Ch 5 – Vertical Motion & Stability

Ch 5 – Vertical Motion & Ch 5 – Vertical Motion & StabilityStability


• IntroductionIntroduction– In the previous chapter, we concentrated In the previous chapter, we concentrated

on the causes and characteristics of the on the causes and characteristics of the wind; that is, the horizontal part of three-wind; that is, the horizontal part of three-dimensional atmospheric motions.dimensional atmospheric motions.

– In this chapter, we examine vertical In this chapter, we examine vertical atmospheric motions.atmospheric motions.

– Although vertical motions are often so Although vertical motions are often so small that they are not felt by the pilot, small that they are not felt by the pilot, they are still important in aviation they are still important in aviation weather (Lester, 2006).weather (Lester, 2006).


• IntroductionIntroduction– Very slow upward motions play a key Very slow upward motions play a key

role in the production of clouds and role in the production of clouds and precipitation, and therefore, in the precipitation, and therefore, in the creation of flight hazards, such as poor creation of flight hazards, such as poor visibilities, low ceilings, and icing.visibilities, low ceilings, and icing.

– Gentle downward motions dissipate Gentle downward motions dissipate clouds and contribute to fair weather.clouds and contribute to fair weather.

– Also, the atmosphere is not limited to Also, the atmosphere is not limited to weak vertical movements (Lester, weak vertical movements (Lester, 2006).2006).


• IntroductionIntroduction– Occasionally, turbulent upward and downward Occasionally, turbulent upward and downward

motions are large enough to cause injury, motions are large enough to cause injury, damage, and loss of aircraft control.damage, and loss of aircraft control.

– Clearly, understanding the nature of vertical Clearly, understanding the nature of vertical motions is a useful addition to your aviation motions is a useful addition to your aviation weather knowledge.weather knowledge.

– When you complete this chapter, you will When you complete this chapter, you will understand not only how vertical motions are understand not only how vertical motions are produced, but also what the important effects of produced, but also what the important effects of atmospheric stability are on those motions atmospheric stability are on those motions (Lester, 2006).(Lester, 2006).


• Section A – Vertical MotionsSection A – Vertical Motions

• Section B – StabilitySection B – Stability

• Section C – Stability and Vertical Section C – Stability and Vertical MotionsMotions


• Section A: Vertical Motions – Section A: Vertical Motions – when an air when an air parcel moves from one location to another, it parcel moves from one location to another, it typically has a horizontal component (wind) and a typically has a horizontal component (wind) and a vertical component (vertical motion)vertical component (vertical motion)– Causes – Causes – air may move upward or downward air may move upward or downward

for a number of reasonsfor a number of reasons

•the most frequent causes are convergence the most frequent causes are convergence and divergence, orography, fronts and and divergence, orography, fronts and convectionconvection


• Convergence / DivergenceConvergence / Divergence– Convergence – Convergence – corresponds to a net inflow of air corresponds to a net inflow of air

into a given areainto a given area

• it may occur when wind speed slows down in the it may occur when wind speed slows down in the direction of flow and/or when opposing direction of flow and/or when opposing airstreams meet; figure 5-2airstreams meet; figure 5-2

– Divergence – Divergence – the net outflow from a given areathe net outflow from a given area

•winds may diverge when the wind speed winds may diverge when the wind speed increases in the direction of the flow and/or increases in the direction of the flow and/or when an air stream spreads out in the when an air stream spreads out in the downstream direction; figure 5-2downstream direction; figure 5-2


• ***Embedded thunderstorms are obscured ***Embedded thunderstorms are obscured by massive cloud layers and cannot be seenby massive cloud layers and cannot be seen


• Orography – Orography – air can be forced upward or air can be forced upward or downward when it encounters a barrierdownward when it encounters a barrier– a simple example is orographic liftinga simple example is orographic lifting

•when wind intersects a mountain or hill, it is when wind intersects a mountain or hill, it is simply pushed upwardsimply pushed upward

•on the down-wind or lee side of the on the down-wind or lee side of the mountain, air moves downwardmountain, air moves downward


• Fronts – Fronts – when the atmosphere itself creates an when the atmosphere itself creates an obstacle to the wind, a barrier effect similar to a obstacle to the wind, a barrier effect similar to a mountain can be producedmountain can be produced– when a cold air mass is next to a warm air when a cold air mass is next to a warm air

mass, a narrow, sloping boundary is created mass, a narrow, sloping boundary is created between the two called a frontbetween the two called a front

• Frontal lifting – Frontal lifting – if either air mass moves toward if either air mass moves toward the other, the warm air moves upward over the the other, the warm air moves upward over the cold, dense air mass in a process called frontal cold, dense air mass in a process called frontal lifting or in some special cases overrunninglifting or in some special cases overrunning


• ConvectionConvection– Convective lifting – Convective lifting – as bubbles of warm air as bubbles of warm air

rise in the convective lifting process, the rise in the convective lifting process, the surrounding air sinks; figure 5-6 and occurs surrounding air sinks; figure 5-6 and occurs under unstable atmospheric conditionsunder unstable atmospheric conditions


