Calculating slope of saturation vapour pressure curve
cal_slopeOfSaturationVapourPressureCurve(Tem)
A vector for slope of saturation vapour pressure curve at air temperature T
is air temperature (degrees Celsius).
In the FAO Penman-Monteith equation, where it occurs in the numerator and denominator, the slope of the vapour pressure curve is calculated using mean air temperature.
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. FAO Irrigation and drainage paper No. 56. Rome: Food and Agriculture Organization of the United Nations, 1998.