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
184.108.40.206 DEVELOPING STAGE: 13
220.127.116.11 MATURE STAGE: 14
18.104.22.168 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 va ary 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 an nd 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, th
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 th hree “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 de
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 th
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 th
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, causin.g 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, while 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.
Buoyancy increase in cumulus towers three ways: the freezing of liquid cloud water, release of latent heat, and precipitation of cloud water. The increase in size of a large cumulus cloud caused by convection over land rarely is steady. As the day passes, the cloud surges at a variable rate. The tallest cumulus rise between 1 to 2 km/hr and showers do not occur until near or after midday. The cloud group reaches tropopause and spread predominantly to one side producing the anvil of the cumulonimbus. Thunderstorms have been accurately measured as high as 67,000 feet and some severe thunderstorms attain an even greater height. More often the maximum height is from 40,000 to 45,000 feet. In general, air-mass thunderstorms exten.d to greater heights than do frontal storms. Rising and descending drafts of air are, in effect, the structural bases of the thunderstorm cell. A draft is a large-scale vertical current of air that is continuous over many thousands of feet of altitude. Downdraft speeds are either relatively constant or gradually varying from one altitude to the next. Gusts, on the other hand, are smaller scaled discontinuities associated with the draft proper. A draft maybe compared to a great river flowing at a fairly constant rate, whereas a gust is comparable to an eddy or any other random motion of water within the main current.2.2 THE NECESSARY INGREDIENTS FOR THUNDERSTORMS
Every thunderstorm needs three ingredients:
· Moisture – to form clouds and rain
· Instability – relatively warm air that can rise rapidly
· A lifting mechanism- fronts, sea breezes, and mountains are capable of lifting air to form thunderstorms.
· Sources of moisture
Typical sources of moisture are large bodies of water such as the Atlantic and Pacific oceans as well as the Gulf of Mexico. Winds bring moisture from the ocean over the land area.lift is provided by approaching cooler, drier, more dense air (a cold front)
An unstable air mass is characterized by warm moist air near the surface and cold dry air aloft. In these situations, if a bubble or parcel of air is forced upward it will continue to rise on its own. As it raises it cools and some of the water vapor will condense, forming the familiar tall cumulonimbus cloud that is the thunderstorm. Characteristics of an unstable air mass with warm moist air near the surface with colder and drier air aloft. Air that is forced upward will continue to rise, and air that is forced downward will continue to sink.
Sources of Lift (upward)
Typically, for a thunderstorm to develop, there needs to be a mechanism, which initiates the upward motion, something that will give the air a nudge upward. This is done by several methods.
This heating of the ground and lower atmosphere is not uniform. For example, a grassy field will heat at a slower rate than a paved street. The warmest air, called thermals, tends to rise.
Fronts, Dry lines and Outflow Boundaries
Fronts are the boundary between two air masses of different temperatures. Fronts lift warm moist air. Cold fronts lift air the most abruptly. The cold-front thunderstorm is caused by the forward motion of a wedge of cold air into a body of warm, moist unstable air. Cold-front storms are normally positioned aloft along the frontal surface in what appears to be a continuous line. Under special atmospheric conditions, a line of thunderstorms develops ahead of a cold front. This line of thunderstorms is the prefrontal squall line. Its distance ahead of the front ranges from 50 to 300 miles. Prefrontal thunderstorms are usually intense and appear menacing. Bases of the clouds are very low. Tornadoes sometimes occur when this type of activity is present.
The warm-front thunderstorm is caused when warm, moist, unstable air is forced aloft over a colder, denser shelf of retreating air. Warm-front thunderstorms are generally scattered; they are usually difficult to identify because other clouds obscure them
Dry lines are the boundary between two air masses of different moisture content and separate warm moist air from hot dry air. While the temperature may be different across the dry line, the main difference is the rapid decrease in moisture behind the dry line.
It is the lack of moisture, which allows the temperatures to occasionally be higher than ahead of the dry line. However, the result is the same as the warm moist air is lifted along the dry line forming thunderstorms. This is common over the plains in the spring and early summer.