• Mechanical Turbulence – Mechanical Turbulence – Figure 5-7; chaotic Figure 5-7; chaotic eddies are swept along with the wind, producing eddies are swept along with the wind, producing downward motions on their downwind side and downward motions on their downwind side and upward motions on their upwind sideupward motions on their upwind side– rough air experienced when landing on windy rough air experienced when landing on windy

days is caused by these small scale days is caused by these small scale circulationscirculations


• Gravity Wave Motions – Gravity Wave Motions – under certain under certain circumstances, air may be disturbed by small scale circumstances, air may be disturbed by small scale wave motionswave motions– that is, parcels of air may be caused to oscillate that is, parcels of air may be caused to oscillate

vertically; figure 5-8vertically; figure 5-8– such oscillations that move away from the such oscillations that move away from the

source of the disturbance are called atmospheric source of the disturbance are called atmospheric gravity waves because the earth’s gravity plays gravity waves because the earth’s gravity plays an important role in producing theman important role in producing them

– a mountain wave is one type of gravity wavea mountain wave is one type of gravity wave

Ch 5 – Vertical Motion & Ch 5 – Vertical Motion & StabilityStability• Section B: Stability – Section B: Stability – a stable system may be a stable system may be

defined as one that, if displaced or distorted, defined as one that, if displaced or distorted, tends to return to its original location and/or tends to return to its original location and/or configurationconfiguration– an unstable system is one that tends to move an unstable system is one that tends to move

away from its original position, once it has away from its original position, once it has been displaced or distortedbeen displaced or distorted

– a system with neutral stability remains in its a system with neutral stability remains in its new position if displaced or distorted; figure 5-new position if displaced or distorted; figure 5-99


• Atmospheric Stability – Atmospheric Stability – a condition that makes a condition that makes it difficult for air parcels to move upward or it difficult for air parcels to move upward or downwarddownward– atmospheric instability is a condition that atmospheric instability is a condition that

promotes vertical motionspromotes vertical motions


– Buoyancy – Buoyancy – the property of an object that the property of an object that allows it to float on the surface of a liquid, or allows it to float on the surface of a liquid, or ascend through and remain freely suspend in a ascend through and remain freely suspend in a compressible fluid such as the atmospherecompressible fluid such as the atmosphere


– Archimedes’ Principle – Archimedes’ Principle – when an object is when an object is placed in a fluid (liquid or gas), it will be placed in a fluid (liquid or gas), it will be subjected to a positive (upward) or negative subjected to a positive (upward) or negative (downward) force depending on whether the (downward) force depending on whether the object weighs more or less than the fluid it object weighs more or less than the fluid it displacesdisplaces•can be thought of as the bowling ball / balsa can be thought of as the bowling ball / balsa

wood-in-the-bucket-of-water concept; figure wood-in-the-bucket-of-water concept; figure 5-115-11


– Positively buoyant – Positively buoyant – if a parcel of air is if a parcel of air is displaced upward and becomes warmer than displaced upward and becomes warmer than its surroundings, it is positively buoyantits surroundings, it is positively buoyant

• it will accelerate upward (away from its it will accelerate upward (away from its original position); it is unstableoriginal position); it is unstable

– Negatively buoyant – Negatively buoyant – if a parcel of air is if a parcel of air is displaced upward and is colder than its displaced upward and is colder than its surroundings, it is negatively buoyantsurroundings, it is negatively buoyant

• it will be accelerated downward (back to its it will be accelerated downward (back to its original position); it is stable original position); it is stable


• Determining Atmospheric Stability – Determining Atmospheric Stability – there there are three basic concepts that help determine are three basic concepts that help determine stabilitystability– the dry adiabatic process, atmospheric the dry adiabatic process, atmospheric

soundings and lapse ratessoundings and lapse rates

•Dry adiabatic process – Dry adiabatic process – cooling by cooling by expansion and warming by compressionexpansion and warming by compression


• Adiabatic cooling – Adiabatic cooling – pressure always decreases pressure always decreases with heightwith height– adiabatic cooling will always accompany adiabatic cooling will always accompany

upward motionupward motion

• Adiabatic heating – Adiabatic heating – adiabatic heating will adiabatic heating will always accompany downward motionalways accompany downward motion– the rate of temperature change associated the rate of temperature change associated

with a dry adiabatic process is a constant: 3 with a dry adiabatic process is a constant: 3 degrees Celsius per 1,000 feet (5.4 degrees degrees Celsius per 1,000 feet (5.4 degrees Fahrenheit per 1,000 feet)Fahrenheit per 1,000 feet)


• ***Understand (cloud-free) air flowing ***Understand (cloud-free) air flowing upslope will cool at the rate of upslope will cool at the rate of approximately 3 degrees Celsius per 1,000 approximately 3 degrees Celsius per 1,000 feetfeet


• Soundings – Soundings – a measurement of meteorological a measurement of meteorological conditions between the ground and some higher conditions between the ground and some higher level in the atmospherelevel in the atmosphere

