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Agronomy 541 : Lesson 4a
Derived Temperature Indices
Introduction
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Regular measurements of climatic temperature data include maximum and minimum temperatures at numerous
climatological stations. Interpreting how these temperatures affect crop growth is not always straightforward.
But other indices derived from these data can illustrate how crops should be responding to the temperatures.
While not completely indicative of crop condition, indices such as the Growing Degree Day can be descriptive
when summed over the growing season. There are several other indices which can be derived from maximum
and minimum temperature measurements; these include Stress Degree Days, Aridity Index, Cooling Degree
Days, Heating Degree Days, and Chill Units. These indices have agricultural and personal comfort application.
We will define and discuss applications of these indices in this part of lesson 4.
What You Will Learn in This Lesson:
How temperature and moisture stress in crops can be compared using various indexes.
About temperature indexes.
About combined temperature and moisture indexes.
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Stress Degree Days
Average temperatures and precipitation are the starting point for comparison of weather and climate between
years and regions. Average maximum temperatures and precipitation for Ames, IA (Fig. 4.1) are included for
reference values. Examining historical climate and yield records has produced a statistical relationship for
monthly temperature with yield (Thompson, 1986) (Fig. 4.2). If the July temperature is near normal in the U.
S. Corn Belt, a near normal crop yield is likely. If the July temperature is 4° F above average, the yield might be
diminished by 600 kg/ha, and if the temperature is 4° F cooler than average in July, the yields may be
enhanced by 300 kg/ha. There have been cold temperatures and low yield, and cold temperatures another
year with very high yield, but there is a relationship that gives some predictability. If the temperature is a few
degrees cooler than usual, it helps the yield. This result implies that normal July temperatures are slightly
stressful to crops in the central United States.
Fig. 4.1 Average monthly maximum and minimum temperatures (°F)
and precipitation (in.) for 1951-1997.
Three degrees colder than usual seems to be the optimum. If it is even colder than that, then it starts to drop
off again. Of course, average temperature means average yields. This relationship allows the July contribution
of temperature to yield potential to be evaluated.
The largest single-month statistical effects on the crop are from July and August. This is because of the
stressful conditions that may occur during the critical reproductive periods of those months. The idea with
temperature indices is to try to quantify some of the stress crops may experience.
Fig. 4.2 The response of corn to weather variables in
the five central Corn Belt states.
Although averages are useful for assessing expected conditions, the range and variability are also of great
concern. Most stressful years will occur on the extremes of the range of climatic possibilities, not near the
means. The range of average high and low temperatures during the month of August over the past 100 years
for Ames gives an example (Figure 4.3).
Fig. 4.3 August monthly averaged maximum and minimum temperatures
for Ames, IA.
Study Question 4.1
How many degrees above average were August 1995 maximum temperatures?
°F
Check Answer
Study Question 4.2
How many degrees above average were August 1995 minimum temperatures?
°F
Check Answer
Figure 4.3 indicates that August 1995 was not particularly one of the hotter Augusts of the past century. It is in
the warmest 50 percent, but not in the warmest 10 percent. High temperatures were not stressfully hot.
Minimum temperatures in August 1995 were some of the very warmest nighttime temperatures in the history of
records kept in Iowa.
This is a significant situation. When the overnight temperatures are high, there will be a marked influence on
growing degree days, more of an influence than if daytime temperatures were high. Remember, growing
degree days cut off at 86° F. What will happen to corn development? The growing degree days will be greatly
accelerated by the high overnight temperatures. Compare the expected August growing degree day for 1995
with 1988 and with an "average" August (Fig. 4.4).
Fig. 4.4 Growing Degree Days for 1988 in Ames, IA (Blue line is actual
GDD accumulation; red line is normal GDD accumulation-left axis.
Green line is the difference in accumulated from the average-right
axis.).
Study Question 4.3
How many GDDs are normally accumulated by July 1 at Ames, IA?
GDDs
Check Answer
Study Question 4.4
What was the difference in accumulated actual growing degree days from average on July 1, 1988?
GDDs
Check Answer
Fig. 4.5 Growing Degree Days for 1995 in Ames, IA (lines
are the same as Fig. 4.4).
In the 1995 situation we have enhanced corn growth and GDD accumulation in August without increasing
stress. In 1988 there was a great increase in stress caused by increased maximum temperatures. There are
ways of calculating this stress using temperature values. Note that the GDD accumulation in August 1995
exceeded that if the very stressful 1988 (Fig. 4.4 and 4.5).
FYI : July Temperatures
Monthly averages and other summary information for climatological stations around the state can be found at the ISU
Climatological Data page:
http://www.agron.iastate.edu/climodat
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IN DETAIL : Comparison of Phenomena
When working with comparing amounts or evaluating phenomena for a particular year, always compare it to the
average. It is necessary to make an "eyeball" guess. To do this, place and adjust a ruler on the chart until it is in the
approximate center of the values or estimate what the average temperature may be. Notice the average temperature
for the highs. Note what it was in 1995. Note what it was the previous hot year, 1988 in this case for the high
temperatures. Compare what happened this year with the previous ones and make an estimate, remembering that we
have been looking at growing degree days when we study the factors that influence crop growth.
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Agronomy 541 : Lesson 4a
Derived Temperature Indices
Heat vs. Water Stress
Some measure of assessing the stress on crops is necessary without going into the field to determine the plant
status, since field measurements are not always available. One of these methods uses daily maximum
temperatures, called the Stress Degree Day (SDD). Similar in calculation to the growing degree day, the
concept of the stress degree day is to use temperature only to measure the stress on crops. We often hear
about heat stress on a crop. True heat stress does not occur very often. Experimental work indicates that the
crop is under severe heat stress when the temperature of the air is in excess of 112°> F (44.4°C). Actually,
leaves begin to cook (experience protein breakdown) when the leaf temperature reaches 117°> F (47.5°> C).
