T/G Layout 1

The ratio of the vapor pressure at Point A to the saturation vapor pressure for the initial
temperature (Point C)— expressed in percent — is the relative humidity. As the parcel
cools along the line A-B, its relative humidity increases. When temperatures cool in the
evening, with little change in local moisture levels, relative humidity increases and
reaches a peak just before sunrise.

For a given temperature (T 1 ):

vapor pressure at Point A

= relative humidity (%)

saturation vapor pressure for Point C

Clouds may occur when air is cooled to near its dew point. There are three ways to
cool air to its dew point:

1. advection of warm air over a cold surface


2. mixing air parcels of different temperature and moisture


3. lifting of air to higher levels

advection

The horizontal transfer of any atmospheric property by the wind.

• First, horizontal motion ( a d v e c t i o n )of warm and moist air over a cool surface
  will cause the air parcel to cool and condensation to occur. This is how advection
  fog forms.


• Mixing parcels of different temperature
  and moisture can also result in cloud
  formation. The mixing cloud is
  another application of the Clausius-
  Clapeyron equation (figure 30).
  Parcels A and B are both in the
  unsaturated region of the graph.
  Parcel A is warm and moist and
  Parcel B is cool and dry. When they
  are equally mixed, the final parcel
  has a vapor pressure equal to the sat-
  uration vapor pressure (es) and con-
  densation occurs. Jet aircraft contrails
  are an example of this type of cloud.


• A third way to cool air to its dew
  point is by lifting. Because pressure
  and accordingly, temperature,
  decrease rapidly with height, a rising
  parcel of air will cool rapidly.

figure 30.

Mixing Clouds

T B T C

e A

es

A

B

T A

e C

C

e B