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# tdew_actvpr_fao56

Compute dew point temperature as a function of actual vapor pressure 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 tdew_actvpr_fao56 (
actvpr     : numeric,
iounit [2] : integer
)
```

## Arguments

actvpr

A scalar or array of any dimensionality containing actual vapor pressure (kPa).

iounit

An integer array of length 2 which specifies the units of the input actvpr and returned tmperature(s):

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

• iounit(1)=0 ; degrees C (degC)
• iounit(1)=1 ; degrees K (degK)
• iounit(1)=2 ; degrees Fahrenheit

## Return value

An array with the same dimensionality and type as actvpr containing estimated dew point temperature(s).

## Description

Compute dew point temperature as a function of actual 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 3-11 in Annex 3.

## Examples

Example 1: There is no explicit example in FAO56. This example has been created from other examples.

```
tmin    = 18               ; degC; ounit(0)=0
tmax    = 25
rhmin   = 54               ; %
rhmax   = 82
rhmean  = (rhmin+rhmax)*0.5

iounit  = (/0,2/)          ; degC, kPa
eamnmx  = actvpr_mnmx_fao56 (tmin, tmax, rhmin , rhmax, iounit)  ; eqn 17; 1.70
eamean  = actvpr_rhmean_fao56(tmin, tmax, rhmean, iounit)         ; eqn 19; 1.78

; iounit=> kPa, degC
tdew_mnmx = tdew_actvpr_fao56(eamnmx, (/2,0/))  ; 14.96
tdew_mnmx@actvpr = "derived from actvpr_mnmx_fao56; EQN 17"

tdew_mean = tdew_actvpr_fao56(eamean, (/2,0/)))  ; 15.65
tdew_mean@actvpr = "derived from actvpr_rhmean_fao56: EQN 19"

print(tdew_mnmx)
print(tdew_mean)

```
The (edited) output for 'tdew_mnmx' and 'tdew_mean' is

```
Variable: tdew_mnmx
Dimensions and sizes:	[1]
Coordinates:
Number Of Attributes: 5
actvpr :	derived from actvpr_mnmx_fao56
long_name :	Estimated dew point temperature
units :	degC
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; Annex: EQN 3-11
(0)	14.95786

---
Variable: tdew_mean
Dimensions and sizes:	[1]
Coordinates:
Number Of Attributes: 5
actvpr :	derived from actvpr_rhmean_fao56
long_name :	Estimated dew point temperature
units :	degC
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; Annex: EQN 3-11
(0)	15.64919

```

<<<<<<< .mine Example 2: Consider TMIN (C), TMAX (C), RHMIN, RHMAX, RHMEAN with dimensionality (time, lat,lon). Calculate the dew point temperature using the actual vapor pressure (hPa; iounit(1)=0)). ======= Example 2: Consider TMIN, TMAX, RHMIN, RHMAX, RHMEAN with dimensionality (time,lat,lon). Calculate the dew point temperature using the atual vapor pressure. >>>>>>> .r11754

```                                                    ; iounit => degC, kPa
EAMNMX    = actvpr_mnmx_fao56(TMIN, TMAX, RHMIN , RHMAX, (/0,2/)) ; eqn 17
TDEW_MNMX = tdew_actvpr_fao56(EAMNMX, (/2,0/))
printVarSummary(TDEW_MNMX)

EAMEAN    = actvpr_rhmean_fao56(TMIN, TMAX, RHMEAN, (/0,2/))        ; eqn 19
TDEW_MEAN = tdew_actvpr_fao56(EAMEAN, (/2,0/) )
TDEW_MEAN@actvpr = "derived from actvpr_mnmx_fao56: EQN 19"
printVarSummary(TDEW_MEAN)

```

The (edited) output for 'tdew_mnmx' and 'tdew_mean' is

```Variable: TDEW_MNMX
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: 5
long_name :	Estimated dew point temperature
units :	degC
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; Annex: EQN 3-11

---

Variable: TDEW_MEAN
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: 5
actvpr :	derived from actvpr_mnmx_fao56: EQN 19
long_name :	Estimated dew point temperature
units :	degC
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; Annex: EQN 3-11

```