the same blanket asyou got more adapted to enduring those low temperatures. Adaptation may be simply an ability to sleep whentemperatures are uncomfortably cold for another.

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If you have been working outdoors in the cold a great deal, it will also do the same thing for your hands. Under thesame conditions where other people's hands become numb and non-functional, the adapted person's hands stillwork. An example to illustrate this point concerns some Eskimos on a hunting trip with some local people in southernIdaho a few years ago. These people were all related so their genetic background was not all that different. But theEskimos had just come from Alaska, being well adapted to working in the outdoors. Their relatives, living in southernIdaho, were well adapted to working indoors. Under these conditions on a hunting trip in the mountains of Idaho, thegroup was caught by a sudden and unexpected blizzard. They were all dressed in similar clothing. When the rescuepeople finally located them, the people from Idaho had lost toes and fingers, parts of ears, and a nose due to frostbite. The Eskimos were frozen to death, apparently without frost bite. Their fingers stayed warm and could move tonear the end. The people who survived mentioned this. They said, "They were not cold. We were freezing and ourhands were freezing. They had gone white, would not bend, and were frozen solid. Our friends' hands were notfrozen. They were in good condition, and then suddenly they were dead." Their bodies sent the heat from the centerof their bodies through the blood stream to their extremities to keep their hands and face warm. It cost them theirlives; whereas the non-adapted people's bodies shut down the extremities and kept the core temperature up. So wedo not really know if the adaptation is an adaptation to the extreme case or not. It did not seem to be in that case.Nevertheless, that is the nature of cold environment conditioning.



Effects of Humidity (On Cold Temperatures)

A question often asked is, "What's the effect of humidity in cold weather?" According to most books, having ahigher humidity makes our body evaporate less water, resulting in less cooling. If that theory is extrapolated tovery cool conditions, it would say that a cold and moist atmosphere would result in a person not being asmiserable as if it were a cold, dry atmosphere. But it is not so. The reason is that people wear clothing. To anunclothed person, it is probably correct that in the humid atmosphere you would not be as cold as in the dryatmosphere. But we wear clothing. Often, as the humidity in the air increases, the insulating value of clothingdecreases because of the moisture captured on the fibers of the clothing. This is the wicking effect of moisturein clothing. It can greatly accelerate the heat loss through clothing. Even though the clothes do not appear wet,the humidity in the air may create a thin layer of moisture on the fibers and result in greater conduction, ortransfer, of heat through the clothing.

The person who says, "The coldest I have ever been was that cold, windy day when the humidity was so high,"probably is telling the truth. It probably is realistic that it was because of the humidity reducing the insulatingvalue of the clothing rather than the direct humidity effect on heat transfer from a person if they wereunclothed.

We have introduced the energy budget of an individual, the forms of energy, and the concept that your comfortis determined not only by air temperature, but by wind, humidity, sun, thermal radiation from other sources, andby factors such as clothing, internal heat source, and evaporation. These are the factors of the energy budget.They are significant for animals and for the success and productivity of crops. We will now look more at how thespecific energy budget of crops.



Energy Balance

The concepts of heat index and wind chill involve terms of the energy budget, the flow of energy into and outfrom an object. According to the "law of conservation of energy", all energy absorbed by something will bedissipated. Describing from a balance perspective (energy in = energy out. This assumes no temperaturechange.

Imagine a leaf, perhaps a soybean leaf or leaflet, absorbing shortwave energy from the sun. It is also absorbingsome thermal energy from warm things around it. The leaf is also radiating away energy according to itstemperature (Fig. 6.13 a).

Fig. 6.13a Radiation balance on a

leaf.

The first term is depicting an energy balance in that the leaf absorbs incoming shortwave energy and emitslongwave energy. Q is considered to be the net amount of energy entering or leaving the leaf. If the energycoming in is balanced by that leaving, Q would be 0. The balance (or the net amount of radiation) of theincoming an outgoing radiation is considered the term R.

Q = R

Wind blowing across a leaf can remove or add heat by conduction and convection (Fig. 6.13b), termed C.

Fig. 6.13b Wind effect on leaf energy

balance.

Q = R + C

If the relative humidity or the air is less than 100%, moisture will evaporate from the leaf (Fig. 6.13c). There willbe a resulting heat loss. This is termed E.

Fig. 6.13c Evaporation

(transpiration) from a

leaf).

Q = R + C + E

When dealing with an animal, metabolism, termed M, must be included in the balance (Fig. 6.13d).

Fig. 6.13d Animal activity adds metabolism to the energy

balance.

