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UHI doesn't make a place look warmer than it is. It makes it warmer than it otherwise would be.
That’s a very interesting statement and would be 100% true if thermometers measured only the air temperature. If thermometers behaved thusly we would know that their measurements would give us an accurate sampling of the very-real phenomenon of warmer air temperatures in urban environments.
However, the laws of thermodynamics dictate that thermometers must measure not only the air temperature but also the temperature of the ambient radiation field; to get an accurate reading of the air temperature requires that we minimize or eliminate the radiation portion. This applies to any thermometer regardless of whether it’s urban or rural.
It turns out though that it’s much easier to find locations for thermometers in rural locations that minimize the radiation contribution. This means that thermometers located in urban locations very frequently measure not what we want (the air temperature) but something else (air temperature plus radiation).
So when we look at data from urban locations very often we are not measuring what we want (i.e., warmer air temperatures in urban locations, aka the UHI) but rather an exaggerated form of the UHI (warmer air temperatures plus increased radiation component). This in turn makes Lancerman’s statement true: the apparent “urban heat island” people talk about makes places seem warmer than they actually are because it's not actually the real UHI at all, it's something else.
TL; DR: the so-called urban heat island is often exaggerated because many urban thermometers are not measuring the air temperature.
On the topic of heat island, I think it might surprise some on this thread that for my area of expertise of the Southeast US, urban stations are actually closest to what the natural climate of the area would be like - at least in terms of low temperatures.
Some of you may have noticed airport stations in the Southeast US often have shockingly large diurnal ranges and surprisingly short frost free seasons, quite the contrary to what would be expected of a low elevation, humid climate. This is actually rather artificial: airport stations are cleared of the natural pine forest cover and so cool off far, far more at night than they would if they were forested due to enhanced sky exposure.
It's not like the urban heat island would produce overreading either: most Southeast US cities are not very urban at all despite those cold airports so if anything urban heat island may underread compared to a properly forested station for places like say Valdosta. I've mentioned all the above plenty in the first/last frost threads but hadn't gone quite so into it, so here's a discussion I had on Palmtalk about it: https://www.palmtalk.org/forum/topic...en-cold-spots/
I guess this means that urban heat islands might bring my area of expertise closer to what they would be without the artificial cold instead of making them warmer than they would otherwise be.
And as for how this relates to a longer base than 30 years, that's a problem for the Southeast US. The official stations have started out in good downtown locations but end up getting moved to those very cold airports, meaning the normals can only go back so far before you end up mixing up 2 different places.
Perhaps longer normals will become more feasible many years into the future. But for now it's something I won't necessarily do.
The effect of "artificial" cooling from the clearing of trees is far, far less than the effect of erroneously-high measurements created by poorly-located thermometers. Moreover, although this effect may create somewhat cooler temperatures under certain conditions at certain times of the day during certain times of the year, it can also create notably warmer temperatures at other times of the day and year. Also, the cooling effect of tree clearing is not restricted to the US Southeast but present in any forested region around the world so spatial comparisons are still possible.
Basically, it's not the big deal you think it is.
Finally--and this needs to be heavily emphasized--regardless of the effect size this phenomenon is not actually artificial inasmuch as the number that a properly-placed thermometer in a cleared forest displays still reflects the actual air temperature of that location. In that sense, it is very, very real effect and not at all artificial.
Contrast this to the effect of putting thermometers on football stadiums or parking garages. In those cases the number that such a poorly-placed thermometer displays is no longer the air temperature nor even the urban heat island; the number such a thermometer displays is something else altogether. Such numbers are very much artificial.
The effect of "artificial" cooling from the clearing of trees is far, far less than the effect of erroneously-high measurements created by poorly-located thermometers.
It depends on how erroneously high the poorly located thermometer readings are. You have to remember that the effect of tree clearing can be substantial enough for some locations to have average lows during the season that the artificial cooling becomes pronounced colder than locations 2 degrees to the north that are also artificially cold themselves, even if it's not to the same degree.
This without question matches or exceeds some poorly located, high-reading thermometers for rather obvious reasons.
It should also be noted that Joe in New Zealand told me the following about tree cover vs open field.
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The effect of forest clearing is pretty major around here, where I'm guessing about 30% of subcanopy vegetation isn't able to grow in the open -mature lowland forest around here is an environment that doesn't see temperatures reaching 0C, while only a hundred metres away, cleared land can see dozens of frosts during winter.
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Just on my property, the difference between forested and cleared land can be anywhere from 2C-6C
It is likely that this effect would be even more pronounced in the Southeast US because of both the great frequency of clear and calm nights and the sandy soil that clearage would well-drain, getting less well drained and therefore a poorer radiator with the tree covering.
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Moreover, although this effect may create somewhat cooler temperatures under certain conditions at certain times of the day during certain times of the year, it can also create notably warmer temperatures at other times of the day and year.
