Crop and Climate Calendars - Iowa State Universityagron- · 2011-12-08 · Agronomy 541 : Lesson 3a Crop and Climate Calendars Iowa Crop Calendars If involved with agriculture, the

Agronomy 541 : Lesson 3a

Crop and Climate Calendars

Introduction

Developed by D. Todey and E. Taylor

It is suggested that you watch Video 3A and complete the exercise in the video before continuing with thelesson.

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In much of the world outside the tropics, the growing season is limited by winter temperatures. The farther awayfrom the Equator your location, the less solar radiation is received which shortens your growing season. Thecold temperatures of spring limit the earliest opportunity to plant. A typical planting date is after the averagelast spring freeze. At the end of the growing season, the average first fall freeze limits growing season length.These and other important dates for crop production are the basis of crop calendars. Temperatures are thecontrolling factor for the crop season in the Midwest. In other areas, rains are the controlling factor. Crops aregrown during the rains.

The crop calendar is the fundamental document of agricultural meteorology. Some form of crop calendar existsfor all agricultural locations. In this lesson we will discuss factors associated with and controlling the growingseason and review the resultant crop calendars.

What You Will Learn in This Lesson:

How crop calendars can guide production.What controls the growing season length in the Midwest.Some additional limitations of GDDs.



Historical Perspective

One of the major shrines open to tourists in Beijing, P.R.C., is the Temple of Agricultural Weather (or Temple ofPraying for Crops) (Figure 3.1) located a short distance to the southeast of the Royal Palace (Forbidden City).The shortest day of the year was the "Agricultural New Year" in China. The Emperor performed a ritual of hopefor favorable weather and good harvest in the new year. Apparently the cycle of agricultural activity waschronicled in detail, if not specified, at this shrine.

Fig. 3.1 Temple of agricultural weather in Beijing, China.

Be it family tradition, phase of the moon, or formal guidelines, farmers must have a calendar of operationsassociated with crop production. Some calendars are based on tradition which have a physical purpose. Moresophisticated calendars based on recorded observations are now available.

The first task in the development of a weather-wise production program is the compilation of a comprehensivecrop calendar. The calendar must include average dates of planting and primary phenological events such ascrop emergence, blooming (tasseling), maturity, harvest, etc. Temperature data should be used to compute thenormal growing degree day accumulation associated with the calendar dates. If historical records exist, astatistical distribution of phenological dates should be compiled. Weather information regarding frost, heatstress, precipitation, and evaporation should be included. Temperature is the main factor controlling crops inthe Midwest.



Iowa Crop Calendars

If involved with agriculture, the most important concern that a person has is the crop calendar. Knowingapproximate dates of (if talking about corn) the last spring frost, planting, emergence, tasseling or silking,maturation, and the first fall frost date are essential. Publications of corn and soybean calendars and theclimate information associated with them have been developed. The calendars are of value in understandinglocal crop production for all crop reporting districts of the state.

Crop Calendars for corn and soybeans in the Central crop reporting district of Iowa are displayed in Figure 3.2as an example of such calendars. These are based on 25 years of climatic data. Across the bottom each majorhash mark indicates the beginning of each month April 1, May 1, June 1, etc. On the y-axis values range from 0to 100%, indicating what percent of the crop has reached that stage. The first line is planting. The 50% valueindicates that normally half of the corn crop has been planted on the 10th of May. That is the average day ofbeing 50% planted. Usually by the 20th of May, the crop is more than 90% planted. The crop is about 85%emerged by the 1st of June. The 50% level gives normal dates; the complete line represents the statisticaldistribution of the range of each event over time.

Fig. 3.2 Crop calendar for a) corn and b) soybeans for the Central district

of Iowa.

The dashed line climbing up through the middle of each chart represents the growing degree dayaccumulation. To the far left side of the chart is the growing degree day scale. The normal growing degree dayaccumulation begins on May 1 in Iowa. The chart gives maturity stages and the harvest dates as well.

Study Question 3.1

What is the average date of harvest for corn?

Check Answer

Study Question 3.2

What is the date of the earliest fall freeze?

Check Answer

Study Question 3.3

What is the date 80% of soybeans have usually set pods?

Check Answer



Frost and Freeze Dates

The length of the growing season is defined as the period between the last spring freeze and first fall freeze.The fall freezing date varies across the state of Iowa from the 29th of September in the northwest (the averagedate in Lyon County), to the 7th of October in the southwest. The 4th of October is the average day in thecenter of the state for the first fall 32° F temperatures.

Fig. 3.3 Average day of first fall freeze (28° F) for Iowa.

Most years there is serious concern about whether or not there will be an early frost because great crop lossescan occur when immature crops freeze. In 1994 there was great concern about an early frost. The summer wasabnormally cool, slowing maturity. Fortunately, there was actually a late first frost. Figure 3.4 depicts the date ofthe first frosts for the past 90 years in Ames, Iowa. The Julian day of the frost is indicated by the day of the yearat the left. The average date for frost to have occurred in half of the state is Julian day 280 (October 7).

Fig. 3.4 Average day of first frost (32° F) at the Agronomy Farm Ames,

IA.

Study Question 3.4

What is the average day of first frost?

Check Answer

Study Question 3.5

What are the days of earliest first frosts and of latest first frosts?

earliest: Check Answer

latest: Check Answer

The frost figure (Figure 3.4) indicates an early frost, or a very short growing year in 1902 and some other veryshort years in the early part of the century. Notice that in 1950 there was a slightly shortened crop yearfollowed by no short growing seasons from 1951 until 1974. There were 24 consecutive years with long,average, or longer than average growing seasons in Iowa. This made a difference in the economy of the statebecause many people no longer planted quite as wide a mix of hybrids and varieties. They started to noticethat they never made any money on short season hybrids. Seed companies report that they started to see ashift to long season hybrids by 1970. This ideal period ended in 1974. An early frost occurred in 1974 leaving alot of green plants and moisture in the corn at harvest. Curiously, from 1974 to 1994 there were no longgrowing seasons. After 24 years without a short season, Iowa had 20 years without having a long one. (All ofthem were average or short.) The cause of the climate variation is not known, but the economic impact wassignificant. From the data, it would not be likely to have a repeat run of long or of short growing seasons.Annual variation limits this possibility.

