Chapter 8: Air Stability Flashcards
Anytime air moves upward, it
expands because of decreasing atmospheric pressure
downward moving air is
compressed by increasing pressure
But as pressure and volume change
temperature also changes.
When air expands, it
cools
and when compressed
, it warms
When air expands, it cools; and when compressed, it warms. These changes are
adiabatic
When air expands, it cools; and when compressed, it warms. These changes are adiabatic, meaning that
no heat is removed from or added to the air
When air expands, it cools; and when compressed, it warms. These changes are adiabatic, meaning that no heat is removed from or added to the air. We frequently use the terms
expansional or adiabatic cooling and compressional or adiabatic heating
The adiabatic rate of change of temperature is
virtually fixed in unsaturated air but varies in saturated air.
Unsaturated air moving upward and downward cools and warms at about
3.0°C (5.4° F) per 1,000 feet
Unsaturated air moving upward and downward cools and warms at about 3.0°C (5.4° F) per 1,000 feet. This rate is the
“dry adiabatic rate of temperature change”
“dry adiabatic rate of temperature change” and is independent of
the temperature of the mass of air through which the vertical movements occur.
Condensation occurs when saturated air moves
upward
Condensation occurs when saturated air moves upward. Latent heat
released through condensation partially offsets the expansional cooling
Condensation occurs when saturated air moves upward. Latent heat released through condensation partially offsets the expansional cooling. Therefore,
the saturated adiabatic rate of cooling is slower than the dry adiabatic rate
The saturated rate depends on
on saturation temperature or dew point of the air
Condensation of copious moisture in saturated warm air releases
more latent heat to offset expansional cooling than does the scant moisture in saturated cold air
. Condensation of copious moisture in saturated warm air releases more latent heat to offset expansional cooling than does the scant moisture in saturated cold air. Therefore,
, the saturated adiabatic rate of cooling is less in warm air than in cold air
When saturated air moves downward, it
heats at the same rate as it cools on ascent provided liquid water evaporates rapidly enough to maintain saturation.
When saturated air moves downward, it heats at the same rate as it cools on ascent provided liquid water evaporates rapidly enough to maintain saturation. ……………………….. at virtually this rate
Minute water droplets evaporate
. Larger drops evaporate
more slowly and complicate the moist adiabatic process in downward moving air.
If we force a sample of air upward into the atmosphere, we must consider two possibilities:
(1) The air may become colder than the surrounding air, or
(2) Even though it cools, the air may remain warmer than the surrounding air.
If the upward moving air becomes colder than surrounding air, it
sinks
If the upward moving air becomes colder than surrounding air, it sinks; but if it remains warmer it is
accelerated upward as a convective current
Whether it sinks or rises depends on the
ambient or existing temperature lapse rate.
The difference between the existing lapse rate of a given mass of air and the adiabatic rates of cooling in upward moving air determines if
if the air is stable or unstable.
The colder, more dense surrounding air forces the balloon on
upward
The colder, more dense surrounding air forces the balloon on upward. This air is
unstable
The colder, more dense surrounding air forces the balloon on upward. This air is unstable, and a
convective current develops
the air aloft is warmer. Air inside the balloon, cooling adiabatically, now becomes colder than the surrounding air. The balloon
sinks under its own weight returning to its original position when the lifting force is removed.
The balloon sinks under its own weight returning to its original position when the lifting force is removed. The air is
stable
The balloon sinks under its own weight returning to its original position when the lifting force is removed. The air is stable, and
convection is impossible
In the last situation, temperature of air inside the balloon is the same as that of surrounding air. The balloon will
remain at rest. This condition is neutrally stable; that is, the air is neither stable nor unstable.
Note that, in all three situations, temperature of air in the expanding balloon cooled at a fixed rate. The differences in the three conditions depend, therefore, on the temperature differences between the surface and 5,000 feet, that is, on the
ambient lapse rates
Stability runs the gamut from absolutely stable to absolutely unstable, and the atmosphere usually is in
a delicate balance somewhere in between
Stability runs the gamut from absolutely stable to absolutely unstable, and the atmosphere usually is in a delicate balance somewhere in between. A change in ambient temperature lapse rate of an air mass can
tip this balance
surface heating or cooling aloft can make the air
more unstable
surface cooling or warming aloft often tips the
balance toward greater stability
Air may be stable or unstable in
layers
Air may be stable or unstable in layers. A stable layer may
overlie and cap unstable air; or, conversely, air near the surface may be stable with unstable layers above.