Conditions such as these are rare in Iowa. Table 4.1 lists the number of days and the year when 112°F
(44.4°C) or higher temperatures have been recorded. Note that there have not been any since 1940. Even the
warmest summers of recent history have not produced such extreme temperatures.
Table 4.1 Occurrences of greater than 112°F (44.4°C)
temperatures in Iowa
Heat Stress Occurrences
Year
1894
1901
1911
1918
1930
1934
1936
1939
1940
Number of Days
1
2
2
2
1
9
12
1
1
At any temperature below that, they are not cooking. They may be using an extraordinarily high quantity of
water because of being at a high temperature. Often that which is referred to as heat stress is really water
stress induced by elevated temperature.
At what temperature does this water stress that is induced by high temperatures (or other factors) become a
significant factor? First, look at the optimum temperature for a crop. For an individual plant, probably the
optimum temperature for photosynthesis and for crop development is 92°> F (33.3°C). If the temperature
raises above 92°> F (33.3°C), changes begin to take place because of temperature induced increased
respiration rate. It is possible that various activities are not working correctly in the plant chemistry. Between
92°> F (33.3°C) and 117°> F (47.5°C) plant development decreases rapidly. Remember, whenever the leaf
temperature hits 117°> F (47.5°C), the leaf dies suddenly. As the temperature drops below 92°> F (33.3°C),
the photosynthesis and efficiency of the plant drop off and often stop near the temperature of freezing (around
32°> F, 0°C) (Fig. 4.6).
Fig. 4.6 Crop growth, stress, and death
conditions as a function of air temperatures.
At any temperature there could be water stress. It begins at about 86°> F often enough that air temperatures of
86°> F or higher are considered to induce stress, or be stressfully high temperatures. The base temperature at
86°> F is used as the base temperature calculation point for stress degree days (SDD).
Stress Degree Days are calculated similarly to growing degree days. If the low temperature is 70°> F (21°C)
with a high temperature for the day of 90°> F (32.2°C), the low temperature is set to 86, which has been
determined to be the average base temperature for stress (if the minimum temperature is above 86°> F, use
the minimum temperature. Add that to 90 and divide the sum by 2, giving the average of 88°> F. Subtract the
base, which is 86, to obtain 2 stress degree days for the 24-hour period. Stress degree days are determined in
much the same way as we keep track of the growing degree days by summing daily throughout the season.
Equation 4.1
Study Question 4.5
Yesterday's high was 98°F and the low was 79°F. Compute the 24-hour stress degree day contribution.
SDD
Check Answer
Study Question 4.6
Under conditions of a high of 102°F and a low of 88°F, compute the 24-hour stress degree day contribution.
SDD
Check Answer
Stress degree days are a valuable indicator of anticipated crop yield for the Midwest. Figure 4.7 is a chart of
the accumulated stress degree days for the state of Iowa from 1948 through 1988.
Fig. 4.7 Stress Degree Days for
Ames, IA (1950-1988
The higher the mark, the greater the stress for the year. It is like a golf score. If there is no mark here, there is
no stress during that particular year (1967 for example). Good crop yield would be expected when there was
little heat stress adversely influencing the crop.
Notice that from 1955 through 1973, stress was relatively uniform from year to year. There was some variability,
but not compared to the variability since that period. Some years show low stress, some years, very high
stress. Two high stress years, 1983 and 1988, are notable.
The question immediately becomes one of why the stress on the crops has been increasing during the past few
years, and when will it get back to normal. But the real question is, "What is normal?" To see what normal is,
look at the entire chart and see what it has looked like for 100 years(1900-1988) (Fig. 4.8).
Fig. 4.8 Stress Degree Days for Ames, IA (1900-1999)
Iowa's summer temperatures were rather mild from 1954 through 1973 producing a period of very low stress.
These years are referred to as the "benign years" by climatologists (Baker et al., 1993). There was very little
stress on the crops and very consistent crop yields because the weather was dependable and consistent. In
the Dust Bowl years (1934, 1936), stress was terrifically high. It was much higher than it was in recent years,
even the stressful years of 1983 or 1988. Stress was, also, high in the drought year of 1901.
Remember that SDDs use temperature only. Stress can be caused in other ways. The SDD assumes "average"
soil moisture. Water stress is induced by unavailability of water, whether by extreme atmospheric demand, low
soil moisture levels, or both. At high moisture levels, stress does not begin until temperature reaches 92°> F or
more. However, in dry soils stress may begin at temperatures near 70°> F. The 86°> F base is considered to
represent an average condition. The growth curve might be adapted under water stressed conditions (Fig. 4.9)
as reduced water availability reduces the potential plant growth.
Fig. 4.9 Decreased potential photosynthetic
rate caused by reduced soil moisture levels
and water stress
In 1995 Iowa's crops survived the very high temperatures in July because the water conditions were so near to
ideal that the high temperatures did not cause excessive damage. When the August high temperatures came,
water was limited in about half the state. Crops eventually suffered quite a bit of stress. Stress would not really
begin until after 92°> F with ideal soil, root, and water conditions.
Stress computed on base 86°> F (30°C) may not mean low yield if it happened to be a moist year. However,
there is a close correlation in the state of Iowa between the amount of rain and maximum temperatures (Fig.
4.10).
Fig. 4.10 Average statewide relationship between July
precipitation and maximum temperatures.
Years with high precipitation tend to be the years when the maximum temperature was below average. Years
with near average high temperature in July can receive anywhere between 2 inches and 7 inches of
precipitation. Very warm years, when the high temperatures were greater than 90°> or 92°>, have always been
on the dry side of usual. The driest year was the warmest year. However, the July's second driest year had
near average temperature. But there is a relationship. If the temperatures are cool, conditions tend to be wet.
Generally if the temperatures are hot, conditions tend to be dry. This contributes to the correlation of crop
yields with stress degree days, at least for the state of Iowa (Fig. 4.11).