IEM Climodat can be found at: http://mesonet.agron.iastate.edu/climodat/index.phtml

National Crop Calendars can be found at: http://www.nass.usda.gov/Publications/Reports_By_Date/index.asp.

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IN DETAIL : 32 Degrees F

The crop calendar gave the date for 32° F frost. Crops usually do not kill at 32° F because a plant is filled with morethan water (Fig. 3.2). It is filled with water, certain salts and sugars and other materials that depress the freezingpoint. Often the first 28° F temperature in the fall is considered to be the killing frost (Fig. 3.3).

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IN DETAIL : Julian Calendar Days

Table 3.1 Julian day calendar.

Month Day Julian Day

January 1 1

February 1 32

March 1 60*

April 1 91

May 1 121

June 1 152

July 1 182

August 1 213

September 1 244

October 1 274

November 1 305

December 1 335

*During leap years, add 1 to March-December numbers.



Frost and Freeze Dates (Continued)

Understanding the length of the season for a crop entails knowing the average last spring freeze and averagefirst fall freeze for the particular crop. As mentioned, the first occurrence of 32° does not necessarily signify theend of the growing season for corn. The temperature that kills crops varies. Most grain and forage crops inIowa (corn, soybeans, sudan grass, sorghum) have frost tolerance of 2-3° C below freezing. Crops such asoats and barley have high resistance to freezing of 7-9 °C below freezing, particularly early in the growingseason. Other garden variety crops have much lower freeze tolerance of less than 1° C below freezing.Knowledge of average dates of these different average temperature levels is necessary for planting schedules.Table 3.2 lists the average date of last occurrence of 32°, 30°, 28°, 26°, and 24° F in the period 1951 through1980 for several locations. The variability between locations differs at the various thresholds.

Table 3.2 Average last date of occurrence of fivetemperature cut-offs (°F) for the Central Iowa crop

reporting district, 1951-80.

Location 32° 30° 28° 26° 24°

Ames Apr. 28 Apr. 20 Apr. 14 Apr. 10 Apr. 5

Ankeny Apr. 28 Apr. 18 Apr. 13 Apr. 9 Apr. 5

Fort Dodge May 1 Apr. 27 Apr. 21 Apr. 12 Apr. 8

Grinnell May 6 May 2 Apr. 24 Apr. 17 Apr. 11

Grundy Center May 6 Apr. 28 Apr. 23 Apr. 13 Apr. 10

Iowa Falls May 4 Apr. 28 Apr. 22 Apr. 14 Apr. 9

Marshalltown Apr. 29 Apr. 24 Apr. 14 Apr. 10 Apr. 4

Newton Apr. 26 Apr. 16 Apr. 12 Apr. 9 Apr. 3

Perry Apr. 29 Apr. 22 Apr. 14 Apr. 10 Apr. 6

Toledo May 3 Apr. 28 Apr. 20 Apr. 14 Apr. 8

Webster City May 4 Apr. 29 Apr. 22 Apr. 12 Apr. 7

The first freeze occurrence in the fall is given in Table 3.3. The average first 32°F temperature in the fall forAmes occurs on October 7th, Ankeny, October 10th, and Fort Dodge, October 6th. The first occurrence of the28°F temperature occurs 1-2 weeks later.

Table 3.3 Average first occurrence of five temperaturecut-offs (°F) in the fall for the Central Iowa district, 1951-

80.

Location 32° 30° 28° 26° 24°

Ames Oct. 7 Oct. 13 Oct. 20 Oct. 24 Oct. 29

Ankeny Oct. 10 Oct. 14 Oct. 19 Oct. 25 Oct. 31

Fort Dodge Oct. 6 Oct. 11 Oct. 15 Oct. 22 Oct. 28

Grinnell Oct. 5 Oct. 11 Oct. 12 Oct. 16 Oct. 22

Grundy Center Oct. 6 Oct. 9 Oct. 15 Oct. 19 Oct. 25

Iowa Falls Oct. 1 Oct. 6 Oct. 12 Oct. 17 Oct. 23

Marshalltown Oct. 3 Oct. 8 Oct. 14 Oct. 22 Oct. 27

Newton Oct. 12 Oct. 18 Oct. 22 Oct. 28 Nov. 1

Perry Oct. 4 Oct. 10 Oct. 15 Oct. 23 Oct. 27

Toledo Oct. 5 Oct. 8 Oct. 14 Oct. 19 Oct. 24

Webster City Oct. 3 Oct. 6 Oct. 14 Oct. 20 Oct. 24

Discussion Topic 3.1

What causes the station to station variability? What are the general trends in dates? Do they makesense? Does it seem strange that Fort Dodge has a later average spring freeze than Ankeny?

A 28° F temperature will typically kill some vegetation, while not killing the stems. Some translocation wouldstill occur if crops were not quite mature. A 26° F temperature will usually kill the plant entirely. A 32° Ftemperature may often do some damage to leaves in localized areas, not uniformly over fields. There is widevariability of events associated with frost. Frost may develop when air temperature did not go below 36° F. Itmay happen on a still night with clear sky and low humidity. There may be frost on car windshields or frost onthe grass at an air temperature of 38-40° F. The leaves and car windshields are colder than the air on clear,still nights. Often objects are colder than the ambient air temperature. In actuality, air does not really warm bythe sun, nor does air cool much by itself. It cools by contact with something else. When the sun rises in themorning, the air is usually at its coldest. The sun shines on the side of a building, a tree, or the soil, warmingthe soil, the tree, or the building. As the air moves past the object, it warms by conduction of heat. As soon asthe sun goes down, objects begin to cool and the reverse process applies. Often frost develops on objectseven though the air temperature has not yet cooled to 32° F. This has to do with thermal radiation balance,which will be discussed in Lesson 6.