Outflow boundaries are a result of the rush of cold air as a thunderstorm moves overhead. The rain-cooled air acts as a “mini cold front”, called an outflow boundary. Like fronts, this boundary lifts warm moist air and can cause new thunderstorms to form.
As air encounters a mountain it is forced up the slope of the terrain. Upslope thunderstorms are common in the Rocky Mountain west during the summer.
2.3 LIFE CYCLE OF THE CLOUD
The building block of all thunderstorms is the thunderstorm cell. The thunderstorm cell has a distinct life cycle that lasts about 30 minutes.
2.3.1 Single cell.
The life cycle of a single cell can be separated into three stages:
· Developing stage
· Mature stage
· Dissipating stage22.214.171.124 DEVELOPING STAGE:
Every thunderstorm begins life as a cumulus cloud. The developing stage lasts 5 to 10 minutes. A cumulus cloud begins to grow vertically, perhaps to a height of 40,000 to 60,000 feet. The cloud starts growing upward, driven by the latent heat as water vapor condenses. Strong updrafts prevail throughout the cell and it rapidly builds up into a towering cumulonimbus cloud. The diameter of the cell is between 2 and 8 km. Temperatures within the cell are higher than temperatures at the same level in the surrounding air, intensifying still more the convective currents within the cell. There is usually no precipitation from the storm at this stage of its development since the water droplets and ice crystals are being carried upwards or are kept suspended by the strong updrafts.126.96.36.199 MATURE STAGE:
The mature stage typically lasts between 25 and 30 minutes. During this stage downdrafts develop, and after some time strong updrafts can be found at the leading part and pronounced downdrafts at the rear part of the Cb. The development of downdrafts is a consequence of the cooling by evaporation of the cloud droplets due to entrainment, as well as the falling rain droplets, which accelerate the downdraft even more.
Therefore the strongest downdraft can be found within the lower layers of the Cb. At the same time the updraft steadily weakens. The air beneath the cloud base is saturated due to the evaporation of the rain droplets. The downward transported air of the downdraft spreads horizontally after it reaches the surface, which may lead to the development of squall lines. The end of the mature stage is reached when the out flowing dry air cuts off the incoming supply of moist air. During the mature stage of the Cb the weather events are most intense. Although each cell may last only 20 minutes, the cluster may last several hours. These can produce heavy rainfall, downbursts, moderate-sized hail, and occasional weak tornadoes.188.8.131.52 DISSIPATING STAGE:
The last stage, the dissipating stage, of a Cb is reached when the updrafts in the lower levels of the Cb are replaced by downdrafts. The Cb dissipates, as there is no supply of warm moist air for the updraft.2.4 THUNDERSTORM TYPES
2.4.1 Multi-cells Storms
Although there are times when a thunderstorm consists of just one ordinary cell that transitions through its life cycle and dissipates without additional new cell formation, thunderstorms often form in clusters with numerous cells in various stages of development merging together.
Unlike ordinary single cells, cluster storms can last for several hours producing large hail, damaging winds, flash flooding, and isolated tornados. However this kind of thunderstorm has a very long life span due to the continuous development of new cells, so-called daughter cells. In most cases these daughter cells develop in the right leading part, but sometimes they can also develop on the left side. According to studies, every 5 to 10 minutes such a daughter cell develops. These new cells have a diameter between 3 and 5 km, and the distance to the center of the thunderstorm is approximately 30 km. The daughter cells develop very rapidly and after a short time become the new center of the Multi-Cell Storm (mother cell). This rapid development takes place because the daughter cells develop immediately in front of the mother cell; therefore no kinetic energy is taken away from the cloud. Although the older cells dissolve at the rear part of the complex, the Multi-Cell Storm is still active due to the continuous new development of daughter cells. Investigations have shown that during the life span of a Multi-Cell Storm 30 or more cells can develop. If the sequence of the cells is short the Multi-Cell Storm can change to a Super Cell Storm.
