Campbell Scientific VisualWeather Software User Manual
Page 64
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Table 1. Names, symbols and units of all quantities used in calculations of hourly ET
o
values.
Symbol
Unit
Meteorological Data
Mean hourly temperature
Ta
o
C
Mean hourly RH
RH
percent
Mean hourly wind speed at 2 meter height
u
2
m/sec
Total hourly solar radiation
Rs
MJ/m
2
/hour
Data Related to Station's Location
Elevation
Z
meters
Weather station's longitude
(user-entered)
Lm degrees
Always west of Greenwich line
minutes
(with Greenwich line as 0 degrees)
seconds
Weather station's latitude
(user-entered)
phi degrees
minutes
seconds
North or south of the Equator (user-entered)
N or S
No Units
Standard meridian (midpoint of the time zone)
Lz
degrees
Anemometer position above the ground level
(user-entered)
Ht meters
Data Related to Time
Day of the year
J or DOY
integer
Begin time hour (military time, hour; e.g., 1400)
tBegin
hour
End time hour (military time, hour; e.g., 1500)
tEnd
hour
Standard (military) time at step's midpoint (e.g.; 14.5)
time
hour
Calculated Values
Slope of the vapor pressure curve
Δ
kPa/
o
C
Specific heat capacity at constant pressure
C
p
=0.001103 MJ/kg/
o
C
Ratio of mol weights of water to dry air
ε=0.622
no units
A-10