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:
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.
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.
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?
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 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:
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.
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?
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
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
