PLRB Catastrophe Services
Background on Natural Phenomena Causing Catastrophes
Fujita Scale - F Scale
Saffir/Simpson Hurricane Scale
Glossary of Hurricane Terms
Glossary of Earthquake Terms
Richter Scale
Thunderstorms affect relatively small areas when compared with hurricanes and winter storms. The typical thunderstorm is 15 miles in diameter and lasts an average of 30-minutes. Nearly 1,800 thunderstorms are occurring at any moment around the world. That's 16-million a year.
Despite their small size, all thunderstorms are dangerous. Every thunderstorm produces lightning, which kills more people each year than do tornadoes. Heavy rain from thunderstorms can lead to flash flooding. Strong winds, hail, and tornadoes are also dangers associated with some thunderstorms.
Of the estimated 100,000 thunderstorms that occur each year in the U.S., only about 10% are classified as severe. The National Weather Service considers a thunderstorm severe if it produces hail at least .75" in diameter, wind 58 MPH or higher, or tornadoes.
When are thunderstorms most likely?
In the spring and summer months and during the afternoon and evening hours, but they can occur year-round and at all hours. Along the Gulf Coast and across the southeastern and western U.S., most thunderstorms occur during the afternoon. They frequently occur in the late afternoon and at night in the Plains states.
What do the severe weather forecast classifications indicate?
The severe weather predictions are issued each day by the National Weather Service's Storm Prediction Center and are classified as Slight, Moderate, and High. Each level of risk is expected to produce the following severe weather conditions across an average area of about 50,000 square miles, which is about the size of OK. Since these areas always vary in size, the exact calculations are always different.
Slight - small numbers of severe thunderstorms with relatively low severe weather coverage. There is also a high probability of from 5 to 29 reports of 1" or larger hail and 3 to 5 tornadoes. This is shown as yellow on the
Moderate - greater severe weather concentration with at least 30 reports of hail 1" -plus, 6 to 19 tornadoes, and numerous wind damage reports. This is shown as red on the PLRB Severe Weather Forecast Chart.
High - widespread severe weather outbreak with 20 or more tornadoes, with at least two rated F-3 (158 to 206 MPH winds), as well as 50-plus reports of extreme wind damage (80-plus MPH). This is shown as magenta on the PLRB Severe Weather Forecast Chart.
Hail is a product of thunderstorms. Large hail, by definition, has a diameter of 1" or greater. When large hail occurs, the thunderstorms producing them are classified as severe. Those storms are also capable of producing strong gusty winds, lightning, and tornadoes.
Large hail is developed when there is a strong updraft. The thunderstorm updraft must be strong enough to support the weight of the hailstones long enough for them to grow to a large size. The size of hailstones at the point of impact is affected by the number of updraft cycles and air temperature at the ground level. Warm weather (over 55
° F) hailstorms tend to produce larger and harder hail, while cold weather (below 50° F) hailstorms produce smaller and softer hail. The amount of melting is affected by a number of factors, including: The distance between the freezing level and the ground; the mean temperature of the downdraft air between the freezing level and the ground; and the size of the hailstones.Hail Diameter Size Description
1/4" - pea size
1/2" - marble size
3/4" - dime size
7/8" - nickel size
1 1/4" - quarter size
1 1/2" - walnut or ping pong size
1 3/4" - golf ball size
2" - hen egg size
2 1/2" - tennis ball size
2 3/4" - baseball size
3" - teacup size
4" - grapefruit size
4 1/2" softball size
A North American tornado is one of the more severe windstorms that can affect insured property. Only the hurricane equals or surpasses the tornado in severity. Occurring mostly east of the Rocky Mountains, tornadoes can be expected generally between March and October. Meteorologists caution that tornadoes can occur in any month of the year, if the weather conditions allow formation of the storm.
Tornadoes form when a mass of warm, moist air collides with a mass of cool, dry air above it. The collision forces warm air upward, creating a low-pressure area at the ground. Surrounding air moves in to fill the vacuum.
