Occasional Thoughts on Climate Change

[tacoman25]

24 minutes ago, bluewave said:

If you look at the JJA 20 year trend from 1995 to 2024, then the high/low trend is nearly identical for both stations. The minimums are rising at a faster pace for both locations. Obviously, Phoenix gets the most attention since the actual temperatures are significantly higher. But the temperature increase at both locations has been the same even at significantly different altitudes.

https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/city/time-series/USW00003103/tmin/3/8/1995-2024?trend=true&trend_base=10&begtrendyear=1995&endtrendyear=2025&filter=true&filterType=binomial

1995-2024 temperature trend 

Phoenix……max…+0.8°F/Decade….min…+1.0°F/Decade

Flagstaff…..max…+0.7°F/Decade…min…..+.1.1°F/Decade

 

That's 30 years. And obviously the exact starting point matters...depending on if you start in 1970, 1980, 1995, 2005, 2010 you'll get differing trend numbers.

But again, July/August is the primary monsoon season (rarely does it start until late June/early July, sometimes not until mid July). Including June makes about as much since if not less than including September.

If I go as far back as the data allows (1933) and plot JJAS avg temp trend for Flagstaff, it's .3F/decade. For Phoenix, it's twice as much with .6F/decade. Same trend for JJA if you prefer that.

[donsutherland1]

21 minutes ago, GaWx said:

 Chris,

 Don and I have had discussions about the UHI portion of the warming at Phoenix over the last 30-40+ years due to the tremendous population growth. We agreed that the UHI portion isn’t at all insignificant if I’m recalling this correctly.

@donsutherland1

Yes, that’s correct. Phoenix has a pronounced UHI effect. One sees it particularly in the explosive increase in 90 or above low temperatures.

[tacoman25]

All trendlines aside, in practical, very recent terms:

- Phoenix was 3 degrees above their long term average in July 2025. Flagstaff was exactly average.

- Phoenix was a ridiculous 9 degrees above their long term mean in July 2023. That same month, Flagstaff was about 5 degrees above theirs.

- Phoenix hasn't had a June below their long term mean since 2009. Flagstaff's most recent was 2023.

- Phoenix's hottest year on record was 2024. Flagstaff's was 1981.

 

[tacoman25]

3 minutes ago, donsutherland1 said:

Yes, that’s correct. Phoenix has a pronounced UHI effect. One sees it particularly in the explosive increase in 90 or above low temperatures.

The last time Phoenix had an August with average low temps below their long term mean was 1990.

Flagstaff did it in 2017, 2016, and 2014. Along with a bunch of other years since 1990.

[bluewave]

13 minutes ago, tacoman25 said:

That's 30 years. And obviously the exact starting point matters...depending on if you start in 1970, 1980, 1995, 2005, 2010 you'll get differing trend numbers.

But again, July/August is the primary monsoon season (rarely does it start until late June/early July, sometimes not until mid July). Including June makes about as much since if not less than including September.

If I go as far back as the data allows (1933) and plot JJAS avg temp trend for Flagstaff, it's .3F/decade. For Phoenix, it's twice as much with .6F/decade. Same trend for JJA if you prefer that.

The 2020s has produced unprecedented summer warmth for both locations. Flagstaff and Phoenix both had their warmest summers on record in 2024 by a significant margin. All the 2020s summers have been in the top 10 for warmth which has never happened before.

Time Series Summary for Flagstaff Area, AZ (ThreadEx) 10 Warmest Summers dense rank sorting by temperature 
Click column heading to sort ascending, click again to sort descending.

1	2024	68.4	0
2	1981	66.6	0
3	2002	66.3	0
4	2025	66.2	13
5	2021	66.1	0
6	2020	65.8	0
-	2007	65.8	0
-	1974	65.8	0
-	1940	65.8	0
7	2018	65.6	0
8	2022	65.4	0
-	2008	65.4	0
-	1980	65.4	0
9	2012	65.3	0
-	1946	65.3	0
10	2023	65.2	0
-	2017	65.2	0
-	1996	65.2	0
-	1977	65.2	0

 

Time Series Summary for Phoenix Area, AZ (ThreadEx)10 Warmest Summers dense rank sorting by temperature  
Click column heading to sort ascending, click again to sort descending.

1	2024	98.9	0
2	2023	97.0	0
3	2025	96.7	13
-	2020	96.7	0
4	2015	95.1	0
-	2013	95.1	0
5	2019	94.9	0
-	2007	94.9	0
6	2011	94.8	0
-	2002	94.8	0
-	1981	94.8	0
7	2022	94.7	0
8	2017	94.6	0
-	2016	94.6	0
-	2003	94.6	0
9	2006	94.5	0
10	1989	94.4	0
-	1988	94.4	0

[rclab]

12 minutes ago, bluewave said:

The 2020s has produced unprecedented summer warmth for both locations. Flagstaff and Phoenix both had their warmest summers on record in 2024 by a significant margin. All the 2020s summers have been in the top 10 for warmth which has never happened before.

Time Series Summary for Flagstaff Area, AZ (ThreadEx) 10 Warmest Summers dense rank sorting by temperature 
Click column heading to sort ascending, click again to sort descending.

1	2024	68.4	0
2	1981	66.6	0
3	2002	66.3	0
4	2025	66.2	13
5	2021	66.1	0
6	2020	65.8	0
-	2007	65.8	0
-	1974	65.8	0
-	1940	65.8	0
7	2018	65.6	0
8	2022	65.4	0
-	2008	65.4	0
-	1980	65.4	0
9	2012	65.3	0
-	1946	65.3	0
10	2023	65.2	0
-	2017	65.2	0
-	1996	65.2	0
-	1977	65.2	0

 

Time Series Summary for Phoenix Area, AZ (ThreadEx)10 Warmest Summers dense rank sorting by temperature  
Click column heading to sort ascending, click again to sort descending.

1	2024	98.9	0
2	2023	97.0	0
3	2025	96.7	13
-	2020	96.7	0
4	2015	95.1	0
-	2013	95.1	0
5	2019	94.9	0
-	2007	94.9	0
6	2011	94.8	0
-	2002	94.8	0
-	1981	94.8	0
7	2022	94.7	0
8	2017	94.6	0
-	2016	94.6	0
-	2003	94.6	0
9	2006	94.5	0
10	1989	94.4	0
-	1988	94.4	0

Amazing BW ….. 3 of the top 5 occurred within the last 4 years. As always …..

[tacoman25]

43 minutes ago, bluewave said:

The 2020s has produced unprecedented summer warmth for both locations. Flagstaff and Phoenix both had their warmest summers on record in 2024 by a significant margin. All the 2020s summers have been in the top 10 for warmth which has never happened before.

