Vertical Motion and Cloud Formation & Atmospheric Stability 11&12

Chapter 12

Vertical motion and cloud formation

The Adiabatic Process

The adiabatic process defined:
When a parcel of air expands and cools or compresses and warms with no interchange of heat from its surroundings
When a parcel of air is moved aloft, pressure decreases, this allows the parcel to expand
The expansion of the parcel means the molecules travel farther within this parcel, energy is used by the molecules and their speed decreases
We already know that temperature and molecule speed are directly related, so with a relative speed decrease comes a decrease in temp
It is by this process clouds form when the temp cools to the dew point
If the air descends, the process is reversed like in the Chinook wind

The Adiabatic Process

As long as the parcel does not lose any of its moisture, the process is reversible
Known as the Reversible Adiabatic Process
However if rain or snow falls from the cloud and leaves the parcel, when the parcel sinks it cannot recover this loss
Net evaporation is less than net condensation in this circumstance
This is known as the Irreversible Pseudoadiabtic Process

Lapse Rates

As long as the parcel of air is unsaturated the rate of adiabatic cooling or warming is constant
Unsaturated means less than 100% relative humidity
This is referred to as the dry adiabatic lapse rate
3°C /1000 feet or 5.5°F/1000 feet
The average lapse rate is
2°C/1000 feet
The moist adiabatic lapse rate
1.2°C/1000 for warm parcels, lots of latent heat released
to 3°C/1000 for colder ones, not so much latent heat
Superadiabatic lapse rate
Greater than 3°C/1000 but less than 8°C/1000
Autoconvective lapse rate
Greater than 8°C/1000
Dew point also changes with a change in altitude but at a much lower rate
.5°C/1000 feet
The graph shows an actual sounding measuring the temperature and dew point

Lapse Rate and Moisture

DETERMINING BASES

Temp and dew point converge at a rate of about 4.4º F or 2. 2º C per 1000 feet
Given: Temp 14º C Dew Point 3º C
T-Dp/Convergence Rate
14-3=11
11/2.2= 5000

Unsaturated Air Example

Saturated Air Example

Descending Air Example (Irreversible Pseudoadiabatic)

At 5,000 feet, both the temperature and dewpoint of the air parcel are 6 °C, which indicates that it is saturated.
As the parcel descends, it quickly becomes unsaturated.
Its temperature increases 3 °C per 1,000 feet, while its dewpoint increases at 0.5 °C per 1,000 feet.
The temperature-dewpoint spread increases while relative humidity decreases until the parcel reaches the surface.
Note that the parcel is now much warmer and drier at the surface then when it began the vertical motion process in Figure 11-2.

Vertical Lifting

The book lists 4 types of lifting:
Orographic effects
Frictional effects
Frontal lift
Buoyancy

Orographic Lifting

Orographic lifting occurs when an air mass is forced from a low elevation to a higher elevation as it moves over rising terrain.
As the air mass gains altitude it quickly cools down adiabatically, which can raise the relative humidity to 100% and create clouds and under the right conditions, precipitation.
Note how the temp is cooler on the windward side than the leeward side

Orographic lifting

Any time terrain causes a lifting of the air there is usually a rain shadow created on the leeward side
The most extreme example of this is the Atacama Desert in Chile
It is in fact rain shadowed from the Andes to the east as well as the Chilean Coast range to the west
Some weather stations have never recorded rainfall

Frictional Effects

Frictional effects refers to the process of descending air associated with a high pressure and ascending air associated with a low pressure

Frontal Lifting

This is also known as Frontal Wedging
This may take place along both a cold front or a warm front
The worst weather is often associated with a fast moving cold front because of this phenomenon

Buoyancy

Usually air becomes buoyant when cooler air moves over warmer ground
We covered examples of this when we talked about land and sea breezes, mountain and valley winds and lake effect
Thermals may drive this process as well and results in fair weather cumulus cloud formation as well as Air Mass Thunderstorms

Cloud Forms (The Core Four)

