AOPA Pilot Magazine - October 2014 - 97

WEATHER WORDS

H

Isallobars. These are lines of
equal surface pressure change.
Forecasters locate stations having
rising or falling barometric/altimeter values in order to see where
low pressure is forming-and
where low pressure centers are
likely to move. Not surprisingly,
lows move to locations where surface pressures are already falling.
Here's a chart with isallobars showing the idea, with "R" signifying rising
pressures, and "F" indicating falling pressures.

into a low pressure center, the conditions
aloft have to be just right.
The triggering mechanism is most
often a trough aloft-a U-shaped pressure pattern that can be seen on constant
pressure charts at the 500-millibar (about
18,000 feet) or 300-millibar (about 30,000
feet) constant pressure charts. As air flows
around the bend at the apex of the "U" and
then makes its way northeastward, some
interesting things happen.
The upper atmosphere develops a
counterclockwise spin, or vorticity, and
the air downstream of this spin zone fans
out in a divergent pattern. What does this
mean at the surface? Well, if air is diverging
aloft, the air at the surface must converge
and rise. Add the spin from the vorticity
and you have a low pressure center at the
surface. Now the air masses around the
surface low begin to rotate and collide with
each other, creating cold and warm fronts.
Presto-there's your classic low pressure
model. And it's easily visible on a surface
analysis chart.
At a glance, you can deduce the weather
conditions within each sector of the model.
The parent low, marked by a prominent
"L," has the lowest pressures, the strongest lifting forces, and therefore some of
the worst weather.
To the south and southeast of the low
is the warm sector. That's where you'll find
the most unstable air, the warmest temperatures, highest humidities, and highest
density altitudes. To the northwest of the
typical low pressure model is the cold sector, with its colder air, highest pressures,
and lower humidities.

Between the cold and warm sectors, of
course, will be a cold front which marks
the boundary between them. Being denser
than the warmer air ahead of it, the cold
front wedges its air beneath that warmer
air, creating cumulus clouds and perhaps
lines and clusters of thunderstorms.
To the north of the warm sector and
east of the low pressure center is the
warm front. Here, less-dense, warmer
air rides up and over the colder air mass
ahead of it. The result is widespread stratus clouds, elevated convection in the
warmer months, and large swaths of icing
conditions in the winter months.
After a few days, our typical frontal
model loses its upper-air support. The low
aloft draws the surface low away from the
juncture of the surface cold and warm fronts,
and a stretch of low pressure-an occlusion-
extends to the north or northwest.
This extension is an occluded front,
which is also well-known for creating
adverse flying weather. Occlusions are signs
that the surface low pressure system is dying.
Soon, the surface low is detached from the
one aloft, and all that's left is the kind of
benign, stationary temperature boundary
that started the whole process to begin with.
That's the life cycle, in a nutshell.
All surface low pressure systems and
fronts aren't the same, of course, but this
description fills the bill for many of them.
See a trough aloft heading east? See surface pressures dropping ahead of it? It
shouldn't be a surprise to see what happens over the next few days.
AOPA
EMAIL tom.horne@aopa.org

www.aopa.org/pilot AOPA PILOT | 97

2015
ary 14--17,turday
Janu esday Sa
Wedn

Florida
Sebring, 444
863.655.6 -expo.com
n
rt-aviatio
www.spo

.
y. Buy
ry. Fl
T
http://www.sport-aviation-expo.com http://www.aopaadvertising.org http://www.aopaadvertising.org http://www.aopa.org/pilot
AOPA Pilot Magazine - October 2014

Table of Contents for the Digital Edition of AOPA Pilot Magazine - October 2014