Time Series Summary for Flagstaff Area, AZ (ThreadEx) 10 Warmest Summers dense rank sorting by temperature 
Click column heading to sort ascending, click again to sort descending.

1	2024	68.4	0
2	1981	66.6	0
3	2002	66.3	0
4	2025	66.2	13
5	2021	66.1	0
6	2020	65.8	0
-	2007	65.8	0
-	1974	65.8	0
-	1940	65.8	0
7	2018	65.6	0
8	2022	65.4	0
-	2008	65.4	0
-	1980	65.4	0
9	2012	65.3	0
-	1946	65.3	0
10	2023	65.2	0
-	2017	65.2	0
-	1996	65.2	0
-	1977	65.2	0

 

Time Series Summary for Phoenix Area, AZ (ThreadEx)10 Warmest Summers dense rank sorting by temperature  
Click column heading to sort ascending, click again to sort descending.

1	2024	98.9	0
2	2023	97.0	0
3	2025	96.7	13
-	2020	96.7	0
4	2015	95.1	0
-	2013	95.1	0
5	2019	94.9	0
-	2007	94.9	0
6	2011	94.8	0
-	2002	94.8	0
-	1981	94.8	0
7	2022	94.7	0
8	2017	94.6	0
-	2016	94.6	0
-	2003	94.6	0
9	2006	94.5	0
10	1989	94.4	0
-	1988	94.4	0

Right, no one is disputing it's been hot in Flagstaff. It just hasn't warmed as much as Phoenix, as those stats clearly show. It's a distinct difference.

- Two of Flagstaff's top 3 warmest summers happened more than 20 years ago. Meanwhile, those same two summers are tied for 6th hottest in Phoenix, and their three hottest are the last three summers.

- Flagstaff has 5 summers among their 10 warmest that occurred 2007 or earlier. Phoenix has 2.

- Flagstaff has a total of 7 20th century summers on their list. Phoenix has 3.

- Phoenix's hottest summer is more than 4 degrees warmer than their hottest 20th century summer. Flagstaff's is less than 2 degrees warmer.

[bluewave]

1 hour ago, GaWx said:

 Chris,

 Don and I have had discussions about the UHI portion of the warming at Phoenix over the last 30-40+ years due to the tremendous population growth. We agreed that the UHI portion isn’t at all insignificant if I’m recalling this correctly.

@donsutherland1

UHI acts as an amplifier of mostly the low temperatures in and around Phoenix leading to the rapid increase in 80° minimums there. But the rapid increase in 100°+ maxes is evenly distributed across, urban, suburban, and rural locations. It’s the rapid warming of the planet which is driving the increasing intensity of the heat.

So the UHI acts to keep the urban centers warmer at night than the outlying areas. But both areas as seeing steep increases in the minimum and maximum temperatures. So without the background warming of the climate, the UHI alone wouldn’t be nearly as significant. 

 

[Typhoon Tip]

15 hours ago, bluewave said:

UHI acts as an amplifier of mostly the low temperatures in and around Phoenix leading to the rapid increase in 80° minimums there. But the rapid increase in 100°+ maxes is evenly distributed across, urban, suburban, and rural locations. It’s the rapid warming of the planet which is driving the increasing intensity of the heat.

So the UHI acts to keep the urban centers warmer at night than the outlying areas. But both areas as seeing steep increases in the minimum and maximum temperatures. So without the background warming of the climate, the UHI alone wouldn’t be nearly as significant. 

 

It just gets exhausting trying to explain this to the people that can’t synthesize global perspectives; probably as a native intellectual limitation. Which unfortunately is precisely what is needed if somebody’s going to understand how global warming works  

Dimes to donuts the majority of the people in the denier frame of mind are narrow perspective types

[chubbs]

12 hours ago, Typhoon Tip said:

It just gets exhausting trying to explain this to the people that can’t synthesize global perspectives; probably as a native intellectual limitation. Which unfortunately is precisely what is needed if somebody’s going to understand how global warming works  

Dimes to donuts the majority of the people in the denier frame of mind are narrow perspective types

Yes, UHI and global warming are both happening. The presence of one doesn't diminish the other. This thread/board is littered with whataboutism. What about this, what about that. Many things can be true at the same time. That doesn't change the big picture. Scientists from the 1970s would not be surprised by the warmer world we have today or that Phoenix has a heat island.

[FPizz]

12 hours ago, Typhoon Tip said:

It just gets exhausting trying to explain this to the people that can’t synthesize global perspectives; probably as a native intellectual limitation. Which unfortunately is precisely what is needed if somebody’s going to understand how global warming works  

Dimes to donuts the majority of the people in the denier frame of mind are narrow perspective types

How come if lows are higher, that doesn't make highs also higher since the starting point for the day is already a higher temp?

When I see reports of some cities around the world planting more trees in cities and then saying high temps went down by 5 degrees during the day for example, is that not showing that UHI also effects the high temps?  

Not denying anything, but just wondering.  Especially about the 2nd question because I see that talked about on many "green" type forums and how many cities should adopt that to lower daytime temps (obviously because of more shade).  

[Typhoon Tip]

3 hours ago, FPizz said:

How come if lows are higher, that doesn't make highs also higher since the starting point for the day is already a higher temp?

When I see reports of some cities around the world planting more trees in cities and then saying high temps went down by 5 degrees during the day for example, is that not showing that UHI also effects the high temps?  

Not denying anything, but just wondering.  Especially about the 2nd question because I see that talked about on many "green" type forums and how many cities should adopt that to lower daytime temps (obviously because of more shade).  

anyway... the bold is a good question.  There's a lot of complexity in there but the simplest explanation is water vapor.   When there is more water vapor that has been heated, the total atmosphere holds more energy that way - needs more energy to keep water in gaseous form.  That keeps the temperature up at night.  However, in the day, it can also hold the temp down some, because it takes more energy to heat WV than dry air.  The complexity is the relativity of those two.  It's still 100/72 on July 10 in Boston ...and that's enough to roast one's nuts.. but, a profile of 104/66 has about the same thermodynamic quotient.  

Ha ha ... wasn't intending to be all guilty like.  It's really more a frustration pointed at the general circumstance.   It's a numbers game.   Let's think about this logically for a minute.  There are 8+ billion human brains walking the Earth.  What are the odds that 100% of them are completely capable of objectively intellectualizing CC, and then responding to it with a cogent sanity that is reflected in their response to it and behaviors after the fact?   

The answer is 0%   .... In other words, there is 0% chance that 100% will get it.  