In severe storms, cloud tops can rise high into the air, reaching the level of strong horizontal winds that blow the tops into the classic anvil shape often reported around tornadoes. When this happens, more air is pulled upward in something like a chimney effect.
The air moving in toward the "chimney" begins to turn counterclockwise and then rises in a twisting column of extreme violence. The funnel will form along the base wall of a severe thunderstorm. The funnel cloud may stay aloft, or it may extend to the ground. If it reaches the ground, the storm is known as a tornado.
The inside of the funnel is hollow, with the inside pressure much lower than the outside air pressure. The temperature is also low inside the funnel. The low temperature causes water vapor to condense, which gives the funnel a black appearance. Debris caught up in the funnel makes it appear even darker as it moves along its path.
Much of the destructive power of the tornado is caused by wind. A portion of the damage to property is caused by the low-pressure area within the hollow funnel. Additional damage is caused by torrential rain, which often accompanies tornadoes.
What is a tornado?
It is a column of violently rotating winds extending down from a thunderstorm cloud and touching the surface of the earth.
What is the difference between a tornado and a funnel cloud?
A funnel cloud is also a column of violently rotating winds extending down from a thunderstorm; however, it does not touch the earth.
When are tornadoes most intense?
Tornadoes which develop from thunderstorms that occur early in the season have a tendency to be most severe.
Which way do tornadoes rotate?
Tornadoes generally turn counter-clockwise with wind speeds that vary from under 100 to 300 miles per hour.
How is tornado intensity rated?
It is rated on the Fujita Wind-Damage Scale, developed in 1971 by Dr. Ted Fujita of the University of Chicago and Allen Pearson, Director, National Severe Storm Forecast Center, Kansas City, MO. It is also known as the F-Scale.
F0 (winds 40-72 MPH)
Gale Tornado - Light damage. Knocks over chimneys and billboards, breaks branches off trees.F1 (73-112 MPH) Moderate Tornado - Moderate damage. Peels surface off roofs, moves mobile homes, destroys attached garages.
F2 (113-157 MPH) Significant Tornado - Significant damage. Snaps or uproots trees, tears off roofs, destroyed mobile homes, pushes boxcars over.
F3 (158 - 206 MPH) Severe Tornado - Severe damage. Removes roofs and walls from well constructed homes, overturns trains, lifts and tosses cars, uproots most trees in a forest.
F4 (207-260 MPH) Devastating Tornado - Devastating damage. Levels well constructed homes; generates large airborne missiles, including cars.
F5 (261 - 318 MPH) Incredible Tornado - Incredible damage. Lifts strong frame homes off foundations, sweeps them away, and dashes them to pieces; debarks trees; badly damages steel reinforced concrete structures.
F6 (319 - 379 MPH) Inconceivable Tornado - Very unlikely. The small area of damage they might produce would probably not be recognizable along with the mess produced by F4 and F5 winds. Missiles, such as cars and refrigerators, would do serious damage that could not be directly identified as F6 damage. If this level is achieved, evidence for it might only be found in some manner of ground swirl pattern, for it may never be identifiable through engineering studies.
Important Reminder: The size of the tornado is not necessarily an indication of its intensity. Large tornadoes can be weak, and small tornadoes can be violent. Large tornadoes can also be strong and small tornadoes can be weak. The Fujita F scale is based on damage, as measured following the passage of the storm, not the appearance of the funnel.
• Occurs with all thunderstorms.
• Averages 93 deaths and 300 injuries each year.
• Causes several hundred million dollars in damage to property and forests annually.
• The action of rising and descending air within a thunderstorm separates positive and negative charges. Water and ice particles also affect the distribution of electrical charge.
• Lighting results from the buildup and discharge of electrical energy between positively and negatively charged areas.
• The average flash could light a 100 watt bulb for more than 3-months.
• Most lightning occurs within the cloud or between the cloud and ground.
• A person's chances of being struck by lightning are estimated to be 1 in 600,000.
