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Written by MENG Chunlei （Beijing Meteorological Institute， China Meteorological Administration， Hetian Meteorological Bureau）
Half of the Earth's population lives in cities， and the heat island effect "heats" the lives of urban people. Eggs can be cooked on the asphalt roads； the temperature of air conditionersis getting higher and higher； even the winter is becoming warmer and warmer. What are the mystery behind these phenomenon？ Is a good thing， or a bad thing？ What is the reason of these heaten islands？ How to deal with is？ Come and have a look！
What on earth is the “heat island”？
At the beginning of 19th century， in his book， Climate of London， British climatologistLuke Howard for the first time forwarded the concept of “heat island effect”， refering to the environmental phenomena that the ground temperature on the islands is higher than that of the surface of the sea water around it.
Into the 21st century， due to urban population concentration， industrial development， traffic congestion， air pollution， and the city's buildings are mostly built in stone and concrete with its low heat capacity， high thermal conductivity， coupled with the building itself to block or weaken the wind， the average annual temperature of the city can be 2℃ higher than the suburbs， or even more.
In the spatial distribution of temperature， the city is like a warm island， thus form the urban "heat island" effect. With the rapid development of urban construction， the urban "heat island" effect has become more and more obvious.
3 major hazards of urban heat island
effect
In the central area of the urban heat island， the surface temperature is high and the atmosphere is ascending. The air pressure is different from the surrounding area， and the atmosphere near the ground converges to the central area， thus forming a low pressure vortex in the urban center. As a result， greenhouse gases （sulfur oxides， nitrogen oxides， carbon oxides， hydrocarbons and other atmospheric pollutants） formed by combustion of fossil fuels in life， industrial production and transportation are concentrated in the central area of the heat island and endanger people’s health and even life. Urban "heat island" effect has three major hazards.
A large number of pollutants gather in the center of the heat island， the concentration of the surge in direct stimulation of the human respiratory mucosa， light cough， runny nose， whereas severe cases will induce respiratory diseases. Air pollutants also irritate the skin， causing dermatitis. Some substances， if entering into the eye will stimulate the conjunctiva， causing conjunctivitis. Some substances can not only damage the human kidney， but also the human nervous system. High temperatures， especially when the humidity is high， is easy to cause heat stroke. People living in the central area of heat island will be emotional irritability， apathetic， depressed depression， bringing endless trouble to work and life. Increased temperature will accelerate the photochemical reaction rate， so that near-surface atmospheric ozone concentration increases， affecting human health.
Urban "heat island" has impact on the residents living and consumption impact. In order to reduce indoor air temperature and indoor air circulation， people use air conditioning， electric fans and other electrical appliances， which need a lot of electricity， leading to the adverse impact of the national economy， and easily causing "air conditioning disease"， which also has adverse effects on human health.
Causes of urban heat island effect
Understanding the major hazards of urban "heat island"， we must take measures to alleviate its effect to “cool” the city. To take effective mitigation measures， we must understand its major causes， which are as following：
1.Underlying Surface
The urban "heat island" effect is affected by the surface features. There are a large number of artificial structures in the city， such as concrete， asphalt pavement， a variety of building walls， etc.， having changed the thermal properties of the underlying surface. These artificial structures have fast heat absorption and heat capacity. In the same solar radiation conditions， they heat up faster than natural underlying surface （green， water， etc.）， so the surface temperature is significantly higher than the natural underlying surface.
There are a large number of impervious surface in the urban surface， whose water content and evaporation is small， and can not pass through the evaporating to cool down. The heat mostly enters the air in the form of sensible heat， causing the air to heat up.
At the same time， the absorption rate of sunlight on the urban surface is higher than that of the natural surface， which can absorb more solar radiation and make the air get more heat and more temperature. In addition， the reduction of urban green space， trees and water bodies is also a major cause of urban "heat island".
Green space can absorb solar radiation， and most of the absorption of radiation energy is transformed into chemical energy during the plant transpiration and in photosynthesis， hence the energy that can be used to increase the ambient temperature is greatly reduced. Green plants in the garden， through the transpiration， continue to absorb heat from the environment， reducing the ambient air temperature. 2.Artificial heat source
The city has a large number of boilers， heaters and other energy-consuming devices and a variety of motor vehicles. These machines and human life activities consume a lot of energy， and most of the heat to the urban atmosphere. Plant production， transportation and residential life are required to burn a variety of fuels， and every day in a large number of heat emissions.
3.Lack of wind
City-intensive buildings， vertical and horizontal roads and bridges， constitute a relatively rough city underlay， and thus increase the resistance of the wind， reduce the its speed， hence the heat is not easy to lose.