So that leaves us in a predicament that some portion of 8+ billion are incapable of getting it.   What is that number?    If it is even 10 percent, we're still  talking 800,000,000+ brains powered by carbon fartin' industrialized environmental destruction. This predisposition leads to reticence to the concepts and acceptance of CC caused by carbon fartin', less likely to react proactively in prevention.  And, 800,000,000+ is still enough people that even if the remaining 7+ billion were to completely accept and adapt their technology and principles of living around preventing anthropomorphic GW, we're all still doomed...  They are doomed because 800,000,000+ is enough to ensure environmental collapse still takes place in spite of everyone else. It's kind of a scary untenable scenario really.

 

[FPizz]

1 hour ago, Typhoon Tip said:

anyway... the bold is a good question.  There's a lot of complexity in there but the simplest explanation is water vapor.   When there is more water vapor that has been heated, the total atmosphere holds more energy that way - needs more energy to keep water in gaseous form.  That keeps the temperature up at night.  However, in the day, it can also hold the temp down some, because it takes more energy to heat WV than dry air.  The complexity is the relativity of those two.  It's still 100/72 on July 10 in Boston ...and that's enough to roast one's nuts.. but, a profile of 104/66 has about the same thermodynamic quotient.  

Ha ha ... wasn't intending to be all guilty like.  It's really more a frustration pointed at the general circumstance.   It's a numbers game.   Let's think about this logically for a minute.  There are 8+ billion human brains walking the Earth.  What are the odds that 100% of them are completely capable of objectively intellectualizing CC, and then responding to it with a cogent sanity that is reflected in their response to it and behaviors after the fact?   

The answer is 0%   .... In other words, there is 0% chance that 100% will get it.  

So that leaves us in a predicament that some portion of 8+ billion are incapable of getting it.   What is that number?    If it is even 10 percent, we're still  talking 800,000,000+ brains powered by carbon fartin' industrialized environmental destruction. This predisposition leads to reticence to the concepts and acceptance of CC caused by carbon fartin', less likely to react proactively in prevention.  And, 800,000,000+ is still enough people that even if the remaining 7+ billion were to completely accept and adapt their technology and principles of living around preventing anthropomorphic GW, we're all still doomed...  They are doomed because 800,000,000+ is enough to ensure environmental collapse still takes place in spite of everyone else. It's kind of a scary untenable scenario really.

 

Thanks, I figured it was something like that.  It is pretty amazing how that happens that it can even out for the high temps.  

Something like this I picture see posted often in numerous ways showing the difference, in a city setting,  how having trees lowers temps everywhere from the air to the ground we walk on (and has been done around the world proving it works).  Wouldn't this be an example of UHI during the day?  Or, would we say the temps on the lower picture are not accurate for the air since it is under a canopy of trees?  So the lower picture is like Central Park temps which people in my forum argue about every day of the week and the top picture is actually the accurate one for temps?  Here is where this pic was from https://symsoil.medium.com/trees-climate-change-and-community-878280498546   I see this movement talked about often though for cities and it would be nice if adopted by many more.  

  

[TheClimateChanger]

5 minutes ago, FPizz said:

Thanks, I figured it was something like that.  It is pretty amazing how that happens that it can even out for the high temps.  

Something like this I picture see posted often in numerous ways showing the difference, in a city setting,  how having trees lowers temps everywhere from the air to the ground we walk on (and has been done around the world proving it works).  Wouldn't this be an example of UHI during the day?  Or, would we say the temps on the lower picture are not accurate for the air since it is under a canopy of trees?  So the lower picture is like Central Park temps which people in my forum argue about every day of the week and the top picture is actually the accurate one for temps?  Here is where this pic was from https://symsoil.medium.com/trees-climate-change-and-community-878280498546   I see this movement talked about often though for cities and it would be nice if adopted by many more.  

  

That's a stupid graphic measuring surface temperature, not near surface air temperatures. Certainly, being shaded can cool the air temperature as well [not to that extent], but surface air temperatures are supposed to be observed in a location free from shading and obstructions.

[TheClimateChanger]

2 hours ago, Typhoon Tip said:

anyway... the bold is a good question.  There's a lot of complexity in there but the simplest explanation is water vapor.   When there is more water vapor that has been heated, the total atmosphere holds more energy that way - needs more energy to keep water in gaseous form.  That keeps the temperature up at night.  However, in the day, it can also hold the temp down some, because it takes more energy to heat WV than dry air.  The complexity is the relativity of those two.  It's still 100/72 on July 10 in Boston ...and that's enough to roast one's nuts.. but, a profile of 104/66 has about the same thermodynamic quotient.  

Ha ha ... wasn't intending to be all guilty like.  It's really more a frustration pointed at the general circumstance.   It's a numbers game.   Let's think about this logically for a minute.  There are 8+ billion human brains walking the Earth.  What are the odds that 100% of them are completely capable of objectively intellectualizing CC, and then responding to it with a cogent sanity that is reflected in their response to it and behaviors after the fact?   

The answer is 0%   .... In other words, there is 0% chance that 100% will get it.  

So that leaves us in a predicament that some portion of 8+ billion are incapable of getting it.   What is that number?    If it is even 10 percent, we're still  talking 800,000,000+ brains powered by carbon fartin' industrialized environmental destruction. This predisposition leads to reticence to the concepts and acceptance of CC caused by carbon fartin', less likely to react proactively in prevention.  And, 800,000,000+ is still enough people that even if the remaining 7+ billion were to completely accept and adapt their technology and principles of living around preventing anthropomorphic GW, we're all still doomed...  They are doomed because 800,000,000+ is enough to ensure environmental collapse still takes place in spite of everyone else. It's kind of a scary untenable scenario really.

 

Excellent post, Tip. The part I emboldened in is an excellent point.  One I have made numerous times. When you factor that into the equation, there is NO central US warming hole. It disappears. Current summers FEEL much hotter than summers of the past, including the Dust Bowl, which were the driest (least humid) on record. And while some may say "heat index" is subjective, it's really just a proxy for the wet bulb temperature reading. And that can, in fact, become deadly, even at relatively modest dry bulb temperatures with full (or near full) saturation.

Anyways, I often see certain people who deny (some people on here get offended with the term "denier" even though they have used that term profusely in the past, but I digress)... people who deny climate change often say highs are of more value, because lows are more affected by UHI. I would actually argue the exact opposite, and say high temperature readings are more impacted by siting/exposure characteristics and equipment, and low temperatures give us a better look at actual trends.

[TheClimateChanger]

17 minutes ago, TheClimateChanger said:

Excellent post, Tip. The part I emboldened in is an excellent point.  One I have made numerous times. When you factor that into the equation, there is NO central US warming hole. It disappears. Current summers FEEL much hotter than summers of the past, including the Dust Bowl, which were the driest (least humid) on record. And while some may say "heat index" is subjective, it's really just a proxy for the wet bulb temperature reading. And that can, in fact, become deadly, even at relatively modest dry bulb temperatures with full (or near full) saturation.

