What is Humidity: Definition and 127 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. O

    Arcing and conductivity and object interference

    Im trying to figure out if I were to have two points with 10cm of humid ocean air between them and a fly or mosquitoes flew between the points, if they would get zapped and create an electrical arc, or if the bug would act as an insulator and be avoided by the arc? also, could the bug trigger...
  2. C

    B Some questions about humidity and storing DVDs in a closed cardboard box

    If I have a dvd disk inside those plastic boxes suitable for dvd disks in black, if I have an electronic board inside a bag or papers inside any plastic bag and after that all of them inside a closed cardboard box when I closed the cardboard box or bag or dvdbox it was in rainy or humid weather...
  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. G

    I Does high humidity air transfer sound better than dry air?

    Does high humidity air transfer sound better than dry air?
  5. 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...
  6. Cheatcode

    Lawn/Garden How can I optimize my DIY humidifier to improve its longevity and efficiency?

    I am making my own humidifier, the store bought heat based humidifiers seem to all die very fast from how hard the water is where I live. I am pumping air from an aquarium air pump (A) into a sealed gallon of water (B) through an airstone (C) so that bubbles (D) rise through water and hydrated...
  7. G

    Question about humidity and plastic and rubber

    Is it possible to maintain an environment with low humidity to preserve electronic devices from oxidation without this low humidity drying out plastic and rubber from electronic devices and DVD discs that are plastic and in that environment? in my room I have two digital hygrometers and an air...
  8. 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...
  9. G

    Question about plastic bags and humidity

    What material is the plastic market bag made of? does this material retain (hold) moisture in the bag and everything in the bag or around the bag becomes damp? i put some bags inside a cardboard box and inside the cardboard box i also put dvd cases and i am worried if the bags will hold...
  10. kyphysics

    I Does Humidity "Disperse" and "Even Out" in Enclosed Space?

    I have a dehumidifier (just one) that is placed indoors in the house in a "central" location (maybe not perfectly center, but close). Obviously, the local humidity around the machine will get sucked out of the air and turn to water. But, would the humidity in a different part of the house be...
  11. S

    I Calculate humidity raise/drop when opening a window

    Hello, I am making a very simple device. It will measure temperature and humidity outside the window and also measure temperature and humidity inside the room. And depending on the collected data, the device will calculate whether u will drop humidity in the room if you open the window or will...
  12. S

    Raising humidity - how much water is needed?

    Hello, my first post here so nice to meet you all :) I have a question. Let's take a small room, about about 16 m^3 (172 feet^3). This room has a very low humidity, especially in the winter. It is around 35%, i would like to raise it to 55% with a custom made swamp cooler. I am trying to figure...
  13. 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...
  14. 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?
  15. 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...
  16. Yud

    I Condensation as humidity control

    Context: Small living room, Moderate amount of humidity if the room doesn't get ventilation but is cold outside. Question: If I use some object that can stay in a cold temperature enough to cause condensation (like having a cold beer sitting for a while) and remove the condensed liquid every...
  17. F

    How does high humidity affect suction pressure & discharge pressure?

    In Chapter 40 (titled Typical Operating Conditions) of my textbook Refrigeration and Air-Conditioning Technology, the authors list several different objectives for readers at the beginning of the chapter. One of the stated objectives is for the reader to be able to describe how humidity affects...
  18. 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...
  19. 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...
  20. T

    Synthetic fibers (rope) that absorb water and shorten

    Hello. I have an engineering idea, an invention. I want to make a watering valve for plants that opens the water when the soil moisture content is too low. I am looking for a material that (1) would not rot, and (2) absorb water and contract at a significant rate to open a valve (millimeter...
  21. Asamad

    Absorption of humidity: NaCl vs. silica gel

    What is the absorption ratio of the water humidity of each in a given period of time, is there any proof?
  22. K

    How to calculate : Mass flow of water vapor in the air (kg/h)

    Hello Homework Statement Lets say i have the given parameters: air volume flow : V = 36 m3/h Relative humidity: RH=33% air mixing ratio : X = 5.5 (1g H2O) / 1kg (DRY AIR) (calculated from RH) air temperature : T= 20 C So i know how much mass of water is the air compared to the air's "dry air"...
  23. acvtre

    HVAC DIY glovebox or closed fume-hood?

