Lecture #10: 
Atmospheric Stability (Continued) and its Applications;
Thunderstorm Development
Friday, 9 February 2001

Text Reading for Lecture #9
Atmospheric Stability (Read pages 160-175)
Cloud Development (Read pages 167-173)

Weather last night

OU Forecast model (coarse resolution)

OU Forecast model (finer resolution)

Review from last time:

  Stable Atmosphere -- Parcel is unsaturated
and always colder than the environment.

How does the situation change if the parcel is
saturated?

Stable Atmosphere - Parcel is saturated and
always colder than the environment.

Let's now consider the opposite extreme, that is,
the rising parcel is everywhere WARMER than
the environment.  Such an environment is said
to be ABSOLUTELY UNSTABLE, because
whether saturated or not, the rising parcel always
will be warmer than its environment.

  Unstable Atmosphere -- Parcel is unsaturated
and always warmer than the environment.

  Unstable Atmosphere -- Parcel is saturated
and always warmer than the environment.

In most cases, especially during the thunderstorm
season, the atmosphere is CONDITIONALLY
UNSTABLE, the condition being whether the
rising air is saturated or unsaturated. 

OKC Sounding at 10 pm last night - instability?

Sample Stability Problem

   T (deg C)       Height (km)
   ------------        ----------------

       34                        0
       31                        1
       28                        2
       21                        3
       10                        4
        2                         5
      -15                        6
      -15                        7
      -10                        8
        -6                        9
        -6                      10

  Plotted "sounding data" from above

  Dry adiabatic lapse rate;  parcel
starting temperature same as environment.

Dry adiabatic lapse rate;  parcel
starting temperature warmer than environment.

Dry adiabatic lapse rate;  parcel
starting temperature much warmer than
environment.

  Parcel now becomes saturated and,
thereafter, cools at the moist adiabatic lapse rate.

Note cloud base (condensation level)
and cloud top (region where parcel is no longer
warmer than the environment).  This is how
forecasters determine the anticipated maximum
vertical extent of thunderstorms.

The difference in temperature between
the rising parcel and the environment, added for each
level where the parcel is WARMER than the
enviornment, produces an area that is proportional
to the amount of energy available to the thunderstorm
updraft.  This is called the Convective Available
Potential Energy (CAPE), and the bigger the area,
the stronger the updraft. 

  A real thermodynamic diagram, shown
here, is used by forecasters to assess storm potential.
It is basically the same as the schematic shown above.

CLOUD AND THUNDERSTORM
DEVELOPMENT

Mechanisms of cloud formation:

  Note how the clouds form from discrete
"parcels" or thermals of air rising.  This is why cumulus
clouds grow in "spurts".

For an atlas of clouds, click HERE.

Orographic uplift and rain shadowing.

Consider the early stages of thunderstorm development
and how the environment changes with time: