Sunshine duration is calculated from the hourly sunshine time (which is calculated in minutes by the models, e.g. in the hour from 2 to 3pm, the forecast model predicts 50 minutes of sunshine). For the daily sunshine duration, the hourly sunshine time is added for the entire day.
In the daily weather forecast overview on the start page, you find the number of sunshine hours expected for the day.
The selection of the pictograms is based on the sunshine duration as well. Here, the following points have to be considered:
- The hourly pictograms are selected optimistically, e.g. if one hour might have sunny spells for like 30 minutes, a somewhat sunny pictogram is selected already. This reflects our better reception of short sunny moments, compared to totally overcast skies.
- The 3h-section on the start page shows hourly pictograms only, e.g. the 11am pictogram shows the sunshine duration from 10-11am.
- In conclusion, it is possible that the daily pictogram shows neither sunshine duration nor a pictogram with sun (because the sunshine duration isn't relevant enough over the curse of the whole day, and not enough to display in the pictogram), but that there are pictograms with sun in the 3h-section and the 1h-detailview. Additionally, it even can happen that the only sunny pictograms are in the 1h-detailview but nowhere else.
- Contrarily, it also is possible that there is a sunny daily pictogram and that the sunshine duration are displayed with 1 hour or more, but that there are no sunny pictograms in the 3h-section. This happens when the sun shines in the hours between the ones displayed (e.g. from 11-13 pm, while 10-11 and 13-14 are displayed), or when the sunshine duration is too short to trigger any sunny pictogram, but in sum still is more than 1 hour.
The sunshine duration is calculated for a flat, non-mountainous surface. Places in valleys or in the shadow of hills or large buildings may observe shorter sunshine durations (especially on sunny days), depending on the horizon levels.
It is possible to calculate the influence of the horizon on the duration of the sunshine (downscaling), but this is computationally so intensive that it would delay the data display. We are therefore currently not offering such a downscaling and rely on the understanding of the users that they adjust the information on the duration of sunshine to their local conditions. Our detailed hoursly display will help with this adjustment.
The colour on the meteograms is white for the night and yellow for the day and is dependent on the possible total sunshine hours per day (time from sunrise till sunset).
Furthermore, the sunshine duration is also used for finding the most sunny weather with where2go.