Cirriform
Nimbus
Cumuliform
Startiform

The Height of Clouds

FAA Version
Note the high clouds in the temperate regions only go to 40,000 on their chart, memorize the temperate regions

CLOUD CLASSIFICATIONS
High clouds – cirrus, cirrocumulus, cirrostratus
Middle clouds – altocumulus, altocumulus castellanus, altostratus nimbostratus
Low clouds – stratus, stratocumulus, cumulus
Extensive Vertical Dev. – cumulus, cumulonimbus, cumulo congestus

The Ten Basic Clouds
There are 10 modifications or combinations to the four core clouds
Often there are features of two or more categories
We can divide these by height starting with the highest first
High Level Clouds
Cirrus (Ci)
Cirrocumulus (Cc)
Cirrostratus (Cs)
Mid Level Clouds
Altocumulus (Ac)
Altostratus (As)
Nimbostratus (Ns)
Low Level Clouds
Cumulus (Cu
Cumulonimbus (Cb)
Stratocumulus (Sc)
Stratus (St)

CLOUD CLASSIFICATIONS
nimbo or nimbus = rain
2 classifications of rain clouds:
stratus and cumulus
Nimbostratus and cumulonimbus
In order to produce significant precipitation, clouds usually need to be 4,000 feet thick or more
http://www.weather.gov/jetstream/cloudchart.html

CIRRUS
CIRROCUMULUS
CIRROSTRATUS
ALTOCUMULUS
ALTOCUMULUS LENTICULAR
STACKED LENTICULAR
ALTOCUMULUS MACKERAL
ALTOCUMULUS FLOCCUS
ALTOCUMULUS UNDULATUS
KELVIN-HELMHOLTZ BILLOWS
ALTOCUMULUS CASTELLANUS
ALTOSTRATUS
CUMULUS
CUMULUS HUMILIS
CUMULUS MEDIOCRIS
TOWERING CUMULUS
CUMULONIMBUS
CUMULONIMBUS MAMATUS
CUMULUS CONGESTUS
CUMULUS CALVUS
CUMULUS PILEUS
CUMULONIMBUS INCUS
STRATOCUMULUS
STRATUS
CUMULUS FRACTUS
STRATUS FRACTUS OR PANNUS
NIMBOSTRATUS
ROLL CLOUD
SHELF CLOUD
Occurs around a T storm
WALL CLOUD
Occurs around T Storms
CUMULONIMBUS CAPILLATUS INCUS

FALL STREAK OR HOLE PUNCH CLOUD
Asperatus Clouds
NOCTILUCENT CLOUDS
Much is not know about these
Thought to be ice crystals in the thermosphere
Possibly caused by rocket engine exhaust which is 95% water
Look for the long blue streaks at sunset
NACREOUS CLOUDS
McMurdo station Antarctica
High polar stratospheric clouds
Composed of water, nitric acid and sulfuric acid

CHAPTER 13

Atmospheric Stability

Atmospheric Stability

Atmospheric stability is the property of the ambient air that either enhances or suppresses vertical motion of air parcels
This concept is very similar to aircraft stability

Adiabatic Change (Review)

Heat is neither added or taken from an outside source
Heating and cooling is caused by compression and expansion
Adiabatic cooling – rising air cools because of expansion
Adiabatic heating – descending air warms because of compression

Stable vs Unstable

A stable atmosphere resists any upward or downward displacement
An unstable atmosphere allows upward and downward movement
Environmental lapse rate is the lapse rate of the surrounding air
The environmental lapse rate compared to the lapse rate of the air that is rising tells us if the atmosphere is stable or unstable
Whether the air is saturated or unsaturated dictates the lapse rate

Saturated Air

Has a lower adiabatic lapse rate
Lift a parcel of air
The parcel cools to the dew point
Moisture condenses
Latent heat from vapor to water releases heat and lowers the lapse rate
This causes the parcel to become buoyant and further drives the process

Unsaturated Air

Has a higher adiabatic lapse rate
Parcel of air is lifted mechanically
The parcel will cool adiabatically because of expansion but at a faster rate because of the absence of latent heat
This parcel of air will be colder than the surrounding air and settle downward resisting vertical movement