National freeze and frost dates (Fig. 3.5) vary with latitude and other small-scale features. What featuresmight affect freeze dates or a large scale?

Fig. 3.5 Average dates of first autumn freeze

(32°F)

Fig. 3.6 Average dates of last spring freeze

(32°F)



Growing Degree Days

A type of crop calendar based on recent temperature conditions can be created using a method of assessingthe amount of growth which has occurred in a season, called thermal time. Most predictive pest models andcrop models use meteorological data as the principal real-time input. For crop development or for pestdevelopment temperature expressed as "heat units" or "growing degree days" is often the principal variable(introduced in Lesson 1). This part of the lesson will expand on the GDD concept.

While the assumption of GDDs is that corn grows linearly, that is not the case.

Figure 3.7 illustrates with an S-shaped curve the response of corn to temperature. A growing degree dayassessment of growth assumes a linear growth response. In actuality, corn begins to grow very slowly, as notedwhere both curves hit the axis at the bottom on the figure between 50° and 60° F. It grows rapidly between 70°and 80° F; at 85° or 90° F it hits the top of the curve. Temperatures above 92° F begin to become detrimentalto the growth of the crop, and the curve starts to move over to the right-hand side.

Fig. 3.7 GDD linearly simulated growth and

actual corn plant growth influenced by

temperature.

That is the way corn grows. The line that is an approximation of the average right up the middle of the curve isthe way the government THINKS corn grows. This is the growing degree day representation of corn growth. Agrowing degree day is a straight-line representation, so if the temperatures are hovering around 60° F, theactual growth line is far below the straight line. Actual growth of the corn is considerably below the predictedgrowth. If the temperature is 74° F, right in the middle where the two lines cross, the corn grows exactly at thepredicted speed, or the growing degree day, line. If the temperature is 82° F, the corn is growing much fasterthan would be predicted. Growing degree days are a straight-line representation of the curved line whichactually represents the growth of corn.

In 1995 Iowa had a more than adequate amount of moisture in the soil during the early part of the growingseason (Figure 3.8). There was a great deal of difficulty getting corn planted around the state. Some placesnever did get planted. The soil conditions were such that 92° would still have been optimal into July. The firstreal heat wave of the summer was damaging to cattle and to poultry because of the high humidity and theextreme temperatures. There was not significant damage to the crop. The high temperatures were almost ideal

for the crop. The crop development did catch up regardless of aspects that were behind schedule.

Fig. 3.8 Palmer Drought Index for April 1995. Note that

Iowa was wet to very wet. (Increasing positive

numbers are wet, negative numbers are dry).

Temperatures well into the 90s did very little damage for two reasons: (1) the crop had plenty of moisture, and(2) it did not need much additional moisture because the humidity was at record high levels. That was rough onthe cattle, rough on the people, disastrous to the poultry, and ideal for the crops because the crop did not usevery much water in an atmosphere with high humidity.

The next heat wave came when conditions were not perfect (Figure 3.9), and the temperatures in the 90scaused the crop to begin to degrade. In the third heat wave, degradation became more obvious. The one inSeptember 1995 took its toll in yield of Iowa's crops.

Fig. 3.9 Palmer Drought Index for September 1995.

Iowa was slightly to moderately dry.

What is the temperature where stress begins if ideal conditions do not exist? As conditions become less thanideal, the optimal temperature begins to creep down to some lower value. On average, it is at 86° F. The worseconditions are, the lower that value gets until there could be stress even to where it would be too cold for the

crop to grow. Nevertheless, over a range from around 80° to around 92° (Figure 3.10), the best temperaturefor the crop to grow varies depending upon the other conditions, most often the moisture available to the crop.The average optimal temperature is 86° F and assumes that soil moisture is something less than optimum, theroots are a little bit damaged; there is a little bit of disease; therefore, stress is considered to start when thetemperature exceeds 86° F. It is not always true, but that is the consideration.

Fig. 3.10 Reduction in development because of

stress.

Growing degree days are a linear approximation of the growth of the crop. Growing degree days are not at alltimes an accurate description of the growth of corn; first, because the corn does not have a linear growthresponse with respect to temperature. It grows according to a curve, and the growing degree days are not acurve. Second, growing degree days do not work perfectly.

Recall the crop response if temperatures are near the 50° mark where growth is just beginning. Why is thegrowth diminished? The actual growth curve is behind the predicted or growing degree day curve (Figure3.11). If the temperature averages somewhere around 72°, then the forecast growth and the actual growth willbe the same. If the temperature averages around 80°, the plant grows faster than growing degree days predict,and the crop will show greater development than growing degree days would indicate. This is an inherentinaccuracy or an inherent weakness that is associated with the concept of the growing degree day.

Fig. 3.11 GDD approximation of actual growth.

The linear approximation sometimes

underestimates or overestimates

development.

People may become irritated by the growing degree days method, and they ask, "Why do we not get somethingbetter?" That would be a good thing to do--get something better than the growing degree day, but considerwhat can be done by using growing degree days to predict crop growth and determine how much improvementis really needed.

First, recall the crop calendar from earlier in this lesson. The crop calendar simply indicates that if a crop isplanted by a certain day, by another certain day it is expected to be out of the ground or entering a differentgrowth stage as the development of the crop progresses. The calendar, in days of the year, is not a bad way tolook at crop development and the things that happen to the crop. But, some years the crop does not grow asfast as the calendar would say. It may deviate from average by quite a bit. The crop calendars show how muchgrowth deviates from average. A crop may be a week ahead or behind at emergence and further off than thatat silking (if corn is the crop being considered). If growing degree days are used, one would have someanticipation as to whether or not the crop will be ahead of schedule or behind the calendar schedule. Aregrowing degree days of value? They are good if they improve on the simple calendar. That is really all thatneed be said about them--"If they are better than the calendar on the wall, then they have contributedsomething." The growing degree day is not a perfect model of crop growth or development. It needs only to bebetter than we had with the wall calendar.