That's why I said when it comes to low temperatures at least. I am aware it can do the latter, hence my explicit specification.
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Also, the cooling effect of tree clearing is not restricted to the US Southeast but present in any forested region around the world so spatial comparisons are still possible.
Not necessarily - they would only be if the sites were artificially cold to the same degree, which is almost certain to vary among 2 random sites for rather obvious reasons, and even then only if the other place got similar amounts of clear, calm nights the Southeast US does that allow the artificially cold sites to be as consistently artificially cold as they are (tbf I don't know if they do or not).
No studies have been done to my knowledge on places outside the Southeast US being artificially cold, so I'll use it as a proxy to demonstrate the former point - there is variation in the Southeast US on the degree of artificial cooling that leads to already-artificially-cold sites being warmer than others further south on clear, calm nights, and undeniably the same variation would happen elsewhere - making comparison difficult.
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Basically, it's not the big deal you think it is.
Completely disagree - and so would the studies that showed this artificial cooling happens.
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Finally--and this needs to be heavily emphasized--regardless of the effect size this phenomenon is not actually artificial inasmuch as the number that a properly-placed thermometer in a cleared forest displays still reflects the actual air temperature of that location. In that sense, it is very, very real effect and not at all artificial.
I'm not saying it's artificial in that sense. What I'm saying is that the actual air temperature that thermometer is reading, no matter how accurate it is, is artificially cold in terms of the low temperature that is reached.
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Contrast this to the effect of putting thermometers on football stadiums or parking garages.
2 questions:
-Are you referring to WeatherSTEM stations? It sounds like you are.
-If you are, where is the source that said they are located there? I haven't been able to find anything on their specific placement.
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In those cases the number that such a poorly-placed thermometer displays is no longer the air temperature nor even the urban heat island; the number such a thermometer displays is something else altogether. Such numbers are very much artificial.
You should know that:
-If you are referring to the WeatherSTEM stations, I've been recommended these over email by the National Weather Service, and they literally even use those stations.
-The readings are not consistent with a poorly placed thermometer in an urban environment - while lows are of course warmer, highs are generally colder, as would be expected from removal of the same nighttime-cooling soil during the day and the terrain change from sand to clay.
-This is a professional weather company, I trust them enough to quality-ensure stations the National Weather Service uses and recommends. It is for this reason and the first that I doubt that they are placed where you say they are, or if they are the stations are probably mounted sufficiently far from the roof to not cause poor readings.
-Naturally good air draining topography of a high elevation hill where the weather station is already significantly increases the lows, this together with the clay<-sand soil would be true even if there was no urbanization to speak of.
Not so artificial after all, I would think.
By the way a question on the first comment:
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Originally Posted by Ed's Mountain
That’s a very interesting statement and would be 100% true if thermometers measured only the air temperature. If thermometers behaved thusly we would know that their measurements would give us an accurate sampling of the very-real phenomenon of warmer air temperatures in urban environments.
However, the laws of thermodynamics dictate that thermometers must measure not only the air temperature but also the temperature of the ambient radiation field; to get an accurate reading of the air temperature requires that we minimize or eliminate the radiation portion. This applies to any thermometer regardless of whether it’s urban or rural.
It turns out though that it’s much easier to find locations for thermometers in rural locations that minimize the radiation contribution. This means that thermometers located in urban locations very frequently measure not what we want (the air temperature) but something else (air temperature plus radiation).
So when we look at data from urban locations very often we are not measuring what we want (i.e., warmer air temperatures in urban locations, aka the UHI) but rather an exaggerated form of the UHI (warmer air temperatures plus increased radiation component). This in turn makes Lancerman’s statement true: the apparent “urban heat island” people talk about makes places seem warmer than they actually are because it's not actually the real UHI at all, it's something else.
TL; DR: the so-called urban heat island is often exaggerated because many urban thermometers are not measuring the air temperature.
Isn't the urban heat island BECAUSE of the influence of the radiation on the air temperature? How do we determine that we're measuring the influence of the radiation but not the radiation itself?
Last edited by Can't think of username; 06-27-2023 at 11:41 AM..
-Are you referring to WeatherSTEM stations? It sounds like you are.
-If you are, where is the source that said they are located there? I haven't been able to find anything on their specific placement.
Not just WeatherSTEM but also all the problematic wunderground stations. As far as WeatherSTEM goes, their locations are easy to pin down plus there are all sorts of photos of them on the Internet. Here they are installing one in Delray Beach. There's more radiation here than in my microwave; very unlikely the shield on the sensor can cope.
If it's not the air temperature then it's artificial.
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Isn't the urban heat island BECAUSE of the influence of the radiation on the air temperature? How do we determine that we're measuring the influence of the radiation but not the radiation itself?
Radiative heating of the air is just one mechanism; the net effect of all mechanisms put together results in a difference (usually an increase) in the air temperature. So the bottom line is, we need to measure the air temperature and only the air temperature to see the combined results of all these mechanisms. The only way to do this is by shielding the sensor and placing it where ambient radiation is minimal. This is feasible in rural locations, airports, or even city parks but much more difficult on the roof of a parking garage.
