tennessee gardening climate...tennessee master gardener handbook 118 offcal tmg instructor copy a ll...

Chapter 5Tennessee Gardening

Climate

Learning Objectives1. Become familiar with the common

climate variables that are most important in gardening

2. Understand how to obtain weather and climate data from sources such as the National Climatic Data Center

3. Learn the different aspects of weather and how it relates to gardening

4. Learn to do calculations with some simple climatic statistics, considering local gardening

5. Learn about automatic sensors and how they can be used in gardening

Tennessee Master Gardener Handbook 118

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All agriculture, including gardening, is dependent on weather. People are at the mercy of Mother Nature when

dealing with her in the form of droughts, dam-aging winds and untimely freeze. However, people can also take advantage of what she has to offer – bountiful sunshine, opportune rains and pleasant temperatures. Master Gardeners can take advantage of the information about weather and climate to help make better deci-sions about when and what to plant in a gar-den. In addition, automated weather sensors have become much more affordable in recent years and present an exciting opportunity for more accurate data for gardeners.

Tennessee is one of the most climatically diverse regions in the United States because of its topography. The annual average tempera-tures range from 62 degrees F in the lower southwest corner to 49 degrees F in the upper elevations of the Appalachian Mountains. The environmental lapse rate, or the rate of decrease of temperature with elevation, is gen-erally about 1degree F per 1,000 feet.

Annual rainfall is lowest in the upper northeast region of the state due to the rain shadow effect of the mountains. Annul rainfall is highest in the high-elevation, windward parts of the Smoky Mountains. Tennessee winters are generally mild, allowing for the production of a wide variety of crops through-out most of the state. An exceptionally long growing season, other than in elevations above 1,500 feet, is also conducive to a wide variety of crops.

WeatherWeather refers to the real-time, current, or the near real-time, several hours or days, atmo-spheric conditions that are experienced, such as a hot summer afternoon or a thunderstorm. During a weather forecast, the variables com-monly reported include temperature, rela-tive humidity, wind speed, wind direction, barometric pressure and precipitation. Real-time weather information usually comes from the television station’s set of weather instru-ments, which are usually located outside of the building, and from the local airport such as Knoxville, Nashville or Memphis. For the near real-time forecast, television and radio stations get weather information from the National Weather Service, which is funded by taxes, or from a private weather service.

ClimateClimate refers to the long-time trends in atmospheric conditions, such as annual average temperature and rainfall. Climatology, the science of climate, relies heavily on statistical tools to summarize the same weather variables mentioned above. It can often be difficult to find climate summaries for a specific location. However, one can obtain long-term weather data from the National Climatic Data Center and create an individual one. The National Climatic Data Center is the agency responsible for archiving weather data and for producing climatic summaries.

Agricultural climatology, or agroclimatol-ogy, is climatology as applied to the effects of climate on crops; it is a major emphasis of this chapter. Bioclimatology is the branch of climatology that deals with the relations of cli-mate and life, especially the effects of climate on the health and activity of human beings and on animals and plants. Because there is so much overlap between agricultural climatology and bioclimatology, the terms are often used interchangeably.

Microclimatology is the study of microcli-mate, or the climatic structure of the air space that extends from the surface of the earth to a height where the effects of the immediate character of the underlying surface no longer

Tennessee Gardening Climate

Calculating Frost Dates

To calculate the average last frost and first freeze dates, research the NOAA web site for the record for your area. Normal or average temperatures or rain are tracked and determined based on 30 years.


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can be distinguished from the general local climate.

Climate Elements Important to GardeningThe climatic elements most important to gar-dening include air temperature, soil tempera-ture, relative humidity, solar radiation, wind speed and direction, and precipitation.

Air TemperatureCardinal TemperatureCardinal temperatures are the minimum and maximum temperatures that define the limits of growth and development of an organism. They also define the optimum temperature at which growth proceeds with the greatest rapidity. Cardinal temperatures may vary with the stage of development. Most gardening handbooks will list the cardinal temperatures of crops.

