Following is an email sent to the Friends of Five Points listserve by Dr. John Knox, Josiah Meigs Distinguished Teaching Professor at UGA. Dr. Knox is a meteorologist.
Dr. Knox wanted you to know that "it is an off-the-cuff, from-memory explanation. I didn't refer to
the research literature while writing it, so it's not authoritative."
Dear Five Points friends,
Several people have contacted me in the past couple of days about a weather question.
As I understand it, the question is: why does it seem like storms
approach Athens from the west, split in two and go around the city, and
then sometimes rejoin once they are past Athens?
Thanks for asking!
As many of you know, I'm a meteorologist on the faculty at UGA. So here's my take on the question:
What you see is real--for once, this isn't a selective-memory
situation where you remember the one time it does happen and forget the
100 times it didn't.
The most likely reason for the examples that have been mentioned to me, with west-to-east-moving storms, is the
urban heat island (UHI) effect.
You will be proud to know that UGA professors Tom Mote and Marshall
Shepherd, and some of their graduate students, have been leaders in UHI
research for the past 20+ years. Along with other researchers around
the world, Tom and Marshall have detected and quantified
the impact that the bubble of hot air around cities on storms both
upwind and downwind of the cities. Go Dawgs!
Let me explain a little more about the UHI:
The urban heat island is an invisible bubble of hot air that is a dome
over cities. The bigger the city, the bigger the dome. It can go up
thousands of feet (but not all the way up). But even small cities--even
towns!--can have some UHI effects.
This bubble of hot air originates from both heat retention and heat
production in urban areas. Asphalt absorbs sunlight and gets really
hot; buildings put out heat; at night, the buildings block the asphalt
from cooling off to the sky, because you can't even
see the sky from the ground in a big city! So, as many of you know,
downtown Atlanta can be quite a bit warmer than, say, Peachtree City or
Lawrenceville on a lot of nights. The effect is also sometimes
noticeable during the day, but it's a noisier signal
(lots going on during the day). But, the bottom line is this: imagine
that there is a dome of warmer air, proportional to city size, over
every town and city larger than, oh, 10,000 people. Which puts Athens
firmly in the category of having a UHI. (You
can quantify the UHI at the ground in Athens; my son did so in middle
school for a science project! But the important thing here is to
conceive of the invisible-but-real dome of air
above Athens.)
OK, so now you're visualizing these bubble-domes over cities. These
domes are just air, but they have a kind of inertia/integrity to them.
They are invisible, but they are also a bit
immovable as well.
Which means: when smaller-scale weather systems approach these domes,
they don't just push the domes out of the way. Bigger systems do; a
cold front, a mid-latitude cyclone, a hurricane--they are big enough to
temporarily remove the UHI, before it regenerates
a day/night or a couple of days/nights after the big weather system
moves through. But thunderstorms are comparably sized to the UHI of
Athens, and frankly even smaller than the big UHI over Atlanta.
So, smaller-scale winds and weather systems like thunderstorms and
low-level winds perceive a UHI as a kind of invisible mountain.
What do wind and smaller, moving weather systems do when they encounter a mountain?
They either go over it, or go around it.
Those are the two choices. Can't go underground. Can't stop.
Going over a mountain, visible or invisible, means you fight
gravity. Gravity is a powerful force. That is not the Nature-preferred
option.
Going around is energetically much easier to do.
You may have now figured out where this argument is headed. Here's the
scenario and explanation for what y'all have reported seeing:
- Line of thunderstorms approaches Athens from the west.
- Know that thunderstorms are forming from rising air. This is important.
- Line starts encountering the west edge of the Athens urban heat island.
- The air with the line of thunderstorms splits and goes around the UHI, on the left and on the right.
- This splitting of the wind leads to surface-level divergence near
and in Athens. We teach our freshmen in Intro to Weather and Climate
that surface divergence = downward motion from above.
- The part of the line that moves through Athens still has rising
air, but it is counteracted to some extent by the sinking air due to
the surface divergence caused by the winds splitting around the UHI of
Athens. Result: weaker updrafts in the Athens-area
thunderstorms, which in turn weakens the thunderstorms in the Athens
area.
- Meanwhile, the splitting of the winds around Athens leads to surface-level
convergence on either side of Athens--in this scenario, that would
be on the far NW and far SW sides of the greater Athens area.
Surface-level convergence leads to additional
rising motion in the storms on the far NW and SW sides.