The diagram shows that the daughter cell (n) develops from the so-called shelf cloud (n+1). This lasts approximately 15 minutes. After an additional 15 minutes the daughter cell becomes the center of the Multi-Cell Storm. The center of the storm is shown with the cell (n-1). The cell is now in its mature stage and strong up and downdrafts can be found. 15 minutes later the cell has finished its mature stage and dissolves at the rear part of the Multi-Cell Storm (n-2).
2.4.2 The Super cell Storm
Super cell thunderstorms are a special kind of single cell thunderstorm that can persist for many hours. Super cells are also known to produce extreme winds and flash flooding. They are characterized by a rotating updraft (usually cyclonic), which results from a storm growing in an environment of significant vertical wind shear.
Wind shear occurs when the winds are changing direction and increasing with height.
The most ideal conditions for super cells occur when the winds are veering or turning clockwise with height. For example, in a veering wind situation the winds may be from the south at the surface and from the west at 15,000 feet. Beneath the super cell, the rotation of the storm is often visible as well. This rotating updraft is called a mesocyclone.
The lowering in the photograph (bottom) represents the wall cloud.2.4.3 Squall Lines
Sometimes thunderstorms will form in a line, which can extend laterally for hundreds of miles. These “squall lines” can persist for many hours and produce damaging winds and hail.
The rain cooled air or “gust front” spreading out from underneath the squall line acts as a mini cold front, continually lifting warm moist air to fuel the storms.
Often along the leading edge of the line a low hanging arc of cloudiness will form called the shelf cloud. Gusty, sometimes damaging outflow winds will spread out horizontally along the ground behind the shelf cloud.2.4.4 Wall Clouds
Wall clouds are a visible manifestation of the mesocyclone at low levels, another words, the wall cloud contains significant rotation The tornado often forms from within the wall cloud, with the funnel cloud descending to the ground2.4.5 Tornadoes and funnel cloud
A tornado is a vortex of rapidly moving air associated with some severe thunderstorms Tornadoes that travel across lakes or oceans are called waterspouts. Winds within the tornado funnel may exceed 500 kilometers per hour. High velocity winds cause most of the damage associated with these weather events.
Tornadoes also cause damage through air pressure reductions. The air pressure at the tornado center is approximately 800 millibars (average sea-level pressure is 1013 millibars). The destructive path of a tornado is usually about half a kilometer wide, and usually no more than 25 kilometers long.3.CAUTIONS AND NOTIFICATIONS FOR AIRMAN
3.1 THE DANGERS OF FLYING IN OR CLOSE TO A THUNDERSTORM
Visual observations of the weather within the thundercloud from aircraft are difficult because of the speed with which they pass through the thunderclouds, and man has yet to devise an instrument that will measure all hydrometers in the cloud.
As you all know AIRSPEED = GROUND SPEED + WIND SPEED
So it is obviously that a sudden change of wind speed can change the airspeed and therefore, change the amount of lift keeping the airplane in the air. If the airplane is going slow, like on take off or landing, this can be very dangerous. Most of the Microburst incidents happen in a thunderstorm. An aircraft flying through a downburst may experience the following sequence of events. As the aircraft enters the edge of the downburst, it encounters an increased headwind. This headwind increases the lift of the aircraft and therefore the altitude of the aircraft. The flight crew may attempt to correct the altitude increase with a decrease in engine power or increase in descent angle.
Next, the aircraft passes through the microburst core where it encounters an abrupt change from headwinds to down flow winds, which results in a loss of lift and altitude. Soon afterward, the aircraft crosses into a region of tailwinds. This additional wind change further decreases lift, causing the aircraft to lose more altitude.
Altitude loss is even greater if flight corrections to compensate for the initial headwind are still taking place. If the wind shear encounter began at a low altitude, such as it would during a takeoff or landing, the overall loss of altitude may cause the aircraft to fly directly into the ground with catastrophic results.
