The featured chart presents a time series of horizontal visibility and cloud coverage reports from the Mason City airport. The top panel tries to mimic the sky coverage amount and level reports by coloring areas in blue that are reported to have clear skies and areas in darker colors that are overcast. The skinny panel shows the horizontal visibility reports. The combination of the two are very interesting over this past weekend with reduced horizontal visibility being reported at the same time as clear skies! How can that be? Well, the automated reports are coming from different sensor packages that are looking in different parts of the lower atmosphere. The clear skies report is coming from a sensor that is attempting to look above any low level fog that may be happening and indeed we are generally cloud free above the fog, so that is why these two values can be so different. We have recently been plagued by a very shallow layer of cold air trapped near the surface by a strong inversion aloft. This has prevented much warming of air temperatures during the daytime and the persistent fog with the moisture that is trapped in this layer as well.

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