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Lesson 15

Lesson 15. Adiabatic Processes Hess, McKnight’s Physical Geography , 10 ed. 138 – 139 pp . Adiabatic Processes. Recall prior to spring break that as the temperature of an air parcel decreases , its relative humidity increases.

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Lesson 15

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

  2. Adiabatic Processes • Recall prior to spring break that as the temperature of an air parcel decreases, its relative humidityincreases. • 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.

  3. Adiabatic Cooling • Clouds are the result of condensation • This 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, namely rising air

  4. 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

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

  6. 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 • 5°F per 1000 feet

  7. 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

  8. 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

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

  10. 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

  11. Example 1

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

  13. 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

  14. Lapse Rates • We know 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)

  15. 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

  16. Environmental Lapse Rate (ELR)

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

  18. Absolute Stability

  19. Absolute Instability

  20. Conditional Instability

  21. Final Thoughts • http://www.youtube.com/watch?v=Ox33TFnfWbE • Please omit pages 85, 86, 91, and 92 from your homework.

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