Turbulence. Turbulence, associated with thunderstorms, can be extremely hazardous, having the potential to cause overstressing of the aircraft or loss of control. Thunderstorm vertical currents may be strong enough to displace an aircraft up or down vertically as much as 2000 to 6000 feet. The greatest turbulence occurs in the vicinity of adjacent rising and descending drafts. Gust loads can be severe enough to stall an aircraft flying at rough air (maneuvering) speed or to cripple it at design cruising speed.
Maximum turbulence usually occurs near the mid-level of the storm, between 12,000 and 20,000 feet and is most severe in clouds of the greatest vertical development.
Severe turbulence is present not just within the cloud. It can be expected up to 20 miles from severe thunderstorms and will be greater downwind than into wind. Severe turbulence and strong out-flowing winds may also be present beneath a thunderstorm. Micro bursts can be especially hazardous because of the severe wind shear associated with them.
The turbulence associated with clouds types is:
· St – slight
· Ci, Cs, Cc, Ac, As – nil or slight except when Ac cas or when merging into Cb
· Sc – moderate
· Ns – moderate but may be severe near base
· Cu, TCu, and Cb – Generally severe but may be catastrophic and include the downbursts
Lightning. Static electricity may build up in the airframe, interfering with operation of the radio and affecting the behavior of the compass. Trailing antennas should be wound in. Lightning blindness. may affect the crew’s vision for 30 to 50 seconds at a time, making instrument reading impossible during that brief period. Lightning strikes of aircraft are not uncommon.
The probability of a lightning strike is greatest when the temperature is between -5ºC and 5°C. If the airplane is in close proximity to a thunderstorm, a lightning strike can happen even though the aircraft is flying in clear air. Lightning strikes pose special hazards. Structural damage is possible. The possibility of lightning igniting the fuel vapor in the fuel cells is also considered a potential hazard.
The lightning first occurs between the upper positive charge area and the negative charge area immediately below it. Lightning discharges are considered to occur most frequently in the area bracketed roughly by the 3.2°F and the 15°F temperature levels. However, this does not mean that all discharges are confined to this region; as the thunderstorm develops, lightning discharges may occur in other areas and from cloud to cloud, as well as from cloud to ground.
St. Elmo’s fire. If an airplane flies through clouds in which positive charges have been separated from negative charges, it may pick up some of the cloud’s overload of positive charges. Weird flames may appear along the wings and around the propeller tips. These are called St. Elmo’s fire. They are awe-inspiring but harmless. It the airplane flies in the vicinity of a cloud where negative charges are concentrated, its positive overload may discharge into the cloud. In this case, it is the airplane, which strikes the cloud with lightning! The electricity discharges cause a noisy disturbance in the lower frequency radio bands but do not interfere with the very high frequencies. This precipitation static, as it is called, tends to be most severe near the freezing level and where turbulence and up and down drafts occur.
Hail. Hailstones are capable of inflicting serious damage to an airplane. Hail is encountered at levels between 10000 and 30000 feet. It is, on occasion, also encountered in clear air outside the cloud as especially active cells throw it upward and outward.
Icing. In a thunderstorm is encountered at or above the freezing level in the areas of heaviest turbulence during the mature stage of the storm. The altitudes within a few thousand feet of the freezing level, above or below, are especially dangerous.
Pressure. Rapid changes in barometric pressure associated with the storm cause altimeter readings to become very unreliable. The barometric pressure ahead of a thunderstorm falls abruptly as the storm approaches then rises quickly when the rain comes, and returns to normal when the storm subsides. Occasionally after a storm, the pressure falls below normal. Then rises to near normal again. All this can happen in a matter of 10 to 15 minutes.
Rain. The thunderstorm contains vast amounts of liquid water droplets suspended or carried aloft by the updrafts. This water can be as damaging as hail to an aircraft penetrating the thunderstorm at high speed. The heavy rain showers associated with thunderstorms encountered during approach and landing can reduce visibility and cause retraction on the windscreen of the aircraft, producing an illusion that the runway threshold is lower than it actually is. Water lying on the runway can cause hydroplaning, which destroys the braking action needed to bring the aircraft to a stop within the confines of the airport runway. Hydroplaning can also lead to loss of control during take-off.
