What causes these "anti-microclimates"?

[General Sylvester D. Palm]

Recently I was looking at some USDA hardiness zone maps for Texas, Louisiana, and Mississippi and noticed something off about three different towns. These three towns that I have posted pictures of all seem to have a lower hardiness zone than the surrounding area which I would think should be the opposite. I thought that the asphalt and heat the buildings and roads create could potentially bump the hardiness zone up half a zone like most large cities. Why would these three towns be half a zone below the surrounding area?

The three towns in question are: Pearsall, TX - Baton Rouge, LA - Vicksburg, MS

 

[Chester B]

I bet it has to do with elevations.  Like low areas surround by higher ground where cold air funnels in and sits.  It could also be high elevation, temps drop the higher you go.

[mnorell]

They are not "anti-microclimates." They are examples of the poor quality of these maps created at great cost and with much fanfare by the "PRISM" group in Oregon...but went across like a lead balloon to anyone who tracks weather/climate. It was announced during the many years of their development that these would take into account local variations with incredible detail, etc. and, sadly, that just isn't reflected in the maps. Even small urbanized areas can create a marked difference in microclimate. I used to live in Natchez, Mississippi, in the downtown grid just a few blocks from the bluff overlooking the Mississippi River. The downtown area, except for a couple of air-drained cold-spots (mostly creeks/gullies/"bayous"), is a zone 9a, while the surrounding countryside is mostly 8b or even colder (I had a Davis weather station as did/do several others there, and tracked the data for years). One of the problems is that NOAA often uses airports as defining weather stations for metro areas. The Natchez airport is located north of town on a grassy plain more inland from the river, and is markedly colder than the downtown area.

The maps also don't seem to take local geography and air-drainage into account. One example that I regularly experienced when driving back and forth from Baton Rouge to Natchez was the journey up and down the Homochitto Ridge, about 20 miles south of Natchez. On a cold, still, clear night, you could leave Woodville (a town to the south of the ridge) and, while driving up and over that ridge near the settlement called Doloroso, the car-thermometer would climb roughly 10 degrees; and back down as one traversed on down the other side into the Homochitto River basin. Yet this is not reflected on this map. I know there are many more examples of this (and of course this and any other warm slopes or frost-pocket/river-course low spots could have easily been modeled/generated with a desktop computer using USGS topo data).

Likewise, Baton Rouge, which is a fairly large, sprawling city nowadays, does have a warmer microclimate than the surrounding area. This is especially marked in the downtown/Government St. and Garden District areas. In the latter, there are large numbers of Live Oaks sheltering the streets and homes, adding several additional degrees to the Urban Heat Island. Also, many areas of Louisiana have large bodies of water, sometimes lakes, sometimes rivers, sometimes just boggy, swampy zones that aren't readily visible on maps. And it is in general a quite rural (or small town) area, and that makes it tricky to estimate just how the microclimates come and go. (There are some South Louisiana folks here on this forum who could elaborate on that, I'm sure.) And again, the Baton Rouge Airport is located north of town in a somewhat colder area (and temps there would usually track within a degree or two of the Natchez downtown grid, despite being 75 miles south of Natchez). So I would assume they relied on the airport readings to either define or bias the zone definition for the city of Baton Rouge.

I can't speak to the issue of Vicksburg, but I would think it's relatively small downtown area would be an 8b. I remember in the past looking at the weather there, and NOAA used to use the airport between Vicksburg (which sits atop a bluff) and Tallulah, Louisiana (on lower, flat land...and I believe somewhat colder). So I suppose it's possible this may have biased Vicksburg down, but I never really did track it or  spend any time there, so can't speak to it directly.

And if you really want to get an earful, try asking some of the Tallahassee people on this forum how they feel about their entire city and surrounding rural area, which has a crazy assortment of microclimates, being painted with a broad "8b" brush by PRISM and the USDA...and also the western half and interior of peninsular Florida, where winter lows are surprisingly variable over short distances...in short, there are many, many shortcomings to using this map, and the best thing is for you to observe local weather stations on wunderground, and hopefully get a station of your own so you really know your situation.

And regardless of zones and microclimates, remember in the Deep South to subtract about 15-20 degrees from your zone when selecting large foundation trees, because every 20-50 years such temperatures are likely to occur, and I don't think anyone wants a monster tree to remove after a 1989-style freeze. In Natchez, even centuries-old live oaks there lost large limbs in the 4F low and lengthy ice-storm that Christmas. Many similar nightmares occurred throughout the south that year, including in Houston, Baton Rouge and New Orleans. 1835, 1899 and 1940 were rough analogs to that year, and there have been many others in between that were close.

