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Agronomy 541 : Lesson 12a
Convection
Introduction
Developed by D. Todey and E. Taylor
It is suggested that you watch Video 12A and complete the exercise in the video before continuing with the
lesson.
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On a sunny afternoon, a breeze blows through the trees creating a gentle rustling sound, familiar to everyone.
This pleasing sound has a more scientific association than creating an attractive afternoon sound. It helps to
transfer excess heat away from the leaves on the trees. It serves to reduce the temperature difference between
the leaves and the surrounding air. This heat transfer, called convection will be discussed in this lesson.
What You Will Learn in This Lesson:
Definitions of convection.
About flow around an object.
How surfaces affect the flow near them.
Agronomy 541 : Lesson 12a
Convection
Convection
Atmospheric convection is simply the effect of wind. Wind is important, not just because it blows things away,
breaks things, and is associated with severe storms. Transport of heat and moisture by the wind is called
convective transfer. Heat, as it relates to leaves, is the main item that we will look at. Large objects are not as
likely to be at air temperature as small objects. That is a generalization, of course, but there are plenty of
examples. Large pipes go from the automobile engine to the radiator. The radiator is just a series of small
pipes or tubes with cooling fins attached to them.
Radiators in an automobile do not cool by radiation. They are designed to cool by convection. Air is forced
through the fins of the radiator, either by the motion of the automobile, or by the fan next to the automobile
radiator. The movement of the air carries the heat away from the small pipes and the cooling fins in the
radiator.
There is no convection in outer space. Lack of air disallows heat transfer via convection. Having no
convection present, things cool by radiation. Spaceships cool by radiation. An astronaut on a space walk
outside a spaceship cools by radiation. Also, if the sun is shining on the astronaut, they heat by radiation. But
there is no air temperature effect, no wind effect, because those things do not exist there. A radiator designed
to cool a spaceship is quite different from a "radiator" that we see on an automobile engine.
Agronomy 541 : Lesson 12a
Convection
Convection
Thermal Boundary Layer
The temperature of small objects is strongly influenced by air temperature. The temperature of the air is
strongly influenced by the temperature of large surfaces. Figure 12.1 shows a car. The top of the car is a large
surface. Cut a tiny piece out of the top of a car, maybe an inch or two wide and six inches long, and set that
piece near the top of the automobile. The piece is sitting next to the top of the car. It will also be flat and at the
same orientation as the top of the car.
Fig. 12.1 A car is parked in the sunlight. A small piece of metal
(a), similar in composition and orientation to the sun, is placed
above the car top (b).
Study Question 12.1
In the sun of the afternoon, which will be warmer: the top of the car or the little piece of metal?
Top of the car
Piece of metal
Check Answer
Agronomy 541 : Lesson 12a
Convection
Convection
Thermal Boundary Layer (cont.)
If it is parked in the sun, the top of the car will be warmer than the piece of metal. It would be difficult for air to
move over such a large object as the car and carry away the heat, whereas air will move very freely around the
small piece of metal.
A very small object is tightly coupled to the atmospheric conditions. A large item is de-coupled from the
atmosphere. A large item can have a different temperature than the air of its environment. The top of the car
can heat up to a much warmer temperature than the air around it. The asphalt of a sidewalk can become much
warmer than the air a foot above the sidewalk. A small piece of asphalt, broken off and suspended in the air,
will be close to the air temperature. It will not heat up greatly if it is suspended in the air. Again, it is a case of
size influencing the effectiveness of convection.
A thin object, such as a pencil lead, suspended in the air, must of necessity be very close to the air
temperature. Again that is because the air can move freely around it. Even though the lead may be a dark
color, absorbing a lot of sunlight, the coupling to the atmosphere and freedom of air moving around it will keep
it close to air temperature. Convection will quickly carry away heat that the item gains from the radiation
impinging on it.
The molecules of air near the hood of the car (Figure 12.2) are at the same temperature as the metal. A few
molecules away, the temperature will still be close to the surface temperature, but somewhere in between that
and the temperature of free air. At some distance from the car, the air temperature will be the same as the
environmental air. The atmosphere will no longer be influenced by the surface. The layer where the
temperature of the surface directly influences the atmosphere is known as the boundary layer. It is an
unstirred layer adjacent to a surface.
Fig. 12.2. When air (a) does not move or moves slowly across a car, a
boundary layer (b) forms where air is stagnant, immediately adjacent to
the car's surface. If the car begins to move the boundary layer will be
wiped away at a speed of 20 mph or so.
There is a temperature gradient across the boundary layer. If the temperature of the car were the same as the
temperature of the atmosphere, there would be no thermal boundary layer. Only where there is a temperature
difference next to a surface will there be a boundary layer.
This layer of stagnant air essentially insulates the object from the atmospheric environment around it.
Boundary layers can exist in any fluid. Air and waters are fluids. Air is a gas, water is a liquid, but are both
fluids. Wherever there is a fluid, there can be a boundary layer. Temperature changes across a solid material
are referred to as a thermal gradient, or when you are referring to temperature changes across a uniform
substance not in contact with a boundary. But in a fluid in contact with an object, a boundary layer can be
formed.
The boundary layer will vary in thickness depending on the temperature difference and other factors including
air flow. The boundary layer over the automobile may be an inch or two thick when the air is still. When the car
is moving, wind will likely wipe away this boundary layer. The air striking the front of the car will essentially
remove the boundary layer. Perhaps where the air is not striking directly, there will still be a thin layer, but often
the car surface will cool to the air temperature of the environment once it is moving. Stagnant air insulates an
object from the environment, and boundary layers exist in all fluids. These are important facts to remember.