Severe Thunderstorms are defined as convective storms with frequent lighting, accompanied by local wind gusts of 97 kilometers per hour, or hail that is 2 centimeters in diameter or larger. Severe thunderstorms can also have tornadoes. Movement of the severe storm is usually caused by the presence of a mid-latitude cyclone cold front or a dry line some 100 to 300 kilometers ahead of a cold front. In the spring and early summer, frontal cyclones are common weather events that move from west to east in the mid-latitudes.
At the same time, the ground surface in the mid-latitudes is receiving elevated levels of insulation, which creates ideal conditions for air mass thunderstorm formation. When the cold front or dry line of a frontal cyclone comes in contact with this warm air it pushes it like a bulldozer both horizontally and vertically.
If this air has a high humidity and extends some distance to the east, the movement of the mid-latitude cyclone enhances vertical uplift in storm and keeps the thunderstorms supplied with moisture and energy. Thus, the mid-latitude cyclone converts air mass thunderstorms into severe thunderstorms that last for many hours. Severe thunderstorms usually extend all the way to the tropopause and can be seen in satellit.e imagery as storms producing overshooting tops.
Severe thunderstorms dissipate only when no more warm moist air is encountered. This condition occurs several hours after nightfall when the atmosphere begins to cool off.3.2 THUNDERSTORM AVOIDANCE
Because of the severe hazards enumerated above, attempting to penetrate a thunderstorm is asking for trouble. In the case of flight, airplane pilots, the best advice on how to fly through a thunderstorm is summed up in one word—DON’T.
Detour around storms as early as possible when encountering them enroute. Stay at least 5 miles away from a thunderstorm with large overhanging areas because of the danger of encountering hail. Stay even farther away from a thunderstorm identified as very severe as turbulence may be encountered as much as 15 or more nautical miles away. Vivid and frequent lightning indicates the probability of a severe thunderstorm.
Any thunderstorm with tops at 35,000 feet or higher should be regarded as extremely hazardous. Avoid landing or taking off at any airport in close proximity to an approaching thunderstorm or squall line.
Do not fly under a thunderstorm even if you can see through to the other side, since turbulence may be severe. Especially, do not attempt to fly underneath a thunderstorm formed by orographic lift. The wind flow that is responsible for the formation of the thunderstorm is likely to create dangerous up and down drafts and turbulence between the mountain peaks.
Reduce airspeed to maneuvering speed when in the vicinity of a thunderstorm or at the first indication of turbulence.
Do not fly into a cloud mass containing scattered embedded thunderstorms unless you have airborne radar.
Do not attempt to go through a narrow clear space between two thunderstorms. The turbulence there may be more severe than through the storms themselves. If the clear space is several miles in width, however, it may be safe to attempt to fly through the center, but always go through at the highest possible altitude. When flying around a thunderstorm, it is better to fly around the right side of it. The wind circulates anti-clockwise and you will get more favorable winds. If circumstances are such that you must penetrate a thunderstorm, the following few simple rules may help you to survive the ordeal:
1) Go straight through a front, not across it, so that you will get through the storm in the minimum amount of time.
2) Hold a reasonably constant heading that will get you through the storm cell in the shortest possible time.
3) Before entering the storm, reduce the airspeed to the airplane’s maneuvering airspeed to minimize structural stresses.
4) Turn the cockpit lights full bright. (This helps to minimize the risk of lightning blindness.) Check the pilot head. Fasten seat belts. Secure loose objects in the cabin.
5) Try to maintain a constant attitude and power setting. (Vertical drafts past the pitot head and clogging by rain cause erratic airspeed readings.)
6) Avoid unnecessary maneuvering (to prevent adding maneuver loads to those already imposed by turbulence).
7) Determine the freezing level and avoid the icing zone. Avoid dark areas of the cell and, at night, those areas of heavy lightning.
