Guide to knowledge of urban planners: factors affecting urban climate
Radiation and sunshine in cities
Urban heat balance and urban heat island effect
Urban wind and local circulation
Urban precipitation and water balance
Urban air pollution and its relationship with urban climate
Urban Climate, Urban Planning and Urban Construction
4. 1 factors affecting urban climate
Cities are not only influenced by regional climate factors such as local latitude, atmospheric circulation, land and sea location, topography and geomorphology, but also by the heat and water vapor released by human activities (production and life), thus forming a local climate different from that of suburbs and villages. We usually call it urban climate.
The scope of urban climate mainly includes three parts: urban cover, urban boundary layer and tail smoke layer.
In the city's high-intensity economic activities, a lot of energy is consumed. According to statistics, a city with a population of one million consumes 3,000 tons of coal, 2,800 tons of oil and 2,700 tons of natural gas every day, and at the same time emits about 150t of dust, 150t of sulfur dioxide, 450t of carbon monoxide and 100t of nitrogen oxides. These dust and harmful gases enter the air, which will change the composition of the atmosphere, affect the transparency of urban air and the budget of radiant heat energy, weaken visibility, provide rich condensation nuclei for clouds, and affect the climate in many ways. If pollutants exceed the self-purification capacity of the atmosphere, it will also cause urban air pollution.
Due to the heat emitted by urban residents' living and production activities, such as household stoves, heating, factory production, public transportation, human and animal metabolism and other energy burning, cities have increased a lot of extra heat income than suburbs. This kind of artificial heat can approach or exceed the solar radiation heat of some middle and high latitude cities. For example, in Hamburg, Germany, the daily heat generated by burning coal is 167J? Cm2, the heat gained by direct solar radiation and sky radiation in winter is 175J? Square centimeter. In Moscow, artificial heat is more than three times that of solar radiation, which has a significant impact on urban warming.
In addition, due to the different water supply and drainage methods between urban and rural areas, a certain amount of "artificial steam" will enter the atmosphere during combustion and some industrial production processes, resulting in obvious differences in water balance between urban and rural areas.
4.2 Radiation and sunshine in cities
The total solar radiation in cities is less than that in rural areas.
High pollutant concentration? Less direct radiation? Is there much scattered radiation? Total radiation reduction
The reflectivity of urban underlying surface is small.
This is especially true in winter. Low reflectivity means high absorption rate.
Generally speaking, there is little difference between the solar radiation absorbed by urban ground and that absorbed by rural areas.
The total sunshine hours and sunshine percentage in cities are less than those in rural areas.
1 There are many air pollutants and clouds, and the transparency is low;
Convective clouds caused by heat island effect often appear.
There are obvious differences in sunshine areas within cities.
This is caused by building shading, which mainly depends on the street trend and the ratio of building height to street width: h/d.
The north wall is completely shaded in winter and sunny twice a day in summer, but the time is not long; The south wall is once a day, but it decreases with the increase of solar declination.
Urban heat balance and urban heat island effect
thermal balance
A large number of artificial heat inputs: industrial production, household stoves, air conditioning and refrigeration, motor vehicle emissions, winter heating, etc.
The thermal conductivity of the underlying surface is 3 times higher than that in rural areas, and the thermal capacity is 1/3 times higher than that in rural areas, so the thermal storage capacity is large.
Thermal income is much higher than that in rural areas.
urban heat island effect
Urban heat island-the phenomenon that the temperature in the city is higher than that in the surrounding suburbs, is one of the most typical characteristics of urban climate, which is common in both middle and high latitudes and low latitudes.
The urban heat island effect can be analyzed from two aspects:
Comparison of simultaneous temperatures in cities and suburbs
Comparison of temperature before and after the historical development of the same city
Comparison of temperature between urban and suburban areas
Tu-r— heat island intensity = temperature difference between the center of heat island and its suburbs at the same time and at the same height (above the ground 1.5m).