Measurements and Instrumentation

Air temperature is measured with a thermom-eter and may be manually or automatically reported. Manual thermometers include the max/min “U” thermometer (Figure 1), the glass thermometer (Figure 3) and the digital Maximum Minimum Temperature System (MMTS), which is used by many National Weather Service Cooperative Stations, (Figure 3) Automatic temperature sensors are electron-ic thermocouples or thermistors that send data signals to a data logger located in an automatic station.

A max/min “U” thermometer can be very useful in building a temperature database for a garden. To build a temperature database, read the thermometer at the same time every day and compare it with the maximum and minimum temperatures reported in the local newspaper, radio or TV. After several weeks of recording temperatures, a pattern should emerge that shows how the garden climate compares with the climate at the nearby weather station. If located in a large city, the garden may be subject to an urban heat island effect. This will result in temperatures, espe-cially at night, which may be several degrees higher than the temperature reported from a local airport.

Agricultural Meteorology and Micrometeorology

Agricultural meteorology is a subdisci-pline of meteorology that relates weath-er to crop production. It generally looks at very small spatial and temporal scales, and will not be covered in this chapter. However, data from agricultural meteo-rological stations do provide weather data related to gardening that can be very useful when summarized.

Micrometeorology is the part of meteo-rology that deals with observations and processes in the smallest scales of time and space, approximately smaller than 1 km and less than a day. One example of micrometeorology is local processes.

Figure 1. Max/Min “U” Thermometer


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Glass Maxi-mum (left) and Mini-mum (right) Thermom-eters

Figure 2.

Figure 3. Homemade Weather Station

E. Barometer

B. Rain Gauge,

A. Psychrometer/Hygrometer,

C. Wind Vane

D. Anemometer


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Building a Wind Vane

Materials ▪ 3 ft x 3 ft piece of thin wood or plastic ▪ Bicycle wheel, skate wheel or lazy susan ▪ Saw ▪ Screws ▪ Plastic or metal letters, “N, E, W, S” ▪ Sand paper or file

Procedure1. Cut an interesting shape from wood or plastic; make a point at one end. Round all edges except the bottom.

2. Using the screw, mount the shape to a swivel.

3. Install your wind vane on top of a tall pole or on the roof of a shed or house.

4. Keep a log of the wind direction as observed several times a day.

Building a Rain Gauge

Materials ▪ Sharp scissors ▪ Fine mesh screen ▪ Nail file ▪ 2 two-liter bottles (tops removed) ▪ Permanent marker ▪ Ruler ▪ Duct tape ▪ Vegetable oil ▪ Bleach (optional)

Procedure1. Carefully cut a two-liter bottle in half to make a funnel. File any sharp edges.

2. Place the cut bottle onto the uncut bottle so that the spouts are touching and in line.

3. Tightly tape the bottle spouts together. Place the screen over the funnel opening and press it slightly inward; tape it in place.

4. Mark the rain gauge up the side in 1/4-inch (or 1/2-cm) graduations with a permanent marker-for more accu-rate readings, tape a ruler to the side of the bottle.

5. Pour enough vegetable oil into the funnel to cover the bottom of the rain gauge in a thin layer. Put the screen in place over the gauge.

6. Place the rain gauge outside, preferably in a shady, but not covered, spot.

7. Decide on a time period to wait before collecting data, but periodically check the gauge and put a few drops of bleach in it to retard critter growth.

8. Keep a record of rainfall totals for each day and the average amount of rainfall over the time period you specify.

** Do not come in contact with the water, as it may contain pathogens

▪ 2 alcohol-filled air thermometers- They must read exactly the same temperature when placed side by side out of direct sunlight

▪ Clear packing tape ▪ Cotton shoelace- the hollow type ▪ 1- or 2-liter bottle with the label removed ▪ Water, distilled is best but tap will do


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Building a Psychrometer

Materials

▪ 2 alcohol-filled air thermometers- They must read exactly the same temperature when placed side by side out of direct sunlight

▪ Clear packing tape ▪ Cotton shoelace- the hollow type ▪ 1- or 2-liter bottle with the label removed ▪ Water, distilled is best but tap will do ▪ Thread ▪ Awl- A long pointed spike ▪ Relative humidity chart

Procedure

1. Punch a hole in the side of the bottle about an inch from the bottom. Heating the awl will make a per-fect hole. The same thing can be done with a hot nail held with tongs. Use great caution when doing this so you do not burn yourself or others. Once the hole is made, place the hot object into cold water.

