Checking my phone for the temperature, I see that its currently 91 degrees but it feels like 99. Got me wondering how thermometers work/how they came to be and how they calculate “what it feels like.”
In: Technology
Humans cool ourselves through evaporative cooling. We sweat, the sweat evaporates and takes substantial heat with it.
The ability for water to evaporate depends on the temperature, the relative humidity, and the air pressure. A “dry” 90 F means water evaporates quickly, while a wet 90 F means water does not evaporate well. The wet 90 F feels much hotter to humans.
The classic way to measure this phenomenon was with a thermometer with a wet cloth wrapped around it. This gives you the wet bulb temperature, which will be below the ambient air temperature.
Even in places that reach 110+ F, nowhere on earth ever reaches a wet bulb temperature above 100 F. In fact, sustained exposure to wet bulb temperatures in the 90 F range is highly fatal.
The most comfortable weather has a wet bulb temperature around 60 F or lower. The most miserable/hot places on earth tend to be in the mid 80 F range. 90+ is very rare. The heat index is the “feels like” temperature and is based on this concept.
Generally thermometers can have a couple of mechanisms.
The “normal” analog thermometers beside the window you might know have a liquid (mercury or an ethanol solution) which expands when heating up and therefore rising up.
Digital Thermometers often have resistance based systems. There a current flows through some sort of cable, which changes according to the temperature. This cable is put in the medium you want to measure.
Your phone however doesn’t measure the temperature directly. It gets its data from weather stations in your area.
An alternative measurement of temperature is the measurement of infrared radiation (used by satellites or heat cameras). Because everything gives away radiation (which is essentially light but with a different wavelenght – like so red you can’t see it anymore), and the energy of this radiation depends on the temperature of the body you are able to calculate it back. It is essentially what you feel if you are near somebody and you fell there warmth.
How the “feeled temperature” is determined I am not sure, but I would guess that factors like wind go into that calculation. You can imagine that your body (around 37 °C/100 °F/310 K) heats up the air near you. If this air is blown away you feel colder as the heat from your skin is transfered to the new air (if its colder). If the air stays around you, you will feel warmer. You can feel this effect when its windy or when running.
for digital thermometers:
some materials change their conductivity based on temperature, so forcing electricity to flow through this material and seeing how that electricity changes tells you the temperature the material is experiencing.
Thermometers often exploit the fact that materials expand/contract depending on temperature, and different materials do it at different rate. The “feels like” temperature is a mix of the real temperature and the humidity. Humidity decreases the ability for sweat to remove heat from the body, so it makes the body heat up more than the same temp with low humidity.
It’s just like a traditional thermometer, measuring expansion or contraction of a liquid. However, the thermometer in your phone is much smaller, and it doesn’t measure liquid.
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