Weather and the Production of Burley Tobacco

Home , Burley (tobacco), Curing of tobacco, Types of tobacco

52 BULLETIN AMERICAN METEOROLOGICAL SOCIETY

JAMES M. BEALL U. S. Weather Bureau

ABSTRACT

Tobacco is an economically important crop in the United States. It is grown under a variety of climatic conditions each imposing its particular microclimatological and micro- meteorological effects. The curing of burley tobacco depends particularly on the proper control of relative humidity. A humidity forecasting project started in 1954 in Kentucky by the Weather Bureau with the cooperation of the University of Kentucky has resulted in that some farmers using the service have received up to 7 cents a pound more than parity for their tobacco. Consequently the Weather Bureau has decided that the service is well worth while.

OBACCO as an economically important ley is the number one cash crop in the State. The crop in the United States goes back to number of acres of tobacco cultivated during re- T the early colonial days in Virginia. The cent years has been reduced because of govern- use of tobacco was first recorded by Columbus in ment controls, but because of better agronomic Cuba in 1492 when he saw it being smoked by the practices the yields have steadily increased. Even American aborigines. The use of tobacco was with this technological advance tobacco is still made popular in the French Court by Jean Nicot, virtually unmechanized. Each tobacco plant is French Ambassador to Portugal. The drug nico- handled at least six times from seed to market. tine derived its name from this proponent of its The fact that it is still a crop of hand labor is il- use. Sir Walter Raleigh made tobacco popular in lustrated by examination of the statistics which the Court of St. James in the sixteenth century. show that only three percent of the arable land of Since then its use has spread world-wide. Kentucky is planted in tobacco yet this crop uti- Tobacco is a weed of the nightshade family. lizes about twenty-seven per cent of the available Other members of this indigenous American plant farm labor force in the State. family are tomato, potato, egg plant and green That weather accounts for more than fifty per- pepper. The distinguishing feature of tobacco is cent of the variability in the yields of the cereal the habit forming nicotine content of the leaf. crops is a fact that has been long recognized. We Because environment is not a critical factor in know subjectively that weather is an important growth its culture has spread to many soil types factor in the production of high quality even and climates from Canada and Scandinavia in the though we know of no work that has correlated sub-arctic to Cuba and Sumatra in the tropics. weather and yield or quality of tobacco. The This spread of its culture has resulted in several proper utilization of weather forecasts in farm distinct varieties. The best cigar wrappers come management decisions will pay increased divi- from Cuba in the Tropics. The best oriental or dends to the farmers of Kentucky. Turkish tobacco comes from Macedonia in the First we will discuss the life cycle of the to- Balkans. The best bright leaf comes from the bacco plant from seed to the sale of the leaf, and Carolinas. The best burley comes from the Blue then we will discuss how weather affects each Grass area of Kentucky. The present discussion phase of production. The seed is extremely small. will be limited to burley. Three hundred thousand to three hundred fifty The trend in the sale of finished tobacco to the thousand are needed to make one ounce. They consumer has been a phenomenal rise in the use of are not planted directly in the field since the seed- cigarettes. In the last 30 years cigarette sales lings are subject to injury from cold, insects, dis- have soared 500 per cent. Associated with this eases and weed competition. Tobacco in Ken- development has been the increase in the use of tucky is planted in seed beds six to nine feet wide burley tobacco which is blended with other to- and sometimes in excess of a hundred feet long baccos to give the desired qualities which the early in the spring. Between May 20 and June 10 cigarette manufacturers extol. the seedlings are transplanted into the fields. The Over seventy percent of the burley grown in the plants are cultivated several times during the sum- USA is produced in the State of Kentucky. Bur- mer to keep down the weed competition for food