2. Be sure the tips are cut off the shoestring and then cut about 2 inches of shoestring and slip it over the bulb of one of the thermometers. Carefully tie it in place with thread.

3. Cut a small piece of packing tape. Position the bulb of the shoestringed thermometer about 1/8 inch over the hole. Be sure the top of the thermometer is aligned with the top of the bottle. Tape the ther-mometer to the bottle. Tape the other thermometer parallel to the first one and about 1/4 inch away. Put a strip of tape around the bottle and both thermometers to make sure they do not fall off.

4. Push the shoelace through the hole. Put room temperature water in the bottle until it reaches just below the hole.

5. Wait 5 to 10 minutes and read both thermometers. There will be a difference in the two. Use the chart below to calculate the relative humidity.

6. Keep a record of the daily humidity for a few weeks. Next to your entries, describe the way you feel on those days.

7. The dry-bulb temperature can also be used to record the air temperature.


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Building a BarometerMaterials

▪ Empty coffee can ▪ Large, heavy-duty latex balloon ▪ Heavy rubber band ▪ Scissors ▪ Coffee stirrer ▪ 3x5 card ▪ Duct tape ▪ White glue

Procedure1. Smoothly tape the rim of the coffee can so the metal edge is completely hidden but remains open. Be sure

the tape smoothly extends down the side of the can an inch or more. To effectively do this, tape around upper side of the can leaving at least 1/2 inch of tape sticking up over the rim. Use scissors to make 8 to 10 cuts in the protruding tape straight down to the can rim. Fold the tape down and smoothly stick it to the inside of the can.

2. Cut the filler hole off the balloon and discard. Stretch the balloon tightly over the taped edge and secure it with a rubber band. Make the rubber band as tight as you can.

3. Put a drop of white glue in the center of the stretched balloon. Put the coffee stirrer on the glue and position it so that it protrudes about 1/2 inch over the edge of the can.

4. Tape the 3x5 card on the side of the can so that it extends over the top and is close, but not touching the coffee stirrer.

5. Mark the card at the tip of the stirrer. It isn’t necessary to put numbers there.

6. Write the current barometric pressure in a journal. Determine if the pressure is high, low or “somewhere in between.” This will be your baseline pressure. Be sure to note the position of the mark on the 3x5 card corre-sponding to the pressure.

7. Repeat step 6 through several cycles of weather. Be sure you have several highs and lows marked on your card and that you have entered all the information in your journal.

When you become accustomed to the way your barometer works, you will have a tool that predicts the weather. Determine how the barometric pressure correlates to present weather.


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Building an Anemometer

Materials

▪ Old bicycle wheel assemblies ▪ Bicycle speedometer ▪ 5, 2-liter soda bottles ▪ 1-gallon plastic milk container ▪ 4-foot wood closet rod ▪ Hammer ▪ 3 “L” shaped wood brackets ▪ Drill with drill bits ▪ Nuts and bolts ▪ Washers ▪ Level ▪ File ▪ Fine sand-paper ▪ Wood screws ▪ Twist ties

Procedure1. Clamp the closet rod firmly in place on a workbench. Drill a shallow hole in one end of the closet rod with a diam-

eter slightly smaller than the threaded screw on the bicycle wheel’s axle.

2. Be sure the caps on the 2-liter bottles are securely tightened. Cut off the bottoms of the bottles. File or sand any rough or sharp ends until smooth.

3. Drill 4 holes in the wheel using a drill bit the diameter of the bolts you have. The holes should be at 0 degrees, 90 degrees, 180 degrees and 270 degrees on the wheel. Drill a second hole 4 to 5 inches from of the first ones. It’s important that the distances be exactly the same for each new hole. Otherwise, your wheel will be out of balance. You now have 2 holes apiece (8 holes total) through which you will secure a nut and bolt (the screws must face outward).