STABILITY AND INSTABILITY

If the parcel is warmer than the ambient air it will continue to rise – thus unstable
if the parcel is the same temp as the ambient air it is neutrally stable
if the parcel is cooler than the ambient air it will sink – thus stable
An inversion will produce a condition known as absolute stability
This happens when the environmental lapse rate is less than the moist adiabatic lapse rate

Stability Types

The book lists 5 types of atmospheric stability but there are only 4:
Absolute Stability
Neutral Stability
Absolute Instability
Conditional Instability

Absolute Stability

Neutral Stability

Absolute Instability

Conditional Instability

Stability And Instability

Orographic lifting – is an air mass being forced up a mountain slope
Cooling from below increases stability
Heating from below decreases stability
Thermals are a sign of instability
Inversions are an example of stability

Clouds Signposts Of The Sky

Characteristics of stable and unstable air
Stable air
Stratiform clouds
Poor vis
Steady precipitation
Smooth ride
Remember if air is lifted and there is no Latent heat it cools at the same rate or faster and will be stable
Unstable air
Cumuliform clouds
Good vis
Showery precipitation
Turbulence
If air is lifted and there is Latent heat released through condensation, the lapse rate will be less and therefor warmer than surrounding air

Clouds Signposts Of The Sky

Remember as air is lifted because of Latent heat it cools slower
this causes further instability
this continues until:

The temp matches or gets cooler than the ambient temp or
It hits a cap of stable air

Sometimes it will be strong enough to push into the stable air causing embedded T storms

Process That Effect Stability

Wind effects
Stability increases when
Cold air advection into the bottom of a column
Warm air advection into the top of a column
Stability decreases when
Warm air advection into the bottom of a column
Cold air advection into the top of a column

Process That Effect Stability

Vertical air motion
When a column of air descends it shrinks and warms due to adiabatic compression
The upper part shrinks more than the lower part and becomes warmer
The lower part can’t shrink more because it’s on the surface
This can produce an isothermal lapse rate or an inversion and is very stable
This process is known as Subsidence and can effect large chunks of air
The process is reversed if the air ascends

Process That Effect Stability

Diurnal temperature variation
Diurnal refers to the difference between day time and night time temps
Mainly influenced by
Surface type
Latitude
Sky cover
Water vapor content of the air
Wind speed
Temperature variation is maximized
Over land
At low latitudes
With a clear sky
Dry air
Light wind
Conversely, temperature variation is minimized
Over large bodies of water
At high latitudes
With a cloudy sky
Moist air
Strong wind

Lifted Index

Lifted Index is a common measure of atmospheric instability.
Its value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level.
Negative values indicate instability – the more negative, the more unstable the air is, and the stronger the updrafts are likely to be with any developing thunderstorms.
However there are no “magic numbers” or threshold LI values below which severe weather becomes imminent.

The CAPE

Convective Available Potential Energy is a measure of the amount of energy available for convection.
CAPE is directly related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather.
Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.
However, as with other indices or indicators, there are no threshold values above which severe weather becomes imminent.
CAPE is represented on an upper air sounding by the area enclosed between the environmental temperature profile and the path of a rising air parcel, over the layer within which the latter is warmer than the former. (This area often is called positive area.)

Measurements of Stability

Kind of humorous the FAA mentions this because they did away with the Lifted/K index chart a few years ago
However you can still find it here
http://twister.sbs.ohio-state.edu/upper/charts/k-index

The CAPE

http ://wxmaps.org/pix/avnsr.pw.html
Remember these measurements all determine stability based on the lapse rate of the air being lifted to the lapse rate of the surrounding air

The Skew-T

The Skew-T shows
Temperature
Drawn at a 45° angle (hence the name)
Pressure
These are horizontal in millibars
Dry Adiabats
Red curved lines starting at the bottom
Represent the rate unsaturated air cools
This is the dry adiabatic lapse rate
Moist Adiabats
Green curved lines starting at the bottom
Represent the rate saturated air cools
This is the moist adiabatic lapse rate
Mixing Ratio
Mass of water vapor compared to mass of dry air
Dew point
Wind speed/direction

Lifting Condensation Level

The height at which a parcel of air becomes saturated when lifted dry adiabatically.
To obtain the LCL, start with the surface temperature and follow up the dry adiabat until it cross the saturation mixing ratio of the surface dew point.