• The air near a lightning strike is heated to 50,000
• A cloud-to-ground lightning strike begins as an invisible channel of electrically charged air moving from the cloud toward the ground. When on channel nears an object on the ground, a powerful surge of electricity from the ground moves upward to the cloud and produces the visible lightning strike.
• Lightning often strikes outside of heavy rain and may occur as far as 10 miles away from any rainfall.
• Straight-line wind is responsible for most thunderstorm wind damage.
• Winds can exceed 100 MPH.
• One type of straight-line wind, a small area of rapidly descending air beneath a thunderstorm known as the downburst, can cause damage equivalent to a strong tornado.
Wind Speed Estimate Description:
25-31 mph - large branches in motion; whistling heard in utility lines
32-38 mph - whole trees in motion; inconvenience felt walking against wind
39-54 mph - twigs breaking off trees; wind generally impedes progress
55-72 mph - damage to chimneys and tv antennas; pushes over shallow rooted trees
73-112 mph - peels surfaces off roofs; windows broken; light mobile homes pushed or overturned; moving cars pushed off roads
113-157 mph - roofs torn off houses; cars lifted off the ground
The hurricane presents one of the greatest severity risks in property insurance. It is one of the more destructive and violent storms on earth. Hurricanes occur in many parts of the world, but our primary focus at PLRB is the storms, which can affect the Gulf Coast and the Eastern Seaboard of the United States.
Hurricanes are named to give each storm a clear identity. This practice avoids confusion in issuing warnings about particular storms, especially when more than one storm is in progress. The names are selected by the World Meteorological Organization of Geneva, Switzerland. The name lists rotate over a five-year period, but if a named storm has been especially damaging the name is retired.
The hurricane season in the Atlantic, Caribbean, and the Gulf of Mexico is June through November. Hurricanes generally form over warm tropical seas, which provide them with the necessary components of heat and moist air.
The ocean surface may reach a temperature of 80
° F or more. As the water surface heats, water evaporates rapidly into the air, forming water vapor, a gas. The air above the water is moistened by the gas.When the weather conditions are correct, this large area of moist air begins to rise. Surrounding air is drawn into this column of moist air. When the air begins rushing in, the column begins to rotate in a counter clockwise direction, in the Northern Hemisphere, because of the earth's rotation. In the southern hemisphere these storms, which are usually referred to as cyclones, rotate in a clockwise direction. In fact, meteorologists refer to these low-pressure air systems as cyclones in both hemispheres. Not all cyclones become hurricanes, as many dissipate after a few hours of activity.
Those low-pressure systems, which survive to become hurricanes, continue to grow with the amount of water vapor condensing. The condensation process creates heat, making the rising column of air warmer. Eventually the condensation forms a cloud, and steady rains begin occurring.
During this process the trade winds are carrying the storm westward. Its intensity will increase, and the pressure may continue to fall. The eye of the hurricane will form when the winds reach about 75 miles per hour, and the pressure falls to a very low reading in the center of the storm system, ranging in diameter from 10 to 20 miles.
Seawater rises slightly within the eye of the hurricane, sometimes as much as one (1) meter above sea level. Heavy rains fall, with most of the water coming from the cloud formation surrounding the eye. The wind surrounding the eye has a greater velocity than wind within the eye. Obviously, the wind and rain surrounding the eye of the storm create the damage.
Once the hurricane makes landfall, its energy will begin to dissipate because there is no longer enough warm moist air being fed into the storm system to keep it alive. The hurricane is not benign once on land, however. It can produce inland tornadoes and flooding over hundreds, even thousands, of miles from the point of entry on the coast.
The damages are caused by strong winds and flooding. The coastal flooding is known as storm surge damage. This is where the hurricane's wave action, and extremely high tides, carry salt-water inland with great force, causing damage in its wake.
Hurricane strength is listed on a scale called the Saffir Simpson Hurricane Scale. When meteorologists refer to a category of storm, they're referring to the following scale.
