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# satvpr_water_bolton

Estimate the saturation vapor pressure over water using Bolton's equation 10.

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 satvpr_water_bolton (
t          : numeric,
iounit [2] : integer
)

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

## Arguments

t

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

iounit

An integer array of length 2 which specifies the units of the input t and returned saturation vapor pressure:

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

• iounit(1)=0 returns units hPa
• iounit(1)=1 returns units Pa
• iounit(1)=2 returns units kPa

## 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

Use Bolton's equation 10 to estimate saturation vapor pressure over water. The quoted accuracy is 0.3% or better for -35C < t < 35C.

``` Reference:
Bolton, David, 1980:
The computation of equivalent potential temperature
Monthly Weather Review, vol. 108, no. 7 (july),  p. 1047, eq.(10)
http://dx.doi.org/10.1175/1520-0493(1980)108<1046:TCOEPT>2.0.CO;2
```

## Examples

Example 1: Show simple usage and compare with satvpr_mean_fao56 which uses slightly different empirical constants. The difference in results is small.

```  t   = 15.0                         ; degC ; iounit(0)=0 , degC
iounit(1)   , hPa, Pa, kPa
esat_0  = satvpr_water_bolton(t, (/0,0/))   ; 17.04077 hPa
esat_1  = satvpr_water_bolton(t, (/0,1/))   ; 1704.077 Pa
esat_2  = satvpr_water_bolton(t, (/0,2/))   ; 1.704077 kPa
print(esat_0)

esat_fao56 = satvpr_mean_fao56(t, 0)     ; 1.70535 kPa

```
The output is:
```
Variable: esat_0
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	[1]
Coordinates:
Number Of Attributes: 5
long_name :	saturation vapor pressure over liquid water
units :	hPa
NCL :	satvpr_water_bolton
doi :	http://dx.doi.org/10.1175/1520-0493(1980)108<1046:TCOEPT>2.0.CO;2
info :	Equation 10 of reference
(0)	17.04077

```