[RedRabbit]

The other above posters are probably correct.

However, I’ll also mention in some cases it could be as simple as them using bad data. USDA’s maps can be pretty flawed.

[Chester B]

I agree, no one should takes these maps as 100% accurate, its just a guideline.  You'll see people on here post about how they are only one street from a half zone warmer.  That's simply not the case.

[General Sylvester D. Palm]

1 hour ago, mnorell said:

They are not "anti-microclimates." They are examples of the poor quality of these maps created at great cost and with much fanfare by the "PRISM" group in Oregon...but went across like a lead balloon to anyone who tracks weather/climate. It was announced during the many years of their development that these would take into account local variations with incredible detail, etc. and, sadly, that just isn't reflected in the maps. Even small urbanized areas can create a marked difference in microclimate. I used to live in Natchez, Mississippi, in the downtown grid just a few blocks from the bluff overlooking the Mississippi River. The downtown area, except for a couple of air-drained cold-spots (mostly creeks/gullies/"bayous"), is a zone 9a, while the surrounding countryside is mostly 8b or even colder (I had a Davis weather station as did/do several others there, and tracked the data for years). One of the problems is that NOAA often uses airports as defining weather stations for metro areas. The Natchez airport is located north of town on a grassy plain more inland from the river, and is markedly colder than the downtown area.

The maps also don't seem to take local geography and air-drainage into account. One example that I regularly experienced when driving back and forth from Baton Rouge to Natchez was the journey up and down the Homochitto Ridge, about 20 miles south of Natchez. On a cold, still, clear night, you could leave Woodville (a town to the south of the ridge) and, while driving up and over that ridge near the settlement called Doloroso, the car-thermometer would climb roughly 10 degrees; and back down as one traversed on down the other side into the Homochitto River basin. Yet this is not reflected on this map. I know there are many more examples of this (and of course this and any other warm slopes or frost-pocket/river-course low spots could have easily been modeled/generated with a desktop computer using USGS topo data).

Likewise, Baton Rouge, which is a fairly large, sprawling city nowadays, does have a warmer microclimate than the surrounding area. This is especially marked in the downtown/Government St. and Garden District areas. In the latter, there are large numbers of Live Oaks sheltering the streets and homes, adding several additional degrees to the Urban Heat Island. Also, many areas of Louisiana have large bodies of water, sometimes lakes, sometimes rivers, sometimes just boggy, swampy zones that aren't readily visible on maps. And it is in general a quite rural (or small town) area, and that makes it tricky to estimate just how the microclimates come and go. (There are some South Louisiana folks here on this forum who could elaborate on that, I'm sure.) And again, the Baton Rouge Airport is located north of town in a somewhat colder area (and temps there would usually track within a degree or two of the Natchez downtown grid, despite being 75 miles south of Natchez). So I would assume they relied on the airport readings to either define or bias the zone definition for the city of Baton Rouge.

I can't speak to the issue of Vicksburg, but I would think it's relatively small downtown area would be an 8b. I remember in the past looking at the weather there, and NOAA used to use the airport between Vicksburg (which sits atop a bluff) and Tallulah, Louisiana (on lower, flat land...and I believe somewhat colder). So I suppose it's possible this may have biased Vicksburg down, but I never really did track it or  spend any time there, so can't speak to it directly.

And if you really want to get an earful, try asking some of the Tallahassee people on this forum how they feel about their entire city and surrounding rural area, which has a crazy assortment of microclimates, being painted with a broad "8b" brush by PRISM and the USDA...and also the western half and interior of peninsular Florida, where winter lows are surprisingly variable over short distances...in short, there are many, many shortcomings to using this map, and the best thing is for you to observe local weather stations on wunderground, and hopefully get a station of your own so you really know your situation.

And regardless of zones and microclimates, remember in the Deep South to subtract about 15-20 degrees from your zone when selecting large foundation trees, because every 20-50 years such temperatures are likely to occur, and I don't think anyone wants a monster tree to remove after a 1989-style freeze. In Natchez, even centuries-old live oaks there lost large limbs in the 4F low and lengthy ice-storm that Christmas. Many similar nightmares occurred throughout the south that year, including in Houston, Baton Rouge and New Orleans. 1835, 1899 and 1940 were rough analogs to that year, and there have been many others in between that were close.

That was just what I needed, thanks for the in depth explanation. I figured that it had something to do with the creators of the map not getting correct data. It just didn't make any sense as to why those places would be colder.

[bubba]

Do not trust Maps or the data. What grows…