Archive for the ‘Weather Physics’ Category
The cause of last Thursday’s terrible airplane crash near Buffalo, New York may never be know for sure. In any case, such investigations take many months. There was a brief period of time when there were some significant doubts as to the role of icing in the accident – partly due to the embryonic stage of the investigation hindered by the complexities of carefully sorting debris of the aircraft and the house from the remains of the victims. What a difficult job that must be.
Yesterday and today there have been more indications that icing was responsible. If anything accurate can be said about the nature of the environment which produces icing conditions it is that it is fickle. Just as the surface has its own micro-climatology, so do clouds. It appears to me that the plane that crashed must have entered an icing environment which might have been severe. There is also a possibility of some form of mechanical and/or instrument failure. The final determination could very well point to a combination of unfortunate happenings.
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THE HYDROLOGIC CYCLE, A CLASSICAL TOPIC
IN NATURAL SCIENCE COURSES
I’ve taught the hydrologic cycle many times in geology, meteorology, physical oceanography and environmental science classes. It’s always been a pleasure but I’ve never had enough time. All of these were college courses and in almost every case the text book covered the subject adequately. However, the manner in which water moves and changes in our natural environment is so very interesting that a few pages in a text with a traditional drawing and an hour lecture from me simply does not do the subject justice. Water is such a remarkable compound – I can’t find the words to explain how very interesting it is and how mysterious it can be at times considering the amount of scientific attention it has received through the years. There is still so much to learn.
So, it is with excitement that I look forward to a 6-hour course that I am scheduled to teach in May to the Senior Institute enrollees at Central Florida Community College. In 37 years of full-time college teaching (and 4 years part-time) I never had the opportunity to devote so much time to the subject. The method I intend to use is my own “idea” but surely it has been done before – that is, to follow water step-by-step as it goes from one phase or one environment to the next. My presentation won’t be a journey without side trips and backtracking. There are multiple manners in which water can transform and/or move with interesting little anomalies along the way. With 6 hours to utilize I will be able to discuss aspects that were only fleetingly mentioned in my previous hour-long presentations e.g.: Capillary action, deposition, glacial calving, influent groundwater movement, juvenile water, super-cooled droplets, and much more.
SUPERCOOLED CLOUD DROPLETS AND ICING
I feel fairly certain that some people who read this have had the experience of having rain freeze upon impact with their vehicle’s windshield. Some would assume that the freezing occurs because the windshield is so very cold. That is usually not the case. Instead, the liquid droplets were probably at a temperature well below “freezing” and the impact with the windshield itself triggered the instant freezing. Hopefully, the “defrosting” vents can keep the windshield warm enough so that the ice can be quickly cleared. Now, imagine what it must be like if the surfaces being iced are the windshield and wings of your aircraft in flight – as well as other aircraft surfaces (e.g. propellers, fuselage, horizontal stabilizers)!
Today, February 15, 2009, the mere thought of super-cooled droplets hauntingly reminds me that in addition to the marvelous beauty of water’s multifaceted journeys and transitions through our natural environment, there are some insidious elements that can become deadly in this modern world. Of course, I’m thinking specifically of the recent terrible aircraft accident responsible for 50 fatalities near Buffalo, New York.
For a short while since the accident it appeared that icing might have been the culprit or perhaps a contributing factor in causing the aircraft to make its sudden rapid descent (apparently almost immediately after the application of flaps). At the time other aircraft in the vicinity were reporting icing. HOWEVER, AT THE TIME OF THIS WRITING, NEWS RELEASES HAVE INDICATED THAT THE NATIONAL TRANSPORTATION SAFETY BOARD CLAIMS THAT ICING APPEARS NOT TO HAVE BEEN A FACTOR. The changing of the airfoil’s shape upon flap engagement might have triggered the rapid descent – an apparent stall leading to a flat spin. That would indicate either insufficient air speed at the time of flap deployment or some type of catastrophic failure. SINCE MANY AVIATION ACCIDENTS HAVE BEEN CAUSED BY ICING – AND IT WILL REMAIN A PROBLEM FOR AIRCRAFT FOR A LONG TIME TO COME, I SHALL CONTINUE.