If there is wind, the boundary layer becomes thin and may disappear altogether. Because air can move freely
around small objects, small objects always have thin boundary layers whether or not there is wind. There is a
temperature gradient across the thermal boundary layer.
Agronomy 541 : Lesson 12a
Convection
Convection
Moisture Boundary Layer
There is also a moisture boundary layer. If a wet object is suspended in the air, the moisture evaporating (water
molecules breaking free from the surface) will saturate the air next to this object. Moving away from the
evaporating surface, the amount of water vapor in the air will decrease until the moisture content would be the
same as the background humidity of the surrounding atmosphere. Any surface which has water that can
evaporate will set up a moisture boundary layer. The depth and strength of the boundary layer are influenced
by the moisture gradient and wind flow. In this lesson, the thermal boundary layer is emphasized first. Boundary
layers also apply to the surface of the Earth in regard to the rest of the atmosphere. The temperature
structure near the surface is shown in Fig. 12.3.
Fig. 12.3 Temperature structure (a) and moisture structure (b) over and
some distance away from a grass field. Different surfaces react
differently at different times of day in regard to heat and moisture transfer
at the surface. But some distance away from the surface all the profiles
become similar to the background temperature and humidity becoming
affected by the surface.
An example of a moisture boundary layer was displayed in Lesson 7 discussing dew. Water evaporating from a
moist surface helps saturate the air near it. Air can mix away more of the moisture in windy conditions. The
saturated and near-saturated air are blown away by the wind. So the evaporation rate continues strongly.
Discussion Topic 12.1
How would these profiles change during the day?
Study Question 12.2
When does a plant use most water?
windy day
calm day
Check Answer
FYI : Surface Heat
Heat and moisture from the surface warm and evaporate water into a layer 1-2 km thick above the surface. Above this
point, the atmosphere is essentially insulated or unaffected by changes in the surface. The diurnal (day-night)
changes of temperature and moisture in the atmosphere are seen in the boundary layer above the surface.
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Agronomy 541 : Lesson 12a
Convection
Convection
Free Convection
Two types of convection are associated with a boundary layer--free convection and forced convection. Forced
convection is associated with wind or something forcing air to move. Free convection is only associated with air
moving because of contact with an object of temperature that contrasts with the temperature of the air. An
object, warmer than the air surrounding it, warms air in the boundary layer causing buoyancy and a resulting
movement of air.
Figure 12.4a shows an aspen leaf, with a plume of warm air rising above the leaf. Why is air rising from the
leaf? The leaf is warmer than the air. The warm air next to the leaf is less dense than the air of the surrounding
environment. The effect is similar to that of a hot air balloon. The warm air over the leaf is less dense.
Therefore, it is buoyant and begins to rise being displaced by cooler air. Smoke rising from a cigarette
smoldering on an ashtray is another example of free or buoyant convection. The formation of cumulus clouds
on a sunny day is another example.
If the leaf were colder than the air, which often happens at night, the picture would be different. A plume of cold
air would be seen sinking off the leaf toward the ground. Air in contact with the cold surface in turn becomes
cold and more dense than the air in the atmosphere around it.
This happens with all kinds of leaves. Here, shown in dark rather than light, is warm air rising off of a pine
bough that is in the sun (Figure 12.4 ). The little line that you see is a thermocouple thermometer trying to
measure the temperature in that plume.
Fig. 12.4a. White-colored areas are warm, buoyant
air rising from the warm surface of a leaf.
Fig. 12.4b. Buoyant air rising from a pine bough.
This is free convection. The boundary layer is very visible. Nothing is stirring the air near the leaf. Temperature
is influenced by the leaf, and where the temperature gradient occurs between the leaf and the atmosphere. So
a boundary layer is visible all around the leaf.
What can be seen in this mystery picture Figure 12.5? This is the face of a little girl. She has a boundary layer
around her face, in dictated by the white layer. A mixed area is seen where she is breathing.
If a fan were turned on, this entire boundary layer would soon look like the disturbed area where the girls are
breathing. Then it would almost disappear altogether and look more like the air in the room that can be seen
away from the face.
Fig. 12.5. Boundary layer around a girl's face. It is disrupted
when she is breathing.
Experiencing a sauna is a good way to understand boundary layers. You may wish to review the Finnish
sauna story from Lesson 6.
The same situation can occur in a different fluid, water. Very warm water in a bathtub can be endured by sitting
very still. Your body will absorb some of the heat from the water around you, cooling it. Splashing in the tub
disrupts the boundary layer of cool water next to the skin, and then the water is too warm to enjoy.
FYI : Plume of Warm Air
Of course, this cannot be seen clearly under ordinary circumstances. The reason the plume of warm air can be seen
rising above the leaf in the figure is because of special optical effects; there is a point source of light illuminating the
leaf in front of a spherical mirror. This mirror looks like the inside of a split-open Christmas ornament. The
configuration is sensitive to the index of atmospheric refraction (air temperature differences).
This similar effect was discussed in lesson 6 in describing mirages. When strong direct or reflected light becomes
polarized, the effect can often be more dramatically seen. Cool air, environmental air, and warmer air have density
differences that can be seen. The photographic technique used in Figure 12.4 is known as the Schlieren Process.
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Agronomy 541 : Lesson 12a
Convection
Convection
Strength of Convection
The amount of buoyancy or convection can be quantified since the amount of temperature gradient over the
boundary can vary. The greater the temperature difference, the greater the amount of heat transfer from the
warm to cold surface can occur. Thus, the strength of the convection is proportional to the amount of energy
transferred from a surface by convection, called C for convection. The transfer is proportional to the difference
in temperature between the leaf (T L) and the air (Ta).
