1. CLOUDS AND TROPOPAUSE 3
1.1 CLASSIFICATION OF CLOUDS 3
1.2 THE HIGHT OF TROPOPAUSE 4
1.3 CLOUDS IN RELATION TO THE TROPOPAUSE 6
2. CUMULONIMBUS CLOUDS IN DIFFERENT SINOPYICAL SITUATIONS 8
2.1 CLOUDS OF VERTICAL DEVELOPMENT 8
2.2 THE NECESSARY INGREDIENTS FOR THUNDERSTORMS 10
2.3 LIFE CYCLE OF CB CLOUD 12
2.3.1 SINGLE CELL STORM. 13
126.96.36.199 DEVELOPING STAGE: 13
188.8.131.52 MATURE STAGE: 14
184.108.40.206 DISSIPATING STAGE: 14
2.4 THUNDERSTORM TYPES 14
2.4.1 MULTI CELLS STORM 14
2.4.2 SUPER CELL STORM 15
2.4.3 SQUALL LINES 16
2.4.4 WALL CLOUDS 17
2.4.5 TORNADOES AND FUNNEL CLOUD 17
3.CAUTIONS AND NOTIFICATIONS FOR AIRMAN 18
3.1 THE DANGERS OF FLYING IN OR CLOSE TO A THUNDERSTORM 18
3.2 THUNDERSTORM AVOIDANCE 24
3.3 INFORMATION FOR A PILOT 24
3.4 IN-FLIGHT ADVISORIES (WARNINGS) 25
The atmosphere/flight environment is forever in a state of constant physical change, giving rise to weather conditions, which vary throughout the range of an extremely large scale. The airman not only lives at the base of this sea of air, but also navigates and flies through it. The weather, therefore, is a matter of vital concern to him, particularly conditions such as fog, ice formation, thunderstorms line squalls, all of which presents particularly unusual hazards to flying.
To a pilot knowledgeable in the science of meteorology, clouds are an indication of what is happening in the atmosphere. The location and type of cloud are evidence of such weather phenomena such as fronts, turbulence, thunderstorms, and tell the pilot what type of conditions may be expected during flight.
1. CLOUDS AND TROPOPAUSE
Clouds are continuously in a process of change and appear, therefore in an infinite variety of forms. It is possible, however, to define a limited number of characteristic forms, frequently observed all over the world, into which clouds can be broadly grouped. A classification of the characteristic forms of clouds, in terms of „genera,“ „species“ and „verities“ has been established.
1.1 CLASSIFICATION OF CLOUDS
GENERA SPECIES VARIETY
Cirrus fibratus, uncinus, spissatus, castellanus, floccus intortus, radiatus, vertebrates, duplicatus
Cirrocumulus stratiformis, lenticularis, castellanus, floccus Undulates, lacunosus
Cirrostratus Fibratus, nebulosus Duplicatus, undulatus
Altocumulus Stratiformis, lenticularis, castellanus, floccus translucidus, perlucidus, opacus, duplicatus, undulatus, radiatus, lacunosus
Altostratus translucidus, opacus, duplicatus, undulates, radiatus
Stratocumulus Stratiformis, lenticularis, castellanus Translucidus, perlucidus, opacus, duplicatus, undulates, radiatus, lacunosus
Stratus Nebulosus, fractus Opacus, translucidus, undulatus
Cumulus Humulis, mediocris, congestus, fractus radiatus
Cumulonimbus Calvus, capillatus
ETAGE CLOUD GENERA HIGHT
Tropical Region Temperate Region Polar Region
High CirrusCirrostratusCirrocumulus 20,000-60,000 ft 16,000-45,000ft 10,000-26,000ft
Middle AltostratusAltocumulus 6,500-26,000 ft 6,500-23,000 ft 6,500-13,000ft
Low StratusStratocumulusNimbostratusCumulusCumulonimbus 0-6,500 ft 0-6,500 ft
By convention, the part of the atmosphere in which clouds are usually present has been vertically divided into three „étages“: high, middle and low. The base of the following cloud genera is normally found in the étage indicated:
Surface and aircraft observations have shown that clouds are generally encountered over a range of altitudes varying from sea level to the level of the tropopause, i.e. to 18 kilometers (60,000 feet) in the tropics, 13 kilometers (45,000 feet) in middle latitudes and 8 kilometers (26,000 feet) in polar regions.
