Most city dwellers know what it is like to return from a summer's day out in the country and suddenly feel a blast of heat upon arrival back in town. The phenomenon of "heat islands," as these concentrations of urban warmth are called, is a worldwide one. And as cities continue to grow, the temperature rises with them.

A number of factors combine to create the effect: the use of building materials, such as concrete, that store the heat, the reduction in wind circulation because of building development and the high energy output from buildings and cars. And rising temperatures due to climate change is exacerbating the whole problem.

In central Europe, urban warming is most troublesome during summer nights.

"In extreme situations the nights in the city center are eight degrees [Celsius] warmer than in the surrounding countryside," according to Professor Dieter Scherer, a climate researcher from the Technical University in Berlin. The old and the young find the extreme heat particularly hard to cope with.

A problem that is not going to go away

Experts say that long-term strategies are needed to combat the effect. Berlin may now already feel distinctly autumnal, but, if this unseasonably warm April was anything to go by, summer days will soon be back again.

One of the most significant ways to combat climbing temperatures in cities is by increasing the amount of greenery. Parks do not store as much heat as built-up areas, and they provide a cooling effect as water evaporates into the air from leaves and vegetation by the process of transpiration.

Recent research conducted by Professor Scherer has shown that when it comes to green space, it is not just size that matters. The study found that a network of small and medium-sized green spaces were actually more effective at helping to reduce temperatures than a smaller number of large parks. During balmy nights, only the residents living right next to the big parks profited from the cooling effect.

Benjamin Bongardt, a postgraduate student in applied climatology at the University of Duisburg-Essen, also recently found that even a 10-meter square (108 square feet) park can make a significant difference.

Urbanites take matters into own hands

When it comes to vegetation, there is also plenty of scope for city residents to take things into their own hands -- both legally and illegally. Community gardens are becoming increasingly popular, as is guerilla gardening. Here, disused or neglected land is taken over and transformed.

The environmental organization, the Grüne Liga (Green League), provides advice for apartment block owners and tenants who want to turn their grey courtyards into green oases.

In Berlin, the city authorities no longer fund these measures, but some individual districts continue to do so and there is continued interest from the grassroots.

"Many tenants have taken the initiative themselves. They are very committed. There is a lot of interest particularly in the inner city districts," said Karen Thormeyer, press spokeswoman for the Berlin branch of the Grüne Liga.

Relief-giving plants

"The more plants there are, the more transpiration takes place. You can even use the facades of buildings. You can do a lot with trailing plants and they really don't take up any space," Thormeyer added.

Roof planting is also popular, although Professor Scherer from the Technical University warns that if the plants are allowed to dry out, then green roofs can actually heat up more than conventional ones, as can parks themselves.

In Stuttgart, even the space between the tracks of the local railways have been planted with grass. The southern German city has long been concerned with environmental matters, partially because its location in a hollow encircled by hills is far from favorable.

The city's climatology department, now headed by Professor Jürgen Baumüller, has been in existence for almost 70 years. "Our goal is to help ensure that the circulation of air is taken into account in city planning," said Baumüller.

Change of habits necessary

"People will probably also have to learn to change their habits, for example by reducing their car and overall energy use. And rather than opening all their windows when it's hot, they'll have to close their shutters like people do in southern countries," he added.

Scientists at the department of applied climatology at the University of Duisburg-Essen, are also helping city planners get the bigger picture and thus combat the problem of heat islands.

One of the main thrusts of their research is producing climatological maps for city planners. Producing an analysis for Gelsenkirchen, a city in the middle of the Ruhr region, Germany's biggest conurbation, was the most interesting challenge so far, according to climatologist Andreas-Bent Barlag.

"The maps help city planners see how they can bring about improvements in the city's climate. We can help minimize the effect caused by a new development or we can tell people to build differently with more gaps so that the wind can get through," Barlag said.