Humidity, Condensation, and Clouds
Met 10 Lecture 5
Dr. Craig Clements
San José State University
Circulation of water in the atmosphere
Hydrologic Cycle
Water in the atmosphere
 Definitions:
– Evaporation: Process where a liquid changes into a gas
– Condensation: Process where a gas changes into a liquid
– Precipitation: Any liquid or solid water that falls
from the atmosphere to the ground.
(i.e. RAIN!)
Water freely
evaporating and
condensing
Since more water
molecules are
evaporating than
condensing, then net
evaporation is
occurring.
Lid on:
Now, evaporation
and condensation are
equal. The air above
water is now called
‘saturated’.
The humidity is now
100%
Condensation

The process by which water vapor changes to a
cloud droplet

Water vapor molecules may ‘stick’ to
condensation nuclei and grow (billions) to
eventually form cloud droplet.

Examples of condensation nuclei include:
a. Dust
b. Salt
c. Smoke
 Condensation occurs primarily as temperature cools:
-colder the molecules more likely they are to ‘stick’ to
other molecules
Evaporation, Condensation and
Saturation
 saturation
 condensation nuclei
• In very clean air, about 10,000 condensation nuclei
are typically found in one cubic centimeter of air,
a volume approximately the size of your fingertip.
Humidity: What is it?
 Humidity refers to any one of a number of ways of
specifying the amount of water vapor in the air.
We can compare the weight (mass) of the water vapor
with the volume of the air in the parcel to obtain the
water vapor density or absolute humidity.
We can compare the weight of the water vapor with the
volume of air in the parcel with the total weight of the air
and obtain the specific humidity.
Vapor Pressure
 The total pressure inside an air parcel is the sum of the
pressures of each individual gas…nitrogen, oxygen and
water vapor.
At 1000 mb (sea level) nitrogen(78%) has a pressure of
780 mb, oxygen (21%) has a pressure of 210 mb. The
partial pressure of water vapor (1%) would be 10 mb.
The number of water vapor molecules is small compared
to total number of air molecules in the volume.
Actual Vapor Pressure is a good indicator of the amount
of water vapor in the air.
Saturation Vapor Pressure
 Saturation vapor pressure describes the amount of
water vapor needed to make the air saturated at any
given temperature.
Air that is saturated:
number of molecules
escaping the water surface
= amount returning.
At higher air temperatures,
it takes more water vapor
to saturate the air.
Relative Humidity
 The relative humidity (RH) is the ratio of the amount of
water vapor actually in the air to the maximum amount
of water vapor required for saturation at that particular
temperature and pressure.
 It is the ratio of the air’s water vapor content to its
capacity:
 RH =
water vapor content
water vapor capacity
Relative Humidity
 RH =
water vapor content
X 100 percent
water vapor capacity
Relative humidity is given as a percent. 50% RH means
that the air contains ½ the amount required for saturation.
A change in RH can be brought about by two primary ways:
1. By changing the air’s water vapor content
2. By changing the air temperature.
Diurnal Temperature and RH variation
Measuring Humidity: Sling psychrometer
A psychrometer consists of two glass thermometers with one
covered with a wick (cloth) that is wet. This measures the
‘wet-bulb’ temperature. Water vapor evaporates from the
wick and the bulb cools. The difference between the dry bulb
and wet bulb is the wet bulb depression.
Dew Point Temperature
Dew point represents the temperature to which air would
have to be cooled (with no change in air pressure or moisture
content) for saturation to occur.
Dew point is a good indicator of the air’s actual water vapor
content.
High dew points indicate high water vapor content, low dew
points = low water vapor content.
Dew points
Fig. 4-8a, p. 86
Dew and Frost
On calm, clear nights, the surface cools
rapidly by what process?
Air near the ground cools to the dew point
quickly, reaching saturation.
Water vapor condenses on blades of grass at
the ground, forming tiny specks of water
called dew.
Dew and Frost
Frost
When the dew point is below freezing (now
called the frost point), frost forms which is
composed of tiny ice crystals.
Water vapor changes directly into ice without
becoming liquid first– called deposition.
When the air’s relative humidity reaches about 75%, some of it
may begin to condense on tiny floating particles of sea salt and
other substances — condensation nuclei— that are
hygroscopic (“water seeking”) in that they allow water vapor to
condense onto them when the relative humidity is considerably
below 100%.
As water collects onto these nuclei, their size increases and the
particles are large enough to scatter visible light. Haze forms.
As RH approaches 100%, the particles grow larger and
eventually becoming visible to the naked eye…forming a cloud!
A cloud is a visible aggregate of tiny water droplets or ice
crystals suspended in the air.
Some are only found in the high atmosphere and some touch the
ground.
Cirrus clouds
Cirrocumulus clouds
Altocumulus clouds
Stratocumulus clouds
Cumulus clouds. Small cumulus clouds such as these are
sometimes called fair weather cumulus, or cumulus humilis
stratus clouds
Condensation trail– from an airplane.