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CityCLIM’s NEW Individual WeatherMap

Weather perception: How hot do you feel?

Weather and climate perception are subjective and depend strongly from individual to individual. A common anecdotal example is that people from cold regions tend to feel that temperate regions are warm, whereas people from hot regions would perceive that same temperate location as cold. This is called “acclimatization”: people from certain climates have gotten used to the temperature there, which will affect their sensitivity levels to other climates.

There are also known differences between the comfort temperatures of men and women, with

studies consistently showing that women prefer warmer temperatures. Age plays a factor as well:

older people tended to have a lower thermal acceptability level than younger people, meaning that

their thermal perception may not be different, but their tolerance to the extreme temperature is

lower. Other factors like seasonal fluctuations, how many layers of clothing you are wearing, activity level, health status, and social factors (our desire to conform with others, our knowledge/attitudes) can all affect thermal comfort and risk.

The Heat Index

When you look for the weather on the news or your favourite weather website, you are usually

presented with the air temperature. However, many other environmental variables can affect how

hot you feel, other than the air temperature. Examples are: humidity, windspeed, degree of shadow, and amount of vegetation in the area.

The Heat Index is one way to give a more accurate idea of “how hot it really feels”. The calculation

factors in relative humidity along with the air temperature. The idea is that your body interacts with

the surrounding temperature through sweating, which is affected by humidity: evaporation through

sweat is a cooling process for the human body, therefore if sweat cannot evaporate effectively (e.g.

when humidity in the air is already high), then the body would feel hotter than the actual air

temperature. There is direct relationship between the air temperature and relative humidity and

the heat index: as the air temperature and relative humidity increase, the heat index increases, and

vice versa. Below you can find a Heat Index chart showing how hot you would feel, given a certain air temperature and humidity. For example, you may read in your newspaper that tomorrow will be 29°C and think to yourself that it sounds like a nice summer day to spend outside. However, you may not know that the humidity tomorrow will be very high (let’s say 90%), meaning that you would feel like the temperature was 39°C. With this information, you might want to stay inside where it is cooler, in order to avoid heatstroke.

Heat index chart showing thermal perception after factoring in air temperature and relative humidity (from: B. Diffey, “Time and Place as Modifiers of Personal UV Exposure,” International Journal of Environmental Research and Public Health, vol. 15, no. 6, p. 1112, 2018).

The Individual Weather Map

RTL Luxembourg are working with Meteologix and Helmholtz Centre for Environmental Research (UFZ) to create a special function in the RTL app and website for citizens to view their individual weather map. Users can indicate subjective temperature preference, and the map will indicate with a “traffic light system” areas where they can expect to feel a range of optimal thermal comfort, and where they should avoid at particular times, for example during certain heat events.

The Individual Weather Map uses the Heat Index, so the “Comfort Temperature Range” that the user can select and that is presented on the map already factors in the air temperature and the relative humidity. This gives a more accurate representation of how hot it really feels to a person.

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