Atmospheric Stability Chapter 4

Learning Targets #7- I can describe adiabatic temperature changes and differentiate between the dry adiabatic rate and the wet adiabatic rate. #8- I can describe the four mechanisms that cause air to rise.

Review Changes in water vapor Condensation can occur from: an increase in the amount of water vapor mixing cold air with warm, moist air Lowering the air temperature to the dew or frost point

Diabatic and Adiabatic Processes A process in which energy is added to or removed from a system. Ex: a pot of water placed over a stove Ex: air in contact with a warm surface EX: air over a cool surface Diabatic processes are often responsible for the formation of fog A process in which temperature changes but no heat is added to or removed from a substance. Ex: pumping up a bike tire Ex: releasing air from tire Adiabatic processes are often responsible for the formation of clouds

Adiabatic Temperature Change As air is heated it expands becoming less dense, and as a result, lighter. Because it is lighter, it rises upwards above the cooler air. As it does so, this air continues to expand. This is because there is less pressure higher in the atmosphere, allowing the air molecules to spread out more. Temperature changes but no heat is added to or removed from a substance are said to be adiabatic Condensation occurs when water vapor is cooled enough to change to a liquid – produces dew, fog, or clouds How does it work? As air is heated…. If no heat is added or removed from a system, work performed by the air (the expansion of the gas) causes a decrease in internal energy (a decrease in temperature), and work performed on the gas (compression) leads to warming.

Adiabatic Temperature Change Dry Adiabatic Lapse Rate (DALR): The rate at which a rising parcel of of unsaturated air cools 10oC/1000m (5.5oF/1000ft) Lifting Condensation Level (LCL): the altitude at which a parcel of air cools sufficiently and condensation/deposition commences. Saturated Adiabatic Lapse Rate (SALR): the rate at which saturated air cools Once air condenses, it cools more slowly Latent heat is released Occurs above lifting condensation level 5oC/1000m (3.3oF/1000ft) to 9oC/1000m

Dry Adiabatic Lapse Rate

Processes that Lift Air Air resists vertical movement. Air near the surface stays near the surface, air up high tend to remain up high Four mechanisms that cause air to rise Orographic Lifting Frontal Wedging Convergence Localized convection lifting

Process that Lift Air

Orographic Lifting Occurs when elevated terrains such as mountains act as barriers to the flow of air As air ascends, adiabatic cooling often generates clouds and precipitation Rain Shadow Desert: as air reaches the leeward side of the mountain, much of the moisture has been lost. If the air descends, it warms adiabatically making condensation and precipitation unlikely

Orographic Lifting When air approaches a topographic barrier, it can be lifted upward or deflected around the barrier.

Orographic Lifting

The Sierra Nevada forms a major barrier to winds that generally blow from west to east. This promotes enhanced precipitation on the windward side and a rainshadow on the leeward side.

Rainshadow Effect

Frontal Wedging Masses of warm and cold air collide to produce fronts The cooler, denser air acts as a barrier over which the warmer less dense air rises (this is frontal wedging) Mid-latitude cyclones Cold fronts cause uplift as cold air advances on warmer, less dense air.

Frontal Wedging Uplift occurs along a warm front when warm air overruns the cold wedge of air ahead of it.

Convergence Whenever air in the lower troposphere flows together, lifting happens. When air flows in from more than one direction, it has to go somewhere Florida mid-afternoon thunderstorms (pg 113)

Localized Convection Lifting Unequal heating of Earth’s surface may cause pockets of air to be warmed more than surrounding air. Thermals: pockets of rising hot air Warm parcels of air rise above the lifting condensation level, clouds form, which can bring about mid-afternoon showers.

Convection lifting

Learning Target Study Guide Fill in Learning Targets 7 and 8 on your LTSG.

Learning Targets #9- I can define environmental lapse rate and understand how it determines the stability of air. #10- I can define the three fundamental conditions of the atmosphere and recognize weather conditions associated with stable and unstable air. #11: I can list and describe the factors that modify the stability of air.

The Critical Weathermaker Why do clouds vary so much in size, and why does the resulting precipitation vary so much? Stability of air A parcel of air rises and its temperature ______________ due to __________________ of the air. Stability is determined by comparing the parcel’s temperature to temperature to that of the surrounding air. Stable air resists vertical displacement (parcel is cooler and more dense than surrounding air) Unstable air does not resist vertical displacement (parcel is warmer and less dense, than the surrounding air) Decreases; expansion

Environmental Lapse Rate ELR-The rate of temperature decrease with height in the troposphere. 6.5oC per kilometer (3.6oF per 1000 feet) Not constant (highly variable) Can vary during the course of the day with fluctuations of the weather as well as seasonally from place to place. Not to be confused with ALRs ELR applies to still air. ALR applies to parcels of rising air.

