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Posted for "Edmond de Chazal" <edechazal@home.com>:
Thought I'd relate an interesting icing experience to the group. Here's what I wrote to the meteorologists at DTN and their response:
Good evening, I recently encountered what appeared to be moderate mixed icing at 12,000 ft just east of Erie Pa with the air temp at -10 degrees Celsius. I was in the clear until the lower stratus layer slowly rose to meet me. Upon entering the top of the layer, ice immediately began to form and the turbulence started, like a gentle roller coaster. The moisture impact on the windscreen sounded like light rain. The layer did not appear more than about 4000 ft thick. The sun had just set. The aircraft is an experimental Lancair, a composite aircraft with a very smooth wing. TAS was about 180 knots but dropped immediately as the ice began to form.
I'm curious if this encounter is typical. I was surprised at the rate of accumulation. Also, how does moisture exist in liquid form at -10 degrees? I'm thinking that it should freeze to ice in the cloud. Will it if left long enough? Should I always expect ice in clouds near the freezing level, or is there an operational advantage to understanding droplet size and lifting rates?
Thanks,
Ed de Chazal
Their Response:
Pure water suspended in the air, does not freeze until a temperature of -40C is reached. When water vapor condenses or freezes, it prefers to condense or freeze onto a surface (preferably some liquid water or ice already present). For a drop to attempt to freeze on its own, a small cluster of molecules must first align to form a nucleus that the rest of the drop can crystallize around. The constant thermal motion in a drop can easily frustrate these attempts to organize, allowing the water to stay liquid. In the clear atmosphere the only available nucleation surfaces are tiny solid particles suspended in the air such as dust, smoke particles, and sea salt. These particles are called condensation nuclei and have diameters of only 2 micrometers or so (0.002 millimeters). The reason that the droplets remain so readily liquid is that there are very few ice nuclei in a typical cloud. When liquid water is present at below freezing temperatures, it is called supercooled water. If it gets cold enough (-40 C), these supercooled water droplets will freeze even in the absence of ice nuclei. This is called spontaneous nucleation.
Icing is often the most common and significant at the tops of clouds. Any clouds that contain supercooled liquid droplets are capable of producing icing. Most of the icing will occur with temperatures from 0 to -20 degrees C. When the air temperature gets colder than -20C, there typically aren't many liquid droplets left, so the threat of icing diminishes. Clouds where only ice crystals are present will not cause icing.
Jeff Johnson
DTN Weather Services, which becomes meteorlogix on October 23
11400 Rupp Drive
Burnsville, MN 55337
For those of us who recall water freezing at 0 degrees C, this is news. It explains a lot for me. Also, an engine vibration kicked in about a minute after encountering the icing. It stayed as I climbed out of the layer. I attribute this to prop ice with a piece of ice probably shedding at that moment causing the prop imbalance. My feeling at this point is that if you can hear something against the canopy and the temp is below freezing, you've got trouble. Since the droplets are large enough to make a sound at impact. For a very imformative explanation of the icing mechanism go to http://www.nws.noaa.gov to the aviation weather section and look at their experimental icing prediction pages. Click the 'about..." screens to get a good theoretical explanation.
Best Regards,
Ed de Chazal
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