c$Author: fredfox $
c$Date: 2002-05-08 18:47:45 $
c$Revision: 1.2 $
c$Source: /weru/cvs/weps/weps.src/util/solar/daylen.for,v $

      real function   daylen(dlat,idoy,riseangle)

c     + + + PURPOSE + + +
c     This function calculates the daylength (hours) for any simulation
c     site based on the global position of the site, and day of the
c     year.  The inputs for the function are day of the year, and latitude
c     of the site.

c     + + + KEYWORDS + + +
c     day length

c     + + + ARGUMENT DECLARATIONS + + +
      integer idoy
      real dlat
      real riseangle

c     + + + ARGUMENT DEFINITIONS + + +
c     idoy   - Day of year
c     dlat   - Latitude of the site, degrees (north > 0, south < 0)
c     riseangle - angle of earths rotation where sunrise occurs
c                 this varies depending on whether you are calculating
c                 direct beam, civil twilight, nautical twilight or
c                 astronomical twilight daylength

c     + + + LOCAL VARIABLES + + +
      real b, dec, e, h, coshr

c     + + + LOCAL DEFINITIONS + + +
c     dec    - declination of earth wrt the sun (degrees)
c     e      - Equation of time (minutes)
c     h      - Hour angle (degrees)

c     + + + COMMON BLOCKS + + +
      include 'p1unconv.inc'

c     + + + FUNCTION DECLARATIONS + + +
      real declination
      real hourangle

c     + + + END SPECIFICATIONS + + +

c     declination angle (dec)
      dec = declination(idoy)

c     sunrise or sunset hour angle
      h = hourangle(dlat, dec, riseangle)

c     Calculate the length of the day
      daylen= 2.0*h/15.0

      return
      end