Equation 12.1
The greater the temperature difference between the leaf and the air, the more heat is transferred and carried
away from the leaf. If the leaf and air are at the same temperature, C=0 and no heat is transferred. The larger
the difference, the greater the heat transfer will be.
Additional factors affect how effectively the heat can be transferred. Wind will increase the amount of air
crossing the leaf. The effect of wind is labeled V for velocity. The greater the wind, the more heat will be carried
away. Then the temperature difference will be reduced as the boundary layer is removed and more heat is
carried away. The dimension of the object reduces it's ability to be near the background temperature. Dividing
by D, the dimension of the object accounts for this. The larger D is, the less heat per unit area will be carried
away from the object.
The car in Figure 12.1 is very large. It could be as much as 6 ft. (2 m) across. This number is divided by 2 and
the energy loss cut in half. It does not work exactly that way. It works according to the square root. If the wind
speed is divided by the dimension, and then square root is taken, and then of course this is multiplied by a
constant.
Equation 12.2
Almost all equations have a constant, called K1 in this case.
FYI : Constants
Sometimes constants are necessary to fit an equation to what is observed in nature. Other times they are just to
make the units come out right to convert from one type of a unit to another. In this case it does both.
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Agronomy 541 : Lesson 12a
Convection
Effects on Convection
The constant has a value, which depends on the size of the object that we are working with. A leaf, with a width
called D, has a maximum width Dmax. Its maximum length is Wmax.
This is related to the D in the equation. But it is not exactly the D that is in the equation. The D that is in the
equation is the characteristic dimension of this leaf. The characteristic dimension is the dimension of a
rectangular plate that would behave the same as a leaf. It would be the closest rectangular plate that has the
same heat transfer and that would have W and D according to the formula to convert from the maximum
dimension of the leaf to the dimension of the rectangular plate. That is the D in our equation. That's why we
were concerned about it last time. The characteristic dimension of the leaf is the "D" in the equation.
The value of the constant varies by leaf dimension. If W is equal to or less than 5 cm (2 in.) then K would be
1.13 x 104, if the length of the leaf is more than 5 cm (2 in.), then the K will have a value of 6.98 x 103.
Equation 12.3
The formula indicates that the amount of heat carried away from a leaf depends on the difference in
temperature between the leaf and the air, the square root of the wind speed, and the square root of the
dimension.
Study Question 12.3
How strong a wind is necessary to double the amount of heat transferred as compared to a 1 mph wind?
mph
Check Answer
Study Question 12.4
What is the value of convection if:
Leaf Temp
= 35°C
Air Temp
= 32°C
Wind Velocity
= 10 cm/s
Leaf dimension = 2 cm
ergs/cm2s
Check Answer
A similar situation occurs with leaf dimension. The larger something is, the warmer it will be or the larger the
temperature difference between it and the environment. The car top will be much warmer than the little one inch
portion of the top. A two inch item won't be twice as warm as a one inch item. It takes a 4-inch one to be twice
as warm as a one inch one. These are things to keep in mind in gauging the relative efficiency of heat transfer.
Agronomy 541 : Lesson 12a
Convection
Effects on Convection
Effects of Location on Heat Transfer
Now we are starting to see why leaf size is important. We talked a great deal about leaf size. Leaves will likely
be smaller on the southwest side than on the northeast side of a tree (Figure 12.6). Heat transfer is important
to ecological adaptation and to plant productivity.
Fig. 12.6 Leaves selected from the SW (left) and the
NE side of a tree. The smaller leaf, from the harsh
environment on the SW part benefits from the
convective cooling more than would a leaf of greater
dimension.
If the large leaf were on the south side of the tree, being about four times the dimension of the small one, it
would have twice the difference between it and the air temperature. The little leaf may get 1-2° F (0.5° C-1.1°
C) above air temperature while the larger might get 2-4°F (1.1°C-2.2°C) above air temperature in the same
locality. If the air temperature gets up to 100°F (38°C), this could be very deleterious to the larger leaf. It could
be harmful because of the temperature or because of the amount of water it requires. The small dimension
would have a real advantage in keeping the leaf closer to air temperature. It is considered an adaptive
advantage to have a small leaf in a very sunny, warm environment.
The formula that we looked at is written for forced convection since air speed is included. In nature, there is
almost never a case of free convection only. Convection is almost always forced convection in nature. Some
breeze or wind is always existing. Completely calm conditions are rare.
Usually, for analytical work it is sufficient to give a minimum value to air speed. The wind never goes to 0. It is
assigned a value such as 0.1m/s. That will either be the low value of forced convection, or represent the
equivalent of the free convection for the objects. That would usually work out suitably if we needed to calculate
what was happening to leaf temperature, water use, photosynthesis or any of the wind sensitive parameters we
might want to approximate.
Agronomy 541 : Lesson 12a
Convection
Effects on Convection
Integument
The leaf surface, color, and texture can also influence convection. Some soybeans may have a significant
number of trichomes (hairs) on the leaves (Fig. 12.7); some have few, if any.
Fig. 12.7 Pubescence on a
developing soybean leaf
There has been some interest from the standpoint of soybeans. Is there an advantage to having a leaf with a
lot of hair on it, or should they be bald (glabrous is the word botanists use). Which do we want for the leaves,
and what difference could it possibly make? One thing that has been observed, some insects do not lay eggs
on a soybean leaf which has a great deal of hair on it. They tend to lay their eggs against the rib of the
soybean leaflet. If they can't get their eggs right next to that rib, then they don't lay eggs on the leaflet. This is
an indication that perhaps the pubescence is a protection from the predation of some types of insects. Some
plant breeding work has been done to see if it will act as a biological control of some of the insect pests of our
crops.
