| NATS 101 Lecture 9 |
| Why temperature decreases with
altitude Precipitation |
| NowÉ Vertical Stability |
| Tennis Basics |
| Air Molecules Act Similarly |
| Rising Air Cools-Sinking Air Warms |
| Rising air parcel expands because pressure decreases with altitude | |
| Expansion requires that the molecules do work against surrounding air | |
| This uses up some of their internal energy which results in their temperature decreasing | |
| Assuming no transfer of heat across parcel walls (adiabatic expansion), cooling rate is 10oC/km |
| Relative Humidity |
| Air with a RH=100% is said to be saturated. | |
| RH depends on air temperature (SVP). | |
| RH changes by either changing airÕs water vapor content or the airÕs temperature. |
| Adiabatic Cooling-Warming |
| Rising, Saturated Air Cools Less |
| As a saturated parcel rises and expands, the release of latent heat offset some of 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 |
| SVP and Temperature |
| As air parcel is lifted, it expands | ||
| decreasing its temperature, | ||
| decreasing its SVP | ||
| Also decreases water vapor density, VP and dew point | ||
| As a result, its relative humidity increases | ||
| If lifted high enough, RH=100% and cloud forms | ||
| Continued lifting causes more cooling | ||
| more water condenses out | ||
| Slide 10 |
| Moist Flow over a Mountain |
| Brain Burners |
| Rising unsaturated (clear) air, and all sinking air | |
| Temperature changes at Dry Adiabatic Rate (DAR) of 10oC/km | |
| Dew point changes at rate of 2oC/km | |
| Rising saturated (cloudy) air | |
| Temperature 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 |
| NATS 101 Precipitation Processes |
| Supplemental References for Precipitation processes |
| Danielson, E. W., J. Levin and E. Abrams, 1998: Meteorology. 462 pp. McGraw-Hill. (ISBN 0-697-21711-6) | |
| Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6) |
| Cloud Droplets to Raindrops |
| A raindrop is 106 bigger than a cloud droplet | |
| Several days are needed for condensation alone to grow raindrops | |
| Yet, raindrops can form from cloud droplets in a less than one hour | |
| What processes account for such rapid growth? |
| Terminal Fall Speeds (upward suspension velocity) |
| Collision-Coalescence |
| Big water drops fall faster than small drops | |
| As big drops fall, they collide with smaller drops | |
| Some of the smaller drops stick to the big drops | |
| Collision-Coalescence | |
| Drops can grow by this process in warm clouds with no ice | |
| Occurs in warm tropical clouds |
| Warm Cloud Precipitation |
| As cloud droplet ascends, it grows larger by collision-coalescence | |
| Cloud droplet reaches the height where the updraft speed equals terminal fall speed | |
| As drop falls, it grows by collision-coalescence to size of a large raindrop |
| Mixed Water-Ice Clouds |
| Clouds that rise above freezing level contain mixture of water-ice | |
| Mixed region exists where Temps > -40oC | |
| Only ice crystals exist where Temps < -40oC | |
| Mid-latitude clouds are generally mixed |
| SVP over Liquid and Ice |
| SVP over ice is less than over water because sublimation takes more energy than evaporation | |
| If water surface is not flat, but instead curves like a cloud drop, then the SVP difference is even larger | |
| So at equilibrium, more vapor resides over cloud droplets than ice crystals |
| SVP near Droplets and Ice |
| Ice Crystal Process |
| Since SVP for a water droplet is higher than for ice crystal, vapor next to droplet will diffuse towards ice | |
| Ice crystals grow at the expense of water drops, which freeze on contact | |
| As the ice crystals grow, they begin to fall |
| Accretion-Aggregation Process |
| Summary: Key Concepts |
| Condensation acts too slow to produce rain | |
| Several days required for condensation | |
| Clouds produce rain in less than 1 hour | |
| Warm clouds (no ice) | |
| Collision-Coalescence Process | |
| Cold clouds (with ice) | |
| Ice Crystal Process | |
| Accretion-Splintering-Aggregation |
| Examples of Precipitation Types |
| Temp Profiles for Precipitation |
| Summary: Key Concepts |
| Precipitation can take many forms | |
| Drizzle-Rain-Glazing-Sleet-Snow-Hail | |
| Depending on specific weather conditions | |
| Radar used to sense precipitation remotely | |
| Location-Rate-Type (liquid v. frozen) | |
| Cloud drops with short wavelength pulse | |
| Wind component toward and from radar |
| Assignment |
| Topic - Precipitation Processes | |
| Reading - Ahrens p121-134 | |
| Problems - 5.14, 5.16, 5.17 | |
| Topic – Atmospheric Pressure | |
| Reading - Ahrens pg 141-148 | |
| Problems - 6.1, 6.7, 6.8 | |
| Assignment for Next Lecture |