T/G Layout 1

figure 29.

An example illustrating cloud formation is given in figure 29. The starting point for the
p a rcel of air is Point A. At this temperature (T 1 ) and water vapor pre s s u re (e 1 ), the parc e l
of air is not saturated with respect to water vapor. That is, it is positioned below and to
the right of the saturation line (es). If the parcel is cooled with no change in moisture, it
will move along the line A-B. When it reaches point B, its vapor pre s s u re (e 1 ) is equal to
the saturation vapor pre s s u re (es) for that temperature (Td) and condensation occurs.
The temperature at point B is known as the dew point temperature or dew point.

Dew Point Temperature (T

d

)

If ambient conditions are: T = T

1

, e = e

1

,

then the system is not at equilibrium with respect to H

2

O,

that is e

1

< e

s

for temperature = T

1

.

But what happens if we cool the air, with e

1

constant, so that T = T

d

?

T

d

T

1

e

1

super

saturated

unsaturated

e

s

B A

e

s

1

T (°C)

C