19-6
19.6 Hazards. A thunderstorm can pack just about every aviation weather hazard into one vicious bundle. These hazards include: lightning, adverse winds, downbursts, turbulence, icing, hail, rapid altimeter changes, static electricity, and tornadoes.
19.6.1 Lightning. Every thunderstorm produces lightning and thunder by definition. Lightning is a visible electrical discharge produced by a thunderstorm. The discharge may occur within or between clouds, between the cloud and air, between a cloud and the ground, or between the ground and a cloud.
Lightning can damage or disable an aircraft. It can puncture the skin of an aircraft. It can damage communication and electronic navigational equipment. Nearby lightning can blind the pilot, rendering him or her momentarily unable to navigate either by instrument or by visual reference. Nearby lightning can also induce permanent errors in the magnetic compass. Lightning discharges, even distant ones, can disrupt radio communications on low and medium frequencies. Lightning has been suspected of igniting fuel vapors, causing explosion; however, serious accidents due to lightning strikes are extremely rare.
19.6.2 Adverse Wind. Adverse winds are always found within thunderstorms and often many miles away from the precipitation area. Crosswinds, gusts, and variable winds/sudden wind shifts can lead to a crash during takeoffs, approaches, and landings. The area along and immediately behind the gust front is particularly dangerous because this is where rapid and sometimes drastic changes in surface winds occur.
19.6.3 Downburst. Shower and thunderstorm cells sometimes produce intense downdrafts called downbursts that create strong, often damaging winds. Downbursts (see Figure 19-7) can create hazardous conditions for pilots and have been responsible for many low-level wind shear accidents. Smaller, shorter-lived downbursts are called microbursts.
A downburst is especially dangerous to airplanes when it is encountered when climbing from takeoff or approaching to land. During this phase, the aircraft is operating at relatively slow speeds. A major change of wind velocity can lead to loss of lift and a crash.
Figure 19-7. Downburst Life Cycle (1-10 km)
FORMATION - IMPACT - DISSIPATION - Evaporation and Downdraft quickly Downdraft moves precipitation drag accelerates and away from point of forms downdraft strikes the ground impact A microburst is particularly dangerous during landing if the pilot has reduced power and lowered the nose in response to the headwind shear (see Figure 19-8). This leaves the aircraft in a nose-low, power-low configuration when the tailwind shear occurs, which makes recovery more difficult. It can cause the airplane to stall or land short of the runway.
Figure 19-8. Landing in a Microburst Glideslope At point X, the aircraft enters the microburst zone where a headwind causes it to balloon above the normal glideslope. At the center of the microburst, point Y, there is a downdraft which causes the aircraft to sink. At point Z, the aircraft enters the most lethal zone where a sudden tailwind causes the aircraft to lose airspeed.
19-7
19-8 Pilots should be alert for indications of a microburst early in the approach phase, and ready to initiate a missed approach at the first indication. However, it may be impossible to recover from a microburst encounter at low altitude.
19.6.4 Turbulence. Turbulence is present in all thunderstorms. Severe or extreme turbulence is common. Gust loads can be severe enough to stall an aircraft at maneuvering speed or to cause structural damage at cruising speed. The strongest turbulence occurs with shear between updrafts and downdrafts. Outside the cumulonimbus cloud, turbulence has been encountered several thousand feet above, and 20 miles laterally from, a severe storm.
The wind-shear zone between the gust front and surrounding air is very turbulent airspace. Oftentimes, the surface position of the gust front is denoted by a line of dust or debris along the ground, or a line of spray along bodies of water. Sometimes the gust front shear zone is denoted by a shelf cloud (see Figure 19-9), which forms as warm, moist air is lifted by the gust front. Shelf clouds are most common with multicell line thunderstorms.
Figure 19-9. Thunderstorm with Shelf Cloud
19.6.5 Icing. Thunderstorms produce abundant supercooled liquid water above the freezing level. When temperature in the upward current cools to about -15 °C, much of the remaining water vapor sublimates as ice crystals; and above this level, the amount of supercooled water decreases. However, supercooled water can exist at temperatures as cold as -40 °C in the presence of vigorous upward vertical motion, such as in a thunderstorm updraft.
19-9 Supercooled water freezes on impact with an aircraft. Clear icing can occur at any altitude above the freezing level, but at high levels, icing may be rime or mixed rime and clear. The abundance of supercooled water makes clear icing very rapid between 0 °C and -15 °C, and encounters can be frequent in a cluster of cells. Thunderstorm icing can be extremely hazardous.
19.6.6 Hail. Hail is precipitation in the form of balls or other irregular lumps of ice produced by thunderstorms. Thunderstorms that are characterized by strong updrafts, large supercooled liquid water contents, large cloud-drop sizes, and great vertical height are favorable to hail formation.