Anyways, I often see certain people who deny (some people on here get offended with the term "denier" even though they have used that term profusely in the past, but I digress)... people who deny climate change often say highs are of more value, because lows are more affected by UHI. I would actually argue the exact opposite, and say high temperature readings are more impacted by siting/exposure characteristics and equipment, and low temperatures give us a better look at actual trends.

Speaking of lows:

 

[FPizz]

59 minutes ago, TheClimateChanger said:

That's a stupid graphic measuring surface temperature, not near surface air temperatures. Certainly, being shaded can cool the air temperature as well [not to that extent], but surface air temperatures are supposed to be observed in a location free from shading and obstructions.

That shows near surface too (the top temperature on it) and there are a million examples online from sites you probably whack to.  I will wait for Tips answer since he is actually smart and has the credentials and you really didn't answer anything at all.

[TheClimateChanger]

25 minutes ago, FPizz said:

That shows near surface too (the top temperature on it) and there are a million examples online from sites you probably whack to.  I will wait for Tips answer since he is actually smart and has the credentials and you really didn't answer anything at all.

Ok, well, I don't believe that. It's not 10C (18F!) warmer in a sunny neighborhood versus a shady one if the thermometer is properly sheltered or aspirated. My car thermometer doesn't register anywhere near that difference and it's housed like a foot or so above the asphalt.

[Typhoon Tip]

4 hours ago, FPizz said:

Thanks, I figured it was something like that.  It is pretty amazing how that happens that it can even out for the high temps.  

Something like this I picture see posted often in numerous ways showing the difference, in a city setting,  how having trees lowers temps everywhere from the air to the ground we walk on (and has been done around the world proving it works).  Wouldn't this be an example of UHI during the day?  Or, would we say the temps on the lower picture are not accurate for the air since it is under a canopy of trees?  So the lower picture is like Central Park temps which people in my forum argue about every day of the week and the top picture is actually the accurate one for temps?  Here is where this pic was from https://symsoil.medium.com/trees-climate-change-and-community-878280498546   I see this movement talked about often though for cities and it would be nice if adopted by many more.  

  

 

The idea of tree and/or vegetation modulation on heat is again water vapor related.   There's also a component of straight shade helping to block the sun.  These two factors are important in that whole idea.   

Vegetation "sweats."  It's called transpiration when it sources from plants. Water evaporates from that source, and that lowers the temperature.  Although that does add water to the atmosphere in the form of vapor, but you'll end up with slightly elevated DPs in the vicinity of vegetation, with slightly lowered temps.   As that is ongoing, the cooler resulting air is drawn downward because of course cooler air is denser.  This principle is illustrated well in the bottom photo - although I can't attest to those actual numbers, but in so far as getting the point across.. that's the idea there. Contrasting, the upper photo doesn't have this process. 

This is a local cooling effect, however.  The temperatures in those respective schematics are in the micro meteorology.  Which, per course ... what one experiences in a city/urban setting on a hot day is in fact within a micro-meteorological realm.  That would be neighborhood vs neighborhood... Unless the given cityscape is pervasively and comprehensively forested and so forth, it calls into questions the efficacy of offsetting a UHI effects.  The collection of sun-exposed concrete surfaces, such as downtown, parking lots... the edifices of the buildings too, to mention the heat output from industrial scaled AC exhaust.   It's a tricky math.   Lot of summing multiple separate integrals in that calculus.

My personal belief is that the UHI capacity goes up with warming CC, and goes back down with cooling CC.  I suspect the growth may not be entirely linear, either ( as in going up together).  Because different materials have different storing capacitance ... so that changes the contribution into the "UHI bubble" based on different energy source and sinks.   It's a coupling of material sciences, with atmospheric science, which is not likely trivial.  But intuitively, doesn't seem like that's a swift calculation to make. Just hypothetically... a sunny day in Phoenix AZ on July 10, 1972 may calculated a UHI factor of some 15F over the surrounding country, but that may be closer to 20F in 2025 given the same initial conditions and sunny day.   See, the climate is 1C warmer, but there is a disproportionately larger UHI response.    

 

 

[FPizz]

53 minutes ago, Typhoon Tip said:

 

The idea of tree and/or vegetation modulation on heat is again water vapor related.   There's also a component of straight shade helping to block the sun.  These two factors are important in that whole idea.   

Vegetation "sweats."  It's called transpiration when it sources from from plants. Water evaporates from that source, and that lowers the temperature.  Although that does add water to the atmosphere in the form of vapor, but you'll end up with slightly elevated DPs in the vicinity of vegetation, with slightly lowered temps.   As that is ongoing, the cooler resulting air is drawn downward because of course cooler air is denser.  This principle is illustrated well in the bottom photo - although I can't attest to those actual numbers, but in so far as getting the point across.. that's the idea there. Contrasting, the upper photo doesn't have this process. 

This is a local cooling effect, however.  The temperatures in those respective schematics are in the micro meteorology.  Which, per course ... what one experiences in a city/urban setting on a hot day is in fact within a micro-meteorological realm.  That would be neighborhood vs neighborhood... Unless the given cityscape is pervasively and comprehensively forested and so forth, it calls into questions the efficacy of offsetting a UHI effects.  The collection of sun-exposed concrete surfaces, such as downtown, parking lots... the edifices of the buildings too, to mention the heat output from industrial scaled AC exhaust.   It's a tricky math.   Lot of summing multiple separate integrals in that calculus.

My personal belief is that the UHI capacity goes up with warming CC, and goes back down with cooling CC.  I suspect the growth may not be entirely linear, either ( as in going up together.  Because different materials have different storing capacitance ... so that changes the contribution into the "UHI bubble" based on different energy source and sinks.   It's a coupling of material sciences, with atmospheric science, which is not likely trivial.  But intuitively, doesn't seem like that's a swift calculation to make. Just hypothetically... a sunny day in Phoenix AZ on July 10, 1972 may calculated a UHI factor of some 15F over the surrounding country, but that may be closer to 20F in 2025 given the same initial conditions and sunny day.   See, the climate is 1C warmer, but there is a disproportionately larger UHI response.    

 

 

Thank you for this well thought out response.  Makes sense to me.  Appreciate it.  Micro-meteorological is a good term for a city landscape.

[bluewave]

The summer JJA UHI effect in the Arizona South Central climate division is really pronounced for low temperatures at the Phoenix Sky Harbor International Airport. But the high temperatures there are fairly similar to the surrounding stations.

The data below tracks the warmest Phoenix summers during the 1980s, 2000s, and 2020s. Unfortunately, several stations have left the network and new ones have joined.

