Can Relative Humidity Reach 100%?

Drowning ?

Del : It's time to clock off. Meet you down the Red Lion for some 100 % humidity

Yes, whereupon it may rain.

Sir,

why can't always RH% be 100% above sea?

Because evaporation will continue till the water vapor pressure in air reaches it's saturation pressure for the corresponding temperature of air/water vapor, which means RH is 100% OR the source of liquid water exhausted.

There's too much atmospheric air to absorb the water vapor.

If you put a cover on an aquarium, you might get 100%RH.

Relative Humidity, what it's related to is the saturation point of water vapor in the air. This is depends on the temperature of the air. Since we can not control the weather, no we can not maintain it at 100%.

It can reach and maintain a RH% of 100% quiet easily, just look up at the sky on a cloudy day, or at a fog bank some morning, out in the rain as well!

Outside of micro temperature/pressure variation any volume of fog/cloud is at 100% RH this is easily proven by the fact that the water is in a particulate state and visible in the volume.

Keep increasing total water/air content in a given volume far above 100% RH and you'll get precipitation as the water molecules surface tension and gravity overcome gas dynamics.

Fog.

Ahhh yes the way I feel before my coffee in the morning. From now on I will have to tell my wife "I am feeling a bit humid this morning".....

And if she replies the same the humidity will.........................well you get the idea

Fog. It sure seems like fog is a stable suspension of water droplets supported by Brownian movement in moist air at equilibrium with the water in the fog. This sure sounds like the amount of water in standard volume (m³) is greater than 100% of the amount dissolved at equilibrium in the air, and thus greater than 100%. However, the question is not "can relative humidity surpass 100%". An interesting side issue is how much air is in a standard volume of dry air at 20 degC and 1 atm compared with the amount of the same mixture of gases in moist air, and since water is lighter than air (18 vs 29), is the fog lighter than if the fog were replaced by dry air? Just s'posin'.

Of course it can, ask anyone who has lived in Singapore, a dry day is only 95% Relative Humidity and quite seldom.....100% was the average day RH.

Someone is confusing RH, with water. All RH is sis the maximum amount of water vapour that the atmosphere can carry. Nothing more, nothing less.....

Europeans getting off a plane in Singapore in the 60's, when you had to walk to the terminal, sometimes had trouble taking breaths in the steamy atmosphere.....(been there done that!).

Nowadays you don't have the walk outside......

Dear Friend,

Yes. It is possible to have 100 % R.H., provided, the air is STILL, over the pond or sea, and no wind movement is there. But always some movement of air will be always there.

DHAYAANANDHAN.S

I think that comment #9 gave you the final answer that you are looking for.

All the others have explained the RH and some hinted to the same.

In a nut shell: 100% RH is achieved in patches of the atmosphere where we see fog or cloud. This is due to the air being in continuous mouvement. The dry air replaces the wet air due to temperature variations, atmospheric disturbance, earth rotation, Solar spot heating etc.=> Winds...Wet patches (Clouds ...) will shed their water as rain when the conditions suddenly change at any spot..... recycle.

Tiny nucleation sites cause water to change phase from gas to liquid near saturation conditions. In normal air there are enough of these nucleation sites to for water vapor to condense below 100% rh. Under special laboratory conditions it is possible to exceed 100% rh without condensation. This is called supersaturation.

I have read 100% RH on my weather stations many times. It most often happens when you have a hot or very warm late afternoon or evening and you have heavy rain or thunderstorms followed by a cooler evening/nighttime. The drop in temperature with the same Absolute humidity that is prevailing will cause the Relative Humidity to increase up to the 100% mark.

GA.

Absolutely correct.

You forgot the last line though! You wrote:-

The drop in temperature with the same Absolute humidity that is prevailing will cause the Relative Humidity to increase up to the 100% mark....

.....and the rest of the "over 100 %" to form as dew on cooler objects and the like.

Which is why it will never exceed 100%.

I do believe that the point at which liquid starts condensing out is also called the "Dew point", but it is also a function of temperature and may occur also before 100% RH is reached....(if my memory serves me well!)

