Monday Oct 6, 2008

2. Monday Oct 6, 2008 • Discuss weather • Quiz and homework • Chapter 5 … Stability • Clouds • Chapter 6 … Forces and Balances

3. Oct 13 Forecast • Jet Stream Rainfall/Temperature

4. This Weekend :: 500 MB • Jet Stream Thuur FriFri Sat Sunn

5. This Weekend :: Surface • Jet Stream Fri Thur Thuur Sat Sun Sat

6. Important terms • Adiabatic processes • Lapse rate • Adiabatic lapse rate • Environmental lapse rate • Compression and expansion

7. Visualize Stability Imagine the concept of air stability as if a parcel of air was a ball being displaced …or moved. If the ball was in a valley it would tend to roll back to its original location ==== STABLE If the ball was on a flat plane it moves to a new position and stays there ==== NEUTRAL If the ball was on top of a hill it would accelerate away from its original position ==== UNSTABLE

8. Stabiity • We will be comparing Air Parcels • Think of parcels as air in balloons • If balloon is helium filled what happens to it? • Why???

9. Stability…more balloons • You have three balloons…and you are standing on top of Sutro Tower (500 ft)…the air temperature is 75 F from the surface to 1000 ft (isothermal) • The air inside balloon 1 is 100 F • The air inside balloon 2 is 75 F • The air inside balloon 3 is 50 F • What happens if you let go of all 3?

10. Adiabatic Process • Assumed: A parcel of air does not mix with its environment or exchange heat energy with its environment • Rising parcels expand and cool • Descending parcels compress and heat • If a parcel rises or descends adiabatically you can predict is temperature using the ADIABATIC lapse rate

11. Lapse Rate • Rate at which temperature changes with altitude Idealized Lapse Rates • Dry adiabatic lapse rate is 10C per km • Moist adiabatic lapse rate is 6C per km Environmental Lapse Rate • Measured Lapse Rate (e.g. RAOB)

12. How Will We Define Stability in the Atmosphere? • We look at the temperature of a rising parcel of air and compare it to the measured (environmental) lapse rate • Remember … our rising parcels of air have defined lapse rates … but the measured lapse rate changes constantly

13. Physical States of Water • The three physical states of matter that we normally encounter are solid, liquid, and gas.   Water can exist in all three physical states at ordinary temperatures on the Earth's surface.  • When water is in the vapor state, as a gas, the water molecules are not bonded to each other. High energy state • When water is in the liquid state, the bonds break and re-form continually. Medium energy state. • When water is in the solid state, as ice, the molecules are bonded to each other in a solid crystalline structure. Low energy state.

14. Latent Heat • Each time water changes physical state, energy is involved. • The important thing to remember is that each time water changes state, energy is absorbed or released, so the surrounding air either heats or cools. • Latent means hidden, so latent heat is "hidden" in the water molecule--we can't feel it, but it is there. • To get the molecule of water vapor to become liquid again, we have to take the energy away, that is, we have to cool it down so that it condenses (condensation is the change from the vapor state to the liquid state).  When water condenses, it releases latent heat and heats the air around it. • Basic rules: Evaporation cools the air, condensation heats the air

15. PHASE TRANSITIONS Water is the only substance that can be found in the atmosphere in the gaseous, liquid and solid phases

16. Energy Released or Absorbed During Phase Changes Can Be Quantified

17. Phase Changes of Water • Water is unique in that it is the only constituent of the atmosphere that exists naturally in all three phase • Phase changes are critical to the development of storms

18. Why Is Moist Adiabatic Lapse Rate Lower Than Dry? • Moist air means saturation…RH=100% • When moist air rises and cools, condensation occurs • Condensation means vapor  liquid

19. Why Is Moist Adiabatic Lapse Rate Lower Than Dry? • Condensation (Vapor state  Liquid state) warms the atmosphere • So…a rising parcel of moist air cools more slowly than a rising parcel of dry air because a second process is at work that warms it. • How much? In lower atmosphere condensation process warms air @ 4C per 1km. • Dry adiabatic lapse rate is -10C per km • Moist adiabatic lapse rate is -6C/km (-10 + 4 =-6)