Convection Condensation Level

The height at which a parcel of air, if heated from below, will rise adiabatically until it is saturated (humidity equals 100%).
This is the level of the flat bases of cumulus clouds.
To obtain the CCL, start with the surface dew point and follow the saturation mixing ration line up until it crosses the temperature curve.

Convection Temperature

The surface temperature at which convective clouds will begin to form from heating of the ground.
To obtain, begin at the convective condensation level (CCL) and follow the dry adiabat down to the surface level and read the temperature at that point.

Level of Free Convection

The level at which a parcel of saturated air becomes warmer than the surrounding air and begins to rise freely.
To obtain, begin at the Lifting Condensation Level (LCL) and follow the moist adiabat up to where it intersects the temperature line.
There are times when no LFC will occur as parcel will always remain cooler than the surrounding atmosphere and therefore will never rise freely.
When this occurs, the atmosphere is absolutely stable.

Usefulness of the Skew-T

Snow

The atmosphere is very moist as indicated by the small amount of separation between the air temperature (red line) and the dew point (blue line). Even though the air temperature increases a few hundred feet above the ground (a temperature inversion) the air temperature, throughout the entire atmosphere, remains below freezing.
So, when precipitation begins, it will be in the form of snow and will remain frozen as snowflakes reaching the ground.

Ice Pellets

As with the previous sounding, the atmosphere is very moist. So much so, the air temperature and dew point are the same from near 900 millibars (3,000 ft. / 1,000 m) to a little above 700 millibars (10,000 ft. / 3,000 m).
At the surface, an arctic cold front had moved south of the observation station with an air temperature well below freezing. The air temperature begins to decrease with height (which is normal) dropping from 23°F to 12°F (-5°C to -11°C).
However, the density of the arctic air is such that it lays close to the ground with its vertical extent fairly small, in this case only about 3,000 feet (1,000 meters) deep. Above 900 millibars (3,000 ft. / 1,000 m) the air becomes considerably warmer. This area is called an inversion, where temperature change with height is ‘inverted’ as it increases with height instead of typically decreasing with height. This inversion is often also referred as a ‘warm nose’.
Eventually, the temperature of the atmosphere will return to the typical decrease with height (near 800 mb) and will continue to cool until it falls to below freezing again (about 720 mb).
While there may be some precipitation forming as rain in the warm ‘nose’ region where the air temperature is above freezing, the vast majority of precipitation will form as snow in the colder below freezing air above the inversion.
As snow falls into the ‘warm nose’, it melts into a liquid drop/rain. Then the liquid drops fall back into the arctic air mass (near the ground) that is cold enough and deep enough for the liquid to freeze into ice pellets before reaching the ground.

Freezing Rain

The basic pattern for freezing rain is similar to ice pellets. The main difference is the cold, dense air near the surface is very shallow and/or the ‘warm nose’ is large or very warm or both. The melted snow does not have sufficient time to freeze before is reaches the ground.
Therefore, precipitation falls as rain but then it freezes upon contact with a surface such as a tree, power line, automobile or bridge. As a general rule, elevated surfaces will ice first because the ground cannot keep them warm, allowing them to cool to the air temperature quickly. This allows the rain to freeze to ice on contact with these surfaces.
Elevated surfaces may be capable of accumulating ice as soon as the air temperature falls below 32°F (0°C). Road surfaces in contact with the ground will generally begin to ice when the air temperature falls to 28°F (-2°C).

Hurricane

Inside hurricanes, the velocity of the air helps keep the air mixed. Therefore, other than the normal decrease with height, variations in temperature (and dew point) are fairly minimal.

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