According to reports and statistics of FAA, more than 65% of delays in the NAS (National Airspace System) are caused by weather and related factors. Furthermore, the weather still plays a significant role in a number of incidents and accidents recorded in the aviation industry. Despite the fact that reports by the NTSB (National Transport and Safety Board) regularly indicate human errors as the direct cause of the accidents, weather is the sole contributor in more than 23% of all the aviation cases. The total financial impact of weather nationally is estimated to be three billion U.S dollars annually including injuries and damages, delays and unplanned cost of operation. Various aspects of weather contribute independently in making weather a hazard to the aviation industry. Among the several aspects of weather, thunderstorm is the most common and severe due to its intensity (Henry, 1930). This paper seeks to analyze thunderstorm as the most serious weather hazard to the aviation industry.
Definitions and Formation
Thunderstorm is a small, strong weather system that forms powerful winds, substantial rain accompanied by lightning and thunder. The weather department defines a thunderstorm as any storm with lightning and thunder formed by a cumulonimbus cloud normally accompanied by heavy rainfall and wind. Sometimes, thunderstorm is accompanied by hail stones and snow. Thunderstorms accompanied by hail stones are identified as hail storms. The thunderstorm is sometimes called electric storm or thundershower. It can be present in any place under two circumstances: the atmosphere must be unstable, the lower air near the surface of the earth must be moderately hot and moist. Statistically, more than a hundred lightnings strike the earth and two thousand thunderstorms occur around the globe every second. Thunderstorms happen as a result of warm, moist air rapidly moving up. The air will then cool down and condense to form cumulonimbus clouds (CBs) that can go up to kilometers high.
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Convective outlook is a definite weather forecast report given in a graphical form and an analysis by the NWS-SPC that gives specific information about the storm by describing slight, moderate, and high risks.
- Downdraft is a column of air that moves towards the earth’s surface at a high speed accompanied by precipitation that falls as droplets or thunderstorm. A sturdy downdraft can form a microburst.
- Updraft is a small current of air that moves up. If the air has adequate level of moisture, the moisture condenses to form cumulus cloud or cumulonimbus.
- Thurnderstorms are among the most beautiful phenomena of the atmosphere, but to pilots, these are one of their nightmares and a highly hazardous condition when encountered during flight. The formation of thunderstorms takes place through a process known as convection, which is the movement of energy in a heating form. The heating of the atmosphere takes place unequally causing imbalance in the atmosphere that the thunderstorms try to correct. There are three significant aspects needed for a thunderstorm to be formed. These include moisture, upward movement and instability of the atmosphere.
Enough moisture is needed for the formation of clouds. Convection occurs normally without the presence of visible clouds, moisture is the reason for a visible cloud that also pushes the convection to carry on. As the warm air moves up, the temperature drops and the moisture in the air condenses to form cloud droplets. During the process of condensation, a lot of heat is released which in turn makes the moving air stay afloat and continue to rise up. The lifting of the air in the atmosphere takes place in a number of ways apart from the convection. Other ways include the low pressure systems, mechanical lift, contacts between thunderstorms, surface weather systems and the jet stream. Whichever way the air is lifted, as long as it makes the neighboring environment warmer, convection will go on. The rate at which the air cools is known as the lapse rate and is the degree of atmospheric stability. Beyond the top levels of the atmosphere, the air normally cools at a rate of 2.8o F to 5.5o F per a hundred feet. In a case whereby the rising air cools at a slower rate than the lapse rate, it remains comparatively warmer than the surrounding air, hence it will continue to rise.
As the rising air reaches the highest point of the atmosphere or the dew point, it forms water in the form of droplets and ice. Since ice and water are denser than air, they begin to fall to the ground through the clouds merging with other droplets along the way and increasing in their size. The movement of the droplets to the ground happens at a high speed, hence will trigger a downdraft of cold moisture and air spreading out on the surface of the earth. This will result in the formation of the strong winds that are commonly referred to as thunderstorms. Typically, thunderstorms can occur in any geographic location, but most frequently they occur in mid-latitude areas due to collisions of the moderately hot and cold air (Harding, 2011).
The formation of thunderstorms takes place in three stages which are:
- Towering cumulus stage: is the stage after conviction has kicked over and the formation of a cloud can be seen. These formed clouds consist of liquid water. During this stage, a vertical movement within the cloud is seen and the hazards related to aviation include icing and turbulence.
- Mature stage: this stage is dominated by both downdrafts and updrafts, and there is the formation of precipitation. The thunderstorm at this stage is made up of water, highly cooled water and ice. Lighting is also produced at this stage.
- Dissipating stage: this is the last stage of thunderstorm formation. The updrafts have reduced or stopped and the storm is majorly covered by downdrafts. There might be a slight formation of precipitation, which will eventually lead to reduction of moisture. The thunderstorm at this stage is mainly made of ice.