4. Mount each of the bottles in same direction when mounting them on the wheel. The open ends and top will lie on the circumference of the wheel. Mark the bottles where the bolts touch. Drill or punch holes at those locations.

5. Put washers on the bolts. (You will now have a bolt through a hole, a nut on the bolt and a washer on the bolt.) Slip a bottle in place on the bolts. Reach in the bottle and put a washer on the bolts. Be careful, in case you left any sharp edges! Then tighten the nuts in place. Do the same for the other bottles.

6. If you have room, you can mount another 4 bottles on the wheel.

7. Turn the wheel assembly sideward and place the axle screw over the hole in the closet rod. Hammer the screw into the hole.

8. Mount and level the anemometer to the side of a shed or construct a base from 24 x 24 x 3/4 inch sealed plywood. L-brackets will make it easy to attach the rod using wood screws.

9. Attach the speedometer to the wheel according to manufacturer’s instructions. Attach it to the closet rod, or any-where you can see it well. If it is not waterproof, design a cover for it.

10. Cut the bottom from a 1-gallon plastic milk container to use as a rain shield for the axle. Poke holes around the cut end to insert twist ties. Attach the ties securely to the spokes. Plastic sheeting can also be used to cover a larger area.

11. Be sure the axle is accessible so you can oil it periodically.

12. Take readings several times a day; more often as weather changes are apparent.

Source: Discovery Education. Retrieved from http://school.discoveryeducation.com/lessonplans/activities/weather-station/index.html


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The standard height for taking air tempera-ture is 5 feet above a sodded surface with good exposure. The thermometer must be either located in the shade- but not attached to a building where the building temperature will exert too much influence on the thermometer- or within a radiation shelter. The National Weather Service currently uses two types of shelters for the cooperative weather stations: an

MMTS shelter (Figure 4) or a cotton-region shelter, which is also called a Stevenson shelter or an instrument shelter. (Figure 5) Electronic thermometers are usually housed in a shelter similar to the MMTS shelter.

Data, Statistics and SourcesMost cooperative stations record only daily maximum and daily minimum temperatures with some automated weather stations report-ing hourly average temperatures. First-order stations, such as those found at airports, also record hourly instantaneous temperatures. Additionally, local sources such as libraries and schools, utility companies, the Better Busi-ness Bureau, and real estate offices may offer summarized temperatures, such as average daily or monthly temperatures. Climatological data taken from nearly 100 stations in Ten-nessee from 1971 until 2000 can be found at the Golden Gate Weather Services and the Southern Region Climate Center.

Locations in Tennessee that Record Soil Temperature

▪ Knoxville Experiment Station ▪ Crossville Experiment Station ▪ Greeneville Experiment Station ▪ Milan Experiment Station ▪ Jackson Experiment Station ▪ UT Martin Experiment Station ▪ Springfield Experiment Station

Table 1. Differences between Wet- and Dry- Bulb Temperature Readings in Degrees F

