Keeping Heat Out of the City

A roof, a tree and a street can all influence how much heat a city collects. They work at different scales, but share a useful purpose: reducing the heat that people and buildings must deal with later. This is the starting point of passive cooling. Instead of depending entirely on powered equipment to remove heat, a city can change its surfaces, vegetation and layout. The approach makes cooling a question of urban design as well as a question of which air conditioner to buy.

屋顶、树木和街道都能影响城市积聚多少热量。它们在不同尺度上发挥作用，却有一个共同且有用的目的：减少人们和建筑之后必须应对的热量。这就是被动降温的出发点。城市可以改变表面、植被和布局，而不完全依赖耗能设备移除热量。这种方式让降温既成为城市设计问题，也成为选择哪种空调的问题。

The United Nations Environment Programme put this approach within a broader strategy in its Cooling and Extreme Heat Factsheet, published on January 16, 2025. The factsheet identifies three connected priorities: passive cooling, improved energy efficiency in buildings and equipment, and the phase-out of climate-warming gases used in cooling equipment. These priorities address different parts of the problem. Preventing heat from entering a building reduces the work required of its cooling system; better equipment can then perform the remaining work with less energy.

联合国环境规划署在2025年1月16日发布的《制冷与极端高温事实简报》中，把这种方式纳入了更广泛的策略。简报提出三个相互关联的重点：被动降温、提高建筑和设备的能源效率，以及逐步淘汰制冷设备中使用的导致气候变暖的气体。这些重点针对问题的不同环节。阻止热量进入建筑，可以减少制冷系统需要承担的工作；随后，更好的设备可以用更少的能源完成剩余工作。

Why does urban design matter? Roads and buildings often replace vegetation that previously provided shade and natural cooling. Asphalt and concrete absorb solar energy and later release heat into their surroundings. Vehicles, factories and air conditioners also add waste heat. Together, these processes help explain the urban heat island effect, in which a city is warmer than nearby areas. UNEP's discussion of natural cooling describes streets and buildings as places where changes can both reflect sunlight and encourage breezes. The outdoor environment therefore deserves attention alongside indoor temperature settings.

城市设计为什么重要？道路和建筑往往取代了原本提供遮阴和自然降温的植被。沥青和混凝土吸收太阳能，之后向周围释放热量。车辆、工厂和空调也会增加废热。这些过程共同帮助解释城市热岛效应，也就是城市比附近地区更温暖的现象。UNEP在讨论自然降温时指出，改变街道和建筑既可以反射阳光，也可以促进微风流动。因此，除了室内温度设定，室外环境也值得关注。

Trees offer two useful mechanisms. Their leaves intercept sunlight, providing shade for people and surfaces below. They also release water through their leaves, helping cool the surrounding air. UNEP compares this process with the way sweating helps humans stay cool. That comparison is helpful, although a tree is not a machine with a fixed cooling output. For a hypothetical school improvement project, planners might compare a shaded walking route with an exposed playground. Their question would be where protection is most needed, rather than simply how many trees could fit on the site.

树木提供两种有用的机制。树叶拦截阳光，为下方的人和表面遮阴。树叶还会释放水分，帮助冷却周围空气。UNEP把这一过程比作人类通过出汗保持凉爽的方式。这个比较有助于理解，但树木并不是制冷输出固定的机器。以一个假设的学校改造项目为例，规划者可能会比较一条有遮阴的步行路线和一个暴露在阳光下的操场。他们要问的是哪里最需要保护，而不只是场地能容纳多少棵树。

Reflective surfaces tackle a different stage of the heat problem. A reflective roof sends more incoming sunlight away, so less solar energy is absorbed at that surface. This helps explain why roof materials belong in a cooling discussion even though residents may rarely see them. Shade prevents sunlight from reaching a surface; reflection changes what happens when sunlight arrives. Neither mechanism requires a compressor to operate. Their value lies in reducing heat gain before powered cooling has to respond, rather than promising to make every room comfortable under every condition.

反光表面针对的是热量问题的另一个阶段。反光屋顶把更多照射过来的阳光反射出去，因此该表面吸收的太阳能更少。这有助于解释为什么屋顶材料应该纳入降温讨论，即使居民可能很少看到它们。遮阴阻止阳光到达表面；反射则改变阳光到达之后的情况。这两种机制都不需要压缩机运转。它们的价值在于，在耗能制冷设备必须作出响应之前减少热量进入，而不是承诺在所有条件下都能让每个房间舒适。

Buildings and streets must also be considered together. UNEP describes connecting green spaces as a way to create wind corridors that reduce local temperatures. Its broader recommendation includes urban design suited to the climate and buildings that support passive cooling. The interpretation here is that ventilation should be planned as part of a place, rather than treated as an isolated window feature. A proposal for a new neighbourhood could examine how air moves between buildings and how shaded routes connect public spaces. Such a proposal would still need local evidence before anyone could claim a particular temperature reduction.

建筑和街道也必须结合起来考虑。UNEP指出，连接绿地可以形成降低局部温度的通风廊道。其更广泛的建议包括适合当地气候的城市设计，以及支持被动降温的建筑。本文的解读是，通风应该作为一个地方整体规划的一部分，而不是被当成孤立的窗户功能。一个新社区方案可以研究空气如何在建筑之间流动，以及有遮阴的路线如何连接公共空间。在有人声称能降低某个具体温度之前，这类方案仍然需要当地证据。

The practical challenge is to combine these measures without overlooking people who need reliable cooling. UNEP's 2023 discussion of urban heat includes early warnings and cooling centres alongside increased greenery. That combination matters: changing a roof or growing a tree cannot be assumed to provide immediate protection during dangerous heat. Passive measures can reduce the burden on mechanical systems, while efficient equipment remains part of the response. The factsheet's attention to cooling gases adds another dimension, because the environmental consequences of cooling involve more than electricity consumption alone.

实际挑战在于，把这些措施结合起来，同时不忽视需要可靠制冷的人。UNEP在2023年关于城市高温的讨论中，把预警和避暑中心与增加绿化并列提出。这种组合很重要：不能假定改造屋顶或种植树木就能在危险高温期间提供即时保护。被动措施可以减轻机械系统的负担，而高效设备仍然是应对措施的一部分。简报对制冷气体的关注又增加了一个维度，因为制冷的环境影响不仅涉及用电量。

A useful city cooling plan therefore has several layers. It can limit incoming heat, improve the outdoor environment, make buildings perform better and provide efficient cooling where it is needed. UNEP's sustainable cooling handbook presents city and district approaches rather than a single universal solution. In this reading's interpretation, the strongest proposal would explain both its physical mechanism and whom it serves. A cooler roof, a shaded pavement and a protected public room solve different problems. Their contribution becomes clearer when they are designed to work together.

因此，一份有用的城市降温计划应当包含多个层次。它可以限制热量进入、改善室外环境、提高建筑性能，并在需要的地方提供高效制冷。UNEP的可持续降温手册介绍了城市和街区层面的多种方法，而不是单一的通用方案。按照本文的解读，最有说服力的方案会同时解释其物理机制和服务对象。更凉爽的屋顶、有遮阴的人行道和提供保护的公共房间解决的是不同问题。当它们被设计为协同发挥作用时，各自的贡献就更清楚了。