1.2 THE HIGHT OF TROPOPAUSE
The height of the tropopause depends on the location, notably the latitude. It also depends on the season. Thus, it is about 16 km high over Australia at year-end, and between 12 – 16 km at midyear, being lower at the higher latitudes. At latitudes above 60, the tropopause is less than 9 -10 km above sea level; the lowest is less than 8 km high, above Antarctica and above Siberia and northern Canada in winter. The highest average tropopause is over the oceanic warm pool of the western equatorial Pacific; about 17.5 km high, and over Southeast Asia, during the summer monsoon, the tropopause occasionally peaks above 18 km. In other words, cold conditions lead to a lower tropopause, obviously because of less convection. Deep convection (thunderstorms) in the Intertropical Convergence Zone, or over mid-latitude continents in summer, continuously push the tropopause upwards and as such deepen the troposphere. This is because thunderstorms mix the tropospheric air at a moist adiabatic lapse rate.In the upper troposphere, this lapse rate is essentially the same as the dry adiabatic rate of 10K/km. So a deepening by 1 km reduces the tropopause temperature by 10K. Therefore, in areas where (or at times when) the tropopause is exceptionally high, the tropopause temperature is also very low, sometimes below -80° C. Such low temperatures are not found anywhere else in the Earth’s atmosphere, at any level, except in the winter stratosphere over Antarctica.
On the other hand, colder regions have a lower tropopause, obviously because convective overturning is limited there, due to the negative radiation balance at the
surface. In fact, convection is very rare in Polar Regions; most of the tropospheric mixing at middle and high latitudes is forced by frontal systems in which uplift is forced rather than spontaneous (convective). This explains the paradox that tropopause temperatures are lowest where the surface temperatures are highest.
The tropopause height does not gradually drop from low to high latitudes. Rather, it drops rapidly in the area of the subtropical and polar front jets. Especially when the jet is strong and the associated front at low levels intense, then the tropopause height drops suddenly across the jet stream. Sometimes the tropopause actually folds down to 500 hPa (5.5 km) and even lower, just behind a well-defined cold front. The subsided stratospheric air within such a tropopause fold (or in the less pronounced tropopause dip) is much warmer than the tropospheric air it replaces, at the same level, and this warm advection aloft (around 300 hPa) largely explains the movement of the frontal low (at the surface) into the cold air mass, a process called occlusion.
1.3 CLOUDS IN RELATION TO THE TROPOPAUSE
Experiences of pilots have confirmed that the tops of most cirrus clouds are at or below the tropopause. In the midlatitudes, the tops of most cirrus cloud layers are at or within several thousand feet of the polar tropopause. Patchy cirrus clouds are found between the polar tropopause and the tropical tropopause. A small percentage of cirrus clouds, and sometimes-extensive cirrostratus, may be observed in the lower stratosphere above the polar tropopause, but mainly below the level of the jet stream core. The cirrus clouds of the equatorial zone also generally extend to the tropopause. There is a general tendency for the mean height of the bases to increase from high to low latitudes more or less paralleling the mean tropopause height, ranging from 24,000 feet at 70°to 80°atitude to 35,000 to 4,000 feet or higher in the vicinity of the equator. The thickness of individual cirrus cloud layers is generally about 800 feet in the midlatitudes. The mean thickness of cirrus clouds tends to increase from high to low latitudes. In polar continental regions in winter, cirrus clouds are virtually based at the surface. In the midlatitudes and in the tropics, there is little seasonal variation.
Jet Stream Cirrus
This photograph taken from about 320 kilometers (200 miles) above the Earth shows a band of cirrus clouds produced by a westerly jet stream that stretches across the Red Sea from Sudan to Saudi Arabia. The contained uniformity of the cloud formation reflects the narrow track of the jet stream moving from left to right across the frame. The shuttle photo shows that the cloud band comprises a series of distinct and precisely spaced roll clouds. A rolling motion creates these in the upper level air current.
Florida Squall Line
This spectacular, low-oblique photograph shows a convective line of thunderstorms associated with a passing cold front over Florida. A shadow from the height of the thunderstorms, caused by early morning sunlight, can be seen traversing the scene southwest to northeast. The clouds in the storm system rise to about 16,500 meters (55,000 feet). The V-shaped cloud structure is normally associated with cold fronts that cross the Gulf of Mexico and Florida in late winter and early spring. Severe thunderstorms and tornadoes usually occur with this type of storm system. At the time this photograph was taken, weather stations across Florida reported severe thunderstorms, strong winds, hail, torrential rains, and numerous tornadoes
These are cumulus thunderheads near Sao Paulo, Brazil. This perspective conveys something of the energy that drives these cloud columns to punch up into the atmosphere. The tops of massive thunderhead storm clouds can tower up to 18,000 meters (60,000 feet) in the tropics.
Cumulus Cloud Tops
This oblique photograph, acquired in February 1984 by an astronaut aboard the space shuttle, shows a series of mature thunderstorms located near the Parana River in southern Brazil. With abundant warm temperatures and moisture-laden air in this part of Brazil, large thunderstorms are commonplace. A number of overshooting tops and anvil clouds are visible at the tops of the clouds. When the rising cumulus columns meet the tropopause, or base of the stratosphere, at about 15,000 kilometers (50,000 feet), they reach a ceiling and can no longer rise buoyantly by convection. The stable temperature of the stratosphere suppresses further adiabatic ascent of moisture that has been driven through the troposphere by the 5-6.8 degree/kilometer (8-11 degree/mile) lapse rate. Instead, ice clouds spread horizontally into the extended cirrus heads seen in this photograph, forming the „anvil heads“ that we identify from the ground. The finer, feathery development around the edges of some of the thunderheads is glaciations – water vapor in the cloud is turning to ice at high altitude. Storms of this magnitude can drop large amounts of rainfall in a short period of time, causing flash floods.