• Radiosondes – Radiosondes – the most common the most common meteorological soundings are made via freely meteorological soundings are made via freely rising, unmanned, instrumented balloons called rising, unmanned, instrumented balloons called radiosondes or rawinsondesradiosondes or rawinsondes


• Lapse Rates – Lapse Rates – an important stability an important stability measurement that can be determined from a measurement that can be determined from a soundingsounding

•the change of temperature with altitude for the change of temperature with altitude for a given atmospheric layera given atmospheric layer

– Lapse rate (LR) = T (bottom) – T (Top) / DELZLapse rate (LR) = T (bottom) – T (Top) / DELZ

•T (bottom) = temperature at the bottom of T (bottom) = temperature at the bottom of the layerthe layer

•T (top) = temperature at the top of the layerT (top) = temperature at the top of the layer

•DELZ = thickness of the layerDELZ = thickness of the layer


• Dry adiabatic lapse rate (DALR) – Dry adiabatic lapse rate (DALR) – the rate at the rate at which the temperature of a dry parcel of air which the temperature of a dry parcel of air decreases as it ascends is also a useful reference decreases as it ascends is also a useful reference in stability determinationsin stability determinations– equal to 3 degrees C per 1,000 feet; figure 5-equal to 3 degrees C per 1,000 feet; figure 5-

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• Isothermal layer – Isothermal layer – no change in temperature no change in temperature with height (LR = 0)with height (LR = 0)

• Inversion layers – Inversion layers – temperature increases with temperature increases with height (LR < 0)height (LR < 0)

• Surface-based inversions – Surface-based inversions – often form at night often form at night and may be the source of wind shear problemsand may be the source of wind shear problems

Ch 5 – Vertical Motion & Ch 5 – Vertical Motion & StabilityStability• Stability EvaluationStability Evaluation

•Stability criteria; figure 5-15; figure 5-16; Stability criteria; figure 5-15; figure 5-16; figure 5-17figure 5-17

– select the layer in the sounding in which you are select the layer in the sounding in which you are interestedinterested

– within the layer, compare the actual LR and DALRwithin the layer, compare the actual LR and DALR– determine which of the following stability criteria are determine which of the following stability criteria are

satisfiedsatisfied

•LR > DALR – absolutely unstableLR > DALR – absolutely unstable

•LR = DALR – neutralLR = DALR – neutral

•LR < DALR – stableLR < DALR – stable

Ch 5 – Vertical Motion & Ch 5 – Vertical Motion & StabilityStability• Section C: Stability and Vertical MotionsSection C: Stability and Vertical Motions

– ***A stable air mass is more likely to have ***A stable air mass is more likely to have smoother air than an unstable air masssmoother air than an unstable air mass

– ***The formation of either predominantly ***The formation of either predominantly stratiform or predominately cumuliform stratiform or predominately cumuliform clouds depends upon the stability of the clouds depends upon the stability of the air being liftedair being lifted


– ***Conditions favorable for the formation ***Conditions favorable for the formation of a surface-based temperature inversion of a surface-based temperature inversion are clear, cool nights with calm or light are clear, cool nights with calm or light windswinds

– ***The stability of an air mass is ***The stability of an air mass is

decreased by heating it from belowdecreased by heating it from below

SummarySummary

• Vertical motions in the atmosphere are Vertical motions in the atmosphere are critical for aviation because of their role in critical for aviation because of their role in the production of turbulence, clouds, and the production of turbulence, clouds, and associated phenomena.associated phenomena.

• You have learned that upward and You have learned that upward and downward motions are forced by fronts, downward motions are forced by fronts, mountains, warm surfaces, and mountains, warm surfaces, and converging and diverging airstreams converging and diverging airstreams (Lester, 2006).(Lester, 2006).

SummarySummary

• Additionally, the resulting vertical motions Additionally, the resulting vertical motions are magnified or suppressed, depending are magnified or suppressed, depending on the atmospheric stability.on the atmospheric stability.

• The understanding of stability has required The understanding of stability has required you to study and understand the concepts you to study and understand the concepts of buoyancy and the adiabatic process.of buoyancy and the adiabatic process.

• With these tools, you have learned how With these tools, you have learned how atmospheric stability is evaluated by atmospheric stability is evaluated by examining atmospheric temperature examining atmospheric temperature soundings (Lester, 2006).soundings (Lester, 2006).


• IntroductionIntroduction– The information in this chapter is basic The information in this chapter is basic

to later discussions of a wide variety of to later discussions of a wide variety of topics ranging from clouds and weather topics ranging from clouds and weather of large-scale cyclones, to of large-scale cyclones, to thunderstorms, to small-scale clear air thunderstorms, to small-scale clear air turbulence (Lester, 2006).turbulence (Lester, 2006).

Ch 5 – Vertical Motion & Stability

Documents

vertical atmospheric

vertical motionsch

vertical motion stabilitych

nature of vertical motions

air stream

air parcel

air mass moves

warm air mass