The degree or intensity of a thunderstorm is determined by the composition of the mature stage. The first stage of the formation is the cumulus clouds, but most cumulus do not end up as storms unless in the presence of a dominant updraft. The cumulus stage thunderstorm can last for up to one and half an hour while the dissipating type may last for up to one day and move for one thousand miles. Pilots are known to have flown through the cumulus storms with little or no incidences, but should consider this their luck since all storms are unpredictable and can cause lethal outcomes (“What causes lumpy lobes of clouds? (Local) (Ask the weather guys) (Column)”, 2009).
Types of Thunderstorms
Pilots can come across thunderstorms of different types and intensity. The types of thunderstorms include Single Cell thunderstorm. Single Cell or common thunderstorm is the type that normally occurs during warm and humid summer conditions. The storm may be severe, accompanied by hail stones and microburst winds.
- Multi Cell or Thunderstorm Cluster. It is a thunderstorm that is formed in clusters and has several cells. These storms can spread across large areas. Sometimes, independent cells within the group can move in a different direction from the whole system.
- Squall Line. A squall line is a constricted group of active storms that often form during or before a cold season in humid, unstable air. It can also be formed in unstable air in any front. The line can be long, wide or severe.
- Supercell. This is a storm that lasts for long and is known to cause approximately all the relevant tornadoes, and most of the hailstones larger than the size of an average mango in America.
Hazards of Thunderstorms and How It Affects Aviation
All thunderstorms have elements which are hazardous to aviation and happen in numerous combinations. Despite the fact that not all thunderstorms represent a hazard, it is very difficult to decide which type of a hazard thunderstorm represents. The effects of a thunderstorm in aviation depend on the hazards it contains. Below are the major hazards to aviation and their effects.
The most severe thunderstorms pull air at a higher force into the base of the cloud. The air coming in might have a rotation motion, thus forming a strong rotation movement from the base and right into the cloud. The speed of the movement can go over two hundred knots and form a room of very low pressure inside the cloud which will gather debris, dust and any solid thing. The rooms of low pressure will form a conical-shaped cloud which extends down from the base of the cumulonimbus. If an aircraft enters a tornado, it is 100% that it will lose its sense of control and the structural body can be ripped into pieces. Clouds related to intense thunderstorms may have a hidden tornado.
Thunderstorms have possible hazardous turbulence, and a severe one can demolish an aircraft. The strongest turbulence inside a cloud happens between downdrafts and updrafts. Pilots can encounter mild turbulence at the edge of a cloud many kilometers above the cloud. It can also be seen at about twenty miles from the edge of an anvil cloud. It is difficult for the pilots to maintain a constant height in the event of turbulence, and by steering to do so, it intensifies stress on the aircraft. Stress is minimized when a plane is maintained at constant altitude. The effects of turbulence vary from jostling and rocking of the aircraft that are very uncomfortable for the passengers, to sharp accelerations that can lead to injury and loss of control. Apart from being dangerous, turbulence also has a major impact on the aviation industry due to the operating costs associated with delays and re-routing of planes. In the most recent incident, a passenger on board a U.S plane flying to Japan died from the effects of turbulence (Galea et al., 2006).
All thunderstorms are accompanied by lightning. The NWS reports describe lightning as a hazard, but in real sense it is not a criterion used to estimate the severity of a thunderstorm because it is misleading in the sense that a lightning can strike more than ten miles from a thunderstorm. Lightning can discharge into space, strike the ground, or a different cloud. Lightning can also strike an aircraft, tearing off or puncturing the frame, and can destroy the electronic equipments for communication and navigation. Research has also found out that lightning sparks can ignite fuel fumes causing an explosion of the aircraft, although very rarely. Lightning strikes happening nearby can blind the pilot, making them unable to navigate the aircraft properly. It can also cause irreversible errors on the magnetic compass. Lightning discharges waves that interfere with radio communications at both frequencies. Despite the fact that the intensity of lightning is not directly related to the other phenomena of the storm, severe storms, however, have a high rate of lightning.
In a thunderstorm, updrafts carry a lot of water in a liquid form and large droplets. When moved above the ice points, the water becomes highly cooled. As the upward moving air continues to cool to negative fifteen degrees, the remaining water vapor crystallizes and forms ice. The highly cooled water freezes on the body of the aircraft with significant consequences. Icing can take place at any height above the freezing point, but mostly at higher levels, pilots can encounter ice in a group of cells. Formation of ice in thunderstorms can be very hazardous to aviation.