Unauthenticated | Downloaded 09/25/21 10:08 PM UTC VOL. 38, No. 2, FEBRUARY, 1957 53 and water. Harvesting begins in late August or mum protection. A tight fence, farm buildings, the first of September. The plants are then placed dense woodland, or a bamboo hedge can all serve in tobacco barns to begin the curing process which to break up the wind. takes from six weeks to two months. In the late The heat loss in advective situations is a func- fall and early winter the stalks are taken down and tion of the wind velocity. The purpose of the wind the leaves stripped off, sorted and made ready for break is to reduce the wind velocity and therefore the market. During December and January the decrease the advective heat loss. tobacco leaves are sold at auction in the ware- The plants are transplanted into the fields about houses. The care and culture of tobacco is vir- the last week of May. The ideal sequence in tually a year-round operation. spring weather as far as tobacco is concerned is for Seed bed preparation begins in the fall. It is the weather to be fairly dry long enough for the necessary to sterilize the soil in order to eliminate farmer to prepare the field followed by a slow soil-borne diseases, insects and weeds. Alternate steady rain to moisten at least the first six inches freezing and thawing aids in the sterilization of the soil just prior to transplanting. As soon process. The traditional method is by burning as the plants are in another short rain is needed wood on the bed for at least half an hour. Later to get the new root system established. Field steam was introduced as a method of sterilization. cultivation starts about two weeks after trans- Chemical sterilization of seed beds is now be- planting and it should be dry for several days for coming more widespread. this field operation. During the summer the soil Seeds should not be planted until the soil tem- should never become so dry as to place stresses peratures have reached an average daily value of on the plants. 50 degrees Fahrenheit. Greenhouse experiments The tobacco plant, being essentially a weed, is have shown the lower threshold of germination of able to withstand periods of more adverse weather tobacco seeds to be 45°F; the upper limit above than some of the other kinds of field crops with- which they will not germinate to be 95 °F and the out appreciable differences in yield. However, optimum germination temperature to be 78°F. the nicotine content of the tobacco is markedly The micro-meteorological aspect of seed bed higher if the plants have withstood periods of location becomes important largely from the stand- drought than if they have an adequate moisture point of reducing the frost hazard and securing supply to insure steady growth. Physiologists the maximum heating. Location on a south slope have shown that a plant experiencing continuous will insure faster germination and a larger per- growth has a different chemical makeup than one centage of usable plants. An effective wind break that has had severe stresses placed on it. In ad- will decrease the losses due to advective freezes. dition other factors of quality are adversely af- Some phenomenal differences between the cli- fected in the drought periods such as the thicken- mate of the southern slope versus the northern ing of the leaf, smaller leaf size and smaller total slope of a small hill have been reported by Geiger number of leaves. and others. The reasoning is quite simple. Con- Harvesting is a process that requires dry sider the heat received on a unit area normal to weather for at least two successive days. The the direct rays of the spring sun. As the projec- tobacco stalk is cut near the ground and impaled tion of this unit area is rotated the amount of heat on a stick about four and a half feet long. Six received per unit area on the projection is re- plants are usually arranged on each stick. The duced as a function of the cosine of the angle be- plants are left overnight in the fields to wilt. The tween the plane normal to the sun's rays and its wilting serves two purposes; first it reduces the projection. Carr [1] has shown experimentally moisture carried into the barn and second, it makes in 1943 in Georgia that under similar conditions the leaves easier to handle without danger of in- a five per cent slope to the south yielded twice as jury. The sticks of tobacco are then hung in the many tobacco plants as a five per cent northerly barn to begin the curing process. Eighty percent slope. Plants on east and west slopes produced of the green weight of the tobacco must be lost intermediate results. in the curing process. For each five thousand The micro-meteorology of temperatures in the pounds of newly housed tobacco two tons of water lee of shelter belts is important since freezing must be lost before the curing process is complete. winds are one damaging agent of young, tender Curing of tobacco is not a simple desiccation plants. A knowledge of the direction of the local process. A complicated series of chemical changes cold winds would be invaluable in the placement takes place inside the leaf. These changes will not of these shelter belts in order to secure the maxi- occur when the leaf is dry. The green color