Current GDD Accumulation and departure from average may be found athttp://www.ipm.iastate.edu/ipm/degreeday.

The growing degree day concept has been around for a long time. It originated somewhere around 1860 by abotanist (perhaps DeCondole) who was studying the effect of temperatures on plants and plant development.By 1900 the growing degree day was known in the United States as what is called the "National WeatherService Growing Degree Day," the one described above.

In 1900, when publications first reported growing degree days, no farmers were using them. They weredeveloped in Iowa, Illinois, and Indiana specifically for corn with the assumption that corn begins to grow at 50°F and begins to come under stress at 86° F, based on the data available up to that time.

By 1980, about half the farmers in Iowa were using growing degree days to aid in selection of hybrids and thevarieties planted. About 3 percent of the farmers were using growing degree days as an aid in pest controlbecause insect development is dependent on growing degree days much as is crop development.

The growing degree day was a little more accepted according to the 1991 survey. It showed that more than athird of the farmers were using growing degree days in evaluating pests, pest treatment, pest development,and pest control. Almost 90 percent were aware of growing degree days and considered them in selection ofhybrids.



Growing Degree Days

Application: Black Cutworm Moth

Growing degree days are available for some aspects of pest control. In some areas they are used. Remember,corn does not develop according to a linear relationship but according to a curve. However, a linearrelationship almost perfectly describes the development of black cutworms. They grow according to growingdegree days at base 50°, just as corn does. But they grow and develop according to a straight line (Fig. 3.12),whereas, corn does not. When this was realized, a real advancement was made in understanding the treatmentof black cutworms and protection of crops from black cutworms.

Fig. 3.12 Comparison of corn and black cutworm moth

development with temperature.

Study Question 3.6

When is the economic damage by black cut worms greatest?

60°F72°F80°F

Check Answer

Usually, when black cutworms are active, temperatures average about 72°. Under those conditions, one blackcutworm, in the average year, during its lifetime would eat four corn plants. If, however, soil temperatures arenear 80° at that time, the crop grows relatively better than the worms. Under those conditions, there areapproximately 2.3 plants destroyed per worm; that is from 4 to 2.3 at a higher temperature. It is possible that

worm eats a certain weight of food during its life, so if the corn is growing rapidly, it does not need so manyplants to harvest the amount of food that it needs. And, so the amount of damage drops down considerably(Fig. 3.13).

Consider another case. If the temperatures are hovering around 60° F at the time the cutworms are damagingcorn, the corn is growing slowly and staying small while the cutworms are growing quite rapidly. The worst caseis 12 plants per worm. Therefore, when a person has scouted a field and determined that a cutworm problem isgoing on there, the first question asked is, "How many plants have already been damaged?" The secondquestion is, "What is the stage of the cutworms?" They are measured, and according to a gauge the stage, theinstar, of the cutworms can be determined. So their development is known. A written record is made of theinstar of the cutworms, the growth stage of the crop, and the percent of the crop that has already beendamaged. Those are the three items needed to determine whether it is advisable to treat to prevent furtherdamage from the worms. With only those three bits of data, it is assumed that the temperature is 72° and thateach worm is eating about four corn plants during its lifetime.

Fig. 3.13 Corn plants eaten during black-cutworm lifetime at various

temperatures.

So the next question is, "What is the temperature at this time, and what is it expected to be for the rest of thelife stage of the cutworms (until they are done eating corn)?" If the temperature is going to be down around 60°F, there is need to treat the field. Economically it will be an advantage to treat the field at the much lower levelof initial damage (and with a smaller number of cutworms) than if the temperature is at a more normal level or iswarmer than usual (around 80°) where damage would indeed be smaller. This can have a very large economicimpact on the individual who is making the decision as to the treatment of the crop. A similar story can beexperienced with many other insect pests.



Growing Degree Days

GDD Inaccuracies

How the growing degree calculation is made and one of the limitations (that growing degree days are linear andcrop development is not linear) have been pointed out. There are other limitations. One that should be obviousfrom the rules previously stated is the limitation of what to do with the low or the base temperature. If thethreshold temperature, called THR, is 50° F, the actual low temperature is 50° F and the high temperature is70° F, there were 10 growing degree days during the 24-hour period. During the period, the temperaturewarmed up to 70° F, and later assume it cooled back down to 50° F. If lines connect the temperature values,the area under the "curve" represents crop growth/development potential (Fig. 3.14). However, temperatureseldom varies between the daily low and high value in a manner linear with time. Hence, the "area under thecurve" is not totally realistic.

Fig. 3.14 GDD assumes temperature changes linearly throughout the day. Click

on figure for actual temperature change.

Now, observe a third inaccuracy of the growing degree day, the condition wherein the low temperature fallsbelow 50° F. At 50° or above, and with averaging over some days, errors would be small. Consider that thehigh was 70° but the low temperature fell to 40°. Remember the rule. If the low temperature is below 50°, call it50°. So, move it up and call it 50°. The growing degree day accumulation for that day remains at 10. Eventhough the night temperature had been 40°, there would still be a 10 just like during the previous day. There issome error involved in this (Fig. 3.15). The plant did not start growing until the temperature reached 50. Sothere were some hours between the 40° and 50° when it would be assumed that the plant was growing, but itreally was not. This error gives the plant more growing degree days than the plant actually recognized orreceived. So there is an area of error and, of course, the area of error may be compounded by the eveningcooling as well.

Fig. 3.15 Error in GDD calculation. The GDDs for this day would be

10. But the growth area would actually be less because the crop

would not grow until temperatures moved above 50 °F.