Effect of ELR on Stability Consider this: ELR= 5oC per 1000 m Air at 1000 m = 5oC cooler than the surface, and so on Parcel = 25oC If it rises to 1000 m, it expands and cools adiabatically at 10oC per 1000 m Parcel is 5oC cooler than surrounding air; more dense; would sink back down

Fundamental Conditions Absolute Stability ELR is less than the SALR Temperature inversion create most stable conditions ELR of 5oC per 1000 m and SALR of 6oC per 1000 m At 1000m, surrounding air temp is 15oC; rising parcel has cooled to 10oC (adiabatically) The parcel is more dense and would resist vertical motion; even if it were forced above the LCL, it would be cooler and more dense and would have a tendency to return to the surface Temperature inversions-when air temperature increases with altitude Occur on clear nights as a result of radiation cooling at Earth’s surface. (the ground and the air next to it will cool more rapidly than the air aloft, causing an inversion) Occur in winter when warm air from the Gulf of Mexico invades the snow-covered surface of the midcontinent. (anytime warmer air overlies cooler air, the resulting layer is extremely stable and resists vertical mixing)

Fundamental Conditions Absolute Instability ELR= greater than the DALR If the ascending parcel of air is warmer than the surrounding air, it will continue to rise because of its buoyancy Occurs most often during the warmest months and on clear days when solar heating is intense The lower-most layer of the atmosphere is heated to a much higher temp than the air aloft, resulting in a steep ELR and a very unstable atmosphere. Instability produced mainly by strong surface heating is generally confined to the first few kilometers of the atmosphere. Above this height, temp drops more slowly with altitude, creating a more stable temperature pattern. Clouds produced by surface heating lack vertical height and rarely produce violent weather.

Fundamental Conditions Conditional Instability Moist air has an ELR between the DALR and SALR ELR is between 5oC and 10oC per 1000 meters. The atmosphere is considered conditionally stable with it is: Stable with respect to an unsaturated parcel of air, but Unstable with respect to a saturated parcel of air Figure 4-26 Parcel is warmer than surrounding air for first 2000 meters With release of latent heat above the LCL, the parcel becomes warmer than surrounding air Here, the air will continue to rise on its own due to buoyancy (LFC-level of free convection-altitude to which a parcel of air must be lifted for it to become buoyant and rise on its own) Conditional instability depends on whether or not the parcel is saturated

Stability and Daily Weather Stable air forced aloft produces clouds that are widespread and are not very “tall”; precipitation is light to moderate, if any Unstable air produces towering clouds, usually producing heavy precipitation. Conclusions: On an overcast, dreary day with light drizzle, the atmosphere is _________. Cauliflower-shaped clouds appear to be growing, the atmosphere would be ____________. On a foggy day, the atmosphere would be __________. Stable Unstable Stable (the air resists vertical mixing; if it were mixing freely with the “dry” layer above, evaporation would get rid of the fog)

How does stability change? Instability is enhanced by: Intense solar heating warming the lowermost layer of the atmosphere. The heating of an air mass from below as it passes over a warm surface. General upward movement of air caused by processes such as orographic lifting, frontal wedging, and convergence. Radiation cooling from cloud tops. Stability is enhanced by: Radiation cooling of Earth’s surface after sunset. The cooling of an air mass from below as it traverses a cold surface. General subsidence (downward airflow) within an air column. Changes in stability occur as air moves horizontally over a surface having a very different temperature than the air. Subsidence stabilizes the air. Uplift enhances instability.

In summary… When air is allowed to expand, it cools and when air is compressed, it warms. These are called adiabatic temperature changes (heat is neither added nor subtracted). DALR=10oC per 1000 m SALR=5oC to 9oC per 1000m; slower cooling rate above LCL LCL-the altitude where a rising parcel of air has reached saturation and condensation begins) When air rises, it expands and cools adiabatically. If it is lifted high enough, it will eventually cool to its dewpoint temp and clouds will form. Four mechanisms that life air are: Orographic lifting: air is forced to rise over a mountainous barrier Frontal wedging: warmer, less dense air is forced over cooler, denser air along a front Convergence: a pile-up of horizontal airflow resulting in an upward flow Localized convective lifting: unequal surface heating causes localized pockets of air to rise because of their buoyancy.

In summary… When air rises, it cools and eventually produces clouds. Stable air resists vertical displacement; clouds have little vertical thickness; precipitation, if any, is light. Unstable air rises because of it buoyancy; clouds are towering and frequently accompanied by heavy rain. The stability of air is determined by comparing the ELR (environmental lapse rate) to a parcel of air: Absolute stability: ELR=less than SALR Absolute instability: ELR=greater than DALR Conditional instability: ELR=between the SALR and DALR Any factor that causes air near the surface to: become warmed in relation to the air aloft increases the air’s instability. be chilled results in the air becoming more stable. Processes that alter atmospheric stability result from temperature changes caused by vertical or horizontal air movements, but daily temperature changes are important as well.