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Stability and Cloud Development

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Stability and Cloud Development

Chapter 7

- 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.

Cloud Development – Orographic Lifting

Windward side

Leeward side

LCL – lifting condensation level

Rain Shadow

Air is forced to rise over mountains.

Whenever air flows together, lifting results

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