The main influence factors on microclimate explained
If you ever stood in front of a bonfire on a chilly night, or 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 are taking a look at its major influencing factors, 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 feels1. 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 days2. Let’s go over the factors one by one.
Air temperature is the most straightforward factor and is relatively 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 above ground3, 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 quickly becomes clear that air is not perfectly mixed where humans tend to move around. The reason is that air is not primarily heated by the sun, but by 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 some time!4 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 like radiation and evaporation. 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 temperatures and strong wind, this can lead to severe wind chill. In summer, conversely, it helps to keep us from overheating. The effect in summer, however, does not entirely come 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. For more detailed information and calculations about this, see our article about human thermoregulation.
Having talked about evaporation, we now arrive at humidity: Evaporation from our skin into the air is only possible if the air is not already fully saturated with moisture, and evaporation also slows down the higher the degree of saturation. This is exactly what 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 moister 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 apparent 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 radiation are readily felt when moving into shade 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 influenced 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 even such complex systems and give architects and urban planners the information they need to create places that not only look good, but also feel good.
In this case, feeling has nothing to do with emotions and is a fairly - if not completely - objective measure. The feeling that a certain microclimate incites is there to incentivize behavior that keeps our body functioning in an optimal way. Besides “felt temperature”, scientists also use the expression “physiological temperature”. If you are exposed to dangerously high “felt” temperatures, your body cannot operate anymore due to overheating, and you will become increasingly incapacitated and eventually die. ↩
The opposite situation in cold weather conditions is often called “wind chill”. It can be equally dangerous, but is easily mitigated by technical means, in particular insulation. On our page about human thermoregulation you can find some more details. ↩
If the forecast is derived from one of the global weather models (as most forecasts are - e.g. from the ECMWF), there is also the problem of resolution: the calculation grid is 10 by 10 kilometers in most cases, and small-scale elevation differences do not even factor into this. ↩
You can easily try this for yourself: just hold your hand over a hot surface on a sunny day - next to the surface the air will feel significantly hotter than when you move your hand away a meter or so. For more information also take a look at our article about surface and air temperatures in deserts ↩