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I see a lot of uninformed people on this thread thinking 35C wet bulb temperature isn't such a big deal and proposing various ineffective tips to cool down. Le

I see a lot of uninformed people on this thread thinking 35C wet bulb temperature isn't such a big deal and proposing various ineffective tips to cool down. Let me explain what the issue is, in simple words: WET BULB TEMPERATURE IS NOT TEMPERATURE YOU SEE ON A THERMOMETER. (please, don't nitpick, I am aware of wet bulb thermometers) Wet bulb temperature is combination of temperature and humidity. The simple layman way to understand it is it says what minimum temperature an object can attain by evaporating water off it, in perfect conditions (forced airflow, full shade, no internal heating, etc.) When air is dry, even if it is hot you can evaporate sweat and cool down -- the wet bulb temperature is low in dry air. You can survive on Sahara because while the air can be very hot, it is also dry. You sweat and you cool down. As the humidity rises, the wet bulb temperature approaches dry bulb temperature and at 100% humidity they are both the same. When air is 100% humid you cannot cool down by sweating at all -- the sweat will not evaporate in 100% humidity regardless of temperature. Meaning that an air with 35C and 100% humidity has 35C wet bulb temperature but so can hotter air that has less than 100% humidity. The hotter air the lower humidity is needed for the object to cool down to 35C. Wet bulb of 35C is a limit of human endurance. Even if you lie down in shade and do nothing and are completely healthy, your body is barely able to remove enough heat to survive. Any higher temperature and you WILL eventually die. People who need to work (like move around at all) or are sick or are small (kids) will die faster. The only way to resolve this issue is to dry the air somehow. Things that don't work: -- going into shade. You are still going to die, just a little bit slower. -- having a fan. It can't cool you down, because water will not evaporate to cool anything below wet bulb temperature. You are going to die. -- going into cellar. The air will get cooler but the humidity will rise. You are going to die, unless you can cool the air DRY BULB temperature below 35C. At which point your cellar will have water condensing on its walls which means you can remove humidity from air. It will be shitty, wet, 100% humid, hot place, to sit half a day in with your family, kids and neighbors that don't have an expansive cellar like you, but at least you will survive. Yay... The issue is in areas that tend to be very hot and susceptible to this phenomenon, people tend to not have means to dry the air. They either don't have money for equipment or they have open homes (who keeps their home shut in a climate like that) assuming they have home at all. From economic point of view, the problem is that even under wet bulb temperature of 35C some people will already be dying and others will have to stop whatever they are doing, which is not good for your economy.

krisoft 5y ago
> please, don't nitpick, I am aware of wet bulb thermometers I don’t want to nitpick. Everything you wrote is correct as far as I know. I only want to add to it. I think understanding what a wet bulb thermometer is crucial to understand the term. And it is a very simple thing: you take a totally normal thermometer, you put it in the shade. So far everything is normal, this is how you measure air temperature since forever. You then put a little sock on the end of the thermometer and make it wet. Super simple. On a traditional mercury thermometer the end is a little glass sphere where the mercury is stored. This glass sphere is called the “bulb”. A wet bulb thermometer is just a thermometer where the bulb is wet. Super simple. Why would you do this? Obviously this “wet bulb” thermometer will measure a lower temperature than the dry one. This is because evaporation of the water takes away energy from the thermometer. The faster the water can evaporate bigger the difference you will see between the dry and the wet thermometers. If the air is saturated with water vapor then the two measurements will be the same. If the air is dry you will have lots of evaporation and the wet bulb will be way colder than the dry one. So if all you have is two thermometers a sock and a bucket of water you can measure the relative humidity in this indirect way by comparing the two temperatures. An other way to think about a wet bulb thermometer is that it is a very crude modell of a sweating human. You keep the sock wet and that simulates the thermometer sweating. The temperature the termometer reaches is the limit of temperature you can cool yourself down using evaporative cooling.
- lmilcin 5y ago
  Yes, this is completely correct. I would add the wet bulb thermometer needs large evaporation area (so the sock cannot be too wet and definitely you can't put it in the bucket) and/or some airflow around it. Otherwise it will show some temperature between actual dry bulb and wet bulb temperatures. In practice, nobody seriously uses wet bulb thermometers outside of school experiments because they are such a PITA. It is easier to just measure dry bulb temperature and humidity separately and then look up wbt in a table. In general measuring temperatures correctly is a tricky business, what with temperature gradients in the objects you try to measure and the thermometer itself. I just recently needed to explain to a bunch of people why there is no such thing as "temperature in the sun" and that you can pick any temperature between WBT and around 5800K and it is going to be a correct temperature "in the sun" depending on properties and shape of the object you are trying to measure..

krisoft 5y ago

> please, don't nitpick, I am aware of wet bulb thermometers I don’t want to nitpick. Everything you wrote is correct as far as I know. I only want to add to it. I think understanding what a wet bulb thermometer is crucial to understand the term. And it is a very simple thing: you take a totally normal thermometer, you put it in the shade. So far everything is normal, this is how you measure air temperature since forever. You then put a little sock on the end of the thermometer and make it wet. Super simple. On a traditional mercury thermometer the end is a little glass sphere where the mercury is stored. This glass sphere is called the “bulb”. A wet bulb thermometer is just a thermometer where the bulb is wet. Super simple. Why would you do this? Obviously this “wet bulb” thermometer will measure a lower temperature than the dry one. This is because evaporation of the water takes away energy from the thermometer. The faster the water can evaporate bigger the difference you will see between the dry and the wet thermometers. If the air is saturated with water vapor then the two measurements will be the same. If the air is dry you will have lots of evaporation and the wet bulb will be way colder than the dry one. So if all you have is two thermometers a sock and a bucket of water you can measure the relative humidity in this indirect way by comparing the two temperatures. An other way to think about a wet bulb thermometer is that it is a very crude modell of a sweating human. You keep the sock wet and that simulates the thermometer sweating. The temperature the termometer reaches is the limit of temperature you can cool yourself down using evaporative cooling.

lmilcin 5y ago
Yes, this is completely correct. I would add the wet bulb thermometer needs large evaporation area (so the sock cannot be too wet and definitely you can't put it in the bucket) and/or some airflow around it. Otherwise it will show some temperature between actual dry bulb and wet bulb temperatures. In practice, nobody seriously uses wet bulb thermometers outside of school experiments because they are such a PITA. It is easier to just measure dry bulb temperature and humidity separately and then look up wbt in a table. In general measuring temperatures correctly is a tricky business, what with temperature gradients in the objects you try to measure and the thermometer itself. I just recently needed to explain to a bunch of people why there is no such thing as "temperature in the sun" and that you can pick any temperature between WBT and around 5800K and it is going to be a correct temperature "in the sun" depending on properties and shape of the object you are trying to measure..