So several outlying stations don’t have a continuous record since 1981. This makes it difficult for a precise measurement of the UHI change since 1981. 

The one common denominator has been a steady rise in both the low and high temperatures for Arizona stations with a long enough period of record for comparison. 

 

AVG Max Temp Data for June 1, 2024 through August 31, 2024 Warmest Summer On Record 
Click column heading to sort ascending, click again to sort descending.

PHOENIX AIRPORT	WBAN	110.4
BUCKEYE 5N	COOP	110.2
YOUNGTOWN	COOP	110.0
GILA BEND 2SE	COOP	109.9
ROBSON RANCH	COOP	109.8
TEMPE ASU	COOP	109.4
EAST MESA	COOP	108.9
BARTLETT DAM	COOP	108.6
CASA GRANDE	COOP	108.3
CASA GRANDE NATL MONUMENT	COOP	108.0
APACHE JUNCTION 5 NE	COOP	107.6
MORMON FLAT	COOP	107.0
MESA FALCON FIELD	WBAN	106.8
SCOTTSDALE MUNICIPAL AP	WBAN	106.8
CASA GRANDE MUNICIPAL AP	WBAN	106.5
PHOENIX DEER VALLEY MUNICIPAL AP	WBAN	106.4
STEWART MOUNTAIN DAM	COOP	106.1
FOUNTAIN HILLS	COOP	106.0
PICACHO 8 SE	COOP	105.8
CAREFREE	COOP	105.6
BOYCE THOMPSON ARBORETUM	COOP	104.0
PINNACLE PEAK	COOP	102.0

 

AVG Min Temp Data for June 1, 2024 through August 31, 2024
Click column heading to sort ascending, click again to sort descending.

PHOENIX AIRPORT	WBAN	87.5
MESA FALCON FIELD	WBAN	84.3
SCOTTSDALE MUNICIPAL AP	WBAN	84.1
YOUNGTOWN	COOP	83.3
PHOENIX DEER VALLEY MUNICIPAL AP	WBAN	82.0
MORMON FLAT	COOP	81.6
GILA BEND 2SE	COOP	81.5
EAST MESA	COOP	81.4
BUCKEYE 5N	COOP	81.3
FOUNTAIN HILLS	COOP	80.2
CASA GRANDE MUNICIPAL AP	WBAN	79.7
APACHE JUNCTION 5 NE	COOP	78.9
PICACHO 8 SE	COOP	78.9
CASA GRANDE	COOP	78.7
PINNACLE PEAK	COOP	78.5
CAREFREE	COOP	77.7
TEMPE ASU	COOP	77.6
BOYCE THOMPSON ARBORETUM	COOP	77.6
CASA GRANDE NATL MONUMENT	COOP	76.8
ROBSON RANCH	COOP	76.2

 

AVG Max Temp Data for June 1, 2007 through August 31, 2007 Warmest Summer 2000s
Click column heading to sort ascending, click again to sort descending.

GILA BEND 2SE	COOP	108.2
LAVEEN 3 SSE	COOP	108.1
MARICOPA 4 N	COOP	107.5
TONOPAH	COOP	107.2
BARTLETT DAM	COOP	107.0
PAINTED ROCK DAM	COOP	106.7
CASA GRANDE	COOP	106.3
PHOENIX AIRPORT	WBAN	106.3
EAST MESA	COOP	106.0
YOUNGTOWN	COOP	105.5
TEMPE ASU	COOP	105.2
ARIZONA CITY	COOP	105.2
SACATON	COOP	104.9
CASA GRANDE NATL MONUMENT	COOP	104.7
STEWART MOUNTAIN DAM	COOP	104.7
LITCHFIELD PARK	COOP	104.4
PICACHO 8 SE	COOP	104.1
FLORENCE	COOP	104.0
APACHE JUNCTION 5 NE	COOP	104.0
MORMON FLAT	COOP	103.8
FOUNTAIN HILLS	COOP	103.6
PHOENIX DEER VALLEY MUNICIPAL AP	WBAN	103.6
SCOTTSDALE MUNICIPAL AP	WBAN	103.5
CAREFREE	COOP	102.9
WICKENBURG	COOP	102.6
AGUILA	COOP	102.3
WITTMANN 1SE	COOP	102.0

 

AVG Min Temp Data for June 1, 2007 through August 31, 2007
Click column heading to sort ascending, click again to sort descending.

PHOENIX AIRPORT	WBAN	83.6
SCOTTSDALE MUNICIPAL AP	WBAN	81.0
MORMON FLAT	COOP	80.5
PHOENIX DEER VALLEY MUNICIPAL AP	WBAN	80.0
YOUNGTOWN	COOP	79.5
LITCHFIELD PARK	COOP	79.3
GILA BEND 2SE	COOP	78.7
PAINTED ROCK DAM	COOP	77.9
FOUNTAIN HILLS	COOP	77.8
EAST MESA	COOP	77.0
PICACHO 8 SE	COOP	76.7
PINNACLE PEAK	COOP	76.4
MARICOPA 4 N	COOP	76.0
CAREFREE	COOP	76.0
FLORENCE	COOP	75.9
TONOPAH	COOP	75.6
STEWART MOUNTAIN DAM	COOP	75.5
APACHE JUNCTION 5 NE	COOP	75.1
TEMPE ASU	COOP	75.1
CASA GRANDE NATL MONUMENT	COOP	75.0
CASA GRANDE	COOP	74.9
WITTMANN 1SE	COOP	74.9
ARIZONA CITY	COOP	74.3
SACATON	COOP	74.2
BARTLETT DAM	COOP	74.2
HORSE CAMP CANYON ARIZONA	RAWS	71.1
WICKENBURG	COOP	70.7

 

 

AVG Max Temp Data for June 1, 1981 through August 31, 1981
Click column heading to sort ascending, click again to sort descending.

GILA BEND 2SE	COOP	110.3
BUCKEYE	COOP	109.9
LITCHFIELD PARK	COOP	108.6
CASA GRANDE	COOP	108.0
CASA GRANDE NATL MONUMENT	COOP	107.9
SACATON	COOP	107.6
TONOPAH	COOP	107.2
PHOENIX CITY	WBAN	106.9
MARICOPA 4 N	COOP	106.9
FOUNTAIN HILLS	COOP	106.8
YOUNGTOWN	COOP	106.7
FLORENCE	COOP	106.5
PHOENIX AIRPORT	WBAN	106.1
MESA	COOP	105.8
TEMPE	COOP	105.7
CHANDLER HEIGHTS	COOP	105.3
SCOTTSDALE	COOP	105.1
BARTLETT DAM	COOP	104.7
DEER VALLEY	COOP	104.5
STEWART MOUNTAIN DAM	COOP	103.8
ELOY 4 NE	COOP	103.8
LAVEEN 3 SSE	COOP	103.7
MORMON FLAT	COOP	103.3
WICKENBURG	COOP	103.2
SOUTH PHOENIX	COOP	102.4
AGUILA	COOP	101.6

 

 

AVG Min Temp Data for June 1, 1981 through August 31, 1981
Click column heading to sort ascending, click again to sort descending.

