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Title pretty much states the question. How much hotter do air conditioning units make it outside in a large city like NYC, Chicago, etc?

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good question. I speculate that it's not a significant contribution. – Xurtio Jul 17 '13 at 17:05
It really depends on many many factors, but a Fermi type answer might be possible. – Ali Jul 17 '13 at 17:07
Related: Urban heat island (UHI): "Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI.[2][6][7]" Unfortunately none of the references are available to me. – Keep these mind Jul 18 '13 at 17:22

This isn't a direct answer, but a relevant comment on the question that is too long to be a comment on this system.

The heating you seem to be considering is only about total electrical power dissipated. Whether some of that is used for air conditioning is irrelevant. In the steady state, the cooled buildings will cool the air, except for the power used to run the air conditioners. A air conditioner that consumes 1 kW of electrical power will cause the same overall heating in steady state as a 1 kW electrical space heater or toaster or hair dryer. In the short term, the air conditioner moves heat from inside to outside, but in the long term that heat gets back inside.

The real question is therefore how much electrical consumption increases the averge air temperature. You should be able to get figures for electrical consumption by doing a little digging. However, relating that power to temperature rise will be tricky. To take some stab at it you have to make some assumption about how much air is flowing around the city, which of course varies dramatically with wind. Pick some representative wind speed, pick some height, then calculate the mass of new air that has to be heated per unit of time. That will give you nice and neat figure for what is at best a wild-ass guess.

Note that dissipation of electrical power in a city is probably a small contribution to the local temperature rise. The stone and asphalt heated by the sun without the benefit of evaporative cooling like leaves have is very likely a much larger factor.

The temperature rise in cities has been measured, so you could do the heated air flow analisys in reverse and take a stab at the extra power required to heat the air. I think you'll find that is way more than the total electrical power delivered to the city. All those car and truck engines sitting in traffic probably add up to more heating power than the delivered electrical power, but I'm still guessing that the solar power not countered by evaporative cooling of leaves dominates by a lot.

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Have a look at urban heat effect.

An urban heat island (UHI) is a metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon.1 The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. Seasonally, UHI is seen during both summer and winter. The main cause of the urban heat island is modification of the land surface by urban development which uses materials which effectively retain heat. Waste heat generated by energy usage is a secondary contributor

tokyo heat island

Tokyo, an example of an urban heat island. Normal temperatures of Tokyo go up more than those of the surrounding area.

An interesting study is the effect of urban heat islands on temperature measurements and the trend of global warming due to bad siting of the temperature sensors.

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