OVERVIEW:

We will discuss potential temperature; adiabatic cooling and warming; inversions; stability; and convective and mechanical lift.

OUTLINE:

1. Review of the Ideal Gas Law. The density of an airmass may be reduced by adding either heat or water vapor.

2. Potential temperature (theta) -- Temperature that a parcel of air would have if it were brought to 1000 millibars (a typical pressure value for the Earth's surface).

2.1. Parcels of air moving upward (downward) in the atmosphere are cooled (heated) by adiabatic expansion (compression).

2.2. A parcel of air at 500 millibars may have a much colder in situ temperature than a parcel immediately below it at 1000 millibars, but it may also have a higher potential temperature. The way to compare the two directly is to bring the 500 millibar parcel to 1000 millibars dry adiabatically.

2.3. Dry adiabatic cooling/heating rate -- Preserves potential temperature. Usually given in degrees Celsius per meter or degrees Celsius per kilometer. Rate near the Earth's surface is usually taken as 9.8 degrees C per kilometer.

2.4. Moist adiabatic cooling/heating rate -- A parcel of rising air will cool adiabatically and force condensation of its water vapor content. The condensing water vapor releases latent heat and slows the rate of cooling. Thus the moist adiabatic cooling rate is slower than the dry rate.

2.5. Rising parcels cool dry-adiabatically until reaching saturation (temperature equals dew point), then cool moist adiabatically. Descending parcels warm dry-adiabatically.

3.1. What is a Skew-T?

3.1.1. A Skew-T is a thermodynamic diagram or graphical calculator that enables non-digital manipulation of the Ideal Gas Law and other thermodynamic equations. It is used for determining the characteristics of a vertical profile of the troposphere and lower stratosphere.

3.1.2. Isopleths on the Skew-T are:

3.1.2.1. Isobars [mbs] -- Lines of equal atmospheric pressure. Straight, brown horizontal lines.

3.1.2.2. Isotherms [dC] -- Lines of equal temperature. Straight, equidistant brown lines that run diagonally upward from the lower left.

3.1.2.3. Dry Adiabats -- Lines of equal potential temperature. Slightly curved brown lines that intersect the 1000 mb isobar at intervals of 2 dC, and run diagonally upward toward the upper left.

3.1.2.4. Saturation (Moist) Adiabats -- Lines of equivalent potential temperature. Curved green lines, diverging upward from 1000 mbs and tending to become parallel to dry adiabats.

3.1.2.5. Mixing Ratio [g/kg] -- Lines of equal water vapor mixing ratio. Dashed green lines running diagonally upward from the lower left.

3.1.2.6. Skew-T's also have a number of other isopleths, and a height scale (kilometers and kilofeet) on the right side.

3.2. Skew-T's and environmental lapse rates.

3.2.1. The environmental lapse rate is the rate at which the atmosphere's temperature changes with height. A positive lapse rate indicates that the temperature is falling with height. A negative lapse rate indicates that the temperature is rising with height.

3.2.2. U.S. Standard Atmosphere -- Plotted on the Skew-T as a thick brown line. Depicts the average temperature profile of the atmosphere above the CONUS.

3.2.3. Observed profile data from balloon soundings -- Routinely plotted on Skew-T diagrams. Depicts the most recently observed (00 or 12 GMT of the current day) temperature and lapse rate of the atmosphere above the point from which the balloon was released.

3.3. Inversions -- Negative lapse rate in the troposphere (temperature warms with height). There are several different types. Some are:

3.3.1. Radiation inversions -- Surface based; relatively shallow. Caused by surface cooling during calm, clear, stable conditions.

3.3.2. Frontal inversions -- The transition zone between two airmasses turned into the horizontal plane.

3.3.3. Subsidence inversions -- Settling currents beneath strong high pressure systems results in adiabatic warming. Usually doesn't reach the Earth surface.

4.1. An unstable condition is one where the denser air is above the lighter air. Gravity will try to reverse this situation. (The troposphere is inherently unstable.)

4.2. A stable condition is one where the denser air is below the lighter air. (Inversions within the troposphere and the entire stratosphere are stable regions.)

4.3. Stability classifications:

4.3.1. Absolute Instability -- Environmental lapse rate causes atmosphere to cool faster than the dry adiabatic rate. In other words, a parcel rising from the surface, cooling at either the dry adiabatic or moist adiabatic rate, will be warmer than the surrounding environment, and will continue to rise on its own.

4.3.2. Conditional Stability -- Environmental lapse rate is between the dry and the moist adiabatic rate. In other words, a parcel rising from the surface and cooling at the dry adiabatic will be cooler than the surrounding environment, and thus fall back to its original position. But a parcel rising and cooling at the moist adiabatic rate will wind up warmer than the environment, and will therefore continue to rise on its own.

4.3.3. Absolute Stability -- Environmental lapse rate causes atmosphere to cool more slowly than the moist adiabatic rate. In other words, a parcel rising from the surface, cooling at either the dry adiabatic or moist adiabatic rate, will be colder than the surrounding environment, and will fall back to its original position.

4.4. An airmass that is initially stable may be destabilized if lifted by some means.

4.5. The stability of an airmass may be changed by horizontal temperature advection. Cold-air advection (CAA) in the low levels will stabilize the airmass, while CAA in the high levels will destabilize the airmass. Warm-air advection (WAA) in the low levels will destabilize the airmass, while WAA in the high levels will stabilize the airmass.

5.1. Mechanical -- Associated with fronts. Saturation occurs at the lifted condensation level (LCL).

5.2. Convective -- Associated with surface heating. Saturation occurs at the convective condensation level (CCL).

5.3. Turbulent -- Associated with strong low-level winds and turbulent mixing in the atmospheric boundary layer. Saturation occurs at the mixing condensation level (MCL).

LAB:

1. Plot the simplified atmospheric profile data through 400 millibars.

2. Analyze Skew-T for the following:

2.1. Surface mixing ratio and saturation mixing ratio.

2.2. Surface vapor pressure and saturation vapor pressure.

2.3. Surface wet bulb temperature.

2.4. Surface lifted condensation level (LCL) and surface convective condensation level (CCL).

2.5. Showalter Stability Index.

2.6. Identify inversions.

2.7. Forecast maximum and minimum surface temperatures.

HOMEWORK:

1. Read Lutgens and Tarbuck chapter 4 (pp. 99 - 117), and review chapter 1 (pp. 16 - 20).

2. Review Skew-T cards on-line.

3. Plot and analyze another Skew-T for the same parameters as you did in lab. You may print out a pre-plotted skew-t from the Plymouth State College Weather Page. See "PSC Wx Pages: Make Your Own...," then "RAOB Soundings: Diagrams/Data." Select "Skew-T Log P Diagram" and put in station identifier "KGYX" to plot the Gray, Maine sounding.