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# satvpr_temp_fao56

Compute saturation vapor pressure using 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_temp_fao56 (
temp       : numeric,
iounit [2] : integer
)
```

## Arguments

temp

A scalar or array of any dimensionality containing temperatures.

## Return value

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

## Description

Compute saturation vapor pressure 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 11 of Chapter 3.

This is Tetens' Formula: 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
es_minC = satvpr_temp_fao56(tminC, (/0,2/))   ; 1.705 kPa
es_maxC = satvpr_temp_fao56(tmaxC, (/0,2/))   ; 3.075 kPa

; ---
tminK   = tminC+273.16                  ; degK
tmaxK   = tmaxC+273.16
; iounit(0)=1 , degK
es_minK = satvpr_temp_fao56(tminK, (/1,2/))   ; 1.705 kPa
es_maxK = satvpr_temp_fao56(tmaxK, (/1,2/))   ; 3.075 kPa

; ---
tminF   = 1.8*tminC + 32                ; deg Farenheit
tmaxF   = 1.8*tminC + 32
; iounit(0)=2 , deg Farenheit
es_minF = satvpr_temp_fao56(tminF, (/2,2/))   ; 1.70535
es_maxF = satvpr_temp_fao56(tmaxF, (/2,2/))   ; 3.07465

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

```

Example 2: Consider T(time,lat,lon) with sizes (3,73,144) and the temperatures have units of degK. Here iounit(0)=1.

```
satvpr  = satvpr_temp_fao56(T, (/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 :	saturation vapor pressure
units :	kPa
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 11

```