Saffir/Simpson Hurricane Scale
|
Category |
Central Pressure (millibars) |
Wind MPH |
Surge Feet |
|
1 |
980 or more |
74-95 |
4 - 5 |
|
2 |
965 - 979 |
96 - 110 |
6 - 8 |
|
3 |
945 - 964 |
111 - 130 |
9 - 12 |
|
4 |
920 - 944 |
131 - 155 |
13 - 18 |
|
5 |
less than 920 |
155 and higher |
18 and higher |
The Rating System Defined
Category 1
Category 2 Maximum sustained winds of 96 to 110 mph. Some trees blown down, some damage to roofs, windows, and doors. No major damage to buildings, except mobile homes. Most coastal roads under water.
Category 3 Maximum sustained winds of 111 to 130 mph. Large trees blown down, mobile homes destroyed. Some structural damage to small buildings. Windows, roofs, and doors damaged. Serious flooding near the coast. Waves batter coastal structures.
Category 4 Maximum sustained winds of 131 to 155 mph. Roofs blown off many small residences. Heavy damage to roofs, windows, and doors. Flooding extends well inland. Major damages to coastal structures from storm surge.
Category 5 Maximum sustained winds greater than 155 mph. Massive damage to windows and doors, roofs blown off many small buildings, some complete building failures. Major damage to lower floors of all ocean front structures from storm surge. Extensive flooding of low-lying areas.
Gale Warnings: May be issued when winds of 39 to 54 mph are expected.
Hurricane: Pronounced rotary circulation with a constant wind speed of 74 mph or more.
Hurricane Eye: A relatively calm area near the center of the storm, lasting from several minutes to an hour or more, and ending suddenly when winds return from the opposite direction, often with even greater force than the first storm winds.
Hurricane Warning: Issued when hurricane conditions are expected within 24 hours or less.
Hurricane Watch: Issued when there is a threat of hurricane conditions within the next 24 to 36 hours.
Small Craft Warning: Small craft operators are advised to remain in port and not venture into open sea.
Storm Surge: An extremely high tide, topped by violent waves, produced by winds and low pressure associated with a hurricane. Most hurricane fatalities occur when victims drown in the storm surge.
Storm Warnings: May be issued when winds of 55 to 73 mph are expected. If a hurricane is expected to strike a coastal area, gale or storm warnings usually will be issued before the hurricane warning.
Tropical Depression: A disturbance that has developed a rotary circulation at the surface. It maintains a constant speed of 38 mph or less.
Tropical Disturbance: A moving area of thunderstorms that maintains its identity for 24 hours or more.
Tropical Storm: Rotary circulation with a constant wind speed ranging from 39 to 73 mph.
Tropical Wave: A westward-moving trough of low pressure that sometimes produces significant showers along its path and may develop into a tropical depression.
Flooding can occur in all parts of the United States. Flash flooding is common following heavy rains and is the leading thunderstorm killer, with nearly 140 deaths annually. River flooding is common following heavy rainfall, spring melting, and so on. Coastal flooding is common following a hurricane.
However flooding occurs, the result on structures and property is disastrous. The hydraulic forces exerted in a flood are very powerful. As water strikes a structure it causes impact or frontal damage. As the water recedes it creates pressures, which can remove soil supporting the structure, possibly causing collapse.
Flooding is not a covered peril under most property policies. It is covered under the National Flood Insurance Program.
Sleet - Formed when snow falling through the atmosphere encounters a warm layer above the surface. It changes to rain. When the layer is thick enough and cold enough, the rain will form solid ice pellets, or sleet.
Freezing Rain - Formed when snow falling through the atmosphere encounters a warm layer above the surface. It changes to rain. If the air becomes cold again near the surface, and is too shallow for ice pellets to successfully form, the rain will become supercooled. It will freeze on contact with the surface. Freezing rain can cause catastrophic damage. The weight of the ice can cause tree limbs to fall and power lines to snap. It can also make roadways a slick as an ice rink, leading to loss of vehicle control.
