What is Relative humidity: Definition and 52 Discussions

Humidity is the concentration of water vapour present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher humidity in cool air than warm air. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 28 g (0.99 oz) of water per cubic metre of air at 30 °C (86 °F), but only 8 g (0.28 oz) of water per cubic metre of air at 8 °C (46 °F).
Three primary measurements of humidity are widely employed: absolute, relative and specific. Absolute humidity describes the water content of air and is expressed in either grams per cubic metre or grams per kilogram. Relative humidity, expressed as a percentage, indicates a present state of absolute humidity relative to a maximum humidity given the same temperature. Specific humidity is the ratio of water vapor mass to total moist air parcel mass.
Humidity plays an important role for surface life. For animal life dependent on perspiration (sweating) to regulate internal body temperature, high humidity impairs heat exchange efficiency by reducing the rate of moisture evaporation from skin surfaces. This effect can be calculated using a heat index table, also known as a humidex.
The notion of air "holding" water vapor or being "saturated" by it is often mentioned in connection with the concept of relative humidity. This, however, is misleading—the amount of water vapor that enters (or can enter) a given space at a given temperature is almost independent of the amount of air (nitrogen, oxygen, etc.) that is present. Indeed, a vacuum has approximately the same equilibrium capacity to hold water vapor as the same volume filled with air; both are given by the equilibrium vapor pressure of water at the given temperature. There is a very small difference described under "Enhancement factor" below, which can be neglected in many calculations unless high accuracy is required.

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  1. Gstoettenmayr

    Volume change when cooling a saturated volume of air

    TL;DR Summary: How much does a bag with 100 % water saturated air shrink in volume when it is cooled? A bag filled with 100% water saturated air at 85 degrees C (inside and outside) at a pressure of 738 torr inside and outside is cooled to 50 degrees C (inside and outside bag) and the pressure...
  2. S

    B Moist air - does it rise or drop?

    A very simple question that i found some contradicting answers about. It's well known that cold air generaly will drop and hot air will rise. That is why if we open a window during winter time, it will feel almost like a wind is blowing through the window. That is great property that can be used...
  3. S

    I Relative humidity question (cooling the air in a room)

    Hello everyone, I have a question about calculating Relative humidity. If I have a surrounding with 30 degrees Celsius and RH of 30%. I am cooling the surrounding with an air conditioner that works at 15 degrees Celcius. At the end of the process, the room reaches 25 degrees Celcius. In this...
  4. Jonathon

    How to Factor Vapor Retarders into Relative Humidity & thus Mold Risk

    Relative humidity can be calculated if you know dewpoint and temperature - by formulae such as the August-Roche-Magnus approximation. All methods - all formulae - ignore vapor resistance (perms). Surely scientists/engineers have a way to then, as a subsequent step, bring vapor retarders into...
  5. J

    Relative Humidity and Temperature Changes when adding Heat

    Can anyone help me with this? -How much heat must be added to 3.5 m3 /s of moist air with a dry bulb temperature of 10°C and a relative humidity of 60% to raise the temperature of the air by 17°C? • What will be the relative humidity of the air once this heat is added? • What is the power...
  6. T

    How can I calculate relative humidity in helium?

    Hello, I want to calculate the relative humidity in Helium. I only have the variables "dew point temperature", "gas temperature" and "gas pressure". So I tried with Magnus and Antoine equation. But I read that the Antoine equation is very unprecise. So do you know how to calculate the relative...
  7. Rub3y

    At which temperature will the relative humidity in a pipeline be 10%?

    In a pipeline is an organic stream (mainly benzene and hydrocarbons, 60 m3/h) which contains 11 g/s liquid water. At which temperature will the relative humidity in the pipeline be 10 %? Is this possible to calculate? If not, which extra information would you need?
  8. Rub3y

    How much liquid water do I need to get a relative humidity of 10%?

    I need to calculate how much liquid (mass) could cause a relative humidity of 10% in a pipeline. The pressure in the pipeline is 38 barg and the temperature is 105 °C. I calculated the partial pressure of the water which would be 3,74 bar (with formula relative humidity and antoine...
  9. N

    How do I compare indoor and outdoor relative humidity levels?

