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Fact: We all know that during the day concrete absorbs heat and releases it during the night, making urban areas hotter than rural areas.

I observed that after sunset the ambient temperature is going down until late at night when it actually starts to feel hot.

I would assume that soon after sunset the temperature should decrease slowly until sunrise, but that's not the case.

So, I would like to understand which phenomena take place from sunset to sunrise in urban areas.

Take into account that I live in Athens (Greece) where there is a huge urban area and the sea is nearby.

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The phenomena that take place is one aspect of the Urban Heat Island effect. The large thermal mass (aka thermal inertia or heat capacity) of the urban fabric heats up from solar radiation during the day. Once the night-time air temperatures drop below the temperature of the buildings, they give up some of that heat again, heating the urban air. The rate at which this happens, and the rate at which the heat is kept in the city, will depend on wind speeds, the size of the city and the urban layout, amongst other things. So the place where you measure temperature, will affect the pattern you see, depending on how close to a building the measurement is, where in the city it is, and what the wind patterns are.

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Thank you for the clarifying response. Now it makes sense. –  jester Jul 11 '11 at 9:32
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