Do other things affect heat transfer? What would it do to the transfer of heat from the leaf, or the transfer of
water from the leaf? If there is enough hair on the leaf, even a substantial wind does not come into contact with
the leaf surface. The boundary layer has a minimum thickness of the thickness of the hairs. A leaf with a
minimum boundary layer or insulation from the environment for both water loss and temperature difference
between the air and the leaf that is proportional to the amount and thickness of the hairs, or the pubescence,
on the leaf. This could be a major factor under some conditions.
A few little hairs on the leaf might have quite the opposite effect. In talking about forced convection, we
sometimes may hear the term laminar (or layered) flow (Figure 12.8a), and turbulent flow (Figure 12.8b). Hair
on a few rough places on a leaf may cause air that was otherwise flowing smoothly over the leaf to become
turbulent and hence carry away more energy because it is not in layers of air now. It is mixed air. Remember we
said that the boundary layer is a layer where the air is really essentially stagnant. If it becomes turbulent, it's
not stagnant any more.
Fig. 12.8a & b Laminar flow occurs over a smooth surface (left); a rough
surface produces turbulent flow (right).
Let's look at an example of a golf ball (Figure 12.9). The golf ball has little craters or dimples in it. What do
these dimples do? They keep the air from flowing smoothly over the golf ball in flight. They cause it to become
turbulent and rough. Then the air would carry more heat away from the golf ball, but who cares about the heat?
It also has less drag on the golf ball if it is turbulent. If it is turbulent, there is less drag and the golf ball will go
faster and farther. The golf ball with dimples on it will fly farther and truer if it had some spin to it (it would have
less hooking and slicing), because of the dimples.
Fig. 12.9 The dimples in a golf ball
serve to disrupt air flow and
reduce atmospheric drag on the
ball.
A fellow a few years ago "someone" decided to build a golf ball that, instead of dimples, had pimples. The
pimpling almost doubled the effectiveness of the dimples in disrupting the air flow, making it turbulent and
reducing the air resistance. The ball of the same weight and size went 30 percent further with the same person
hitting. The PGA outlawed it. Nevertheless, it was a more effective golf ball, and it was easier to hit in a straight
line. There was less hooking and slicing even than the fabulous one we have with the dimples. I suppose it
would have messed up everything, because a person would have been able to drive further and truer with the
pimpled rather than the dimpled ball.
When taking observations of a weather balloon from the atmospheric, observers like weather balloons to
ascend at a nice, uniform rate. One of the problems with the balloon ascending is that the wind blows it around.
Any slight breeze affects its rise. Rising through the various densities of the air deforms the balloon and
changes the ascent rate in addition to other factors.
A person named Jim built a pimpled weather balloon. It has bumps all over the outside of it to cause some
turbulence around the balloon as it rises. It rises at a much more uniform rate, being less influenced by these
minor perturbations of the atmosphere that used to make things a real headache for balloon watchers. The
Jimsphere (Figure 12.10), or the weather balloon with the pimples on it, has sort of become the standard and a
much more effective weather balloon.
Fig. 12.10 Jimsphere; protrusions disrupt laminar
air flow causing the balloon to rise in a less erratic
manner than an otherwise identical smooth
surfaced balloon. (AURA/ST SCI/NASA)
Back to the disruptions of air flow at the leaf. The density of hairs can be great enough that it does not disrupt
the air flow. There is not room for the air to break into little currents. Then the hairs would make the boundary
layer thicker. Similarly, additional dimples would be worse for the golf ball. There is an ideal size to those
pimples on the ball, or to the dimples in the ball. Likewise, there is an ideal number of hairs on the leaf or of
both density and length of the hairs on the leaf.
Agronomy 541 : Lesson 12a
Convection
Effects on Convection
Wind and Water Use
Here's a question for people that wash clothes. If you hang wet clothes on a line and the wind blows, do the
clothes dry faster or slower? The clothes will dry faster in the windy environment rather than in the dry
environment. This is usually true in Iowa, but not everywhere. There was an interesting study published by Dr.
Curtiss of Iowa State University, who was studying soybeans. He made a statement about the amount of water
used by soybeans of a certain leaf area and published how wind influenced the water use of soybeans.
A Professor Clumb from Rutgers immediately sent a letter to the editor of the Botanical Journal and said, in
essence, "Not so. I've made careful measurements, and the greater the wind, the less evaporation, the less
water soybeans use. Dr. Curtiss has done something wrong." Dr. Curtiss came back and showed that his
measurements were very careful, scientific. The wind made the soybean use more water. Dr. Clumb responded
to this. There were in excess of eight exchanges, as to whether or not a soybean used more water as the wind
speed increased.
A fellow from Texas, then, sent a letter to the editor stating he had done a careful experiment to see which of
these two eminent scientists was correct. He found that wind had absolutely no influence on the water use of
soybeans. He made many repetitions of his data and wind did not influence the water use of the soybean.
There the discussion ended. To settle the argument requires mechanistic modeling, not available at that time.
The relationship of water use (transpiration) to wind can be computed for different conditions of temperature,
humidity, and solar radiation (Figure 12.11). Under one condition making the wind four times as great will
double the water use, of the soybean. Under another condition of a little bit brighter sunlight, wind diminishes
water use. There is a condition, like in Texas, where wind speed increases don't change evaporation.
