Monday, 28 January 2013

Impacts of tornadoes

Impacts of tornadoes
A tornado is one of the most intense and destructive wind found on the Earth’s surface. Due to the fact that they as most common to form along a cold front mean that they can, and regularly do form in groups over wide areas. They are very destructive because the funnel wall has a very large lifting force. A large tornado has the ability to move large objects weighing as much as 300 tonnes tens of metres and can move large houses hundreds of metres. As well as being able to move large objects, tornadoes have the ability to suck up vast quantities of water. In the past, tornadoes have been seen to completely drain rivers and completely exposing the bed.
But this is not the most destructive part of a tornado. Explosions inside the tornado occur due to a large dip in pressure that happens in a very short time period. Buildings with higher air pressure’s to that of the tornadoes cause the explosion. When the high pressure is unable to escape fast enough, a large explosion is created, these explosions also produce vast amounts of heat, which can actually be the main cause of death through severe burning.
Tornadoes alone killed over 12,000 people between 1916 and 2000. Although now there is on average 200 less deaths a decade due to warning and awareness. In 1927 a town of 1200 was wiped out in under 90 seconds with 26% of the population being either dead or severely injured. Although now death toll has reduced, costs of repairs are still large. In 1974 315 people were killed in a 16 hour period between Alabama and Canada but caused damages which cost $500millionUSD.
+National Geographic +Geology +USGS Earthquake Notification BOT +Earthquake Update +Plate Tectonic

Resources used
  • Environmental Hazards-Assessing risk and reducing disaster (Fifth edition) Keith Smith and David N. Petley
  • Natural Hazards (Second Edition) Edward Bryant

Climatic Hazards: Tornadoes

What is a Tornadoe?

A tornado is atmospheric storm which consists of rotating column of air which extends down from cumulonimbus cloud (thunder storm cloud). They are usually only small in size and only last for a short period of time. They are often linked with supercell thunder storms.

How are Tornadoes formed?

They form because of the interaction between warm air and cold air. They are always formed over ground as the heat from the ground warms the air making it unstable through the day so that it rises. When it rises forms a cumulonibus cloud which coverges with the colder air. The air is then deflected by the Coriolis force and it spins upwards. This rotation is then aided by the differing wind speeds and direction at differnet altitudes. Supercell Thunderstorms are best at creating tornadoes due to there strong upward movement and imense downdraught. Near the rear of the storm a funnel of air can begin to appear which can lower and touch the ground forming the tornado.

Where do tornadoes appear? 

They are mostly seen in the middle latitudes but can also be linked with the updraught created by large hurricanes. The USA is one of the most common places for tornadoes to form with over 75% of the worlds recorded tornadoes being in this area. This is due to  the interaction between the warm air from the south over the Gulf of Mexico meeting with the colder air from the Arctic making perfect conditions for the formation of tornadoes. They have become so common in the southward stretch between Nebraska and Texas that the area has been named "Tornadoe Alley".

What effect do Tornadoes have?

Damage caused by tornadoes is often due to the high wind speeds which can reach over 500km per hour. There is also the lifting force of the funnel whcih can move large objects. They also create large preasure differential between the outside of buildings and there inside which can lead to them exploding outwards creating large ammouts of debriswhich does large ammounts of damge as it is blown around.

Acces to geography - Hazards - by Malcolm Skinner (pages48 to 51)
Natural Hazards - by Edward Bryant (pages 93 - 96)

Thursday, 24 January 2013

The hazards posed and some causes of tropical storms.

 I spent a large proportion of time in vein, but in the end I managed to scrounge these little gems from two academic journals published in 1994 and in 1991. Some quite interesting stuff here:

Types and causes of tropical storms:
The largest and most common type of tropical storm is the temperate or mid-latitude cyclone that forms as a result of rising air motion induced by waves in the jet stream. These low pressure systems derive their energy from the strong horizontal temperature gradients and are called "cold core" cyclones by meteorologists. Mid-latitude cyclones are the familiar low pressure regions that produce the majority of the cool season precipitation. In most of these cyclones, the pressure gradient between the centre and the outer margins of the systems is modest, averaging about 5 mb/100 km. Consequently, the associated wind fields are relatively weak, ranging from 20 to 30 kmh
However, there are times when the inducing factors, such as the presence of a substantial temperature contrast as the cyclones cross the Gulf Stream, or a sharp difference in temperature between land and the ocean, can combine to produce very strong temperate cyclones off the mid Atlantic coast. Tropical cyclones and their mid-latitude counterparts bear little resemblance apart from their cyclonic winds. Tropical cyclones are warm-core systems, in that the air temperatures at their centers are higher than those in the surrounding regions.
These cyclones form over the open ocean and get their energy from the evaporation of warm ocean water. The low pressure centre of a hurricane is augmented and strengthened in part by a massive ring of thunderstorms which release enormous amounts of heat into the storm system. The most powerful hurricanes will commonly have a 20 mb/100 km pressure gradient, which results in more powerful wind field.
The strongest winds are usually concentrated over a much smaller area than those associated with a mid-latitude cyclone. Also rather interestingly, less than five percent of all tropical disturbances develop into hurricanes. Seems bizarre but understandable from the explanation.
Coastal Storm Hazards. Robert Dolan and Robert E. Davis. Journal of Coastal Research, Special Issue No. 12. COASTAL HAZARDS: PERCEPTION,SUSCEPTIBILITY AND MITIGATION (1994), pp. 105 Coastal Education & Research Foundation, Inc.