PHOENIX AIRPORT	WBAN	83.4
PHOENIX CITY	WBAN	81.9
GILA BEND 2SE	COOP	78.9
PHOENIX SOUTH MOUNTAIN	COOP	78.9
LAVEEN 3 SSE	COOP	78.1
FOUNTAIN HILLS	COOP	77.5
YOUNGTOWN	COOP	77.5
TONOPAH	COOP	77.2
DEER VALLEY	COOP	77.1
LITCHFIELD PARK	COOP	76.9
MORMON FLAT	COOP	76.6
STEWART MOUNTAIN DAM	COOP	76.2
BARTLETT DAM	COOP	75.8
MESA	COOP	75.5
SUPERIOR	COOP	75.1
MARICOPA 4 N	COOP	75.0
BUCKEYE	COOP	74.8
FLORENCE	COOP	74.7
CHANDLER HEIGHTS	COOP	74.6
TEMPE	COOP	74.5
CASA GRANDE NATL MONUMENT	COOP	74.3
ELOY 4 NE	COOP	74.2
CASA GRANDE	COOP	73.2
SOUTH PHOENIX	COOP	73.1
AGUILA	COOP	71.1
SCOTTSDALE	COOP	70.3

 

 

 

 

[donsutherland1]

Phoenix is on track to record its fourth hottest summer on record. All four will have occurred since 2020. The ranking will be as follows:

1. 2024
2. 2023
3. 2020
4. 2025

 

[tacoman25]

On 8/19/2025 at 7:33 PM, Typhoon Tip said:

It just gets exhausting trying to explain this to the people that can’t synthesize global perspectives; probably as a native intellectual limitation. Which unfortunately is precisely what is needed if somebody’s going to understand how global warming works  

Dimes to donuts the majority of the people in the denier frame of mind are narrow perspective types

I haven't seen anyone in this thread denying global warming or claiming warming is UHI-driven. 

[tacoman25]

12 hours ago, TheClimateChanger said:

Anyways, I often see certain people who deny (some people on here get offended with the term "denier" even though they have used that term profusely in the past, but I digress)... people who deny climate change often say highs are of more value, because lows are more affected by UHI. I would actually argue the exact opposite, and say high temperature readings are more impacted by siting/exposure characteristics and equipment, and low temperatures give us a better look at actual trends.

Who in this thread is denying climate change?

Your argument here makes little sense to me. Properly sited sensors have reliable, correct high temps, and that's why there's scientific standards for the ones that are used for NOAA/NWS data.

Low temps, on the other hand, vary more wildly based on location and that can't be fixed through better siting. But it does make sense that sensors that have a long period of record in a single location that has seen the least amount of changes around it are going to provide the most reliable temperature record. 

[bluewave]

Another reason that the high temperatures in desert cities can be similar or cooler to the outlying areas is due to something called the urban cooling island. It’s another reason greening urban desert cities is so important. Since it can lead to a 2C cooling inversion during the day. 
 

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL066534

During the day, downtown areas appear to be cooler than the suburbs in six cities out of eight: Abu Dhabi, Kuwait City, Las Vegas, Phoenix, Biskra, and Bikaner, with values ranging from the −5.33°C in Abu Dhabi to the −0.06°C in Las Vegas during the summer. This effect can be partially explained considering that bare soils (prevalent in the suburbs) absorb more sunlight than urban surfaces due to their low reflectivity, hence leading to the relatively higher temperature of suburban areas [Georgescu et al., 2011]. This thermal gradient between the city center and suburbs can become even more pronounced when irrigated vegetation is present in the urban sites, resulting in heat advection, subsidence, and strong latent heat fluxes over the city. Also, the gradient can be modulated through the local wind regime and larger-scale atmospheric circulation alike, resulting in additional surface temperature variability across the diverse cities [Oke, 1987].

SUHI values referred to urban + vegetation sites are, in general, lower than the ones for the downtown sites, resulting in a further reduction of the temperature from few tenths of degrees to the −2.36°C in Phoenix at day in summer. It is, in fact, during the summer that the difference between urban and suburbs testing sites becomes wider: this phenomenon may be connected with intensive urban irrigation, eventually resulting in a sharp decoupling between vegetation and precipitation and to a lowering of the effects of heat stress on plants [Jenerette et al., 2013]. An exception is here represented by Bikaner, where from June to August, the southwestern monsoon carries sporadic spells of intense precipitation, which lower more easily the temperatures in bare and “permeable” suburban substrates than in “impervious” man-made surfaces [Kharol et al., 2013].

In agreement with previous results [Lazzarini et al., 2013; Lazzarini and Ghedira, 2014], Abu Dhabi displays the sharpest UCI effect, probably associated with the fact that it is located on an island [Frey et al., 2006; Wulfmeyer et al., 2014] and that the strong sea breeze regime triggered by monsoonal winds during the summer can induce a significant reduction of the atmospheric boundary layer depth, with a consequent mitigation of sensible heat fluxes along the coast [Eager et al., 2008]. In contrast, Riyadh and Doha do not show any inversion effect in the UHI, although the summer SUHIs have lower values than the winter ones on average. Again, this is consistent with previous results [Gamo, 1996; Alghamdi and Moore, 2014; Hashem and Balakrishnan, 2014; Sasidharan et al., 2009] and could be a consequence of the scarce vegetation coverage in these two cities (landscaping activities are limited) and urban design [Alghamdi and Moore, 2014].

During the night, however, the classical UHI phenomenon is restored: bare soil surfaces cool down faster than man-made and vegetated surfaces, resulting in strong positive SUHI values across all the cities of the ensemble. Therefore, HDCs are not immune from the UHI effect but rather oscillate between a diurnal UCI effect and a classic UHI at night. Since only two cities (Abu Dhabi and Kuwait City) out of the six displaying this diurnal UCI/nocturnal UHI alternation are close to a water body, breezes do not seem to have a dominant role in determining these common patterns, albeit additional local forcing on fluxes could arise from mild slopes and anabatic/katabatic flows [Yoshino, 1984; Rendón et al., 2014; Chemel et al., 2008; Sun et al., 2009; Brazel et al., 2010]. Nonetheless, proximity to the sea remains an important factor in reducing temperature ranges (see, e.g., the case of Abu Dhabi, Doha, and Kuwait City). In contrast, vegetation abundance and typology could be among the main causes of UCI. Transpiration patterns, in fact, largely depend on photosynthetic pathways: in C3 and C4 plants, for example, transpiration predominantly takes place during the day [see Caird et al., 2007, and references therein], while in Crassulacean acid metabolism (CAM) plants carbon uptake mainly occurs at night, under condition less conductive to water loss via transpiration [Bartlett, 2014].