The environmental parameters determine the magnitude and sign of the effect of wind on water use, shown in
Figure 12.11. Everyone was right; everyone was wrong. The measurements were good, and the effect of wind
was correctly assessed for each case in the Curtiss vs. Clumb debate. The error was in the extrapolation of a
local observation to the universe of possibilities. Even the best of scientists may err in assuming a specific case
is universal.
Fig 12.11 Water use under varying wind
speeds and solar-radiation conditions.
Study Question 12.5
Under what radiation conditions does wind have little effect on transpiration?
977
697
558
279
Check Answer
In this case, we're looking at a difference in the amount of sunlight and thermal radiation that the leaves are
absorbing at a certain air temperature. Changing the air temperature and relative humidity, or the amount of
energy, sunlight, and thermal energy that the earth is absorbing, would produce a different set of curves.
You could never do enough experiments to find out in which cases the soybean uses more water, no difference
in water, or less water with wind to get them all. Combining the convection formula above with the water use
formula and the sunlight absorption formula, we can instantly calculate what the advantage is.
Another effect on the transpiration rate is evaporation. The evaporation from a leaf is expressed as:
Equation 12.4
and
Equation 12.5
Where E is the amount of energy used in evaporation of water, L is the latent factor to compute the energy
related to evaporation of water, s (TL) is the density of water vapor in saturated air at leaf temperature, s (T A)
is the density of water vapor in saturated air at air temperature, RH is the relative humidity in the free air, ri is
the stomatal resistance to water vapor loss and Ra is the resistance of the boundary layer to the diffusion of
water vapor. K2 is a constant, D is the characteristic dimension of the leaf, W is the characteristic length of the
leaf and V is the wind speed. The expression is an adaptation of Fick's law: the loss of water vapor is driven by
the vapor density (or vapor pressure) difference between the evaporating surface (cells in the leaf) and the
free atmosphere and is limited by the conductivity of the medium to water vapor. The conductivity is often
replaced by resistance to the diffusion of water vapor as expressed above.
It is generally assumed, for computation that the surface of a leaf cell is saturated with pure water and that the
relative humidity of the air spaces within a leaf is at 100%. None of these three assumptions is strictly correct.
However, any error introduced by the assumptions is usually negligible.
What it really comes down to is two things are happening when the wind blows. First, the wind is removing the
thermal boundary layer, as it blows. As the thermal boundary layer is removed, a warm leaf is cooled. A cool
leaf does not evaporate as much water as a warm one. The evaporation decreases because it has cooled. The
same wind has removed the moisture boundary layer. Water would evaporate easier. Whichever effect
predominates dictates the situation. If removing the moisture boundary layer allows a leaf to evaporate more
freely while keeping the temperature about the same, evaporation will increase. If a leaf was much warmer than
the air temperature, cooling it down to air temperature would reduce the evaporation. The temperature
difference could be a larger effect than the boundary layer for moisture.
The two boundary layers, the boundary layer for moisture and the thermal boundary layer, both behave
similarly. They are influenced by leaf dimension and wind. Both influence evaporation, one directly, the other
indirectly, through the leaf temperature. It is seldom intuitive as to which will be dominant.
Study Question 12.6
Doubling the relative humidity of the air will do what to evaporation?
Increase
Decrease
Check Answer
Study Question 12.7
Decreasing the leaf dimension does what to the evaporation?
Increase
Decrease
Check Answer
Agronomy 541 : Lesson 12b
Storms
Introduction
Developed by D. Todey and E. Taylor
It is suggested that you watch Video 12B and complete the exercise in the video before continuing with the
lesson.
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The topic of storms covers a variety of different topics. The Glossary of Meteorology (1953) defines a storm as
"any disturbed state of the atmosphere, especially as affecting the earth's surface and strongly implying
destructive or otherwise unpleasant weather." This definition leaves various different manifestations of storms.
The local scale of storm usually refers to a summer-time convective event, including thunder, rain, hail, high
winds, and possibly tornadoes. A larger scale event is a winter storm with freezing rain, snow, and cold. All of
these are of concern because of their potential loss of life or damage to property.
What You Will Learn in This Lesson:
About the thunderstorm life cycle.
About the climatology of different storms.
Reading Assignments:
pg. 293-304—Aguado and Burt
Agronomy 541 : Lesson 12b
Storms
Storms as Killers
Media has made the killer nature of storms much more apparent to us. Killer storms anywhere on the globe are
brought directly into our homes. The same media are able to warn us of impending storms in our area. Thus,
the number of deaths due to storms are decreasing.
National Weather Service statistics from 1994 provide the number of casualties associated with the variety of
severe weather: winter storms, 81; tornadoes, 69; lightning, 69; flash floods, 59. Injuries are also reported;
winter storms, 2859 ; tornadoes, 1087; lightning, 484; and flash floods, 33. This is an interesting statistic in that
the trend of casualties associated with storms has winter storms being the number one killer of people in the
United States. Lightning, heretofore, has been the most serious in taking of human life, followed by tornadoes
and flash floods. The world as a whole would probably have to put the floods associated with hurricanes in as
the largest killer. The reason for this is the high population concentration in hurricane and flood-prone areas of
Southeast Asia. Some staggering statistics result from cyclones (as hurricanes are called in the Eastern
Hemisphere) in Bangladesh, the Philippines, and other areas of southeast Asia.
While the number of deaths is decreasing, the dollar value of property damage is increasing. Some
discussion of these can be found at the Federal Emergency Management Agency (FEMA) website.
Fig 12.12 Damage in Florida from Hurricane Andrew (1992).
Most of the decrease in deaths can be attributed to improvements in forecasting. While the prospect of precise
long range forecasts are still a long way off, short term forecasts are improving steadily for all storms discussed
in the next two lessons.
