Urban Climate: Formation and Impacts | 城市气候的形成与影响

📚 Urban Climate: Formation and Impacts | 城市气候的形成与影响

Urban climate refers to the local climatic conditions that differ significantly from those of surrounding rural areas, created by the modification of natural surfaces and atmospheric processes through urbanisation. This article examines how cities generate distinctive heat, wind, moisture and pollution patterns, and evaluates their environmental and human consequences.

城市气候是指因城市化过程改变自然下垫面和大气过程而形成的局地气候，其条件与周边乡村地区显著不同。本文探讨城市如何产生独特的热量、风、湿度和污染格局，并评估其环境与人文影响。

1. The Concept of Urban Climate | 城市气候的概念

Urban climate is a local-scale climatic system produced by the replacement of vegetation and soil with buildings, roads and other impervious surfaces. Compared with rural areas, cities typically show higher air temperatures, reduced wind speeds near the surface, lower humidity, greater cloudiness and increased precipitation downwind of the urban centre. These differences are most pronounced on calm, clear nights when radiational cooling is strong in the countryside but suppressed in the city.

城市气候是由植被和土壤被建筑物、道路及其他不透水表面替代而产生的局地气候系统。与乡村相比，城市通常表现出更高的气温、近地面风速降低、湿度减小、云量增多以及城市下风方向降水增加。这些差异在晴朗无风的夜晚最为明显，此时乡村辐射冷却强烈，而城市却受到抑制。

2. The Urban Heat Island (UHI) | 城市热岛效应

The urban heat island is the most famous feature of urban climate, referring to the condition in which urban air and surface temperatures are warmer than those of the surrounding countryside. On average, a city of one million people may be 1–3 °C warmer than its rural hinterland, but under ideal anticyclonic conditions, the difference can exceed 10 °C. The greatest warming occurs at the urban canopy level (rooftop to ground) and is strongest at night.

城市热岛是城市气候最著名的特征，指城市空气和地表温度比周边乡村更暖的现象。平均而言，一座百万人口的城市可能比其乡村腹地高 1–3 °C，但在理想的反气旋条件下，温差可超过 10 °C。增温最显著的区域位于城市冠层（屋顶至地面）范围内，且夜间最强。

3. Causes of the Urban Heat Island | 城市热岛的形成原因

Reduced albedo and heat storage: Building materials such as concrete, brick and asphalt have a lower albedo than vegetation, so they absorb more shortwave radiation during the day and re-emit longwave radiation at night.
Anthropogenic heat release: Heat from vehicle engines, air conditioners, power plants and industrial processes is released directly into the urban atmosphere.
Reduced evaporation: Urban surfaces are impermeable, so less water is available for evapotranspiration. Energy that would be used to evaporate water instead warms the air as sensible heat.
Urban canyon geometry: Tall buildings trap longwave radiation by reducing the sky view factor, limiting outward radiation loss and slowing the development of nocturnal inversions.
Pollution blanket: Aerosols and greenhouse gases from urban emissions absorb and re-emit outgoing longwave radiation, further reducing heat loss.

反照率降低与热量储存：混凝土、砖和沥青等建筑材料反照率低于植被，白天吸收更多短波辐射，夜间再发射长波辐射。

人为热释放：机动车发动机、空调、发电厂和工业过程所产生的热量直接释放到城市大气中。

蒸发减少：城市地表不透水，可用于蒸散的水分较少。原本用于蒸发水的能量转而以显热形式加热空气。

城市峡谷几何结构：高层建筑通过减小天空视角因子来捕获长波辐射，限制向外辐射损失，并延缓夜间逆温的形成。

污染覆盖层：城市排放的气溶胶和温室气体吸收并再发射外逸长波辐射，进一步减少热量损失。

4. Factors Controlling UHI Intensity | 控制热岛强度的因素

UHI intensity is not constant but varies with weather, location and time. The strongest heat islands occur under high pressure, with calm winds and cloudless skies, because these conditions maximise radiational cooling in rural areas while urban heat storage continues. Wind speed is critical: light winds allow the heat island to remain spatially coherent, whereas strong winds mix the air and reduce the temperature contrast.

