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• Total cloud cover in the 10-day ensemble meteogram is the instantaneous forecast value in oktas (eighths of the sky covered by cloud).  In the 15-day extended ensemble meteogram it is the daily average of ENS forecast values at 06, 12, 18 and 24UTC.  When all members have 0 cloudiness (clear sky) or 8 oktas cloudiness (overcast), there is no line or box at all.  Note:
  • When the forecast is very uncertain and all cloud amounts are more or less equally likely, the columns cover almost the whole range from 0 to 8 oktas, which can be wrongly interpreted as ““overcast””.
    

  • An alternative display (currently only available within the ecCharts meteogram platform) has a circle divided clockwise into eight arcs, each arc representing 1/8 cloud cover. So, for example, the arc covering 45°-90° represents 2/8 cloud cover. The shading within each arc is proportional to the number of members that forecast this particular degree of cloud cover or more.
    
    

• Total precipitation in the 10-day ensemble meteogram is the accumulated precipitation (sum of convective and large-scale) over 6hr periods (00-06UTC, 06-12UTC, etc).  In the 15-day meteogram it is the accumulated precipitation over 24hr periods (00-24UTC).  Note:

  • On the 10-day meteogram the box-and-whisker plot locations align with the end of the 6 hour period.

  • Probabilities for intervals longer than the 6hr and 24hr time intervals cannot be deduced from the ensemble meteogram.

  • Periods of probabilities >0% in every interval can be wrongly interpreted as uninterrupted rain.

  • Consideration of the median alone can be wrongly interpreted as protracted dry spells.

  • The precipitation shown on the ensemble meteograms cannot be directly intercompared inter-compared as the rainfall range (y-axis) varies from one location to the next and from one forecast to the next.  The rainfall range is chosen separately for each ensemble meteogram so that 100% of the predicted values are covered for the 15-day ensemble meteograms, and at least 90% of the predicted values are covered for the 10-day ensemble meteograms (if the top of the distribution is beyond the scale maximum the largest 6-hourly total is shown at the top as red numbers).

• 10m wind speed in the 10-day ensemble meteogram is the instantaneous forecast value in m/s.  Note this is the mean speed, not the diagnosed gust.  In the 15-day ensemble meteogram as it is the 24-hour wind-speed average of ENS forecast values at 06, 12, 18 and 24UTC.  Note:
  
  • The peaks of the whiskers should not be interpreted as wind gusts.  ENS products related to gusts should be used (e.g. CDF diagrams).
    
    
• 10m wind direction (only shown in the 15-day ensemble meteogram) is the daily distribution of directions obtained by taking each 6-hourly forecast step for the day (50 members x 4 forecast steps at 06-12-18-24UTC) and allocating it to the relevant octant.  The area of an octant is proportional to the probability of that wind direction (i.e. to the proportion of forecasts falling in that octant).  The probability of each octant is shown by shading light (low) to dark (high).  Note:
  • The wind roses shown on the ensemble meteograms cannot be directly intercompared as each is scaled to the size of the most populated octant. The size of the wind rose does not refer to wind speed.
    
    
• 2m temperature  in the 10- day ensemble meteogram is shown as instantaneous forecast values at 6-hourly intervals.  In the 15-day ensemble meteogram it is shown as daily maximum and minimum temperatures (in °C).  Note:
  • The forecast temperature is adjusted by using a 6.5K/km lapse rate applied across the difference between the station height (as displayed in the title) and the ENS orography (the relative heights of ENS and true orography are shown in the top right corner of the meteogram web page.  In some instances, some information is also included in the temperature panel title of the meteogram itself).

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