8) Do not use the autopilot. It is a constant altitude device and will dive the airplane to compensate for updrafts, causing excessive airspeed, or will cause the plane to climb in a downdraft creating the risk of a stall.3.3 INFORMATION FOR A PILOT
To minimize the hazards to air navigation that are constantly being manufactured in the so-called weather factory, a vast world-wide meteorological organization has been built up, to collect, analyze and broadcast information relative to the ever changing flight environment or the upper air. There are some instruments and ways to measure the weather: weather radar, weather satellites, airplanes that gather data, computers information analyses.
Airborne weather radar is one of the best instrument aids that a pilot can have in locating and avoiding thunderstorms. It is able to detect and display on the cockpit radar screen any significant weather that lies ahead on the flight route.
. DOPPLER RADAR is a key forecasting tool
All weather radars send out radio waves from an antenna. Objects in the air, such as rain drops, snow crystals, hail stones or even insects and dust, scatter or reflect some of the radio waves back to the antenna. All weather radars, including Nexrad, electronically convert the reflected radio waves into pictures showing the location and intensity of precipitation.
Doppler radars also measure the frequency change in returning radio waves. Waves reflected by something moving away from the antenna change to a lower frequency. Waves from an object moving toward the antenna change to a higher frequency.
The computer that’s a part of a Doppler radar uses the frequency changes to show directions and speeds of the winds blowing around the raindrops, insects and other objects that reflected the radio waves. Scientists and forecasters have learned how to use these pictures of wind motions in storms, or even in clear air, to more clearly understand what’s happening now and what’s likely to happen in the next hour or two.
METAR is routine weather report issued at hourly or half-hourly intervals. It is a description of the meteorological elements observed at an airport at a specific time.
Example: METAR KPIT 091955Z COR 22015G25KT 3/4SM R28L/2600FT TSRA OVC010CB 18/16 A2992 RMK SLP045 T01820159
SPECI is special weather report issued when there is significant deterioration or improvement in airport weather conditions, such as significant changes of surface winds, visibility, cloud base height and occurrence of severe weather. The format of the SPECI report is similar to that of the METAR and the elements used have the same meaning. The identifier METAR or SPECI at the beginning of the weather report differentiates them.
A METAR/SPECI report usually contains the following information:
Originating airport name, METAR/SPECI issue time, wind direction/speed/gust, wind direction variation,
visibility, runway visual range, weather during time of observation, cloud, air temperature/dew-point, QNH (pressure measured at airport with adjustment made to suit aeronautical use), weather during the past hour but not at time of observation, wind shear information, trend-type landing forecast.
Example: EYVI 251730 120V18009KT 3000 TSSHRA SCT008CB 14/12 Q1012=
A TAF conveys the following meteorological information:
Originating airport name, TAF validity period, wind direction/speed/gust, visibility, weather, cloud, icing, turbulence, significant change of weather elements, maximum and minimum temperature.
When forecast temperatures are included in accordance with regional air navigation agreement, the maximum and minimum temperatures expected to occur during the period of validity of the aerodrome forecast should be given, together with their corresponding times of occurrence. An amended forecast may be issued subject to certain criteria and is prefixed by the keyword TAF AMD.
TAF VHHH 160400Z 160606 13018KT 9000 BKN020 BECMG 0608
SCT015CB BKN020 TEMPO 0812 17025G40KT 1000 TSRA SCT010CB
BKN020 FM1200 15015KT 9999 NSW BKN020 BKN100 TN26/22Z TX30/06Z=3.4 IN-FLIGHT ADVISORIES (WARNINGS)
Warn pilots of potentially hazardous weather. They include SIGMETs, CONVECTIVE SIGMETs, AIRMETs, and Center Weather Advisories (CWA). SIGMETs warn of hazardous conditions of importance to all aircraft i.e. severe icing or turbulence, dust storms, sandstorms, and volcanic ash. AIRMETs warn of less severe conditions which may be hazardous to some aircraft or pilots. SIGMETs
are issued as needed. AIRMET bulletins are issued routinely and supplement the Area Forecast (FA). CONVECTIVE SIGMETs are issued hourly for thunderstorms in the continuous U.S. Center Weather Advisories, issued as needed, are detailed advisories of conditions which meet or approach SIGMET or AIRMET criteria.