热岛强度并非恒定，而是随天气、地点和时间而变化。最强热岛出现在高压控制下，风小且无云，因为这些条件使乡村辐射冷却达到最大，而城市储热仍在继续。风速至关重要：微风使热岛在空间上保持连贯，而强风混合空气并减小温差。

Seasonally, UHI intensity is often greatest in winter, especially in mid-latitude cities where space heating adds anthropogenic heat and the lower sun angle reduces solar warming of rural surfaces. Topography also matters: cities in valleys or basins, such as Los Angeles or Beijing, experience stronger inversions and thus more intense heat islands than cities on open plains.

季节上，热岛强度通常在冬季最大，尤其在中纬度城市，因为取暖增加人为热且太阳高度角低减少了乡村地表的日照增温。地形也起作用：位于山谷或盆地中的城市，如洛杉矶或北京，逆温更强，热岛也强于开阔平原上的城市。

5. Urban Wind Patterns and the Canyon Effect | 城市风场与峡谷效应

Urban structures dramatically alter wind flow. At the regional scale, the urban heat island generates a weak pressure gradient that induces a country breeze, bringing cooler air from the periphery towards the city centre. This circulation is strongest during warm afternoons when the temperature contrast peaks, and it plays a role in ventilating polluted urban air.

城市结构大幅改变风场。在区域尺度上，城市热岛产生微弱的水平气压梯度，引发乡村风，将周边较凉的空气带向市中心。这种环流在午后温差最大时最强，对城市污染空气的通风起到作用。

At the street scale, buildings channel wind along canyons, producing funnelling effects. When wind blows parallel to a street, it accelerates; when it blows perpendicular, it creates rotor eddies and downdraughts, leading to uncomfortable and unpredictable gusts for pedestrians. The presence of tall buildings can also deflect high-level winds downwards in a process called downwash, carrying pollutants emitted from tall chimneys to ground level.

在街道尺度上，建筑物将风沿峡谷引导，产生狭窄效应。当风向与街道平行时，风速增大；当风向垂直时，则形成旋转涡流和下曳气流，给行人带来不适且不可预测的阵风。高层建筑还会将高空风向下偏转，即下洗作用，把高烟囱排出的污染物带到地面。

6. Urban Moisture and Evaporation | 城市湿度与蒸发

Cities are generally drier than the countryside. Impervious surfaces prevent infiltration and reduce the surface area for evaporation; storm drains rapidly remove water, and vegetation is scarce. Consequently, relative humidity in urban areas is often 5–10 % lower than in rural surroundings. However, absolute humidity may sometimes be higher because of combustion processes that produce water vapour and cooling towers that emit plumes of steam.

城市通常比乡村干燥。不透水表面阻止下渗并减少蒸发面积；雨水系统快速排水，植被稀少。因此，城市相对湿度通常比乡村低 5–10 %。但绝对湿度有时可能更高，因为燃烧过程产生水蒸气，冷却塔排放蒸汽羽流。

This drier atmosphere, combined with high temperatures, increases the vapour pressure deficit, which intensifies plant water stress in urban parks and gardens. It also means that urban residents experience greater physiological discomfort during heatwaves, partly because the body’s cooling mechanism depends on the ability to evaporate sweat.

这种干燥大气与高温相结合，增大了饱和蒸汽压差，加剧了城市公园和花园中植物的水分胁迫。这也意味着热浪期间城市居民感受到更大的生理不适，部分原因是人体散热机制依赖汗液蒸发能力。

7. Urban Precipitation and Cloudiness | 城市降水与云量

There is substantial evidence that cities enhance cloudiness and precipitation, especially in and downwind of the urban area. Several mechanisms are involved. First, the urban heat island increases convective instability, making moist air more likely to rise and form cumuliform clouds. Second, aerosols from traffic and industry act as cloud condensation nuclei, promoting the formation of droplets at lower supersaturations. Third, roughness from buildings causes mechanical turbulence and slows storm systems, increasing the duration of rainfall over the city.

