Tropical/heat island effect

Heat island effect refers to the phenomenon that the temperature in one area is higher than that in the surrounding area. The high temperature area in the center is like an island protruding from the sea, so it is vividly called a heat island. The common phenomenon is the urban heat island effect, that is, the temperature of the city is higher than that of the suburbs. There are also non-urban heat island effects, such as the heat island effect on the Qinghai-Tibet Plateau.

Heat island effect-urban heat island effect

Urban heat island effect refers to the phenomenon that urban temperature is higher than suburban temperature, which is one of the typical characteristics of urban climate. At the beginning of the 20th century, Reich Howard, a British climatologist, first mentioned the term "heat island effect" in his book Climate in London.

On the near-surface isotherm map, the temperature in the suburbs is relatively low, while the urban area forms an obvious high temperature area, like an island exposed from the water, which is called "urban heat island". Generally, the urban heat island center is about 65438 0℃ higher than the surrounding suburbs, and the highest can reach above 6℃. Under the action of urban heat island, the near-surface heat island circulation blows from suburbs to urban areas. Urban heat island enhances air convection.

Heat Island Effect —— The Cause of Urban Heat Island Effect

The heat island effect is mainly caused by the comprehensive influence of urban population density, the heat discharge from factories and vehicles, the release of residents' living energy and the characteristics of urban building structure and underlying surface. The intensity of heat island has obvious diurnal and seasonal variations. The diurnal variation is strong at night and weak during the day, and the maximum value appears in the clear and windless night. The maximum heat island intensity observed in Shanghai is above 6℃. Seasonal distribution is also related to urban characteristics and climatic conditions. Beijing is strongest in winter. The summer is the weakest, and the spring and autumn are in the middle. Shanghai and Guangzhou are the strongest in June 5438+ 10. The average temperature difference between urban and rural areas is about 65438 0℃, such as 0.7 ~ 65438 0.0℃ in Beijing, 0.5 ~ 65438 0.4℃ in Shanghai and 0.5 ~ 65438 0.5℃ in Los Angeles. In rural areas, the temperature often reverses at night. The existence of horizontal temperature difference makes the warm air in the city rise and spread to a certain height, while the airflow in the nearby countryside sinks and converges along the ground, forming a heat island circulation, which is called "country wind". This flow field is especially obvious at night. Urban heat island also affects the air humidity, cloud cover and precipitation of the city to a certain extent. The effects on plants are early germination and flowering, delayed defoliation and dormancy. Urban heat island effect. On the other hand, there are dense buildings in the city, and asphalt and cement pavements have higher heat capacity (can absorb more heat) than the soil and vegetation in the suburbs, but the reflectivity is small, which makes the city absorb and store more solar energy than the suburbs during the day, and the temperature in the city is still higher than that in the suburbs at night. Urban heat island takes the city center as the center, a strong warm air flow rises, while relatively cold air sinks over the suburbs, thus forming suburban circulation. Under the action of this local circulation, various pollutants in the air gather over the city. Without strong cold air, urban air pollution will be aggravated, and human living environment will be destroyed, leading to various diseases and even death. There is also a classification of urban heat island effect in Shanghai, which holds that there are three causes of heat island: first, the surface properties of cities and suburbs are different, and the thermal properties are quite different. Urban reflectivity is low, heat absorption is high, and evaporation consumes less heat. Second, the man-made heat emitted by urban areas is greater than that in suburbs; Third, the urban air pollution concentration is high, and there are many aerosol particles, which plays a role in heat preservation to a certain extent. Air pollution plays a very complicated and special role in urban heat island effect. The concentration of air pollutants from industrial production, transportation and daily life is particularly high in urban areas, just like a thick blanket covering the city. During the day, it greatly weakens the direct radiation of the sun and slows down the temperature rise in urban areas. Sometimes it will have an "island" effect in the city. At night, it will greatly reduce the heat loss caused by effective long-wave radiation on the surface of the city, play a role in heat preservation, and make the city "cool" slower than the suburbs, forming a night heat island phenomenon.

Heat Island Effect —— Harm of Urban Heat Island Effect

Heat Island Effect Under the influence of the "heat island effect", clouds and fog will increase over the city, which will cause harmful gases and smoke to accumulate over the urban area and cause serious air pollution. Many human diseases are caused by the "heat island effect"

Medical research shows that environmental temperature is closely related to human physiological activities. When the ambient temperature is above 28 degrees, people will feel uncomfortable. No matter how high the temperature is, it will easily lead to irritability, heatstroke and mental disorders; When the temperature is higher than 34 degrees, with frequent heat waves, it can also cause a series of diseases, especially the incidence of cardiovascular and cerebrovascular diseases and respiratory diseases, and the mortality rate is obviously increased. In addition, high temperature can accelerate the photochemical reaction rate, thus increasing the concentration of harmful gases in the atmosphere and further endangering human health.

