Meteorology Name _________________________________

Thermodynamic Diagram (Stability)

I. Introduction (For this lab, PLEASE READ chapter 6 in Ahrens)

A thermodynamic diagram (also called an adiabatic chart) is a pressure-temperature graph on which is plotted an atmospheric vertical profile or “sounding.” These charts enable meteorologists to put together a vertical picture of the atmosphere. Among other things, thermodynamic charts are used to assess atmospheric stability, predict cloud base levels and estimate the chances for thunderstorms.

Here is a sample thermodynamic diagram. In the lab room you will be given two of these:


II. Reading the thermodynamic diagram

On a thermodynamic diagram, the ordinates (green horizontal lines) are lines of equal pressure known as isobars. They are labelled in millibars.

The abscissas (green vertical lines) are lines of equal temperature known as isotherms. They are labelled in Celsius degrees.

The straight green lines that slope upward to the left are called “dry adiabats.” These indicate the cooling or warming rate of dry air as it is raised or lowered. Always use the dry adiabats when raising or lowering unsaturated air. The adiabats are labelled in Kelvin degrees according to their potential temperature, which is the temperature that air would have if it were moved to 1000 mb.

The nearly straight blue lines inclined a bit to the left of the isotherms are lines of constant mixing ratio. This means that they represent the number of grams of water vapor in each kilogram of dry air (g/kg).

The sloping red lines are called moist adiabats. They represent the temperature lapse rate for saturated air as it is raised. Always use the moist adiabats when raising or lowering saturated air.

III. Useful Definitions

Air parcel: A small representative sample of air.

Adiabatic: The way to describe a process in which no heat is being added or removed.

Mixing ratio (w): In a sample of air, the mixing ratio is the ratio of the mass of water vapor to the mass of dry air. It is expressed in grams of water vapor per kilograms of dry air (g/kg). To find the mixing ratio, go to the intersection of the dew point with the given pressure. For example, at 900 mb, a dew point of -19.5°C is equivalent to a mixing ratio of 1 g/kg, the blue line which intersects there.

Saturation mixing ratio (ws): The mixing ratio an air parcel would have if saturated. To find ws, go to the intersection of the temperature with the given pressure. For example, at 900 mb, find the temperature of -10°C. No blue mixing ratio line intersects here, but the 2 g/kg line is close. Interpolate (guess) ws to be 2.1 g/kg.

Relative Humidity (RH): The ratio (in percent) of the amount of water vapor to the maximum amount the air could hold if saturated. To find the RH, use w and ws:

RH = (w/ws) x 100%

IV. Atmospheric Stability

Stability of air depends on how an air parcel will behave if forced upward or downward away from its initial position. There are three possibilities:

1. The air parcel will move back toward its initial position. This is a stable situation, simply called stability.

2. The air parcel will stay in the new position. This is called neutral stability.

3. The air parcel will move away from its initial position ON ITS OWN. This is instability. It is important because, if pushed upward a small amount, the parcel will accelerate upward, eventually becoming saturated and continuing upward as a precipitation-producing cloud.

To assess stability on a thermodynamic diagram, lift a parcel from any level along the dry adiabat if it is unsaturated. If the parcel is saturated, lift along the moist adiabat. If at any pressure level the parcel is warmer than the sounding at that pressure, the parcel is unstable. ALWAYS COMPARE THE PARCEL’S TEMPERATURE TO THE TEMPERATURE OF THE SOUNDING AT THE SAME LEVEL. Never compare temperatures at different levels.



1. Plot the following sounding on the thermodynamic diagram. Plot each temperature as a small circle and connect the circles with ruled lines. Plot the dew point as small x’s and connect the x’s with ruled lines.

Pressure (mb) Temperature (°C) Dew point (°C)

1000 9 5

900 12 12

800 2 -17

700 -5 -8

600 -10 -16

500 -25 -30

400 -8 -30

2. What is the temperature (C) at the following levels?

900 mb _____ 850 mb_______ 550 mb _______ 425 mb _______

3. At what pressure level(s) can the following dew points be found? (Find all levels)

10°C _____ 0°C ________ -10°C __________________

4. Find the mixing ratio (w), the saturation mixing ratio (ws) and calculate the relative humidity for the following pressure levels:

Pressure (mb) w (g/kg) ws(g/kg) RH (%)

1000 _______ _______ _______

900 _______ _______ _______

800 _______ _______ _______

700 _______ _______ _______

500 _______ _______ _______

5. If the air at 1000 mb is lifted to 700 mb, will it be colder or warmer than the surrounding air? Does this represent a stable or an unstable condition? Why?

6. If the air at 900 mb is lifted to 700 mb, will it be colder or warmer than the surrounding air? Does this represent a stable or an unstable condition? Why?

7. If air is lifted from 900 mb to 500 mb, how many grams of water vapor would be condensed from each Kilogram of air? (Hint: Lift the parcel. The mixing ratio tells you how many grams of water vapor are actually in 1 Kg of air to start.)

Grams condensed: ______________

In a logical, clear manner, show how you got your answer:

(you may draw pictures if you wish. Show all calculations)

8. If air rises on its own from 900 mb but doesn’t stop at 500 mb, where does it stop rising? (Hint: To rise, the air must be unstable so it must be warmer than the sounding. Draw it on the diagram to see where this is no longer true).

If you are standing outside watching this parcel rise, what do you see at the level where it stops?

Answer both questions here:

Portions of this lab were used by permission from Mississippi State University’s Graduate School of Geo-Science.


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