Hurricanes and Tornadoes

Tornadoes
With the amount of media coverage dedicated to American storm-chasers, you'd think the US had the monopoly on these twisters. It doesn't. It might come as a shock, but the United Kingdom is actually the world's most tornado-prone nation.



This fact was calculated by the late Dr Fujita of Chicago University. He devised the standard method of measuring tornado intensity. Fujita figured that since Britain has an average of 33 tornadoes every year in an area 38 times smaller than the USA, you're twice as likely to witness a tornado here.



How tornadoes happen
# Warm and cool airstreams collide
# A rotating area of low pressure storm clouds form
# Air within a low pressure front rises, creating a strong upward draught like a vacuum cleaner
# This draws in surrounding warm air from ground level, causing it to spin faster and faster
# These strong air currents can create a vortex - a spiralling funnel of wind - that can reach speeds of 300mph
# Where the funnel touches the ground, it creates a path of concentrated destruction, rarely more than 250m across




Heavy objects, like cars and cows, can be sucked up and flung around like confetti, and houses appear to explode. This is because air pressure within the vortex is extremely low. Inside the building the air pressure is normal, so when the tornado passes over, the air inside the building expands, creating an explosion.Wind speeds in tornadoes can vary from 72 to almost 300mph. Fortunately, only 2 percent of all tornadoes have winds greater than 200 mph.




Hurricanes



By definition, a hurricane is fierce rotating storm with an intense centre of low pressure that only happens in the tropics. In south-east Asia they're known as typhoons and in the Indian Ocean, cyclones.

They cause high winds, huge waves, and heavy flooding. In 1998, Hurricane Gilbert produced 160mph winds, killing 318 people, and devastating Jamaica. A tropical storm can only be classified as a hurricane if it sustains wind speeds above 73mph or force 12 on the Beaufort Scale. Each year about 50 tropical storms reach hurricane status.

One of the most powerful of all weather systems, hurricanes are powered by the heat energy released by the condensation of water vapour. However, the conditions have to be exact for a hurricane to form, with the sea's surface temperature being above 26.5°C.




How hurricanes happen
Air above warm tropical water rises quickly as it is heated by the sea. As the air rises it rotates or spins creating an area of low pressure, known as the eye of the storm. The eye can be clearly seen on satellite pictures, and is usually eerily calm.

The hurricane only moves slowly at speeds of 20-25mph bringing torrential rain and thunderstorms and very strong winds. However, they also cause flooding on low lying coastlines with a phenomenon known as a 'storm surge'.




Storm surge
This is caused by the intense low pressure at the eye of a hurricane, combining with the effect of strong winds. The sea rises 1cm for every millibar of pressure - if the pressure is 930 millibar, the sea surge will be about 80 cm. Hurricanes can raise the seas surface by as much as 4m.




The hurricane winds push the surge along in front of its path. When this surge hits low-lying coasts, the effects can be devastating. In addition to the sea surge, flooding can also result from torrential rain falling from the storm clouds.

Once it reaches the mainland, a hurricane may cause widespread damage for a few days, but with no warm water to supply heat, they quickly die out.

Earthquakes

When the Earth moves
Earthquakes are caused by the motion of tectonic plates - individual sections that make up the Earth's surface like panels on a football. Immense strain accumulates along fault lines where adjacent plates meet. When the rock separating the plates give way, sudden seismic ground-shaking movement occurs.

Of course, if we want to know why earthquakes happen, we need to dig a little deeper.



Centre of the Earth
The Earth is made up of three main layers:
# The core is at the centre of the Earth
# The mantle is a mobile semi-molten layer around the core
# The outer-shell of the Earth is called the crust. Scientists call this the lithosphere - it's the part we're on now

The crust is made up of 12 individual tectonic plates. Below the sea, they can measure three to six miles (4km-9.6km) thick and under land this increases to 20-44 miles (32km-70.8km). Below the crust, radiation from the Earth's core heats the semi-molten mantle to temperatures of over 5000°C.

