Is Fog solid or gaseous mater? Can it reach the triple point of water? As per my research, fog rises only if its density is less than that of water as water vapor does, a fact that is forcing me to classify fog as a gaseous mater. Yet the only condition for fog to exist is when temperature is below the triple point of water,where we prospect to have liquid on transition to solid. This fact violates the idea for the triple point of water.

  • 7
    $\begingroup$ In reality the fog are very small droplets of water. So it is liquid fase. $\endgroup$ Jan 19, 2017 at 10:17
  • 1
    $\begingroup$ It is not the triple point separating liquid and vapor. A saturating point is good enough to make the phase change. Fog is also not in equilibrium state which can fail the state equation. The process is dynamic. Liquid droplet undergoes phase change from liquid to vapor when the temperature increases, e.g. droplets falling down. And vapor undergoes phase change from vapor to liquid when it moves up due to buoyancy. $\endgroup$
    – user115350
    Jan 19, 2017 at 19:24
  • 2
    $\begingroup$ Is dust floating in the air a gas or a solid? $\endgroup$
    – Samuel
    Jan 19, 2017 at 22:38

4 Answers 4


It is a colloid in which the dispersed phase is liquid and the medium in which it is dispersed is gas. This type of colloid is called aerosol.


Standard everyday fog consists of a mixture of liquid water droplets suspended in water vapour in the air (it is an example of a 'liquid aerosol' colloid). A nice summary of the formation is given by the National Geographic website encyclopaedic entry about fog as:

Fog shows up when water vapor, or water in its gaseous form, condenses. During condensation, molecules of water vapor combine to make tiny liquid water droplets that hang in the air. You can see fog because of these tiny water droplets. Water vapor, a gas, is invisible.

There has to be a considerable amount of humidity and often, the presence of very small microscopic aerosols (dust, sea-salt etc etc) to condense on.

  • $\begingroup$ The triple point diagram refers to 100% H2O, not humid air. Not sure, but I think it applies to the partial pressure of H2O in air, at least neglecting chemistry (ideal gas approximation, no liquid-phase solutes). So then the diagram would predict an ephemeral fog with no frost nor dew when the temperature is 0.01°C and the partial H2O pressure is 6 mBar. $\endgroup$ Jan 20, 2017 at 6:43
  • $\begingroup$ Low partial pressure of H2O simply means low absolute humidity. At 0°C, 6 mBar is about 100% relative humidity which agrees with my description. $\endgroup$ Jan 20, 2017 at 8:23
  • 1
    $\begingroup$ If I remember correctly, the pre-fog microscopic aerosols about which water vapor condenses to form fog and cloud are sometimes called nucleation sites, in case any readers are looking for key words for further research. $\endgroup$ Jan 21, 2017 at 5:11

Fog is not a pure substance, so don't search for its phase. It's a mixture of water, water vapor and air. So No single PV or PT or any kind of diagram can represent its state.

BUT, As water vapor act approximately like gas, as long as we deal with "shot"s of the system, and because this mixture is approximately uniform in composition everywhere, we can say it is like water and water vapor system.

That's because It really act like that in practice, pressure will be constant for some isothermal compression as this PVT surface for water suggests.

The approximated system therefor lies in "vapor and liquid" part of this PVT surface.

If you intersect a horizontal plane with this surface, In between the critical point and the triple line. The resulting curve will be where approximated fog can exist. It is way higher than the pressure of the triple line.


Source: http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/pvtexp.html#c1

  • 5
    $\begingroup$ If you're going to include a picture in your answer that you didn't draw yourself, you should at least mention where you found it. Of course, it would generally be even better if you'd respect their copyright and not repost their images without permission at all. (If you do have permission from the author of the HyperPhysics site to use that image, it would be good to mention that in your answer, too.) $\endgroup$ Jan 19, 2017 at 14:36
  • $\begingroup$ Note that you can also have fog in which the water particles are solid. Hereabouts it's called 'pogonip'. $\endgroup$
    – jamesqf
    Jan 19, 2017 at 19:14
  • $\begingroup$ Fog is not homogeneous. All colloids except gas in gas are heterogeneous. $\endgroup$
    – Mitchell
    Feb 18, 2017 at 19:23
  • 1
    $\begingroup$ @BhavyaSharma I meant we can call it a thermodynamic system because it is uniform in composition. I will replace it if saying homogeneous brings technical problems. $\endgroup$
    – AHB
    Feb 19, 2017 at 11:45

Fog is a colloid, which is a type of mixture in which microscopic particles of one substance are suspended in a continuous medium. A colloid is distinguished from other types of suspension in that the suspended particles do not settle out of the suspension over practical time scales.

Both the suspended substance in a colloid and the medium in which it is suspended can be solid, liquid, or gas, except that there are no known gas-in-gas colloids. Different combinations of phases in colloids have different names. Liquid-in-gas colloids, like fog, and solid-in-gas colloids, like smoke, are called aerosols. Other well-known names for different kinds of colloids include foams (gas-in-liquid), emulsions (liquid-in-liquid), and gels (liquid-in-solid).

  • $\begingroup$ «there are no known gas-in-gas colloids. » because gasses are all miscable? Is there some reason to suppose how there might be such a thing? $\endgroup$
    – JDługosz
    Jan 19, 2017 at 21:13
  • $\begingroup$ That is the reason. I don't know of any reason to expect any exceptions, but then I wouldn't have expected this answer: ( worldbuilding.stackexchange.com/a/68103/21092 ) so I'm inclined to be cautious about stating absolutes. $\endgroup$
    – Nobody
    Jan 20, 2017 at 18:51
  • $\begingroup$ This answer implies that there may exist a solid (ice) in air colloid as well and its not clear to me if that would also be called fog. If it is cold enough couldn't this type of "fog" also exist? $\endgroup$
    – Octopus
    Jan 20, 2017 at 21:13
  • 2
    $\begingroup$ @Octopus - There certainly is the possibility of an ice-air colloid. This is what forms "sun dogs". $\endgroup$
    – Hot Licks
    Jan 20, 2017 at 21:38
  • $\begingroup$ @HotLicks And cirrus clouds and contrails perhaps? $\endgroup$ Jan 21, 2017 at 5:14

Not the answer you're looking for? Browse other questions tagged or ask your own question.