When icing was being blamed, I suspected that some critical errors might have been made in the cockpit. At best, my notions were intuitive – or, on the other end of the spectrum, unfair during such an early stage in the investigation. Nevertheless, a surprising amount of information has been made available during this embryonic phase – partly due to the fact that the flight recorders are advanced models and they were in very good shape. There is no need for me to dwell on factors that can cause a plane to become unstable when icing occurs – suffice it to say that airfoils lose “lift efficiency” quickly when ice buildup changes their shape and of course the weight of the ice accumulation can also be a huge factor. I do not know what kind of air speed indicators are installed on that type of aircraft but I do know that icing can cause false readings on some types. Icing can also cause problems at air intakes and oil cooler intakes of some aircraft.
IN THE FIRST PLACE?
The cause of the icing is a surprise to most people. Though icing can occur on a plane’s very cold surface when it descends into “warm” clouds whose temperatures are above freezing, the vast amount of problematic icing occurs when the liquid droplets themselves are below what we traditionally consider freezing temperature. These droplets consist of what is called supercooled liquid water (SLW). Water in cloud droplets can get as cold as about negative 40 degrees Celsius (which is the same as negative 40 Fahrenheit) without freezing.
When liquid water freezes (box 3 to 4 in the illustration above) the water molecules align in a crystalline fashion. But in order to do so they need one of two things: 1) either a freezing nuclei whose surface acts as a template to initially “show” the molecules how to (or trigger the molecules to) line up, or 2) some molecules themselves must be jolted (or jiggled) such that for at least an instant they are arranged so they can act as a template or model for the rest to follow. The likelihood of such alignment occurring in undisturbed droplets is slim. This would not be true of most fresh water at the surface, such as in lakes because there are microscopically-sized particles available in the water to act as templates. On the other hand, water that has condensed and remains in the air is very “clean” by comparison.
An aircraft flying though supercooled cloud droplets causes considerable rapid stirring to set the stages for freezing upon impact with that aircraft – just as supercooled raindrops freeze upon impact with trees and suspended wires in those notorious, damaging ice storms.
The first three links below show convincing demonstrations of liquid water freezing as a result of hexagonal ice crystal seeding. The ice crystals provide the template which “shows” the liquid water what to do in order to become solid. In the third example when the water freezes and builds up a small mound on the wooden post, I suspect that the split second ideal alignment of some water molecules (while pouring) provoked the freezing.
In this 4th example you will see that a jolt causing a sloshing of the water in the small amount of air space at the top of the bottle allows for enough water movement so that for an instant a hexagonal orientation occurs among some molecules causing a very rapid “follow the leader” freezing all the way down to the bottom of the bottle.
Just as condensation and deposition give off heat, freezing is also exothermic. This is probably why some of the water remains in the liquid state. If the SLW is not very much colder than “freezing” temperature, the heat given off during freezing will cause the remaining liquid to acquire enough heat to teeter over to the liquid side.
Use the search term “supercooled water” on YouTube.com and you will find many other video demonstrations.
WHEN WATER FREEZES IT EXPANDS, BECOMING LESS DENSE. THIS EXPLAINS WHY SOLID WATER FLOATS UPON LIQUID WATER.
If you compare box 3 and 4 in the illustration in this post, you will see why water expands and becomes less dense upon freezing. To establish the hexagonal grid necessary for ice, the molecules can’t be as close together as they were when they were in the cold liquid stage.
Information on supercooled liquid water would have eventually been posted here if the Continental Express Flight 3407 disaster had not occurred. It is regrettable that the accident played a role in my posting this information at this time. I offer my sympathy to all who have broken hearts over the loss of a loved one and all others adversely effected.
Finally, the information in this post about SLW and icing merely scratches the surface compared to that which is known. But, that which is not understood is formidable.