Course 5 - Section 11 - Thunderstorms Flashcards
Thunderstorm Size
A thunderstorm is a weather phenomenon produced by cumulonimbus cloud (CB)
Individual thunderstorms are generally no more than 10NM in diameter but tend to develop in clusters of two or more
Thunderstorm activity may therefore cover a very large area. CBs associated with thunderstorms may extend vertically as high as 70,000 feet
What is required for thunderstorms to form?
- Unstable Air
- Lifting Action to get air moving upward
- High moisture content in the air
What are the three stages of thunderstorm formation?
- Cumulus stage
- Mature stage
- Dissipating stage
Thunderstorm formation: Cumulus stage
The cumulus stage is characterized by the formation of a cumulus cloud that gradually builds up into a towering cumulus
The cloud mostly has updrafts that support the water droplets within the cloud. No precipitation occurs at the cumulus stage
Thunderstorm formation: Mature stage
The mature stage of thunderstorm is characterized by the onset of precipitation and is where the highest incidence of precipitation occurs.
Lighting, microbursts, gust fronts, hail and tornadoes can also occur at this stage
Updrafts within the cloud may attain 6000 feet per minute (FPM). As the water droplets grow, they start falling and dragging air down with them, producing downdrafts in the 2000 FPM range
Violent turbulence is associated with these updrafts and downdrafts
Hail only occurs in the mature stage of a thunderstorm
Thunderstorm formation: Dissipating stage
The onset of precipitation during the mature stage causes thunderstorms to dissipate
During the dissipating stage, the precipitation cools down the lower part of the cloud. It also breaks the updrafts that are the main source of its energy
The top of the cloud spreads out into an anvil form
What are the two types of thunderstorms (list)
Air mass thunderstorms
Frontal thunderstorms
Air Mass Thunderstorms
Air mass thunderstorms form when warm, moist, and unstable conditions exist in the atmosphere
While all type of thunderstorms are severe, air mass thunderstorms are sometimes called “ordinary” or “garden variety” thunderstorms because they USUALLY HAPPEN IN ISOLATION
Air mass thunderstorms usually form as a result of CONVECTION, ADVECTION OR OROGRAPHIC LIFT
List the types of air mass thunderstorms
Convection thunderstorms
Advection thunderstorms
Orographic thunderstorms
Convection thunderstorms
Convection thunderstorms are caused by the uneven heating of the ground
These types of thunderstorms usually occur in the afternoon or early evening and dissipate late in the evening or overnight
With convenction thunderstorms, clouds are scattered over a large area
Advection thunderstorms
Advection thunderstorms are caused by cold air passing over a warm surface. They may occur over land or over water
Orographic thunderstorms
Orographic thunderstorms are caused by air being lifted by sloping ground. These storms may occur day or night and they dissipate with a change in wind direction
With orographic thunderstorms, clouds form a stationary line
Air mass thunderstorms: Clusters
Air mass thunderstorms can form individually or in clusters
Indvidiaul air mass thunderstorms are call SINGLE-CELL THUNDERSTORMS
Large clusters of air mass thunderstorms are collectively known as SUPERCELLS
Frontal Thunderstorms
Frontal thunderstorms are caused by humid or unstable air being lifted by the frontal surface.
Clouds form in a solid line several hundred miles long in various stages of development
Frontal thunderstorms are most often associated with cold fronts, however they can also occur with warm fronts
What are the two types of frontal thunderstorms
Cold Frontal Thunderstorms
Warm Frontal Thunderstorms
Cold Front Thunderstorms
Cold front thunderstorms occur day or night, and they dissipate with the movement of the front
Clouds are in a line parallel to the front
Warm Front thunderstorms
Warm front thunderstorms are often embedded in stratiform clouds and are not as visible as cold front thunderstorms
Like cold front thunderstorms, they occur day or night, and they dissipate with the movement of the front
List the ten areas of concern when flying near thunderstorms
-Turbulence
-Hail
-Lightning
-Icing
-Squall lines
-Tornadoes
-Low ceiling and visibility
-heavy rain showers
-microbursts
-gust fronts
Thunderstorms and Turbulence
Turbulence is found in many types of weather and may be light, moderate or severe. Turbulence affecting aircraft can range from just a annoying bumps to conditions severe enough to cause structural damage
-The strongest turbulence in a cloud occurs during the mature stage in the transition zone between updrafts and downdrafts generally between 12,000 and 20,000 feet
-below the cloud, downdrafts that race towards the ground cause low level wind shear to be present. Wind shear refers to a change in windspeed, wind direction or both, over a short distance resulting in a tearing or shearing effect
-above the cloud, turbulence is also possible (up to several thousand metres) and outward up to 30 km
Severe turbulence is present in all cumulonimbus clouds. Severe turbulence can affect aircraft by damaging airframes and causing abrupt changes in aircraft speed and altitude
Roll Clouds
a roll cloud is a low level, horizontal, tube-shaped accessory cloud completely detached from the base of a cumulonimbus cloud. Roll clouds are associated with thunderstorms and turbulence
Often a roll cloud will appear on the leading edge of the storm and will mark the position of the eddies or swirls where the wind changes direction. This is also known as the SHEAR ZONE and is most prevalent with cold fronts and squall lines
The roll cloud is also associated with the downdraft reaching the ground, also known as a gust front, and is often the first drastic change in surface wind conditions ahead of an approaching storm
Thunderstorms and Hail
Hail occurs in the MATURE STAGE of thunderstorm cells that have more intense updrafts
Hail will occur most from the surface up to around 10,000 to 15,000 feet. Hail could happen as high as 45,000 feet
If the clouds are built diagonally, hail that has been carried in the updraft may be case out into the clear air surrounding the cloud from the sides or from the overhanging anvil cloud
Hail can cause damage to aircraft, windshields, outside instrument sensors and even airframes
What are the four types of lightning
In cloud (sheet lightning)
Cloud to cloud
cloud to ground
distant
Lightning and thunderstorms
Lightning is the visible electric discharge produced by a thunderstorm.