- Hence, the storms that are to the north and south of Athens not only are not fighting
against sinking air; they are getting an extra boost from the
surface convergence! Result: much stronger updrafts in the
thunderstorms on either side of Athens!
- Presto: as the line
approaches and moves through Athens, you see the approaching line weaken
magically in the Athens area and intensify on either side of it!
- Ironic
denoument: as the line of storms moves to the east edge of Athens, the
reverse happens. The split winds that wrap around the UHI come back
together on the east edge, causing surface
convergence. Which means that the storms that had weakened right
over Athens, suddenly fire back up and get more intense a county or two
to the east!
- To the untrained or trained observer alike, the
evolution of this situation makes Athens look like it has a magic spell
on storms that weakens them as the storms move into the area, and then
re-strengthens them just after they depart.
Does this sound like what y'all have observed? Pretty close?
In this scenario, I did not invoke anything about topography--elevation,
mountains, etc. That's because the change in topography from Atlanta
to Athens isn't huge. There is a drop in altitude from Atlanta, but I
am not aware that there are distinct
differences between north-of-Athens trajectories vs. south-of-Athens
trajectories vs. headed-for-Athens trajectories. I might be wrong. But
I think it's a harder argument that's less convincing than the sequence
I described above that's related to the UHI.
BUT...
This story changes if you are talking about storms and bad weather and
winds coming from the NORTHWEST. This is something that Tom, Marshall,
and I talk about regularly.
If you had an old-timey raised-topography map of north Georgia, and you
moved your finger from NW to SE (same angle every time) from north far
Georgia to a) locations south of Athens, b) Athens, and c) north of
Athens, your finger would tell you something.
It would tell you that there is some altitude loss ("downsloping") from
far north Georgia to, say, Madison, GA. For Athens and places to the
north of Athens, though, your finger would tell you there is a LOT MORE
downsloping of wind coming from far north
Georgia to NW Georgia from about Athens northward. A LOT MORE. (This
would be easy to explain in person, but is harder in words.)
Downsloping winds lead to surface divergence. Surface divergence adds a downward component to air--sinking air. Storms and precipitation and even clouds require
rising motion.
And so: the varying topographic features of far north Georgia result in a
die-out of storms, precipitation, and even sometimes clouds in
northeast Georgia in situations where the wind is blowing NW to SE. The
direction of the wind is critically important. This argument does not
hold at all for west-to-east winds, or east-to-west winds, or
south-to-north winds. Just NW-to-SE winds. Meteorologists call this a
"downsloping effect" or in the West a "rain shadow
effect." What we notice in Athens is that, often, a forecast of
thunderstorms for Athens due to NW-to-SE approaching thunderstorms turns
into a dud, a bad forecast. Areas to our south get more stormy weather
than we do. Have you ever noticed that? I've
even seen NE Georgia have a patch of clear skies due to topographic
downsloping with clouds surrounding NE GA, again in NW-to-SE winds.
This is real. Anyone else notice this?
In conclusion: both the invisible "topography" of the urban heat
island, and the very visible topography of far north Georgia, can and
do affect storms and precipitation in the Athens area. Both of these
effects tend to make us less stormy and a little
drier than surrounding parts of north Georgia, including Atlanta.
Atlanta has a bigger UHI, with detectable downwind effects ( UGA
scientists have published on it). But there's no corresponding
downslope for Atlanta--in fact, Atlanta is on a plateau,
so there's a bit of upsloping that at least doesn't hurt thunderstorm
development and may help it some.
I hope this answer helps. I'm not the expert on it, but this is how I
teach it to undergraduates and how I personally have observed it.
It's not obvious. Both my climatologist-wife and I were surprised at
the magnitude of the effects when we moved to Athens--especially the
mountain downsloping effect, and another mountain-driven effect known as
"the wedge" or "cold air damming." (But the wedge/cold
air damming are for another time.) Just know that you are not stupid for not fully understanding this; you
are observant to have picked up on this; these subjects are
research-grade topics; and though often non-experts will rely on
selective memory and make mountains out of molehills, in this particular
case y'all have correctly picked out a phenomenon that
is due to mountains and is not a molehill! (Rim shot.)
Thanks for reading, sorry for the length.
John Knox
Dr. John A. Knox
Josiah Meigs Distinguished Teaching Professor, Department of Geography
Undergraduate Coordinator, Atmospheric Sciences Program
Room 139, Geography/Geology Building
University of Georgia
Athens, GA 30602
