The sun is a source of energy, it’s inherent to the summer season, it warms us up and recharges our batteries, but when there’s too much of it – it’s too much for us too. Especially in cities.
While 39% of the world’s population lived in cities in 1980, today it’s about 55%, and this is expected to rise to 66% by 2050. Slovakia is on a similar upward trend, with more than half of the population now living in cities (53.5% in 2021). Built-up areas are becoming hotspots for the effects of climate change, and it is in Central Europe that cities are highly vulnerable to heat waves, droughts and flash floods.
Why is this and what is meant by the term ‘urban climate’ or ‘urban heat islands’?
Urban climate is defined as a local climate that differs from that of adjacent rural areas as a result of urban development. Urban Heat Island (UHI), in turn, defines urban development that has been shown to have higher temperatures compared to its surroundings (rural or less built-up areas in the countryside).
There are a number of factors that cause heat islands or influence urban climate. Cities are characterised by densely built-up areas with impermeable surfaces. The limited and diminishing amount of natural features in cities is considered to be a major factor. These are vegetation and water bodies that cool the environment by providing shade, evaporating surface water and carrying water from plant leaves into the air.
Hot urban environments and surfaces (e.g. pavements, roads, buildings) provide less shade than natural landscapes, resulting in increased temperatures. Lack of green space, or the complete absence of it, and water have been shown to hinder natural cooling. Another factor is the materials (e.g. metals, asphalt, concrete) that are used by man in the construction of buildings and infrastructure. Compared to vegetation, water and natural materials, these materials absorb more heat during the day and subsequently emit more heat at night. Buildings in cities can also be a problem as they slow down the wind speed (i.e. ventilation of the city), leading to higher air temperatures. The latter can also be increased by human activity itself and its effects, the so-called waste heat discharged into the environment (e.g. heat emissions from transport, heating and air-conditioning of buildings and industrial facilities).
What temperatures are we talking about in relation to the urban tree effect?
Just the shade of a single tree can provide shelter from the heat in a sweltering summer, and when that single tree is part of an urban park it has a very strong cooling effect. Results of research carried out abroad show that shaded surfaces can be 11-25 °C cooler than the maximum temperatures of unshaded surfaces. A simple measurement was also carried out in neighbouring Bohemia, where a thermal imaging camera was used to measure the temperatures on the pavement in different parts. On the part of the pavement where the shade from the trees was falling, values of 27-30 °C were measured. A few steps away, in a place where there were no trees, the temperature on the pavement rose to 40 – 45 °C, a difference of almost 20 degrees. The important thing to realise is that even planting and caring for trees in towns and cities has certain rules in order to have the desired effect.
Why do heat islands need to be addressed?
On hot summer days it is necessary to think about the fact that high temperatures have a negative effect on our organism. Heat waves are especially dangerous for the elderly and sick people, as cardiovascular diseases increase the risk of death from overheating. After all, we all know for ourselves – how do we feel when it is too hot in summer?
More interesting info here https://www.meteoblue.com/sk/products/cityclimate/index
Author. Lenka Blinová, PhD.