An individual unit of hail is called a hailstone. Hailstones can range in size from a pea
(0.25 inch diameter) to larger than a softball (4.5 inch diameter). Hailstones that are 3/4 inch in diameter and larger can cause significant damage to aircraft and make it difficult to control. A hailstone was collected at Vivian, South Dakota, on July 23, 2010, that measured 8 inches in diameter, 18.62 inches in circumference, and weighed 1.93 pounds (see Figure 19-10).
Figure 19-10. Vivian, South Dakota Record Hailstone
Hail is most frequently found in the interior of continents within the mid-latitudes and generally confined to higher elevations within the tropics. In the United States, hail is most common across the Great Plains region east of the Rocky Mountains. Hail is more common aloft and at higher elevations because the stones begin to melt when they fall below the freezing level, and the smaller stones may melt into raindrops before they reach the surface.
19-10 When viewed from the air, it is evident that hail falls in paths known as hail swaths. They can range in size from a few acres to an area 10 miles wide and 100 miles long. Piles of hail in hail swaths have been deep enough to require a snow plow to remove them, and occasionally hail drifts have been reported.
19.6.7 Rapid Altimeter Changes. Pressure usually falls rapidly with the approach of a thunderstorm, then rises sharply with gust frontal passage and arrival of heavy rain showers in the cold downdraft, falling back to normal as the storm moves away. This cycle of pressure change may occur in as little as 15 minutes.
19.6.8 Static Electricity. Static electricity, a steady, high level of noise in radio receivers, is caused by intense corona discharges from sharp metallic points and edges of flying aircraft. It is encountered often in the vicinity of thunderstorms. When an aircraft flies through clouds, precipitation, or a concentration of solid particles (e.g., ice, sand, or dust), it accumulates a charge of static electricity. The electricity discharges onto a nearby surface or into the air, causing a noisy disturbance at lower frequencies.
The corona discharge is weakly luminous and may be seen at night. Although it has a rather eerie appearance, it is harmless. It was named “St. Elmo’s Fire” by Mediterranean sailors, who saw the brushy discharge at the top of ship masts.
19.6.9 Tornado. A tornado is a violently rotating column of air in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. When tornadoes do occur without any visible funnel cloud, debris at the surface is usually the indication of the existence of an intense circulation in contact with the ground. A waterspout is any tornado occurring over a body of water.
Tornadoes can occur almost anywhere in the world, but are most common in the central and eastern United States during spring afternoons and evenings. They typically last only a few minutes and travel a few miles, but can persist for more than 90 minutes and track more than 100 miles in extreme cases.
On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex is typically a few hundred yards in diameter, but can range in width from less than 10 yards to over 2 miles. Wind speeds are typically estimated on the basis of wind damage using the Enhanced Fujita Scale (Enhanced F Scale) (see Table 19-1).
19-11 Table 19-1. Enhanced Fujita Scale (Enhanced F Scale) for Tornado Damage EF-Rating Class 3 Second Wind Gust Description Relative Frequency mph km/h EF-0 Weak 65-85 105-137 Gale 53.5% EF-1 Weak 86-110 138-177 Weak 31.6% EF-2 Strong 111-135 178-217 Strong 10.7% EF-3 Strong 136-165 218-266 Severe 3.4% EF-4 Violent 166-200 267-322 Devastating 0.7% EF-5 Violent >200 >322 Incredible <0.1% Note: The Enhanced F Scale is a set of wind estimates (not measurements) based on damage. The 3 second gust is not the same wind as in METAR/SPECI surface observations.
Note: Confirmed tornadoes with no reported damage (i.e., those that remain in open fields) are always rated EF-0.
Over 80 percent of all U.S. tornadoes are produced by supercell thunderstorms. Multiple tornado occurrences associated with a particular large-scale weather system is termed a tornado outbreak. On rare occasions, one supercell can produce multiple tornadoes over many hours.
An aircraft entering a tornado is almost certain to suffer structural damage. The tornado vortex has been documented to extend to the cloud top in violent tornadoes.
20-1 CHAPTER 20. WEATHER RADAR
20.1 Principles of Weather Radar. The most effective tool to detect precipitation is radar.
Radar, which stands for Radio Detection and Ranging, has been utilized to detect precipitation since the 1940s. Radar enhancements have enabled more precision in detecting and displaying precipitation.
The radar used by the National Weather Service (NWS) is called the Weather Surveillance Radar-1988 Doppler (WSR-88D). The prototype radar was built in 1988.
It is essential to understand some principles of weather radar. This will allow you to correctly interpret WSR-88D images. This section will also include a comparison between some WSR-88D principles and aircraft radar principles. These comparisons will help explain the strengths and limitations of the WSR-88D and aircraft radar.
20.1.1 Antenna. The antenna (see Figure 20-1) alternately emits and receives radio waves into the atmosphere. Pulses of energy from the radio waves may strike a target. If it does, part of that energy will return to the antenna.
