Ticker for January 26, 2009

                
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January 26, 2009   January 26, 2009   January 26, 2009   January 26, 2009



The Law of Unintended Obsequences

As the threat of icing continues to threaten the region, here are a
couple of Mesonet "rules of thumb" for winter watching. These work
most of the time (but certainly not all the time) for winter weather.

First of all, it turns out that wind sensors, mounted at 10 meters
(about 33 feet) will often grind down to zero wind speeds as they take
on ice. In this way, they can serve as a "canary in the coal mine"
to indicate elevated icing. Actually, it's probably more accurate to
say that they serve as a "bird on a wire", because elevated icing
can wreak havoc on power grids and limbs (that impact power grids).

Here's an example from earlier this afternoon:



Notice that the Mesonet Quality Assurance staff has removed the wind
obs from a swath south of the Oklahoma City area. These sensors had
already taken on some ice at map time.

This map is updated every five minutes at:



Closer to the ground, the rain gauge can indicate whether precip is
freezing at about knee level (the height of Mesonet rain gauges).
Checking a meteogram can help. Here's an example from the Vanoss
Mesonet station, as of 3:30 pm:



Notice that the wind sensor (at 33 ft) stopped producing meaningful
numbers at about noon. However, precipitation (green trace) continued
to increase after noon, which indicates that at least some of the
precip was still flowing through the rain gauge (about 18 inches high).

Taken together, the meteogram hints (but certainly not definitively)
that ice may be accruing on elevated structures (limbs, wires), but
not yet accumulating on ground-level surface.

Meteograms are also updated every 5 minutes at the Mesonet web page:
http://www.mesonet.org/

Of courses, bridges and overpasses have their own rules and will
freeze LONG before ground surfaces.