With this error, introduced by assuming that the temperature was 50° even though it was only 40°, plants donot grow or develop as fast as anticipated.

There are three inherent errors: First, the inherent error that growing degree days assume linear growth (Fig.3.11); second, the "shape" of the temperature trend with time (Fig. 3.14); third, the error of adjusting theminimum temperature up to the base when we calculate, so there is a little area of mistake if things under thearea of the curve are considered (Fig. 3.15).

Now, someone might say, "Okay, fix it. An account needs to be made for this area that has an error." This hasbeen done. A lot of work was done at Iowa State University, primarily by entomologists, to do just that. During a24-hour period, there will be a minimum temperature that existed at some time,usually just before sunrise. Soonafter the sun comes up, the temperature starts to increase, usually quite rapidly. Then the increase ratediminishes, and a peak is reached for the day. Temperature will fall to whatever the low for the next night mightbe. It may be different from the previous night. Nevertheless, it will taper off. This peak will probably comearound 1 p.m. standard time or at least some time in the afternoon. The low point will probably come around 6a.m., actually just before sunrise or just the minute or two after sunrise.

A temperature curve of what might happen during a day is shown in Figure 3.16 (warming up quite quicklywhen the sun comes up, usually if it's a clear day, hitting a peak around 1 p.m. standard time, then tapering offslowly until 6 a.m. the next morning to the minimum for the 24-hour period). Someone might say, "We would dobetter to draw a curve something like a sine curve rather than a growing degree day straight line that indicateschanges from 6 a.m. to 1 p.m. and then another straight line for the period 1 p.m. to 6 a.m. the next day." Amethod was developed for calculation of growing degree days called the "sine wave method" that assumes thatthe day heats up slowly and reaches a peak around 1 o'clock. It then cools off in a uniform way according to asmooth sine curve.

Fig. 3.16 Sine wave approximation to daily temperature.

A growing degree day calculation according to a sine curve makes it possible to correct one of the errorsdescribed earlier. If the minimum temperature were somewhat below threshold (Fig. 3.16b), the temperature ofthe day rose, and attained a peak, the sine wave would define the hour it will cross the threshold temperature.The plant will grow according to the area beneath that curve and be considerably more accurate than thestraight line method.

GDD Advantages

This more accurate way is not generally used. An interesting study was done a few years ago. An entomologistin a neighboring state expressed some criticism. He said, "Iowa developed the sine method of calculatinggrowing degree days, while Iowa is not using them. Iowa is using growing degree days by the old WeatherService method which has been used since 1900." The reply was, "There are two reasons we do that! The firstreason is, Iowa has used the Weather Service method since 1900. Everyone has that record; hence, ongoingmeasurements and data are consistent with the historical record. The second reason is, data indicate that thegrowing degree day method (after the inaccuracies of the growing degree days were adjusted to base 50)gives better results with the Black cutworms than the method that uses the sine wave calculations."

The results are better using the original method, for black cutworms under field conditions at least, than forusing the sine wave method. The sine wave method is better for use with insects reared inside the laboratory.For those that are growing outside, the straight line, old-fashioned method turns out to be the best. Why itturned out to be the best is not really known. Some ideas have been suggested. The worms move around toplaces having different temperatures, Maybe their activity is sort of compensated for in the error in the method.Certainly the worms do move around. If the temperatures get too hot, they dig a hole in the ground and godown to where it is cool enough for them to be comfortable. When the temperatures are appropriate on thesurface, they come up to the surface and feed and do the things that cutworms do, as observed by theentomologists and this author. The results of cutworm activities are the reasons why our growing degree daycalculations seem to be acceptable for Black cutworms.

There is one more reason to use the old-fashioned method for calculation, which is really the overridingreason, the accuracy of the system. When making a calculation, one does not need to have any calculationbetter than the accuracy of the system.

What is the biggest error in the system? Is it the calculation of the growing degree day, or is it the measurementof the temperature of the air itself? A thermometer is not perfect. The thermometer reads to the nearest degreeor records something on the order of ±1° F. There is a 1° error just in the thermometer. What about the erroracross a field? From one side of the field to the other? On a south-facing slope, and a north-facing slope, the

plants grow differently. Even if the soils are the same, there is different response to temperatures, differentemergence times, and different development rates. The temperature variation across the field itself may be onthe order of 5° F. If there is a variability of 5° across the field itself, the error that comes from not calculatinggrowing degree days, the very best way pales next to the error of the temperature variability of theenvironment.

When growing degree days are observed, the hope is that this process is something a little better than the cropcalendar that was considered previously. The crop calendar gives the average dates of planting, the averagedate of emergence, the dates of tasseling, and the dates of maturity as calendar dates. The growing degreeday adjusts for temperature and indicates whether or not the temperature effect has been significant enough tomove away from the average dates, and about how far. That can be of some benefit, and that is all it is--justSOME benefit. There is no reason to think that a growing degree day will be a perfect model for what a crop willdo.

Agronomy 541 : Lesson 3b

Radiation and Climate

Introduction


It is suggested that you watch Video 3B and complete the exercise in the video before continuing with thelesson.


Energy and radiation are emitted by all objects. This is how we are able to see other objects. The mostimportant source of radiation emitted, though, is the sun. Solar radiation is recognized as driving processesthat occur on the Earth. Weather as well as plant and animal life are all dependent on solar radiation. How thesolar radiation reaches the surface to provide the heat and radiation necessary is not as straight-forward as itmay seem. The fate of solar radiation on its path through the atmosphere and its resulting effects on thesurface and atmosphere are discussed in this lesson.


About the components of the electromagnetic spectrum.How radiation heats the atmosphere.The difference between longwave and shortwave radiation.

