Hot nights in the city

New research has shown that those people living in urban areas are likely to be more exposed to heat stress during the night than those living in rural areas. This additional heat stress means the impacts of climate change on the health of people living in cities is likely to be much greater, especially in areas of urban expansion.

The reason behind this is because of the way cities modify climate.

Replacing the natural landscape with urban areas alters the atmosphere in two primary ways.

·      Urban structures are more efficient at storing heat than natural landscapes and this heat is released when the sun goes down.

·      Urban surfaces are generally impervious, which means that water stored in the ground is unable to evaporate into the atmosphere, limiting evaporative cooling.

Of less importance is that energy is often trapped in city environments by the multiple reflections of outgoing radiation. There is also additional heat generated by human activity.

In the past, research into the urban heat island effect has traditionally been studied from the perspective of temperature only. But other variables like humidity, wind and radiation are also key factors that affect human comfort.

If temperature were used as the only indicator of heat stress then the effects of cities would be partly misestimated.

Earlier research has showed that wind and radiation do not change dramatically when cities replace the natural environment but humidity does. The higher the humidity the harder it is for a body to cool down during warm periods.

With climate change we know that both temperature and humidity will increase. At the same time, cities are expected to expand with a growing urban population. Thus it is very important to estimate the heat-stress levels people living in these areas will be exposed to.

Our latest research examines how the combined impact of climate change and urban expansion affect heat stress. It revealed quite different responses to heat stress during the day and night.

During the day, there is very little difference in temperature between new urban areas and the natural environment. This means changes to daytime temperatures will mostly be driven by global warming.

Humidity in new urban areas during the day will increase at a lower rate than in natural areas, so during the day the net effect may actually be slightly less heat stress impacts than in the surroundings.

However, we find a very different impact overnight.

At night there is almost no difference in humidity between the natural surroundings and developments on the fringes of the city. The difference between the two areas can be found in temperature as the built structures of fringe suburbs emit heat that has accumulated through the day back into the atmosphere. So, as opposed to daytime, temperature differences dominate nighttime heat-stress contrasts, while humidity differences just play a minor role.

The result is that warmer nights due to urbanization and climate change makes it much harder for the body to recover from the heat of the day. If adverse conditions persist over successive nights, the body may not be able to recover from the heat stress of the day.

Research indicates that persistent warm temperatures over a number of nights during a heat wave tend to correlate with increases in hospital admissions, particularly for vulnerable members of the community.

Even where serious health impacts do not occur, many parts of the population are affected by significant sleep disruption.

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