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Selected Questions from students in the "course" and the answers they received: 1. Why is the stated average lapse rate different in different publications? I.E. The different texts are referring to the phenomena of cooling that takes place as one rises in altitude and elevation. The difference between 2 and 3 degrees refers to how much moisture is in the parcel of air as it rises. To put vapor into the air requires the addition of energy into the "system" which slows down the rate of cooling as the air rises. Dry air cools faster. Older texts also used degrees F which makes it more confusing. 2. There is a line in that article "Collision With The Sky" about avoiding gravity waves where they say "watch out for K-H clouds." I don't know what a K-H cloud is..... Kelvin-Helmholz clouds form patterns similar to waves cresting at a beach. They form from up-and-down motions of the atmosphere and from drier air flowing faster over moister air. The University of Arizona has an interesting lesson slide on these clouds : http://www.atmo.arizona.edu/students/courselinks/fall00/nats101-mullen/lecture22/sld028.htm (Note: this link is no longer active) 3. In flying higher altitude flight levels I have numerous times come across C/B's in a line where the overhangs stretch out and touch the next Cell area, sort of linking all the cells together. If one "HAD" to go through at some point and there was no jetstream present where would you suggest. The answer I have to give you is to avoid going closer than 20 miles to any severe cell. My suggestion is to go around the line or divert to get more gas and "wait-it-out" before flying that direction. 4. In lesson one, concerning stability and the effects of moisture, you state "With more moisture, the dew point temperature is higher so clouds will form with less cooling". Could you explain why the dew point temperature is higher with more moisture? When air cools the molecules get closer together. The simplest analogy I can think of is to look at a wet sponge and a dry sponge. The wet sponge only requires a little squeeze (or a little cooling) to get out water, but a dry sponge requires a harder squeeze (or a lower temperature). 5. Greetings, I am curious if, should you ever find the need to cross a stationary front over the Atlantic, which area of the front is less critical and is it better inside the cloud layers or on top of the Strata surrounding this front. Sometimes, this lines extend in a diagonal line from Florida, all the way up to beyond Bermuda, well over a 1000 nm. I have chosen to cross inside the clouds and found smoother air, any suggestions. These lines include cells from Fl 290 to FL 350 and higher sometimes. We are going outside the NWA for this answer... 6. Is there link that will discuss the mechanics of the precipitation "dragging down" the cold, high altitude air? I'm curious as to how this happens with thunderstorms and with other forms of precipitation. Is air trapped along the boundary layer of the drop? How does this work?? Thanks for the interest and question. If we find links that are useful we'll add them. You are getting into cloud physics where there are still a lot of unknowns and theories -- and not much copied to the Internet yet. In major cloud systems like Thunderstorms there exists strong updrafts and downdrafts -- keeping the cloud building. Sometimes the downdrafts go all the way to the ground and bring precipitation along with them; some downdrafts are dry. You may note a cool breeze at the ground ahead of a thunderstorm before the rain hits you. Looking at the precipitation alone ---- rain will fall when it gets heavy enough and gets out of the strong updrafts -- it could bring air parcels along with it -- that would be called "entrainment." This would be similar to water in a creek dragging sediment from the edges and bottom. If you can get a hold of a text on cloud physics it should help you more. 6. How do you pronounce "derecho"? How often do they occur and when was the last one recorded? Derecho is pronounced "day-RAY-cho." They probably occur every day someplace on earth. It is essentially a family of downburst winds coming out of a large cluster or a squall line of thunderstorms and can produce damaging straight-line winds over hundreds of miles long. They move primarily from northwest to southeast. 7. A question, not really in the direct line of the presentation: How far do the actual charges travel during a lightning event? I know that during electrical current flow in a conductor such as a wire, each individual charged particle travels only a surprisingly short distance, and not particularly fast, although charge may be very quickly transferred over a large distance. I guess that the mean free path increases considerably once a plasma is formed. Dr. Krider, One of the students from the National Weather Association 'Thunderstorms And Flying' internet course had a very interesting question on lightning, which I bet you'll just love! I don't know the answer and was hoping you'd be so kind as to help. If I understand the question correctly, the student wants to know how far does the actual free electrical charge travel along a single return stroke in a lightning flash, as opposed to the more typical question of distance between subsequent flashes. I could answer the later question, since the distance between subsequent flashes has been fairly well studied for lightning safety and protection (mean ~3 NM). Even the distance between subsequent strokes within a flash has been studied (~3 Km). But I've never heard of any research into how far the electric charge actually moves. The text books and science movies always imply the charge moves along the whole length of the stroke, i.e. a free electron in the lower part of the cloud makes it all the way to the soil. But that's also the way the references show charge flowing in metal wires, and we know that's not right -- individual charge moves only a tiny fraction of an inch, even if the current is flowing along miles of wire; it's the 'holes' that are really flowing the long distance. I presume it's analogous in lightning. The 'holes' flow along the entire return stroke, while the individual free electrons actually only move a distance on the order of their Mean Free Path (MFP). But I have no idea as to the MFP for free electrons under return stroke conditions. - At a shear guess, several tens of meters, as in a stepped leader distance. Maybe a bit more, since it's not pushing through cold dense unionized air like a stepped leader. As I said, I don't have any idea as to the right answer. But since this is exotic lightning physics, I thought you'd be able to help, if anybody can. Didn't I tell you it was a great question! Thanks in advance for sharing your insight! William P. Roeder The student is correct. The mean free path in the channel is only about 10 times larger than in the environment (near sea level) so the distance is quite short. How short - I'll defer to your student who is obviously closer to the relevant literature than I am at the moment. Dr. Phil Krider, Uof Arizona |
Updated: 23 Nov 2003