Tornado Lesson

Tornado

Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported nationwide, resulting in 80 deaths and over 1,500 injuries. A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground. The most violent tornadoes are capable of tremendous destruction with wind speeds of 250 mph or more. Damage paths can be in excess of one mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas!


Reference : http://news.bbc.co.uk/2/hi/science/nature/7533941.stm

What causes tornadoes?

Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.

During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.

Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes.

Tornadoes occasionally accompany tropical storms and hurricanes that move over land. Tornadoes are most common to the right and ahead of the path of the storm center as it comes onshore.

Frequency of Tornadoes
Tornadoes can occur at any time of the year.

1. In the southern states, peak tornado occurrence is in March through May, while peak months in the northern states are during the summer.
2. Note, in some states, a secondary tornado maximum occurs in the fall.
3. Tornadoes are most likely to occur between 3 and 9 p.m. but have been known to occur at all hours of the day or night.
4. The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction. The average forward speed is 30 mph but may vary from nearly stationary to 70 mph.
5. The total number of tornadoes is probably higher than indicated in the western states. Sparce population reduces the number reported.

Tornado Safety
What YOU Can Do

Before the Storm:

1. Develop a plan for you and your family for home, work, school and when outdoors.
2. Have frequent drills.
3. Know the county/parish in which you live, and keep a highway map nearby to follow storm movement from weather bulletins.
4. Have a NOAA Weather Radio with a warning alarm tone and battery back-up to receive warnings.
5. Listen to radio and television for information.
6. If planning a trip outdoors, listen to the latest forecasts and take necessary action if threatening weather is possible.

If a Warning is issued or if threatening weather approaches:

1. In a home or building, move to a pre-designated shelter, such as a basement.
2. If an underground shelter is not available, move to an interior room or hallway on the lowest floor and get under a sturdy piece of furniture.
3. Stay away from windows.
4. Get out of automobiles.
5. Do not try to outrun a tornado in your car; instead, leave it immediately.
6. Mobile homes, even if tied down, offer little protection from tornadoes and should be abandoned.

Tsunami Lesson

What is a tsunami?

Tsunami is the Japanese name given to large waves that sometimes devastated the shores and ports of Japan. A tsunami is a wave in the ocean but it is very different to normal waves.

Tsunamis have very long wavelengths. Crest to crest they measure between 10 and 500 km and they travel through the ocean at more than 700 km/h. Sometimes there appears to be just one wave but often there are multiple waves travelling a few minutes apart.

Wave height [amplitude] may not appear to be great in the open ocean (and often goes unnoticed) but unlike normal waves the tsunami is moving the entire water column, all the way to the sea floor! The water depth therefore has a major influence on the behaviour and appearance of the wave. In addition because of the wavelength, the first sign of the arrival of a tsunami may actually be the sea level falling and bays appearing to empty.

In deep open water the wave is almost impossible to see although modern instruments can detect it. However, as the wave approaches shore and the water shallows it slows down. The wave rapidly bunches up as the faster rear sections catch up with the slower front sections resulting in the wave growing in height the closer it gets to shore. This effect is enhanced if the near-shore sea bed provides a long gradual shallowing. Many tsunamis are barely distinguishable from normal sea waves but some turn into monsters rising 30 metres above the shore line! The damage along a shore line may vary because of the influence the local shape of the sea floor has on wave behaviour.

Bays and harbours that are funnel shaped also suffer more from a tsunami because they concentrate the effects. Damage in these areas is further increased by the sloshing backwards and forwards of the water, just like in a bathtub!

Reference : http://news.bbc.co.uk/2/hi/science/nature/7533972.stm

What causes a tsunami?

Unfortunately tsunamis have been given numerous names in the past that are misleading. Even the word tsunami meaning ‘harbour wave’ is misleading!