Fig. 4.11 Yield reductions (bu/Ac) caused by increased July and
August heat stress. The brown line indicates yield reductions when
soil moisture is limited. The purple line indicates when soil
moisture is high.
Fi
4 11 i di
t
th
ff
t f
il
i t
dh
t t
i ld Wh
il
i t
i hi h
Figure 4.11 indicates the effect of soil moisture and heat stress on crop yield. When soil moisture is high on
July 1, there is no yield reduction, when soil moisture is limited, increasing heat stress reduces yield
significantly.
How much yield reduction would you expect from Jul/Aug. heat stress of 250 with insufficient soil moisture?
bu/Ac
Check Answer
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Aridity Index
Using temperature is the simplest method of quantifying stress. But in the previous section, we discussed how
crop stress may vary by temperature depending on the moisture involved. Extremely high (>93° F)
temperatures will stress a crop no matter what the soil moisture content. Between 82 and 93° F (27.8-33.9° C)
stress can be alleviated by sufficient moisture for the crop to transpire and alleviate the stress. Since soil
moisture acts as an alleviating factor in assessing stress, it must be accounted for when determining the
amount of stress.
The best method would include soil moisture, wind, solar radiation, crop stage, and as many other other factors
as possible. But soil moisture values and other factors are not regularly measured, are rarely available over a
large area, and what measurements exist have little detail. Since the source of soil moisture is rainfall, total
precipitation over some period (usually a month) can be used as a proxy for soil moisture.
The least stressful conditions are obviously those when temperatures are below average and rainfall above
average. The most stressful conditions occur when above average temperatures happen during periods of
below average rainfall. The other contingencies produce intermediate conditions since the temperature and
precipitation values oppose each other (Fig. 4.12).
Fig. 4.12 Categorization of possible crop stress based on
monthly average temperature and total precipitation.
Classifying these stresses gives some indication of conditions. Saying a plant is under stress is understood.
But the extent of the potential stress cannot be ascertained from the contingencies in Figure 4.12. It is possible
to have more stress in contingencies 1 or 4 than in 3 (Fig. 4.12) depending on the situation. Knowing the extent
of potential stress is important. A numeric value of the stress would best classify the amount of stress. To force
the rainfall to appear similar to the soil moisture, the value must be integrated over a longer period of time.
Since monthly rainfall and temperatures are usually easily available, they are used for this analysis.
Stress occurs under drought or arid conditions. Knowledge of the drought severity is useful, then, in
determining the stress. An index combining rainfall with temperature to measure the stress is named the Aridity
Index (AI) (Harouna and Carlson 1994), defined using monthly maximum temperature and precipitation as
Equation 4.2
Equation 4.3
Equation 4.4
The index is defined comparing monthly averages for maximum temperature and precipitation to their average
values. The T' and P' are normalized by the monthly standard deviation. The normalization indicates how
different from average a month is compared to how variable the month can be. When temperatures are above
average and precipitation is below average, the aridity index is positive, signaling droughty conditions. When
opposite conditions prevail the index is negative.
Study Question 4.9
What is the aridity index when the July average maximum temperature was 99.1°F and the precipitation total
was 2.5 inches? (July average monthly maximum temperature is 89.2°F with a standard deviation of 5.7°F,
July average precipitation is 3.7 inches with a standard deviation of 2.4 inches)
Check Answer
For comparison, absolute extremes of the aridity index for a single month in Iowa have a magnitude of about
5.0. Summers of 1934 and 1936 contained extremely hot and dry conditions with AI values from 5-6 in parts of
the state. Ames, IA values confirm the arid conditions of the 1930s (Figure 4.14a). The summers of 1992 (cool
temperatures) and 1993 (heavy rainfall) recorded some of the lowest with AI values between -5 and -6 (Figure
4.14b).
Fig. 4.14a Ames, IA aridity index (Eq. 4.2) for the 1930s.
Fig. 4.14b Ames, IA aridity index (Eq. 4.2) for the 1988-98.
Other conditions modify what the crop is experiencing, but the AI gives a good first approximation of extremes
of climate. Warm and wet conditions or cool and dry conditions produce opposing values of the AI. This is
another advantage of the AI. These countering effects can be somewhat confounding. Using the AI helps
discern what conditions actually exist. It also provides historical context of how certain drought periods compare
to other historical droughts.
IN DETAIL : Normalized
Assessing how much a monthly value deviates from what is expected in a month requires, not only knowledge of the mean, but
of the variability expected in the month. The method of normalization can be done a number of different ways. Usually, the
standardizing value is the standard deviation. This transforms each month to a scale which can be intercompared.
Fig. 4.13 Precipitation distribution for two months with the same mean
precipitation but different variability.
Study Question 4.8
In which situation will a 1 in. (2.54 cm) monthly precipitation deviation be less likely?
Fig. 4.13a
Fig. 4.13b
Check Answer
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Agronomy 541 : Lesson 4a
Derived Temperature Indices
Other Drought Indexes
The ultimate goal of the AI is to measure potentially droughty conditions. Numerous other indices have been
developed over time to assess the climatic situation to describe the severity of drought. Two common indices
reported are the Crop Moisture Index and the Palmer Drought Index (Palmer, 1965). These are regularly
published throughout the year to numerically indicate the short-term and long-term water conditions.
The Palmer Drought Severity Index (PDSI) is a long-term measure of drought conditions (Fig. 4.15). It takes
into account temperature, precipitation, and the available water content of the soil. It then uses the water
balance of inputs and outputs from the soil to assess the severity of the drought or wet period (Table 4.2).
Certain criteria are defined which indicate the beginning of a drought. Since it accounts for long periods of
dryness, even a few months of near normal precipitation may not end the period of drought.
Table 4.2 PDSI Classifications for Dry and Wet Periods.
