# UHI magnitude calculation

Dear all,
I have a doubt about the parameters to be included in the UHI magnitude:
In the User guide the UHImax is defined as "the maximum magnitude of the UHI effect for the city (or more precisely, the difference between Tair,ref and the maximum temperature observed in the city)."

Where Tair,ref is defined as:

I was thinking of calculating the Tair,ref temperature as the average of summer temperatures (differentiating between daytime and nighttime) over the last 3 years in the rural area.

In this case, following my logic, the UHImax, is the difference between Tair,ref (calculated as described above) and the average of summer temperatures (differentiating between daytime and nighttime) of the last 3 years in the city.
*Not therefore the maximum (absolute) value ever recorded in the city.

Am I wrong?

Thank you,
Antonio

Hi @antobaro ,

Thanks for writing in with your question.

In your case, `UHImax` equals the maximum temperature observed in the city during the period of interest (the last 3 years), during day and night, respectively, MINUS summer `Tair,ref` for day and night, respectively. I suppose you are correct in that you probably ought not subtract the maximum value ever recorded there if you are only interested in the last 3 years. Perhaps the User Guide language could be more precise here, but all input data should be relevant for the temporal period of interest (only). So if you’re interested in the last 3 years, only input data from the last 3 years. If you’re interested in summer, only use data from the summer. When investigating night versus day, be sure to differentiate your input data so that they only cover night and day, respectively. The model is time period agnostic, so it’s up to you, the user, to ensure the data cover the appropriate period. The only time-related element the model distinguishes between is related to the calculation of cooling capacity. For daytime calculations, be sure to rely on shade, albedo, and ET index values, but for nighttime, these factors are irrelevant of course, so instead be sure to use data on building intensity. But again, the model is unaware if input data cover day or night, so the user must manually adjust and select the appropriate method.

You may also find these other forum posts useful:

I hope this helps your understanding,
Jesse

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Yes, it seems logical to me to use homogeneous periods to calculate the UHI max .

This would also imply to use, in addition to the homogeneous time periods, also HOMOGENEOUS METHODOLOGIES of calculation for Tair,ref (rural temperature) and urban temperature (this in my opinion is less clear in the user guide).

That is, if my Tair,ref is calculated as the AVERAGE of the last 3 years of rural temeperature, my UHI max should be calculated as the difference between the AVERAGE of the temperatures recorded in the city in the last 3 years and the AVERAGE of rural temperatures recorded in the last 3 years.

UHImax= TMEANurban-TMEANrural

Similarly if I take as Tair,ref THE MAXIMUM temperature recorded in the last 3 years in the rural area, then my UHImax becomes the difference between the MAXIMUM temperature recorded in the last 3 years in the city, minus the MAXIMUM rural temperature (Tair,ref).

UHImax=TMAXurban-TMAXrural

Obviously always comparing homogeneous time periods (night, day, summer temperatures only, etc).

In the former case we show the difference in average temperatures, in the latter the difference in maximum temperatures.

Tell me if I’m wrong, you certainly have a much more accurate understanding of how the model works than I do, I’m just interfacing with it for the first time and as a user.

Best,
Antonio

Dear @jesseG,
Excuse me, but I have a doubt always on this issue of Tair, ref and UHI calculation.
In my study area, besides rural areas, I have natural/semi-natural areas where the average temperature is lower than both urban and rural areas.
So, also considering the equation that governs the Tair output, wouldn’t it be more appropriate to set the Tair, ref as the temperature recorded in these areas?
In these natural areas in fact HMi=1 and Tair=Tair,ref; which is not the case for the rural area.

Thank you,
Antonio

Hi @antobaro ,

Thanks for continuing the conversation. These are good and fair questions.

@chris would you be able to clarify Antonio’s doubts and answer his questions posted here on April 4th and April 12th, 2022?

Thanks,
Jesse

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Thanks @jesseG for the mediation.

Hopefully @chris can answer that as well!

Thanks again!

Hi @antobaro,

These are some great questions and I hope I can answer them to your satisfaction!

What you choose as the UHI magnitude parameter depends on what you are trying to model. If you are trying to model the largest heatwave a city has ever experienced, the UHI magnitude would be the difference between the non-urban reference temperature for the region (rural or natural) and the highest recorded urban temperature during that heatwave. You can differentiate between daytime and nighttime UHI magnitude if you wish.

If you are trying to look at a more statistical average UHI impact, you can choose the difference in long term average temperatures between the reference and urban areas (in your case over the past three years). Otherwise we recommend this source for seasonal UHI magnitudes (Global Surface UHI Explorer | Center for Earth Observation); the values are conservative but they are available globally.

I agree with your point about homogenous methodologies: you don’t want to be comparing the highest temperature ever recorded in the city with a long term average non-urban baseline temperature. Either compare maximums in both or averages in both, mixing the two would raise some eyebrows.

As to your final point about reference temperature (Tref), the difference between rural and natural areas is, in the case of the user guide, semantic. Tref should be the lowest temperature you expect from your study area, which likely occurs in a more naturalized environment. ‘Rural’ in the users guide is a misnomer: what we really mean is the lowest non-urban temperature that the heat island will build upon. So feel free to use a temperature from the naturalized areas of your study area rather than a ‘rural’ reference.

Cheers,
Chris

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Perfect @chris, very useful! Thanks

Just a note on this Global Surface UHI Explorer | Center for Earth Observation
I’ve looked at it and it’s certainly useful for determining UHImax, but, it seems to me, less so for Tair,ref because the data I’ve looked at only presents the differences and not the exact temperatures measured.

Let me know if I’m wrong or missing something, thanks.

Antonio

You are definitely correct. I typically use averaged temperature data when determining non-urban reference, which you will have to search for from other sources.

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