Reading Assignments:

pg.31-39—Aguado & Burt, 4th pg: 31-41, Energy Transfer

pg.60-68—Aguado & Burt, 4th pg: 61-69, Solar/Energy Processes



Radiation and Temperature

All objects emit radiation. This allows us to see objects. If no radiation were emitted, everything would be black.Radiation from most objects is emitted at relatively cool temperatures. Objects which emit at warmertemperatures produces other effects. Use as an example the computer in front of you. You can feel heatcoming off the computer when a hand is placed over it because heat is produced inside by tubes or otherelectronic devices operating inside. Warm air is rising from the vents that are in the back of the machine. Thisis convective heat coming off the computer, warm air rising. Also some heat can be felt coming off the side ofthe computer. A hand held about 8 inches away from the side, and then turned toward the wall of the room, willsense more warmth coming from the computer than from the room to one's hand. This is heat that is beingradiated. All objects that have temperature, which is anything that has temperature above absolute 0, areradiating heat, and any heat that is felt coming off the side is radiated heat. The heat coming off the top ismade up of two kinds of heat. Radiated heat is coming off the top along with convected heat (the warm airrising from the machine) (Fig. 3.17).

Fig. 3.17 Types of heat emitted from a computer.

Now, imagine if a pot-bellied stove containing a hot fire were in the room. Soon a hand would not becomfortable for an extended period of time near the its side because the stove would become so hot thatradiation would overheat the hand (Fig. 3.18). Still warmth could be felt coming from the stove.

Fig. 3.18 Types of heat from a pot-bellied stove.

If the stove were stoked up as hot as possible, one would actually begin to see it turn red hot. An incandescentlight, that has an adjustment or a rheostat to dim the lights in a room, can be dimmed from a nice white light toyellow, and then may change to red and then dark red, as the power is decreased. As things cool off, theircolor changes. As a stove gets red hot, it gives off heat. It could become white hot as it gets up to thetemperature of most light bulbs and of the sun.

Radiation emitted by an object can vary. Most of the radiation we usually experience is associated with thatwhich we can see (Fig. 3.19). But there is a great deal more electromagnetic energy which we cannotexperience visually. This includes the ultraviolet (associated with “sunburn”) and infrared that we may feel asheat on our body but cannot see (Fig. 3.20). Radiated energy can be measured by electronically. Electronicinstruments can estimate the temperature of the radiating source by the wavelength of the emitted radiation(spectrometer) as the wave length of the peak of emission shortens as temperature increases or by the totalamount of emitted radiation (bolometer) as the total emitted energy is proportional to the fourth power of theabsolute temperature of the radiating object.

Fig. 3.19 Visible light and the electromagnetic spectrum.

Study Question 3.7

What is the peak wavelength of the sun?

400 nm500 nm600 nm700 nm

Check Answer

In the middle of the day when the sun is highest in the sky it has the least atmospheric interference andappears as a yellow-orange color. Looking at Fig. 3.19 this indicates the sun must be emitting somewhere inthe visible spectrum. The temperature of an object indicates what temperature it emits at and what the peakradiation emitted by the object is. The sun emits radiation at many wavelengths (Fig. 3.20) but the peak is atabout 0.6 m in the visible spectrum. The peak of emission is related to where the object emits the mostradiation and how it appears.

The examples describe the nature of short- and long-wave radiation. Everything radiates. If radiating at a cooltemperature, the radiation is from the longwave part of the spectrum (Fig. 3.19). Longwave radiation is infraredenergy that cannot be seen. As a fire or light bulb gets hotter, the wavelength of peak emission shortens andthe color goes from reds to orange, etc.

The sun emits shortwave radiation (Fig. 3.20) which can pass through a piece of glass, or pass through theatmosphere. The radiation from the sun is absorbed by the earth. Longer wave, or infrared radiation, is thenradiated from the warm earth. This long-wave radiation cannot get through the glass. Glass appears black,

even the clearest glass appears black to infrared radiation. It cannot get through the glass to allow any escapeof the heat. So the greenhouse heats up. Light gets in, and the infrared cannot pass directly out.

Fig. 3.20 Intensity (or relative amount of energy) contained in solar radiation and

terrestrial (Earth radiation).

A representation of the wavelength of the light is given in Figure 3.21. The energy coming from the sun is at a

wavelength from 0.1 µ (1µ = 10-6 m.) to 3 µ, with most of the sun's energy being at 0.5 µ. The wavelength ofvisible light is about 0.3 to 0.8 µ. Ultraviolet is a shorter wavelength than the visible. Infrared wavelengths varyfrom 0.8 to 3.0 µ The sunlight strikes the earth and is converted to heat. The Earth radiates energy at a peakwavelength of about 10 µ (Fig. 3.20). Earth's radiation, striking something such as water or carbon dioxide inthe atmosphere, does not get through and ends up, to some extent, trapped. There is a small window at 10 µwhere some of this radiation escapes through the atmosphere. This enables satellites to measure thetemperature of the earth's surface. A discussion about satellite sensing is in Lesson 9a.

Fig. 3.21 Wave characteristics



Radiation Interactive

Solar radiation is the driving force for all processes in the atmosphere. The sun obviously heats theatmosphere and causes other effects. But how this occurs is not necessarily direct. Spend a few minutesthinking about how the sun heats the atmosphere and how that interaction could be changed. Then click andload the java simulation and follow the directions in the simulation. Answer the study questions below thesimulation to test your understanding.

Try This!

Click here to load the simulation.

When the 'Authentication Required' window opens you will need to enter your ISU netID, password, and thedomain name IASTATE to access the simulation.



Radiation and Atmosphere

Earth, the solar body the third closest to the sun at 93,000,000 miles (148,800,000 km) away, is greatlyaffected and influenced by the sun. The earth is bombarded by 126 trillion horsepower per second from thesun's output. But because of the distance of the Earth from the sun, only one half of 1 billionth of the energybeing radiated by that celestial body is absorbed. Even though the Earth intercepts only a minute fraction ofthe total output of the sun, the amount the Earth intercepts is very large.

Fig. 3.22 Earth's orbit around the sun.