4.00 or more
3.00 to 3.99
2.00 to 2.99
1.00 to 1.99
0.50 to 0.99
0.49 to -0.49
-0.50 to -0.99
-1.00 to -1.99
-2.00 to -2.99
-3.00 to -3.99
-4.00 or less
Extremely wet
Very wet
Moderately wet
Slightly wet
Incipient wet spell
Near normal
Incipient dry spell
Mild drought
Moderate drought
Severe drought
Extreme drought
Fig. 4.15 Palmer Drought Severity Index for the summer of 1936
(averaged from April to August).
Study Question 4.10
According to the PDSI, what was the drought condition in the southwest district of Iowa in 1936?
Incipient wet spell
Mild drought
Moderate drought
Severe drought
Check Answer
Plotting of historical values of the Palmer Drought Index for states, single climatic districts, or over the country
can be found at http://www.cdc.noaa.gov/USclimate/USclimdivs.html.
Several modifications to the PDSI have produced other indices are also available. The most common one is the
Crop Moisture Index (Palmer 1968). The CMI uses weekly temperature and precipitation in addition to the
previous week's CMI. This index is has much shorter response time, describing short-term crop needs.
Beneficial rainfalls in the middle of a drought may produce positive CMIs while the long-term drought continues.
It uses the same scale of the PDSI and is normalized to be comparable across the country. One shortcoming is
that the CMI begins each season at 0. During protracted several year droughts, moisture levels may be dry to
start the year providing an erroneous zero point for measurement.
The current Crop Moisture Index can be found at http://www.usda.gov/oce/waob/jawf/wwcb/color_cmi.gif.
A series of the different Palmer indices (current and historical) is available from the National Climatic Data
Center (NCDC) provide several different drought measurements and current precipitation conditions (Fig. 4.16)
Fig. 4.16 Monthly indices of the Palmer drought series for 1934 and
1992 for the central reporting district of Iowa.
Included on these figures are the monthly precipitation, the monthly standardized precipitation, and the
modified Palmer and hydrological Palmer indexes. These are again shorter-term modifications to the original
Palmer.
Since the meaning of drought differs from area to area and user to user, new indices are continually being
developed to create a single number to define the drought conditions for an area and for a particular use.
These may be found in the literature or in searching web sites.
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Cooling Degree Days
Other temperature based indices are used for other climatic assessment. The cooling degree day is a measure
of how much energy would be required to heat or to cool a commercial building or a home during the
summertime. To arrive at the amount of energy required subtract 65 from the daily average air temperature.
The daily maximum and minimum are used to produce the daily average. No cut-offs are used, thus, the
equation for cooling degree days is
Equation 4.5
The assumption is if the temperature averages 65° F, no air conditioning or furnace use is needed. The
assumption is that there is some heating due to solar radiation. We assume a normal relationship with wind. If
the temperature averaged 65° F, that means it probably was 75° F in the daytime and 55° F at night. People
closed their windows at night unless they wanted the house to cool.
If the temperatures average 75° F, then 10 CDD accumulated during the 24-hour period. If the average is 85°
F, there were 20 and so forth.
The record of cooling degree days for the entire state of Iowa during the past (almost a century) is presented
as Fig. 4.17.
Fig. 4.17 Total annual cooling degree days since 1900.
Circled E's indicate an El Niño event (to be discussed in
Lesson 11b).
Sometimes there was a very high requirement. If there had been air conditioning during the Dust Bowl, the
requirements would have been approximately 1600 units for the season. During very cool summers, the
demand was about one-fourth of that. The cool summer near the bottom of the figure is 1915, which was the
coolest summer that we have had historically. We have come close to it during the most recent years. A great
deal of variability is apparent. From the hottest to the coolest is a factor of four.
About 1938, the variability from year to year, rather than being so great, became almost limited (Fig. 4.17). The
high temperatures and the low temperatures were confined to a narrow range compared to those before 1940,
and compared to temperatures since that date. This had an interesting impact on the economy of our society.
Many power plants were being built, and many natural gas lines were laid during the 1960s. There was
conjecture concerning just how much electrical requirement would be necessary for heating or cooling a
building. Planners considered how much had been required in the last few years and noted a specific required
amount. They projected that no more than that level would be needed. They made some estimates of what the
capacity of the natural gas pipelines and what the capacity of the power plants should be, assuming that these
were extraordinary years.
During the past few years, we have exceeded the limits of those ranges (Figure 4.17), resulting in electrical
shortages for air conditioning during hot summers. We have had some cool summers wherein utilities have not
made any money. In recent years, more variable conditions have exceeded the benign years when there was
very little stress on the crop. Note the 20 years on the chart that would have had the highest air conditioning
requirements and the greatest cooling requirements tend to be the 20 years that also had the lowest crop
yields. You can find more recent CDD data on the ISU Climatological Data Page.
Go to ISU climatological data page: http://mesonet.agron.iastate.edu/climodat/index.phtml
A small "E" is written some of the years in Figure 4.17; these "E's" were the summers an El Niño was occurring.
The curious thing to note here is that no El Niños occurred during the hottest 20 years. El Niños tended to
occur during average temperature or cooler than average temperature years. The temperature in Iowa does
not determine whether or not there will be an El Niño. It is the El Niño that determines whether or not Iowa will
be hot during the summer. There has never been a blistering hot summer, when the temperatures persist
above 95° F (55° C), when an El Niño has been going on. Several El Niños during the 1990s have helped
produce cooler growing seasons. More about El Niños will be discussed in Lesson 11b.
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Heating Degree Days
The Heating Degree Days (HDD) are calculated in a similar manner to the cooling degree day calculation.
These can be viewed in two ways: (1) either 65 - the average air temperature = the heating degree days, the
amount it would take to heat your house over the winter,
Equation 4.6
or (2) if calculation of the cooling degree days is negative, the result is the same as a heating degree day.