When air is cooling and first becomes saturated, condensation or sublimation begins to form clouds. Whether the air is stable or unstable within a layer largely determines
cloud structure
Since stable air resists
convection
Since stable air resists convection, clouds in stable air form in
horizontal sheet-like layers or “strata
Since stable air resists convection, clouds in stable air form in horizontal, sheet-like layers or “strata.” Thus, within a stable layer, clouds are
stratiform
Adiabatic cooling may be by
upslope flow
Adiabatic cooling may be by upslope flow as illustrated in the following figure; by
lifting over cold, more dense air; or
by converging winds.
Cooling by an underlying cold surface is a
stabilizing process
Cooling by an underlying cold surface is a stabilizing process and may produce
fog
If clouds are to remain stratiform, the layer must
remain stable after condensation occurs
Unstable air favors
convection
A “cumulus” cloud, meaning
heap
Unstable air favors convection. A “cumulus” cloud, meaning “heap,” forms in a
convective updraft and builds upward
Unstable air favors convection. A “cumulus” cloud, meaning “heap,” forms in a convective updraft and builds upward. Thus, within an unstable layer, clouds are
cumuliform
Thus, within an unstable layer, clouds are cumuliform; and the vertical extent of the cloud depends on the
depth of the unstable layer.
Initial lifting to trigger a cumuliform cloud may be
the same as that for lifting stable air
Initial lifting to trigger a cumuliform cloud may be the same as that for lifting stable air. In addition, convection may be set off by
surface heating
Initial lifting to trigger a cumuliform cloud may be the same as that for lifting stable air. In addition, convection may be set off by surface heating. Air may be
unstable or slightly stable before condensation occurs
Initial lifting to trigger a cumuliform cloud may be the same as that for lifting stable air. In addition, convection may be set off by surface heating. Air may be unstable or slightly stable before condensation occurs; but for convective cumuliform clouds to develop, it must be
unstable after saturation
Initial lifting to trigger a cumuliform cloud may be the same as that for lifting stable air. In addition, convection may be set off by surface heating. Air may be unstable or slightly stable before condensation occurs; but for convective cumuliform clouds to develop, it must be unstable after saturation. Cooling in the updraft is now at
the slower moist adiabatic rate because of the release of latent heat of condensation
Initial lifting to trigger a cumuliform cloud may be the same as that for lifting stable air. In addition, convection may be set off by surface heating. Air may be unstable or slightly stable before condensation occurs; but for convective cumuliform clouds to develop, it must be unstable after saturation. Cooling in the updraft is now at the slower moist adiabatic rate because of the release of latent heat of condensation. Temperature in the saturated updraft is
warmer than ambient temperature, and convection is spontaneous.
Initial lifting to trigger a cumuliform cloud may be the same as that for lifting stable air. In addition, convection may be set off by surface heating. Air may be unstable or slightly stable before condensation occurs; but for convective cumuliform clouds to develop, it must be unstable after saturation. Cooling in the updraft is now at the slower moist adiabatic rate because of the release of latent heat of condensation. Temperature in the saturated updraft is warmer than ambient temperature, and convection is spontaneous. Updrafts
accelerate until temperature within the cloud cools below the ambient temperature. This condition occurs where the unstable layer is capped by a stable layer often marked by a temperature inversion
cumuliform clouds
Vertical heights range from
the shallow fair weather cumulus to the giant thunderstorm cumulonimbus - the ultimate in atmospheric instability capped by the tropopause.
When unstable air lies above stable air
- convective currents aloft sometimes form middle and high level cumuliform clouds.
- In relatively shallow layers they occur as altocumulus and ice crystal cirrocumulus clouds.
- Altocumulus castellanus clouds develop in deeper mid-level unstable layers.
Merging Stratiform and Cumuliform
A layer of stratiform clouds may sometimes form in a mildly stable layer while a few ambitious convective clouds penetrate the layer thus merging stratiform with cumuliform. Convective clouds may be almost or entirely embedded in a massive stratiform layer and pose an unseen threat to instrument flight.
