Windthrow Lee E. Frelich Jun 24, 2003 17:03 PDT  ENTS: Its been an exciting day in MN. A severe thunderstorm went through last night at 4:00 AM. The cottonwoods got their crown thinning that they needed. The street I walk down to get to campus was closed due to a hackberry tree (about 7' cbh) having been deposited across the street. All afternoon, severe weather warnings have been flying nonstop. Some places have had 10 inches of rain. There are four tornado warnings are in effect right now, including one only nine miles away from me. Too bad I can't see it--its raining too hard. It has been interesting to see how each species of tree responds. Hackberry split at their first main fork. Bur oaks and elms give up small branches about an inch in diameter at the end of major limbs. Sugar maple fold up in the wind so they only have a small surface area, and they either split, or the tree uproots. Cottonwoods have branches about 10 feet long stripped from the tree. A tree is essentially an a circulation system that is designed to maximize surface area exposed to the atmosphere, so they can photosynthesize. But they have to pay a price when the wind is extreme. I had better turn off the computer now--tornado and heavy lightning are approaching and computer-killing power surges are likely! Lee   Windthrow Lee E. Frelich  Jun 25, 2003 11:11 PDT  ENTS: Things are getting back to normal (except those whose houses we levelled by tornadoes, crushed by trees, or washed away by floods--I was lucky I just have mud from a minor flood in the garage) after last nights storms. Minneapolis had four separate warnings: tornado warnings at 6:30 and again at 9:30, a severe thunderstorm warning at 12:00 am and a flash flood warning that lasted all night an extended into morning. At 6:00 pm last night, a weather balloon released in Minneapolis showed winds of over 100 mph blowing straight up from the ground. With a dewpoint of 77 degrees, and a strong jet stream directly overhead, these vertical winds made for an exceptionally unstable atmosphere. What goes up must come down, and all the moisture those updrafts fed into the towering thunderheads came back down in the form of a dozen tornadoes, golf ball sized hail, 4-10 inches of rain, and several downbursts. Once again, streets were closed this morning by debris (mostly trees) and many freeways were closed by floodwaters. The small town of Buffalo Lake just west of Minneapolis had the worst with a 1/2 mile wide F2-F3 tornado and 10 inches of rain. Four isolated super cell thunderstorms that each produced tornadoes west of Minneapolis converged at about midnight to form one almost stationary Massachusetts-sized thunderstorm that produced downbursts, tropical storm intensity downpours and about 50 strokes of lightning per minute throughout the night over Minneapolis. We should see a lot of browning on the edge of leaves now, just from the physical abrasion of the wind and heavy rain. We will also get light brown flecks on leaves from ozone, which is created by lightning as it heats up the air and converts O2 to O3. This thunderstorm created a major ozone event. Lee Windthrow Lee E. Frelich Jun 25, 2003 11:22 PDT  Gary: To answer your question about tree geometry, I think it is pretty well fixed genetically. What throws the genetic pattern off is injury by drought, wind, lightning, etc. Of course, old trees accumulate injuries that change their growth form, so they develop a lot of character. There are several different growth patterns that trees can be classified into that relate to the frequency of buds along a stem, what angle the new shoots grow from the stem when the buds open, and whether the buds are organized in whorls, alternate or opposite, and the degree to which apical dominance limits the growth of side branches. In the tropics there are about 20 different growth forms, which are shown in the book by Halle, Oldeman and Tomlinson, which as I recall was published about 20 years ago. A few simple parameters like bud placement, branching angle, and apical dominance can be combined in many ways to make unique and recognizable growth patterns for thousands of tree species around the world. With respect to wind out analysis of MN blowdowns shows that tree species with narrow crowns on top of a tall trunk (forest grown aspen, black spruce and balsam fir) are the most susceptible. Trees that retain low branches (white cedar) and broad crowned species (maples and oaks) are less susceptible to blow down. Lee Windthrow Leverett, Robert Jun 25, 2003 12:04 PDT  Lee: I read an article a number of years ago in Scientific American that discussed the geometry of tuliptree leaves in heavy winds. As I recall the species does a remarkable thing by curling its leaves into cylinders that allow wind to blow past and through, thus minimizing wind resistance. I wish I had saved the article. It made quite an impression. The storm system you described sounded scary if not downright terrifying. I have been in a few of those mid-western storm systems that produced lightening flashes saturating the atmosphere. They are awesome and serve to remind us that natural events still rule. Given the frequency and intensity of storms in Minneapolis, how do they affect the awareness and feelings of ordinary citizens toward nature/natural events relative to say Philadelphia, Atlanta, or Los Angeles?    Bob Windthrow Lee E. Frelich Jun 25, 2003 12:45 PDT  Bob: A lot of trees streamline their leaves during high winds (i.e. the blade is parallel to the wind, presenting a tiny surface area). I have not heard of trees rolling up their leaves, but then there is so much to know about trees that I have barely started to acquire knowledge. Regarding people's attitude towards natural events, you may have noticed that no one was killed, and people are seldom killed here even though we have violent weather. That's because people here are very aware of weather safety, which is taught in the schools, and are always ready. Buildings and infrastructure are engineered to handle extreme events, so we have relatively little damage, and people just sit back and enjoy the storms. People in Minnesota are pretty strongly connected to nature and like to see all the variety it produces, or they wouldn't live here. People from the two coasts (and especially LA) don't think of weather as something that could kill them at any moment. My neighborhood always has lots of visitors from more benign climates, and it is fun to watch them look around and wonder why they are suddenly the only ones left in the middle of the park when the sky turns black, or deep luminous green like it was last night, or the tornado siren sounds. After a few minutes (but just before the tornado arrives) one of us will go out and tell any panicking CA residents what to do (i.e. stop gathering under large open grown trees and go in a building). Lee Windthrow Paul Jost Jun 25, 2003 13:10 PDT  Lee, Bob, I'm just hypothesizing here, but I'll give it a try: A "flat" leaf is never really flat which is probably a really bad thing when wind is concerned. Any slight curvature in the leaf while exposed to wind in a direction parallel to the approximate plane of the leaf (wind blowing at the edge of the leaf showing the smallest possible surface area to the wind) will cause it to act like a wing due to the pressure differences caused by moving air traveling different path lengths along the top and bottom surfaces of the leaf. Ooooh... nasty run-on sentence. Anyway, a slightly curved or nearly flat leaf will always flip around in the wind while a leaf that curls into a tube will align itself with the wind. The nearly flat leaf the flops around and periodically exposes it's surface to the full speed of the wind will likely get twisted or torn off while a leaf that forms a tube might survive by being more aerodynamically stable and properly oriented with minimal surface area to the wind at all times. I'm just guessing about this but it seems to make sense to me... Paul Jost Re: Snowfalls capture the record hunter's imagination   Lee E. Frelich   Jun 11, 2004 10:30 PDT  Bob: Our thunderstorms can produce derechos (clusters of several downbursts) with gusts of 150 mph and sustained winds of 120 mph. They can cover an area 10-15 miles wide and 150-200 miles in length. They reach the size and intensity of a category 3 or 4 hurricane, and cause comparable damage, although the spatial pattern of damage is different. It is probably several hundred years between such events at any one spot on the ground. A more typical severe thunderstorm that most places experience once a decade has winds 70-80 mph. Lee Home