    If my inside relative humidity level is 50% at 73 F, and my outside relative humidity level is 97% at 36 F, how can I tell if I open my door, whether I will lose inside humidity into the atmosphere? The real life scenario is that my wife complained that the humidity was too low in the house...
  10. K

    Free Air Delivered and Relative humidity

    Homework Statement Air is drawn into a compressor at normal temperature and pressure (NTP) and compressed to a pressure of 6 bar gauge. After compression the air is delivered at 1.2m^3/min and cooled to a temperature of 30C, at which point condensate is collected at the rate of 2 litres per...
  11. C

    I Does moisture escaping from powders decrease chamber vacuum?

    I'm drying some wet powders using vacuum oven and start seeing some condensation on the window after few minutes. The vacuum level stays pretty constant which puzzles me as i thought that the moisture escapes from the powder into the air will increase the chamber pressure or decrease the vacuum...
  12. wasup23

    I How does relative humidity drop below 100%

    How is it that once the dew point is reached, moisture continues to condense. What is causing the imbalance of partial water vapor pressure? Does the rise in atmospheric temperature simply pass up the evaporation rate too quickly?
  13. Robert James Liguori

    I What are some formulas for calculating humid air density?

    I'm trying to compile a complete list of available humid air density formulas that work with air pressure, temperature and dew point (or RH). If you know any, please advise. Thanks, Robert
  14. sistruguru

    Relative Humidity (RH) affecting atmospheric attenuation of light?

    Ok, so I have another question that online searches have not been able to produce. Everything I've seen online references sound waves, or RF signals, or electromagnetic waves. I know that the reason we can view sunsets and sunrises safely is because of atmospheric attenuation - the amount of...
  15. Galbi

    Water evaporation by negative pressure and 0% of RH

    Greetings! I'm trying to evaporate water in some samples. I'm trying 3 methods. Baking samples with 100°C, normal air pressure, and normal RH. Put the samples in 0% RH, 23°C, and normal air pressure. Put the samples in container of -15 psi air pressure, 23°C, and normal RH. The total amounts...
  16. E

    I Relative Humidity Inside a not so Perfectly Sealed Housing

    Hey all, As environmental physics isn't my forte, I have a problem that has been annoying me for about a month now at work and I am still getting no where with it... so any help with this head ache is much appreciated. The problem at hand is as follows. A sealed metal housing is purged with...
  17. C

    B Relative Humidity 10 kilometers (6.2 miles) away....

    Hi, I'm reading about 50% relative humidity in my house about 10 kilometers (6.2 miles) away from the sea. How can water vapor from the sea reach me 10 kilometers away? I'm expecting zero or very low relative humidity.. does it mean if you live the ocean, you won't get relative humidity below...
  18. O

    Relative humidity of the air entering a compressor

    Homework Statement Air is drawn into a compressor at normal temperature and pressure (N.T.P.) and compressed to a pressure of 6 bar gauge. After compression the air is delivered at 1.2m3 min–1 and cooled to a temperature of 30°C, at which point condensate is collected at the rate of 2 litres...
  19. T

    A few questions about relative humidity

    I have a couple of wonderings about relative humidity under different circumstances, During hot weather periods RH gets high. Is this because water evaporates from oceans and such? Shouldn't how weather result in low RH since hot air can hold more moisture? During the winter RH usually gets...
  20. A

    How temperature affects relative humidity

    I have a fan in crawlspace that I can turn on to draw in fresh air. This could reduce the relative humidity by raising the air temperature, and lowering the moisture density. However, air temperature doesn't go up very much because the dirt and foundation wall tend to keep the air at around 55...
  21. 3

    Should You Leave Water in Your Vacation Home to Raise RH?

    I hope it is ok to post a rather pedestrian question to those of you who may laugh. We own a vacation home in Phoenix and continually folks insist we should leave pails of water in the house while we're away to help raise the RH and keep construction materials from drying out, cracking etc. This...
  22. F

    Relative humidity and the science behind cigar storage

    I recently got heavier into cigar smoking and want to start storing cigars at home in a humidor. Like most things that become a sort of hobby of mine, I research the hell out of my new fascination. So while learning about humidors, I discovered that the cigar community is split on whether or...
  23. R

    Always 100% relative humidity over water? Supersaturation?