Day/night ranges in the downtown sites appear smaller than the ones in the suburbs, while the ranges at the different sites tend to be amplified during the summer, with the exception of Bikaner, where again the precipitation events associated to the Indian summer monsoon could play an important role in modulating suburban LSTs. These features can be explained considering the lower heat capacity of suburban bare soils, allowing for quick variations of temperature.

Owed to the decoupling with precipitation, the seasonal variability of NDVI within urban areas is extremely weak. However, during the summer, the elevated temperatures characterizing the HDCs can still trigger intense water stress in urban vegetation [Julien et al., 2011], leading to a summer reduction of the vegetation coverage in general, in contrast with what is generally observed in temperate regions.

Following Zhou et al. [2013], Figure 2 shows the SUHI seasonal cycle evolution for day (Figure 2a) and night (Figure 2b). SUHI is calculated both as the difference in LST between urban + vegetation and the suburbs sites (green lines and symbols) and between downtown and the suburbs (in red). Observations are marked with light dots (grey for SUHI^(Dt) and green for SUHI^{(U + V)}), while monthly average values are indicated by black triangles (SUHI^{(U + V)}) and filled circles (SUHI^(Dt)), and the corresponding standard deviation of LST and SUHI is represented in the form of horizontal and vertical error bars. Thick continuous lines represent the SUHI^{(U + V)} and SUHI^(Dt) seasonal cycle after band-pass filtering of fine temporal scales up to 1 week with a Morlet wavelet of central frequency ω₀=6 [Mallat, 2008]. The SUHI seasonal dynamics still displays clear diurnal UCI signature, and urban + vegetation sites show the highest potential for cooling in HDCs. However, the seasonal evolution of the SUHI strongly varies from city to city, pinpointing how not a single factor but rather the interplay between urban form, vegetation abundance, and local to regional climatic forcing are ultimately driving the thermal regimes of these cities. Also, the role of vegetation is not always univocal: in Biskra during the day, and in Phoenix at night, urban + vegetation sites appear to be warmer than their downtown counterparts. In both cases, the urban + vegetation test sites are located on a mild slope, facing north in the case of Biskra and southwest in the case of Phoenix. The topographic slope and the exposition to sunlight during the day could play a more prominent role than the sole vegetation coverage in these two cases. In Phoenix, for example, the urban + vegetation test site could be affected at night by a thermal belt effect [Yoshino, 1984], otherwise absent in the downtown site at the bottom of the valley, more likely subject to katabatic flows. It is also clear that in the presence of extended urban vegetated areas, plants phenology, irrigation systems, and specific microclimatic conditions can all contribute to the evolution of the SUHI.

We performed a comparative analysis of the SUHI effect of cities located in hot desert environments (HDCs) using satellite data processing techniques. The main aim was to identify common patterns in the thermal regime of HDCs, testing at the same time the occurrence of the UCI effect observed in single studies dedicated to arid land cities. During the day, an average SUHI inversion of around 2°C was confirmed in six cities from a pool of eight. This inversion has been often associated with the abundance of irrigated vegetation in HDCs, which creates large areas where surface temperatures are mitigated through evaporative cooling.

Despite that, the night LST analysis showed the standard UHI effect in all the cities. During the year, the variation of LST mainly showed a biseasonal mode, with a more evident inversion of SUHI during summer months. A comparative analysis of how the percentage of NDVI/ISA relates to LSTs also highlighted how medium dense urban sites with dedicated green areas are the ones showing the highest SUHI inversion probably due to the combined effect of shadowing and plant transpiration. However, all the cities displayed peculiar characteristics, pointing out how specific microclimatic and aridity conditions need to be taken into account in the SUHI assessment. Surface energy balance can be retrieved, and further variables (i.e., surface roughness and wind data) can be added to the picture, in order to better understand the microclimate of cities located in hyperarid regions.

Seasonality of vegetation is expected to have a marginal role in HDCs due to the decoupling of vegetation from precipitation through irrigation [Jenerette et al., 2013]. In contrast, the typology of vegetation (C3, C4, or CAM) used in the landscaping could play a crucial role in mitigating (or reversing) UHI effects in desert cities. Also, the role of different irrigation strategies, as well as the link with desalination and its potential long-term impacts, needs to be addressed.

 

 

[Typhoon Tip]

Ho, great ...

https://phys.org/news/2025-08-ozone-planet-thought.html

[donsutherland1]

1 hour ago, bluewave said:

Another reason that the high temperatures in desert cities can be similar or cooler to the outlying areas is due to something called the urban cooling island. It’s another reason greening urban desert cities is so important. Since it can lead to a 2C cooling inversion during the day. 
 

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015GL066534

 

During the day, downtown areas appear to be cooler than the suburbs in six cities out of eight: Abu Dhabi, Kuwait City, Las Vegas, Phoenix, Biskra, and Bikaner, with values ranging from the −5.33°C in Abu Dhabi to the −0.06°C in Las Vegas during the summer. This effect can be partially explained considering that bare soils (prevalent in the suburbs) absorb more sunlight than urban surfaces due to their low reflectivity, hence leading to the relatively higher temperature of suburban areas [Georgescu et al., 2011]. This thermal gradient between the city center and suburbs can become even more pronounced when irrigated vegetation is present in the urban sites, resulting in heat advection, subsidence, and strong latent heat fluxes over the city. Also, the gradient can be modulated through the local wind regime and larger-scale atmospheric circulation alike, resulting in additional surface temperature variability across the diverse cities [Oke, 1987].

SUHI values referred to urban + vegetation sites are, in general, lower than the ones for the downtown sites, resulting in a further reduction of the temperature from few tenths of degrees to the −2.36°C in Phoenix at day in summer. It is, in fact, during the summer that the difference between urban and suburbs testing sites becomes wider: this phenomenon may be connected with intensive urban irrigation, eventually resulting in a sharp decoupling between vegetation and precipitation and to a lowering of the effects of heat stress on plants [Jenerette et al., 2013]. An exception is here represented by Bikaner, where from June to August, the southwestern monsoon carries sporadic spells of intense precipitation, which lower more easily the temperatures in bare and “permeable” suburban substrates than in “impervious” man-made surfaces [Kharol et al., 2013].