    Hi everybody, I'm a PhD student working on solution-processed kesterite PV cells. The material is spin-coated and I personally believe that its wetting properties are influenced by environmental parameters like temperature and humidity. To control these parameters, or at least reduce their...
  24. A

    I How to Control Basement Humidity Without Warming Air

    not exactly sure where to post this, or if this is the right place but, my problem is controling humidity in my basement, what id like to do is be able to let in the cool nights air without making the basment humid what I am doing now is running a large vollume dehumidifyer unit (90pt/perday)...
  25. oobgular

    Change in humidity with altitude

    Homework Statement So I sent a humidity sensor up in a weather balloon, which gave a reading for relative humidity. I eventually want an absolute humidity, but I am unsure whether I need to correct the output based on the changing density and atmospheric pressure with altitude. Homework...
  26. B

    I Humidity when T is not constant in space

    Hi all! How is humidity related to temperature in a system where temperature is constant in time but not in space? As an example: If we have humid air trapped between two parallel walls with T1 and T2 respectively, how does humidity behave along the line from one to the other wall? Is absolute...
  27. C

    Calculate the moisture content of air

    Homework Statement (Simplified) During tests at cliffside power station, the following data was recorded from the cooling tower. A cooling tower has a 9 cell draft design, each diameter is 10m. Data D_o = 10m V_exit = 14.4 m/s T_exit(15 celsius) = 288.15K Ambient conditions : T(15 celsius) =...
  28. G

    Humidity & Discharge: Molecular Mechanism & Role of Polarity

    It is known that increased humidity helps discharge a charged body. I was wondering what is the molecular mechanism of that process. It is not clear to me why increased number of well separated uncharged water molecules in the air will help the process of discharge. Does the polarity of water...
  29. 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?
  30. 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...
  31. ervays

    Temperature, humidity and gas sealed box

    Hi, I'm working on my thesis and I need to calibrate some gas sensors that I'm building. I need controlled conditions(temp, RH and a known gas concentration) in order to calibrate the sensors. I'd love to know some idea to make a sealed box in which I trace and vary those parameter. I've...
  32. D

    Why when I freeze a container does the humidity go down?

    I am working on a project to stockpile and freeze photographic film and paper for later use. This is not for academic or commercial purposes, but just for personal use. I've been experimenting and am stumped as t the following: I place a package of film in a ziplock bag in an environment of...
  33. G Cooke

    I Can Temperature Changes Alter Humidity in a Completely Filled Closed System?

    We know that for a closed system, isobaric heating decreases the humidity and isothermal compression increases the humidity. But assuming that we start with the volume completely filled with humid air, is it not true that any increase in temperature must also increase the pressure? If so...
  34. Ravi Singh choudhary

    Vapour pressure vs saturation pressure; too much confusion

    It was only the psychrometry came; I read about vapour pressure is equal to the saturation pressure at 100% relative humidity. While before even in the textbooks both terms are used frequently as same. I fully understood what saturation pressure is; learned during phase change phenomenon of...
  35. Alfreds9

    Water wicking up porous media and air humidity ?

    Hello, I'd like to know which of these 3 example I sketched would have a steadier and higher air humidity at measuring point, assuming same conditions (except those illustrated as different, like porous media thickness and measuring point), water level and air turbulence within larger container...
  36. K

    I The influence of humidity when creating a vacuum

    Hello folks, I have two simple questions and some ideas about it:1. Why is it harder to create a vacuum and takes more time to evacuate the chamber when the air inside is humid? Humid air is lighter than dry air. So I have no idea why it takes longer...2. Why do you use nitrogen to vent the...
  37. 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...
  38. 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...
  39. 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...
  40. D

    Calculate humidity from dry/Wet bulb temp

    Homework Statement Dry-bulb Temperature 46'C Wet-bulb Temperature 34'C P=101.3kPa with these, I have to 'calculate' Specific Humidity(w), Relative humidity(φ), Specific Enthalpy(h)Homework Equations Specific Humidity w=0.622*Pv/Pa when Pa=P-Pv Relative Humidity φ=w*P/[(0.622+w)*Pg] Specific...
  41. N

    Why does ice grow in the freezer rather than evaporate?

    Hi! I hope I put this in the right place. I apologize if I didn't. We we're having a discussion here, as to why frost builds up in the freezer. The discussion has been something like this: Guy 1: The water in this room temperature bowl will weigh less and less over time. This is because...
  42. 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...
  43. 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...
  44. MexChemE

    Converting humidity ratio to specific humidity

    Hello PF! I have a very simple question on psychrometry. I had an argument with one of my professors regarding these two quantities, humidity ratio and specfic humidity. Humidity ratio is defined as mass of water vapor per unit mass of dry air. It's the one you can read from a psychrometric...
  45. Delta2

    Humidity definitions and measurement devices

    I know there are three definitions of humidity. Absolute , relative and specific. Can you tell me if on a typical commercial device (such as the one i have in a clock i have) that measures humidity as percentage is it the relative humidity that it measures? Can the percentage become greater...
  46. R

    Atmospheric humidity for water production.

    *Suggestions for Water Production for California.* California is still in the midst of a huge drought: Annie Sneed Science 03.23.15 7:00 am California’s About to Run Out of Water. We Have to Act Now. http://www.wired.com/2015/03/californias-run-water-act-now/ There appear to be however...
  47. P

    Will Boiling Water Create Humidity?

    Is this a way to "humidify" your house? Will boiling water create humidity?