definition
"Green Island Effect" means that the temperature of a certain area (about 3 hectares) is lower than that of surrounding buildings.
The forest is the highest vegetation. A special climate can be formed in a forest area and under the canopy. Forests can reduce the daily and annual changes of temperature, reduce the surface wind speed, increase relative humidity, increase precipitation and form a forest microclimate. This is the green island effect of the forest.
principle
Because the forest can change the wind direction, weaken the wind speed, stop the wind and sand, and play the role of wind and sand fixation and soil conservation, large-scale afforestation often becomes one of the effective measures to transform the microclimate. China's "Three North" region is characterized by heavy wind and sand, less precipitation and strong evaporation. Large-scale afforestation and windbreak construction have played a great role in changing the climate in these areas and promoting agricultural and animal husbandry production.
Greening can adjust the temperature and play the role of being warm in winter and cool in summer. In hot summer, the huge leaf area of trees and lawns can shade the sun, which can effectively reflect the solar radiation heat and greatly reduce the direct sunlight to the ground. Trees can reduce their own temperature and increase the humidity of the nearby air by evaporating water through their leaves. Therefore, the temperature of green space in summer is 3 ~ 5℃ lower than that of non-green space, and it can be reduced by about 10℃ than that of building area. Therefore, in a green place, people will feel fresh air, which can provide people with a good environment for cooling down and heatstroke. In the cold winter, in the green space with many trees, the temperature of the forest and its back can be increased by 1 ~ 2℃ because trees can reduce the wind speed and the invasion of cold air.
practical application
(A) Shanghai's large-scale greening began to show the "green island effect."
200 1 07 05 The first day of plum blossom in Shanghai was extremely hot, and the high temperature lasted for several days. At this time, the new green space in Shencheng is like a huge "air conditioner", which begins to play a pleasant "green island effect" and makes the hot air a little cool.
According to the recent comparative test results of Yan 'an Middle Road Greenland, the largest "green lung" in the downtown area of Shanghai, the total area is 230,000 square meters, and the 7.4 hectares of Greenland built in the first phase reduced the monthly average temperature in the surrounding areas by 0.6 degrees Celsius. After the project is fully completed recently, experts predict that the temperature will drop by about 1 degree Celsius. In fact, a large area of green space has natural heat absorption and cooling function, which is very obvious for alleviating the "heat island effect" in the central city.
Shanghai's urban land resources have always been precious, and it is known as "an inch of land and gold". Huangpu, Luwan and Jing 'an, located in the central city, are one of the areas with the strongest "heat island effect" because of the high-rise buildings, concentrated commercial buildings, dense houses and busy traffic. With the acceleration of the transformation of the old city, the Shanghai Municipal Government is far-sighted and has invested a lot of money to build a large-scale public green space in Zhong Yan at the junction of the three districts, so that it can exert the best ecological and environmental effects, which not only embodies the elegance of an international metropolis, but also is an important measure for Shanghai to promote the coordinated development of economy, society, population, resources and environment.
In recent years, Shanghai regards ecological environment construction as an important content to improve the comprehensive competitiveness of the city. The per capita public green area in urban areas reaches 4.6 square meters, and the green coverage rate reaches 2 1%. Last year alone, seven parks including Central Park, Hongqiao Central Garden, Jinqiao Park and Huacao Park were newly built, and 30 public green spaces of more than 3,000 square meters were built.
As a new city in Shencheng, Pudong New Area has a unique green environment and has become a bright spot to attract domestic and foreign investors. Road greening throughout the region is characterized by high, dense, thick, thick and colorful, which is pleasing to the eye; Last year, 7,700 trees of all kinds were newly planted, which made the greening landscape present a diversified pattern; The 100 hectare ecological forest built by Pudong International Airport, Huanglou Town and Liutuan Town has raised the ecological environment of the new area to a new level. The per capita public green space exceeds 10 square meter, which makes Pudong avoid the "heat island effect" in the process of urbanization.
The construction of large-scale green space has improved the real estate value of surrounding areas. After the completion of Zhong Yan Greenland, the rent of nearby office buildings soared from 7,000 yuan to more than 10,000 yuan; With the progress of green space construction, the real estate opposite Huangxing Theme Park, a newly-built large-scale green space in Yangpu District, is booming. Residents around the green space have become the biggest beneficiaries of the ecological environment.
