Geoscience Reference
In-Depth Information
air. At the same temperature and pressure, the moister air has the same
number of molecules as the drier air, but some of the heavier air molecules
are replaced by lighter vapor molecules. Rearranging the Ideal Gas Law for
moist air, gives:
a
= [
P
/(
R
d
T
)] / [
1 + 0.61
(
r
v
/
r
a
)] = [
P
/(
R
d
T
)] / [
1 + 0.61q
]
d
is a constant so assuming the temperature and pressure are the same, if
a parcel of air is moister than its surroundings (i.e. if
q
is greater) its density
is less than the surrounding air and it will tend to rise. It is the effect of
this greater buoyancy which is allowed for by re-expressing potential
temperature as virtual potential temperature.
(h) In convective conditions parcels of air heated to a temperature above that
of the surrounding atmosphere near the ground can often keep on rising to
the cloud condensation level because both the air in the parcel and the sur-
rounding air cool with height. However, the ascending air cools at the dry
adiabatic lapse rate and the surrounding air cools less quickly so there can
be situations where ascent is suppressed prior to reaching the level at which
water vapor in the rising air parcels saturates, see answer 1(d).
(i) Once at the cloud condensation level, condensation and cloud formation
begins. This releases latent heat which further warms the air thus tending
to make the air more buoyant and enhancing its further ascent within the
cloud.
(j) In mid-latitude clouds with a temperature of -25°C all the phases of water
(solid, liquid or vapor) are likely to be present. In such clouds the Bergeron-
Findeison process is likely to be the most important process responsible for
cloud particle growth.
Answer 7
(a) Very open situations are not necessarily always the best rain gauge sites
because near-ground wind speeds tend to be higher and wind-related
blow-in/blow-out gauge errors possibly higher. Consequently an optimum
site might be surrounded by obstructions but should be located in a flat
open area of short mown grass and should be sufficiently far from up-wind
of obstructions that they all subtend a vertical angle of less than 30°. Ideally,
the gauge would be placed at the center of a pit in the ground (say) 1-2 m
across such that the top of the gauge is level with the ground. This avoids
splash-in errors. The top of the pit should be covered with an open mesh
(plastic mesh is cheap and easy to find) so that it has a similar aerody-
namic roughness to that of the surrounding grass. If this is done the near-
surface wind flow is essentially parallel to the ground and the effect of
wind on the gauge is minimized. Such a site might look like that shown
in Fig. 26.9.