All fluids when heated - even molten rock - are affected by a process called convection. The makes hot liquid rise to displace cooler liquid, creating a current. Tectonic plates effectively float on the mantle, like croutons in a bowl of super-heated soup. But these plates and constantly moving due to the convection current.

Of course, they creep along very slowly - roughly the same speed your fingernails grow. Even at this sub snail-pace, the effects can be devastating. The combined annual force of earthquakes is equal to 100,000 times the power of the atomic bombs that flattened Hiroshima.




The point where the seismic activity occurs is the epicentre, where the earthquake is strongest. But it doesn't always end there, seismic waves travel out from the epicentre, sometimes creating widespread destruction as they pass.


Earthquakes hit Britain on a fairly regular basis. In fact, there can be as many as three in a week. Luckily, the geology beneath us is stable, and these tremors usually go unnoticed.

Tsunami

History
"Tsunami" is the Japanese word meaning tidal wave. A tidal wave is a large sea wave caused by a submarine earthquake or volcanic explosion. When the ocean floor is tilted or offset during an earthquake, a set of waves is created. These waves are similar to the concentric waves generated by an object dropped into the water.

Usually tsunamis move entirely across an ocean to the shore. A tsunami can have wavelengths of 60 to 120 mi and may reach speeds of 800 km/h. When the wave enters shallow waters, the wave, which may have been half a meter high out at sea, grows rapidly. When the wave reaches the shore, it may be 50 ft. high or more. Tsunamis have incredible energy because of the great volume of water affected. They bring waves of destruction capable of killing thousands of residents along the coast. Towering walls of water have struck populated coastlines with such fury that entire towns have been destroyed. In 1896 a population of 20,000 in Sanriku, Japan were wiped out. Tsunamis have resulted in waves as high as 135 ft. above normal sea level.
When a tsunami strikes the shore, it creates a number of waves with troughs that are lower than normal sea level. Each following wave is higher than the one before it. The period between waves is 10 to 30 minutes. This usually gives people ample time to escape to high ground after the first wave.



Big waves
Following the world's biggest recorded earthquake (measuring 9.5 on the Richter Scale) off the coast of Chile in 1960, a series of waves created havoc around the Pacific Rim. It caused 56 deaths in Hawaii, 32 deaths in the Philippines, and 138 deaths in Japan - 10,000 miles (16,000km) away.



Tsunamis
A tsunami (pronounced tsoo-nah-mee) is a chain of fast moving waves caused by sudden trauma in the ocean. They can be generated by earthquakes, volcanic eruptions, or even the impact of meteorites. Tsunami are also incorrectly known as tidal waves, but unlike tidal waves they are not caused by changes in the tides.

They are most common around the edge of the Pacific, where more than half of the world's volcanoes are found. These seismic surges can assault coastlines, often with little or no warning. Rocks weighing as much as 20 metric tonnes have been plucked from sea walls and carried 180m inland.

Wave power
Tsunamis aren't like wind-generated waves that rhythmically roll onto a beach. A tsunami can have a wavelength (ie distance between wave crests) in excess of 100km (60 miles) and there may be an hour between them. They travel at great speeds across an ocean with hardly any energy losses and are barely noticeable out at sea.

Over the deep Pacific Ocean, a tsunami travels at about 800kph (500mph). If an earthquake happened in Los Angeles, a tsunami could hit Tokyo quicker than you could fly between the cities by jet. The tsunami caused by the earthquake off the coast of Indonesia in late 2004 travelled 4,500km (2,800 miles) to Somalia in East Africa in just seven hours.

As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it behaves like a normal wave - only with more muscle.

Shallow water slows the tsunami and its height grows. Tsunamis batter the coast with tremendous amounts of energy. They can strip sand from beaches, tearing up trees, and even obliterating whole towns. Some have been known to reach as much as 30m above sea level.