Air Temp. Difference between Wet- and Dry-Bulb Readings in Degrees F

(Dry Bulb) 1 2 3 4 5 6 7 8 9 10 15 20 25 30 35

20 85 70 55 40 26 12

25 87 74 62 49 37 25 13 1

30 89 78 67 56 46 36 26 16 6

35 91 81 72 63 54 45 36 29 19 10

40 92 83 75 68 60 52 45 37 29 22

45 93 86 78 71 64 57 51 44 38 31

50 93 87 80 74 67 61 55 49 43 38 10

55 94 88 82 46 70 65 59 54 49 43 19

60 94 89 84 78 73 68 63 58 53 48 26 5

65 95 90 85 80 75 70 66 61 56 52 31 12

70 95 90 86 81 77 72 68 64 59 55 36 19 3

75 96 91 86 82 78 74 70 66 62 58 40 24 9

80 96 91 87 83 79 75 72 68 64 61 44 29 15 3

85 96 92 88 84 80 76 73 69 66 62 46 32 20 8

90 96 92 89 85 81 78 74 71 68 65 49 36 24 13 3

95 96 93 89 85 82 79 75 72 69 66 51 38 27 17 7

100 96 93 89 86 83 80 77 73 70 68 54 41 30 21 12

105 97 93 90 87 83 80 77 74 71 69 55 43 33 23 15

110 97 93 90 87 84 81 78 75 73 70 57 46 36 26 18

115 97 94 91 88 85 82 79 76 74 71 58 47 37 28 21


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Just how much manipulation and summari-zation of the temperature data from the nearby station one can achieve for individual purposes is dependent upon the time scale of data, hourly versus daily for example, and the period of record. To calculate daily means or monthly average maximum or minimum temperatures, a record of at least 10 years is recommended, although 30 years is considered optimal. It is also best to use the most current set of data, such as 1971-2000.

Growing SeasonsThe annual pattern of average temperatures, which can be seen by the graph in Figure 6, can be used to estimate the start of a crop’s growing season. Currently, the most common method for measuring the length of the grow-ing season is to count the number of days be-tween the average dates of the last killing frost or freeze in the spring and the first killing frost or freeze in the autumn. This is known as the frost-free season or the freeze-free season. To calculate the average frost or freeze dates for spring and fall, at least 20 years of dates are necessary.

Figure 4. MMTS Shelter

Figure 5. Cotton-Region Shelter

Figure 6. Montly Average Temperatures at Knoxville

Obtaining Weather Data

One can obtain the raw daily data from the National Climatic Data Center web-site, the address is in the Resources sec-tion of this chapter. However, unless ac-cessing this site from a public domain, there is a substantial fee to download data. Considerable expertise in Microsoft Excel®, or a similar spreadsheet program, is required to develop one’s own tem-perature summaries and charts. Current monthly climatological summaries can also be found at The National Climatic Data Center

(NCDC) website.


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Low Temperatures, Freeze and FrostLow temperatures in the spring and fall can cause delayed planting, frost damage or complete destruction of plant materials. There are two types of freeze that can occur: advec-tive and radiation. An advective freeze results from the penetration of a large cold front into the area. It is usually accompanied by windy conditions. Generally, it is very difficult to use frost protection for this type of freeze. The only option is supplemental heat or moving susceptible plants indoors, especially if the temperature is expected to drop below 24 degrees F.

A radiation frost typically occurs on clear nights with little or no wind when the outgo-ing radiation is greater than the incoming ra-diation and the cool air near the surface creates a stable temperature inversion near the ground.

Freezing also occurs when there is cold air drainage into a low spot in a garden. Although the surrounding area of higher elevation might have temperatures above freezing, the temper-ature in the low spots might be 5 to 10 degrees F colder.

There are many terms to describe the type of injury sustained by plants in low or freezing temperatures. A black frost, also called a hard frost, is a dry freeze with respect to its effects upon vegetation, that is, the internal freezing

of vegetation unaccompanied by the protective formation of hoarfrost frost. A black frost is al-ways a killing frost, and its name derives from the resulting blackened appearance of affected vegetation. A hard freeze is a freeze in which seasonal vegetation is destroyed, the ground surface is frozen solid underfoot, and heavy ice is formed on small water surfaces such as puddles and water containers.

Hoarfrost is a deposit of interlocking ice crystals, called hoar crystals, formed by direct deposition on objects. These objects usually have a small diameter and are freely exposed to the air. Examples include tree branches, plant stems, leaf edges, wires and poles. A killing freeze is any occurrence of air temperature below 32 degrees F that kills annual vegeta-tion without the formation of frost crystals on surfaces. Chilling injury is the physiological damage to plant parts and tissues in the tem-perature range from about 32 to 68 degrees F, depending on the crop.

Temperature extremes can provide useful information about crop adaptation. The USDA produces a hardiness zone map that indicates which types of perennial plants cannot be grown in an area due to the chance of tem-peratures occurring that are outside the range for that plant’s growth.

Growing DegreesAnother important climatic variable that is based on air temperature is the growing de-gree-day (GDD), or heat unit. GDD is a heat index that relates the development of plants, insects and disease organisms to environmen-tal air temperature. The equation to calculate GDD is:

Tavg – BT = GDD **Tavg = The daily average temperature and BT = The base temperature

A corn heat unit is a modification of GDD with both upper and lower temperature thresholds. All temperatures above 86 degrees F are set to 86 and all temperatures below 50 degrees F are set to 50 before calculation of daily mean temperature. The reference temperature, or base temperature, for corn heat units is 50 degrees F. Growing degrees can also be calculated on an hourly basis if data are available. These are referred to as growing degree-hours.

