Windthrow Lee E. Frelich
Jun 24, 2003 17:03 PDT 


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!



Windthrow Lee E. Frelich
 Jun 25, 2003 11:11 PDT 


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.


Windthrow Lee E. Frelich
Jun 25, 2003 11:22 PDT 


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.


Windthrow Leverett, Robert
Jun 25, 2003 12:04 PDT 


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?   


Windthrow Lee E. Frelich
Jun 25, 2003 12:45 PDT 


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).

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 


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.