All tsunami are caused by the sudden displacement of large volumes of water. All are the result of violent events with enough power to displace large volumes very rapidly. However, tsunami may be caused by events that are not local to the tsunami site. Because the waves have been generated by huge releases of energy and they travel so effectively through the deep ocean some tsunami are caused by events that literally happen on the other side of the world.

The usual causes of a tsunami are:
an earthquake
- most tsunamis are caused by submarine earthquakes but not all submarine earthquakes cause tsunamis. Movement on the fault must have a vertical component that generates sufficient displacement to set a tsunami running

a landslide
- underwater landslides or coastal landslides that fall into the ocean can displace enough water to create a tsunami. Sometimes the landslides are caused by earthquakes.

a volcanic eruption or explosion
- submarine explosions, caldera collapse and massive pyroclastic flows can all cause sufficient displacement of water to generate a tsunami.

impact by a meteorite
- large meteorites have a high probability of landing in the ocean and causing a tsunami given that about two thirds of the surface of the Earth is covered by water.

Earthquake Lesson

What is an earthquake ?

In ancient times earthquakes were thought to be caused by restless gods or giant creatures slumbering beneath the Earth.

In Japan earthquakes were thought to be caused by a monster catfish (Namazu) that lived under Japan. In this picture people are punishing the catfish for causing a large earthquake in 1855.

The great Lisbon earthquake and subsequent tsunami of 1755 caused massive destruction and had a huge effect on European scientific and philosophical development.

The early Greek philosophers developed a theory that earthquakes were caused by movements of gases trying to escape from underground.

What Causes An Earthquake ?


Reference : http://news.bbc.co.uk/2/hi/science/nature/7533950.stm

An Earthquake is a sudden tremor or movement of the earth's crust, which originates naturally at or below the surface. The word natural is important here, since it excludes shock waves caused by French nuclear tests, man made explosions and landslides caused by building work.

There are two main causes of earthquakes.

Firstly, they can be linked to explosive volcanic eruptions; they are in fact very common in areas of volcanic activity where they either proceed or accompany eruptions.

Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type.

Terminology

An earthquake can be likened to the effect observed when a stone is thrown into water. After the stone hits the water a series of concentric waves will move outwards from the center. The same events occur in an earthquake. There is a sudden movement within the crust or mantle, and concentric shock waves move out from that point. Geologists and Geographers call the origin of the earthquake the focus. Since this is often deep below the surface and difficult to map, the location of the earthquake is often referred to as the point on the Earth surface directly above the focus. This point is called the epicentre.

The strength, or magnitude, of the shockwaves determines the extent of the damage caused. Two main scales exist for defining the strength, the Mercalli Scale and the Richter Scale.

Earthquakes are three dimensional events, the waves move outwards from the focus, but can travel in both the horizontal and vertical plains. This produces three different types of waves which have their own distinct characteristics and can only move through certain layers within the Earth. Lets take a look at these three forms of shock waves.

Types of shockwaves

P-Waves
Primary Waves (P-Waves) are identical in character to sound waves. They are high frequency, short-wavelength, longitudinal waves which can pass through both solids and liquids. The ground is forced to move forwards and backwards as it is compressed and decompressed. This produces relatively small displacements of the ground.
P Waves can be reflected and refracted, and under certain circumstances can change into S-Waves.


Particles are compressed and expanded in the wave's direction.
S-Waves
Secondary Waves (S-Waves) travel more slowly than P-Waves and arrive at any given point after the P-Waves. Like P-Waves they are high frequency, short-wavelength waves, but instead of being longitudinal they are transverse. They move in all directions away from their source, at speeds which depend upon the density of the rocks through which they are moving. They cannot move through liquids. On the surface of the Earth, S-Waves are responsible for the sideways displacement of walls and fences, leaving them 'S' shaped.