Crop Calendars

One can develop a crop calendar for each of the garden crops that are grown by knowing the cardinal temperatures and relating them to the average tem-peratures at the specified location. The cardinal temperatures for many crops can be found in any crop or vegetable production handbook.

The graph above shows the monthly average temperatures for Knoxville. The best planting date for lettuce is when the daily average temperature reaches 41 F, or sometime in late February. For beans, the threshold temperature is 57 F, which occurs sometime in early April.


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ChillingMany deciduous fruit trees require a certain amount of chilling during the dormant period to produce fruit the following season. A chill unit is an index calculated from air tempera-ture to estimate fulfillment of plant dormancy requirements and the ability to start spring-time growth. A commonly used base tempera-ture is 45 degrees F. Chilling requirements can also be calculated on an hourly basis if data are available. These are referred to as chill hours.

Heating and Cooling DegreesOther commonly reported derivatives of temperature are heating degree-days and cooling degree-days. These are based on a base temperature of 65 degrees F. They can be used to estimate the air conditioning or heating requirements of supplemental buildings such as sheds and greenhouses.

Soil TemperatureAlthough soil temperature is recorded at a few locations in Tennessee, the available daily maximum and minimum soil temperature data are available from the The National Climatic Data Center (NCDC) website. Soil tempera-ture in the spring dictates how soon a crop can be successfully planted. Many gardening books list soil temperature requirements for optimal germination in the spring. Soil temperature is important because it affects microbial growth and development, organic matter decay, root development, and water and nutrient absorp-tion by roots. The size, quality and shape of storage organs are also affected by soil tem-perature. Darker soils absorb more radiation and warm up faster than light colored soils and sandier soils are generally warmer than fine-textured soils. Moist soils tend to be cooler than dry soils. Soil temperature is often measured with a Palmer Soil Thermometer.

Relative HumidityThe humidity in the air is reported much less frequently than temperature, and it is recorded at a much smaller number of weather stations. The most commonly reported variable is rela-tive humidity (RH), or how much water the atmosphere holds in relation to the maximum amount it can hold at that pressure and tem-perature. Relative humidity may be expressed as a daily maximum and minimum or as an hourly observation. Dew point or dew point

temperature is the temperature that a given air particle must be cooled to, at constant pressure and water vapor content, in order for saturation to occur. It is related to humidity. For example, if the current temperature is 75 degrees F and the dew point is 75, the relative humidity is 100% because the air must be saturated.

Dew point temperature, especially in the spring, gives a quick estimate of the minimum temperature for the following morning. For example, if the forecast is for a dew point of 28, there is a good chance that the minimum temperature the next morning will be very close to 28 degrees F. However, if a weather front rolls through that evening, then this estimate will not be accurate.

Relative humidity can be combined with temperature data to estimate disease sever-ity. Leaf wetness duration is the length of time plant surfaces are continually exposed to liquid moisture. Leaf wetness duration is often related to plant disease infection periods. For example, a run of days with high temperatures and RH is likely to result in a greater incidence of plant disease. Knowing the relationship between a disease of concern and humidity, one can develop a simple model to estimate the optimal spray times, rather than spraying at a given time interval.

Humidity is usually measured with an electronic humidity probe, which is often located on the same probe as the temperature probe. Manually, humidity is measured with a hair hygrometer or a sling psychrometer (See Figure 3A).

Solar RadiationAll plant life is dependent on the radiant energy of the sun, solar radiation. Insolation (Incoming Solar Radiation) is the amount of solar radiation received at the earth’s surface. Solar radiation can be considered as moving in waves. It is measured in watts per square me-ter, langleys per square centimeter or particles called quanta. Quanta are measured in micro-mols. The important aspects of solar radiation include light quality, light intensity and light duration. There is very little climatic informa-tion about light quality, or the distribution of wavelengths in the incoming radiation. Pho-tosynthetically Active Radiation (PAR) is the electromagnetic energy in the 400-700 nm, which is the visible wavelength range.