The question, then, becomes, what happens to the energy from the sun? Energy reaching the top of theatmosphere does not all reach the surface of the earth. Properties of the atmosphere intercept variousamounts of radiation.

Study Question 3.8

Which does not affect the amount of incoming solar radiation?

cloudsparticulates in the airCO2

time of year

Check Answer

Of 100 units of energy reaching the top of the atmosphere, six are scattered back into space by particles in theatmosphere. Clouds (especially water clouds) reflect 20 more units back into space. Another 19 units areabsorbed by the atmosphere and clouds. This leaves only 55 units of solar energy that reach the surface. Fourunits of these are reflected by the surface back into space (Fig. 3.23). Thus, just over half the incoming solarenergy is absorbed by the surface. These are global averages and would obviously change when looking atindividual locations or day to day weather.

Fig. 3.23 Solar radiation reaching the top of the

atmosphere is partitioned on its path through the

atmosphere. Only about half of the energy

reaches the surface.

How much solar radiation reaches the surface drives the weather and climate. All of our weather is determinedby that energy coming from the sun, and when the sun strikes the earth's surface. Any radiation that does notreach the surface has little effect on weather or climate. But solar radiation simply reaching the surface doesnot fully explain the effects of radiation on the surface. When solar radiation reaches the surface, two situationscan result, the radiation is absorbed by the surface or it is reflected by the surface. Very little is transmitted intothe soil. So little is transmitted that it is ignored. Absorbed radiation is used for internal processes and otherfunctions which will be discussed. But some radiation usually is not absorbed. If radiation is not absorbed, it isreflected.

An example of reflected radiation is when comparing the visual effect of snow. When snow is freshly fallen andwhite it is very bright, reflecting a great deal of eye to your light. Snow absorbs about only about 25% of thesun's energy which strikes it; some snow is darker and some lighter. Powder snow is an example of a lightersnow. Regardless of particular form, snow absorbs about 25% of the sunlight and reflects about 75% of thelight back up into the sky. Hence, snow is considered to have a high albedo. Albedo is the percentage ofradiation the surface reflects.

A forest will absorb about 95% of the light of the sun that strikes it. A plowed field might absorb 95% as well.Sand is brighter, absorbing less than the forest. Water surface absorption of sunlight depends on the anglelight strikes it. The more vertical the rays striking the water, the more is absorbed. The angled the sunlight, themore is reflected. Sometimes, one gets a glint off the water, known as specular reflectance. Different surfaceshave albedos that influence the amount of solar energy absorbed and retained by the earth. Combining all thesurfaces as measured by satellite data, creates a total earth albedo of about 0.30 (Table 3.4).

Table 3.4 Albedos of various surfaces.

Surface Albedo (%)

Fresh snow 75-95

Clouds (thick) 60-90

Clouds (thin) 30-50

Ice 30-40

Sand 15-45Grassy field 10-30

Dry, plowed field 5-20

Water 10*

Forest 3-10

*daily average, water varies greatly by the angle sun hits it.

What happens to radiation when it reaches the surface is then a function of the surface itself. This can have agreat impact of weather and climate. Snow has a huge impact on how the surface is warmed as do severalother surfaces. This should help explain some of the temperature differences seen over the different surfacesin the radiation simulation.



Net Radiation Partitioning

After reaching the surface, it can again be reflected. Most of this radiation returns to space having little effecton weather or climate. Some of the reflected radiation from the surface can be reflected back to the surface byclouds. But solar radiation does not have much influence on the atmosphere. Solar radiation must be absorbedat the surface to work in the atmosphere.

Once radiation has reached the surface and been absorbed, two different processes can occur.

1. The radiation can be absorbed by the surface, heating the surface of the earth, the plant, etc. This willraise the temperature of the surface (called sensible heating. The radiation absorbed is used tochange temperature.

2. The radiation can evaporate water. When water exists on a surface, solar radiation can add energy tothe water molecules. Adding this energy will cause them to evaporate from the water surface. Thispartitioning of the energy is used to change the phase of water from liquid to vapor. The energy is usedwithout a corresponding temperature change. Therefore this heating is called latent heating. Latentrefers to the heat which "disappears" without causing a temperature change.

The result is that the same amounts of incoming solar radiation cause different temperature changes at thesurface depending on the type of surface. Several different locations and climates are depicted in Figure 3.24The surface type and climate are very intertwined with how the radiation is partitioned.

Fig. 3.24 Partitioning of incoming (net) radiation between sensible heat (SH)

and latent heat (LH) for different areas of the globe.

For example, Manáos, Brazil at 3° S is located in a very tropical, very moist region. Most radiation here is usedto evaporate water from the moist surfaces. Little sensible heating occurs. Aswan, Egypt is located in a sub-tropical desert where little water exists to evaporate. At Manáos, this is consistent throughout the year. Otherlocations vary by time of year. Iowa would be a place that would vary temporally. This is a second factor whichcan be related to the radiation simulation. Which surfaces heated most rapidly, which the least?

Radiation partitioning has an effect on plants directly. When open water exists on a plant, such as when dewforms, radiation is initially partitioned to evaporate water from the surface. Thus, the plant will stay cooler untilthe water is gone. Then radiation will go to heat the plant.

Study Question 3.9

Which would have a smaller temperature range between day and night?

ManáosAswan

Check Answer

This warming of the surface causes longwave radiation to be emitted (Fig. 3.21) warming the air next to it byconduction. Unlike shortwave radiation which passes through the atmosphere, longwave radiation does notpass through the atmosphere. It is absorbed by carbon dioxide, water, and ozone in the atmosphere. Thesemolecules then re-emit the energy back toward the surface continuing to warm the air.

This is why the atmosphere is talked about as acting like a greenhouse. Shortwave radiation passes throughwhile longwave radiation does not. It is absorbed by the above gasses and used to warm the atmosphere.



