Tuesday, December 27, 2011

Cloud Phase

As a rule, I don't like to write about accidents; I prefer to write about preventing them. Still, I read about accidents, and I was interested in Scott Dennstaedt's analysis of the weather factors in last week's tragic crash of a TBM-700 in New Jersey. I have a little TBM-700 time, working with an owner who wanted to get his ATP who quickly realized that it was far more cost effective to do so in a simulator. It's an amazingly capable airplane.

But, as I said, I don't write about accidents.

Dennstaedt's analysis of the weather factors included a weather "product" (as the National Weather Service people call it) that's new to me, the Cloud Phase chart. It took me a second to realize what this means: in chemistry we learn that matter is generally in one of three phases, solid, liquid, or gas. So the cloud phase refers to the water in the cloud. If it's solid, that's ice, which is less likely to stick to an airframe; if it's liquid, though, and cold enough, then it is more likely to stick to the airplane and freeze.

Here's a recent example, showing the cloud phases for the Eastern USA. The most important color is baby blue, indicating cold liquid water. The Ohio Valley, Michigan, and northern New England have cold liquid water clouds.

Compare that with the "Maximum Icing Severity Chart" available from aviationweather.gov. This shows the potential for heavy ice throughout the Ohio Valley, Michigan, and northern New England, with SLD (Supercooled Liquid Droplets) potential in northern New England. This seems to be associated with an occluded front that runs from a low centered near Kingston, Ontario (CYGK) that runs to New York City (KJFK), which becomes a cold front that seems to be forming a wave off the DelMarVa peninsula off Dover, Delaware (KDOV).

It should be obvious that low pressure over a Great Lake will form ice, but in cases where the situation is less obvious the cloud phase chart could be a useful aid in making a go-no go decision.

Now here's the trick: it's really hard to find the chart. I finally found it at a NASA-Langley website. Click on the Conus links under GOES EAST or GOES WEST, then use a drop-down menu to access the Cloud Phase chart.

Not easy, but nothing about icing is ever easy.



At December 30, 2011 at 8:09 PM , Anonymous Patrick Minnis said...

If you look further down the menu, you will find that there is an icing potential map that gives the probability and intensity of the icing. It even provides information about icing under the red cloud areas in the phase chart.


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