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It is known that cities tend to be warmer than the surrounding landscapes, and this is known as the "urban heat island effect".

In the summer there is greater heat output from air conditioning systems. In the winter there is greater heat output from heating systems (heat pumps aside). Are these a substantial contributor to the effect, or are there other causes?

Summary of question: What are the causes of the urban heat island effect, and are there specific times of the year when the effect is stronger?

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    $\begingroup$ While cooling the inside, the AC units release heat outside. $\endgroup$ – milancurcic Jun 28 '14 at 13:31
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    $\begingroup$ Good question! A quick search says the answer is "yes". Winter and summer are when the urban heat island effect is most noticeable. However, I have a world cup game to watch right now that takes precedence over research. $\endgroup$ – David Hammen Jun 28 '14 at 20:09
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    $\begingroup$ Suggesting a substantial edit, but I hope I have retained the original intent - see what you think. $\endgroup$ – Semidiurnal Simon Jul 1 '14 at 8:17
  • $\begingroup$ @SimonW I approved the edit -- a huge improvement! Well-written questions increase the chances of an answer, and make the site look like a more worthwhile time investment to any passers-by who are thinking about creating an account. $\endgroup$ – Pont Jul 1 '14 at 9:46
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As with so many concepts in geography, I don't think there is a definitive answer -- it would very likely vary from city to city and from year to year. Urban heat island (UHI) effect is affected by so many different variables, such as albedo (reflectivity of surfaces), specific heat of the materials receiving and releasing radiation from the sun, thermal conduction into the ground, cloud cover, wind, and as you mentioned, anthropogenic heat production.

Throughout the year in a city with four distinct seasons, most of these variables change independently of each other (except for probably the specific heat of the materials). For instance, in a hypothetical city that receives a great deal of snow, albedo might be much higher in winter due to the high reflectivity of snow covering low-reflectivity surfaces (such as asphalt, roofs, etc.), which might reduce its contribution to UHI by causing fewer surfaces to absorb radiation from the sun. However, as mentioned in the question, anthropogenic contributions to UHI Effect might be higher in winter in the hypothetical city from latent heat from buildings, water/sewer pipes, vehicles, and so on. Further, wind patterns might shift in winter, bringing in more cool air in winter rather than summer, which would counter the increase from the anthropogenic sources.

Also, impacts from urban heat island are actually generally most pronounced during dusk, which is when much of that short-wave radiation from the sun (which was being stored all day in the sun) is released as long-wave radiation. With that in mind, another factor that could contribute is day-length. Many cities with four distinct seasons are in the mid-latitudes, which have variable day lengths between the seasons. Longer daylight in summer, then, would suggest that the UHI Effect would be worse in summer, as there would be more time for the surfaces to absorb short-wave radiation from the sun.

Great question - sorry for not definitively answering it. It appears there are some gaps and conflicts in the limited research on this topic.

A grad student at UMN suggests that UHI effect is worse in winter (without really explaining why), while TR Oke suggests it is worse in summer, while James Voogt suggests that UHI effect is generally stronger in summer and winter compared to spring and autumn (in the mid-latitudes).

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The urban heat island (UHI) effect scales with population of a metropolitan area... the bigger the city the larger the UHI effect. UHI is not due to more heat being generated through anthropogenic activity (e.g. air conditioners) but rather it is primarily a land use change issue (e.g. from vegetative to urban). Vegetation is able to regulate heat and moisture much better than concrete and building surfaces, which trap more heat during clear sky days and then radiate that heat throughout the night. The urban heat island effect is strongest at night, when the daytime had significant direct heating of the surface (e.g. clear sky conditions), and when the atmospheric boundary layer temperature inversion is close enough to the ground to prevent ventilation / mixing with the air above.

From http://en.wikipedia.org/wiki/Urban_heat_island :

There are several causes of an urban heat island (UHI). The principal reason for the nighttime warming is that the short-wave radiation is still within the concrete, asphalt, and buildings that was absorbed during the day, unlike suburban and rural areas. This energy is then slowly released during the night as long-wave radiation, making cooling a slow process. Two other reasons are changes in the thermal properties of surface materials and lack of evapotranspiration (for example through lack of vegetation) in urban areas. With a decreased amount of vegetation, cities also lose the shade and cooling effect of trees, the low albedo of their leaves, and the removal of carbon dioxide. Materials commonly used in urban areas for pavement and roofs, such as concrete and asphalt, have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the surrounding rural areas. This causes a change in the energy balance of the urban area, often leading to higher temperatures than surrounding rural areas. Other causes of a UHI are due to geometric effects. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection and pollution from dissipating. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere. As UHI raises the temperature of cities, it will also increase the concentration of ozone in the air, which is a greenhouse gas. Ozone concentrations will increase because it is a secondary gas, aided by an increase in temperature and sunlight.

From http://www.epa.gov/heatisland/about/index.htm it says:

As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. These changes cause urban regions to become warmer than their rural surroundings, forming an "island" of higher temperatures in the landscape. Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F (27–50°C) hotter than the air, while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining. In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings.3 On a clear, calm night, however, the temperature difference can be as much as 22°F (12°C).

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