Heating degree days respond similar to cooling degree days. If very little heating were required, winters would
have been very warm and vice versa. Heating degree days for Ames, IA are plotted in Figure 4.18.
Fig. 4.18 Total winter Heating Degree Days for Ames,
IA since 1900.
Study Question 4.11
If the daily maximum temperature was 64°F and minimum was 38°F, how many HDDs were accumulated?
units
Check Answer
Heating degree days give a measure of how much heating is necessary during a winter. We never know from
year to year just how much our heating should cost unless we keep track of the heating degree days (Fig.
4.19). Heating degree days across the bottom of the chart go from 0 to 2000; the natural gas consumption to
heat the home from 0 to 400. If gas is used for more than heating the home, you would have some variability
here since the furnace pilot light continuously uses a small amount even if you are not heating your house.
Note some very straight lines relating the number of heating degree days accumulated during the season to
the natural gas consumption by the home. There are two sets of lines on the figure. One household was on the
upper line in the winter of 1971. They had insulation improvement in their home for energy conservation,
moving them down to the lower new curve, reducing their bill by about 30% for the same type of year. Assuming
on their heating bill, the weather conditions were the same, they could estimate that they saved 30%.
Fig. 4.19 Natural gas consumption for an Ames, IA residence for
three different winters (Carlson, 1991).
An interesting thing happened involving home heating. One salesman came by to a lady's home and told her
he could save her 25% on her heating bills if she would have more insulation put into her house. She had more
insulation put into her house, but her heating bill went up. She was upset about that and wanted to sue the
insulating company. They had installed insulation appropriately. It turned out just that there had been a warm
winter the year previous to the insulation addition and an extremely cold winter year following. Nevertheless, the
insulating company had done a good job. Heating degree days are really the only way to evaluate the situation
since heating degree days are a measure of how cold the season was in total.
Heating degree days and cooling degree days are used extensively by the utility industry to track electrical and
natural gas usage during the year.
Heating degree days and cooling degree days are kept track of by the National Weather Service and published
regularly.
Current Iowa CDD and HDD data for individual cities can be found at:
http://iwin.nws.noaa.gov/iwin/ia/climate.html. Or for other states by changing the state postal identifier in the
URL.
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Chill Units
A final topic, if not of direct farm interest, will be of personal and yard interest. Particularly interesting, in
growing fruit trees, is the concept of the chill hour. The chill hour was defined many years ago as the numbers
of hours when the temperature is less than 45° F (7.2°C).
Equation 4.7
The old way of accumulating chill hours was to count how many hours the temperature stayed below 45° F
(7.2°C) during the winter. People who had a large peach or apple orchard would always put a
hygrothermograph (a recording thermometer) and humidity recorder in their orchard. The instrument which
made a chart that looks like the one in Figure 4.20. A line would be drawn at the 45° F (7.2°C) mark. The hours
the temperature stayed below the line were counted daily all winter; the hours above 45° F (7.2°C) were
ignored.
Fig. 4.20 Continuous temperature trace produced by a thermograph.
What good does it do to keep track of chill hours for the winter season? Insects that over-winter in an area,
trees and seeds that are adapted to an area, and probably most things that are perennial, that grow from year
to year, have a chill requirement, known as a vernalization requirement. The plant seed or insect must spend a
specified number of hours at a temperature of 45° F (7.2°C) or less for it to grow. Peaches are particularly
sensitive to chill hours. This explains why few peaches grow in Iowa.
The chill hour requirement for a peach tree is determined in this manner. After the tree has become dormant,
prune the tree, gather many of the short branches, and put two or three of them in a glass of water. Store the
rest of the small branches in the refrigerator at 40° F. Nothing will happen to the branches in the glass of water.
They will stay dormant. Maybe they will rot. They will not grow or come out in bud. After 100 hours in the
refrigerator, take another couple of branches out of the refrigerator and put them in a glass of water to see if
anything happens to them. After another hundred hours, take another few branches out of the refrigerator and
put them in a glass of water. After perhaps 900 hours, take your branches out of the refrigerator and put them
in a glass of water. In two days fruit buds and blossoms will be all over them. You have just determined that
your peach tree had a chill hour requirement of 900 hours. That is how long the stems had been in the
refrigerator until you took them out and put them in a glass of water to grow. Alberta peaches have a different
requirement than Redskins, for example. They may vary from 800 hours for some of the low chill hour
requirement peaches, such as grow in Georgia and northern Florida, to well over 1500 hours for peaches that
are produced in the states of Washington, Idaho, and Utah. So you need to pick a chill requirement that could
logically be achieved in the state where you are going to be growing the peaches.
Why is this important? Think a little bit about the trees in Iowa. In the springtime in Iowa, sometimes some trees
will be in full bloom, and other trees have nothing blooming. You can almost say that the trees that are in leaf
and in bloom first are from Europe. They were imported from England. Because they are not native, they come
out in blossom or in leaf too early. When they have done that, they become susceptible to freeze damage and
a lot of other problems.
The native trees wait until later to get blossoms on them, usually past the time of frost danger. So what are the
trees doing? They have some chill hour requirement that avoids a January thaw. At the time of the January
thaw only 1000 chill hours may have accumulated. Plants that required 1500 chill hours ignored that warm
stretch and remained dormant. The next time it became warm maybe 1700 chill hours had accumulated, and
the plants start to grow vigorously.
The plant should have enough chill hours so that when it starts to grow vigorously it is past the hard freeze. It is
important to be aware of the latest date that a hard freeze would be expected to occur and result in damage to
the tree. It may be 22° F that would be the hard freeze that could kill trees, if they were beginning to grow.
Leaves and blossoms would die at 26-28°F (-4 to -5°C). Hence, it is necessary that the chill hour requirement
for that tree gets it past that last date of hard freeze. Then the trees in the area should grow vigorously.
