Chapter 11 Geog 202
doppler effect
- A change in sound frequency caused by motion of the sound source, motion of the listener, or both. - A similar effect is used to detect the movement of storms as electromagnetic radiation is shortened or lengthened after hitting the target precipitation
mesocyclones
- A scale of meteorological phenomena typically having horizontal extents of several tens of kilometers - small scale with an area of low pressure/ rotating up draft -less than 20% create tornados
floods
- An overflowing of a large amount of water beyond its normal confines, esp. over what is normally dry land. - This may occur from any combination of a variety of factors, such as excessive snowmelt, heavy precipitation, and waterlogged soils.
11.4 Summarize the characteristics and processes involved in the formation of floods and flash floods.
- Floods can result from prolonged or repeated rain events, especially if they occur over existing snowpacks. This will often take the the form of streams or rivers overtopping their banks. - Flash floods are those that occur when a strong thunderstorm remains over an area sufficiently long to create heavy runoff.
11.2 State measures that can taken to ensure safety from lighting.
- If lightning threatens, even at a distance, it is important to take shelter indoors and to stay away from electrical appliances. - Cars with metal exteriors and hard tops are relatively safe.
11.3 Explain how the different types of thunderstorms form and describe the strong winds that can accompany them.
- In air mass thunderstorms, downdrafts eventually destroy the storm. They undergo a sequence from the cumulus to the mature to the dissipative stages in tens of minutes. - Severe storms: Those that produce large hail, damaging winds, or tornadoes. These storms develop when downdrafts of wind reinforce the storm. Such storms occur within the squall lines, in multi-cellular mesoscale convective complexes. (AKA Supercells)
11.1 Sequence the processes involved in lightning formation and identify the different types of lightning
- Lightning begins when negative electrical charges build up near the base of a cloud and positive charges gather at the top. - In cloud-to-ground lightning, rapidly growing leaders extend downward from the base of the cloud. When they connect with some object at the surface, a visible stroke develops. - Most often a rapid sequence of multiple strokes follows the initial one to produce a lightning flash. Extreme heating of the air within the stroke causes the air to expand explosively and create the sound of thunder.
11.7 Identify aspects of tornadoes as a natural hazard, including areas most at risk for tornadoes, damage, and fatalities. Also explain how tornadoes are rated and forecasted.
- The majority of tornadoes are classified as weak and cause no fatalities. - However, extremely large tornadoes can cause tremendous devastation and loss of life. But these are relatively rare. - Tornadoes hit the United States more than any other place on Earth, and mostly happen in an area called Tornado Alley. This area is defined as a swath near the middle of Texas northeast towards the Great Lakes. This is because this area is very flat and has tropical air coming in from the gulf clashing with cold continental polar air coming from Canada constantly. It's this instability that causes tornadoes. - The enhanced Fujita scale is the ranking scale system that provides information on a tornado's strength by the type of damage done to particular types of structures. - The U.S. Weather Service has a system of tornado watches and warning that alerts the public to threatening weather. - Watches cover fairly large areas where conditions are favorable for the formation of tornadoes. While Warming means that a tornado has been seen and/or touched down to the ground.
flash
- The rapid combination of lightning strokes. - The rapidity and nearness of the strokes often cause the individual strokes to appear as single flickering stroke
Why is it extremely unlikely that a tornado will move from east to west?
- Tornadoes move in the direction of mid-level winds, towards the right of the initial direction of the thunderstorm. - The influence of low-level winds and friction from the ground also acts on tornadoes. So, it is extremely unlikely that a tornado will move from east to west.
downbursts (aka downdrafts)
- violent downdrafts that are concentrated in a local area. - As these strong downward movements of air due to the drag caused by heavy precipitation and the cooling of air as falling precipitation evaporates
gust front
A boundary that separates a cold downdraft of a thunderstorm from warm, humid surface air. On the surface, its passage resembles that of a cold front.
roll cloud
A dense, roll-shaped, elongated cloud that appears to slowly spin about a horizontal axis behind the leading edge of a thunderstorm's gust front.
Stepped leaders
A narrow zone of ionized air that serves as a conduit for an initial lightning strike
shelf cloud
A part of the cloud that protrudes ahead of the main portion of the cloud and above a gust front
ball lightning
A rare and short-lived type of electrification that resembles a glowing mass of air about the size of a basketball
waterspouts
A rather weak whirlwind (narrow rotating column of air) forming over a water surface. Rising and condensing air makes the waterspout visible.
tornado outbreak
A series of tornadoes that forms within a particular region—a region that may include several states. Often associated with widespread damage and destruction. - This means six or more tornadoes with 24 hours from a single weather system
lightning
A sudden spark, or energy discharge, caused when electrical charges jump between parts of a cloud, between nearby clouds, or between a cloud and the ground. This is accompanied by the emission of light.
