Chapter 5
Water vapor pressure (in millibars) is water vapor partial pressure.
water vapor pressure =
× pressure inside parcel
# water vapor molecules
total # of molecules
1 argon atom
3 water vapor molecules
200 nitrogen molecules
54 oxygen molecules
Parcel
(molecules can’t pass
through wall, but
walls can expand and
contract)
Pressure inside parcel = 1015 millibars
Water vapor pressure =
3
(1015 millibars) = 11.8 millibars
258
Adding water vapor molecules, or increasing pressure, or removing non-water molecules
increases water vapor pressure!
Saturation implies that if more water is added to a parcel, water vapor will condense to
form water (liquid) when the parcel comes to equilibrium at that temperature.
Lid
Measure pressure of
water vapor Pwv above
water at each
temperature T, allow to
come to equilibrium.
Pwv
Water vapor
in air
Water
Measure
temperature, T
Hot plate
Pwv
(see figure 5.12)
Pwv is a strong function of T
Temperature
What happens if Pwv approaches Ptotal?
What causes boiling?
Boiling occurs when the saturation vapor pressure equals the total pressure above the
liquid.
As T increases, eventually Pwv will equal Ptotal (open container)
What if container is closed ??
Ptotal will increase as Pwv
Ptotal
So temperature required to boil will increase.
This is the principle of the pressure cooker – water boils at a higher temperature and,
thus, cooks food faster.
Ptotal
Small bubble of water vapor
forms.
If Pwv = Ptotal, the bubble can
survive.
If Ptotal > Pwv, the bubble will
collapse.
Note: For an open container, Ptotal will always equal atmosphere pressure in that
location.
Relative Humidity (RH)
RH =
actual water vapor pressure
saturation water vapor pressure
× 100 percent
Note: Humans are more sensitive to how close air is to saturation than they are to actual
water vapor content in air.
Dew point = The temperature to which air would have to be cooled (with no change in
pressure or water content) for saturation to occur .
Example: Keeping your house comfortable in Florida in summer
Suppose
Dew point is
overnight low is
75°F
72°F
So, RH will be 100% by morning
Windows were open and fans running overnight, so inside house will be about 72°F and
100% RH by morning.
Now close all windows and doors. At 2pm, outside temperature reaches 95°F and 90%
RH ( heat index over 130 - ouch! )
However, house is well insulated , so temperature inside rises to 80°F due to heat
conduction through walls and roof and radiative heating. What is RH inside the house
now? (assume no water enters house)
From table 1, p 117:
Actual water vapor pressure ~27mb
Saturation water vapor pressure at 80°F = 35mb
So, RH =
27
= 77%
35
heat index ~ 82
⇒ very warm but not deadly!
A sling psychrometer measures the wet-bulb temperature, which is the lowest
temperature that can be attained by evaporative cooling.
A large wet-bulb depression indicates a low RH.
Humans sweat to keep their bodies cool.
As water evaporates from wet surface, energy is required (latent
heat). This energy is supplied by the wet surface and this loss of
energy (heat) cools the surface.
Evaporation becomes more and more difficult as air gets closer to saturation.
At saturation, condensation = evaporation.
Condensation gives off heat and warms surface.