Julia code to calculate leaf wetness using latitude, day of year and relative humidity. Not functional, just a start at translation from R, see leaf_wet.R gist.

leaf_wet.jl

function leaf_wet(wth = wth, simple = TRUE)

    rh = diurnal_rh(
      doy = wth[, DOY],
      rh = wth[, RHUM],
      tmin = wth[, TMIN],
      tmax = wth[, TMAX],
      tmp = wth[, TEMP],
      lat = wth[, LAT]
    )
    if simple
      lw = length(rh[rh >= 90])
    else
      w = rh
      x = (rh - 80) / 15
      w[rh > 95] = 1
      cs_1 = rh < 95
      w[cs_1] = x[cs_1]
      w[rh < 80] = 0
      lw = sum(w)
    end
    return lw
  end

  function diurnal_rh(rh, tmin, tmax, tmp, doy, lat)
    vp = saturated_vapour_pressure(tmp) * rh / 100
    hrtemp = diurnal_temp(lat,
                          doy,
                          tmin,
                          tmax)
    es = saturated_vapour_pressure(hrtemp)
    rh = 100 * vp / es
    return rh = findmin(100, findmax(0, rh))
  end

# Calculate hourly temperature for a given day
function diurnal_temp(lat, doy, tmin, tmax)
  dayl = day_length(lat, doy)
  nightl = 24 - dayl
  sandhya = 0.5 * dayl
  sunris = 12 - sandhya 
  sunset = 12 + sandhya
  
  x = [[dayl] [nightl] [sunris] [sunset] [tmin] [tmax]]

  function hourly_t(x)
    hr_temp = zeros(24) 
    for hr in 1:24
      # period a: hours between midnight and sunrise;
      if hr < x[3]
        tsunst = x[5] + (x[6] - x[5]) * sin(pi * (x[1] / (x[1] + 3)))
        hr_temp[hr] = (x[5] - tsunst * exp(-x[2] / 4) + (tsunst - x[5]) * 
          exp(-(hr + 24 - x[4]) / 4)) / (1 - exp(-x[2] / 4))
      elseif hr < x[4]
        # period b: hours between time of sunrise and sunset
        hr_temp[hr] = x[5] + (x[6] - x[5]) * sin(pi * (hr - x[3]) / (x[1] + 3))
      else
        #  period c: hours between sunset and midnight;
        tsunst = x[5] + (x[6] - x[5]) * sin(pi * (x[1] / (x[1] + 3)))
        hr_temp[hr] = (x[5] - tsunst * exp(-x[2] / 4) + (tsunst - x[5]) * 
          exp(-(hr - x[4]) / 4)) / (1 - exp(-x[2] / 4))
      end
    end
    return hr_temp
  end
end

# Calculate day length for a given latitude on a given date
function day_length(lat, doy)
    if lat > 90 || lat < -90
      error("latitude values must fall between -90 and 90 degrees")
    end
    doy = mod(doy, 366)
    # William C. Forsythe and Edward J. Rykiel and Randal S. Stahl and Hsin-i Wu
    # and Robert M. Schoolfield. Ecological Modeling, Volume 80 (1995) pp. 87-95,
    # "A Model Comparison for Daylength as a Function of Latitude and Day of the
    # Year", <DOI: 10.1016/0304-3800(94)00034-F>
    P = asin(0.39795 * cos(0.2163108 + 2 * atan(0.9671396 * tan(0.00860 * (doy - 186)))))
    a =
        (0.014540572569940837 + sin(lat * 0.01745) * sin(P)) /
        (cos(lat * 0.01745) * cos(P))
    a = min(max(a, -1), 1)
    dl = 24 - (24 / pi) * acos(a)
    return dl
end

# Calculate saturated vapour pressure, es
function saturated_vapour_pressure(tmp)
    0.61094 * exp((17.625 * (tmp)) / ((tmp) + 243.04))
end