有充分证据表明城市增强云量和降水，尤其在城市及其下风方向。涉及几种机制。第一，城市热岛增加对流不稳定性，使潮湿空气更易上升形成积状云。第二，交通和工业产生的气溶胶作为云凝结核，在较低的过饱和比下促进水滴形成。第三，建筑粗糙度产生机械湍流并减缓风暴系统移动，延长城市降雨历时。

Observational studies of large cities such as London, Paris and St Louis indicate that summer rainfall may be 10–20 % greater within 30–50 km downwind of the urban centre. This “urban rain shadow” effect is highly seasonal and strongest during warm convective storms, whereas winter stratiform precipitation shows little urban influence.

对伦敦、巴黎和圣路易斯等大城市的观测研究表明，城市中心下风向 30–50 km 范围内夏季降雨量可能增加 10–20 %。这种“城市雨影”效应具有很强的季节性，在暖季对流性风暴中最强，而冬季层状降水受到的城市影响很小。

8. Fog and Air Pollution | 城市雾与空气污染

Urban fog is a complex phenomenon. In the past, industrial cities experienced thick, persistent smog combining smoke and fog, commonly known as London-type smog. Today, photochemical smog, formed by the reaction of nitrogen oxides and volatile organic compounds in sunlight, is more common in sunny cities. Urban fog itself forms when sufficient moisture and condensation nuclei exist; since cities have abundant particles but limited moisture, urban fog tends to be less frequent than rural radiation fog, but it is denser and longer-lasting where it occurs.

城市雾是一个复杂的现象。过去工业城市经历过浓稠且持久的烟雾，即烟与雾结合，通常称为伦敦型烟雾。如今，光化学烟雾，由氮氧化物和挥发性有机物在阳光下反应形成，在阳光充足的城市更为常见。城市雾在水分和凝结核充足时形成；由于城市颗粒物丰富但水分受限，城市雾频率往往低于乡村辐射雾，但一旦出现则更浓且持续更久。

Air pollution interacts with urban climate in a reciprocal way. The urban heat island weakens the surface inversion, which can slightly improve vertical mixing at night. However, the presence of an elevated inversion layer aloft traps pollutants below it, creating a persistent haze dome. This haze reduces incoming solar radiation, lowers visibility and modifies the urban energy balance by absorbing both solar and terrestrial radiation.

空气污染与城市气候相互作用。城市热岛削弱地面逆温，可略微改善夜间垂直混合。然而，高空存在逆温层会将污染物限制在下方，形成持久霾盖。这种霾减少到达地面的太阳辐射，降低能见度，并通过吸收太阳辐射和地面辐射改变城市能量平衡。

9. Solar Radiation and Daylight in Cities | 城市太阳辐射与日照

Due to atmospheric pollution, urban areas typically receive 10–20 % less global solar radiation than rural areas. Dust and particles scatter and absorb sunlight, with the greatest reduction in the ultraviolet component. On the other hand, the diffuse fraction of radiation increases because scattering redirects sunlight from all directions. This means that shaded areas receive more scattered light than in the countryside.

由于大气污染，城市接收的太阳总辐射通常比乡村少 10–20 %。尘埃和颗粒散射并吸收阳光，其中紫外线减少最明显。另一方面，散射辐射比例增加，因为散射使阳光从各个方向到达。这意味着阴影区域接受到的散射光比乡村多。

Building geometry also reduces daylight at street level, especially in dense high-rise districts where the sky view factor is small. Conversely, sunlight reflected between facing glass facades can create localised glare and excessive heating, raising cooling loads in summer. Urban planners increasingly use solar access regulations to protect street-level daylight while allowing high-density development.

