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# prsatm_tz_fao56

Compute atmospheric 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 prsatm_tz_fao56 (
t          : numeric,
z          : numeric,
P0     [1] : numeric,
Z0     [1] : numeric,
iounit [3] : integer
)
```

## Arguments

t

A scalar or array of any dimensionality containing temperature. See iounit(0) argument.

z

A scalar or array of any dimensionality containing elevation (m). If not a scalar, the size and shape must match t.

P0

A scalar specifying the atmospheric pressure at sea level. Most commonly: P0= 1013.0 hPa, 101300 Pa, 101.3 kPa See iounit(0) argument.

Z0

A scalar specifying the reference level elevation (m) corresponding to P0. Most commonly: Z0=0.0 (m).

iounit

An integer array of length 3 indicating the units t, P0 and the returned pressure value(s).

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

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

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

## Return value

An array with the same dimensionality and type as t containing estimated atmospheric pressure (kPa).

## Description

Compute atmospheric pressure 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 7 of Chapter 3 or equation 3-2 in Annex 3.

## Examples

Example 1: Replicate example in Annex 3.

```  P0  = 101.3         ; kPa
z0  =  0.0          ; m

t   = 20.0          ; degC
z   =  0.0          ; m

prs = prsatm_tz_fao56(t, z, P0, z0, (/0,2,2/)  ; 81.756  kPa
print(prs)

```
The output for 'prs' is

```     Variable: prs
Type: float
Total Size: 4 bytes
1 values
Number of Dimensions: 1
Dimensions and sizes:	[1]
Coordinates:
Number Of Attributes: 4
units :	kPa
long_name :	atmospheric pressure
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; Annex: EQN 3-2
```

Example 2: Match the 50-1000 elevation values in Annex 2: Table 2.1

```            ; match Table 2.1
zz    = ispan(50,1000,50)
tt    = conform(zz, 20.0, -1)
pres  = prsatm_tz_fao56(tt, zz, P0, z0, (/0,2,2/) )

printVarSummary(pres)
print("")
print("Annex 2: Table 2.1")
print(zz+"  "+pres)
```

The (edited) output is:

```     Variable: pres
Type: float
Total Size: 80 bytes
20 values
Number of Dimensions: 1
Dimensions and sizes:	[20]
Coordinates:
Number Of Attributes: 4
units :	kPa
long_name :	atmospheric pressure
url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
info :	FAO 56; Annex: EQN 3-2

Annex 2: Table 2.1

z     prs
(0) 	 50   100.711
(1)	100   100.125
(2)	150    99.542
(3)	200    98.961
(4)	250    98.383
(5)	300    97.808
(6)	350    97.235
(7)	400    96.665
(8)	450    96.099
(9)	500    95.534
(10)	550    94.973
(11)	600    94.414
(12)	650    93.857
(13)	700    93.304
(14)	750    92.753
(15)	800    92.205
(16)	850    91.659
(17)	900    91.116
(18)	950    90.575
(19)      1000    90.038
```