March 12, 2008

Atmospheric Stability (Continued)

n We are going to extend the tabular method for lifting air parcels to include a determination of atmospheric stability. The bottom line is this … if a lifted air parcel ever becomes warmer than the air surrounding the parcel, the atmosphere is said to be unstable for lifted parcels. If a lifted parcel does not become warmer than the air surrounding the parcel, the atmosphere is said to be stable for lifted parcels.

o Before going through a numerical example, we will briefly explain the physics behind the italicized statements above.

§ Recall that air parcels always adjust their sizes so that the air pressure inside the parcel equals the air pressure outside the parcel.

§ Recall the gas law that was introduced in the 1/30/08 lecture

Pressure = (Temperature) x (Number Density) x (Constant of proportionality)

or we can say that Pressure is proportional to (Temperature) x (Number Density)

§ Since the air pressure inside the parcel is the same as the outside pressure

{(temperature) x (number density)}_{inside parcel} = {(temperature) x (number density)}_{inside parcel}

§ If the temperature inside the parcel is greater than the temperature outside the parcel, then the number density inside the parcel must be smaller than the number density outside the parcel. This means that a parcel of warm air surrounded by cooler air, is less dense than the surrounding air, and will rise upward.

§ Basically, warm air rises!

§ This works the same way in any fluid. For an object to float or rise upward in a fluid, the object must be less dense than the surrounding fluid. I think you all have an idea about what does and does not float in water. It is basically the same principle to decide whether or not an air parcel will move up or down.

o Demonstration: Try to float a can of Pepsi and a can of diet Pepsi (easy to do). Try it in class.

o Demonstration: Helium filled balloons, cooled by liquid nitrogen (too much preparation). We can talk about doing this, but I am not going to do this in class.

n Numerical Example for stability. The environmental temperature is the temperature of the air surrounding the air parcel. This will always be given to you. In practice this information is know based on data collected by radiosondes. Again black print indicates what you will be given and red indicates what you are expected to fill in. The stability column will not be provided on quizzes. It is up to you to figure that out.

Elevation (meters)	Environmental Temperature (°C)	Stability?	Parcel Temperature (°C)	Parcel Dew Point Temperature (°C)	Comments
7000	-19	Stable	-20	-20	↑
6000	-15	Unstable	-14	-14	↑
5000	-10	Unstable	-8	-8	↑
4000	-4	Unstable	-2	-2	↑
3000	5	Stable	4	4	Latent heat release; condensation occurring
2000	14	Stable	10	10	Saturated; level where cloud begins to form
1000	22	Stable	20	10	Unsaturated
0	30	Neutral	30	10	Unsaturated

n Sketch a picture of what the cumulonimbus (thunderstorm) cloud would look like under these conditions. Point out the anvil and why it forms.

n Show video clip of thunderstorms developing over the Catalinas.

Some notes on Predicting Violent Storms

Meteorologists use a similar method to assess the potential for thunderstorms and severe weather:

n Lift parcels upward to see if they become unstable. This must be done for the current measured atmospheric conditions as well as for forecasted conditions later in the day.

n The more unstable the atmosphere, the greater the potential for violent storms.

o The depth of the unstable layer is important.

o The greater the positive difference between the parcel temperature and the surrounding air temperature, the more unstable the atmosphere.

o

n Question to answer. If the atmosphere is unstable, will parcels be lifted high enough (by one of the four mechanisms listed on the previous page) to reach the unstable layer? This must occur to “release the instability” if storms are to occur.

n Another way to state this … The instability provides the potential to form thunderstorms. The degree of instability can be assessed by lifting hypothetical parcels upward. In actuality, one can determine the potential energy available for forming thunderstorms. But storms will only form if air parcels are lifted up to the altitude where they become unstable.

Thunderstorms (Chapter 10)

n Thunderstorms form when moist, unstable air is lifted vertically into the atmosphere. The four basic mechanisms that force air to rise vertically upward were discussed in the last lecture. Any one of these can initiate thunderstorm development.

n Immediately after lifting begins, a rising parcel of warm moist air begins to cool. At a certain elevation the dew point is reached resulting in condensation and the formation of a cumulus cloud. During condensation, large quantities of latent heat are released. Much of the energy used in thunderstorm development comes from the release of latent heat. It is the heat released by condensation within a cloud that permits the rising air to stay warmer than its surroundings, and thus to be buoyant through great depths.

n Cumulonimbus clouds can reach heights of 20 kilometers above the Earth's surface. Severe weather associated with thunderstorms includes hail, strong winds, lightning, intense rain, flash flooding, and tornadoes.

n Thunderstorm development requires an unstable atmosphere. The more unstable the atmosphere, the potentially more violent the storm can become. You should understand how each of these 3 changes act to make the atmosphere more unstable:

1. Warm the air in the lower troposphere (jut above the surface). Rising parcels which start out warmer have a better chance of becoming warmer than the surrounding environment as they are lifted.

2. Cool the air aloft (well above the surface in the middle troposphere). Colder environmental conditions above the surface make it more likely that lifted surface parcels will be warmer than the surrounding environment.

3. Add more water vapor to the air in the lower troposphere (just above the surface). More water vapor in a rising parcel means that more latent heat will be released during cloud formation and increases the likelihood that the parcel will become warmer than the surrounding environment.

n Overview of different types of thunderstorms. We will use an in-class handout.

n Go over the life cycle of an ordinary, single cell thunderstorm using an in-class handout and figure 10.1 in the textbook.