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Old 03-24-2021, 09:44 PM
 
Location: Fort Worth, Texas
4,876 posts, read 4,177,263 times
Reputation: 1908

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Ok. So the purpose of this discussion thread is to see what other C-D weather enthusiasts feel about the influence of dry climates on even what are usually thought of as humid climate zones. For me personally, I think that the influence of semi arid climates begins in areas that border the prairie ecosystems, yes even the tall grass prairie ecosystem. So where do you personally feel that the influence of the semi arid climate zones begin? I would say that it starts in eastern Illinois at its eastern most limit or more likely west of the Mississippi River.Where would you draw the lines for slight, moderate or strong semi arid influences on the climate of the United States? Discuss
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Old 03-25-2021, 07:46 AM
 
1,965 posts, read 1,241,388 times
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If you really think about it, arid influences manifest across the entire country. I enjoy reading some of the older discussions that took place on this forum, and learned lots of interesting facts from them - but one that stood out to me was that it was almost impossible to avoid a relative humidity of ~30% or less, regardless of where you were in the country. And it's pretty obvious why.

Pure polar/arctic air is really desiccating and dry. Everyday people don't really think of it, because they see such airmasses in association with wintry precip events. But the air is practically bone dry, as can be seen how it creates deserts like the Gobi. The cold incursions of the continental US ensure that the native vegetation has to adapt to the sheer amount of moisture loss seen with the cold, dry air - whether its the complete winter deciduousness of mid-latitude forests in hotter, humid summer areas, or complete coniferous in the taiga where the colder air is more dominant throughout the year.

Even in the Southern US, notice also that many of the plant genuses - pines, oaks, etc - also are found in drier, med climate regions. And when you look closely at some of the species, like the iconic southern live oak (quercus virginiana), you find that the leaves actually have a somewhat sclerophyll character, which is an adaptation needed to restrict moisture loss. And since cold snaps in the South don't often come with winter precipitation, combined with the daytime warmups, the environments can really get the full force of aridity coming from these cold airmasses. And there's no escape in Florida, the warm winter dry season might actually amplify the seasonal aridity - hence the savannah-like environment seen in the Everglades, as well as the fact that it is one of very few areas east of the Mississippi (along with southern Georgia) where it is one of the few areas east of the Mississippi with aridity indices match or break the threshold.

The fact that you can find plant species like yucca, prickly pear, etc all the way to the Eastern Seaboard tells you all you need to know about the pervasiveness of arid influence across the US. It may not be as apparent in our current climate period, but there definitely might have been stronger aridity pulses in the past, as well as possible going into the future. It definitely won't surprise me to learn if areas of Florida were once drier savannah, woodland, etc, hence the remnant species like scrub jay where the closest relatives are all out in Western North America.

Last edited by kemahkami; 03-25-2021 at 08:00 AM..
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Old 03-25-2021, 01:13 PM
 
Location: Fort Worth, Texas
4,876 posts, read 4,177,263 times
Reputation: 1908
^^^^^^^^

Very useful and insightful information; very informative, thanks
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Old 03-25-2021, 02:36 PM
 
519 posts, read 472,433 times
Reputation: 295
Quote:
Originally Posted by ScrappyJoe View Post
If you really think about it, arid influences manifest across the entire country. I enjoy reading some of the older discussions that took place on this forum, and learned lots of interesting facts from them - but one that stood out to me was that it was almost impossible to avoid a relative humidity of ~30% or less, regardless of where you were in the country. And it's pretty obvious why.

Pure polar/arctic air is really desiccating and dry. Everyday people don't really think of it, because they see such airmasses in association with wintry precip events. But the air is practically bone dry, as can be seen how it creates deserts like the Gobi. The cold incursions of the continental US ensure that the native vegetation has to adapt to the sheer amount of moisture loss seen with the cold, dry air - whether its the complete winter deciduousness of mid-latitude forests in hotter, humid summer areas, or complete coniferous in the taiga where the colder air is more dominant throughout the year.

Even in the Southern US, notice also that many of the plant genuses - pines, oaks, etc - also are found in drier, med climate regions. And when you look closely at some of the species, like the iconic southern live oak (quercus virginiana), you find that the leaves actually have a somewhat sclerophyll character, which is an adaptation needed to restrict moisture loss. And since cold snaps in the South don't often come with winter precipitation, combined with the daytime warmups, the environments can really get the full force of aridity coming from these cold airmasses. And there's no escape in Florida, the warm winter dry season might actually amplify the seasonal aridity - hence the savannah-like environment seen in the Everglades, as well as the fact that it is one of very few areas east of the Mississippi (along with southern Georgia) where it is one of the few areas east of the Mississippi with aridity indices match or break the threshold.

The fact that you can find plant species like yucca, prickly pear, etc all the way to the Eastern Seaboard tells you all you need to know about the pervasiveness of arid influence across the US. It may not be as apparent in our current climate period, but there definitely might have been stronger aridity pulses in the past, as well as possible going into the future. It definitely won't surprise me to learn if areas of Florida were once drier savannah, woodland, etc, hence the remnant species like scrub jay where the closest relatives are all out in Western North America.
Humidity =/= rainfall (example- the Persian Gulf), but this is a very good post!
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Old 03-25-2021, 07:17 PM
 
1,965 posts, read 1,241,388 times
Reputation: 1589
Quote:
Originally Posted by psyche_da_mike24 View Post
Humidity =/= rainfall (example- the Persian Gulf), but this is a very good post!
I know that. What I meant is that people in the US often don't think of the dryness associated with the wintertime cold air-masses, due to how they occur - in the wake of wintertime extratropical cyclones that bring ice, snow, and/or rain. It's especially noticeable in the South - people talk about damp, "humid cold" conditions in, say, South Louisiana, even though the airmass responsible came from the Arctic over land (i.e. modified continental polar/Arctic or cP/cA) rather than being a pure marine airmass (i.e. maritime polar, mP).

Regarding deserts in general, if the humidity/dewpoint is really high, it can actually counteract the evaporation! Hence the coastal fog deserts in South America, South Africa, etc. Even with hotter deserts like the Persian Gulf and Middle East, the high humidity assists in reducing evaporation - probably assists with coconuts in Oman, or the relatively high biodiversity of Socotra.
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