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# rhum_fao56

Compute relative humidity 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 rhum_fao56 (
actvpr    : numeric,
satvpr    : numeri,c,
optrh [1] : integer
)
```

## Arguments

actvpr

Actual vapor pressure.

satvpr

Saturation vapor pressure (same units as actvpr).

optrh

Argument which specifies if the returned relative humidity should be in percent (%; optrh=0) or fractional (optrh=1).

## Return value

An array with the same dimensionality and type as actvpr containing estimated relative humidities.

## Description

Compute relative humidity the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper 56 entitled: Crop evapotranspiration - Guidelines for computing crop water requirement . Specifically, see equation 10 of Chapter 3.

Saturation vapor pressure acn be readily calculated by various function: satvpr_mean_fao56, satvpr_mean_fao56.

## Examples

Example 1: The actual vapour pressure is 1.91 kPa. The mean saturation vapour pressure is 2.39 kPa

```  ea = 1.91     ; kPa
es = 2.39     ; kPa

rhum_0 = rhum_fao56(ea, es, 0)   ; 79.91%
printVarSummary(rhum_0)

rhum_1 = rhum_fao56(ea, es, 1)   ; 0.7991
printVarSummary(rhum_1)

```
The output for 'rhum'_1 is

```     Variable: rhum_1
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	[1]
Coordinates:
Number Of Attributes: 4
long_name :	relative humidity
units :	fraction
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 10
```

Example 2: Consider satvpr(time,lat,lon) and actvpr(time,lat,lon) with sizes (3,73,144).

```  RHUM  = rhum_fao56(actvpr, satvpr, 0)
printVarSummary(RHUM)
```

The output for 'RHUM' is

```     Variable: RHUM
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 :	relative humidity
units :	%
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; EQN 10
```