Weather
Lecture 5: Stability (Quiz 1)
1. Terminology
(1). Convection: rising air current (vertical motion).
(2). Advection: wind or horizontal movement of air current.
(3). Turbulence: irregular air motion.
(4). Adiabatic process
No energy or mass
exchange between an air parcel and its
surrounding air when the air parcel rises or subsides
(sinks).
(5). Environmental air or ambient air: Air surrounding an
air parcel.
(6). Air parcel:
any given volume of air (can be any size)
2. Lapse rates
(1). Environmental lapse rate or
existing lapse rate or ambient lapse rate:
A. The observed vertical
change of air temperature with increasing
altitude.
B. Determined from
radiosonde observation.
(2). Adiabatic lapse rate (air parcel’s
lapse rate)
A theoretical
lapse rate determined by the first law of thermodynamics
(conservation
of energy).
A. Dry
(unsaturated) adiabatic lapse rate
(A).
Unsaturated air parcel.
(B).
-1 oC/100 m (-10 oC/1000 m)
or -5.4 oF/1000
means air temperature decreases
with increasing altitude.
B. Wet
(saturated) adiabatic lapse rate
(A). Saturated air parcel within clouds or fogs.
(B). Varies according to moisture content
(C). About -0.6 oC/100 m (-6 oC/1000 m) or - 3.2 oF/1000
ft in
the lower atmosphere.
Release of latent heat during condensation adds heat to
the saturated air parcel as it rises. Hence, wet adiabatic
lapse rate is smaller than dry adiabatic lapse.
3. Stability
The tendency of the resistance of an
air parcel against the vertical
displacement exerted by an external force or forces (orographic uplifting,
solar heating, and weather systems)
(1). Stable:
A. The air parcel tends
to resist against the vertical displacement
when uplifted by an
external force or forces.
B. The air parcel’s
temperature is colder than its surrounding air at the
same height.
C.
Small environmental lapse rate [< Dry adiabatic
lapse rate,
or < 1 oC/100
m (or 10 oC/1000 m), ignore the minus sign].
D. Inversion and/or
isothermal (air temperature increases or remains
constant with altitude). Environmental or ambient lapse rate.
E. An air parcel tends to sink if uplifted from the ground surface.
Air parcel’s temperature decreases more than (small environmental
or ambient lapse rate) than its environmental (ambient) air and
hence is colder than its environmental when uplifted to a given
height. Cold air
parcel tends to sink, resisting against uplifting.
(2). Unstable
A. The air parcel tends
to exert no resistance against the vertical
displacement when uplifted.
B. The air parcel’s
temperature is warmer than its surrounding air at
the same height.
C.
Large environmental lapse rate (> Dry adiabatic lapse rate,
or > 1 oC/100
m or 10 oC/1000 m).
D. An air parcel tends to continually rise if uplifted from the ground
surface.
Air parcel’s temperature decreases less than (large environmental
or ambient lapse rate) than its environmental (ambient) air and
hence is warmer than its environmental when uplifted to a given
height. Warm air parcel continues to rise (no resistance against
uplifting).
(3). Conditional stability
A.
Stable for unsaturated air parcel (environmental lapse rate
< 10 oC/1000
m).
B.
unstable for saturated air parcel
(environmental lapse rate
> 6 oC/1000
m).
C. Environmental lapse rate is between dry and wet
adiabatic lapse
Rates (Environmental lapse rate is between 10 oC/1000 m
and
6 oC/1000 m).
(4). Neutral stability
A. Saturated air parcel:
Environmental lapse rate equals wet adiabatic
lapse rate.
B. Unsaturated air
parcel: Environmental lapse rate equals dry adiabatic
lapse rate.
* Since dry
(unsaturated air parcel) and wet (saturated air parcel in
clouds or fogs) adiabatic
lapse rates are constant (does not change),
stability is
determined by the environmental or ambient lapse rate
only. We assume the
wet adiabatic lapse rate in the lower
atmosphere is
constant since it does not vary much!
4. Weather
(1). Stable: stratus clouds, fog, smog, haze, drizzle-type rain (if there is rain,
light rains or small raindrops lasting for a long time)
(2). Unstable air: cumulus, no fog and less smoggy or hazy, shower rain
(if any, heavy rain in short-time).
**Please keep
in mind, the sky may be clear with either stable or
unstable air. It requires other conditions to form smog,
haze, fog,
cloud and precipitation.