S-waves move particles at 90° to the wave's direction.
L-Waves
Surface Waves (L-Waves) are low frequency transverse vibrations with a long wavelength. They are created close to the epicentre and can only travel through the outer part of the crust. They are responsible for the majority of the building damage caused by earthquakes. This is because L Waves have a motion similar to that of waves in the sea. The ground is made to move in a circular motion, causing it to rise and fall as visible waves move across the ground. Together with secondary effects such as landslides, fires and tsunami these waves account for the loss of approximately 10,000 lives and over $100 million per year.


L-waves move particles in a circular path.
Tectonic Earthquakes

Tectonic earthquakes are triggered when the crust becomes subjected to strain, and eventually moves. The theory of plate tectonics explains how the crust of the Earth is made of several plates, large areas of crust which float on the Mantle. Since these plates are free to slowly move, they can either drift towards each other, away from each other or slide past each other. Many of the earthquakes which we feel are located in the areas where plates collide or try to slide past each other.

The process which explains these earthquakes, known as Elastic Rebound Theory can be demonstrated with a green twig or branch. Holding both ends, the twig can be slowly bent. As it is bent, energy is built up within it. A point will be reached where the twig suddenly snaps. At this moment the energy within the twig has exceeded the Elastic Limit of the twig. As it snaps the energy is released, causing the twig to vibrate and to produce sound waves.

Perhaps the most famous example of plates sliding past each other is the San Andreas Fault in California. Here, two plates, the Pacific Plate and the North American Plate, are both moving in a roughly northwesterly direction, but one is moving faster than the other. The San Francisco area is subjected to hundreds of small earthquakes every year as the two plates grind against each other. Occasionally, as in 1989, a much larger movement occurs, triggering a far more violent 'quake'.

Major earthquakes are sometimes preceded by a period of changed activity. This might take the form of more frequent minor shocks as the rocks begin to move,called foreshocks , or a period of less frequent shocks as the two rock masses temporarily 'stick' and become locked together. Detailed surveys in San Francisco have shown that railway lines, fences and other longitudinal features very slowly become deformed as the pressure builds up in the rocks, then become noticeably offset when a movement occurs along the fault. Following the main shock, there may be further movements, called aftershocks, which occur as the rock masses 'settle down' in their new positions. Such aftershocks cause problems for rescue services, bringing down buildings already weakened by the main earthquake.

Volcanic Earthquakes

Volcanic earthquakes are far less common than Tectonic ones. They are triggered by the explosive eruption of a volcano. Given that not all volcanoes are prone to violent eruption, and that most are 'quiet' for the majority of the time, it is not surprising to find that they are comparatively rare.

When a volcano explodes, it is likely that the associated earthquake effects will be confined to an area 10 to 20 miles around its base, where as a tectonic earthquake may be felt around the globe.

The volcanoes which are most likely to explode violently are those which produce acidic lava. Acidic lava cools and sets very quickly upon contact with the air. This tends to chock the volcanic vent and block the further escape of pressure. For example, in the case of Mt Pelee, the lava solidified before it could flow down the sides of the volcano. Instead it formed a spine of solid rock within the volcano vent. The only way in which such a blockage can be removed is by the build up of pressure to the point at which the blockage is literally exploded out of the way. In reality, the weakest part of the volcano will be the part which gives way, sometimes leading to a sideways explosion as in the Mt St.Helens eruption.

When extraordinary levels of pressure develop, the resultant explosion can be devastating, producing an earthquake of considerable magnitude. When Krakatoa ( Indonesia, between Java and Sumatra ) exploded in 1883, the explosion was heard over 5000 km away in Australia. The shockwaves produced a series of tsunami ( large sea waves ), one of which was over 36m high; that's the same as four, two story houses stacked on top of each other. These swept over the coastal areas of Java and Sumatra killing over 36,000 people.

By contrast, volcanoes producing free flowing basic lava rarely cause earthquakes. The lava flows freely out of the vent and down the sides of the volcano, releasing pressure evenly and constantly. Since pressure doesn't build up, violent explosions do not occur.