The STS 41-B crew photographed this pinwheel of anticyclone clouds over the southern hemisphere of the Pacific Ocean. The ground winds at the center of this cyclonic system reach 80 kilometers per hour (50 miles an hour). Circular storms in the northern hemisphere produce spiraling clouds with a clockwise pattern,
southern latitude storms have a counterclockwise cloud motion.
Eye of Typhoon Yuri
This photograph shows the bowl-shaped eye (center of photograph) of Typhoon Yuri in the western Pacific Ocean just west of the Northern Mariana Islands. The eye wall descends almost to the sea surface, a distance of nearly 45,000 feet (13,800 meters).
Finally, there exists a group of clouds, rarely or occasionally observed, not included in the main classification.
Some of these „special clouds“ consist for the greater part or in their entirety of non-aqueous liquid or solid particles. Included in the „special“ clouds are the nacreous clouds, which by day resemble pale cirrus, but after sunset, are characterized by brilliant colors. They occur at altitudes between 21 and 30 kilometers (70,000 and 100,000 feet). The physical constitution of nacreous clouds is still unknown. Measurements have shown that their altitude ranges from 75 to 90 kilometers (250,000 to 300,000 feet). Their physical composition is also unknown, but they are believed to be composed of fine, cosmic dust particles possibly with a thin, outer layer of ice. Noctilucent clouds become visible after sunset. Other special clouds include clouds from fires produced by the fine combustion products. These may appear as dark, cumulus or cumulonimbus clouds but usually are rapidly dispersed and carried great distances by the wind, spreading to resemble thin, stratiform veils. Clouds from volcanic eruptions, explosions, and industrial activities are also considered in the „special cloud“ category.
2. CUMULONIMBUS CLOUDS IN DIFFERENT SINOPYICAL SITUATIONS
2.1 CLOUDS OF VERTICAL DEVELOPMENT
The bases of this type of cloud may form as low as 1500 feet. They are composed of water droplets when the temperature is above freezing and of ice crystals and super cooled water droplets when the temperature is below freezing.
Cumulus (Cu). Dense clouds of vertical development. They are thick, rounded and lumpy and resemble cotton balls. They usually have flat bases and the tops are rounded. They cast dense shadows and appear in great abundance during the warm part of the day and dissipate at night. When these clouds are composed of ragged fragments, they are called cumulus fractus.
Towering Cumulus (TCu). Cumulus clouds that build up into high towering masses. They are likely to develop into cumulonimbus. Rough air will be encountered underneath this cloud. Heavy icing may occur in this cloud type.
Cumulonimbus (Cb). These clouds are much larger and more vertically developed than cumulus clouds, which form, in a more stable atmosphere. They can exist as individual towers or form a line of towers called a squall line often present at cold fronts. Underneath they are dark and menacing. At a distance they rise up like huge White Mountains when the Sun shines on them, and are commonly topped with anvil-shaped heads.
To introduce and discuss the importance of cumulonimbus clouds, one must understand the many processes and stages of cumulonimbus formation. The first stage or process involves the production of showers within these clouds. Next, the vertical development, which describes the rising towers to the tropopause, will be discussed. Precipitation separates cumulonimbus clouds from other cumulus clouds. The cumulonimbus can be associated with severe weather, which includes, tornadoes, waterspouts, and funnel clouds. Brief to prolonged heavy rain, hail, sleet, and flash floods are common characteristics of cumulonimbus weather patterns. Strong winds and lightning accompany these clouds to create a very severe storm system The visual aspect of shower formation frequently shows that it appears that showers fall only from clouds whose top extends to a particular height level. The generated rains are functions of height above the base and depend on certain properties such as condensed water concentration and updraft speed. The cloud depth required varies from day to day, but is often well defined.The maritime air only requires heights of less than 2 km. This takes places so that the tops are below the 0 C level. Inland, where the surface is warmer, the required depths increase to allow for below freezing temperatures needed in the cloud tops. The first visible signs of shower development in large cumulus can often be detected by observing the dissolution of the cloud towers. Radar observations of shower formations provide valuable information. It shows cloud location, evolution of size distributions and phase of the cloud particles.
The appearance of the ice phase in cumulus above the 0 deg C level comprises a considerable fraction of the cloud water. This period is very important in the formation of a shower. A cloud whose top is above the active ice nuclei formation level of about –10 deg C can be expected to produce showers regardless of the coalescence of the cloud droplets.