Stability and Cloud Development

1. Chapter 6 Stability and Cloud Development

2. Stability & Cloud Development • This chapter discusses: • Definitions and causes of stable and unstable atmospheric air • Processes that cause instability and cloud development

3. Stability & Movement A rock, like a parcel of air, that is in stable equilibrium will return to its original position when pushed. If the rock instead departs in the direction of the push, it was in unstable equilibrium.

4. Movement & Temperature Rising air expands, using energy to push out, which slows and adiabatically cools the air. A parcel of air may be forced to rise or sink, and change temperature relative to environmental air, which is sampled using radiosonde balloons.

5. Dry & Moist Adiabatic Rates A parcel of rising dry air cools at approximately 10° C/km, while latent heat released into the parcel by condensing vapor keeps the moist adiabatic lapse rate nearer to 6° C/km. Because the environmental air cools more slowly, the parcel of air held by the helicopter is always cooler and sinks back to its original position.

6. Stable Atmospheric Conditions Cold air below warm air is considered stable because cold air has a higher density, and when displaced will gravitate back below the warm, lighter air. A morning atmosphere has this stable vertical profile, keeping fog unmixed.

7. Subsidence Inversion Cooler surface air is called a temperature inversion, caused by radiational cooling, cold advection, or subsiding air that shrinks in total thickness. As it sinks and shrinks, its top lowers a greater distance and warms more than the air resting at earth's surface.

8. Absolutely Unstable Atmosphere Absolutely unstable conditions indicate that a lifted parcel of air, whether dry or moist, will be warmer than the surrounding environmental air, and hence continue to rise.

9. Conditionally Unstable Conditions are initially stable when the environmental lapse rate (LR) is between the dry and moist adiabatic LR. Air that is dry is always cooler and sinks, but when the parcel's dew point is reached it cools more slowly, and may become warmer than the surrounding air, creating instability.

10. Stability Summary Environmental temperatures determine stability for rising parcels of air. The atmosphere is absolutely unstable when the environmental lapse rate (ELR) is steeper than the dry LR, absolutely stable when the ELR is less steep than the moist LR, and conditionally unstable otherwise.

11. Heat as an Instability Trigger As the environmental lapse rate (ELR) steepens (ie, drops more rapidly with height), it becomes more unstable. Heating air below, through contact with a hot surface or fire, will cause the ELR to steepen. Cooling air aloft by clouds radiating energy or cold advection will also increase instability.

12. Mixing as an Instability Trigger Winds that mix upper and lower atmospheric air will steepen the environmental lapse rate. Wind (forced convection) will warm descending and lower air while cooling rising and upper air.

13. Layer of Air Instability Lifting a stable layer of air causes it to stretch out vertically, with the top section experiencing a greater elevation change and more cooling than the lower section, even when both cool at the dry adiabatic rate.

14. Convective Instability & Storms Lifting a stable layer of air that is saturated below and unsaturated aloft will cause the moist air to cool more slowly, and create a very steep and unstable environmental lapse rate. Severe thunderstorms are associated with such processes.

15. Convection & Clouds Pockets of warm air rise as thermals with invisible water vapor, and at the dew point temperature condensation creates the cloud base. Rising air from below is replaced by sinking air from above, creating areas of blue sky.

16. Cumulus Clouds & Clear Sky

17. Development of a Cumulus Cloud The panels reveal vertical trends for temperature and moisture during cloud development. The level of free convection is where rising air becomes warmer than environmental air.

18. Stability & Cloud Height Environmental lapse rates (ELR) determine the stability, and as the time of day changes the ELR, the cloud profile can change.

19. Cumulus to Cumulonimbus

20. Topography & Clouds Winds blowing moist air toward a mountain will experience orographic uplift to an elevation where dew point is reached and clouds are formed. When the condensed moisture falls as rainfall, the leeward side of the mountain is kept in a rain shadow.

21. Mountain Wave Clouds As moist air condenses into clouds on the windward side of the mountain, sinking and swirling winds on the leeward side shape a lenticular mountain wave cloud.

22. Changing Cloud Forms Uniform winds may align a large region of cirrocumulus and stratocumulus clouds into rows of cloud streets.

23. Billow “KH” & Castellanus Clouds Rapid changes in wind speed with elevation can create wavelike billows at the cloud top, while conditionally unstable air above a cloud may cause lifting and condensation in the shape of castles.

24. Mixing & Stratocumulus When air is stable and close to saturation, then forced wind mixing of the atmosphere: a) steepens the environmental lapse rate b) moves moist low-level air aloft The cooler and moister air aloft then condenses into a deck of clouds.