Q = R + C + E + M

The energy balance of an object is then written as:

Equation 6.1

We are initially describing a plant and will not worry about metabolism. Energy is absorbed from the sun , theground, and the whole environment around the plant. It absorbs a percentage, and re-radiates a certainamount of energy. The wind can supply energy or carries it away. Evaporation usually carries energy away withthe evaporation of water. If dew is forming on a leaf, it adds energy to the system. Although these signs say "+",they could be + or -. Dew could be forming or evaporation could be taking place. The wind could be adding ortaking away energy. Usually all of these terms are taking energy away from a system. The balance of factors onthe soybean leaf result in a certain amount of energy, Q, which determines the temperature of the leaf, "TL."

The temperature of the leaf is a crucial thing. There is, for example, an optimum leaf temperature forphotosynthesis. The optimum varies according to the "type" of plant, and pre-conditioning, etc.

Fig. 6.14 Net photosynthesis varies by leaf temperature for different

plant types

Net photosynthesis (often indicated as nPs) may be limited by the temperature of the leaf. At some lowtemperature, photosynthesis is 0 or very low. As temperature goes up it can increase assuming that there is

ample light, that it is a healthy plant and has nutrients it needs. Photosynthesis has an optimum temperature.When temperature gets beyond the optimum, photosynthesis falls off rapidly. This optimum is probablysomewhere around 90°F(32°C). There is a 0 photosynthesis point, probably somewhere around 50°F(10°C).The leaf may die if its temperature exceeds 117°F (47°C). These are some effects of the temperature of theleaf determined primarily by its environment.

The leaf does respond to the environment in ways that do influence leaf temperature. The soybean leaf acts asif hinged. The leaflets under stress will all be vertical. Under ideal conditions, they will all be horizontal. Corndoes a similar thing. You will see wide leaves under ideal conditions, perhaps slanted towards the sun. Instressful conditions, they roll or curl and changes the leaf area and exposure. The young corn under stresshas a pineapple appearance(Fig 6.15).

Fig. 6.15 Corn reacting to stress by rolling its leaves into a pineapple

appearance.

Leaves of the grass in your lawn (bluegrass), much like the corn, curl and fold in half lengthwise, cutting theleaf area in half, and color goes from rich green to darker gray.

Leaf folding and rolling change the amount of energy absorbed. Changing the angle of exposure to the sunhas a similar effect. Also the evaporation is altered by the modified evaporating surface area that is exposed tothe atmosphere. These also change the effect of the wind, often in a more complex way. Each of these factorsinfluence leaf temperature.

Evaporation may take place on the surface or from within the leaf (transpiration). Most of the transpiration fromleaves is from stomata. The leaf has some control, be it passive or physiological. It appears that there is someadaptive control. Evaporation rates influence evaporative cooling and water stress on the leaf.

The factors that determine the temperature of the leaf are similar to the factors influencing the temperature of ahuman. The human adds metabolism to those factors. Combined, they determine the temperature of either theleaf or the being. It all can be expressed by the energy balance equation. We will consider the energy balanceconcept in connection with heat and water stress in a later section.

Agronomy 541 : Lesson 6b

Atmospheric Composition and Structure

Introduction

It is suggested that you watch Video 6B and complete the exercise in the video before continuing with thelesson.



The blue skies and clouds we see from the ground are only part of the atmosphere. While most of the weatheroccurs in the lowest portion of the atmosphere, numerous layers higher above the surface of the earth haveimportant properties and effects on our everyday life. In this lesson we will examine the composition of theatmosphere and the various layers. We will also discuss aspects of temperature change throughout theatmosphere.


About the general structure of the atmosphere.Why temperature varies in the atmosphere.About the various atmospheric layers

Reading Assignments:

pg. 4-19—Aguado and Burt



Atmospheric Structure

The atmosphere is divided vertically into layers. Each has specific properties involving the earth and itsweather and climate. Layers are named and differentiated by their temperature profile (Fig 6.16). Layers arenamed ending in -sphere or -pause. The -pause layers are neutral (no temperature change with height). Theseare the transition layers between the main levels, the -spheres.

Fig. 6.16 Average atmospheric

temperature profile and associated

levels.

The troposphere is the lowest layer of the atmosphere, extending from the surface up to ten kilometers at thetop of the tropopause. The first part is the troposphere from the surface up to about 10,000 to 15,000 feet.Then the tropopause extends from there up to 6 miles, about 20,000 or 30,000 feet. This is the layer wheremost atmospheric events such as clouds and storms that affect life take place. The temperature near theground averages about 20°C (68°F). Well into the tropopause, the temperature is about -40°C/°F. Theatmosphere cools about 5°F for each 1000 feet or 10°C/km (a dry atmosphere).