Under the action of "heat island effect", the temperature in every place in the city is not the same, but presents a closed high temperature center. In these high-temperature areas, the air density is low, the air pressure is low, and it is easy to produce cyclone updraft, which makes all kinds of waste gas and chemical harmful gases around constantly replenish the high-temperature areas. Under these harmful influences, residents in high-temperature areas are prone to diseases of digestive system or nervous system, in addition to bronchitis, emphysema and asthma.

The serious urban heat island effect not only affects people's normal life and work, but also restricts the further improvement of people's quality of life and the further development of the city. Therefore, it is of great significance to study the technical methods to reduce the urban heat island effect and take various measures to alleviate its impact, which will improve people's quality of life and maintain the sustainable development of the city.

Urban heat island effect may occur all year round. The main influence on residents' life and consumption is the heat island effect in high temperature weather in summer. In order to reduce the indoor temperature and make the indoor air circulate, people use air conditioners, electric fans and other electrical appliances, which need to consume a lot of electricity. For example, at present, the electricity consumption of the United States 1/6 is used for refrigeration, which needs to pay 40 billion US dollars. High temperature weather also has adverse effects on human health. Related research shows that. No matter how high the temperature is, it is easy to cause symptoms such as irritability, heatstroke and mental disorder; If the temperature continues to be higher than 34℃, it can also lead to a series of diseases, especially the incidence of cardiovascular and cerebrovascular diseases and respiratory diseases, and the mortality will increase significantly. In addition, the temperature rise will accelerate photochemical reaction, increase the ozone concentration in the near-surface atmosphere, and affect human health.

Heat island effect-the role of trees in reducing heat island effect

Plant cover reduces heat island effect. Like forest trees, the contribution of urban forests to reducing carbon dioxide in the atmosphere and their carbon sequestration capacity vary with the growth and death of trees, tree species composition, age structure and forest health. Growth rate and life span reflect the storage capacity of carbon dioxide. Although fast-growing trees can absorb more carbon dioxide in the early stage, they can absorb more carbon dioxide in the early stage.

Taking hybrid poplar with fast growth but short life cycle and sugar maple with slow growth but long life cycle as examples, hybrid poplar can absorb about 2460 kilograms of carbon dioxide in a 30-year life cycle, while sugar maple can absorb about 3225 kilograms of carbon dioxide in a 60-year life cycle.

Heat Island Effect —— A Method to Avoid or Reduce Urban Heat Island Effect

Urban heat island effect (1) should protect and increase urban green space and water area, because urban water and green space have a considerable weakening effect on urban heat island effect in summer. Urban green coverage rate is inversely proportional to urban heat island intensity. The higher the green coverage rate, the lower the intensity of urban heat island. When the coverage rate is greater than 30%, the urban heat island effect is obviously weakened. When the coverage rate is more than 50%, the effect of reducing heat island in green space is extremely obvious. The concentrated green space with a scale of more than 3 hectares and a green coverage rate of more than 60% is basically equivalent to the temperature of the natural underlying surface in the suburbs, that is, the heat island phenomenon is eliminated, and a low-temperature zone centered on green space is formed in the city, which has become an excellent environment for people's outdoor recreation activities.

(2) The intensity of urban heat island is strengthened with the development of the city, so we should control the urban population density and building density while controlling the urban development, because densely populated areas are also high-density areas and high-energy-consumption areas of buildings, which often form high-temperature areas.

The increase of industrial zones will increase the heat island effect. (3) For example, Beijing is located in the middle of the plain, surrounded by mountains on three sides. Due to the influence of mountain valley wind, the wind direction from south to north prevails. North at night and south during the day. Therefore, when expanding the new urban area or rebuilding the old urban area, we should appropriately widen the north-south streets to strengthen urban ventilation and reduce the intensity of urban heat island. When planning buildings, we should combine the local wind direction and don't take all the buildings away. At the same time, it is best to tear down the walls of some units and build fences to increase air circulation.

(4) Water surface, wind force, etc. It is also an effective factor to alleviate urban heat island. Water has a large heat capacity, and under the condition of absorbing the same heat, the temperature rise is the smallest, showing a lower temperature than other underlying surfaces; Evaporation and heat absorption on water surface can also reduce the temperature of water body. Wind energy can take away the heat of the city and alleviate the urban heat island to some extent.