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changes to yellow as the chlorophyl breaks down. The yellow darkens to tan, brown or red depending on the curing conditions and the type of the leaf. One of the measures of tobacco quality is its cured color. Other types of tobacco have col- ors that are desirable all the way from the bright yellow of flue cured Carolina tobacco through the almost black of the Louisiana perique. The most desired burley color is a very light tan. Other changes take place in the leaf during the curing process. Starches are broken down to sugar which in turn is used up in the oxidation process as car- bon dioxide and water. The complex nitrogen compounds are broken up into simple amino acids and ammonia. Malic acid decreases while the citric acid increases and a portion of the nicotine disappears. FIG. 2. Schematic diagram illustrating the effect of Air cured burley tobacco that has dried out wind and outdoor relative humidity on a barn filled with rapidly is just like hay and would sell for about fresh tobacco. the same price as hay instead of from 30 to 60 cents a pound which tobacco brings. The farmer has considerable control of the climate in his barn by using ventilators or louvers on the side of the barn as well as the large doors in the barn ends by which means he can control the air movement through the curing tobacco. An- other measure of control over the climate in the barn is added by heating. Coke heaters are gen- erally used in Kentucky. In the Wisconsin and Connecticut cigar tobacco areas charcoal is widely used. In some places liquid petroleum gas heaters are used. Jeffrey [4] has worked out experimentally the diagram (FIGURE 1) for the relationship of mean daily relative humidity to the color to which burley will cure. As far as he could determine tobacco suffered no adverse effects due to the diurnal variations of humidity as long as the mean daily value was in the optimum range. Tobacco left at above 75 percent relative humidity for 42 hours suffered deterioration which was noticeable to experienced tobacco graders. This deterioration is accelerated at higher temperatures. The color will become darker and the weight of the leaves becomes lighter due to matter being used up in the oxidation process. The most serious losses in curing occur due to a mold or bacterial disease called house burn in Kentucky and pole rot or shed burn in the northern air-cured areas. This disease occurs when the relative humidities are sustained at 90 percent or more for a period long enough for the disease to become endemic. During the early fall when a FIG. 1. Psychrometric chart showing relationships be- two or three day warm frontal type of rain keeps tween temperature, relative humidity, and quality of the humidity and the temperatures high, it be- cured tobacco as judged by the color of the tobacco leaf. comes necessary to heat the barns in order to in-

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FIG. 3. Schematic diagram illustrating the effect of sun FIG. 5. Actual temperature distribution in the experi- when the influence of wind is negligible. mental barn, ventilators open. sure proper curing and eliminate the possibility of established the following environment-curing re- house burn. lationships. O'Bannon has investigated the micrometeor- 1. Good quality burley tobacco can be cured at ology of tobacco barns in order to determine the temperatures from 65°F to 95°F provided the best management practices. FIGURES 2, 3 and 4 relative humidity is near optimum. are from his work published as University of 2. The optimum relative humidity for curing Kentucky Agricultural Experiment Station Bulle- most burley tobacco is 65 to 70 percent. tins 444 and 501. 3. Burley tobacco cured at too high relative hu- Actual distributions of temperature and humidity is darker than if cured at optimum condi- midity when the ventilators were closed are shown tions and a loss in weight results. in FIGURES 7 and 8. 4. Burley tobacco cured at too low relative hu- A summary of the research conducted by the midity tends to be mottled, with yellow areas Kentucky Agricultural Experiment Station has

FIG. 4. Schematic diagram showing the combined effects FIG. 6. Actual relative humidity distribution observed of sun, wind and outdoor relative humidity. concurrently with the data in FIGURE 5.

