These are real time meteograms produced by the Experimental Eta SREF. Here is a sample set of meteograms for Chicago's O'Hare Aiport, Illinois (725300) for the period beginning at 06Z on the 25th (the 25/06 at the bottom left) and ending at 00Z on the 28th (the final 6-hr increment beyond the 27/18 at the lower right corners).
TRACE 1: Visbility - computed from the calculated from the Stoelinga-Warner algorithm, values in kilometers - maximum value is 12 km; as with all traces, the solid, darker line represents the ensemble mean.
TRACE 2: Low Cloud Fraction - in per cent.
TRACE 3: Cloud Base Pressure - in millibars, can reflect either stratiform or convective cloud. This ensemble mean is computed slightly differently than most. If more than half of the ensemble members have an undefined value (no cloud in the column exceeding the threshold, the mean will not exist at that time. Otherwise, the mean is the average of all the different bases; ensemble members with no cloud in the column are omitted from the computation. This can clearly be seen in the 3rd trace above; the ensemble mean only exists at some times, and there are times when a couple of members predict a cloud base, but there are not enough members to meet the criterion for computing an ensemble mean.
TRACE 4: Lifted Index - degrees Celsius.
TRACE 5: Dew Point Depression - difference between surface temperature and dew point, degrees Fahrenheit; can be used to identify times when low clouds and fog are possible but not accounted for by the visibility computation.