20. Stability: Comparing the Measured Lapse Rate to the Adiabatic Lapse Rate Unstable Stable Dry Adiabatic Stable Unstable Moist Adiabatic

21. Stability…What happens to rising air • A parcel of air is less dense than its environment if it is warmer than the environment…it is therefore unstable and will rise So storm clouds (convective clouds) form when the atmosphere is unstable, and rising motions dominate

22. Condensation Level • The level where saturation first occurs in a lifted parcel of air • A parcel rises at the Dry Adiabatic lapse rate before saturation, Moist Adiabatic after saturation. • Also called the Lifting Condensation Level

23. Level of Free Convection • When a parcel of air is warmer than its environment it is less dense (buoyant) and rises freely • The level buoyancy occurs is the level of free convection

24. Illustration of LCL and LFC (pg 91) Parcels cool at dry adiabatic rate until saturated…the LCL Parcel rises further at the moist adiabatic rate until it becomes warmer than the environment (LFC) Once warmer than the environment, the parcel rises freely parcel rises

25. Cumulus Clouds…Bottom and Top

26. What Causes Air to Rise??? Frontal Lifting Orographic Lifting Convergence Can you name another ???

27. Stability Indices • Meteorologists have developed indices to determine stability through a large vertical layer of the atmosphere. • These indices are used to determine the potential for thunderstorm development • RAOB data are typically used to calculate each index • Each index provides slightly different info

28. Stability Index Examples • Lifted Index:(LI=T(env) @ 500 - T (lifted to 500)lower values (below 0) show unstable air. Lifting begins at 50 mb above surface, • Showalter Index: Similar to LI except lifting begins at 850 mb • CAPE Index: Convective Available Potential Energy (area of positive buoyancy on RAOB) • Total Totals: Combinations of T and Tdp at 850, 700 and 500 mb levels • SWEAT Index: (Severe WEAther Threat), includes weighted TT index, 850 mb T, 850 and 500 mb wind speeds and wind direction shear.

29. Stability Indices

30. Clouds and Cloud Types Clouds are generally classified based on characteristics, such as, altitude, appearance, or origin. Altitude distinctions apply to those clouds that fit in various layers of the atmosphere as follows: high clouds - have bases above 18,000 feet middle clouds - have bases between 7,000 and 18,000 feet low clouds - have bases below 7,000 feet fog - cloud in contact with the ground multi-level clouds... vertically thick spanning multiple layers orographic clouds - distinct clouds that form via interaction between wind and mountainous terrain features

31. Clouds and Cloud Types Latin Root Translation Cumulus = heap Stratus= layer Cirrus = curl of hair Alto = middle Nimbus = rain

32. Clouds and Cloud Types Most clouds owe their existence to upward vertical motion of air Clouds are often associated with weather producing phenomena fronts, troughs, surface low pressure systems (convergence) topography

33. Fourteen Cloud Types Cumulus Congestus Fair weather Cumulus Cumulonimbus Cirrocumulus Altocumulus Stratocumulus Cirrus Nimbostratus Cirrostratus Altostratus Stratus Fog Contrails Lenticular

34. Homework Due Oct 13 You are lifting a parcel of dry air adiabatically from sea level to a height of 3 km. If the temperature of the parcel at sea level is 30 C, what is the temperature at 3km? assume the RH of the parcel is less than 100% during lifting) If the environmental temperature at 3km is 5 C, is the parcel of air stable or unstable? (1 pt each) You are lifting a parcel of moist air adiabatically from sea level to a height of 3 km. If the temperature of the parcel at sea level is 30 C, what is the temperature at 3km? If the environmental temperature at 3km is 5 C, is the parcel of air stable or unstable? (1 pt each) Define lifting condensation level (1 pt)