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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.

See Also

Crop & Evapotranspiration functions, relhum, relhum_ttd

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