The stratosphere comprises the next 6 miles, being between the 12 and 18.6 mile height (expressed inkilometers, about a 30 km height). The stratosphere is basically an unmixed or stratified layer of theatmosphere, hence the name "stratosphere."The wind is usually not turbulent there, but there is generalcirculation movement. Things that are trapped in the stratosphere often remain in layers for a long time. Dustor gas from a volcano that penetrates into the stratosphere may be layered for a considerable period of time.Mount Pinatubo in 1991 erupted sending volcanic ash into the atmosphere which remained throughout 1992.This produced beautiful sunsets and cooled the surface of the earth by reducing incoming solar radiation.

The stratosphere includes the ozone layer. At the ozone layer, temperature actually begins to warm withaltitude. The ozone, among other things, is responsible for that. Ozone absorbs a great deal of solar energynear the ultra-violet end of the spectrum, heating the air. Well into the stratosphere, temperature may exceedfreezing (30 miles). Above this level only 1% of the atmospheric remains.

From about 30-50 miles (50 -85 km) temperatures cool again in the mesosphere. There are very few moleculesof air existing at this level. Above the mesopause at about 50 miles (85 km) is the thermosphere. Interactionbetween solar radiation and the very few air molecules at this level produce artificially high temperatures.These temperatures would not be felt by humans, though.

The aurora develops 60 miles away from the earth. This is an area of ionization sometimes resulting in thenorthern lights or the southern lights. They seem to be centered from 60-80 miles above the Earth. The aurora,occurring near the Arctic Circle or even further north, is occasionally visible in the central U.S. Rarely has itbeen visible as far south as Texas. The nature, causes, and effects of the ionized layers will be discussed later.

IN DETAIL : Atmosphere Temperature

Why the stratosphere doesn't mix with the troposphere.

A typical temperature structure near the surface of the earth is

Fig. 6.17

Here temperature decreases with height. Thus, warmer less dense air near the surface rises and colder more dense air aloftsinks and the atmosphere mixes.

Fig. 6.18

Discussion Topic 6.2

Why do you see cumulus clouds during the afternoon, but not at night? What changes during the day?

In Figure 6.16, the temperature structure in the stratosphere is different.

Close Window

Figure 6.16 Atmospheric temperature profile

The temperature rises with height. Warmer air is above colder air. This condition is more stable because the air densities haveequilibrated. The atmosphere will mix very little compared to the troposphere below.



Upper Air Charts

Figure 6.16 depicted the profile at a single point up through the atmosphere. This is usually measured byradiosondes (weather balloons) rising through the levels. Certain levels of the atmosphere are significant inunderstanding what the weather is doing and will do. At various pressure levels in the atmosphere,meteorologists draw charts which detail conditions such as temperature, winds, and height of that pressurelevel above the surface, the 500mb surface will vary in height because of temperature differences (Fig. 6.20).

Fig. 6.20 Warm air expands raising the height of the 500mb surface

while cooler air compresses and lowers the 500mb level (or level of

1/2 of the atmosphere).

Temperature differences produce hills and valleys in the height of the pressure surface. These hills and valleysare depicted as highs and lows on pressure levels maps (Fig. 6.21).

Fig. 6.21 Where the 500mb level is high are called ridges (denoted by

H). Low levels are troughs (denoted by L). Solid lines are the height of

the 500mb surface above the ground. Click on image to view larger

version.

The 500 millibar chart (Fig. 6.21) is commonly used to express conditions at the atmospheric mid-point. The500 mb chart is the key to what is happening in meteorology. Conditions half-way up through the atmospherewill influence and are indicative of situations in both the lower half and the upper half of the atmosphere.

From the waves in the atmosphere, dry locations and regions likely to be wet may be anticipated. It is called the500-mb heights chart, and the elevation of the 500-mb pressure is depicted much the same as terrestrialelevations on a topographic map. The "ridges" and "valleys" indicate where the 500-mb changes its elevationbecause the pressure is high, or because the pressure is low. These define the so-called troughs and ridgesand allow some assessment of where temperature and moisture conditions. Ridges act like high pressurecausing sinking air and cloudless areas. Troughs are associated with areas of low pressure and create risingmotion, clouds, and precipitation. In Figure 6.21 the coasts would probably be experiencing clouds andprecipitation while the Rockies and Western Plains would be drier and warmer.