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WeatherSTEM is now in use in 192 locations in 13 states, including 162 in Florida.
Among the local locations is the rooftop of the St. Augustine Street parking garage, a few hundred feet away from Doak Campbell Stadium...
Not just WeatherSTEM but also all the problematic wunderground stations. As far as WeatherSTEM goes, their locations are easy to pin down plus there are all sorts of photos of them on the Internet. Here they are installing one in Delray Beach. There's more radiation here than in my microwave; very unlikely the shield on the sensor can cope.
You should know that they are putting the weather stations on tall masts (which is visible in that image) that would solve the problem of radiation. This is the exception to the rule of problematic roof stations.
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In general, therefore, roofs are very poor locations for air temperature, humidity, wind and precipitation observations unless the instruments are placed on very tall masts.
I trust that this is working because as I alluded to, the high temperature-increasing heat radiation a rooftop would give off is discordant as can be with the colder high temperatures - especially during heatwaves, when the radiation would be more pronounced yet they remain colder. All the average highs, usual maximums, and record highs (the latter 2 from heatwaves ofc) of the WeatherSTEM stations I use are colder than the airport.
Plus, as I also said, I trust the National Weather Service and professional weather companies to quality check stations they use/set up/recommend to those that email them - and this is consistent with having passed that quality check on both ends.
As for other wunderground stations, they are likely to be accurate in my opinion because they have the exact same going on. They tend to be colder for highs at the same time they have the warmer lows, as again would be expected from removing them from the artificially cleared sandy soil - not the warmer highs radiation would give. This is, again, discordant with radiation being a problem.
You should also know they are valued on Palmtalk, by people who set up their own stations - probably because of just this. I trust Palmtalk as well.
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Radiative heating of the air is just one mechanism; the net effect of all mechanisms put together results in a difference (usually an increase) in the air temperature. So the bottom line is, we need to measure the air temperature and only the air temperature to see the combined results of all these mechanisms. The only way to do this is by shielding the sensor and placing it where ambient radiation is minimal. This is feasible in rural locations, airports, or even city parks but much more difficult on the roof of a parking garage.
Thank you for the explanation. The masts solve this problem for the roofs, but I completely agree, with no mast it's not a good thing to have because as you mentioned it's artificial and so isn't going to get you anywhere.
When choosing a time period for climate normals there is a balance between making it long enough to smooth year-to-year variation and short enough to give an indication of the climate as it is now, rather than what it was in the past due to longer term changes. 30 years seems a good balance between the two.
If we look at say Sydney you can see what kind of a difference it makes. I've only chosen Sydney as the BOM has records going back a long way that are easily accessible.
Using 1858-2020 data the annual mean maximum is 21.8C. For 1991-2020 it is 22.8C
1858-2020 has a coldest month mean maximum of 16.4C. For 1991-2020 it is 17.9C
So you can easily see that if we use an inappropriately long period, the averages will be a poorer predictor of what this year, next year or a year a few years in the future will be like than a more appropriate period such as 1991-2020.
You should know that they are putting the weather stations on tall masts (which is visible in that image) that would solve the problem of radiation. This is the exception to the rule of problematic roof stations.
Besides, putting a thermometer on top of a tall mast creates all sorts of new problems. For starters, you are no longer measuring the temperature at 2 meters above the ground so comparisons with other sites are not possible.
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Originally Posted by Can't think of username
Thank you for the explanation. The masts solve this problem for the roofs, but I completely agree, with no mast it's not a good thing to have because as you mentioned it's artificial and so isn't going to get you anywhere. ...
Masts don't solve the problem, they create new problems. How tall is very tall? For a mast to be tall enough to reduce the radiation effect of a 1.4 acre 7 storey parking garage like the FSU one would mean a very tall mast indeed. There's a reason standards for measurement of temperature exist and why those standards do not include "very tall" masts.
From the 'purest' climatological standpoint, yep, I agree that 30 years come short to summarize the influence of some big scale process.
But I think that after all, the thing about a 30 year period has to do more with human geography and let's say, 'sociology'. And in that case it's very appropiate IMO. If you think twice 30 years is roughly the median age of the population, so that amount of years matches perfectly with what a generation perceives as the climate during their times and their daily lives. So, that's my take.
Some of you may have noticed airport stations in the Southeast US often have shockingly large diurnal ranges and surprisingly short frost free seasons, quite the contrary to what would be expected of a low elevation, humid climate.
Elevation by itself has zero effect on diurnal ranges. How many times do I have to hear this myth...it's quite obvious that you need flat, valley-floor geography to get high diurnal ranges, not necessarily elevation. In fact in many cases elevation reduces diurnal range as cloud cover is increased on the whole-- this is especially true in wet climates.
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