Hazards associated with tropical storms in the mid-Atlantic region specifically:
One topic simply wasn't enough for me; I couldn't contain myself and I stumbled across this which seems quite self-explanatory but it's not the sort of thing I think about when I'm day dreaming. But apparently it is not the wind fields associated with tropical and extratropical storms that represent the most serious hazard, but rather the waves and storm surges that the winds produce. Sooner or later, all waves produced by these storms end up dissipating most of their potential and kinetic energy in the coastal zone. The growth of waves in a storm from the initial ripples and wavelets is governed by three factors: the wind speed, its duration, and the distance (fetch) of water surface over which it blows. With an increase in any of these factors, the height of the waves and the potential storm surge in creases. Wave hindcasting methods and storm surge predictions were developed by extension of the basic physics of wave generation. Wave heights generated by hurricanes typically range from 5 m to 15 m
Temporal Variation of Tropical Cyclones in the North Atlantic Basin. Anthony J. Vega and Mark S. Binkley. GeoJournal, Vol. 23, No. 4, Caribbean Hurricanes (April 1991), pp. 313 Springer

Cyclones cause huge damage every year, with loss of life and considerable damage to buildings as well. They normally happen around the Indian ocean, however can happen anywhere where the conditions are right.  They are usually caused by several triggers, 1) warm or moist air, 2) wind blowing in different directions 3) Low wind, known as low wind shear 4) sufficient distance from the equator causing spinning. 

They are low pressure systems over the tropics or sub tropics with organised convention. They are normally 5/6 miles high and 300/400m wide. They are normally about 10-15mph, however can be more. Anything above wind speeds of 74mph are considered to be hurricanes. Hurricanes are large cyclones. 

 Predicting cyclones is very difficult, often predictions from the sea are used, however, predicting them is considered a fine skill. Prediction is also done by people at specialist tropical cyclone forecasting centres around the globe such as the National Hurricane Center in Miami, Florida who are constantly studying satellite images, instruments and other weather data to detect and track them through their life-cycle.
Notable cyclones that you may have heard of Katrina - 2005 - Katrina was the most costly hurricane on record causing an estimated $75 billion in damage in Louisiana and Mississippi. 



Definition: A mobile,destructive vortex of violently rotating winds having the appearance of a funnel-shaped cloud & advancing beneath a large storm system.

Formation is precipitated by warm moist air near the ground meeting cold, dry air. Tornadoes usually form over land but they can form over tropical oceans. When the ground is heated by the sun, warm moist air rises. As it rises it cools forming massive cumulonimbus clouds and releasing heat that sustains the updraft. The updraft meets winds of different directions and speeds. This together with the rotation of the earth causes it to spiral coutnerclockwise, creating a vortex. The strength of the updraft determines how much of the surrounding air is sucked into the bottom of the tornado. As it rises the jet stream takes effect and causes an extra spin and the characteristic anvil cloud.

There are two ways that Tornadoes are measured: 1)The Fujita Scale and 2) Pearson Scale,
The Fujita scale is based on the speed of the tornado, whereas the Pearson Scale is based on the length and width of the tornado path.

Tornado Alley:
Tornado Alley in the USA has the perfect conditions for the formation of tornadoes. This is due to a number of factors. The first being there is a plentiful supply of warm moist air being brought in by the low level winds from the south and the Gulf of Mexico. The second is the very dry air that comes from the Rocky Mountains and the deserts of northern Mexico. The third is the prevailing westerly winds sometimes accompanied by the powerful jet stream, carrying cool air from the Pacific Ocean.

Stormy Weather by Mark Maslin p30-33.
Hazards by Malcolm Skinner p48-49

Tuesday, 22 January 2013

Introduction Into Tropical Storms

A basic definition to start - Tropical Cyclone - Violent tropical storms between 200-700km in diameter with very strong winds circling a calm eye. (1)

Conditions needed - Sea temperatures to be 26 C and above. Depth of ocean at 70m. Location of 5 N/S of equator for the Coriolis* effect to take place. Low level of convergence of air in low atmosphere. Rapid outflow of air in upper atmospheric circulation. (1)

Occurrence - Latitude of 5 degrees to 20 degrees N/S of equator. Moves Westwards

Structure - Approach -  Temperature falls, pressure falls, winds begin to gust, showers develop.
                  20-30km - Pressure decreases rapidly, 100km/h winds, cumulonimbus clouds give thunder and rain.
                  Eye - Period of calm, pressure very low temperature rises.
                  20-30km - Wind increases further, torrential rain, temperature drops further but pressure begins to rise.
                  Move away - Pressure and temperature rise, rain decreases, sunny intervals, winds drop.

Overall - It happens when warm air from the ocean rises and mixes with cool air above it. This produces storm clouds, rain, and strong swirling winds. When the winds hit 74 miles per hour, the storm is considered a hurricane. (2)

* Coriolis Effect - Results from the earth's rotation causing freely moving objects to veer towards the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

(1) Access to geography - Hazards - Malcolm Skinner, 2003, p38-39
(2)  Scholastic News -- Edition 4; 9/10/2012, Vol. 75 Issue 2, p4-5, 2p