Jet Stream

The 500 mb map (Fig. 6.21) depicts areas when the solid lies are closer together. These are areas of fasterwind speeds, usually associated with the jet stream. The structure of the atmosphere is associated with the jetstreams. Jets usually form at the edges of air masses. Watching where the air masses meet gives a goodindication of jet steam position. Wind velocity in jets is usually very high, often 100-150 mph. They werediscovered during World War II when pilots would report getting to targets over Europe much faster than theycould return. The two jets most common across the United States are the sub-tropical jet(usually positionedover the southern United States) and the polar jet(usually positioned over the northern U.S.). During very coldoutbreaks an Arctic jet may form bringing extreme cold from the North Pole.



Composition of the Atmosphere

Our atmosphere is composed mainly of gases that do not sustain life. Oxygen is an important gas to sustaininganimal life. Carbon dioxide is crucial to plant life. Carbon dioxide composes only about 0.03% of theatmosphere. Oxygen is about 21% of our atmosphere. The vast bulk of our atmosphere is made up of nitrogen(78%).

The nature of the atmospheric gases influences the behavior of the atmosphere. The periodic table ofelements indicates different molecular weights of elements. Different gases have different molecular weights.Ozone, which is O3 (M.W. = 48), is heavier than nitrogen (M.W. = 28). So why is the ozone layer way up high?

Carbon dioxide is heavier than nitrogen. So why aren't all the carbon dioxide and the ozone molecules near theground? They're not enough heavier to sit there very long.

If you put a chunk of dry ice in a pan of water and set it on your table, you will see all of the carbon dioxide boilout of that pan, down onto the table, and spill down onto the floor because it is heavier than the air. But soonyou won't see the carbon dioxide on the floor any more. It will diffuse among all the molecules in the air and berandomly and uniformly distributed throughout the room in a short time (Fig. 6.22).

Fig. 6.22 Over time gases will naturally diffuse to be evenly distributed

throughout an area.

Initially, a high concentration of carbon dioxide is heavy and sinks to the floor. The weight of it will not matter,however, since it is soon well mixed with the other gases. This is because they bounce around off each otherlike a bunch of billiard balls on a pool table. They become mixed into the atmosphere with gravity having littleeffect of their distribution.

Why is the ozone concentrated higher in the atmosphere? First, it is a concentration, but it is not all thatconcentrated. Most of it exists in this layer. It is still diffused and mixed throughout the atmosphere because theozone forms at the level due to the action of the sun and the gas molecules. We will discuss it more at a futuretime. But because it is forming there, it is up high and not because it is lighter than the air. It is actually heavierthan some of the other components of the atmosphere. That doesn't make it sit in a particular place. Otherfactors make it sit in a particular place.

Helium (M.W. = 4) and hydrogen (M.W. = 2) begin to dominate at higher altitudes. The composition of the air ataltitudes above 500 miles is hydrogen, our lightest gas (Fig. 6.23). There are very few molecules of gas left

here; the primary molecules here are hydrogen rather than nitrogen which dominates from 45 miles down to theearth's surface.

Fig. 6.23 Below 45 miles the atmosphere has the

given constituents. At very high levels, only the

lightest gases are found.



Adiabatic Temperature Change

Another application of temperature changes in the atmosphere is in adiabatics, or temperature changecaused by compressing or allowing air to expand. Air flow over a mountain explains this well. Prevailing windsoften force air over a mountain.

Fig. 6.24 Air forced over a mountain

cools drastically, but does not saturate.

Air returns to its upwind temperature

as it sinks.

On this mountain is some forced air movement. Wind is blowing air up over the mountain. Going from about1,000 feet at the base to 7,000 feet at the top, the temperature changes as the air goes over the mountain.The temperature decreases at about 5.5°F per 1,000 feet. Every thousand feet that we go up, the temperaturedrops about 5.5°F. After going up 6,000 feet, the temperature should have dropped by 32°F. Starting at 80°Fon the upwind side of the mountain, and cooling it off by 30°F, the temperature would be something around50°F at the top of the mountain. Then, coming back down the mountain, compression will heat the air back up.It will get back up to its 80°F by the time it gets back down the other side.

This is called adiabatic change of temperature due to compression and decompression of the atmosphere as itmoves over the mountain. Now go over the same mountain, starting at 60°F with a high relative humidity. Whenthe air gets up so high and cools off a little bit, it starts to rain on the mountain. Because the condensation ofmoisture releases heat, the air does not cool as quickly. Heat is being released as we go up as well as coolingbecause of the change in elevation. The air does not cool at the rate of 5.5°F. It cools at the rate of 3.2°F foreach 1,000 feet. Imagine this air that started at 60°F, and rains as it goes over the Sierra Nevada mountains.Reaching 10,000 feet elevation the air is at 18.8°F. Then it comes down the mountain.