However, you do not want to have such a high number of chill hours that during a warm winter it will never be
achieved. That is a problem in Iowa. Sometimes winters do not accumulate more than 1000 chill hours. Other
years the winters are so cold that accumulate 2,000 or 3,000. It is difficult for a specific plant to fit right in and
have the exact number of needed chill hours. For other places in the country, one should choose a plant that
would have enough chill hours to get it through a January thaw and not so many that it would not have
achieved them by spring. Because of this sensitivity to chill hours, peaches do not do very well in the state of
Iowa. Apples also have a chill hour requirement, but not as precise as for peaches. Therefore, except for
disease, apples do fairly well in Iowa.
The chill hours are also very important to agriculture and crop production. The agronomist may not use them
directly, but the entomologist does. Many of the insects that over-winter in the state have a chill requirement
that fits right in with the concept of making sure the insect stays dormant long enough and does not begin its
spring growth and emergence until the spring has really arrived. These are all considerations that go along with
the chill hour requirement.
The chill hour, mentioned earlier, is a somewhat old concept that has been revised slightly as a result of further
observations. It was noted that if the stems of the peach tree that were colder than 32° F (0°C), were not killed,
they still did not ever get their chill. They could be left in refrigeration the full time, taken out, and would not
grow. Refrigerating them at 40° F (6°C) produced no problems. Temperatures below 32° F (0°C) were not
working. So people said, "Aha, the chill hours really should not be just temperature below 45° F, like was set up
in Georgia originally with the research, it should be between 32° and 45° F." The concept worked in Georgia
because temperatures seldom drop below 32°F there. Georgia has many hours between 32° F and 45° F. But
to make them work in Iowa, one must say, "the hours do not count if they are below 32° F, but the temperatures
from 33° F (0.6° C) to 45° F (7.2°C) do count."
With some additional study, it was found that the 33° F (0.6°C) to 45° F (7.2°C) temperatures were not quite
right either. There is an important difference that seems to occur at about 36° F (2.2°C). Between 32° F(0°C)
and 36° F (2.2°C), it took twice as many hours for chilling to occur as between 36° F (2.2°C) and 48° F (8.9°C)
. Some chilling can also occur in a warm refrigerator between 48° F (8.9°C) and 54° F (12.2°C), but only half
as well. Above 54° F (12.2°C) no chilling will occur.
What if you get some warm days in January and February? It turns out that between 54° F (12.2°C) and
temperatures as high as 65° F (18.3°C), chilling is undone. As a result of the differences in chilling time and
temperatures, a different relationship was evident and could not be called a chill hour. It is termed a chill unit,
resembling the unusual curve in Figure 4.21. When the temperature is at 0°C (32° F), no chill unit
accumulation took place. If the temperature warms to 5°C, the chill unit value approached a value of 1. At 7°C
(close to 45° F), conditions were ideal. But as the temperatures rose, the chilling advantage dropped off again.
This procedure became more workable and much more accurate than using the time with temperature of less
than 45° F. Temperatures can now be considered all the way to 65° F (18°C or 19°C) down to 0°C (32° F)
according to these steps. Added units as described, does a a very good job of forecasting when a tree has had
sufficient chill to grow, when an insect has had sufficient chill to emerge, or when seeds have had sufficient
vernalization or chilling that they can grow. Chill time can even be determined by putting the twigs (stems) or
seeds in a refrigerator at 7° C and relating it to this chart as has been described.
Fig. 4.21 Chill unit contribution at discrete
measured temperatures
Equation 4.8 relates chill units to temperature steps by Fig. 4.21. The logic for a spread sheet that computes
the chill contribution for one hour at a given temperature (for hours) is:
Equation 4.8
The number of chill units are not available at climatological stations. But they can be computed using National
Weather Service airport stations or other automated stations such as the ISU Campbell Network. Request
"hourly" data.
ISU Campbell Network
Note: this method has several (minor) updates since the original analysis published by Ashcroft, Richardson
and Seeley from 1965 to 1977. Search for Seeley or simply for Utah Chill Units to locate updates and
modifications. This has become the fundamental method for chill analysis worldwide. When I did the initial
formulation (as a laboratory technician for Ashcroft and Richarson) in 1964, a continuous formula was included
but is not used because the step-wise method is more adaptable to a wide horticultural range of products and
the original curve was strictly valid for only "redskin" peaches.
Agronomy 541 : Lesson 4b
Wind
Introduction
Developed by D. Todey
It is suggested that you watch Video 4B and complete the exercise in the video before continuing with the
lesson.
Podcast Version
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Air movement locally or around the globe is the effect recognized as wind. Air is moving continuously at all
levels of the atmosphere in response to forces placed upon them. All motion is a response to some applied
force. The driving forces for winds occur on all scales. Global winds are driven by differences in the heating of
the Earth. The global-scale circulation and its effects will be discussed in Lesson 7. Synoptic circulations,
which are on the scale of lows and highs, are driven by other energy imbalances. The wind that is observed is
a combination of three main forces which will be discussed in this lesson.
What You Will Learn in This Lesson:
What forces affect wind speed and direction.
What conditions must exist for wind to occur.
About localized wind circulations and their effects.
Reading Assignments:
pg. 182-183—Aguado & Burt, 3rd pg: 112-113, Distribution of Pressure
pg. 187-197—Aguado & Burt, 3rd pg: 117-128, Speed and Direction of Wind
pg. 220-228—Aguado & Burt, 3rd pg: 246-254, Major Wind Systems
IN DETAIL : Wind Naming
Winds are always named by where they are blowing from. Northerly winds come from the north. Likewise, a sea
breeze comes from the sea.
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Agronomy 541 : Lesson 4b
Wind
Pressure Gradient Force
The driving force for wind is the pressure gradient force. When pressure is different from one location to
another, a difference in pressure exists. When a pressure difference exists, a pressure gradient exists. The
pressure gradient is usually indicated by the proximity of isobars (lines of constant pressure) on a surface
weather map. Where several lines are tightly packed on the map, a large pressure gradient exists, where the
lines are spread apart, less of a gradient exists. In a basic sense, more air exists in one place than in another.
