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.

A Stable Atmosphere stabilizing processes nighttime surface radiational cooling; warm air advected to cold surface; air aloft warming (e.g., subsidence inversions) Stable air provides ideal conditions for high pollution levels.

An Unstable Atmosphere destabilizing processes daytime solar heating of surface air; cold air advected to warm surface superadiabatic lapse rates (> 10 C/km) Unstable air tends to be well-mixed.

Conditionally Unstable Air Conditional instability: environmental lapse rate between dry and moist lapse rates Condensation level cloud base Q5: exactly at what height in the figure would Tp = Te? a) above 2 km, b) at 2 km, c) at 1.33 km

Q6: If environmental lapse is greater than dry lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable Q7: If environmental lapse is less than moist lapse rate, the Q8: The earth’s atmosphere is ordinarily Q9: The air T in an unsaturated parcel follows the a) dry lapse rate, b) moist lapse rate, c) environmental lapse rate Q10: If air T increases with height, the air is surely stable. If it decreases with height, the air is: a) stable, b), unstable, c) undecided

Atmosphere Stability Varies