    Hello Is there any way that RH over water in a closed container could not be 100%? Assuming that the walls of the container are at the same temperature as the contents, that sufficient time has gone by for uniform diffusion of water vapour, that we are at usual room temperatures and...
  24. S

    Relative Humidity Changes in a Closed Room

    Homework Statement A room with volume 2000 cubic metres has air at T=25degrees Celsius with relative humidity 80%. Density of water vapor in saturated air at 25deg Celsius is 22.8g/m^3. Temperature and pressure of room remains constant. What mass of water vapor must be removed from this air to...
  25. Rugile

    Archived Fluid statics and relative humidity problem

    Homework Statement A tube (both ends open, height h, diameter d) is immersed into water so that length h1 of the tube is above water. Then the top of the tube is covered tightly and the tube is pulled out of the water so that length h2 is above water. The relative humidity of air is \phi and...
  26. S

    Calculating the relative humidity

    Can anyone help me with the below? Air is drawn into a compressor at normal temperature and pressure (N.T.P) and compressed to a pressure of 6 bar gauge. After compression the air is delivered at 1.2 m3 min-1 and cooled to a temperature of 30°C, at which point condensate is collected at the...
  27. G

    What is the Reference State for Finding Specific Enthalpy on a Mollier Diagram?

    Homework Statement We have a saturated vapor mixture with relative humidity 80% at a termpature of 30 C. Find the specific enthalpy of the mixture. Homework Equations The Attempt at a Solution I missed a lecture and a session due to illness and have fallen behind. We are allowed...
  28. A

    How much water is needed to increase relative humidity in a bunker?

    I have to design a system to increase the relative humidity in a bunker. I would like to use a spray nozzle and service water. How could I determine how much the relative humidity would raiser per volume of water sprayed into the bunker. Known: - Volume of Bunker - Temperature - Initial...
  29. H

    Relative Humidity and Dew Point after compression

    Hello all, Would appreciate some guidance here. I have an air compressor compressing ambient air to 3000 psi into a 425cu-in tank. I have the volumetric flow rate at the exit. If I was looking for relative humidity, dew point, and moisture content/accumulation at the exit, how would I go...
  30. D

    Relative Humidity and feeling of comfort

    Hi, The more humid it is, the more irritable the climate will be. The sweat will not evaporate easily if the relative humidity is high. I have been to two cities in two different countries recently - Madison in USA and Kolkata in India. The humidity in Kolkata is about 65% and that in Madison...
  31. J

    Relative humidity and evaporation

    Homework Statement The air is at 50% humidity and the temperature of the surface of the ocean and the air is 20degrees C. What are the two critical processes that must occur to ensure there is continued evaporation in the absence of external energy sources? Homework Equations Relative...
  32. S

    Air temperature and relative humidity

    In sauna rooms, people spend time in conditions of air temperature over 90° C (194° F) with a relative humidity near 10%. However, if you stick your arm into liquid water that is at a temperature of 90° C (194° F), you will be severely burned in less than one minute. Give two reasons why...
  33. maistral

    Relative humidity calculations.

    This is totally pissing me off, I don't know what the heck am I doing wrong. Alright, so I was given a temperature of 30°C at 30% relative humidity. I have to get the absolute humidity- So I used Antoine; log(P)=7.96681-1668.21/(228+30); P = 31.6869mmHg. 0.3 x 31.6869 = 9.41517...
  34. C

    Evaporation in 100% relative humidity

    hello guys if the water surface is at same temp as air above, and humidity is 100%, do any wataer molecules leave their liquid state to become vapour ie evaporate? I couldn't remember what professor said exactly, so i checked up on internet and it says water can't evaporate at 100%...
  35. R

    Relative Humidity: Find Optimum Off Coil Air Temp.