Immobile aircraft on the ground during a storm and freezing seasons face a problem of ice accumulation on the body surfaces such as controls, spaces between instruments, engines and propellers. These aircrafts will require a lot of maintenance at regular intervals. The presence of ice on the surface of an aircraft, e.g. the wing, raises the drag and can reduce aircraft lift by about thirty percent. This icing on plane surfaces has been identified as the primary cause of airport accidents during takeoff. Ice accumulation on the controls can obscure the speed, altitude and other parameters measuring instruments, which can lead to loss of control, or navigation errors that can further lead to disasters or incurring of extra expenses. Ice also interferes with airport operations, thus reducing the capabilities of the airports.
Formation of ice in the mechanical parts of the plane, e.g. the carburetors, fuel cells, and the engine air intake worsens the performance of the engine leading to decreased engine power. Deposition of ice on the structure of a plane, e.g. the wings increases the weight of the plane, produces false readings on the data-capturing equipment and interferes with the handling of the plane.
Hail along with turbulence is the biggest hazard related to the thunderstorm to the airplanes. Highly cooled drops of water moved beyond the ice points start to freeze. The first drop freezes and other drops attach and freeze on it. As this continues, the hailstone grows in size and forms very big ice balls. Huge hail happens in thunderstorms with updrafts that have built very high in the atmosphere. Finally, the hailstones start to descend at a high speed. Pilots may encounter hail in clear air many kilometers away from the thunderstorm. As the hailstones descend, they melt and may reach the surface as stones, rain or both. Hailstones that are large, over two inches in diameter, can destroy an aircraft in a short period of time. Pilots should be prepared for a possibility of hail in the event of any storm (Wilson & Turner, 1982).
Low Ceiling and Visibility
Strictly speaking, visibility is next to nil in a thunderstorm cloud. Low ceiling between the base of the cloud and the ground and decrease in visibility are a safety concern for all types of aviation. A study containing the statistic reports from the NTSB reveals that low ceiling and visibility were identified as the causative factor for 25% of all accidents in the aviation industry. The hazard is magnified when associated with the other hazards that make operating precision flying instruments difficult.
Effects on Altimeters
Atmospheric pressure normally drops sharply with the approach of a storm. It then rises sharply with the arrival of the downdraft and intense rainfall. After that it adjusts to normal as the storm passes. This sequence of pressure adjustment may take place within fifteen minutes. If the pilot is not provided with a corrected altimeter setting, the altimeter may be in deviation of up to a hundred feet, and this can be disastrous.
Engine Water Ingestion
Different aircraft engines are produced with different limits of water intake. When the speed of updrafts is more than that of the falling rainfall in a thunderstorm, high concentration of water may occur. These waters can be beyond the limit the aircraft engine is designed to ingest. Serious thunderstorms have areas of high water concentration that can lead to failure of one or both engines. Presently, there is no known remedy that can prevent the engine from damage during high water ingestion. Avoiding serious storms is the only effective way of preventing risks from multiple engine failure. In case of this exposure, the pilots should follow the procedures provided in the approved AFM (Airplane Fight Manual).
Between 2000 and present days, two major aviation accidents took place in India alongside other small accidents that are unspecified. In July 2000, an Alliance Air Flight crashed upon takeoff from Patna airport, resulting in a death of sixty passengers. The investigation identified the cause of the accident as multiple engine failure due to ice ingestion. A most recent accident occurred in New Dheli when the pilot lost visibility during storm and the plane crashed.
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Pilots should avoid all types of thunderstorms at all cost, even the Columbus type that looks harmless because all storms have a potential of being highly hazardous. The real danger of a thunderstorm is worse than its outward look. They should not underestimate the strength of a storm and fly through it, even when the observation from the radar shows that it is clear. The pilots that have flown through a thunderstorm without any incidents should consider themselves lucky. The best policy of air safety is to avoid the storm (Wiggins, 2007).
It is recommended that the pilots should neither take off, nor land in the presence of a storm since strong winds can cause loss of control. The pilots should not fly under a storm since there could be presence of invisible turbulence. They should also check the radar for guidelines from to navigate round a storm or for diversion. After the diversion, the pilots should make sure that the flight route is clear before moving ahead on the original route.
Weather is one of the biggest hazards to the aviation industry, and thunderstorm contributes a lot to this. The NTSB has identified thunderstorm as a contributing factor to up to 20% of all aviation accidents and incidents. A thunderstorm is often accompanied by strong winds, heavy rainfall and lightning. The impacts of thunderstorm on aviation range from the actual damages and injuries to the high operation costs that are a result of delays and diversion of flights. Lastly, the pilots should turn on the light of the cockpit to the highest strength so as to avoid the temporary blindness caused by the brightness of lightning.
The formation of a thunderstorm happens in three steps through a process called convection and in the conditions of moderately hot, moist air, atmospheric instability and an upward lift. The hazardous phenomenon of a thunderstorm can be attributed to turbulence, icing, invisibility, hail, water ingestion and lightning. Thunderstorm can be extremely dangerous to the aircraft, hence the pilots should try to avoid it as much as possible.