Radiation Balance

The earth maintains its temperature because of its heat capacity and the radiation properties of the surfaceand of the atmosphere, what we will call the "Greenhouse Effect." The greenhouse effect was first described bythe physicist/mathematician Fourier in the 1800s and was well understood by 1900. Basically, consider thegreenhouse effect of a real greenhouse. The sun's shortwave radiation shines through the glass; the lightstrikes something inside of the room and is converted into heat. The heat is a form of longwave radiation. Thisradiation is trapped by the greenhouse glass, being unable to penetrate it. The heat is trapped, warming thegreenhouse (Fig. 3.25).

Try This! : Greenhouse

Fig. 3.25 Shortwave energy penetrates the glass, heating surfaces

inside the greenhouse. Longwave energy cannot pass through the

glass, trapping heat inside. Thus, the greenhouse is warmed. Click on

the image above to hear an explanation of the diagram.

Carbon dioxide, water, and ozone (at lower levels) in the atmosphere have similar effects to that of glass. Thegreenhouse effect of the atmosphere has similarities to the heat trapping that goes on in a real greenhousethat is made of glass. There are some differences, but the similarities are enough that we can pay at leastsome attention to the concept of greenhouse warming and greenhouse gases. The longwave radiation emittedby the surface is absorbed by the molecules of the mentioned gases. These gases re-emit the energydownward toward the surface keeping the surface warm. Without these gases the energy would be emitted intospace with much less heating of the atmosphere occurring.

Keep in mind that it is the greenhouse effect that makes the earth the place that we enjoy as we have it now.The moon's temperature gets very hot. The daytime on the moon lasts a couple of weeks. One day on themoon is about a month on the earth. But in the day the temperature heats up to above the boiling point ofwater on the moon, and at night it cools off to -238° F. If the lunar day were 24 hours, surface temperaturedifference would be less extreme but still substantial. Without the water vapor and carbon dioxide the moon has

no buffering effect to maintain the heating balance mentioned earlier. The earth would experience similardiurnal fluctuation if we did not have the atmosphere as a blanket around the earth.

Try This! : Radiational Cooling

Transcription of the audio on radiational cooling.

Fig. 3.26 Effect of radiational cooling. Click on the image above to hear

an explanation of the diagram.

Now, return briefly to the computer/heater example. If the computer were next to me, the warmth could be felt allnight, and it would keep me warmer. Put a tree over my head, and it radiates according to its temperature. Itmight be 70° F, but it is radiating heat at 70° F down upon the boys sleeping beneath the tree. If you go outfrom under the tree, you are exposed to the cosmic cold of space. If you would measure the radiation comingdown from the tree, it would say the "sky" is at 70° F, the temperature of the tree leaves. Go out under theclear sky and measure the temperature; the thermometer would say the sky is at -40° F. No wonder you feelcold. You do not have a warm object radiating. You are next to a freezer that's -40° F. There is not sufficientheat coming from the atmosphere to keep you from feeling cold. An example of Mideast ice houses can befound in lesson 7.

A corn grower reported that some of the corn in the field was found frosted the morning(s) following a night thathad been 60 ° or 70° F. There may have been some damage other than frost. However, thermal frost can beseen even when the temperature is quite warm and the night is clear, still, and dry with no humidity, no wind,and no clouds. It is not uncommon for a surface to become 5°, 10°, or even 15° below air temperature. Hence,if air temperature had been 45-50° F, there could have been frost, but would be unlikely at an air temperatureabove 70° F.

It is this radiative cooling and solar heating of the Earth and differential temperatures from place to place on theEarth that give rise to air movements.

Assignment 3.1

Click here for Assignment 3.1

Lesson 3 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?

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FYI : Transcription of The Audio on Radiational Cooling

If the sky is clear, the night is still, and the humidity is low, the earth cools off faster than if it is cloudy and humid. Agood example of the earth-cooling principle was felt on a trip with some Boy Scouts, a few years ago, into the bottomof the Grand Canyon. On the north rim of the Canyon (about 10,000 feet elevation), there was snow on the ground.Descend into the Grand Canyon, temperatures get much warmer. They grow bananas in the bottom. Thetemperature was not going to go below 70o F at night, so a lot of the boys said they would not take their sleepingbags. They would just take a light blanket or sheet to sleep in while they were down there. They walked the bottom ofthe Grand Canyon and laid their bed roll out on the sand. I put mine underneath an oak tree. One of the boys asked,"Why are you sleeping underneath that tree?" "So I do not get cold," I replied. He answered, "Well, it is not going tobe cold." Before morning, all of the Boy Scouts were under the oak tree with me. The relative humidity there wasabout 5% and the sky was perfectly clear. The ground cooled until everyone was very chilly, even though thetemperature itself did not go below 70o F. Under the tree, the temperature felt warmer. It was really the same, but itfelt much warmer because we did not experience radiative cooling (Fig. 3.26).



Introduction


It is suggested that you watch Video 3A and complete the exercise in the video before continuing with thelesson.


In much of the world outside the tropics, the growing season is limited by winter temperatures. The farther awayfrom the Equator your location, the less solar radiation is received which shortens your growing season. Thecold temperatures of spring limit the earliest opportunity to plant. A typical planting date is after the averagelast spring freeze. At the end of the growing season, the average first fall freeze limits growing season length.These and other important dates for crop production are the basis of crop calendars. Temperatures are thecontrolling factor for the crop season in the Midwest. In other areas, rains are the controlling factor. Crops aregrown during the rains.

The crop calendar is the fundamental document of agricultural meteorology. Some form of crop calendar existsfor all agricultural locations. In this lesson we will discuss factors associated with and controlling the growingseason and review the resultant crop calendars.


How crop calendars can guide production.What controls the growing season length in the Midwest.Some additional limitations of GDDs.

Documents

Crop and Climate Calendars - Iowa State Universityagron- · 2011-12-08 · Agronomy 541 : Lesson 3a Crop and Climate Calendars Iowa Crop Calendars If involved with agriculture, the