ATM 10Lecture topics:Stability & Clouds: IntroductionStability & Clouds: IntroductionWhat is Stability?Why examine stability?How is T structure classified?If I know , then I know the stability?How many types of stability are there?How do you tell them apart?What are d and m ?Am I now able to figure out the stability classes?Absolutely StableAbsolutely UnstableConditionally UnstableWhich stability classes are more common?If an absolutely unstable LAYER doesn’t last long, why mention it?How can absolutely unstable air be created? -- 7 waysAm I now ready to discuss how clouds form?ATM 10Severe and Unusual Weatherhttp://atm.ucdavis.edu/~grotjahn/course/atm10/index.htmlProf. Richard GrotjahnLecture topics:Lecture topics:••What happens when air rises?What happens when air rises?••Stability Stability – What is it? – Why is it important?– How is it classified?– How do you know what type is present?– Use of a chart of temperature in vertical–Etc.Vertical Movement & TemperatureVertical Movement & TemperatureA rising air parcel A rising air parcel encounters less pressure encounters less pressure so it expands. so it expands. Expansion uses energy to Expansion uses energy to push out, push out, adiabatically adiabatically cooling the air.cooling the air.A sinking parcel A sinking parcel encounters greater encounters greater pressure and that higher pressure and that higher pressure does work on pressure does work on the parcel thereby the parcel thereby heating it up.heating it up.Recall: Recall: P = P = ρρR TR TFigure 7.2Figure 7.2Stability & Clouds: Introduction• Stability is related to weather several ways:Stability & Clouds: Introduction• Three examples of weather related to stability:– Clouds types linked to stability of the air. Stratus clouds found in stableconditions, cumulus are in unstable conditions. Thunderstorms in very unstable conditions.– Lenticular wave clouds form in stable air– Downslope winds, like the Santa Ana: stable air• But what is stability?stratiformcumuliformlenticularWhat is Stability?• Stability indicates to what extent the temperatureCHANGE IN A LAYER OF AIR resists vertical motion. • Fig. 7.1 illustrates the concept w/ an analogy.Stable – returns toOriginal position.Unstable – keeps goingIn direction of push.Figure 7.1Figure 7.1Why examine stability?• Different types of cloud form for different stability conditions• Most clouds form as air rises -- leading to cooling -- leading to condensation. So, since stability affects the ability of air to rise, stability affects cloud growth.How is T structure classified?• Stability related to temperature change in the vertical.• Temperature change in the vertical is called the lapse rate. • Lapse rate is: Γ = - ∆T / ∆Z• ∆T is T at top of layer minus T at bottom of layer.• ∆Z is the thickness of layer.• Γ >0 usually Fig: ∆T= -15 C, ∆Z=1000m, Γ = 0.015 C/mIf I know Γ, then I know the stability?•Almost• Stability depends on:- Γ value AND on - whether the air is saturated or not• Why saturated? Because if air that is saturated is cooled further, then some vapor must condense. Recall that the condensation adds heat to the air.How many types of stability are there?•Threebasic types, (plus some special cases given specific names)1. Absolutely stable2. Conditionally unstable3. Absolutely unstable• Special case of #3 is inversion: T increases with elevation• Neutral a special case related to saturationHow do you tell them apart?• Compare the actual change of temperature IN A LAYER with two calculated lapse rates called the dry adiabatic and moist adiabatic lapse rates: Γd & Γm.• Example: in clear air, deduce stability from smoke plumeWhat are Γd and Γm ?• dry adiabatic lapse rate: Γd = 10 oC/1000m of elevation change• moist adiabatic lapse rate: Γm < Γd.– Γm depends on amount of water vapor to dry air which in turn depends on T and P.– T change for parcel that is saturated.Am I now able to figure out the stability classes?• Yes! but it is easier to do this using a chart• Compare the actual T change with height to the lapse rates Γd (redline) and Γm (blue line)• Figures 7.3, 7.6, and 7.7 work out the three categories.• Figure 7.8 is a summary:Figure 7.8Absolutely Stable• Γ to the right of Γm • Unsaturated (left), saturated (right)• Recall: P = ρ R TAbsolutely Unstable• Γ to the left of Γd• Unsaturated (left), saturated (right)• Recall: P = ρ R TConditionally Unstable• Γ between Γd and Γm • Unsaturated (left), saturated (right)• Recall: P = ρ R TWhich stability classes are more common?• Atmosphere is most commonly conditionally unstable• Atmosphere can be absolutely stable for long periods• Absolutely unstable conditions can occur but usually are briefIf an absolutely unstable LAYER doesn’t last long, why mention it?• Can facilitate several types of severe weather• Convection could be "thermals" if the air is unsaturated. • If saturation occurs, cumulus type clouds form.How can absolutely unstable air be created? -- 7 ways1. Bring colder air aloft 2. Radiation cools top of layer3. Daytime heating4. Bring warmer air below5. Air moves over a warmer surface6. Mixing the air (fig. 7.10)7. Moving the whole column of air upwards (figs. 7.11 & 7.12)Am I now ready to discuss how clouds form?• Yes! (finally)End of lecture