Listed below are the major concepts that you should know when finished with this unit

Objective 1: Recognize the effects of the expansion or compression on rising or descending parcels of air.

Note the diagram above: as a parcel of air rises it cools due to expansion. Conversely, when an air parcel descends it warms due to compression.

Objective 2: Identify and define the dry and moist adiabatic lapse rates.

The diagram on the left shows the dry adiabatic lapse rate of 5 degrees F per 1000'. Air cools at this rate when no condensation is occurring.

However, if you look at the sketch on the right, it shows air rising at the dry rate of 5 degrees F per 1000' until the cloud level...from that point on, the rising air cools at only 3 degrees F per 1000'. This is known as the moist adiabatic lapse rate. This is due to the fact that condensation is occurring, which releases heat and warms the air.

Objective 3: Analyze the effect this rising air has on weather conditions.

The rising air shown in the diagram above expands and cools, producing clouds, and sometimes precipitation. The most important cloud-producing process in the atmosphere is this adiabatic cooling as the air rises and cools to the dewpoint!

On the other hand, air that descends, such as that shown in the diagram above, tends to warm up due to compression. This warming process often decreases the relative humidity of the air and provides for clear, stable weather conditions.

Objective 4: Calculate the temperature of a parcel of air as it rises through the atmosphere, and identify the lifted condensation level (LCL) and equilibrium levels.

If you look at the diagram above, it shows a rising parcel of air. The parcel has a temperature of 87 F at the surface, and it begins to cool at the dry adiabatic rate of 5 F per 1000'. When the water vapor in the rising parcel begins to condense, the rising parcel cools at the moist adiabatic rate of only 3 F per 1000'. This occurs at about 3,000' in the diagram above. Notice that above the 3,000' level the parcel is only cooling at 3 F per 1000'. The height where the parcel begins to cool at the moist rate is called the lifted condensation level (point A). Note: we will learn how to find the actual convective cloud base height (called the Convective Condensation Level) shown on the above diagram in Unit 8.

Note at point B the air parcel keeps rising as long as it is warmer than the surrounding air.

At some point the rising parcel's temperature will equal that of the surrounding air, and the air will stop rising. This is known as the equilibrium level (point C). Thunderstorm anvils often spread out at the E.L.

Objective 5: Be able to correlate the following pressure readings in millibars with their average heights above the surface.

Plain and simple, this is something you will have to memorize. You will be using these heights throughout the course. So learn them now!

Objective 6: Analyze a skew-t log-p thermodynamic diagram (skew-t for short), and identify the temperature & dewpoint lines, the parcel trajectory, LCL, and E.L. on sample diagrams.

This one is going to take some practice, so be patient. There is a lot of information on a skew-t, but it can be understandable if you take it a step at a time:

The dewpoint temperature is always the left-hand sounding line (usually white, sometimes blue)

The temperature is always the right-hand sounding line (usually white, sometimes red)

The parcel trajectory is the yellow line (sometimes white)

The LCL is the point where the rising parcel begins cooling at the moist adiabatic rate. It is usually indicated by a "kink" or change in slope of the parcel trajectory line.

The Equilibrium level is found where the parcel trajectory crosses the temperature line near the top of the sounding (sometimes in the colder months this is not evident).