< Back
 |
An Exciting Bore: Atmospheric antics and gravity waves over Iowa
10/30/2007
|
|
|
The waves of an undular bore roll over Des Moines on October 3, 2007. [Credit: Iowa Mesonet Skycam]
|
|
On a seemingly normal, if damp and chilly, morning in early October 2007, the citizens of Des Moines, Iowa were treated to a rare and mysterious sight in the
grey skies above them. Rolling past from the west came a gang of thunderstorms, barreling by as if engaged in a giants' ski boat race overhead. Pushed out in
their wake were rolls of frothy moisture—except instead of trailing white rivulets of water, these storms created towering waves of air, giant gravity waves,
that rose and fell among the clouds and formed what's called an undular bore.
Undular bore waves resemble an unusual smaller scale phenomenon of water, called a tidal bore. Certain coastal rivers are prone to these due to peculiarities
of riverbed topography. Imagine a mini-tsunami: when an incoming tide rises rapidly in these waterways, the sudden influx of water pushes a wave forward that
becomes a wall of water several feet high.
Undular bores, on the other hand, are walls of air pushed outward and upward by a weather system, usually a storm. In Des Moines, the fast-moving
thunderstorms slammed into a mass of cold, stable air hunkered down over the city, forcing volumes of atmosphere upward. Gravity dictates that what comes up
must come down—even if what goes up is air—and so the air displaced by the gaggle of storms also crashed down, spawning gravity waves that became visible as
long, dark bands of clouds. Such waves can speed along at 10 to 50 miles per hour and stretch 5 miles between peaks, according to Tim Coleman of the National
Space Science and Technology Center in Huntsville, Alabama. On the ground, Des Moines residents felt the passage of these waves as strange winds that blew
strongly in one direction, then repeatedly spun around to blow in the opposite direction.
Winds blowing toward the radar appear green
and away from the radar appear red in this NEXRAD
image of four undular bore waves trailing thunderstorms
over Iowa. [NOAA/NASA]
This particular bore was created by thunderstorms, but undular bores can also spawn storms themselves, or even feed energy to minor tornadoes to turn them
into real engines of destruction. While witnessing an undular bore is a rare treat, they may not be all that uncommon. Coleman estimates that a bore might
pass over a given point in the United States about once a month. Look to the skies then, the next time you feel the approach of stormy weather. You may just
see something that's not boring at all.
—Jessa Forte Netting
Giant Atmospheric Waves Over Iowa (NASA press release) – watch movie
For Further Study
Middle-atmosphere dynamics, Tidal
bore, Storm, Thunderstorm, Wave(physics)
View all Images of the Week
|
|
|
