1	Chapter 6: Stability
2	Concept of Stability
3	Air Parcel Expands as It Rises
4	Air Parcel Expands As It Rises… Air pressure decreases with elevation. If a helium balloon 1 m in diameter is released at sea level, it expands as it floats upward because of the pressure decrease. The balloon would be 6.7 m in diameter as a height of 40 km.
5	What Happens to the Temperature? Air molecules in the parcel (or the balloon) have to use their kinetic energy to expand the parcel/balloon. Therefore, the molecules lost energy and slow down their motions è The temperature of the air parcel (or balloon) decreases with elevation. The lost energy is used to increase the potential energy of air molecular. Similarly when the air parcel descends, the potential energy of air molecular is converted back to kinetic energy. è Air temperature rises.
6	Adiabatic Process If a material changes its state (pressure, volume, or temperature) without any heat being added to it or withdrawn from it, the change is said to be adiabatic. The adiabatic process often occurs when air rises or descends and is an important process in the atmosphere.
7	Diabatic Process Involve the direct addition or removal of heat energy. Example: Air passing over a cool surface loses energy through conduction.
8	Dry Adiabatic Lapse Rate
9	Lapse Rates A lapse rate is the rate at which temperature decreases (lapses) with increasing altitude. 3 different lapse rates we need to consider: (1) dry adiabatic lapse rate (2) moist adiabatic lapse rate (3) environmental lapse rate
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11	Moist Adiabatic Lapse Rate
12	Phase Changes of Water Latent heat is the heat released or absorbed per unit mass when water changes phase. Latent heating is an efficient way of transferring energy globally and is an important energy source for Earth’s weather and climate.
13	Environmental Lapse Rate The environmental lapse rate is referred to as the rate at which the air temperature surrounding us (or the air parcels) would be changed if we were to climb upward into the atmosphere. This rate varies from time to time and from place to place. A rawinsonde’s thermometer measures the environmental lapse rate.
14	Environmental Lapse Rate The environmental (or ambient) lapse rate is referred to the vertical change in temperature through still air. The environmental lapse rate is not fixed. It changes from day to day and from place to place.
15	An Example of Environmental Lapse rate
16	How to Determine Stability How do we determine where the atmosphere is unstable – under which convective clouds and storms may form? è Answer: Compare the environmental lapse rate with the dry/moist lapse rate
17	Static Stability of the Atmosphere Ge = environmental lapse rate Gd = dry adiabatic lapse rate Gm = moist adiabatic lapse rate Absolutely Stable Ge < Gm Absolutely Unstable Ge > Gd Conditionally Unstable Gm < Ge < Gd
18	Absolutely Stable Atmosphere
19	Absolutely Unstable Atmosphere
20	Conditionally Unstable Atmosphere
21	Real-Life Examples in Topeka, Kansas
22	An Example
23	How Thunderstorm Forms?
24	How to Change Environmental Lapse Rate? During the day, surface insolation gains result in greater heating near the surface than aloft. At night, the situation reverses as terrestrial radiation loss causes near surface chilling à a temperature inversion.
25	Four Ways to Lift Air Upward
26	Stability Indices Environmental Lapse rate Lifted Index = T (environment at 500mb) – T (parcel lifted to 500mb) Showalter Index: similar to lifted index but was lifted to 850mb CAPE (Convective Available Potential Energy): derived from soundings Convective INHibition (CINH) Index K Index Total Totals Index SWEAT (Severe Weather Threat) Index