Rising Steam and Its Temperature
Why does steam always rise in upward direction?
What is the temperature of water vapour in air?
It rises because the vapor-laden air is warmer and less dense than the surrounding air. Unless
the vapor (steam) is very close to the emanation source, its temperature would be very close
to the temperature of the surrounding air.
When hot water evaporates, it evaporates as invisible water vapor. Its density to a good
approximation is given by the ideal gas law (PV=nRT). The molar density (dmol) = (n/V) = P/RT.
At the "normal" boiling point, P = 1 atm, so:
(d) = 1/RT where R is the ideal gas law constant 0.08025 (l*atm/mol*kelvin)
and T = 373.15 K which corresponds to the "normal" boiling point of 100 C.
The density of the surrounding air is approximately 1/RT` where T` ~ 298 K which corresponds to
an ambient temperature of 25 C. The mass density (dmass) of the water vapor at the "normal"
boiling point is also less than the surrounding air: (dmass) = (m/V) = P*M/R*T, where M is the
molecular weight of water and air (18 vs. ~ 28 gm/mol) respectively.
Now when the rising water vapor condenses into small visible droplets and additional 10
kcal/mol of heat is evolved, which further heats the space immediately surrounding the
droplets forcing them to rise more by the convection produced.
At this point things can get more complicated, but interesting, so I will pursue it for
you. If you live in a cold climate, the upper tier of states in the midwest will do, and if the
temperature is very cold -- say 0 F. or lower -- and if the wind is not too high, say less than
20 mph, and finally you have a power plant or other facility that is releasing steam, you will
observe that the plume "hangs up" in the atmosphere (either cumulus or stratus) almost
motionless, depending upon the wind speed. What is happening here is that the water vapor has
condensed into small
droplets of either liquid or solid, but because of the low temperature, that cannot evaporate.
The capacity of the air for water vapor is so small that the tiny droplets "have no where to
go". In addition, now the temperature of the surrounding air has dropped to let's say -40
F = -40C. At that temperature the density of the air is about 30% greater than it is at 25 C.
and in relatively still air the tendency for the droplets to fall to earth is greatly decreased
since it is now diffusion controlled. The droplets may also have picked up an electrical charge
causing them to repel one another, but that is beyond the scope of this response.
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Update: June 2012