Snow - Heavy Snow is at least 6" in 12-hours or at least 8" in 24-hours.
Blizzard - A blizzard, a term first used following an Iowa and Minnesota winter storm on March 14, 1870, is a combination of heavy snowfall, high winds, and freezing temperatures. These storms can cause a wide variety of damage to property. The risks vary from livestock losses due to suffocation through structural collapse from the weight of snow. Agricultural losses present a severe exposure from greenhouse operations to citrus growing operations.
The severe cold will often cause damage to water distribution systems in buildings. The cold weather penetrates the building's protective skin, freezing the water in pipes. As the temperatures warm, the water thaws and the pipes which have cracked by the expanding frozen water burst.
The heavy snows generated by blizzards, sometimes in combination with very strong winds, will cause roof structures to collapse under the added weight. And, as in any respectable windstorm, trees may be blown onto property causing damage.
No natural phenomena seem to create greater fear in people than an earthquake. We all assume the ground is solid. It won't move. When it moves violently in an earthquake people's sense of well being is removed. The shock is profound.
The earthquake potential in the United States exists and presents the possibility of a "mega" catastrophe to all lines of insurance. America has sustained some substantial earthquakes in its history along the east coast (Charleston), in the middle west (New Madrid, Missouri), and along the west coast (Northridge). As the earth's continental plates shift, pressures build. When those pressures are relieved, an earthquake occurs.
As the plates press against one another, the earths crust bends until it breaks, often with violent force. When a portion of the earth's broken crust snaps back into place, it creates a telltale line, or crack, called a fault.
When two of the earth's plates push against one another, an enormous amount of rock is moved beneath the surface. That movement transmits energy and motion in a wave along and beneath the surface of the earth. This is known as the push wave or the compression wave, and travels at about 5 miles per second on average.
As the two plates push against each other and the rocks move about beneath the earth's surface, they rotate. As the rock particles rotate, they transmit energy and motion in a twist wave or a torsion wave. Traveling more slowly than the push wave, the twist wave moves along at about 3 miles per second.
Seismic waves are composed of the push wave, which scientists identify as the P Wave (since it travels faster it is measures first by the seismographic equipment) and the torsion wave, which is identified as the S Wave, short for sinusoidal wave, but more commonly meaning secondary, since it is measured by the equipment as the second shock wave.
Seismologists measure and plot the seismic waves transmitted by earthquakes. Their seismographs record data, which allows calculation of the power or magnitude of the quake. It is often stated as a Richter Scale reading. Named after Charles F. Richter, the American seismologist who first proposed the mathematical scale in a 1935 academic paper, the scale is an open-ended series of numbers, with each number indicating a magnitude 10 times as great as the number below it. A quake with a magnitude of 6 is 10 times greater in power than a magnitude 5 quake.
Another measurement scale used is the Mercalli Scale. Named after geologist Giuseppe Mercalli in 1902, this scale assigns a value level from I to XII to earthquakes. It is not measured on instruments, however. The Mercalli Scale is used to quantify on-the-spot damage assessments by scientists.
The earth experiences more than 100 earthquakes each month. About a half-dozen occur nearly 400 miles beneath the surface, about 30 occur nearly 90 miles beneath the surface, and about 100 occur within 15 miles of the earth's surface. Most are quite low in energy. The number of catastrophic earthquakes is low when compared with the total number of shakes occurring on the earth each year.
Earthquake damage depends to a large extent on the strength of the shake and the strength of the structures shaken. A relatively modest quake can destroy buildings that are not well engineered or constructed. A relatively strong quake may cause little or no damage to structures that are well engineered and constructed.