4.Greenhouse gases
The city's motor vehicles， industrial production and intensive population activities， resulting in a large number of nitrogen oxides， carbon dioxide， dust， these substances can absorb large amounts of energy of heat rediation in the environment， resulting in the well-known "greenhouse effect".
Solutions of urban “heat island” effect
Understand the city "heat island" causes， we can have targeted measures to “cool” the city. The major measures include：
1.Watering
Urban green space is limited， can not take away the heat by natural evaporation. As a result， we can improve the evaporation of artificial water supply， such as road sprinkler， fountain， spray， water mist green watering measures that can increase the artificial evapotranspiration， which will take away more heat.
2.New materials on constructing roads
City roads， squares and other materials with pervious laying. That is， use new water-paved road asphalt to store rainwater， increase surface evaporation hence reduce road temperature.
3.Less artificial heat
As far as possible to civilian coal into liquefied petroleum gas， natural gas and expand the heating area is the fundamental countermeasures. Improve the utilization of energy， change coal as gas. Control the use of air conditioning temperature， improve the quality of building insulation materials to reduce the emission of artificial heat and so on.
4.Reduce pollutant emissions
Move polluting enterprises away from the city， reduce greenhouse gas emissions， further “cooling” the city.
5.Increase the surface albedo
Light colors can reflect more of the solar radiation out. Such as white with the highest reflectivity. A large number of white roofs and walls can be used to increase the surface albedo. 6.Increase vegetation coverage
The results show that the urban green coverage is inversely proportional to the heat island intensity， and the higher the green coverage is， the lower the heat island intensity is. When the coverage is more than 30%， the heat island effect is obviously weakened； when the coverage is greater than 50%， the reducing effect is extremely obvious. Concentrated greenbelt with a size of more than 3 hectares and a green coverage rate of over 60% is basically equivalent to the temperature of the suburbs natural underlying surface， which eliminates the heat island phenomenon.
TIPS：Urban “Heat Island”
Urban "heat island" effect refers to the city "high temperature" due to a large number of artificial fever， high regenerative body such as buildings and roads and green land reduction， and the temperature in the city is obviously higher than that of the outer suburbs. In the near-surface temperature map， suburban temperature changes are very small， while the urban area is a high-temperature zone， like the island of the sea. Because this island represents the high temperature of the urban area， it is vividly known as the urban "heat island".
It seems that the urban heat island effect can also be "broken". In this case， for the sake of our city， everyone is responsible. Let us together for the city's cooling， action up！
Half of the Earth's population lives in cities， and the heat island effect "heats" the lives of urban people. Eggs can be cooked on the asphalt roads； the temperature of air conditionersis getting higher and higher； even the winter is becoming warmer and warmer. What are the mystery behind these phenomenon？ Is a good thing， or a bad thing？ What is the reason of these heaten islands？ How to deal with is？ Come and have a look！
What on earth is the “heat island”？
At the beginning of 19th century， in his book， Climate of London， British climatologistLuke Howard for the first time forwarded the concept of “heat island effect”， refering to the environmental phenomena that the ground temperature on the islands is higher than that of the surface of the sea water around it.
Into the 21st century， due to urban population concentration， industrial development， traffic congestion， air pollution， and the city's buildings are mostly built in stone and concrete with its low heat capacity， high thermal conductivity， coupled with the building itself to block or weaken the wind， the average annual temperature of the city can be 2℃ higher than the suburbs， or even more.
In the spatial distribution of temperature， the city is like a warm island， thus form the urban "heat island" effect. With the rapid development of urban construction， the urban "heat island" effect has become more and more obvious.
3 major hazards of urban heat island
effect
In the central area of the urban heat island， the surface temperature is high and the atmosphere is ascending. The air pressure is different from the surrounding area， and the atmosphere near the ground converges to the central area， thus forming a low pressure vortex in the urban center. As a result， greenhouse gases （sulfur oxides， nitrogen oxides， carbon oxides， hydrocarbons and other atmospheric pollutants） formed by combustion of fossil fuels in life， industrial production and transportation are concentrated in the central area of the heat island and endanger people’s health and even life. Urban "heat island" effect has three major hazards.
A large number of pollutants gather in the center of the heat island， the concentration of the surge in direct stimulation of the human respiratory mucosa， light cough， runny nose， whereas severe cases will induce respiratory diseases. Air pollutants also irritate the skin， causing dermatitis. Some substances， if entering into the eye will stimulate the conjunctiva， causing conjunctivitis. Some substances can not only damage the human kidney， but also the human nervous system. High temperatures， especially when the humidity is high， is easy to cause heat stroke. People living in the central area of heat island will be emotional irritability， apathetic， depressed depression， bringing endless trouble to work and life. Increased temperature will accelerate the photochemical reaction rate， so that near-surface atmospheric ozone concentration increases， affecting human health.