Agronomy 541 : Lesson 12b
Storms
Winter Storms
Advance notice and media coverage of winter storms, like most other weather phenomena, is improving
steadily. Improved forecasting can give 24-48 hour notice of impending winter storm conditions. These are
most often produced by strong low pressure forming over the Plains and moving through the Midwest. A
number of tracks can produce a wide variety of conditions. A track of the surface low to our south (Fig 12.13)
may cause overrunning conditions as discussed in lesson 10 with warm fronts (Fig. 12.14). The low may bring
warm moist air aloft over colder air at the surface. The warm moist air precipitates through the colder air
producing a variety of winter weather.
Fig 12.13 Low track which can bring freezing rain.
Fig 12.14 Warm air being lifted over colder air at
the surface.
Fig 12.15 Clouds and weather produced by Figure 12.14.
If this condition occurs over a long period of time, huge amounts of ice can collect on objects causing power
outages and making travel impossible.
Another track takes a low from the Four Corners area, bringing them northeastward. These can be heavy snow
producers because they tap into and transport large amounts of moisture from the Gulf of Mexico. The track to
particularly watch is for the low center to move between St. Louis and Quincy, Missouri. This track will usually
bring the heaviest snow to central Iowa. Moving the track northward or southward changes the heavy snow
path correspondingly.
Fig 12.16 Track of a low which will produce heavy snow just
northwest of the surface low.
A third track, most well known for bringing very cold air southward from Canada, can bring heavy snow on
occasion. Alberta Clippers are usually fast moving, creating only a few inches of snow or less in their path while
bringing very cold continental Polar or Arctic air out of Canada. A few Alberta Clippers have created memorable
snowfalls. The February 1996 snowfall which caused 14" of snow in central Iowa was an Alberta Clipper.
Fig 12.17 Low track of an Alberta Clipper.
Snowfall over Iowa varies annually. Snow occurs from November through March, but can occur on October,
April and May. A fairly strong gradient in snowfall varies from 25" annually in southern Iowa to 40" in northeast
Iowa (Fig 12.18).
Fig 12.18. Average annual Iowa snowfall measured in inches (1951-1980)
IN DETAIL : Forecasting
The National Weather Service forecasts have become more detailed and will continue to do so in their forecast
condition and timing. During the winter, in advance of an impending storm, a winter storm watch is issued. This means
winter storm conditions could occur within the next 36 hours. Once conditions become assured, warnings are issued.
An upgrade of the watch to a winter storm warning can occur. A winter storm warning means any combination of snow,
sleet, freezing rain, or blizzard conditions, which would cause the disruption of travel or be a threat to life and
property, will occur. If one type of severe weather is expected, a specific warning for that condition may be issued.
The following warnings can be issued:
Blizzard Warning - must have 35 mph or greater wind for more than just 3 hours with visibility less than 1 mile
caused by blowing or drifting.
Heavy Snow Warning - widespread snow accumulations averaging 6 inches or more in 12 hours, or 8 inches during
a 24-hour period.
Ice Storm Warning - more than ¼ inch of ice is expected to form on objects.
Wind Chill Warning - wind chill temperature colder than -50°F.
High Wind Warning - winds greater than 40 mph or regular gusts greater than 58 mph.
Less severe conditions may be handled with an advisory.
Winter Weather Advisory one or more of the following can be expected (a less severe situation than a winter storm
warning).
Snow Advisory more than 3-5" of snow in a 12 hour period.
Freezing Rain Advisory freezing rain (or drizzle) of less than ¼" is expected.
Wind Chill Advisory wind chills of 35°F to 50° F.
Dense Fog Advisory widespread areas of fog reducing visibilities to ¼ mile or less.
Blowing or Drifting Snow Advisory intermittent visibility reduction to ¼ mile or less.
Wind Advisory Sustained winds are 30-39 mph or gusts of 45-57 mph are expected.
These criteria vary around the country and during the winter depending on the timing. Whereas 2" of snow in Iowa
has little effect, 2" of snow can be disastrous in the south. In Iowa, advisories are generally issued during the first
snowfalls to heighten people's awareness, even if 2-3" is expected. By the end of the winter, criteria are lessened to
prevent warning overkill. An advisory of a 2" on top of 10" on the ground is rather pointless unless accompanied by
other severe conditions.
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Agronomy 541 : Lesson 12b
Storms
Thunderstorm Climatology
Most of the warm season rainfall is dominated by convective clouds which lead to thunderstorms. Whenever
conditions are unstable enough, thunderstorms may occur. These conditions may occur any time of year in
Iowa. When the potential for severe weather exists a watch or warning may be issued. In warmer climates,
they are more frequent throughout the year. Iowa has 40 or 50 thunderstorm days per year (Fig 12.19),
occurring mainly in the summer months. Warm winter times can also produce thunderstorms. Even
temperatures near freezing and falling snow can accompany thundersnow. Missouri has 50 or 60 days per
year. Florida has over 100 thunderstorm days per year for two reasons. One, Florida is warmer throughout the
year, allowing more days for thunderstorms to occur. Second, air mass thunderstorms can occur almost any
afternoon in the summer.
Fig 12.19 Average thunderstorm days per year (Lutgens and Tarbuck).
There are very few thunderstorms days in California and along the West Coast. In the Rocky Mountains, 20-30
thunderstorm days occur per year with some of them being thunder snowstorms. Thunderstorms are most
frequent in Florida, in the southeast, the southern Rockies, and the adjacent Great Plains, chiefly in the
summer. Our fall and winter and early spring weather patterns are not thunderstorm patterns; they are frontal
patterns. This is due to the stability of the air that we talked about previously. Thunderstorms seldom occur on
the West Coast where the temperature contrast between the water and the land is minimal. A more uniform
temperature area, doesn't create the instability to develop severe thunderstorms.
