Calculate average monthly day length (rast)

average_daylength_for_month.R

average_daylength_for_month <- function(month, rast) {
  require(chillR)
  require(terra)
  ndays <- lubridate::days_in_month(month)
  jdays <- sprintf("%02d-%02d-2025", 1:ndays, month) |> 
    lubridate::dmy() |>
    lubridate::yday()
  lats <- terra::yFromRow(rast, seq_len(nrow(rast)))
  daylengths <- sapply(lats, function(lat) {
    chillR::daylength(lat, jdays)$Daylength
  })
  mean_daylengths <- colMeans(daylengths)
  out <- terra::init(rast[[1]], NA)
  values(out) <- rep(mean_daylengths, each = ncol(rast))
  out
}

# r <- terra::rast(res = 1)

# jan <- average_daylength_for_month(1, r)

# all <- lapply(1:12, average_daylength_for_month, r) |>
#   terra::rast() |>
#   setNames(month.abb)