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There are a few stations in Tennessee with instruments that record hourly or daily solar radiation. The NSRDB Daily Statistics Files contain summarized monthly data for five locations in Tennessee: Memphis, Nashville, Knoxville, Bristol and Chattanooga. A few stations in Tennessee also record percentage of possible sunshine.

The SunPosition is another resource that provides some very useful data about the sun. If one is aware of latitude, the SunPostion can give information on altitude, azimuth and daylength. The web address of SunPosition is given in the Resources section at the end of this chapter.

Wind Speed and DirectionGenerally, winds are greatest in the late afternoon when there is the greatest differ-ence between air and surface temperatures. Of course, fronts that move through can result in high winds at any time, as can thunderstorms. Winds are generally calmest in the early morn-ing before 9 a.m.

Wind has many influences on vegetable production. Some wind is necessary to replen-ish the CO2 near the leaves. However, excess wind can result in plant lodging, breakage of stems and branches, and destruction of sensi-tive flowers and fruit. High winds and exces-sive temperatures desiccate plants, resulting in wilting and possible yield reductions. Produc-ers in the western U.S. often plant windbreaks to help control water loss and damage.

Wind also affects operations such as pesti-cide application and spray irrigation. Pesticides should never be applied when wind speeds are greater than 5 mph. If winds exceed 10 mph, spray irrigation is ineffective. Therefore, it is important to always wait to spray until winds have decreased below 5 mph.

Wind speed is measured with an anemom-eter that may use a cup (Figure 7) or a sonar device. A handheld anemometer can be helpful to measure wind speed before pesticide ap-plication. Wind direction is measured with a sensor that points into the prevailing winds like the one shown in Figure 8.

Precipitation and EvapotranspirationPrecipitation

The most commonly reported precipitation value by the National Weather Service for Tennessee is daily precipitation. Usually, daily precipitation consists of rainfall collected in a standard eight-inch rain gauge. (Figure 9). Because there is very little snowfall in Tennes-see, very few stations report snow depth. The stations that do have snow melt it and include the water equivalent in the daily precipitation amount. These data, like temperature, can be found as daily, monthly or annual averages at the NCDC website.

Doppler radar, as part of the National Weather Service NEXRAD program, is used to estimate rainfall on a 2.3-mile grid across the United States. Current radar maps from

Figure 7. Anemometer Using a Cup or Sonar Device

Figure 8. Handheld Anemometer


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National Doppler Radar Sites can be found at the National Weather Service’s webpage.

EvapotranspirationEvapotranspiration is the total amount of water lost by a crop, that is the water lost from transpiration through the leaves and evapora-tion through the soil surface. An evaporation pan (Figure 10) is located at each of the UT experiment stations to record daily evapora-tion. Its average pan coefficient is about 0.7; therefore, if the evaporation for one day is 0.3 inches, then the estimated potential evapo-transpiration is 0.21 inches. A crop coefficient relates potential evapotranspiration to crop evapotranspiration and can be used to estimate the water requirements of a crop.

HailHail occurs when heavy updrafts of air cause ice pellets to form. These pellets eventually fall to earth in sizes ranging from a pea to a soft-ball. Hail causes $1 billion worth of damage to U.S. crops in any given year.

DroughtAn agricultural drought occurs when rain-fall has been below normal for a period long enough to negatively affect crops. This con-trasts to a hydrological drought, which is generally of longer duration and affects water supply. Hydrological drought is measured by below-normal streamflow, lake and reservoir levels, groundwater levels, and depleted soil moisture content.

The Palmer Drought Severity Index (PSDI) and the Crop Moisture Index (CMI) are explained very well by the Climate Prediction Center. These websites can be found in the Resources section of this chapter. Both PDSI and CMI are reported per climatic division. Tennessee has four divisions: East, Plateau, Middle and West.

Large producers of crops often use canopy temperature taken by an infrared thermometer (Figure 11) as a rough indicator of the level of crop stress. This may help the producer deter-mine if irrigation is warranted.