In Iowa thunderstorms can occur at any hour of the day. During the summer thunderstorms usually occur at
night. This contrasts with storms along the front range of the Rockies and Florida, where thunderstorms are
usually afternoon events. The maximum in afternoon heating leads to convection and the production of
thunderstorms in these two areas and most of the rest of the United States. Often these storms are air mass
thunderstorms. Iowa thunderstorms (and one in most of the Upper Midwest) usually require the additional
forcing caused by low level winds which occurs at night to produce thunderstorms which are more severe and
longer lived storms.
FYI : Watch or Warning
A severe thunderstorm watch or tornado watch is issued when conditions are appropriate for a severe thunderstorm
or tornado. A warning is issued when a severe thunderstorm is occurring or a tornado has been sighted or indicated
by radar.
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Agronomy 541 : Lesson 12b
Storms
Thunderstorm Life Cycle
Air mass thunderstorms go through stages of development (found on page 118 of the textbook). In the early
stages of development, clouds form in rising air, produced by heating at the surface, and grow, creating tall
columns of clouds in the sky, reaching 25,000 feet. At this point they are not precipitating. Downdrafts
eventually start as the precipitation forming process creates drops that begin to fall. Continued precipitation fall
and cooling of the falling air by melting and evaporation produce downdrafts falling through the storm. The
downdrafts eventually overcome the updrafts. As the thunderstorm begins to end its life, it almost becomes
entirely downdrafts. This life cycle is particular to an air mass thunderstorm. These initially form by rising
bubbles of air in a uniform air mass. Since they kill themselves, they rarely create weather more severe than
some lightning.
Fig. 12.20 Air mass thunderstorm life cycle.
More common in the Midwest are more severe squall-line or supercell thunderstorms. These are more severe
because their tilted updrafts continue to process moisture and feed downdrafts over several hours. These are
more severe, producing heavy rain, hail, high winds and tornadoes. A discussion of these is found in the text
on page 117-126.
Study Question 12.8
What is the temperature when of the cloud when precipitation starts to form?
°C
Check Answer
Hail
Hail was discussed in lesson 5 as a form of precipitation. Hail is also a form of severe weather associated with
thunderstorms. Ice which is too light to fall through an updraft can be recycled in the updraft in a storm. As the
smaller ice is recycled, it collects more water, becoming larger, until it can fall. The stronger the updraft in a
storm, the larger the hail it supports. Very strong storm updrafts are necessary to support large hail. Hail
damage to structures and vehicles in Iowa is fairly rare. Large hail is more common in the High Plains where
storms have extremely strong updrafts (Figure 12.21).
Fig 12.21 Number of hail days per year.
Since hail is more common and larger in the Great Plains, it is of more concern to crops there. Spring storms
can decimate a wheat crop easily. Some area of North Dakota employ cloud seeders to reduce the possibility of
hail. The method used is to seed the cloud with silver iodide. The theory is that by providing more ice nuclei
for hail to form on will produce smaller hail, which will melt while falling. Iowa is susceptible to hail. The state
sustained a large amount of hail damage during the late spring of 1998 (Figure 12.22).
Fig 12.22 Hail damaged corn.
Large hail stones are not as damaging to crops as is small (1/4-3/4 inch) intense hail. Damage to corn and
soybeans can be more severe later in the summer after the plant has grown. Damage to corn early in its life,
while the growing point is underground, can usually be overcome. Soybean damage can also be overcome.
While drought may reduce crop yield by 40% across the Corn Belt once every six years or so, hail is less
general but may be just as severe in local areas. Annual yield loss to hail averages 2.45% for Iowa, 3.56% for
Nebraska, and 3.75% in South Dakota. On the average, the Midwest crop loss to hail is 1.85% per year. In
1980, the loss was near 3.5% Corn Belt-wide. Both hail and tornadoes appear to have a recurring geographic
pattern with an intense band near the Missouri and the Mississippi Rivers.
The peak of the hail damage season is mid-July. Significant losses can occur from May through October, the
entire growing season. It is not uncommon to have 50 hail damage days in a single growing season, but
normally there will be one or two major out brakes that account for much of the damage. The 23 June 1981
storm accounted for 51% of all hail losses for the year in the entire Corn Belt.
Most damaging hail events occur between 3 p.m. and 9 p.m. This is about the same time period found for
tornadoes and for very heavy rainfall. The peak time for thunderstorms in Iowa is between 11 p.m. and 7 a.m.
(Hillaker and Waite, 1985).
IN DETAIL : Downdrafts
These downdrafts reach the ground and spread. As a thunderstorm approaches an area, the downdrafts will usually
precede the storm. Gusts of 30-50-mph can be recorded as these downdrafts hit the earth and then spread out away
from the thunderstorm. Particularly in the direction the storm is advancing, large clouds of dust may be seen over dry
areas.
Initially, air begins descending in a thunderstorm being pulled by the falling rain and ice. As the rain and ice descend
into warmer temperatures, the ice melts and the rain evaporates. These processes require heat to change the
phases. The heat is drawn from the surrounding air, cooling it. This cooled air becomes negatively, buoyant and
begins to sink faster, creating a strong downdraft. In severe cases, these downdrafts can become intense enough to
become a hazard on the ground (see pg. 126-127 in the text).