Figure 20-1. Radar Antenna
The shape of an antenna determines the shape of a beam. The WSR-88D has a parabolic-shaped antenna. This focuses the radio waves into a narrow, coned-shaped beam. The antenna can be tilted to scan many altitudes of the atmosphere.
20.1.2 Backscattered Energy. The amount of energy returned directly back to the radar after striking a target is called backscattered energy (see Figure 20-2).
20-2 Figure 20-2. Backscattered Energy
Targets may include precipitation, clouds, dust, birds, insects, buildings, air mass boundaries, terrain features, etc. Reflectivity is a measurement of the amount of backscattered energy. An echo is the appearance, on a radar display, of the backscattered energy (i.e., reflectivity).
20.1.3 Power Output. The WSR-88D has a peak power output of 750 kilowatts. This allows for better detection of low reflectivity (small) targets in the atmosphere, such as clouds, dust, insects, etc.
Most aircraft radar have a peak power output of less than 50 kilowatts. Therefore, smaller targets are difficult to detect with aircraft radar.
20.1.4 Wavelengths. The wavelength is the distance between two crests or two troughs within the radio wave emitted from the radar (see Figure 20-3). The WSR-88D has a wavelength of 10 centimeters. Most aircraft radars have a wavelength of 3 centimeters. Although shorter wavelengths are better at detecting smaller targets, they are significantly more attenuated than longer wavelengths.
20-3 Figure 20-3. Wavelengths
20.1.5 Attenuation. Attenuation is any process which reduces energy within the radar beam.
This reduces the amount of backscattered energy.
20.1.5.1 Precipitation Attenuation. Precipitation attenuation (see Figure 20-4) is the decrease of the intensity of energy within the radar beam due to absorption or scattering of the energy from precipitation particles.
Figure 20-4. Precipitation Attenuation
Precipitation close to the radar absorbs and scatters energy within the radar beam. Therefore, very little, if any, energy will reach targets beyond the initial area of precipitation. Because of precipitation attenuation, distant targets
(i.e., precipitation) may not be displayed on a radar image.
The amount of precipitation attenuation is related to the wavelength of the radar (see Figure 20-5).
20-4 Figure 20-5. Precipitation Attenuation Versus Wavelength
As the wavelength of the radar decreases, the amount of precipitation attenuation increases.
The WSR-88D’s 10-centimeter wavelength is not significantly attenuated by precipitation. However, aircraft radars, which typically have 3-centimeter wavelengths, have a significant precipitation attenuation problem. As a result, aircraft weather radar typically only shows the leading edge of extreme intensity echoes.
20.1.5.2 Range Attenuation. Range attenuation is the decrease of the intensity of energy within the radar beam as the beam gets farther away from the antenna. If not compensated for, a target that is farther away from the radar will appear less intense than an identical target closer to the radar.
Range attenuation is automatically compensated for by the WSR-88D.
However, most airborne radars only compensate for range attenuation out to a distance of 50 to 75 nautical miles (NM). Targets beyond these ranges will appear less intense than they actually are.
20.1.6 Resolution. Resolution is the ability of the radar to show targets separately.
20.1.6.1 Beam Resolution. Beam resolution is the ability of the radar to identify targets separately at the same range, but different azimuths (see Figure 20-6).
20-5 Figure 20-6. Beam Resolution
Two targets must be separated by at least one beam width (diameter) in order to be displayed as two separate echoes on a radar image.
The WSR-88D has a beam width of 0.95°. Therefore, at a range of 60 NM, targets separated by at least 1 NM will be displayed separately. At a range of 120 NM, targets separated by at least 2 NM will be displayed separately.
Aircraft radar have beam widths that vary between 3 and 10°. Assuming an average beam width of 5° at a range of 60 NM, targets separated by at least
5.5 NM will be displayed separately. At a range of 120 NM, targets separated by at least 10 NM will be displayed separately.
The beam resolution is better for the WSR-88D than aircraft radar (see Figure 20-7).
20-6 Figure 20-7. Beam Resolution Comparison Between WSR-88D and Aircraft Weather Radar
In the example above, the targets (thunderstorms) are at the same range in azimuths for both the aircraft and WSR-88D radar. At 10 NM, the beam width is small enough for both the WSR-88D and aircraft radar to display the thunderstorms separately. At 60 NM, the WSR-88D beam width is still small enough to display both thunderstorms separately. However, the aircraft radar beam width is larger, which results in the two thunderstorms being displayed as one echo.
Note that the beam becomes wider at greater distances from the radar.
Therefore, the beam resolution decreases with increasing range from the radar.
As a result, lines of precipitation may appear to break up as they move closer to the radar. In reality, the breaks in the precipitation were most likely always there.