Calculates daylengths at a given period of the year (R.J. Reader & K. Phelps).

### Option

`LATITUDE` = scalar |
Latitude at which the daylength is to be calculated, positive for northern hemisphere and negative for southern hemisphere; default 52.205 N (Wellesbourne) |
---|

### Parameters

`DAYNUMBER` = variate |
Days of year for which daylengths are required |
---|---|

`DAYLENGTH` = variate |
Calculated daylengths in hours |

### Description

`DAYLENGTH`

calculates a set of daylengths at a given latitude. The numbers of the days during the year for which the daylengths are required should be specified, in a variate, using the `DAYNUMBER`

parameter. The lengths will then be stored in the variate specified by the `DAYLENGTH`

parameter. The latitude is defined by the `LATITUDE`

option, by default `LATITUDE`

=52.205 which is the latitude of Wellesbourne.

Option: `LATITUDE`

.

Parameters: `DAYNUMBER`

, `DAYLENGTH`

.

### Method

The formula by which the daylengths is calculated is given in Sellers (1965).

### Action with `RESTRICT`

If either the `DAYNUMBER`

or the `DAYLENGTH`

variate is restricted, the calculations will be done only for the units not excluded by the restriction.

### Reference

Sellers W.D. (1965). *Physical Climatology*. University of Chicago Press, Chicago, Illinois.

### See also

Procedure: `HEATUNITS`

.

Commands for: Calculations and manipulation.

### Example

CAPTION 'DAYLENGTH example'; STYLE=meta VARIATE Dayno; !(1...31); DECIMALS=0 DAYLENGTH [LATITUDE=52.205] DAYNUMBER=Dayno; DAYLENGTH=Daylength CALCULATE Sunset = 12 + Daylength / 2 CALCULATE Sunrise = 12 - Daylength / 2 CAPTION 'Sunrise and Sunset times for January at Wellesbourne,UK.' PRINT Dayno,Sunrise,Sunset; DECIMALS=2