    [b]the total load in the conditioned space is equal to 320 mBh with a sensible heat ratio equal to 0.90. Outside air equal to 20% of the blower air volume will be used for ventilation. The space setpoint is 75°F db, 45% relative humidity. The outdoor air is 90°F db, 78°F wb. What relative...
  36. M

    Psychrometrics: Dew-point temp from Relative Humidity

    Homework Statement Air/Water vapor mixture, at 26C and 45%RH. What is the Dew-point temperature Homework Equations Tdp = Tsat when pg=pv RH = pv/pg The Attempt at a Solution From the psychrometric chart I can get the Humidity ratio, volume per kg dry air, Enthalpy, and...
  37. N

    Relative Humidity more than 100%

    In a calculation I am getting relative humidity more than 100%. What does it suggests? I know this means liquid water is also present in addition to saturated vapour. How should I calculate liquid water content then? Do I have to calculate activity first? Also what happens when I am considering...
  38. A

    Relative humidity at negative temperature

    as we know at zero degree temp. water condense to form ice. so what will be the relative humidity of a chamber at a temperature below zero degree temp.? will it be nagative or what?
  39. L

    Thermodynamics problem with relative humidity

    Homework Statement After taking a deep breath, the mass of air expelled when I exhale is approximately 2.5g. The intake air is at 18deg dry bulb temperature at 60% relative humidity, and the air exhaled is at 100% relative humidity at a temperature of 32deg. The net mass of water expelled...
  40. P

    Air Density: How to calculate Relative Humidity

    Hi there, I'm trying to calculate air density; I noticed on http://en.wikipedia.org/wiki/Density#Density_of_air" that there is a table that makes it seem that air density is a function of temperature... I suppose I could safely say that temperature has a significant effect on air density...
  41. P

    Calculating the change in relative humidity

    Homework Statement the air temperature is -15 degrees Celsius and the relative humidity is 70 per cent. If the air is brought indoors and heated to 20 degrees Celsius with no added moisture, what is the new relative humidity? Homework Equations RH=e/es x 100% or ws=epsilon*es/p...
  42. K

    Relative Humidity and Vaporization

    Homework Statement If a person breathes 10 liters per minute of air at 68 degrees F and 50% relative humidity, how much water per day must the internal membranes supply to saturate the air at 98.6 degrees F. (assume all the moisture is exhaled) If each gram of water extracts 580 calories as...
  43. L

    Question on relative humidity 2

    Homework Statement The temperature of a room is 25 degree Celsius and its relative humidity (RH) is 60%.If the temperature is lowered to 15 degree Celsius,what fraction of water vapor gets condensed? Saturated Vapour Pressure(SVP25)at 25 degree celsius = 23.7mmHg S.V.P.15=12.8mmHgHomework...
  44. L

    What is the relationship between relative humidity and temperature?

    Homework Statement Temperature of a closed room is 15 degree celsius (T288).Relative humidity(RH)at 15degree celsius =60% Find the RH when the temperature is 20 degrees. S.V.P at 15 degrees celsius=12.67mmHg SVP at 20 degree celsius = 17.36mmHg (ans:45%) Homework Equations The...
  45. L

    Relative Humidity: Ice Block's Effects on Room Air | Homework Questions

    Homework Statement Consider the folowing statements made about the space just above a small block of ice placed in still air,where the room temperature and the relative humidity are 30 degrees and80% (A) Absolute humidity of air in the space just above the iceblock is higher than the absolute...
  46. H

    How much water is needed to raise the humidity in my house?

    I am trying to get an idea of how much water vapour is in the air in my house, and how much water it would take to raise the humidity. Lets say that my house has a floor space of of about 170 sq meters with ceilings of 2.5 meters for a total volume of 425 cubic meters. Assume an air...
  47. J

    How Much Water Must Be Supplied to Saturate Breathed Air and Its Caloric Impact?

    Homework Statement If a person breathes 10 liters per minute of air at 68 degrees F and 50% relative humidity, how much water per day must the internal membranes supply to saturate the air at 98.6 degrees F? (Assume all the moisture is exhaled). I f each gram of water extracts 580 calories as...
  48. Ivan Seeking

    Relative humidity calc at high temps

    I am looking for equations valid at temps up to 500 degrees F, that allows one to calculate the relative humidity for air based on wet bulb and dry bulb temps. Everything that I can find is only accurate at lower temps. For that matter I can't even find a psychrometric chart that goes that high...