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Lesson 15 Adiabatic Processes Hess, McKnight’s Physical Geography, 10 ed. 138 – 139 pp.

Adiabatic Processes Recall prior to Fall break that as the temperature of an air parcel decreases, its relative humidity increases. Once a parcel has cooled to the dew point, it becomes saturated and condensation occurs. Adiabatic cooling is one of the most common ways clouds form.

Adiabatic Cooling Clouds are the result of condensation Again, condensation occurs when an air parcel’s temperature reaches the dew point Air parcels are free to flow horizontally or vertically through the atmosphere ◦ Most commonly, clouds are formed with vertical movement of air parcels, namely rising air

Adiabatic Cooling, cont. As an air parcel rises, it becomes less- pressurized With less pressure exerted on the parcel, it therefore expands This expansion causes cooling and increases RH ◦ Adiabatic cooling

Adiabatic Heating Conversely, as a parcel of air descends it compresses due to higher pressure This is a warming process, therefore the air warms

Dry Adiabatic Rate Consider an air parcel that is unsaturated ◦ RH < 100% If this air parcel rises, it will cool at what is known as the dry adiabatic lapse rate (DALR) ◦ 10 °C per 1000 meters ◦ 10°F per 1000 feet

Dry Adiabatic Rate, cont. As this air parcel rises its relative humidity increases (because it’s expanding) Eventually the air temperature cools and equals the dew point temperature ◦ At this point the RH=100% The elevation that this occurs at is known as the lifted condensation level (LCL) ◦ At this point, condensation occurs and cloud forms

Moist Adiabatic Rate At this point, the air parcel is saturated and the RH=100% If the parcel continues to rise, it will cool even further However, because it is saturated (i.e. 100% RH) it will cool slower (MALR) ◦ 6 °C per 1000 meters ◦ 3.3°F per 1000 feet This is the result of latent heat which is released once condensation begins

Heat Transfer Theory Evaporation is a cooling process because liquid water is converted into a gas, which takes heat energy from the environment.

Final Thoughts… Dry Adiabatic Lapse Rate (DALR): ◦ 10 °C per 1000 meters ◦ 10°F per 1000 feet Moist Adiabatic Lapse Rate (MALR): ◦ 6 °C per 1000 meters ◦ 3.3°F per 1000 feet

Example 1

Lesson 16 Stability Hess, McKnight’s Physical Geography, 10 ed. 143– 147 pp.

Stable vs. Unstable The atmosphere and individual air parcels can be characterized as either stable or unstable. Stability occurs when vertical motion is suppressed. ◦ No clouds form (unless they are forced, i.e. orographic lift) Instability (unstable air) occurs when air tends to rise on its own The temperature of an air parcel relative to the air which surrounds it determines stability

Lapse Rates We know from the previous lecture there are two types of adiabatic lapse rates ◦ Dry adiabatic lapse rate (DALR) ◦ Moist adiabatic lapse rate (MALR) There is a third lapse rate, known as the environmental lapse rate (ELR)

Environmental Lapse Rate (ELR) The ELR is basically the actual air temperature at any height in the atmosphere It is also known as a temperature profile, vertical temperature gradient, or normal lapse rate The average value: ◦ 6.5 °C per 1000 meters ◦ 3.6°F per 1000 feet

Types of Stability/Instability There are three types: ◦ Absolute stability ◦ Absolute instability ◦ Conditional instability Let’s look at examples of each of these…

Absolute Stability

Absolute Instability

Conditional Instability

Homework Due Friday Please omit pages 85, 86, 91, and 92 from your homework that is due this Friday.