The atmosphere is always trying to even-out imbalances. An imbalance in pressure causes winds to blow as
the atmosphere attempts to even out the pressure difference. This is most commonly experienced when a
strong area of low pressure passes over an area. The pressure difference between the low and adjacent high
pressure produces strong winds.
Pressure differences usually occur as a result heating differences. Large-scale heating differences between
the equator and poles produce the general circulation of the atmosphere. The general circulation of winds
around the globe will be discussed in Lesson 7. The most heat is contained near the equator, where most of
the heating occurs. Rising air motion is a general rule here in response to the excess heat. At the poles, colder
temperatures cause sinking motion. This difference in heat drives the general circulation.
This flow pattern describes the general movement of the air. The "warmest" region is the equator, being the
most normal to the sun, not because it is closer to the sun but because it is almost flat with respect to the
radiation from the sun. The Poles are at an angle, causing the sun's radiation to strike the North Pole and the
South Pole at steep angles. The more normal the surface is to the sun, the brighter and warmer a region will
be. The greatest warming is around the equator (Fig. 4.22). Clouds can create some variation; occasionally
there will be greater warming somewhere besides at the equator. But on the average near the Poles the
heating is minimal and near the equator at a maximum. This accounts for the general circulation of winds on the
planet.
Fig. 4.22 Parallel rays striking the Earth are spread more toward the poles
than the equator. The rays, being more normal or more vertical, heat the
equator more efficiently than nearer the poles. This causes a general
temperature difference between the equator and poles and drives the
circulation of the atmosphere.
The heating differences in turn create pressure differences. The cold air sinking at the pole would tend to
produce higher pressure with cold air sinking to the ground. Low pressure would occur with rising air as air is
accelerated away from the ground. The pressure differences cause winds to blow, trying to even out the
pressure differences.
These pressure differences are observed looking at pressure gradients. The difference in pressure produces a
force called the pressure gradient force (PGF). It is defined as:
Equation 4.9
where P is the pressure and Z is the horizontal distance.
The strength of the pressure gradient force can be changed by increasing the pressure difference (P) or
reducing the distance (Z) of the pressure change. When a pressure gradient force exists, the wind will attempt
to balance the force by moving directly from high to low pressure (Fig. 4.23).
Fig. 4.23 The pressure gradient force (PGF) moves directly from high to
low pressure. (click rewind to restart animation)
The surface map (Fig. 4.24) displays an excellent example of the above relationship. The PGF is quite strong
around the area of low pressure in the northeastern United States, while the south-Central United States has a
relatively weak pressure gradient.
Fig. 4.24 Surface weather map with winds, isobars, and low and high
pressure depicted. Click on image to view larger version.
The surface map indicates the surface winds and direction on the barbs and the isobars, lines of constant
pressure. Notice how tightly packed the isobars are off the Eastern Seaboard. A strong pressure gradient
exists and strong winds are blowing. Winds are southerly at 25-30 knots (each full barb represents 10 knots
and each half-barb, five knots). Looking over southern Kansas, little pressure gradient exists; wind responds
with 5-10 knot winds here.
Agronomy 541 : Lesson 4b
Wind
Coriolis Force
The second force, the Coriolis force (or Coriolis effect, CF), is a product of the rotation of the Earth. Newton's
First Law state's that "Every body exists in its state of rest or of uniform motion in a straight line unless it is
compelled to change that state by forces impressed upon it." Therefore, any moving object, which is not
attached to the Earth, should move in a straight line. As the Earth rotates that straight-line motion will change
how something appears to move to someone who is rotating with the Earth. This apparent change in motion is
termed the Coriolis force or Coriolis effect. The equation describing the Coriolis effect is:
Equation 4.10
Study Question 4.12
Where would the Coriolis force be more effective?
Pole
45° N
Equator
Check Answer
The effect of the Coriolis force is to change the direction of the wind. Speed is a factor in determining the
Coriolis force, but the force itself has effect on speed. The wind appears to turn to the right in the Northern
Hemisphere and to the left in the Southern Hemisphere as a result of the Coriolis force. The ultimate result of
the Coriolis turning is to balance the pressure gradient force and creates a wind that blows parallel to the
isobars, called a geostrophic wind (Fig. 4.25).
Fig. 4.25 The Coriolis force (CF) changes the direction of
motion by acting to the right of the flow in the Northern
Hemisphere.
The result of the pressure gradient force and Coriolis force is a wind that begins blowing from high to low
pressure and is turned to the right by the Coriolis force in the Northern Hemisphere. These winds occur above
the surface, where the only two forces acting are the pressure gradient and Coriolis force.
Maps of the upper atmosphere, such as the one pictured at 500 mb (Fig. 4.26) depict the conditions in the
upper atmosphere. Winds blow parallel to the height contours here (similar to isobars).
Fig. 4.26 View of the 500 mb map centered on the North Pole.
Solid lines give some guidance as to what direction the wind
is blowing. This map is particularly useful in determining the
large scale flow patterns of the atmosphere. Click on the
image for a better view.
Study Question 4.13
Where would have the fastest wind speeds at 500mb?
Mexico
Just off the coast of the Carolinas
South-central Canada
Check Answer
Study Question 4.14
Where would winds be blowing from here?
Northeast
Northwest
Southeast
Check Answer
Agronomy 541 : Lesson 4b
Wind
Friction
The third force, which factors into wind's speed and direction is friction. Even though air is a fluid, the air
molecules still rub across the surface of the Earth. Air is also channeled and diverted by buildings, trees, and
hills. All these effects cause friction, a rubbing of the air molecules across the surface. Air well above the
surface experiences little friction, while air nearer the surface experiences more friction. The layer where air is
most effected by friction and the surface is called the boundary layer.