Fig. 6.25 Air forced over the mountain

cools and saturates. After saturation,

rain falls and the air cools more slowly.

After sinking, it warms to above its

upwind temperature.

Try This!

This is a JAVA application of air flow over a mountain. Flow of a leaf is depicted as well as cloud formation onthe mountain picture. The initial conditions may be changed by dragging the condition bars on the right of thenumeric displays. The change of temperature with altitude is given in the graph to the right. Numeric displaysduplicate the conditions.

Set a condition and click on the Start Wind play button. Change the conditions and try again by clicking NewTrial. Try several different ones. The notebook picture gives a table of your attempts. Answer the followingquestions and submit your answers.

Study Question 6.5

Seattle is a very cloudy rainy city. What would Eastern Washington be like, wet or dry?

wetdry

Check Answer

Study Question 6.6

Air starting at 0 elevation is pushed over a 4000 ft mountain saturating at 2000 ft. The initial temperature is70°F. What temperature is it at the top of the mountain?

°F Check Answer

Study Question 6.7

If the air descends down the mountain, what is the resulting temperature (follow-up to Study Question 6.6)?

°F Check Answer

It doesn't rain any more as it comes down and warms at the rate of 5.5° per 1,000 feet. When it gets to thebottom of the mountain it is almost 74°F. It started at 60°F, cooled off as it went over the mountain and camedown the other side warmer than it started. This is the dry side of the mountain. It has been heated because itlost its water. This has a name, called a chinook wind.

A chinook wind has some very interesting effects agriculturally here in our part of the country. A chinook windoften develops in the winter. When the chinook develops in the winter, Canadians talk about the Banana Belt ofWestern Canada, which is just on the east side of the Rocky Mountains. The chinook effect often occurs nearthe Rocky Mountains in Nebraska, in the Dakotas, and in Canada. The air comes from the west coast, movesover the Rockies and comes down on the Plains side warmer than it started out on the Pacific side. It is dry andwarm, bringing a January thaw with desiccating air that is moving down the mountain. The chinook windchanges a lot of things in our environment. There are similar winds in other parts of the world such as SantaAnas in California and Foehn in Switzerland.

A couple of years ago we had a chinook wind that came off of the Rocky Mountains almost making it toChicago. It continued to move at a slow velocity for many days. We had a much warmer than usual winterbecause the Chinook wind essentially continued for six weeks, influencing us all the way across Iowa. We havetwo reasons that we can have a warm winter. It can be warm, moist air from the Gulf of Mexico flowing up intoour part of the world for the winter, which will give us a lot of snow, a cold and wet winter, or a cold and foggywinter. Or it can be chinook effect coming down off the Rockies resulting in a warm and dry winter. So we havehad warm, wet winters, and we have had warm, dry winters. That is why you will often hear that our warmestwinter may very well be our snowiest one, because the moisture is coming up from the Gulf. If it is warmbecause a chinook is blowing, it will be one of our warmest and droughty winters and make things very roughfor winter grains that are planted primarily in the western plains.

Assignment 6.1

Click here for Assignment 6.1

Lesson 6 Reflection

Why reflect?

Submit your answers to the following questions in the Student Notebook System.

1. In your own words, write a short summary (<150 words) for this lesson.2. What is the most valuable concept that you learned from the lesson? Why is this concept valuable to

you?3. What concepts in the lesson are still unclear/the least clear to you?4. What learning strategies did you use in this lesson?



References

Steadman, R. G., 1979: The assessment of sultriness. Part I: A temperature-humidity index based on humanphysiology and clothing science. J. Appl. Meteor., 18, 861-873.

Steadman, R. G., 1979: The assessment of sultriness. Part II: Effects of wind, extra radiation, and barometricpressure on apparent temperature. J. Appl. Meteor., 18, 874-885.



Introduction

It is suggested that you watch Video 6A and complete the exercise in the video before continuing with thelesson.



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 thetemperature of the forehead is usually near 30°C. It is possible that there is a physiological control mechanismto maintain a nearly constant forehead temperature. Under some conditions, the effect of heat loss by the headcan be seen. Maybe you have seen this effect when you have looked at the shadow of an automobile against abuilding 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 willtypically happen when light reflected from a car windshield shines in your window, and then you look at yourshadow on the wall. Often you can see the heat rising from your face or from your hand. It always seems towork 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 cansee wavy lines that may look like water on the road at some distance in front of you. The image of the road isdistorted 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 seeheat 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 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.

Energy Budget: Heat Index and Wind Chillagron- · PDF fileAgronomy 541 : Lesson 6a Energy...

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