Some quakes also produce seismic sea waves known as tsunamis (A Japanese word meaning harbor wave), which are often mislabeled as tidal waves. An earthquake occurring at or near the sea can create the seismic sea wave, which distributes the energy released from the earthquake. Pulsing through the ocean like a sonic boom, they are often not noticeable because of the long wavelength, but are often disastrous when they reach shore. Walls of water between 6 and 60 feet in height can strike the shore at speeds of up to 500 miles per hour. The tsunami usually will strike in a train of 6 to 7 waves, causing catastrophic damage. The American states of Alaska and Hawaii have experienced disastrous tsunamis, destroying large amounts of property and killing hundreds of people. The Pacific Tsunami Warning Center in Honolulu protects people in the Pacific, but property is still exposed to loss.
Earthquake: Officially defined as a temblor or seismic event.
Epicenter: Point on the surface, directly over the hypocenter.
Fault: A break in rock strata or veins that causes a section to become dislocated along the line of fracture.
G Units: Produced by S Waves. A small increase in G Units can exert catastrophic stress on structures. Typically G Units are small but highly destructive.
Hypocenter: Underground point-of-origin.
Liquefaction: Occurs when a passing S Wave suddenly raises the pressure of groundwater in pores between soil grains. The ground liquefies, losing strength, causing failure of structures anchored to that soil.
Mercalli Scale: Named after geologist Giuseppe Mercalli in 1902. It assigns a value level from I to XII to on-the-spot observations.
New Madrid Fault: A fault in the middle Mississippi River Valley affecting the states of Arkansas, Illinois, Mississippi, Missouri, and Tennessee. A series of severe temblors occurred in 1811 and 1812. These shocks were felt along the East Coast from New York to South Carolina. Geologists consider New Madrid an active fault and state the highest risk in the US, outside of California, for an earthquake is along this fault. They warn that central and eastern US geology will transmit the shock waves further thus causing greater damage than a temblor of similar intensity in California.
The Palmdale Bulge: A very active fault area located about 40 miles north of Los Angeles on the edge of the Mojave Desert. The Garlock and the San Andreas Faults merge at this location.
P Waves: Pressure waves, which travel deep into the earth, sometimes penetrating through the planet. These waves are a moving, compressional shock front of energy traveling from particle to particle through rock.
Richter Scale: Named in honor of Dr. Charles Richter, a geologist, California Institute of Technology, who first proposed the mathematical measurement in a 1935 academic paper. This is a logarithmic scale that records the movement of the ground in microns (26,000 microns = 1"). The scale begins at zero and is open ended.
MAGNITUDE APPROXIMATE TNT ENERGY
1.0 6 oz
1.5 2 lbs
2.0 13 lbs
2.5 63 lbs
3.0 397 lbs
3.5 1,990 lbs
4.0 6 tons
4.5 32 tons
5.0 199 tons
5.5 1,000 tons
6.0 6,270 tons
6.5 31,550 tons/1.5 Atomic Bombs
7.0 199,000 tons/9.9 Atomic Bombs
7.5 1,000,000 tons/50 Atomic Bombs
8.0 6,270,000 tons/313.5 Atomic Bombs
8.5 31,550.000 tons/1,577.5 Atomic Bombs
9.0 199,000,000 tons/9,500 Atomic Bombs
San Andreas Fault:
This well known fault lines extends along the California coast from San Diego to San Francisco, a distance of about 500 miles marking the continental boundary between the Pacific crustal plate and the North American plate. It consists of two separately formed faults. The northern half is older and less active with annual slippage or movement of 1/2". The southern half of the fault has a slippage of 2" annually.
Seismogram: Recording on paper of an earthquake.
S Waves:
Sinusoidal waves, also known as secondary wave since they are measured by seismographs as the second shock wave of an earthquake, that move with an up and down pattern on the surface. They produce a shear motion at right angles to the direction in which the wave is advancing. The shaking force can knock down structures.
Tsunamis: Energy transmission of an earthquake via water in the form of wave action. If this occurs following an earthquake there is usually a train of six or seven waves. This type of severe wave action is distinguished by the lack of any wind associated with the damage caused on shore. The State of Hawaii is susceptible to tsunamis. On August 1, 1946, a tsunami struck Hilo with a wave height of 16', killing 175 people and causing tremendous property damage.
