Atmospheric stability

To make a cloud we need:


Definitions
Adiabatic - A process in which heat is neither added nor subtracted from the system.

Diabatic - A process in which heat is added or subtracted from the system, e.g., solar heating, radiation cooling.
 

For example:
When the bicyclist runs over the nail, the air, having a higher pressure than the outside air, will rush out.  The air does work against the atmosphere as it rushes out from the tire.  In doing this work of displacing the outside air, the air from the tire must use some energy.  That energy comes from the kinetic molecular energy.  The kinetic energy of the molecules from the tire slows and the temperature falls.

No heat has been added or removed from the system yet the expanding air cools.
This process is called Adiabatic Cooling.  Also called Expansional Cooling.

This process is reversible.

If we took a pump to compress the air, as we would if we were filling the tire, then the energy used to compress the air is used to increase the kinetic energy of the molecules.  Compression warms the air.
This process is called Adiabatic Warming.
 

For example, if we raise a parcel of air from ground level to 100 meters in height, the temperature will decrease by 1°C. The parcel cools at a rate of 1°C per 100 m or 10°C per km.

The parcel expanded and did work on its environment!

Now, bring the parcel back down to the surface.  The environment did work on the parcel.

This is an adiabatic process and is reversible.

Example 2:  If we use a moist parcel of air (RH = 100%)

The rising air cooled and produced condensation.  The condensation released latent heat so the rising parcel does not cool as rapidly with height as a dry parcel.

Parcel cools only 0.6°C per 100 m (on average).

Moist adiabatic lapse rate = 0.6°C per 100 m.
Remember -- This is an average lapse rate.  The actual one varies!!!
If the moisture falls out of the parcel as rain, the process is not reversible.

Reversible only if no moisture has been removed!

If we (somehow) lift the parcel: It will cool at the dry adiabatic lapse rate.
The parcel will find itself cooler than the environmental (sounding) temperature.
At the same pressure, a cooler parcel will be more dense than the environment.
Being denser, the parcel will descend back to where it came from.
STABLE!

If we (somehow) lift the parcel:  It will cool at the dry adiabatic lapse rate.
The parcel will find itself warmer than the environmental (sounding) temperature.
At the same pressure, a warmer parcel will be less dense than the environment.
Being less dense, the parcel will ascend and move farther from where it came from.
UNSTABLE!

 

If we (somehow) lift the parcel:  It will cool at the dry adiabatic lapse rate.
The parcel will find itself at the same temperature than the environmental (sounding) temperature.
Being the same density, the parcel will not be accelerated in any direction and will remain where it is.
NEUTRAL STABILITY! -- Dry Neutral, or Conditional Instability


 
 

So ...
We can evaluate the stability of an atmospheric layer by comparing the sounding to the dry and moist adiabats.




Things to realize from these diagrams:


Now all we have to do is get the parcel of air lifted.  We can do that in four ways:

  1. Orographic Lifting
  2. Convergence and Divergence
  3. Surface Boundaries
  4. Convection


    5. Orographic Lifting

      Air is forced upward by topography

      Adiabatic Warming (Leeward Side)

      Adiabatic Cooling
      (Windward Side)

      Therefore it is usually wetter on the windward side than on the leeward side.
       

    Convergence and Divergence
      Convergence at the surface (Low Pressure), air rises
      Divergence Aloft


    Surface Boundaries

      Warm and Cold Fronts
      Outflow Boundaries (Thunderstorms)
      Dry Line


    Convection
     

      Lift by heating surface (diabatic)
      Parcels of warm air rise from the surface and mix with the ambient air.
      Responsible for cumuliform clouds.


Cloud Formation
When we lift the air, where will condensation occur?

Depends on the moisture content of the air that is being lifted.
Moist air requires less cooling, hence less lifting, to reach the dew point.
Drier air requires more cooling and more lift to reach the dew point.
The lifting condensation level (LCL) is the altitude, usually expressed as a pressure, at which the lifted air is cooled dry adiabatically to saturation.

Clouds will form at this level.

As an air parcel rises and cools, the saturation mixing ratio decreases.
The actual mixing ratio does not change.
When the parcel cools to the point when the parcel mixing ratio and the saturation mixing ratio are equal, RH will be 100% and a cloud will form.

If lifting continues, the parcel will rise moist adiabatically (making a cloud).

Clouds are a visible manifestation of condensation or deposition in the atmosphere.

How can chance collisions of water vapor molecules lead to the formation of cloud droplets that will be long-lived?

If more water is added such that the atmosphere is supersaturated (RH ~300 %), then water molecules can form a stable droplet.  This process is called homogeneous nucleation.

We can measure the amount of moisture in the air and find that the cloud droplets form when the air just reaches saturation.  Why?

Recall that dew and frost form on grass or other things.  The water vapor molecules need a "gathering place".

Nearly a century ago it was discovered that the atmosphere contains particles that have an affinity for water.  These serve as centers for condensation.  They are called Cloud Condensation Nuclei (CCN).

With CCN, we need much smaller supersaturations (RH >100%).  In nature we find supersaturations on the order of 1.5%.

The atmosphere has plenty of CCN:

CCN are more plentiful near the surface of the earth.  CCN are more plentiful over land rather than the ocean.

The formation of cloud droplets using CCN is called: heterogeneous nucleation.