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] F R O S T P R O T E C T I O N : F U N D A M E N T A L S , P R A C T I C E A N D E C O N O M I C S FIGURE 5.2 Sample weather data during an advection frost near Zamora, California (USA), in March 2002. Sunset was at about 1942 h. Dates are given in USA notation (mm/dd/yy) RAINFALL (mm h -1 ) [ TEMPERATURE (° C) WIND SPEED (m s -1 ) 3.0 2.5 2.0 1.5 1.0 0.5 0.0 16 12 8 4 0 -4 8.0 6.0 4.0 2.0 0.0 dew - point air 360 300 180 240 120 60 0 3/6/02 - 00.00 3/6/02 - 06.00 3/6/02 - 12.00 3/6/02 - 18.00 3/7/02 - 00.00 3/7/02 - 06.00 3/7/02 - 12.00 3/7/02 - 18.00 3/8/02 - 00.00 3/8/02 - 06.00 3/8/02 - 12.00 3/8/02 - 18.00 3/9/02 - 00.00 3/9/02 - 06.00 3/9/02 - 12.00 3/9/02 - 18.00 3/10 /02 - 00.00 WIND DIRECTION (degrees) 96
FROST FORECASTING AND MONITORING For use in the FFST.xls application program, select data only from radiation frost nights. Avoid including nights with wind speeds greater than 2.0 m s -1 and nights with cloud cover or fog. For example, Figure 5.2 illustrates the data selection problem. On 6 March, there were rainy windy conditions, which continued until near noon on 7 March. Then the rain stopped, but the wind changed from a south to a west-northwest wind and the wind speed was high until about 2100 h. A sharp drop in dew-point temperature is typical of the passage of a cold front. Sunset occurred at about 1742 h, so the wind speed was high for more than three hours past sunset. Net radiation was not measured at this site, so information on cloud cover is unknown. However, intermittent cloud cover often follows a cold front. At two hours past sunset, the air and dewpoint temperatures were 5.1 °C and 0.0 °C and the wind speed had just dropped from 2.6 to 1.6 m s -1 . There was still a large drop in temperature after this point, which is not characteristic of a radiation frost. Based on this weather data, the weather conditions on 7–8 March were too windy in the evening and they are not typical of a radiation frost. On 8–9 March, the air and dew-point temperatures were 3.9 °C and 1.4 °C at two hours past sunset and the wind speed dropped earlier in the evening (i.e. near sunset). There was no evidence of cold air advection, so the data from 8–9 March can be input into the FFST.xls application program to determine a forecast model. Note that one can use data from nights when the minimum air temperature does not fall below 0 °C as long as the night had clear skies and calm or little wind. The FFST.xls application program is written in MS Excel for easy input and for graphic as well as tabular output. For as many as 50 nights, the air and dew-point temperature at two hours past sunset are input along with the observed minimum temperature the following morning. A sample input screen with 10 days of input data is shown in Figure 5.3. In Figure 5.3, the input data were used to determine a linear regression of the observed minimum (T n ) versus the air temperature at two hours past sunset (T 0 ), and the results are shown in the “Prediction from Temperature (T p ′)” column. The output equation for T p ′ = b 1 × T 0 + a 1 is shown at the top of the page. To the left of the equation, the root mean square error (RMSE) is shown. This statistic is similar to a standard deviation in that it is a measure of closeness of the predicted and observed values. In Figure 5.3, the RMSE is 0.65 °C for the formula based on the two hours past sunset temperature only. This implies that one deviation about the 1:1 line is approximately 0.65 °C, and two deviations about the 1:1 line would be about 1.3 °C. Assuming that the variation of the RMSE is about the same as a standard deviation, this means that the variation 97
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- F A O E N A N D V I MO N I T O R
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FOREWORD Agrometeorology deals with
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ACKNOWLEDGEMENTS We want to thank D
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LIST OF PRINCIPAL SYMBOLS Roman Alp
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T n °C Observed minimum temperatur
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CHAPTER 1 INTRODUCTION OVERVIEW Whe
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INTRODUCTION Freeze and frost defin
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INTRODUCTION FIGURE 1.2 Mean air an
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INTRODUCTION FIGURE 1.4 Air (T a )
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INTRODUCTION TABLE 1.2 Categories a
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INTRODUCTION example, because bloom
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INTRODUCTION south in Florida in re
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INTRODUCTION assistance might be in
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CHAPTER 2 RECOMMENDED METHODS OF FR
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RECOMMENDED METHODS OF FROST PROTEC
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] F R O S T P R O T E C T I O N : F
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CHAPTER 8 APPROPRIATE TECHNOLOGIES
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APPROPRIATE TECHNOLOGIES frost prot
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APPROPRIATE TECHNOLOGIES fashion. O
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES CROP COUNT
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APPROPRIATE TECHNOLOGIES other crop
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APPENDIX 2 CONSTANTS Specific heat
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- FAO ENVIRONMENT AND NATURAL RESOU
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