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would recommend having the barns well ventilated as soon as the relative humidity is lower outside the barn than it is inside. In the afternoon or early evening when the outside humidity attains the value inside the barn the doors should be closed. During rainy periods or during periods of high humidities it becomes necessary to heat the barns. One concept that is difficult to explain to farmers is the necessity to open the ventilators slightly, preferably on the lee side of the building during the heating in order that the moisture laden warm humid air can be carried out of the barn. During times when the relative humidity is too low and the tobacco in the barn is curing too fast, the recommended practice is to keep the barns closed during the day when the humidities outside are low and open during the night when the FIG. 7. Actual temperature distribution in the experi- outside humidities can add moisture to the curing mental barn, ventilators closed. tobacco. After the tobacco has finished curing the leaves prominent. At very low relative humidity, a green are hygroscopic and respond to changes in the cast or green spots are obtained, especially at low relative humidity of the environment. During temperature. this period it is general practice to keep the barns 5. No evidence of difference in quality has been closed all the time. detected as a result of diurnal fluctuation of con- In the late fall the farmer will take the stalks ditions as compared with constant conditions. down and, with his crew, will strip the leaves off. 6. Cured tobacco is likely to be damaged if ex- Grading and sorting is done on the farm. After posed to a relative humidity above 75 percent for about thirty leaves of the same grade have been longer than necessary to make the leaves pliable collected the bases are tied together with a leaf of enough to handle during the fall stripping process. the same grade into a "hand" of tobacco. The rate of damage increases rapidly with both During "the stripping" season the farmer wants high humidities and high temperatures. the tobacco to absorb enough moisture so that it 7. Experienced handlers can estimate the cur- rent relative humidity of the tobacco with sufficient accuracy by its feel since the leaves are hygroscopic. 8. When using natural ventilation to reduce high relative humidities inside a tobacco barn, the proper time to open ventilators is at the moment when the outside relative humidity falls below the inside relative humidity, and the proper time to close the ventilators is at that moment when the average relative humidity within the barn has been reduced to a minimum. The lowest relative humidity inside a barn may lag behind that outside the barn depending upon the difference between outside and inside conditions. During periods of "normal" weather the day- time humidities fall to near 30 percent during the day and exceed 75 percent at night. The practices recommended by The University of Ken- tucky College of Agriculture Tobacco Specialists in these cases assume that the tobacco retains FIG. 8. Actual relative humidity distribution observed much of its original moisture. Therefore, they concurrently with the data in FIGURE 7.

Unauthenticated | Downloaded 09/25/21 10:08 PM UTC VOL. 38, No. 2, FEBRUARY, 1957 57 can be handled with ease yet not get so wet as to lower the quality. The term "case" is used to designate the degree of moisture content in the leaf in Kentucky. High case designates that the leaf is very moist, medium case indicates there is enough moisture in the leaf so that it will handle without breaking and low case being too dry to be handled with ease. The various case values are shown in FIGURE 9, developed by W. R. Hamon, of percentage of moisture per unit of dry weight of tobacco versus the relative humidity in the air after equilibrium has been reached. In 1954 the Weather Bureau wished to deter- FIG. 9. Effect of relative humidity on the moisture of mine the feasibility of making a relative humidity cured burley tobacco. (After Hamon [6]) forecast which would be of value to the farmers during the tobacco curing season. W. R. Hamon He also reported that several of his neighbors had was assigned to the Weather Bureau office at heeded the warning and that all together the fore- Lexington to work cooperatively on this project casters had saved them "a pile of money." with the University of Kentucky. A group of The operational problem from a meteorological thirteen farmers representing a cross section of the point of view is to localize the forecast to an area tobacco producers were selected as cooperators. within a 25 mile radius of Lexington. Synoptic Each morning Mr. Hamon called these men giving data available over the national weather tele- them the tobacco curing forecast. The forecast printers was used to make the forecast, and in ad- experiment was eminently successful. These dition the values for the minimum relative hu- farmers received on the average 7 cents a pound midity for today, the maximum relative humidity more than parity for their tobacco while most of for tonight and an outlook for tomorrow's rela- the crop sold at support prices. As a result of this tive humidity were added. experiment the Weather Bureau decided that this Lexington is located in one of the gaps of the service was well worth while. upper air network with Dayton 130 miles north- During the 1955 season the Weather Bureau west and Nashville 170 miles southwest. This and the University of Kentucky College of Agri- makes a knowledge of the vertical temperature culture and Home Economics expanded the pro- structure over the station an estimate. gram to cover an area within a 25 mile radius of One approach to the problem of forecasting rela- Lexington. This year the forecasts were dissemi- tive humidities would be to draw charts of rela- nated by radio by the forecasters John C. Hurley tive humidities and forecast directly from these and James M. Beall. The forecasts were pre- charts. Relative humidity is so non-conservative pared by 0530 CST. The advices from the Agri- that this method was not tried. cultural College were appended and the combined Another approach would be to forecast the tem- bulletin was read at 0615 CST over the local radio peratures and the dew point values independently stations. and from them derive the relative humidity fore- In order to overcome the lack of the personal cast. This was the procedure used. relationship that existed in the 1954 experiment, Not much is recorded in the literature of at- the forecasters made it a policy to visit at least tempts to forecast minimum relative humidities. one tobacco barn in the area or discuss the forecast These references are confined to fire-weather in- with the county agent or the tobacco extension vestigators in the western United States and the specialist every day. In this way the forecasters fruit spray forecast investigators working near were able to warn the farmers not to cut tobacco the Great Lakes. Fog forecasting or maximum on Saturday, September 10, 1955 because the fore- relative humidity forecasts have been made for casters were expecting rain on Sunday which years and the techniques are well known. would have damaged the tobacco in the field if it The standard approach to the maximum tem- were cut. Instead they advised that it would be perature forecast in air mass conditions is to de- better to wait until the first part of the next week termine the amount of heating expected and en- when they foresaw more satisfactory tobacco curing ter it on an equivalent area thermodynamic dia- weather. One farmer reported that the forecast gram such as the tephigram. Meyers [7] devel- had saved him the loss of nine acres of his tobacco. oped the methodology in his masters thesis at

