The main influence factors on microclimate explained
If you ever stood in front of a bonfire on a chilly night, went outside on a windy or hot & humid day, you know that the air temperature is only a small part of how we perceive microclimate. In this article we take a look at the major factors influencing this, how they relate to each other and how they can be managed in the context of architecture and urban planning.
First things first: when we talk about microclimate in this context, we mean the environmental conditions that influence how a place in the built environment feels. There are plenty of possibilities to influence this and not only improve quality of living but also mitigate negative effects from climate change. Also, in the context of this article we concentrate on hot summer days. Let’s go over the factors one by another.
Air temperature is the most straight forward factor and is easy to measure and predict, which is why it is readily available from weather reports. However even with something so clear, things are a bit more complicated when looking deeper. Most weather reports, tend to report air temperature at a height of 10 meters, not considering any ground influence, i.e. perfectly mixed average air temperature. If you think of hot air wavering over pavement on a hot summer day, it becomes quickly clear, that air is not perfectly mixed where humans tend to move around. The reason for this, is that air is not primarily heated by the sun, but direct contact with hot surfaces, which in turn are heated by the sun. So air is significantly hotter, after it touches hot surfaces. Just think about a car that has been sitting in the sun for a time! So while the meteorological air temperature is important, the situation can be very different in an urban environment with its many surfaces and changes in wind velocity.
Wind velocity is a major influence factor. Basically it determines, how fast our body would reach the air temperature if there would not be any other factors. Since the air temperature is lower than our body temperature in most cases, there is a cooling effect if the wind is stronger, since more of the relatively cool air particles come into contact with our body. In winter, with very low air temperature and strong wind, this can lead to severe wind chill. In sommer conversely, it helps us to keep from overheating. The effect in sommer however does not come entirely from the cooler air temperature but from the fact, that moving air (usually) helps us to evaporate moisture from our skin. This evaporation process needs a lot of energy (similar to when you evaporate water on a stove) which is taken away from our bodies and that is the major part of the cooling effect of wind in summer.
Talking about evaporation, we arrive at humidity: Evaporation from our skin into the air is only possible, if the air is not saturated and becomes slower the higher the air is saturated with moisture. This is exactly what the relative humidity tells us. At 100% humidity, air is completely saturated and water does not evaporate anymore from our skin (or anywhere else), making this cooling mechanism impossible. Conversely in dry heat, water can readily evaporate from the skin, making is easy to cool down. This however is only true, if there is sufficient movement in the air, otherwise the air around our body saturates. Also air temperature plays a role, since hot air can hold a lot more moisture than cool air. If warm, saturated air cools down, water precipitates out of it and forms fog. Which brings us to the next factor: heat conductivity. While less of an influence than other factors, denser and more moist air will also conduct heat from our bodies faster, even if there is not much wind speed. This is why foggy can quickly change into clammy.
As if all this would not already be complicated enough there is also radiation. We feel infrared radiation as a directed force on our skin - most apparend when you are sitting in front of a fire on a cold night, but usually simply as the solar radiation on a sunny day. The direct effects of solar radations are readily felt when changing into shadow on a hot, sunny day. In the built environment there is also reflected radiation from the ground and buildings that can be a major influence. Not only are there acute effects during the day, but secondary radiation is a major factor for the urban heat island during nights. Rather than cooling down by radiating their heat out into space, the radiation bounces between buildings, keeping cities significantly warmer.
We have already hinted at it: all these factors influence each other in complex dependencies. Architecture determines space and surface. Surface catches radiation, heats air, influences reflection, which in turn influences air temperature, which in turn dictates moisture etc. and all of that is fluenced by the complex wind flow conditions in cities. However, with our multiphysics microclimate simulations, and the computing power of modern information technology, we are able to calculate all this complexity and give architects and urban planners all the information they need in order to create places that do not only look good, but also feel good.