tornadoes
A violently rotating column of air stretching from a cloud to the ground. - So it has two layers of air going different pass each other, causing swirling (wind shear) - The warm air on the ground can lift this swirling - The swirling air pulls some of the clouds down into the vortex - Its' not technically a tornado until it reaches the ground - It is sustained by air cool descending withing the tornado and warm air rising around it
tornado warning
A warning issued when a tornado has actually been sighted in an area or is indicated by radar.
severe thunderstorm warning
An advisory issued by a local office of the National Weather Service indicating that severe thunderstorms are occurring or imminent - Strong jet stream winds - Clashing air masses near the surface: big difference in temperature and moisture - Vertical wind shear - Upper-level inversions
vault or Bounded weak echo region
An apparently empty area on a radar display, where moist air enters a supercell thunderstorm. Water droplets are abundant but are too small to provide a strong radar echo.
haboobs
Desert dust storms triggered by intense downdrafts that cause strong horizontal winds.
Downdrafts
Downward rushes of air caused by winds blowing downward, hitting the land and spreading out in strong gusts of wind
Stages of Tornado Development
First Stage: Warm, moist air rises to meet a layer of warm, dry air. - This inversion inhibits cumulus cloud growth. - Leading to evaporation and cooling of warm/dry air - Warm moist air "builds up" below the inversion layer Second Stage: Eventually the layer of warm, dry air breaks and explosive development occurs Third Stage: Then rapid uplift and wind shear produce tornadoes.
What is the difference between a lightning stroke and a lightning flash?
In cloud-to-ground lightning is caused due to the advance of negatively charged air which is known as the stepped leader. When this leader approaches the ground, a spark is made to move upwards from the ground towards the leader. A pathway is created when a leader and spark connects each other and thus the transfer of electrons takes place. These form bright bands which are known as stroke. The combination of two or more strokes is known as lightning flash.
convective outlooks
Information on the probability of different types of severe storms issued by the Storm Prediction Center
sprites
Large, brief bursts of red light emanating from cloud tops as lightning occurs. This happens because of discharges of positive lightning between an underlying thundercloud and the ground
Describe the current theories regarding the formation of charge separation
Lightning is caused due to the separation of positive and negative charges which is known as charge separation. In cloud positive charges are at the upper surfaces while negative charges lies at the bottom surfaces. When clouds extend above the freezing level a lightning is a caused. This influences the charge separation. The electric field is caused due to the collisions that are made between ice crystal and graupel. When an ice crystal and soft hail are made to collide each other, the liquid droplets migrate to the ice crystal (Riming). This is due to which transfer of a positive charge. Which is being made because there is a movement between the water and ice crystals. A negative charge is developed between the crystal and hailstone. Thus the lightning takes place due to the charge.
sheet lightning
Occurs when the lightning flash is not seen but the flash causes the cloud (or clouds) to appear as a diffuse luminous white sheet. Happens because of in-cloud or cloud-to-cloud discharges.
sweep
One rotation of a radar transmitter/receiver unit to gather data
severe storm and tornado watches
Public notices issued by the Storm Prediction Center advising of the possibility of tornadoes or other severe weather for particular regions
air mass thunderstorms
Relatively small, short-lived (last about 10 minutes) thunderstorms that do not produce powerful winds, large hail, or tornadoes. - Have three stages: (a) cumulus, (b) mature, and (c) dissipative
Air mass thunderstorms vs severe thunderstorms
Severe Thunderstorms (aka frontal storms) - Lifted by the collision of air masses (frontal lifting) - Usually longer, more severe - Tornadoes, hail possible Air Mass Thunderstorms - Relatively mild - Occur within warm, moist air masses, NOT along fronts - Surface Heating: temporary instability
suction vortices
Small, rapidly rotating whirls perhaps 10 m in diameter that are found within large tornadoes. As these whirls within the large tornado that causes the most devastation.
outflow boundary
The boundary separating cooler surface air flowing outward from a downdraft and warmer ambient air
cumulus stage
The earliest stage of air mass thunderstorms in which clouds begin to grow but have not yet begun to produce lightning or precipitation. In more technical terms, this caused due to the convection when unstable air begins to rise. Air mass thunderstorms mostly occurs in the evenings. This air cools adiabatically to form cumulus clouds. These lasts for a very few minutes. The darkness of the sky occurs due to the thickness of the cloud that is formed. A cloud in this stage moves upward at a velocity ay 5 to 20 m/sec.
fair-weather electric field (aka mean-electric field)
The ever-present voltage difference between the surface and the ionosphere, which serves as the background situation for events such as lightning.
