More than ever Rome was facing a water shortage this summer. Every year many Romans leave the city in the hot summer months, but the city attracts many tourists so the use of water does not get significantly less. In ancient times many of the million+ inhabitants lived in 'insulae' and called these days 'dies canincula'. Built up areas - like the ancient insulae - are good at trapping heat and make the city center warmer than its surroundings: the 'Heat Island Effect' (HIE).

A recent study funded by the European Commission shows a worrying trend in a business-as-usual scenario for greenhouse gas emissions. The weather related premature mortality can increase 50 times by the year 2100: mainly due to heat waves, especially in the south of Europe (Increasing risk over time of weather-related hazards to the European population: a data-driven prognostic study. Forzieri et al, Lancet Planet Health; vol. 1 august 2017, 200-208).

Cool paving Roads are covered over with a (off-)white layer to reflect the sun rays. Other benefits: less degradation of the asphalt when it blocks ultraviolet, water and reduced oxidation. Roads will last longer and repairs are better protected (good news for the Rome based association called "Tappini").

Cool cars and bikes Cars painted white or another bright color that reflect more of the suns heat. The airco has less to cool, they are better visible in the dark and therefore safer. Another point: burning a liter of fuel produces heat, harmful pollutants and over a kilogram of water vapor. For a big city like Rome that's a lot of extra humidity. So the logical next step is electric vehicles (EV): cars, scooters and bikes. The Ferrari 512S Modulo design is a good starting point for it has single curved panels that could be equipped with solar cells.

Cool roofs We can paint roofs and walls (off-)white but for historical cities like Rome there has to be a more acceptable solution. We can slowly fase out the darker - heat absorbing - tiles together with a novel idea presented in the next section.

Plants, trees and water also help to cool a city.

In summer trapping heat in a city center is something you want to avoid. But in winter this gives a reduction in the costs of heating. Is there a best of both worlds possible with heat adaptive structures? In nature the flower opens in the summer sun: what if an increase in temperature or sunlight opens a heat reflector?

Temperature activate switches can be made with a strip of two metals fused together. Because both metals react different when the heat increases the combined metal bents. Around the world a number of shape-change polymers have been developed that have a better performance. Even 2 way memory effect polymers that have a distinct start shape en distinct shape when heated.

The heat adaptive foil - in the same color as the original tiles - can be glued on the roof and when heated enough the 'flower' parts curl up and create a reflective surface. As is shown above. With small flowers this can just as well be used on curved surfaces.

With more complex surfaces there is another option: cold they look like closed flowers on short stems. These rows of polymer flowers will always be visible and when the temperature is high enough they open. The open flowers will create white reflectors as is shown next. It is the reverse of the first idea.

The shape is not restricted to 'fowers', other shapes can possibly give a better reflector. Esthetics and behavoir in stormy weather are crucialy important and need to be tested.