Friction has two effects on the wind. Friction opposes the direction of motion by acting opposite to the flow of
air. The force of friction changes the air's speed.
Friction acts to slow the wind by dragging across the surface (Fig. 4.27). The trees, buildings etc. slow the
wind.
Fig. 4.27 Profile of wind speeds in the lower atmosphere.
Friction causes slower speeds near the surface. Upper
level winds experience little friction.
Study Question 4.15
Using the Coriolis equation from the previous page, what happens to the Coriolis force when friction slows the
wind speed?
Increases Coriolis force
No change
Decreases Coriolis force
Check Answer
Since the Coriolis force is reduced by the wind speed decrease caused by friction, the Coriolis force and
pressure gradient force will not balance each other. The balance between the pressure gradient force and the
Coriolis force that existed in geostrophic flow is overcome (Fig. 4.28). The imbalance will cause the pressure
gradient force to dominate produce the flow seen at the surface around high and low pressure areas (which will
be discussed in Lesson 8b. Here winds blow across the isobars in toward low pressure areas and away from
high pressure areas.
Fig. 4.28 As friction slows the wind speed, the pressure
gradient force (PGF) and Coriolis force (CF) no longer are
balanced. When this happens, winds blow across the
isobars.
This combination of forces occurs only in the scale of motion relating to the synoptic-scale and larger. This
scale refers to the scale of low and high pressure systems and larger. The Coriolis force only works here
because of the large-scale motion. When dealing with smaller scale wind flows, pressure gradient is the main
driving force. Look back at Figure 4.24 to see if this applies in reality.
IN DETAIL : Boundary Layer
The meteorological boundary layer is that layer of the atmosphere nearest the surface, which experiences changes
caused by the surface. You examined some boundary layer effects in the radiation simulation in lesson 3b.
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Agronomy 541 : Lesson 4b
Wind
Local Wind Circulations
Localized circulations occur on a scale of tens of kilometers. For example, differential heating between two
fields, a field with a crop on it and a field of bare soil, will produce soil surfaces with different temperatures. The
bare soil has a low albedo. The soil surface does not reflect a great deal of energy, absorbing about 90% of
the sunlight reaching it. The warm soil warms the air next to the soil, causing the warm air to rise. Temperatures
are cooler over the plant-covered surface because less solar radiation is being converted to sensible heat. The
amount of absorbed energy may be a little lower while more is reflected since plants have a lower albedo.
Some of the absorbed solar energy will be used to evaporate water. With a contrast of cool air and warm air, a
pressure difference is created and a circulation is produced. The warmer air would rise with the cooler air
sinking underneath it.
Glider pilots will watch for the difference over a large plowed field or open soil next to a vegetated one because
they can often exploit a local updraft there. They can be dramatic even on a local scale to someone with a
parachute or to a soaring eagle. The rising air currents are called convection.
Sometimes this difference can be quite dramatic. One memorable occasion was in the Alabama-Georgia border
area. The airborne training school, at least at that time, was located in Georgia. They were having a drop of
parachutes across the river in Alabama. As the plane flew by, three or four chutes were dropped over a plowed
field. The rest of them were over meadow. All of the men over the meadow were on the ground quite promptly.
The people over the plowed field took longer to come down. Two of them took several minutes longer than
those dropped over the grassy field. The third fellow's chute was going up rather than down, and it
disappeared over the trees. When the chute got over the trees, there was no longer an updraft and, no doubt,
the chute came down, perhaps in a place that was not comfortable for him. Any place where two surfaces have
heating and cooling differences can produce a similar situation.
Agronomy 541 : Lesson 4b
Wind
References
Baker, D. G., D. C. Ruschy, and R. H. Skoggs, 1993: Agriculture and the recent "Benign climate" in Minnesota.
Bulletin of the American Meteorological Society 74: 1035-1040.
Carlson, R. E., 1991: Heating Degree Days in Iowa relative to home natural gas consumption, conservation
efforts, and long-term trends. Iowa Academy of Science, 98: 159-161
Harouna, S. and R.E. Carlson, 1994: Analysis of an Iowa aridity index in relationship to climate and crop yield.
Jour. Iowa Acad. Sci., 101(1):14-18
Palmer, W. C., 1965. Meteorological Drought. Research Paper No. 45, U.S. Department of Commerce Weather
Bureau, Washington, D.C.
Palmer, W. C., 1968. Keeping track of crop moisture conditions, nationwide: the new Crop Moisture Index, 21:
156-161.
Thompson, L. M. 1986. Climatic change, weather variability, and corn production. Agron. J. 78:649-653.
Thompson, L. M. 1988. Effects of changes in climate and weather variability on the yield of corn and soybean.
J. Prod. Agric. 1:20-27.
Whiteman, D. C. 2000. Mountain Meteorology. Oxford University Press. N.Y., 355p.
Agronomy 541 : Lesson 4a
Derived Temperature Indices
Introduction
Developed by D. Todey
It is suggested that you watch Video 4A and complete the exercise in the video before continuing with the
lesson.
Podcast Version
Full Podcast List
Regular measurements of climatic temperature data include maximum and minimum temperatures at numerous
climatological stations. Interpreting how these temperatures affect crop growth is not always straightforward.
But other indices derived from these data can illustrate how crops should be responding to the temperatures.
While not completely indicative of crop condition, indices such as the Growing Degree Day can be descriptive
when summed over the growing season. There are several other indices which can be derived from maximum
and minimum temperature measurements; these include Stress Degree Days, Aridity Index, Cooling Degree
Days, Heating Degree Days, and Chill Units. These indices have agricultural and personal comfort application.
We will define and discuss applications of these indices in this part of lesson 4.
What You Will Learn in This Lesson:
How temperature and moisture stress in crops can be compared using various indexes.
About temperature indexes.
About combined temperature and moisture indexes.