Download Video Transcript: Brief Tour of a Low Pressure System

Brief "Tour" of a Low Pressure System
We cover a lot of topics in this lesson. But they all come together into a conceptual model of
how high pressure systems and low pressure systems work. And so I want to take a tour of a
surface low pressure system and take a look at some of the different features so you can see how
things fit together with a real weather system on real weather maps.
We'll start at the surface here. This is a surface analysis from 12Z on February 13, 2014. There
was a strong area of surface low pressure along the East Coast, right along the North CarolinaVirginia border.
And this was a fairly significant storm. It dropped heavy snow through the middle Atlantic.
Some places got over two feet. Heavy snow up into New England with blizzard conditions. So
this is a pretty big storm during February, 2014.
And to the south of the storm, to the south of the surface low, we have this blue line with blue
triangles. That's the cold front signaling the leading edge of advancing colder air. Out ahead of it,
we tend to have a warmer, more humid air mass. Perhaps maritime tropical would be fairly
common. And behind it, we have an advancing colder air mass, commonly continental polar, or
even continental Arctic air mass behind the front arriving.
And we can see the temperature contrasts here. To the east of Florida, temperatures are in the
70s. Go back into Florida behind the front and we have temperatures actually in the 30s in
northern Florida, so much colder air arriving behind that cold front.
To the east of the low, the red line with the red semicircles, that's the warm front. That signals
advancing warmer air. To the north of that warm front, the colder air is retreating. And it's the
retreat of that cold air that actually allows the warm air to advance. So we have a maritime
tropical air mass, warm and humid, advancing, while the colder air mass, which is more dense,
retreats.
To the north of the warm front, we generally have overrunning, upward motion as the warm air
glides over the dense cool air near the surface. And that often results in a large area of layered
clouds, which is what we had here on our infrared satellite image from February 13. We see a big
area of layered clouds to the north of the warm front.
It's fairly common to start with very high, thin clouds, cirrus clouds, which thicken gradually into
cirrostratus, and then altostratus, and then down into stratus clouds, and even nimbostratus
clouds once steady precipitation begins north of the warm front. Meanwhile, along and just
ahead of the cold front, we have a thinner band of clouds, which is indicative of the showery
weather that typically accompanies cold frontal passages.
So in addition to what's going on at the surface, I also want to talk about what's going on at 500
millibars, because that's also a very important part of what strengthens the low. And it's the low's
development. So this is a 500 millibar analysis. Shows 500 millibar heights 12Z on February 13,
2014.
We can see a deep trough over the eastern United States. That represents an area with cold air
columns, which makes sense, because the cold front was off the coast here. So behind it, we have
a cold air mass. And we have this deep trough that goes along with those cold air columns.
While vorticity isn't analyzed on this map, I did put in, with a red X, where the vort max was
with the trough axis here so that you can see where it was. X is a common way to mark a vort
max, although sometimes, you don't get an X with a vort max and you have to actually examine
the vorticity field to find a vort max.
But note that the trough axis at 500 millibars is well west of the surface low. Surface low was
just off the coast here near where Virginia and North Carolina come together. So the trough was
well to the west. And that's a common orientation when a system is still strengthening, because
you get divergence to the east of the vort max.
And that divergence aloft helps to remove mass from air columns. And it helps reduce the weight
of the air columns. And the surface pressure decreases. And that really drives a system.
When you have strong, upper level divergence that is stronger than the low level convergence
around the low, the weight of air columns is reduced. And you get a strengthening low. So this
fact that the trough is west of the surface low is the common regime that you see when a surface
low is still strengthening.