You're pretty much right on the mark. From Wikipedia there's a pretty good definition:

The dew point is the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water. Condensed water is called dew when it forms on a solid surface.

The dew point is a water-to-air saturation temperature. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity decreases.^[1]

What I remember is from one Meteorology Course and a lot more from Refrigeration/AC courses and both go back a bit.

Dew Point, now your switching gears a bit. You can have Dew Point with 10% humidity. As it is the correlation between now temperature and moisture content that provides Dew.

The ole Coke Can on the coffee table effect - as the can changes state and gives up its temperature attracting moisture latent warmer air (cold attracts heat) the moisture in then condenses on the can until such a time that the can becomes the same temperature in the room releasing the moisture back into the space. This can be completed in a space that has little humidity.

A comment on the Coke can on the coffee table effect -

when the can at ambient temperature is opened, the escaping carbon dioxide ( with dissolved water) "evaporates" and carries off heat, thus cooling the can and condenses water from the air, leaving the wet ring. Off course, most people take the can from a fridge, confusing the thermodynamics. Then there's the Coke spill ring to consider . . .

I do feel that the tiny effect of the CO2 escaping may be measurable in a laboratory, but it will certainly never be noticed by a human.....

Does the can change its state? Does cold attract heat? Does the can release moisture back into space?

No, it is still a can and the liquid coke is still a liquid. Some CO2 does escape.

I wouldn't use the term "attract". The temperatures do equalize in time through "conduction' but not through "attraction" or "radiation" and I doubt "convection" to any great degree.

When condensation forms it generally slides down the can and forms puddles on the table. It will only evaporate based on the relative humidity. If the room got colder it might just sit there since cold air can not hold as much moisture as warm air. If warm wet air was introduced into the room again it might just sit there if the relative humidity of the introduced air was higher than the existing conditions.

Dew point is important and if the relative humidity was only 10% it would take a long time for condensation to form unless of course the can was frozen.

Ok the can does not change state, it remains a can, however the contense in the can unopened does change state by warming the same as a standard DX cooling HVAC unit in a home. Due to the colder temperatures of the can/coil dew is formed on the can/coil. As stated cool air or cool surfaces will hold less water vapor, thus creating dew and pesky water spots.

Low temperature/pressure from the evaporation of refrigerant in the evaporator coil attracts heat which causes heat transfer by convection/attraction (removing heat and moisture from the air) through the evaporator coil. Thus cooling the room since you just removed the heat that was in the air because it was attracted to the much colder evaporator.

Dear Mr.Andy Germany,

Condensation WILL NOT OCCUR UNLESS THE RH IS 100 %. If you refer the Psychrometry - it will explain that DEW is formed when the air cannot hold any more vapour on account of the fall in AirTemperature. The water vapour which will not be visible to human eyes, gets rejected by the air and mist/droplets formed.

The water in the Atpmospheric air is in the FORM of VAPOUR IN A SUPER HEATED CONDITION. How this Super Heat prevails is due to DALTON's LAW of PARTIAL PRESSURES of the constituents of the Air like, N2, O2, CO2, Water Vapour etc and the total pressure (in the atmosphere at sea level 1.03 Kg/cm^2 or 14.696 psi) is the total sum of the pressures exerted by the constituents of the air.

N2 is 77%, O2 is 21 % ( by weight) etc. etc. and water vapour pressure will be around 1 psi to 1.5 psi 0.07 Kg/cm^2 to 0.105 Kg/cm^2 approx, and for this pressure the Saturation Temp. will be around 39 to 54 Deg.C and when the air temperature falls below this level, the vapour will be rejected by the air and dew/mist forms.

Thanks,

DHAYANANDHAN.S

I did not say anything different.......read my posts with a little more understanding please.....notice what I said about temperature for example......

Its easily misunderstood by many, not just you.....

You said,
" DEW is formed when the air cannot hold any more vapour on account of the fall in AirTemperature."
The truth is that air does not hold water. The same effect would happen in any gas or mixture of gasses, or even in a vacuum. The controlling. mechanism is the partial pressure of water vapor. Condensation (dew point) will occur at all humidity levels up to and including 100% rh. Under special conditions, supersaturation can occur which means that rh can exceed 100%.