Atmospheric Stability Why do cumulus clouds develop? Why are temperature inversions associated with air pollution? Why do hot air or helium balloons rise?

Atmospheric Vertical Stability A stable atmosphere has weaker mixing in the vertical An unstable atmosphere has a large amount of mixing in the vertical

Why is this Important? A stable atmosphere is associated with air pollution, fog, and strong surface temperature inversions in winter. An unstable atmosphere is associated with lots of atmospheric mixing, with good air quality and sometimes convection/thunderstorms. Meteorologists spend a lot of time trying to figure out whether the atmosphere will be stable or unstable

Stable Situation

Stable

Stable

Unstable

Unstable

Unstable

Unstable

Lava Lamp: Instability

Atmospheric Stability Concepts

Stable and Unstable If you push something in stable situation, it will return to its original location

Neutral Move it and it will stay in the place you left it.

In the real atmosphere, we don’t deal with rocks, but with air parcels Reminder. An air parcel is an identifiable collection of air…think of a balloon. As we discuss stability we need to keep in mind some essential ideas.

Essential Ideas The density of an air parcel depends on temperature Increase temperature > less dense Decrease temperature > more dense The equation that expresses this relationship is the perfect (or ideal) gas law. P = r R T P is pressure (Nm-2), r is density (mass/volume, kg m-3), T is temperature (°K), R is the gas constant

Essential Ideas r = P/(RT) With a little algebra, we can solve for density: r = P/(RT) Thus, when pressure is held constant, if T increases than r decreases (less dense)! There is a distinction (and often a difference) between conditions within an air parcel and its environment. Not necessarily the same!

Tp Te Te Up Is not necessarily the same as Tp Temperature Of the Air Parcel Up Temperature Of the Environment The pressure of the parcel and the environment are generally the same

Basic Rules If an air parcel is warmer than it surrounding environment, then it is less dense that the environment as that level and tends to rise. If an air parcel is the same temperature as the environment at that level, it has the same density as the environment at that level and will stay in position. If an air parcel is colder that the environment at the same level, it is more dense than the environment and tends to sink.

None of this should surprise you. A hot air balloon is warmer and less dense than the environment and rises. A lead balloon sinks Helium balloon rises because He is less dense than than the gases in the atmosphere.

But why does hot air rise But why does hot air rise? And why doesn’t all the air in the atmosphere fall down to the surface?

But why does a parcel of less dense air rise? Imagine an air parcel with the same density as the environment PT environment re rp Parcel Up Pb > Pt, so there is an upward force That balances out the weight. Stays put PB Weight

Hydrostatic Balance The difference in pressure between the top and bottom of the air parcel produces an upward directed pressure gradient force. This force balance gravity (the weight of the air parcel) Called HYDROSTATIC BALANCE. Most of the atmosphere is in this balance and that is why the atmosphere doesn’t collapse.

But what happens if an air parcel is warmer than the environment? E.g. a hot air balloon An parcel warmed by number of processes (e.g., warm surface, latent heating, etc.)

Warmer than the environment If the parcel warm, its density is less. PT environment r2 re Parcel Up Pb > Pt, so there is an upward force But the weight is less Thus, the parcel rises PB Weight inside circle is less

So.. If parcel is warmer and thus less dense than the environment at that level it rises If the parcel is is the same temperature as the environment it stays in place If the parcel is colder than the environment, it it is more dense that air of the environment and sinks https://www.youtube.com/watch?v=ydXJVSO-jXU

Meteorologists want to know how stable the atmosphere is by examining an atmospheric sounding..but how?

You need to know one more thing: the dry adiabatic lapse rate The dry adiabatic lapse rate is the rate of change of temperatures with height of an unsaturated air parcel when it gets pushed up or down. Lapse rate: the rate of change of temperature with height G = - DT/Dz So if temperature falls by 5C in 1000 meters (km) moving up in the vertical, the lapse rate is 5C per km.

