Meteorology: Understanding the Atmosphere            Ackerman and Knox



Wake lows

The passage of a dying thunderstorm can sometimes be the cause for high winds in summer. Behind a few storms, a small region of very low pressure called a wake low develops. It's called a "wake low" because it forms behind the path of, or in the wake of, the thunderstorm. The pressure gradient near a wake low is very strong-a large change in pressure over a short distance. Therefore, the air blows straight from high to low pressure. Since the storm is passing, this creates the unusual situation of high winds blowing toward, not away from, a thunderstorm that is dying and leaving your vicinity.

On June 30, 1993, a thunderstorm on the northern fringe of the storms causing the 1993 Upper Mississippi floods triggered a wake low that passed through Madison, Wisconsin. A remarkable change in pressure occurred in Madison during a few short hours early in the morning of the 30th: a drop of 16 millibars in less than 2 hours! Sleepy residents awoke to strong and sustained winds of up to 50 mph, even though the thunderstorm had already passed through the city and was diminishing. This is a reminder that strong winds are related to strong pressure gradients, no matter what the situation is. Here is the co-author's impressions of this particular windstorm in poem form:




                         Written Pentecost 1998,
                            Castelvecchio Pascoli, Tuscany

Awakened by wind
Wearywondering: do
Iowa floodstorms beckon?
No; fluent, it lacks
Spurt and pause, a
Gustless rush sustained
Unpunctuated by thunder.

The shades shimmy.  Stirring, I
Whisper to my scientist-spouse
"Geostrophic adjustment,"
Speaking tongues half-translated from the Greek,
Galilean variant,
Babble of a specialist.
A rare nimbus collapses, spawning
Gush into absence
Bursting headlong lest Earth
Spiral it askance,
Retrospective prophet of senescence.

Shade-trees supine, I shed linens and
Dark-drunkenly staggering in shag
To the slamshut dooró
Passing through soundless stumbling.  A
Severe weather statement scrolls
Assuring end to breeze
Unforeseen seconds hence,
Its source obscured, name
Stricken from public consumption.
Unstanched, the spewing sluiced past

Until sunrise, strewing
Streetslough and spiked squiggles on
Pressure traces to attest.
The tower gauge malfunctioned.
Story unspooled, peers share
Stitched-up passion, since
An eyewitness perspective
Simply isn't publishable.

Soulwhirring but wordless, I
Await a second wind.

© by John Knox