建筑几何结构也减少街道层面的日照，尤其在天空视角因子小的高层密集区。相反，相对玻璃幕墙之间的反射可产生局部眩光和过热，增加夏季冷却负荷。城市规划者越来越多地制定日照权法规，以在鼓励高密度开发的同时保护街道日照。

10. Health and Environmental Impacts | 健康与环境影响

Urban heat islands amplify heatwave mortality. During the 2003 European heatwave, many cities recorded death rates far above rural areas, and the 2022 heatwave in London exceeded 40 °C with hundreds of excess deaths. At night, the absence of cooling in dense urban cores prevents the human body from recovering from daytime heat stress, particularly affecting the elderly, infants and those with cardiovascular diseases.

城市热岛加剧热浪死亡率。2003 年欧洲热浪期间，许多城市记录到的死亡率远高于乡村；2022 年伦敦热浪超过 40 °C，导致数百例超额死亡。夜间，高密度城市核心冷却缺失使人体无法从白天热应激中恢复，尤其影响老年人、婴幼儿和心血管疾病者。

Enhanced precipitation and storms downwind of cities exacerbate flash-flood risks in impermeable urban basins. Polluted fog and photochemical smog cause respiratory and eye irritation, and the interaction of high temperatures with ozone formation creates severe ground-level ozone episodes in summer. Cities must therefore integrate climate-responsive design into infrastructure and public health planning.

城市下风向增强的降水和风暴加剧了不透水城市流域的骤洪风险。污染雾和光化学烟雾引发呼吸道和眼睛刺激，高温与臭氧生成的相互作用在夏季造成严重地面臭氧事件。因此城市必须将气候适应性设计纳入基础设施与公共卫生规划中。

11. Mitigation and Adaptation Strategies | 减缓与适应策略

Green roofs and walls: Vegetation increases evapotranspiration, reduces surface albedo heating and lowers building energy demand. A green roof can reduce roof surface temperature by up to 25 °C in summer.
Cool pavements and reflective materials: Increasing albedo with white roofs or permeable concrete reduces stored heat and lowers sensible heat flux.
Urban ventilation corridors: Preserving wind paths aligned with prevailing breezes allows cool rural air to penetrate the urban core, reducing heat stress and pollution accumulation.
Water bodies and shade trees: Ponds, fountains and tree canopies create cool islands and improve human thermal comfort through shading and evaporation.
Low-carbon transport and emission controls: Reducing anthropogenic heat and aerosol emissions simultaneously lessens UHI and improves air quality, creating co-benefits for climate and health.

绿色屋顶与立体绿化：植被增加蒸散、降低地表加热并减少建筑能耗。绿色屋顶在夏季可降低屋顶表面温度达 25 °C。

凉爽路面与反光材料：使用白色屋顶或透水混凝土提高反照率，减少蓄热并降低显热通量。

城市通风廊道：保留与盛行风向一致的通风路径，使乡村凉空气进入市中心，减少热应激和污染积累。

水体与遮荫树木：池塘、喷泉和树冠通过遮荫与蒸发形成冷岛，提升人体热舒适。

低碳交通与排放控制：减少人为热和气溶胶排放同时减轻热岛并改善空气质量，产生气候与健康的协同效益。

12. Conclusion | 结论

Urban climate is a clear case of human modification of the natural environment at the local scale. Cities change surface energy budgets, water cycles and atmospheric composition, producing warmer, drier and more polluted conditions than surrounding areas. The urban heat island, modified wind fields, increased convection and reduced radiation all influence the lives of billions of urban dwellers. Future urban growth, especially in tropical and subtropical regions, requires climate-sensitive planning that mitigates heat, promotes ventilation and adapts infrastructure to the urban climate that humans have created.

城市气候是人类在局地尺度上改造自然环境的典型案例。城市改变地表能量收支、水循环和大气组成，产生比周边更温暖、更干燥、更污染的条件。城市热岛、改变的风场、增强的对流与减少的辐射影响着数十亿城市居民的生活。未来的城市增长，尤其在南亚和东南亚等热带与亚热带地区，需要气候敏感型规划，以减缓高温、促进通风并使基础设施适应人类所创造的城市气候。

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Urban Climate: Formation and Impacts | 城市气候的形成与影响