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MIT using standard Weather Bureau charts and An interesting note on how this program pays nomograms showing the maximum heating values its own way may be summed up by a statement by months for latitudes. that in the past for each dollar that the tobacco Forecasting maximum and minimum tempera- farmer has received for his crop the federal gov- tures under advective situations is discussed in ernment has collected in excise taxes almost two most textbooks on forecasting. dollars on finished tobacco products. In addition Dew point has long been recognized as a quasi- if the farmer is able to make more income on his conservative air mass property. The forecast of tobacco due to increased quality (his acreage be- this value in practice follows the same reasoning ing the same or less under government controls) and techniques as the forecasting of the air he will have a larger income tax liability. There temperatures. is little doubt in the writer's mind that this pro- A detailed report on the forecast techniques de- gram has paid its own way and that the work in veloped in Lexington by Beall and Hurley is be- tobacco curing forecasts should be expanded. ing prepared. While the program in Kentucky is in the experimental stage it can be reported that REFERENCES their work this year fell well within the limits of [1] Carr, J. M.: "Tobacco Plant Production in the accuracy desired by the farmers to enable them to Coastal Plain of Georgia"; Georgia Coastal Plain Experiment Station Bulletin 38, 1943. control the humidities in their barns to secure the [2] Garner, W. W.: The Production of Tobacco; the optimum curing conditions. Blakiston Co., New York, 1951. [3] Geiger, R.: The Climate Near the Ground; Harvard The weather during the 1955 season was almost University Press, Cambridge, Mass., 1950. ideal from a curing standpoint and the forecast [4] Jeffrey, R. N.: "The Relation of Curing Conditions verification which follows reflects the relatively to Quality in Burley Tobacco"; Kentucky Agricul- tural Experiment Station Bulletin 496, Lexington, stable conditions that prevailed. Ky., 1946. [5] Johnson, J. and Ogden, W. B.: "The Relation of Air Conditions to Tobacco Airing"; Agricultural Tinax Tmin RHm in RHmax RHmean Experiment Station of the University of Wisconsin Research Bulletin 110, Madison, Wis., 1931. Sept. ±2.7 ±4.3 ±7.5 ±10.5 ±8.9 [6] Hamon, W. R.: Weather and Burley Tobacco Cur- Oct. 3.4 5.0 10.5 6.3 8.4 ing; U. S. W. B, Washington, D. C., 1954. Nov. 1-15 ±3.7 ±4.3 ±9.2 ±9.4 ±9.3 (Mimeographed) [7] Meyers, V. A.: "An Investigation of Diurnal Tem- perature Changes"; Masters Thesis, MIT, Cam- bridge, Mass., 1947. The next step is to make this type of micro- [8] O'Bannon, L. S.: "Distribution of Temperature and meteorological forecast available over the entire Relative Humidity within a Burley Tobacco Barn"; Kentucky Agricultural Experiment Station Bulletin burley belt. This will require that micro-meteoro- 444, Lexington, Ky., 1943. logical surveys be conducted to determine the size [9] O'Bannon, L. S.: "Principles of Burley Tobacco of a forecast zone which will have homogenous Barn Operation"; Kentucky Agricultural Experi- ment Station Bulletin 501, Lexington, Ky., 1947. weather patterns including variability of relative [10] Smith, J. W.: Agricutural Meteorology; Macmillan humidity only within tolerable limits. Co, New York, 1920.

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