dissipative stage
The final stage of an air mass thunderstorm in which there is an insufficient inflow of water vapor to offset the loss of moisture by precipitation. This is caused by a cloud base covered with the downdrafts. So when these occupy the entire space the additional water vapor is cut off and thus storm enters into this stage. One can find the disappearance of the precipitation as the sky gets cleared.
volume sweep
The result of multiple sweeps of a radar transmitter/receiver unit, each directed at different angles relative to the horizon. This is to provide a three-dimensional view of storm activity.
charge separation
The separation of positive and negative ions into different parts of a cloud. A necessary precursor for lightning.
hook echo
The shape of a hook on a Doppler radar screen that indicates the possible presence of a tornado. - The rain echo forms the hook pattern as air rotates around the strong updraft. - The updraft is the hollow portion of the hook (looks like a backwards "J" or a 6) - It's where the tornado would most likely be found (if the storm were to produce one).
wall cloud
Thick cloud beneath a rotating thunderstorm, a place where severe weather often develops.
11.6 Explain how, where, and when tornadoes form.
Tornadoes are most likely to form in the spring or early summer, but they can occur at any time of the year. - Many tornadoes (especially those that emerge from supercells) follow the formation of large rotating areas within storm clouds. These storms are called mesocyclones. However, other unexplained processes can also lead to their formation.
11.8 Explain how a waterspout forms
Waterspout are similar in structure to tornadoes over land. They typically form over warm bodies of water, with the lower atmosphere being heated from below to create unstable conditions.
Thunder and Lightning
Which do we notice- thunder or lightning? - Why is there a difference in timing? - Speed of sound: 340 m/s (1115 ft/s) - How far away was the lightning strike if it takes 15 seconds to hear thunder? 3 miles, for each mile it takes 5 seconds for thunder to travel to the point where the lighting was.
11.5 Analyze the geographic and temporal distribution of thunderstorms
Worldwide, thunderstorms are most common in the tropics, especially over interior equatorial Africa. - Within the United States, they are most common in the extreme southern states.
heat lightning
a flash or flashes of light seen near the horizon, especially on warm evenings, believed to be the reflection of distant lightning on high clouds. Not a type of lightning
Mesoscale convective systems (MCSs)
a general clustering of thunderstorms on the order of a few hundred kilometers across
squall lines
a linear band of thunderstorms often found several hundred kilometers ahead of a cold front
funnel clouds
a narrow, rapidly rotating column of air extending downward from a thunderstorm. A funnel cloud becomes a tornado if it reaches the ground. - This is what forms the core of a tornado or waterspout. Originally white, but changes color due to the things it picks up. Like dirt, metal things, etc.
doppler radar
a radar tracking system using the Doppler effect to determine the location and velocity of a storm, clouds, precipitation, etc. - Does this by seeing how fast by the wind is moving and measuring horizontal motions as well as the internal characteristics of clouds
mesoscale convective complexes (MCCs)
a type of mesoscale convective system having an oval or nearly circular shape
dart leader
a zone of ionized air that serves as a conduit for a lightning stroke subsequent to the initial stepped leader in a lightning flash
supercells
an extremely powerful, self-sustaining thunderstorm characterized by intense, rotating updrafts. These very large thunderstorms form an extremely powerful updraft.
enhanced Fujita Scale
classifies the intensity of a tornado, based on wind speeds and specifically on the types of damage that occur on a scale ranging from eF1 to eF5
flash floods
floods in which a dangerous volume of water builds up in a short time due to a heavy thunderstorm staying in one area
cloud-to-cloud lightning
lightning that occurs from cloud to cloud or between negative and positive sections of the same cloud
in-cloud lightning
lightning that occurs when the voltage gradient within a cloud overcomes the electrical resistance of the air. The result is a very large and powerful spark that partially equalizes the charge separation.
St. Elmo's Fire
occurs when ionization in the air causes tall objects to glow as they emit sparks
derechos
powerful, large-scale winds that flow in a straight lines.
stroke or return stroke
the flow of electricity associated with an individual bolt of lightning
entrainment
the incorporation of surrounding, cold and unsaturated air into a cloud
wind shear
the rate of change of wind speed or wind direction over a given distance
mature stage
the second phase of a thunderstorm, where downdrafts move from the base of a cumulonimbus cloud and produce heavy precipitation. Tornadoes touch down during this phase of the storm. So needless to say, this is most dangerous part of the storm.
microbursts
the smallest and most intense downbursts that may produce winds approaching at 170 mph. These downbursts have a wind shear that capable of causing airplanes to crash.
thunder
the sound caused by the rapid expansion of air due to heat being expelled in the surrounding air along an electrical strike
multicell thunderstorms
thunderstorms that contain a number of cells, each in a different stage of development. Contains zones of updraft as well
blue jets
upward-moving electrical ejections from the tops of the most active regions of thunderstorms