There are several different methods for defining the amount of
water vapor in the atmosphere. The definition used depends on
- Specific humidity - measures mass of water vapor
in a fixed total mass of air. In general it is greater in the
tropics (16 g/kg) than in the polar regions (4 g/kg). This definition
is used when we are interested in the actual amount of water that
is in the atmosphere as a gas. The specific humidity remains constant
as long as you do not add or take out water vapor molecules from
the volume of air.
- Mixing ratio - mass of water vapor in a fixed
mass of remaining dry air. Since there are so few molecules of
water vapor in a volume of air, as compared to N2 and O2, the
value of the mixing ratio is similar to the specific humidity.
Changing the temperature of the air parcel does not affect the
parcels mixing ratio.
- Absolute humidity - mass of water vapor in a
fixed volume of air, or the water vapor density. We don't ever
use this definition so don't bother memorizing it!
- Actual vapor pressure - the amount of water
vapor in terms of the amount of pressure that the water vapor
molecules exert. It is expressed in units of mb. We use the actual
vapor pressure when discussing evaporation and condensation. It
is therefore of use when we discuss clouds.
- Saturation vapor pressure - pressure that water
vapor molecules would exert if the air were saturated at a given
temperature, its units are mb. Saturation vapor pressure increases
with rising temperature. Saturation vapor pressure just above
liquid water is greater than that over ice. At temperatures below
freezing it takes more vapor molecules to saturate air directly
above water than it does to saturate air directly above ice. This
is important for precipitation.
- Relative humidity - ratio of the amount of water
vapor actually in the air compared to the amount of water vapor
required for saturation at that particular temperature and pressure.
This is one method that nightly weather persons report. It is
expressed as a percentage. Increasing or decreasing the amount
of water vapor in the air changes the relative humidity. A change
in temperature will also bring about a change in relative humidity.
- Dew point temperature - the temperature to which
air would have to be cooled (with no change in air pressure or
water content) for saturation to occur. When the dew point is
below 32F (0C) it is called the Frost point
- The Heat Index - combines air temperature with relative
humidity to determine an 'apparent' temperature - what the temperature
Boiling point decreases with altitude - boiling occurs when saturation
vapor pressure of escaping bubbles is greater than the total atmospheric
Humidity and clouds
To form clouds we need to change the phase of the atmospheric
water vapor to either a liquid or a solid. To do this we have
to increase the relative humidity to near 100%. We can accomplish
this by either cooling the air or by evaporating water vapor into
the air. Most clouds form by cooling the air through lifting.
From the hydrological cycle we know atmospheric water vapor content
is the surface. Water must evaporate (or sublime) from the surface.
The rate of evaporation is a function of temperature of the water,
the temperature of the air, the relative humidity of the air,
the wind speed and the surface area of the water. Think of the
- They blow out warm air (saturation vapor pressure increases
with increasing temperature)
- The air is fast moving (increased wind speed increases evaporation)
- You rub your hands underneath them (You are spreading-out
the water on you hands - increasing its surface area)
THE HYDROLOGICAL CYCLE