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# wetbulb_stull

Calculate wet bulb temperature at standard sea level pressure (1013.25 hPa) using the method of R.Stull. Available in version 6.4.0 and later.

## Prototype

```load "\$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"  ; This library is automatically loaded
; from NCL V6.2.0 onward.
; No need for user to explicitly load.

function wetbulb_stull (
t          : numeric,
rh         : numeric,
iounit  : integer,
opt        : logical
)

return_val [dimsizes(t)] :  float or double
```

## Arguments

t

Scalar or array containing temperature(s) [ units see iounit ].

rh

Scalar or array containing relative humidity in percent (%).

iounit

An integer array of length 2 which specifies the units of the input t and returned wetbulb temperature:

• iounit(0)=0 input t (degC)
• iounit(0)=1 input t (degK)
• iounit(0)=2 input t (Farenheit)

• iounit(1)=0 returns units degC
• iounit(1)=1 returns units degK
• iounit(1)=2 returns units Farenheit

opt

Currently not used. Set to False.

## Return value

A scalar or an array of the same size and shape as t. The output will be double if t is of type double. The return units are specified by iounit(1).

## Description

Quote from abstract: The equation was found as an empirical fit using gene-expression programming. This equation is valid for relative humidities between 5% and 99% and for air temperatures between -20C and 50C, except for situations having both low humidity and cold temperature. Over the valid range, errors in wet-bulb temperature range from -1C to 0.65C, with mean absolute error of less than 0.3.

```Reference:  Stull, R., 2011: Wet-bulb temperature from relative humidity
and air temperature, J. Appl. Meteor. Climatol.
http://journals.ametsoc.org/doi/pdf/10.1175/JAMC-D-11-0143.1
```

## Examples

Example 1:

```   T  = 20.0            ; degC
RH = 50.0            ; %
TW00 = wetbulb_stull(T, RH, (/0,0/), False)
print(TW00)

T  = 20.0 +273.15    ; degK
RH = 50.0
TW10 = wetbulb_stull(T, RH, (/1,0/), False)
print(TW10)

T  = 1.8*20.0 + 32   ; degF
RH = 50.0
TW20 = wetbulb_stull(T, RH, (/2,0/), False)
print(TW20)
```
The output from each each would be the same as TW00::
```
Variable: TW00
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	
Coordinates:
Number Of Attributes: 3
long_name :	Wet Bulb Temperature via Stull
units :	degC
reference :	http://journals.ametsoc.org/doi/pdf/10.1175/JAMC-D-11-0143.1
(0)	13.69934

```
Example 2:

Compare wetbulb_stull and wetbulb_Wrap (wetbulb). The wetbulb_stull was derived using a pressure of 1013.25 hPa.

```   tc   = 20               ; C
rh   = 50               ; %

tk0  = 273.15           ; K
tdc  = dewtemp_trh((t+tk0), rh) - tk0        ; C

twb_stull = wetbulb_stull(t, rh, (/0,0/), False)   ; C

p    = 1013.25          ; hPa
;;twb  = wetbulb(p, tc, tdc)
twbW = wetbulb_Wrap(p, tc, tdc, (/0, 0, 0/), False)

print(twb_stull)    ;  twb_stull=13.70

print(twbW)         ;  twbW=13.75   ; twb=twbW
```