Urban "heat island" has impact on the residents living and consumption impact. In order to reduce indoor air temperature and indoor air circulation， people use air conditioning， electric fans and other electrical appliances， which need a lot of electricity， leading to the adverse impact of the national economy， and easily causing "air conditioning disease"， which also has adverse effects on human health.
Causes of urban heat island effect
Understanding the major hazards of urban "heat island"， we must take measures to alleviate its effect to “cool” the city. To take effective mitigation measures， we must understand its major causes， which are as following：
1.Underlying Surface
The urban "heat island" effect is affected by the surface features. There are a large number of artificial structures in the city， such as concrete， asphalt pavement， a variety of building walls， etc.， having changed the thermal properties of the underlying surface. These artificial structures have fast heat absorption and heat capacity. In the same solar radiation conditions， they heat up faster than natural underlying surface （green， water， etc.）， so the surface temperature is significantly higher than the natural underlying surface.
There are a large number of impervious surface in the urban surface， whose water content and evaporation is small， and can not pass through the evaporating to cool down. The heat mostly enters the air in the form of sensible heat， causing the air to heat up.
At the same time， the absorption rate of sunlight on the urban surface is higher than that of the natural surface， which can absorb more solar radiation and make the air get more heat and more temperature. In addition， the reduction of urban green space， trees and water bodies is also a major cause of urban "heat island".
Green space can absorb solar radiation， and most of the absorption of radiation energy is transformed into chemical energy during the plant transpiration and in photosynthesis， hence the energy that can be used to increase the ambient temperature is greatly reduced. Green plants in the garden， through the transpiration， continue to absorb heat from the environment， reducing the ambient air temperature. 2.Artificial heat source
The city has a large number of boilers， heaters and other energy-consuming devices and a variety of motor vehicles. These machines and human life activities consume a lot of energy， and most of the heat to the urban atmosphere. Plant production， transportation and residential life are required to burn a variety of fuels， and every day in a large number of heat emissions.
3.Lack of wind
City-intensive buildings， vertical and horizontal roads and bridges， constitute a relatively rough city underlay， and thus increase the resistance of the wind， reduce the its speed， hence the heat is not easy to lose.
4.Greenhouse gases
The city's motor vehicles， industrial production and intensive population activities， resulting in a large number of nitrogen oxides， carbon dioxide， dust， these substances can absorb large amounts of energy of heat rediation in the environment， resulting in the well-known "greenhouse effect".
Solutions of urban “heat island” effect
Understand the city "heat island" causes， we can have targeted measures to “cool” the city. The major measures include：
1.Watering
Urban green space is limited， can not take away the heat by natural evaporation. As a result， we can improve the evaporation of artificial water supply， such as road sprinkler， fountain， spray， water mist green watering measures that can increase the artificial evapotranspiration， which will take away more heat.
2.New materials on constructing roads
City roads， squares and other materials with pervious laying. That is， use new water-paved road asphalt to store rainwater， increase surface evaporation hence reduce road temperature.
3.Less artificial heat
As far as possible to civilian coal into liquefied petroleum gas， natural gas and expand the heating area is the fundamental countermeasures. Improve the utilization of energy， change coal as gas. Control the use of air conditioning temperature， improve the quality of building insulation materials to reduce the emission of artificial heat and so on.
4.Reduce pollutant emissions
Move polluting enterprises away from the city， reduce greenhouse gas emissions， further “cooling” the city.
5.Increase the surface albedo
Light colors can reflect more of the solar radiation out. Such as white with the highest reflectivity. A large number of white roofs and walls can be used to increase the surface albedo. 6.Increase vegetation coverage
The results show that the urban green coverage is inversely proportional to the heat island intensity， and the higher the green coverage is， the lower the heat island intensity is. When the coverage is more than 30%， the heat island effect is obviously weakened； when the coverage is greater than 50%， the reducing effect is extremely obvious. Concentrated greenbelt with a size of more than 3 hectares and a green coverage rate of over 60% is basically equivalent to the temperature of the suburbs natural underlying surface， which eliminates the heat island phenomenon.
TIPS：Urban “Heat Island”
Urban "heat island" effect refers to the city "high temperature" due to a large number of artificial fever， high regenerative body such as buildings and roads and green land reduction， and the temperature in the city is obviously higher than that of the outer suburbs. In the near-surface temperature map， suburban temperature changes are very small， while the urban area is a high-temperature zone， like the island of the sea. Because this island represents the high temperature of the urban area， it is vividly known as the urban "heat island".
It seems that the urban heat island effect can also be "broken". In this case， for the sake of our city， everyone is responsible. Let us together for the city's cooling， action up！