In agreement with previous results [Lazzarini et al., 2013; Lazzarini and Ghedira, 2014], Abu Dhabi displays the sharpest UCI effect, probably associated with the fact that it is located on an island [Frey et al., 2006; Wulfmeyer et al., 2014] and that the strong sea breeze regime triggered by monsoonal winds during the summer can induce a significant reduction of the atmospheric boundary layer depth, with a consequent mitigation of sensible heat fluxes along the coast [Eager et al., 2008]. In contrast, Riyadh and Doha do not show any inversion effect in the UHI, although the summer SUHIs have lower values than the winter ones on average. Again, this is consistent with previous results [Gamo, 1996; Alghamdi and Moore, 2014; Hashem and Balakrishnan, 2014; Sasidharan et al., 2009] and could be a consequence of the scarce vegetation coverage in these two cities (landscaping activities are limited) and urban design [Alghamdi and Moore, 2014].

During the night, however, the classical UHI phenomenon is restored: bare soil surfaces cool down faster than man-made and vegetated surfaces, resulting in strong positive SUHI values across all the cities of the ensemble. Therefore, HDCs are not immune from the UHI effect but rather oscillate between a diurnal UCI effect and a classic UHI at night. Since only two cities (Abu Dhabi and Kuwait City) out of the six displaying this diurnal UCI/nocturnal UHI alternation are close to a water body, breezes do not seem to have a dominant role in determining these common patterns, albeit additional local forcing on fluxes could arise from mild slopes and anabatic/katabatic flows [Yoshino, 1984; Rendón et al., 2014; Chemel et al., 2008; Sun et al., 2009; Brazel et al., 2010]. Nonetheless, proximity to the sea remains an important factor in reducing temperature ranges (see, e.g., the case of Abu Dhabi, Doha, and Kuwait City). In contrast, vegetation abundance and typology could be among the main causes of UCI. Transpiration patterns, in fact, largely depend on photosynthetic pathways: in C3 and C4 plants, for example, transpiration predominantly takes place during the day [see Caird et al., 2007, and references therein], while in Crassulacean acid metabolism (CAM) plants carbon uptake mainly occurs at night, under condition less conductive to water loss via transpiration [Bartlett, 2014].

Day/night ranges in the downtown sites appear smaller than the ones in the suburbs, while the ranges at the different sites tend to be amplified during the summer, with the exception of Bikaner, where again the precipitation events associated to the Indian summer monsoon could play an important role in modulating suburban LSTs. These features can be explained considering the lower heat capacity of suburban bare soils, allowing for quick variations of temperature.

Owed to the decoupling with precipitation, the seasonal variability of NDVI within urban areas is extremely weak. However, during the summer, the elevated temperatures characterizing the HDCs can still trigger intense water stress in urban vegetation [Julien et al., 2011], leading to a summer reduction of the vegetation coverage in general, in contrast with what is generally observed in temperate regions.

Following Zhou et al. [2013], Figure 2 shows the SUHI seasonal cycle evolution for day (Figure 2a) and night (Figure 2b). SUHI is calculated both as the difference in LST between urban + vegetation and the suburbs sites (green lines and symbols) and between downtown and the suburbs (in red). Observations are marked with light dots (grey for SUHI^(Dt) and green for SUHI^{(U + V)}), while monthly average values are indicated by black triangles (SUHI^{(U + V)}) and filled circles (SUHI^(Dt)), and the corresponding standard deviation of LST and SUHI is represented in the form of horizontal and vertical error bars. Thick continuous lines represent the SUHI^{(U + V)} and SUHI^(Dt) seasonal cycle after band-pass filtering of fine temporal scales up to 1 week with a Morlet wavelet of central frequency ω₀=6 [Mallat, 2008]. The SUHI seasonal dynamics still displays clear diurnal UCI signature, and urban + vegetation sites show the highest potential for cooling in HDCs. However, the seasonal evolution of the SUHI strongly varies from city to city, pinpointing how not a single factor but rather the interplay between urban form, vegetation abundance, and local to regional climatic forcing are ultimately driving the thermal regimes of these cities. Also, the role of vegetation is not always univocal: in Biskra during the day, and in Phoenix at night, urban + vegetation sites appear to be warmer than their downtown counterparts. In both cases, the urban + vegetation test sites are located on a mild slope, facing north in the case of Biskra and southwest in the case of Phoenix. The topographic slope and the exposition to sunlight during the day could play a more prominent role than the sole vegetation coverage in these two cases. In Phoenix, for example, the urban + vegetation test site could be affected at night by a thermal belt effect [Yoshino, 1984], otherwise absent in the downtown site at the bottom of the valley, more likely subject to katabatic flows. It is also clear that in the presence of extended urban vegetated areas, plants phenology, irrigation systems, and specific microclimatic conditions can all contribute to the evolution of the SUHI.

 

We performed a comparative analysis of the SUHI effect of cities located in hot desert environments (HDCs) using satellite data processing techniques. The main aim was to identify common patterns in the thermal regime of HDCs, testing at the same time the occurrence of the UCI effect observed in single studies dedicated to arid land cities. During the day, an average SUHI inversion of around 2°C was confirmed in six cities from a pool of eight. This inversion has been often associated with the abundance of irrigated vegetation in HDCs, which creates large areas where surface temperatures are mitigated through evaporative cooling.

Despite that, the night LST analysis showed the standard UHI effect in all the cities. During the year, the variation of LST mainly showed a biseasonal mode, with a more evident inversion of SUHI during summer months. A comparative analysis of how the percentage of NDVI/ISA relates to LSTs also highlighted how medium dense urban sites with dedicated green areas are the ones showing the highest SUHI inversion probably due to the combined effect of shadowing and plant transpiration. However, all the cities displayed peculiar characteristics, pointing out how specific microclimatic and aridity conditions need to be taken into account in the SUHI assessment. Surface energy balance can be retrieved, and further variables (i.e., surface roughness and wind data) can be added to the picture, in order to better understand the microclimate of cities located in hyperarid regions.

Seasonality of vegetation is expected to have a marginal role in HDCs due to the decoupling of vegetation from precipitation through irrigation [Jenerette et al., 2013]. In contrast, the typology of vegetation (C3, C4, or CAM) used in the landscaping could play a crucial role in mitigating (or reversing) UHI effects in desert cities. Also, the role of different irrigation strategies, as well as the link with desalination and its potential long-term impacts, needs to be addressed.

 

 

Phoenix has a five-year master plan for expanding trees/shade. The City is aware of the growing risks its people face from increasingly frequent and intense heat.

https://www.phoenix.gov/content/dam/phoenix/heatsite/documents/ShadePhoenixPlan_Nov13CouncilDraft_topost_EN.pdf