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Agronomy 541 : Lesson 12b
Storms
Tornadoes
Tornadoes are the most violent and damaging part of a thunderstorm. Although not in the center of the
tornado-producing region of the country, Iowa has a number of tornadoes annually, especially in the western
and southwestern parts of the state. Figure 12.23 depicts the annual number of tornadoes per state. The
region of most tornado occurrence is to the south and west of Iowa, peaking in Kansas, Oklahoma, and Texas.
This is the prime area for tornado researchers and tornado chasers in the spring and early summer.
Fig 12.23 Number of tornadoes per state.
Tornadoes in Iowa occur primarily in the months of May, the peak of the Iowa tornado season, and June.
Numbers fall off from these peaks through the summer and into the early spring. During the cold season they
are very rare. But they can occur and have occurred in any month. Why is May the peak of the tornado
season? We have the greatest contrast between the cold air of winter and the warm, moist air of the
approaching summer. Warm air is advected from the south. Cold air is transported aloft from the north. The
contrast of air masses produces a very unstable situation.
The most likely time to have tornadoes in Iowa is 6 p.m. There are very few before noon. After noon the
number increases to 6 p.m. The numbers drop off after 10 p.m. (Figure 12.24). The number of tornadoes over
night is very small compared to the thunderstorm frequency at night. Intense thunderstorms do occur at night.
But the tornado-producing ones usually occur in the late afternoon and evening.
Fig 12.24. Iowa tornado frequency distribution (195971)(Stanford, 1987).
What is the prime direction of movement? Two-thirds of the tornadoes move in from the southwest; about onethird of them from the west or northwest, and less than 2% of them come from an easterly direction. But they
can move any direction. The vast majority that are coming toward you are coming from the southwest or at
least from the western half of the circle.
Tornadoes form only when there is strong thunderstorm activity in the area. The day most likely to have
tornadoes occurs when there is a strong cold front, with the cold front running almost north and south. Warm
moist air will be flowing northward ahead of the cold front. Storms will usually begin over the Plains and move
eastward. Nation-wide the tornadoes are most likely in May, but deaths are most likely in April. There are fewer
tornadoes in April, but deaths are most likely. Fewer total tornadoes occur in April, but the percentage of
severe tornadoes is higher. This is probably due to a few well known April tornado outbreaks.
One outbreak that I experienced, was the outbreak of tornadoes on April 3 of 1974. I was a National Weather
Service employee at the time, stationed in Alabama. As we came to work that morning, we saw that there was a
cold front laying just like the weather situation is pictured on pg. 116 of your book. I remember Clarence came
to work, glanced at the weather map, and said, "Oh, tornado weather." It was no news to any of us. As the day
started to develop, the tornadoes began to develop all the way from Illinois to Alabama. This particular one that
you see making the long path (on page 116) went through the National Weather Service office at Huntsville.
That's not where I was. But I was watching these guys on the computer typing the message: "Tornado on the
runway, coming toward tower. Evacu..." They didn't get the word finished. They left the tower. The tower was
destroyed. We didn't hear anything more from those guys and we didn't know if they were hurt or not. We didn't
hear from there for two weeks until they got their weather office back open up in Huntsville. It was essentially
completely destroyed, but no one was hurt.
The super outbreak was the largest single outbreak of tornadoes on a single day. The length of the path of
these was great. There were 127 tornadoes in that particular outbreak, 315 deaths, and $600 million in claim
damages. Another famous tornado was the Tri-state tornado of March 15, 1925. Reportedly a single tornado,
but more likely a series of tornadoes produced a line of damage which extended from southeastern Missouri to
southern Indiana. There were 695 dead and 2027 injured in this event.
To learn more about this go to the Tornado Project Online.
Tornados are classified by the fujita scale, a scale developed by a retired professor from the University of
Chicago, Theodore Fujita. More about him can be seen in the text on page 126-127. The scale is based on the
damage done by the tornado.
Most tornados care classified as F0-F1, which are fairly weak tornadoes with winds less than 112 mph (Figure
12.25).
Fig 12.25 Breakdown of tornado strengths.
But the strongest tornadoes are the largest killers (Figure 12.26).
Fig 12.26 Breakdown of deaths by tornado class.
A number of tornado myths exist. That large cities do not get hit by tornadoes or that rivers protect places are
not true. Cases that debunk these myths can be found at the Tornado Project Online site.
Assignment 12.1
Click here for Assignment 12.1
Lesson 12 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?
Agronomy 541 : Lesson 12b
Storms
References
Hillaker, Harry J., Jr., and Paul J. Waite. 1985. Crop-hail damage in the Midwest Corn Belt. J. Climate and Appl.
Met. 24:3-15.
Lutgens, F.K. and E.J. Tarbuck; 1992: The atmosphere. Prentice Hall Inc. Englewood Cliffs, NJ.
Stanford, J.L. 1987: Tornado: Accounts of tornadoes in Iowa. Iowa State University Press, Ames, IA.
Agronomy 541 : Lesson 12a
Convection
Introduction
Developed by D. Todey and E. Taylor
It is suggested that you watch Video 12A and complete the exercise in the video before continuing with the
lesson.
Podcast Version
Full Podcast List
On a sunny afternoon, a breeze blows through the trees creating a gentle rustling sound, familiar to everyone.
This pleasing sound has a more scientific association than creating an attractive afternoon sound. It helps to
transfer excess heat away from the leaves on the trees. It serves to reduce the temperature difference between
the leaves and the surrounding air. This heat transfer, called convection will be discussed in this lesson.
What You Will Learn in This Lesson:
Definitions of convection.
About flow around an object.
How surfaces affect the flow near them.