Chapter 6 Cloud Development and Forms
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
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.
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).
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.
The air’s susceptibility to uplift is called its static stability. Statically unstable air 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 neutral air neither 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.
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.
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.
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 convection is the height to which a parcel of air must be lifted for it to become buoyant and to rise on its own.
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.
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.
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.
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.
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.
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.
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
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
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.
Cirrostratus clouds are composed entirely of ice but tend to be more extensive horizontally and have a lower concentration of crystals. When viewed through a layer of cirrostratus, the Moon or Sun has a whitish, milky appearance but a clear outline. A characteristic feature of cirrostratus clouds is the halo, a circular arc around the Sun or Moon formed by the refraction (bending) of light as it passes through the ice crystals.
Cirrocumulus are composed of ice crystals that arrange themselves into long rows of individual, puffy clouds. Cirrocumulus form during episodes of wind shear, a condition in which the wind speed and/or direction changes with height. Wind shear often occurs ahead of advancing storm systems, so cirrocumulus clouds are often a precursor to precipitation. Because of their resemblance to fish scales, cirrocumulus clouds are associated with the term “mackerel sky.”
Altostratus clouds are the middle-level counterparts to cirrostratus. They are more extensive and composed primarily of liquid water. Altostratus scatter a large proportion of incoming sunlight back to space. The insolation that does make its way to the surface consists primarily or exclusively as diffuse radiation. When viewing the Sun or Moon behind altostratus, one sees a bright spot behind the clouds instead of a halo.
Altocumulus are layered clouds that form long bands or contain a series of puffy clouds arranged in rows. They are often gray in color, although one part of the cloud may be darker than the rest and consist mainly of liquid droplets rather than ice crystals.
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.
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.
Cumuliform clouds are those that have substantial vertical development and occur when the air is absolutely or conditionally unstable. Fair-weather cumulus (above) called cumulus humilis, do not yield precipitation and they evaporate soon after formation.
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.
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.
An important characteristic of clouds is their breadth or coverage. When clouds occupy more than nine-tenths of the sky, conditions are said to be overcast. When coverage is between six-tenths and nine-tenths, it is called broken. Scattered clouds occupy between one-tenth and one-half of the sky, and less than one-tenth cloud cover is classified as a clear-sky condition.
The next chapter examines precipitation processes.