(5) Reducing man-made heat release, changing civil coal into liquefied gas and natural gas as far as possible, and expanding heating area are also fundamental countermeasures.

Heat Island Effect —— "Heat Island Effect" of Qinghai-Tibet Plateau

Humboldt, one of the pioneers of modern geography on the Qinghai-Tibet Plateau, who was influenced by the heat island effect, found that the snow line of the high mountains near the equator was about 200 meters lower than that of many high mountains on the Qinghai-Tibet Plateau in mid-latitude. For example, the snow line on the western slope of Gongga Mountain is about 5 100 meters high. The alpine snow line near Quito, Ecuador, near the equator is only about 4800 meters. This is unreasonable: due to the high temperature at the equator, the alpine snow line should usually drop from the equator to the poles and drop to sea level near the poles.

Based on this, Humboldt put forward the theory of "heat island effect" in Qinghai-Tibet Plateau: the main direct heat source of troposphere atmosphere is the ground, or "underlying surface". As the underlying surface of the Qinghai-Tibet Plateau rises in a large area (equivalent to raising the "stove"), its heat is much higher than that of other areas with the same latitude and altitude, even higher than that of areas with the same altitude near the equator.

The "heat island effect" on the Qinghai-Tibet Plateau affected by the heat island effect has a great influence on many environmental factors, such as glaciers and organisms. For example, the vertical natural belt on the south slope of Gongga Mountain is much richer than Emei Mountain at the same latitude, and the distribution boundary of many trees is also in Emei Mountain. This is the principle.

Heat Island Effect —— A Method to Prevent Heat Island Effect

1. Choosing efficient and beautiful greening forms, including street park, roof greening, vertical wall greening and waterscape setting, can effectively reduce the heat island effect and obtain a fresh and pleasant indoor and outdoor environment.

2. The greening management of residential areas should establish a management mechanism combining greening with environment, and formulate relevant local administrative regulations to ensure green land.

3. It is necessary to carry out greening planning for roads, high-altitude corridors and streets with relatively dense greenhouse gas emissions, create a green ventilation system, introduce fresh air from outside the city into the city, and improve the microclimate.

4. To eliminate bare land and dust as an important part of urban management. Except for buildings, hard roads and trees, all floors should be paved with lawns, and even in places where lawns are difficult to grow at the crown projection, broken corn stalks and sawn wood chips should be paved to improve the specific heat capacity of the ground.

5. Build a number of tree-lined avenues, so that the urban area will form a belt-shaped green channel, and gradually form an urban agglomeration layout with green as the isolation belt, thus weakening the heat island effect;

Under the existing conditions, 1 should be considered to control the use of air conditioning, improve the quality of building insulation materials and reduce man-made heat emission; Improve the water retention performance of urban roads. 2. Illuminate the building to increase the heat reflection. 3. Improve energy utilization rate and turn coal into gas. 4. In addition, the "permeable road paving plan" refers to paving roads with new asphalt with strong permeability to store rainwater and reduce road surface temperature. 5. Form a water system around the city to adjust the urban climate.

Heat Island Effect —— Suggestions on Weakening Urban Heat Island Effect

(1) It is necessary to protect and increase urban green space and water area, because urban water and green space play a considerable role in weakening urban heat island effect in summer.

(2) The intensity of urban heat island is strengthened with the development of the city, so we should control the urban population density and building density while controlling the urban development, because densely populated areas are also high-density areas and high-energy-consumption areas of buildings, which often form high-temperature areas.

(3) For example, Beijing is located in the middle of the plain, surrounded by mountains on three sides. Due to the influence of mountain valley wind, the wind direction from south to north prevails. There are many northerly winds at night and southerly winds during the day. Therefore, when expanding the new urban area or rebuilding the old urban area, we should appropriately widen the north-south streets to strengthen urban ventilation and reduce the intensity of urban heat island. (4) Reduce man-made heat release, try to change civil coal into liquefied gas and natural gas, and expand the heating area.

Because the specific heat of water is greater than that of concrete, under the condition of absorbing the same heat, temperature difference will be formed due to different temperatures, which will inevitably increase the circulation speed of thermal cycle. In the process of atmospheric circulation, the water system around the city will play the role of secondary cooling, so that the urban temperature will not be too high, and the purpose of preventing urban heat island effect will be achieved. Dense urban population is also one of the important reasons for the formation of heat island effect. Therefore, in the future new town planning, we can consider leaving only the central government and municipal government, tourism, finance and other departments in the city center, and moving the rest to the satellite city, and then connecting the satellite city with the subway around the city.