Chapter 6 Cloud Development and Forms

1. Chapter 6 Cloud Development and Forms

2. Four mechanisms lift air so that condensation and cloud formation can occur: 1. Orographic lifting, the forcing of air above a mountain barrier 2. Frontal lifting, the displacement of one air mass over another 3. Convergence, the horizontal movement of air into an area at low levels 4. Localized convective lifting due to buoyancy

3. Orographic lifting The upward displacement of air that leads to adiabatic cooling is called orographic uplift (or the orographic effect). When air approaches a topographic barrier, it can be lifted upward or deflected around the barrier. Downwind of a mountain ridge, on its leeward side, air descends the slope and warms by compression to create a rain shadow effect, an area of lower precipitation.

4. Fronts Fronts are transition zones in which great temperature differences occur across relatively short distances. Air flow along frontal boundaries results in the widespread development of clouds in either of two ways. When cold air advances toward warmer air (cold front), the denser cold air displaces the lighter warm air ahead of it (a). When warm air flows toward a wedge of cold air (warm front), the warm air is forced upward in much the same way that the orographic effect causes air to rise above a mountain barrier (b).

5. Horizontal convergence Pressure differences set the air in motion in the effect we call wind. When a low-pressure cell is near the surface, winds in the lower atmosphere tend to converge on the center of the low from all directions. Horizontal movement toward a common location implies an accumulation of mass called horizontal convergence, or just convergence for short.

6. Convection Convection results from heating the air near the surface. The result is an updraft which is often strong enough to form clouds and precipitation.

7. Static stability The air’s susceptibility to uplift is called its static stability. Statically unstableair becomes buoyant when lifted and continues to rise if given an initial upward push; statically stable air resists upward displacement and sinks back to its original level when the lifting mechanism ceases. Statically neutralairneither rises on its own following an initial lift nor sinks back to its original level; it simply comes to rest at the height to which it was displaced.

8. Static stability The stability depends on the temperature of the lifted air parcel compared to its environment. Statically unstableis warmer than the environment after lifting; statically stable is colder than the environment; and statically neutralhas the same temperature as the environment.

9. Static stability The stability depends on the temperature of the lifted air parcel compared to its environment. Statically unstableis warmer than the environment after lifting; statically stable is colder than the environment; and statically neutralhas the same temperature as the environment. Adiabat z T Parcel curve

10. Static stability The stability depends on the temperature of the lifted air parcel compared to its environment. Statically unstableis warmer than the environment after lifting; statically stable is colder than the environment; and statically neutralhas the same temperature as the environment. Adiabat Unstable environment z T Parcel curve

11. Static stability The stability depends on the temperature of the lifted air parcel compared to its environment. Statically unstableis warmer than the environment after lifting; statically stable is colder than the environment; and statically neutralhas the same temperature as the environment. Adiabat Stable environment Unstable environment z T Parcel curve

12. Static stability: unsaturated air Dry and unsaturated air follows the dry adiabat, and stability is relative to DALR DALR Stable environment Unstable environment z T Parcel curve

13. Static stability: saturated air Saturated air follows the saturated adiabat, and stability is relative to SALR SALR Stable environment Unstable environment z T Parcel curve

14. Absolutely unstable air When a parcel of unsaturated or saturated air is lifted and the Environmental Lapse Rate (ELR) is greater than the dry adiabatic lapse rate (DALR), the result is absolutely unstable air.

15. Absolutely stable air When a parcel of unsaturated or saturated air is lifted and the Environmental Lapse Rate (ELR) is less than the saturated adiabatic lapse rate (SALR), the result is absolutely stable air and the parcel will resist lifting.

16. Conditionally unstable When the ELR is between the dry and saturated adiabatic lapse rates the air is said to be conditionally unstable, and the tendency for a lifted parcel to sink or continue rising depends on whether or not it becomes saturated and how far it is lifted. The level of free convectionis the height to which a parcel of air must be lifted for it to become buoyant and to rise on its own.

