Chapter 6 Cloud Development and Forms
Four mechanisms lift air so that condensation and cloud formation can occur: • Orographic lifting, the forcing of air above a mountain barrier • Localized convective lifting due to buoyancy • 2. Frontal lifting, the displacement of one air mass over another • 3. Convergence, the horizontal movement of air into an area at low levels
If a parcel of air rises high enough and cools sufficiently, expansion lowers its temperature to the dew or frost point, and condensation or deposition commences. The altitude (m) at which this occurs is known as the lifting condensation level (LCL) The LCL is the level of the basis of clouds. The rate at which saturated air cools is the saturated adiabatic lapse rate (SALR), which is about 0.5 °C/100 m (3.3 °F/1000 ft).
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.
Important concepts to review • Adiabatic lifting (or adiabatic expansion) • Adiabatic sinking (or adiabatic compression) • Saturation mixing ratio, temperature, dew point and relative humidity • Stability of the atmosphere
Adiabatic lifting (adiabatic expansion) • Air parcel lifts due to increase in buoyancy (warmer and less dense than the surrounding environment) • Volume expands and work is done against the environment. • This occurs too fast to transfer or receive heat from the environment • Because no heat is transferred from or to the air parcel, volume increases and the air mass inside the air parcel cools down • This is known as adiabatic lifting H=400m T=26oC Dry adiabatic lapse rate: 10oC/km or 1oC/100m H=300m T=27oC H=200m, T=28oC H=100m, T=29oC H=0 T=30oC
Adiabatic compression H=400m T=26oC • Air parcel sinks • Volume decreases due to the work done by the environment • This occurs too fast to transfer or receive heat from the environment • Because no heat is transferred from or to the air parcel, volume decreases and the air mass inside the air parcel warms up • This is known as adiabatic compression or adiabatic sink Dry adiabatic lapse rate: 10oC/km or 1oC/100m H=300m T=27oC H=200m, T=28oC H=100m, T=29oC H=0 T=30oC
DALR=Dry adiabatic lapse rate LCL= Lifting Condensation Level Example Parcel is dry and compresses at DALR Lapse rate decreases: condensation warms the parcel 1 km LCL DALR=10oC/km
Impact on vegetation Yellow pine forest at 4,000' on west slope of Sierra Great Basin sage at 4,000' on floor of Owens Valley east of Sierra
Convection produced by surface heating Differences in surface characteristics result in differential heating Usually shows strong diurnal cycle peaking in mid-afternoon
Cumulus clouds develop in summer east of San Diego - combination of topography and heating 10 min later
Regional to Large Scale Fronts are transition zones in which great temperature differences occur across relatively short distances. 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).
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 airresists 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.
Note that in this case the air parcel is warmer than the environment and will continue to rise T=7 oC T=8 oC T=9 oC 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.
absolutely stable air 7o C 8oC 9oC 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.
Absolutely stable air 7oC 8oC 9oC 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.
Example: Stable atmosphere 9.3oC 8.9oC 8.8oC 10.0oC Temperature inversion V 7.0 C 8.0 C V 9.0 C V 10 C If Force parcel upward This mechanism may result in gravity waves
Gravity waves http://www.weathervortex.com/sky-ribbons.htm
If there is enough moisture in the atmosphere gravity waves can result in gravity wave clouds like that: http://www.weathervortex.com/sky-ribbons.htm
When the Environment Lapse Rate (ELR) is between the DALR (Dry Adiabatic Lapse Rate) and the SALR (Saturated Adiabatic Lapse Rate), the air is said to be conditionally unstable, and the tendency for a lifted parcel to sink or rise depends on whether 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.
Conditionally unstable 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.
On the other hand… Air that is unstable at one level may become 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 at the DALR, making it initially warm and buoyant. After penetrating the inversion layer, the rising air is no longer warmer than the surrounding air. The parcel cools more rapidly than the surrounding air and becomes relatively dense, sinks, and eventually comes to rest at an equilibrium level. Very important for the West coast
ENTRAINMENT An air parcel has no barrier to prevent it from mixing. As air rises, considerable turbulence is generated, which causes ambient air to be drawn into the parcel. This process, called entrainment, suppresses the growth of clouds by introducing unsaturated air into their margins causing some of the liquid droplets to evaporate. This typically occurs at the boundary of clouds where turbulence is more intense
INVERSIONS Situations in which the temperature increases with altitude are inversions. Air parcels rising through inversions encounter warmer surrounding air and have strong negative buoyancy. Inversions are extremely stable and resist vertical mixing. Radiation inversionsresult from cooling of the surface. Very common during the winter Clouds are trapped by the inversion layer
Frontal inversions exist at the transition zone separating warm and cold air masses. Frontal Inversion
Subsidence inversions result from sinking air: Very important in the West coast. Why??? Answer: High pressure systems are related to subsidence
SUBSIDENCE INVERSION VERY COMMON IN CA Sundowners and Santana Winds can cause subsidence inversions
Fog Fogis a cloud whose base is at or near ground level.
Radiation fog Radiation fog (ground fog) develops when the nighttime loss of longwave radiation causes cooling to the dew point.
Valley Fog Valley fog forms in mountain valleys, often during the fall, winter and early spring. It is the result of a temperature inversion caused by heavier cold air settling into in a valley, with warmer air passing over the mountains above. It is essentially radiation fog confined by local topography, and can last for several days in calm conditions.
Upslope fog forms when winds blow air up a slope. This flow upwards causes the warm air to cool as it rises, which causes the moisture in it to condense and form fog.
Advection fog Advection fog is created when moist air passes over a cool surface, and the air is cooled. As the air is cooled it reaches its relative humidity saturation point and fog is formed. This kind of fog is common as a warm front passes over an area with significant snow pack
Steam fog Evaporation or steam Fog is a specific type of advection fog. It occurs when you get cold air (1) advancing over warm water or warm (3), moist land surfaces. In this situation, fog forms as water from the surface evaporates into the cold air and then saturates (2) Source: http://www.physicalgeography.net/fundamentals/8f.html Evaportation fog forming over a lake as cool air flows over the warm water. (Source: NOAA Photo Collection Website)
Precipitation fog Precipitation fog forms from the evaporation of falling raindrops
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 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. This is a photograph of cirrostratus clouds near Moab, Utah.Click on image for full size version (66K JPG)Courtesy of Anne Pharamond
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 (midlevel clouds (2000-7000m up) . They are often gray in color, although one part of the cloud may be darker than the rest and consist mainly of liquid droplets.
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.
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 called cumulus humilis, do not yield precipitation and evaporate soon after formation.