Urban heat island stands on the cool ocean in rural areas at home and abroad;
In winter nights, the urban area of Shanghai is 2~5 higher than the suburbs. c;
The annual average temperature in the center of Paris is higher than that in the suburbs 1.7? C
Comparison of temperature before and after urban development
With the development of urbanization, cities are getting warmer and warmer. For example, the annual temperature change in Tokyo can be divided into three stages.
1920~ 1942: The temperature change trend is increasing year by year (urban development).
1942~ 1945: The temperature trend is decreasing year by year (Tokyo was destroyed in large scale during World War II, and the urban heat island effect did not exist).
1945~ 1967: The temperature change trend shows an upward trend year by year (after the war, urban construction recovered rapidly and the temperature began to rise again).
Variation of urban heat island intensity
periodism
Diurnal variation: strong at night, weak at noon during the day.
Annual change: winter and autumn are more obvious than summer and spring, which may be due to the fact that urban heating consumes more energy in winter and releases a lot of man-made heat.
Weekly change: obviously affected by the rest day cycle, weak on weekends, Zhou Qiang.
acyclic
1) critical wind speed: the heat island effect is small when the wind speed is high, and disappears after exceeding the critical wind speed.
2) Cloudy: Strong heat islands often appear in cloudless weather.
Regional differences of urban heat island intensity
The intensity of urban heat island is closely related to the layout, shape and topography of the city. The overall layout is compact, which has a strong warm-keeping effect in the city center. The banded dispersed structure has weak warming effect in the city center. Because of the low wind speed, the heat island effect of basins or depressions is particularly strong, which not only offsets the sinking effect of cold air, but becomes the warmest heat island center.
The size of the city (area, population and density, etc.). ) also affects the intensity of heat island.
The natural landscape near the city and the nature of the underlying surface of the city also play a certain role in the intensity of urban heat island. Wide streets and squares without greening, the temperature rises sharply at noon, drops sharply at night, and fluctuates daily. Boulevard and green square are cooler during the day, and the daily variation of temperature is small.
At this time, due to the air loss near the ground in the suburbs, the air rising from the center of the heat island flows back to the suburbs at a certain height and sinks in the suburbs, forming a slow local heat island circulation, also known as the urban wind system. Near the ground, some winds converge from the suburbs to the city, which is called the country breeze.
It should be pointed out that the rural wind gathered in the city center is not very stable, and it is often intermittent or pulsating (periodic), that is, it blows for a while and stops for a while. The pulse period is about 65438±0.5~2.0h This pulse is especially obvious at night.
Influence of urban development on prevailing wind
With the development of the city, the population increases, the building density and height increase, and the roughness of the underlying surface increases, which often reduces the annual average wind speed of the city.
The average wind speed in cities is smaller than that in suburbs.
The difference of wind speed between urban and suburban areas also varies with time and wind speed: generally, the difference is large during the day and small at night; Summer is big and winter is small.
Local differences of wind in urban overburden
On the whole, the average wind speed of the city is less than that of the empty suburbs with the same height, but there are great local differences in the wind within the coverage of the city. In some places, the wind speed is extremely low; Under special circumstances, the wind speed in some places can also be greater than that in the suburbs at the same time and height. The main reasons for the difference of wind speed in urban overburden are the street trend, width, height, and the form and orientation of buildings on both sides. When the wind blows through the uneven buildings in the city, due to the barrier effect, it will produce different ascending and descending airflow, vortex and flow around, which makes the local change of the wind complicated.
When the prevailing wind meets an airtight building, part of the airflow rises over the roof on the windward side, part of the airflow sinks to the ground, and the other part bypasses the perimeter of the building and flows to the back of the house. When the prevailing wind direction is parallel to the street, the wind speed will increase due to the narrow tube effect. If the wind direction is at a certain angle to the street, the wind will be blocked and the speed will be reduced. The wind speed in the middle of the street is higher than that in the sidewalk near the building. If the wind speed in the center of the street is calculated as 100%, then the wind speed of the sidewalk on the windward side is 90%, and the wind speed of the sidewalk on the leeward side is only 45%. If roadside trees are planted beside the sidewalk, the wind speed will be reduced by 20% ~ 30% when the leaves are lush; In the shade of the park, the wind speed will be reduced by 50%.