Dry adiabatic lapse rate The dry adiabatic lapse rate, Gd , is the rate of change of temperature with height of an unsaturated air parcel. The value is 9.8 C per km When an unsaturated air parcel rises it cools by 9.8C per km. Cooling due to adiabatic expansion. When an unsaturated air parcel sinks it warms by 9.8C per km. Warming by adiabatic compression.

Reminder The term adiabatic is used to denote situations where there are no sources of heating or cooling. No release of latent heat, no radiation, no nothing. It turns out that by comparing Gd with the environmental lapse rate Ge, one can tell the vertical stability of unsaturated air parcels.

Case 1: Environmental lapse rate =dry adiabatic lapse rate Gd = Ge Neutral 1 km .2C .2C Gd = 9.8 C per km 10C 10C 0 km Parcel Environment

Case 2: Environmental lapse rate greater than dry adiabatic lapse rate Ge > Gd Unstable, parcel is warmer than environment 1 km .2C -5C Ge =15C per km 10C 10C 0 km Parcel Environment

Case 3: Environmental lapse rate <dry adiabatic lapse rate Ge < Gd Stable, parcel is cooler than environment 1 km .2C 5C Ge =5C per km 10C 10C 0 km Parcel Environment

We can evaluate stability by plotting the environmental sounding and the dry adiabatic lapse rate on a thermodynamic diagram

Summary Ge > Gd unstable Ge = Gd neutral Ge < Gd stable How can we make the atmosphere unstable? Increase Ge !! How do we do this? Increase temperatures near the surface Cool aloft

Increasing Lapse Rate (warm surface or cool aloft) Z Gd Gd warm Temp

Increasing Lapse Rate Summer/warm time of the year, sun heats the surface. Unstable atmosphere develops, with up and down circulations

Increasing Lapse Rate In winter, cooler air moving in aloft above warm water can produce enhanced lapse rate: instability and post-frontal showers.

In the atmosphere, instability results in convection

Can result in bumpy flights before landing on warm days

https://www.youtube.com/watch?v=vBf1UkSUQQ0

Simulation https://www.youtube.com/watch?v=nnJazkb9mNI

Instability and Convection Can Also Occur in the Kitchen

Now add saturation Instability and convection can be completely dry…air remains unsaturated. But if air has enough water vapor, rising air cools to saturation, producing clouds...usually cumulus clouds But this produces another issue--when air becomes saturated and rises more, water condenses out producing latent heating. Thus, saturated air cools less when rising

Saturated Adiabatic Lapse Rate (a.k.a. moist adiabatic lapse rate) The lapse rate becomes Gs= 6.5 C per km Gs= 6.5 C per km Saturation Gd= 9.8 C per km

How do we calculate instability in a saturated atmosphere. No Problem How do we calculate instability in a saturated atmosphere? No Problem! Just replace with saturated adiabatic lapse rate Ge > Gs unstable Ge = Gs neutral Ge < Gs stable

Easier to get instability Since Gs is less than Gd, it much easier to get instability in a moist, saturated atmosphere—like the U.S. East Coast in summer! Another interesting possibility: Gs<Ge<Gd, called conditional instability: atmosphere is unstable when atmosphere is saturated and stable if unsaturated. Much of the U.S. East Coast is conditionally unstable.

Instability and Great Stability If a sounding shows an unstable layer, one might expect cumulus clouds and maybe thunderstorms, or perhaps turbulence. But what happens if the atmosphere is very stable? Then the atmosphere will be layered with little vertical mixing. Good for air pollution and fog.

Instability can help mix pollutants in the vertical

Inversions Inversion layers, in which temperature increases with height, are VERY stable.

How do inversions form? Two major ways Radiation inversions: cooling at the surface as earth radiates infrared radiation to space.

Subsidence Inversion Sinking air aloft, often associated with high pressure. Both radiation and subsidence inversions can happen at the same time.

Meteorologists evaluate stability by comparing environmental soundings (mainly from radiosonde), with the dry and saturated adiabatic lapse rate shown on thermodynamic charts.