17. Assume the ELR is 0.7 °C/100 m and the air is unsaturated. As a parcel of air is lifted, its temperature is less than that of the surrounding air, so it has negative buoyancy.

18. A parcel starts off unsaturated but cools to the LCL, where it is cooler than the surrounding air. Further lifting cools the parcel at the SALR. At the 200-m level, it is still cooler than the surrounding air, but if taken to 300 m, it is warmer and buoyant. Tutorial ”Stability” Ch 6 (ed4)

19. The ELR can be changed by the advection of air with a different temperature aloft. In (a), the winds at the surface and the 100 m level bring in air with temperatures of 10 °C and 9.5 °C, respectively, yielding an ELR of 0.5 °C/100 m. In (b), the surface winds still bring in air with a temperature of 10 °C. The wind direction at the 100 m level has shifted to northeasterly, and the advected air has a temperature of 9.0 °C.

20. The ELR changes when a new air mass replaces one that has a different lapse rate. Location A has a steeper ELR than does B. As the air mass over Location A moves over B, it brings to that location the new temperature profile.

21. Air that is unstable at one level may be stable aloft. The solid line depicts a temperature profile that is unstable in the lowest 500 m but capped by an inversion. An unsaturated air parcel displaced upward would cool by the DALR (dashed line), making it initially warm and buoyant relative to the surrounding level. After penetrating the inversion layer, the rising air is no longer warmer than the surrounding air, and lifting is suppressed. The parcel continues upward due to its momentum. It cools more rapidly than the surrounding air and becomes relatively dense. After stopping, the air parcel sinks and eventually comes to rest at some equilibrium level.

22. Entrainment An air parcel has no barrier to prevent it from mixing with its surroundings. As air rises, considerable turbulence is generated, which causes ambient air to be drawn into the parcel. This process, called entrainment, is especially important along the edges of growing clouds. Entrainment suppresses the growth of clouds because it introduces unsaturated air into their margins and thus causes some of the liquid droplets to evaporate.

23. Inversions Situations in which the temperature increases with altitude are called inversions. Air parcels rising through inversions encounter ever-warmer surrounding air and have strong negative buoyancy. Inversions are extremely stable and resist vertical mixing. Radiation inversionsresult from cooling of the surface. Frontal inversions exist at the transition zone separating warm and cold air masses. Subsidence inversionsresult from sinking air. Frontal Inversion Subsidence Inversion

24. The Basic Cloud Types High clouds - cirrus, cirrostratus, and cirrocumulus Middle clouds -altostratus and altocumulus Low clouds -stratus, stratocumulus, and nimbostratus Clouds with vertical development - cumulus and cumulonimbus

25. High clouds are generally above 6000 m (19,000 ft). The simplest of the high clouds are cirrus, which are wispy aggregations of ice crystals.

26. Low clouds have bases below 2000 m. Stratus are layered clouds that form when extensive areas of stable air are lifted. Usually the rate of uplift producing a stratus cloud is only a few tens of centimeters per second, and its water content is low. Low, layered clouds that yield light precipitation are called nimbostratus. Seen from below, these clouds look very much like stratus, except for the presence of precipitation.

27. Stratocumulus are low, layered clouds with some vertical development. Their darkness varies when seen from below because their thickness varies across the cloud. Thicker sections appear dark, and thinner areas appear as bright spots.

28. Intensely developed clouds are cumulus congestus. They consist of multiple towers, and each tower has several cells of uplift. This gives them a fortress-like appearance with numerous columns of varying heights. Their strong vertical development implies that these clouds form in unstable air.

29. Cumulonimbus are the most violent of all clouds and produce the most intense thunderstorms. In warm, humid, and unstable air, they can have bases just a few hundred meters above the surface and tops extending into the lower stratosphere. A cumulonimbus is distinguished by the presence of an anvil composed entirely of ice crystals formed by the high winds of the lower stratosphere that extend the cloud forward.