Geoscience Reference
In-Depth Information
identify different air masses and indicates when
latent heat has been released through saturation of
the air mass or when non-adiabatic temperature
changes have occurred.
convection
involves uplift by mechanical forces
such as flow over orographic barriers, frontal
uplift, turbulence due to surface friction or ascent
due to convergent windflow.
Figure 5.2
illustrates an important property of
the tephigram. A line along a dry adiabat (
)
through the dry-bulb temperature of the surface
air (
T
A
), an isopleth of saturation mixing ratio
(
xs
) through the dew-point (
Td
), and a saturated
adiabat (
w
) through the wet-bulb temperature
(
Tw
), all intersect at a point corresponding to
saturation for the air mass. This relationship,
known as Normand's theorem, is used to estimate
the
lifting condensation level
(see
Figure 5.3
). For
example, with an air temperature of 20
B CONDENSATION LEVEL
Rising air cools as air parcels expand and the
relative humidity level of the air increases.
After reaching saturation - 100 percent relative
humidity - condensation occurs and cloud forms
above the condensation level. Convection may
occur as free or forced convection.
Free convection
is caused by density differences in the atmosphere
that give rise to thermals - rising currents due to
differential heating of the atmosphere.
Forced
°
C and a
dew-point of 10
°
C at 1000mb surface pressure on
A
B
Stable case
Unstable case
TROPOPAUSE
ENVIRONMENT
CURVE
A
B
Height
pressure
T
d
PATH
CURVE
T
A
Unstable
SALR
Isothermal
layer
CONDENSATION
LEV
E
L
X
s
I
nversion
DALR
T
d
T
A
Temperature
Figure 5.3
Tephigram showing (A) stable air case - T
A
is the air temperature and T
d
the dew-point; and
(B) unstable air case. The lifting condensation level is shown, together with the path curve (arrowed) of a
rising air parcel. x
s
is the saturation humidity mixing ratio line through the dew-point temperature (see text).
