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# satvpr_mean_fao56

Compute mean saturation vapor pressure using minimum and maximum temperature temperature as described in FAO 56.

Available in version 6.4.0 and later.

## Prototype

```load "\$NCARG_ROOT/lib/ncarg/nclscripts/csm/crop.ncl"  ; This library is automatically loaded
; from NCL V6.5.0 onward.
; No need for user to explicitly load.

function satvpr_mean_fao56 (
tmin       : numeric,
tmax       : numeric,
iounit [2] : integer
)
```

## Arguments

tmin

A scalar or array of any dimensionality containing minimum temperatures.

tmax

A scalar or array of containing maximum temperatures. The units and dimsionality must match tmin.

iounit

An integer array of length 2 which indicates the units of the temp and returned saturation vapor pressure.

• iounit(0)=0 means the input are in degrees C (degC).
• iounit(0)=1 means the input are in degrees K (degK).
• iounit(0)=2 means the input are in degrees F (degF)
If iounit(0)=1 or 2, the tmin and tmax will be converted to degC internally prior to the computation. The input values are unaltered.

iounit(1) refers to the units of the returned saturation vapor pressure

• iounit(1)=0 yields hPa
• iounit(1)=1 yields Pa
• iounit(1)=2 yields kPa

## Return value

An array with the same dimensionality and type as tmin containing mean saturation vapor pressure.

## Description

Compute 'mean' saturation vapor pressure (kPa) as described in the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper 56 entitled: Crop evapotranspiration - Guidelines for computing crop water requirement. Specifically, see equation 12 of Chapter 3.

Tetens' Formula is uised. This is an empirical expression for saturation vapour pressure with respect to liquid water that includes the variation of latent heat with temperature.

Tetens, O. 1930. Uber einige meteorologische Begriffe. Z. Geophys.. 6. 297–309

## Examples

A table which displays saturation vapor pressures for different temperatures is here.

Example 1: Replicate example 3 in Chapter 3. Use different temperature units to illustrate that the answers are identical.

```
tminC   = 15.0                         ; degC
tmaxC   = 24.5
; iounit=(/0,2/) , degC, kPa
esmeanC = satvpr_mean_fao56(tminC, tmaxC, (/0,2/))   ; 2.39 kPa
;---

tminK   = tminC+273.16                 ; degK
tmaxK   = tmaxC+273.16
; iounit=(/1,2/) , degK, kPa
esmeanK = satvpr_mean_fao56(tminK, tmaxK, (/1,2/))   ; 2.39 kPa
;---

tminF   = 1.8*tminC + 32               ; deg Farenheit
tmaxF   = 1.8*tmaxC + 32
; iounit=(/2,2/) , degF, kPa
esmeanF = satvpr_mean_fao56(tminK, tmaxK, (/1,2/))   ; 2.39 kPa
;---
printVarSummary(esmeanF)

```
The output for 'esmeanF' is
```
Variable: esmeanF
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	[1]
Coordinates:
Number Of Attributes: 4
long_name :	mean saturation vapor pressure
units :	kPa
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 11 and 12

```

Example 2: Consider TMIN(time,lat,lon) and TMAX(time,lat,lon) with sizes (3,73,144) and the temperatures have units of degK.

```
SATVPR  = satvpr_mean_fao56(TMIN, TMAX, (/1,2/))
printVarSummary(SATVPR)

```
The output for 'SATVPR' is
```
Variable: SATVPR
Type: float
Total Size: 126144 bytes
31536 values
Number of Dimensions: 3
Dimensions and sizes:	[time | 3] x [lat | 73] x [lon | 144]
Coordinates:
time: [1..3]
lat: [-90..90]
lon: [ 0..357.5]
Number Of Attributes: 4
long_name :	mean saturation vapor pressure
units :	kPa
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 11 & EQN 12

```