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Modified on Friday, 22 July 2011 21:23 by Administrator Categorized as Uncategorized

Here is another calculation method from New Zealand.

Smith (1973) produced and tested a simple forecast scheme, for use in Canterbury, to estimate the next morning’s grass minimum temperature, using an understanding of the maximum rate of cooling prescribed by radiation physics and measuring the weather conditions at 15.00 hrs NZ standard time of the current day: (3) Tg = 1/3 ( T + Td /2 ) - c where Tg is the forecast grass minimum temperature (oC) T is the 15.00 dry-bulb temperature (oC) Td is the 15.00 dew-point temperature (oC) c is a constant c=8 for May. Sept. and Oct. c=9 for Jun. and Aug. provided that: 1. the cloud amount is <1/4 of low or medium clouds and <3/4 high clouds 2. the wind speed is < 5 kt from E or NE (wind off the sea) or < 7 kt from any other direction. For example, a psychrometer reading at 15.00 in September gives a temperature of 6 oC and a dew-point temperature of 3 oC. Tg = 1/3 (6 + 3/2) - 8 = -5.5

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