Hacker News Reader

> 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.

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..

>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

How is humidity typically measured? I was under the assumption a hygrometer uses both dry and wet bulb measurements.

Wet bulb requires it to be... you know... wet.

Not very practical to include in your electronics.

I don't know exact method used but I can assure you, there is no water reservoir in any of my gadgets:)

It looks like I’m confusing a classic psychrometer with more modern ones. The modern ones still use absorption of water to change some physical property like capacitance or resistance but this is also why they need to be continually calibrated against a true dry/wet bulb hygrometer.

My experience is that they tend to get out of cal quite often which can lead to bad scenarios, like if using enthalpy setback for air-handling

Well, wet bulb thermometer works specifically by evaporating water and measuring temperature.

Psychometer is completely different idea.

Yes, measuring humidity reliably is a difficult problem as basically any other measurement that requires direct exposure of eroding/corroding components and fragile electronics to elements.

I believe a psychrometer operates on the same principle.

"A psychrometer, or a wet and dry-bulb thermometer, consists of two calibrated thermometers, one that is dry and one that is kept moist with distilled water on a sock or wick. At temperatures above the freezing point of water, evaporation of water from the wick lowers the temperature, such that the wet-bulb thermometer will be at a lower temperature than that of the dry-bulb thermometer. "

https://en.wikipedia.org/wiki/Hygrometer

The psychrometer just compares the wet and dry bulb temperatures to arrive at a relative-humidity value. I.e., it still operates on the principle of a wet-bulb thermometer.

The traditional 5 dollar analog hygrometer uses a paper/metallic strip coil:

https://www.youtube.com/watch?v=jCUHDFYdG44

I guess I was dancing around the point a bit. I meant the definition of relative humidity uses dry/wet-bulb comparisons but we use all sorts of more pragmatic calibrated proxies to measure it in practice.

> 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.

Don't you mean wet bulb temperature? Why would you need a dry bulb temperature below 35C?

> 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

If we do start seeing this kind of temperature around the world, I expect people will start building underground refugees or repurposing existing structures.

I mean DRY BULB temperature. Reducing dry bulb temperature == removing heat. Reducing wet bulb temperature == removing humidity.

But it doesn't matter a lot, because at 100% humidity (required to spontaneously dew humidity out of air) dry bulb temperature equals wet bulb temperature.

Dry bulb changes indpendently of wet bulb temperature but cannot be lower than wet bulb temperature.

Because when air temperature (dry bulb) reaches wet bulb temperature you are at dew point and when you cool down further all that moisture is going to either cause mist or dew or snow or rime.

In fact, lowering DRY BULB temperature under dew point (WET BULB temperature) is the leading way to remove humidity in nature -- the water precipitates in the form of mist high up in the atmosphere, the mist coalesces into droplets and falls down on air as rain. Alternatively, if the temperature is below zero, it causes snow by sublimation (both wet and dry bulb temperatures are equal.

>Reducing dry bulb temperature == removing heat. Reducing wet bulb temperature == removing humidity.

This isn't the best interpretation. Both reducing humidity and reducing dry-bulb temperature can reduce the amount of energy in the air.

Take a look at a psychrometric chart [1]. If you hold the humidity constant and reduce dry bulb temperature, you indeed reduce the enthalpy (what I interpret you mean by 'heat'). Likewise, if you keep the dry-bulb temperature constant and reduce the humidity you also reduce the enthalpy

To your point, this is why fixed dry-bulb economizers aren't recommended in humid climates as a means of conserving air-conditioning energy by bringing in outside air but enthalpy controllers are allowed across all zones.

[1] https://www.engineeringtoolbox.com/docs/documents/816/psychr...

> going into cellar. The air will get cooler but the humidity will rise.

If we are already counting on a wet bulb temperature of 35C, the humidity is likely already very close to 100%.

Reducing the temperature is likely to be a net good.

That's false.

TLDR: The air gets cooler but still can't cool you. Your body produces heat that has nowhere to go and you still die. Then the walls of the cellar heat up immediately, because cooling 100% humid air requires sinking a lot of energy.

