1 / 22

# stability and cloud development - PowerPoint PPT Presentation

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????

Related searches for stability and cloud development

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'stability and cloud development' - Mia_John

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### 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????

• 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.....

• 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.

• 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?

• 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?

• 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?

• 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:

• 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

• 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

• 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

• 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.

• 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.

• 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

• 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.

• 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

QUESTION

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?

• 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.

Unequal heating of surfaces causes parcels to rise.

Windward side

Leeward side

LCL – lifting condensation level