Figure 9. Standard 8-Inch Rain Gauge

Figure 10. Evaporation Pan

Figure 11. Canopy Temperature by Infrared Thermometer


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Automated Sensors and Stations

The decreasing cost of automated sensors and weather stations allows producers to invest in their own weather equipment. An electronic thermometer can be placed in a freeze-suscep-tible area of an orchard, for example, and can be used to set off an alarm when the tempera-ture falls below a threshold programmed by the user. Automated rain gauges can provide data to estimate crop irrigation needs.

SummaryTo have a successful garden, weather must be considered. Temperature, rainfall, precipita-tion and even relative humidity all can have an effect on what plants can grow in which locations. After learning the concepts in this chapter, you should be familiar with each of the common climate variables that are most important in gardening, you should under-stand how to obtain weather and climate data from sources such as the National Climatic Data Center, you should understand how to figure out growing degree days and finally, you should have an understanding of auto-matic sensors and how they can be used in gardening.

Terms To KnowAgricultural climatology (Agroclimatology)Agricultural droughtAgricultural meteorology (Agrometeorology)Agricultural meteorological stationAir temperatureAnemometerAutomatic weather stationBioclimatologyBlack frost (hard frost)Canopy temperatureCardinal temperaturesChill hourChill unitChillingChilling injuryClimateClimatic elementClimatological dataClimatologyCorn heat unitCotton-region shelterCrop calendarCrop coefficientCrop moisture indexDaily maximum temperatureDaily meanDaily minimum temperatureData loggerDew point (Dew point temperature)Doppler radarEight-inch rain gaugeEnvironmental lapse rateEvaporation panEvapotranspirationExposureFreezeFreeze-free periodFrostFrost-free seasonFrost protectionGrowing degree-day (GDD.)Growing degree-hour (GDH)Growing seasonHard freezeHeating degree-dayHoarfrostHydrological droughtInsolationInstrument shelterKilling freezeLangleyLeaf wetness duration

Companies that Sell Automated Weather Stations and their

Associated Websites

▪ Campbell Scientific, Inc. http://www.campbellscientific.com

▪ Onset http://www.onsetcomp.com ▪ Ben Meadows http://www.ben-

meadows.com ▪ Forestry Suppliers http://www.

forestry-suppliers.com ▪ Weather Matrix http://www.weath-

ermatrix.net/ ▪ Texas Weather Instruments, Inc.

http://www.txwx.com/ ▪ RainWise, Inc. http://www.rainwise.

com/


O�cal TMG

InstructorCopy

Maximum minimum temperature systemMeteorologyMicroclimateMicroclimatologyMicrometeorologyNational Climatic Data CenterNational Weather ServiceNEXRADPalmer Drought Severity IndexPhotosynthetically active radiationPotential evapotranspirationPrecipitationRadiation freeze or frostRain gaugeRainfallRelative humiditySoil temperatureSolar radiationTemperature extremesTemperature inversionThermistorThermocoupleUrban heat islandWeatherWind directionWind vane

Test Your Knowledge1. Why is the general climate of Tennessee

so diverse?

2. What are the most important climate ele-ments to gardening?

3. What is the difference between weather and climate?

4. What are some considerations in setting a thermometer out to see the temperature of your garden?

5. How does wind affect plants?

ResourcesClimate Prediction Center

cpc.ncep.noaa.govGolden Gate Weather Services

ggweather.comNSRDB Daily Statistics Files

rredc.nrel.govSouthern Region Climate Center

srcc.lsu.eduSunPosition Website

susdesign.com/sunposition

The National Climatic Data Centerncdc.noaa.gov

The National Weather Servicenws.gov

The National Weather Service Doppler Radar Mapsweather.noaa.gov/radar/national.html

The National Weather Service: Hailerh.noaa.gov/er/cae/svrwx/hail.htm

The United States Drought Monitordrought.unl.edu/dm/monitor.html

The United States National Arboretumusna.usda.gov/Hardzone/ushzmap.html

tennessee gardening climate...tennessee master gardener handbook 118 offcal tmg instructor copy a ll...

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