If you found yourself in conditions of an actual heat wave as described, you would despair at the fact the air can get cooler in your cellar but it doesn't cool you at all.

Which is counterintuitive but unfortunately that's how physics works.

First, humidity does not have to be very close to 100%.

In fact, in most cases it isn't and the reported waves of 35C WBT are almost all between 42 and 44C in absolute temperature. This means the air has to cool down 7-9 degrees before it even reaches dew point.

7-9 degrees is a huge temperature difference when temperature is over 40C.

Unfortunately the problem isn't the air temperature. The problem is your body's ability to remove heat. And if you take air and cool it down the temperature will drop but humidity will rise. The wet bulb temperature is preserved unless you can somehow remove water from air. And with wet bulb temperature being same 35C your body still doesn't have any way to remove heat and you are still going to die.

Only after the air cools down by 7-9 degrees celsius you can start seeing any effects, but don't be too happy.

Because physics has a nasty trick to play on you.

The effects of water condensing on walls of your cellar is going to be your cellar walls heating up immediately.

Yes, water cools stuff efficiently when it evaporates but just as efficiently it heats it up when it condenses.

Conservation of energy is a bitch. Please, learn some physics, it is really fun. And sometimes terrifying.

So your cellar is very likely to have very limited ability to cool down very hot humid air and condense water out of it.

This is all fascinating stuff to me, I'd never really heard of wet bulb before. Quick question though: how does the pressure difference play into this in a basement? As far as I'm aware below ground pressure and voids under basements tend to cause pressure differentials in even non-AC basements (causing radon issues where I'm from)? I assume this would help a cellar situation in the event of power outage as air even slightly below ground tends to be much cooler than above ground air?

The pressure difference between your backyard and your basement is for all practical purposes null. It is also orders of magnitude smaller than those caused by gusts of wind and changing weather.

I'm not sure this is so simple. Clearly your answer is correct with a largely uninsulated house with a basement only partially below ground (perhaps most of the developing world to be fair). Any semi-sealed home will have heat naturally accumulate (and dissipate) at the top. Apparently the movement of air upward - known as the stack effect - creates a (slight) vacuum pull at the bottom. Of course it's a small effect in normal temperatures but in extreme reverse stack-effect scenarios like you're describing the ground temperature below 3 - 5 feet should result in a basement being lower temperature than the backyard as available air gets sucked through the cooler ground to get into the basement? I'm sure this varies widely by region (ground temps vary in 3 - 5 ft depths) but everywhere this temperature will be cooler than the outside in such weather extremes.

>The effects of water condensing on walls of your cellar is going to be your cellar walls heating up immediately.

While true about the conservation of energy, the drastic differences in both mass and heat capacity of air/earth mean the cellar walls will probably have no perceptible change in temperature.

> TLDR: The air gets cooler but still can't cool you.

Evaporative cooling is not the only kind of cooling that exists.

If this were the case you’d die of heat death standing in the middle of 100% humidity antarctica.

If your cellar is cold enough you’ll be in a much better situation than before. You seem to assume that the moment you open your cellar door, all humidity and temperature will immediately equalize with the outside.

Again, completely wrong. 100% humid 0C air is still 0C wet bulb temperature -- perfectly good for cooling you down.

>Again, completely wrong.

You are saying the same thing as the OP.

There are generally two type of heat in this scenario sensible and latent.

Think about when you heat up water on the stove. If you were to graph the temperature T against the heat energy added Q, you would see the temperature gradually rise as the amount of heat was added. This is the 'sensible' heat. But at a certain point the water would hit boiling temperature. You're still adding heat but the temperature is no longer increasing so what gives? The energy is now going towards phase changing the water from a liquid to a gas. This is the 'latent' heat.

The OP is just saying there are more paths than just latent heat transfer, which is why they said you won't overheat in Antarctica if it's 100% RH. Your statement here is that evaporative (latent) cooling won't happen at 100% humidity but sensible cooling can still occur. You're both on the same page.