What exactly is a heat island?
– Most cities, at least in temperate and high latitudes, have such a thing as urban heat islands. They were first discovered in relation to London a little over 200 years ago by the eminent British amateur meteorologist Luke Howard. Since then, this phenomenon has been confirmed and investigated in almost all major cities and many medium-sized cities.
The urban, urbanized surface is almost always warmer than the countryside outside.
Although there are exceptions. In tropical desert regions, oasis cities, on the contrary, form cold islands – their temperature is lower compared to the surrounding hot desert. But in most parts of the world, cities create heat islands. They are actively studied, this phenomenon is very important.
Generally speaking, heat islands are not limited to big cities. In any settlement – and in small towns, towns and even villages. Each settlement creates its own heat island, the smaller the settlement, the weaker it is. But still, it’s there.
Even individual city gardens create their own local heat islands.
How are heat islands formed?
— There are many factors that create heat islands. First, these are direct heat emissions from anthropogenic activities. First of all, this is the business of city heating, especially in the cold season. It also includes heat emissions from moving cars, as well as metabolic heat generated by human and animal organisms – but this is a negligible part of the total anthropogenic heat flux.
In addition, the appearance of heat islands is affected by special surfaces such as concrete, asphalt pavements with high heat capacity.
Therefore, urban heat islands are especially strong in the evening and at night due to the inertia of the cooling of these surfaces.
Less evaporation of precipitation also affects, because in cities, as a rule, their artificial flow is regulated, and the heat consumption for their evaporation is less than in the countryside.
Outside the city, the temperature is also lower due to the transpiration of plants – the movement and evaporation of water through stems, leaves, flowers. There are very few plants in the city outside of the local forest parklands. Meanwhile, urban forests and parks, on the contrary, create their own so-called “islands of cold” within the city.
The whole city is a heat island, and a park with trees forms an island of cold in it – a local drop in temperature. It has to do with transpiration of plants.
In addition, industrial haze plays an anti-radiation role – in the city, as a rule, there is a high content of suspended particles in the air, and they form a curtain that partially returns heat.
– I’ve heard that in big cities of the USA and European countries, the temperature in the city can be 10-15 °C higher than outside…
– There is the concept of the average density of the heat island over a period of time – for example, over a period of several years. And there is the concept of maximum intensity – one-time indicators in these conditions, where heat islands are very pronounced. As a rule, this is observed during anticyclones, when stagnation, calmness or an extremely weak wind is observed, the sky is especially clear at night. In pronounced anticyclonic weather conditions, heat islands are very pronounced. Therefore, there are different ratings. The maximum intensity of the heat island is observed between 10 and 15 °C, both in the USA and in European cities and Russian cities.
In the last few years, the maximum intensity was recorded by us in Moscow on the night of February 23, 2018 – 11.5 ° C. That’s a lot, but not important for average conditions.
On average, Moscow is 1 °C warmer than its surroundings, and the city center directly where the densest buildings are built is about 2 °C warmer. This is an important indicator.
– How does it affect the microclimate of Moscow, the weather?
– 2 °C is too much.
In the suburbs, in the countryside, spring frosts occur – but not often in Moscow. In many plants, flowering, fruiting is observed earlier, the growing season lasts longer. The maturation of agricultural products is more active and faster. Severe frosts are very rare in Moscow – much more often in the Moscow region. This difference in air temperature has a lot to do with it.
Also, heat islands are associated with lower relative humidity. More precipitation falls in many cities – but vice versa, the effect is uncertain. But, first of all, heat islands still affect the temperature. Not just air – surface temperature, but temperature deep in the ground, temperature at altitude.
— Is Moscow possibly the “warmest” city in Russia?
– Maybe. Moscow has a very large population, 12 million. No other Russian city can even come close to Moscow in terms of population and therefore anthropogenic emissions. However, I have not researched this topic and cannot say with certainty that no other city can compare with Moscow in terms of heat island strength. Geographical conditions can also play a role.
In addition, many cities in Russia have not been studied from this perspective. There are several groups of researchers working in Moscow, including myself with my colleagues and students. The heat island in Tomsk has a group of researchers, one of which is St. He studied the heat island in St. Petersburg. Unfortunately, these studies are not carried out as actively in our country as in some other countries.
– And why?
“The reason is that we have a very cold country with a cold climate. Therefore, the danger posed by heat islands in other countries – the danger of additional heat stress in abnormally hot weather conditions – is not so important to us. The climate in Russia is so cold that heat islands rarely appear as a negative phenomenon.
In our country, on the contrary, heat islands are quite a positive phenomenon, which allows, for example, to reduce the cost of urban heating in winter.
And in other countries – tropical, subtropical – people face very strong heat stress in the heat, because urban heat islands exacerbate this even more. In Russia these are extremely rare cases for such intense heat. We usually have short episodes. A very long catastrophic heat wave occurred in 2010, when a long-lasting blocking anticyclone existed in the Middle Volga region for two months, and for 60 consecutive days the temperature was significantly above the climatic norm. Later, 55 thousand people died on the European side of Russia due to the heat, and in Moscow the temperature was further intensified by the heat island of the capital. But this is an exceptional event, nothing like this has happened before or since.
– Is it important for people living in the most “hot” parts of the city to take some additional protection measures, carefully monitor their health?
“I don’t think it really matters, except for rare cases of abnormally hot weather during the summer months.
– When cities are being built and roads are being laid, is information about heat islands taken into account?
– I think that this information does not affect our decision in any way. In some other countries where heat islands could pose a danger, they are trying to combat additional temperature rises. There are projects of white asphalt, white roofs, green roofs to reduce the heat of these artificial surfaces. The more green spaces in the city, the weaker the heat island as a whole. For this reason, it is important not to reduce the green area areas.
— How big is the heat island’s contribution to climate change?
– On the regional climate scale, the effect is of course noticeable. But on a global scale, it’s still small. Cities occupy only 2% of the planet’s surface. About half of the world’s population live in them, but they take up very little space.
The city is just a hot, hard spot on the planet’s body.
– That is, when the heat islands are not worth fighting?
– There is such a need in countries with a warm climate. In general, such decisions depend on geographic location. They do not fight heat islands everywhere, and everywhere this phenomenon is not so dangerous. And struggle does not bring results everywhere.
In South Korea, they even opened a city river that had been closed for decades, a main road running through the city centre. The road was moved, probably hundreds of millions of dollars spent. It was thought that this would greatly cool the area, but the cooling effect turned out to be very weak. Because the effect of open water, such as green spaces, is often very local and manifests itself in just a few tens of meters on either side of its shores.