Weather and climate in megacities

City dwellers are well aware of the fact that their weather is somehow different. Large cities simply have their own climate. And to some extent they only have themselves to blame. For cities are actually a "causative factor" in changing their own climate conditions. At the same time, they are also affected by these changes in a remarkable way: weather extremes in cities have further-reaching consequences than elsewhere. For insurers, this means huge loss potentials.

Megacities - mega-magnets. More and more people are being drawn to the city, which is where most people live nowadays. Whereas in the 1950s just under 30% of the world's population (at that time around 2.5 billion) lived in cities, today the figure stands at 50% (of six billion) and by 2025 this is expected to rise to 60% (of 8.3 billion).

Industry, commerce and infrastructure, as well as the availability of an abundant energy supply and a transportation system in cities offer good prospects for work and prosperity. Yet it appears that megacities determine and affect the weather and the climate, not only locally but also on a global scale.

Cities are heat islands

Summers and winters mean temperatures are several degrees higher (in some cases up to ten degrees Celsius) in cities than in the surrounding countryside. Weather extremes in the summer such as heat waves or thunderstorms are more frequent, whereas extreme weather conditions are moderated in the cold season (cold snaps, snow).

There is less sunshine ...

Solar radiation is lower in large cities. Industry, traffic and private households emit exhaust gases and other pollutants, as well as dirt and dust particles that do not enter the higher atmosphere but get trapped and hang like a smog dome over inner cities. Apart from having a negative impact on health (smog, respiratory diseases, allergies), they also reduce the level of solar radiation. After all, rust and dust particles reflect and dissipate the rays of the sun.

... and there is a gathering change of wind

Cities also have their own special wind climate, with long, straight canyon-like streets generating jet effects with high wind speeds. In this environment, strong turbulence and gusts can occur mostly on the lee side of tall buildings. The typical skyscraper areas in city centres are therefore predestined for strong gust-like winds.

Megacities influence the climate on a global scale

It is not only the immediate environment that is affected. Cities are large furnaces powering the greenhouse effect: Although they cover only 0.2% of the earth's surface, they emit around 80% of the greenhouse gases that affect the climate. Weather and climate changes are therefore at least partly caused by cities themselves. In addition, the impact of such changes is especially marked in cities, conurbations being particularly prone to damage and severely affected on a regular basis.

Skyscrapers and lightning magnets

The lofty buildings in many large cities (typically skyscrapers, towers, masts, antennas) attract lightning. As electronic equipment is increasingly vulnerable to overvoltages, losses occur not only at the place of the lightning strike but also within a surrounding area of several square kilometres, where they do further damage.

Climate change brings more hail, windstorms and flooding

As a consequence of climate change, we can expect the future to bring more extreme weather events such as windstorm, hail and flooding.

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  Although the silhouette of a conurbation may initially slow down approaching windstorms due to the increasing surface roughness, the storms can still wreak considerable damage in/to cities, partly as a result of the domino effect: When wind speeds are high, roof tiles or cladding may be torn off and damage neighbouring buildings, causing flying debris that in turn leads to more damage to streets and other buildings.
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  Due to the high concentrations of values in cities, (motor vehicles, buildings, public utilities, etc.) a single hailstorm can bring about substantial damage.
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  As urban areas are mostly paved with concrete and asphalt, the water cannot drain off and has to run away on the surface. During and after intense precipitation events the water cannot easily drain away as the canalisation systems are not designed to cope with such quantities of water. As a consequence, flooding occurs regularly after such events.

Megacities depend on a smooth-functioning infrastructure

People in cities are much more dependent on infrastructure (i.e. supply water, electricity, district heating, etc.) than people who live in the country. Furthermore, the latter are more likely to help one another in emergencies. Numerous natural catastrophes over the past years have made it patently clear just how susceptible the infrastructures of large cities are even to small losses and how critical bottlenecks can develop within a very short time.

The decisive point is to take possible city-climate effects into consideration right at the planning stage, including evacuation plans following natural or weather-related catastrophes. Insurers have to expect large loss accumulations in megacities and take particular account of highly insured urban and suburban districts in their scenarios. They must be aware that the weather and climate in megacities often obey their own laws.