In 200 1 year, the per capita public green area in Shanghai reaches 5.2 square meters, and the green coverage rate reaches 22%. Three new parks and 20 3000-square-meter green spaces are progressing smoothly; The construction of 75 hectares of forest belt in the green belt around the city is being implemented; A number of large-scale plantation projects in the suburbs of Shanghai have entered the start-up stage. By then, Shanghai will have more green.
Nowadays, high-rise buildings are more and more closely connected with green space, which has a greater impact on this part of the region. Green space and trees have obviously improved the microclimate. The temperature difference between green land and bare land is compared. The time period is mainly the high temperature from 1 1 to 15. By comparing the temperature difference between arbor forest, shrub forest and lawn and bare land in the park, it is found that the temperature difference between arbor forest and shrub forest is small, that is, the temperature difference between morning and evening and the highest temperature at noon is smaller than that between lawn and bare land, and the heating process is slow. However, the high temperature of lawn and bare land appeared earlier and lasted longer. Generally, the maximum temperature of arbor forest and shrub forest is 1~2 hours later than that of lawn and 3~4 hours later than that of bare land. The duration is shorter than lawn 1~2 hours, and shorter than bare land for 3~4 hours.
People here pointed out that the "green island effect" has brought ecological benefits to Shanghai and become a great wealth for the sustainable development of the city. Some experts have calculated that the 7.4 hectares of Zhong Yan Greenland Phase I only absorbs heat and cools down, which is equivalent to saving 7 1640 kWh required for air conditioning every day, equivalent to 43,700 yuan.
(B) the slow heat island effect of the construction of roof green island in Chengdu
Chengdu has fully implemented the roof greening scheme and tried to alleviate the "heat island effect" in the urban area. The plan will comprehensively standardize the greening facilities of the city's roof gardens, and strive to reach the requirement of per capita roof greening area of 0.5 square meters before the end of the year. It is specially stipulated in Chengdu that the medium and high-rise and multi-storey and low-rise non-sloping roof buildings below 12 floor and below 40 meters in height must be afforested as required; All buildings in Chengdu, which have been built in the past 20 years, have clear property rights and meet the safety requirements of construction, can implement roof greening.
Tropical/heat island effect
The phenomenon that the temperature in one area is higher than that in the surrounding area. It is expressed by the temperature difference between two representative measuring points (that is, the heat island intensity). There are two main types:
urban heat island effect
The comprehensive effects of urban population density, heat exhaustion of factories and vehicles, energy release of residents' daily life, urban building structure and underlying surface characteristics are the main reasons. 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 on a 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 the strongest in winter, the weakest in summer and the middle in spring and autumn. Shanghai, Guangzhou 10 is the strongest. The annual 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. Urban heat island can affect the temperature stratification in the surface layer and reach a certain height. Unstable stratification is dominant in cities during the day, and inversion occurs in rural areas at night. The existence of horizontal temperature difference makes the warm air in the city rise and spread to a certain height, while the nearby rural airflow sinks and converges with the city along the ground, forming a heat island circulation, which is called "country wind", and this flow field is especially obvious at night. Urban heat island also affects urban air humidity, cloud cover and precipitation to some extent. The effects on plants are early germination and flowering, delayed defoliation and dormancy.
Urban heat island effect is one of the typical characteristics of urban climate. It is a phenomenon that the temperature in cities is higher than that in suburbs. On the one hand, the formation of urban heat island is the heat emitted by people in modern big cities; On the other hand, the buildings in the city are dense, and the asphalt and cement pavements have higher heat capacity (can absorb more heat) than the soil and vegetation in the suburbs, while the reflectivity is low, 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 is centered on the city center, where 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.
"Heat Island Effect" in Qinghai-Tibet Plateau
Humboldt, one of the pioneers of modern geography, 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 the middle latitude when he visited the Andes in South America during 1799- 1804. For example, the height of the snow line on the western slope of Gongga Mountain is about 5 100 meters, while the height of the snow line on the mountain near Quito, Ecuador near the equator is only about 4,800 meters. This is unreasonable: due to the high temperature in the equatorial region, 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 has a great influence on many environmental factors, such as glaciers and living things. For example, the vertical natural belt on the south slope of Gongga Mountain is much richer than Emei Mountain with similar latitude, and the distribution boundary of many trees is also in Emei Mountain. This is the principle.