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Stability and Cloud Development - PowerPoint PPT Presentation

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Stability and Cloud Development. Chapter 7. Cloud Development Introduction to atmospheric instability. Q: How and why do clouds form on some days and not on others????

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Cloud Development Introduction to atmospheric instability

  • Q: How and why do clouds form on some days and not on others????

  • Q: Why does the atmosphere sometimes produce stratus clouds (thin layered) while other times we get cumulus, or cumulonimbus clouds to form??

  • The answer is largely related to the concept of atmospheric stability.....

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Cloud Development - stable environment

  • Consider this simple situation of a marble in the bottom of a bowl

  • If you push the marble up the side of the bowl, it will fall back down to the bottom, to it's original position

  • Stable air (parcel)- vertical motion is inhibited

  • If clouds form, they will be shallow, layered clouds like stratus.

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Cloud Development - unstable environment

  • If the marble is on the top of the bowl and you give it a little push, it rolls off the bowl.... does NOT come back to it's original position.

  • This is an unstable situation.

  • Unstable air (parcel)- vertical motion occurs.

  • This commonly produces cumulus or cumulonimbus clouds.

  • So, the question becomes, how does one determine the stability of the atmosphere?

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Assessing Atmospheric Stability

  • To determine whether or not a parcel will rise or sink in the atmosphere, one must compare the parcels temperature (Tp) with that of the environment (Te) at some altitude:

  • If Tp > Te what will the parcel do?

  • If Tp = Te what will the parcel do?

  • If Tp < Te what will the parcel do?

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Assessing Atmospheric Stability 

  • The bottom line - compare an air parcels temperature (Tp) with the environmental temperature (Te) at a given altitude

  • if Tp > Te, parcel rises

  • if Tp = Te parcel does not move up or down

  • if Tp < Te parcel sinks

So, to assess stability,

what two pieces of information do we need? 

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Assessing Atmospheric Stability

  • The vertical temperature profile of the environment and the temperature of the parcel of air

  • Vertical profiles of atmospheric temperature are collected at 12, 00 UTC every day at select NWS offices by launching balloon soundings: 

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Rising air parcels

  • Consider a rising parcel of air, as the parcel rises, it will adiabatically expand and cool

  • Adiabatic - a process where the parcel temperature changes due to an expansion or compression, no heat is added or taken away from the parcel

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Adiabatic cooling

  • Since it takes energy for the parcel molecules to "push out" on the parcel walls, they use up some of their internal energy in the process.

  • The parcel expands since the lower pressure outside allows the air molecules to push out on the parcel walls

  • Therefore, the parcel also coolssince temperature is proportional to molecular internal energy

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Sinking air parcels

  • As the parcel sinks, it will adiabatically compress and warm

  • Adiabatic - a process where the parcel temperature changes due to an expansion or compression, no heat is added or taken away from the parcel

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Adiabatic Warming

  • The parcel compresses since it is moving into a region of higher pressure

  • Due to the parcel compression, the air molecules gain internal energy.

  • Hence, the mean temperature of the parcel increases.

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Dry adiabatic lapse rate

  • As a parcel of air rises, it cools, but at what rate???

  • Rate of temperature change with height is called the lapse rate.

  • Units of lapse rate are °C km-1.

  • Let's first consider an unsaturatedparcel of air

  • Unsaturated parcels cool at a rate of 10°C km-1 - this is called the dry-adiabatic lapse rate.

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Dry adiabatic lapse rate

  • What will be the parcel's temperature be at 1 km?

  • What will be the parcel's temperature be at 2 km?

20 0 C

30 0 C

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Question for Thought

  • Suppose the air pressure outside a conventional jet airliner flying at an altitude of 10 km is 250 mb.

  • Further, suppose the air inside the aircraft is pressurized to 1000mb.

  • If the outside air temperature is -50 ºC, what would be the temperature of this air if brought inside the aircraft and compressed at the dry adiabatic rate to a pressure of 1000mb?

  • Assume that a pressure of 1000mb is equivalent to an altitude of 0 m.

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Moist Adiabatic Lapse Rate

  • For a saturated parcel of air, that is when it's T=Td, then it cools at the moist adiabatic lapse rate = 6°C km-1

  • What will be the parcel's temperature be at 3 km?

  • What will be the parcel's temperature be at 4 km?

14 0 C

8 0 C

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Moist Adiabatic Lapse Rate


Why does the parcel cool

at a slower rate,

6 °C km-1

when it is saturated than

at10°C km-1

when it is unsaturated?

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Dry versus Moist-Adiabatic Process

  • The moist adiabatic lapse rate is less than the dry adiabatic lapse rate because as vapor condenses into water (or water freezes into ice) for a saturated parcel, latent heat is released into the parcel, counteracting the adiabatic cooling.

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Cloud Development – Localized Convection

Unequal heating of surfaces causes parcels to rise.

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Cloud Development – Orographic Lifting

Windward side

Leeward side

LCL – lifting condensation level

Rain Shadow

Air is forced to rise over mountains.

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Cloud Development – Convergence

Whenever air flows together, lifting results

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Cloud Development – Frontal Wedging

Warmer, less dense air, is forced over cooler, denser air.