Lightning is always present in, or in the vicinity of, CBs
Lightning is most prevalent in CBs in the mature stage between the temperatures of +8 and -8 celcius (at +/-5000 feet from the freezing level)
Electricity generated by thunderstorms can cause serious problem for aircraft:
-punctures to the skin
-damage to communication or navigational equipment
-temporary blindness of pilots
-errors in magnetic compasses
-disruption to low and medium radio frequencies
-ignition of fuel vapours (rare)
Static discharges on wind screens, wing tips and propeller tips can cause static on low frequency bands (also known as precipitation static)
Less frequently, and electrical discharge called st elmos fire can appear as a bushy halo around some edges of aircraft
Thunderstorms and Icing
*** Clear and mixed icing is possible in all stages of thunderstorm cloud development, but SEVERE CLEAR ICING is most likely to occur in the UPPER HALF of a thunderstorm cloud IN THE MATURE STAGE, especially when temperatures are between 0 and -25 degrees C
When an aircraft flies at or below freezing temperatures and strikes supercooled water droplets, the droplets freeze and adhere to the aircraft, the result is icing.
Icing is a major hazard to aviation and is a fact of life in canadian aviation. Conditions can range from light and readily controlled, to extreme and disasterous in flight
Dangerous icing can occur in thunderstorm clouds
Thunderstorms and Squall Lines
Squall Lines are narrow bands of highly active thunderstorms. They generally develop ahead of a cold front in moist and unstable air.
Squall lines can form rapidly, reaching maximum intensity during the late afternoon or early evening. They can range from one to several hundred miles in length and may be broken or in a continuous line
A squall line can contain severe thunderstorms with torrential rains, large hail, and tornadoes. It contains all the other hazards generally associated with thunderstorms
Thunderstorms and Tornadoes
A tornado is a violently rotating column of air that extends from the thunderstorm to the ground
Tornadoes sometimes occur with isolated thunderstorms but occur much more frequently with supercell thunderstorms (those associated with cold fronts and squall lines)
They can occur several km outward from teh main area of lightning and precipitation
Tornadoes generate violent turbulence. An aircraft entering a tornado vortex is certain to suffer structural damage. Since the vortex extends well up into the cloud, a hidden vortex could be encountered
Cumulonimbus mammatus clouds
Cumulonimbus mammatus, or mamma clouds, are often an indication of tornadoes and violent turbulence
These clouds are round shaped and hang under teh cloud base or anvil
Thunderstorms and Low Ceiling and Visibility
Visibility is generally near zero within a thunderstorm cloud. The ceiling and visibility can also become greatly reduced in the precipitation between cloud base and the ground
Heavy precipitation can saturate the air, rapidly causing low level clouds called stratus fractus and fog to form. As a result, ceiling and visibility are rapidly reduced turning a visual flight rule approach into an instrument flight rule approach
Aircraft will be restricted by the lower ceiling and visibility, but the situation may become even more severe when other hazards are present, such as turbulence, hail, lighting, and low-level wind shear
Thunderstorms and Heavy Rain Showers
A thunderstorm contains vast amounts of large liquid water droplets
The heaviest rain showers are associated with the MATURE stage of thunderstorm development with the onset of the first downdrafts
Heavy rain showers associated with thunderstorms can CONTAMINATE THE WING SURFACE which may lead to reduction in lift and possible early stall
Heavy rain can REDUCE VISIBILITY and cause refraction on teh windscreen of the aircraft if encountered during approach and landing. This refraction produced the illusion that the runway threshold is lower that it actually is
Water lying on a runway can cause HYDROPLANING, which destroys breaking action of the aircraft and can lead to loss of control
Thunderstorms and MICROBURSTS
a Microburst is a sudden and powerful downdraft within a thunderstorm
Microbursts can cause sudden and dangerous low-level wind shear, resulting in hazardous conditions for takeoff and landing
Thunderstorms and Gust fronts
A gust front is the result of strong downdrafts in the mature stage of a thunderstorm reaching the ground and spreading out horizontally
There is usually significant turbulence associated with gust fronts
Nearly all thunderstorms produce some degree of gust front
Cumulus clouds and their affect on flight
Flying near a CU cloud will result in a BUMPY FLIGHT beneath and in the cloud. There are updrafts below and in the cloud
A CU can develop into a TCU and then a CB
Towering Cumulus clouds and their affect on flight
Flying near a TCU cloud will result in TURBULENCE
ICING is possible in TCU at altitudes above freezing level
There are strong updrafts below and in the cloud
A TCU may develop into a CB
Cumulonimbus clouds and their affect on flight
Flying near a CB Cloud will result in SEVERE TURBULENCE
There also may be SEVERE ICING
CBs are accompanied by dangerous hazards such as
-thunderstorms
-precipitation
-wind
-violent vertical currents
-hail
-lightning
Altocumulus Castellanus clouds and their affect on flight
Flying near an ACC cloud will result in TURBULENCE AND ICING
precipitation may also be present with ACCs
