| NATS 101 Lecture 12 Vertical Stability |
| Tennis Basics |
| Air Molecules Act Similarly |
| Rising Air Cools-Sinking Air Warms |
| Rising air parcel expands | |
| Expansion requires work against outside air | |
| Air molecules rebound from ÒwallsÓ at a slower speed, resulting in a cooler temperature | |
| Assuming no transfer of heat across parcel walls (adiabatic expansion), cooling rate is 10oC/km |
| Adiabatic Cooling-Warming |
| Rising, Saturated Air Cools Less |
| As a saturated parcel rises and expands, the release of latent heat mitigates the adiabatic cooling | |
| Cooling for saturated air varies with mixing ratio. | |
| We will use an average value of 6oC/km for moisture lapse rate | |
| Note: sinking clear air always warms at dry lapse rate |
| Moist Flow over a Mountain |
| Brain Burners |
| Rising and sinking unsaturated (clear) air | |
| Temp changes at Dry Adiabatic Rate (DAR) of 10oC/km | |
| Dew point changes at rate of 2oC/km | |
| Rising and sinking saturated (cloudy) air | |
| Temp cools at Moist Adiabatic Rate (MAR) of 6oC/km | |
| Dew point decreases at rate of 6oC/km |
| Concept of Stability |
| ArchimedesÕ Principle |
| Archimedes' principle is the law of buoyancy. | |
| It states that "any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body." | |
| The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If the density of an object is greater/less than the density of water, the object will sink/float. | |
| Demo: Diet vs. Regular Soda. | |
| http://www.onr.navy.mil/focus/blowballast/sub/work2.htm |
| Absolutely Stable: Top Rock |
| Stable air strongly resists upward motion | |
| External force must be applied to an air parcel before it can rise | |
| Clouds that form in stable air spread out horizontally in layers, with flat bases-tops |
| Absolutely Unstable: Middle Rock |
| Unstable air does not resist upward motion | |
| Clouds in unstable air stretch out vertically | |
| Absolute instability is limited to very thin layer next to ground on hot, sunny days | |
| Superadiabatic lapse rate |
| Conditionally Unstable: Lower Rock |
| Environmental Lapse Rate (ELR) |
| Summary: Key Concepts I |
| Rising unsaturated air, and all sinking air | |
| Temp changes at DAR of 10oC/km | |
| DP changes at rate of 2oC/km | |
| Saturation occurs with sufficient lifting | |
| Rising saturated air | |
| Latent Heating Mitigates Adia. Cooling | |
| Temp and DP cools at MAR of 6oC/km | |
| Note that MAR is always less than DAR |
| Summary: Key Concepts II |
| Vertical Stability Determined by ELR | |
| Absolutely Stable and Unstable | |
| Conditionally Unstable | |
| Temp Difference between ELR and Air Parcel, and Depth of Layer of Conditionally Instability Modulates | |
| Vertical Extent and Severity of Cumulus |
| Assignment for Next Lecture |
| Topic - Precipitation Processes | |
| Reading - Ahrens p121-134 | |
| Problems - 5.14, 5.16, 5.17 |