LFFF SIGMET SST 5 VALID 240900/241100 LFPW
FRANCE UIR ISOL TO OCNL CB OBS/FCST TOP BLW FL380 ON FIR
MARSEILLE S OF 45N MOV N SLW NC=
LFFF SIGMET SST 4 VALID 240700/240900 LFPW
FRANCE UIR ISOL TO OCNL CB OBS/FORCS TOP BLW FL/380 ON FIR
MARSILLE S OF 45N MOV N NC=
GAMET is used to report forecasted or actual en-route weather phenomena that may affect aircraft in flight. An AIRMET (AIRman’s METeorological Information) is used to report forecasted or actual en-route weather phenomena for aircraft flying at low altitude, typically under 100 meters and Visual Flight Rule (VFR) pilots. GAMET and AIRMET are sold as optional modules.
There are three AIRMETs – Sierra, Tango, and Zulu.
AIRMET Sierra describes IFR conditions and/or extensive mountain obscurations.
AIRMET Tango describes moderate turbulence, sustained surface winds of 30 knots or greater, and/or nonconvective low-level wind shear.
AIRMET Zulu describes moderate icing and provides freezing level heights.
After the first issuance each day, scheduled or unscheduled bulletins are numbered sequentially for easier identification.
Example of AIRMET Tango issued for the Salt Lake City FA area:
SLCT WA 121345
AIRMET TANGO UPDT 2 FOR TURB VALID UNTIL 122000
AIRMET TURB.NV UT CO AZ NM
FROM LKV TO CHE TO ELP TO 60S TUS TO YUM TO EED TO RNO TO LKV
OCNL MOD TURB BLW FL180 DUE TO MOD SWLY/WLY WNDS. CONDS CONTG BYD 20Z THRU 02Z.
AIRMET TURB.NV WA OR CA CSTL WTRS
FROM BLI TO REO TO BTY TO DAG TO SBA TO 120W FOT TO 120W TOU TO BLI
OCNL MOD TURB BTWN FL180 AND FL400 DUE TO WNDSHR ASSOCD WITH JTSTR. CONDS CONTG BYD 20Z THRU 02Z
Example of GAMET
PSYS: 12 L 998 HPA N54.0 E06.0 MOV SE 15KT WKN
12 COLD FRONT LINE EHGG – EDDK MOV E 20KT NC
WIND/T: 2000 FT AMSL 230/15KT PS09
FL050 250/25KT PS03
FL100 260/40KT MS07
CLD: BKN SC 2000 FT AMSL /FL050 SCT CB TOP 1400FT
MNM QNH: 1002 HPAINFERENCE
The pilot can today avail himself of last minute weather reports and forecasts along all the regularly established air routes. In addition, he can secure much valuable weather data with reference to areas located off the organized airways. He must, however, possess sufficient and adequate weather sense, to be able to size up and deal with sudden changing conditions, which may be encountered at any stage during flight. Weather conditions can be influenced by great variety of synoptic situations, which determine the flight conditions as well. The most obvious “problems” in the sky, during the flight, can be clouds and associated phenomena. As aviators, we learn in our basic aviation meteorology that thunderstorms are the most dangerous clouds of all species that produce significant hazards that could seriously jeopardize the safety of flying. It has aptly been described as a cumulus cloud gone wild. It is always accompanied by thunder and lightning, strong vertical drafts, severe gusts and turbulence, heavy rain and sometimes hail. It is a weather condition of which a pilot should be enormously respectful. Since over 44,000 thunderstorms occur daily over the earth, every pilot is sure occasionally to come in contact with one Because of the severe hazards enumerated above, attempting to penetrate a thunderstorm is asking for trouble. In the case of flight, airplane pilots, the best advice on how to fly through, close or under a thunderstorm is summed up in one word—DON’T.LITERATURE