Weather Hazards

Hurricanes

A Hurricane is a storm with a violent wind, in particular a tropical cyclone in the Caribbean. It has a wind of force 12 on the Beaufort scale (equal to or exceeding 64 knots or 74 mph).

A comparison of the effects of Hurricane Floyd (Florida) and Hurricane Mitch (Honduras)

This video shows how Fifty six people died in the USA as a result of Hurricane Floyd and 6,000 died in Honduras as a result of Hurricane Mitch. No evacuation plan was possible in Honduras and the emergency management organisations were very differently sourced. The clip highlights the fact that victims of 'natural' disasters are predominantly poor.

Tornadoes

This video is presented by Helen Czerski goes to the USA to experience the power of a real tornado. She meets up with an expert, Josh Wurman, to show how the physics of Earth's unstable atmosphere form tornados, but only when the conditions are right.

Drought

Drought occurs when there is a lack of rainfall over a long period of time, resulting in water shortages for groups of people, activities or the environment. Droughts have a significant impact on agriculture and can harm the economy.

Causes of lack of rain

• Water vapour needs to rise high through the atmosphere in order to condense and bring about rain. However, in areas of high pressure, with the air subsides, water vapour does not rise and no rain or clouds will form. When the high pressure stays in an area for a prolonged length of time the result is drought.
• Mountains affect the movement of air too. Air carrying water vapour will rise higher in order to pass over to the windward side of a mountain. As the air rises it cools causing water vapour to condense bringing about precipitation and when reaching the other side of the mountain it has lost most of its water vapour. The leeward side of a mountain is warmer and drier and in some cases a desert.

If there are long periods of no rain, the land can become extremely dry leading to plants dying, crops failing and possibly even animals dying.

During this time, humans are often forced to stop using hose pipes due to a lack of water.

Floods

Flooding is caused by:

• a large amount of persistent rain
• rapid thawing of snow
• a storm surge
• a combination of high tides and high river levels

In periods of heavy rain, many areas often flood.

This leads to crops being ruined and extensive damage to buildings.

People are often forced to evacuate their homes and in LEDC’s disease can also be spread by contaminated water.

Storm surges

Storm surges are caused by strong winds and low air pressure. When pressure decreases by one millibar, sea level rises by one centimetre. A deep depression, with a central pressure of about 960 mb, causes the sea level to rise half a metre above the level it would have been had pressure been about average (1013 mb). When pressure is above average, sea level correspondingly falls.

Storm surges create large waves. The highest waves wash away protective dunes, batter sea walls and break over coastal defences causing flooding.

The greatest surge on record for the North Sea as a whole occurred on 31 January and 1 February 1953

Thunderstorms

Most thunderstorms are associated with towering clouds known as cumulonimbus. The right conditions for the formation of a thunderstorm are unstable air and a mechanism for causing air to rise.

While air is rising it is said to be unstable. This instability is the result of a rapid fall of temperature with height, as well as a considerable amount of moisture in the air. This process may because by a warm surface; the air near the surface being forced to rise over higher ground or instability within a weather front.

For example, on a summer's day, the land is warmed by the sun, and as the air just above becomes warmer it starts to rise. As it rises it cools, and, if cooled sufficiently, cumulus clouds form at the condensation level. These small, white puffy clouds grow larger and larger as the temperature of the ground increases, causing more warm air to rise. After a time, the top of the cloud turns to ice (usually below a temperature of -20 °C) and streams away in the winds at the level of the cloud top, giving it a characteristic anvil shape.

Lightning

Lightning is a large electrical spark caused by electrons moving from one place to another. Electrons cannot be seen, but when they are moving extremely fast, the air around them glows, causing the lightning flash. The actual streak of lightning is the path the electrons follow when they move.

An atom consists of three basic parts, a proton (which has a positive charge), a neutron (which has no charge) and an electron (which has a negative charge). Electrons cling to the positively charged centre of the atom because they have a negative electrical charge. During a thunderstorm, some of the atoms in the cloud lose electrons while others gain them.

When a cloud is composed entirely of water droplets, there is very little transfer of electrons. As a storm cloud grows in height, the water droplets higher up become cooler. They continue in the liquid state below 0 °C as supercooled water, but eventually they begin to turn to ice, usually at a temperature below -20 °C. These ice particles often collide. When they do, smaller particles lose an electron to the larger, thereby gaining a positive charge.

The small particles are propelled towards the top of the cloud by strong internal winds, while the larger particles start to fall. This causes the top of the cloud to develop a strong positive charge. The larger, negatively charged, ice particles begin to 'capture' supercooled water droplets, turning them instantly to ice and growing, some reaching a sufficient size to start falling. This leads to the base of the cloud becoming negatively charged which, in turn, induces a positive charge on the ground below. In time, the potential gradient between cloud and ground, or between adjacent clouds, becomes large enough to overcome the resistance of the air and there is a massive, very rapid transfer of electrons, which appears as a lightning flash.

Thunder

The word thunder is derived from 'Thor', the Norse god of thunder. He was supposed to be a red-bearded man of tremendous strength; his greatest attribute being the ability to forge thunderbolts. The word Thursday is also derived from his name.

Thunder is the sharp or rumbling sound that accompanies lightning. It is caused by the intense heating and expansion of the air along the path of the lightning. The rumble of thunder is caused by the noise passing through layers of the atmosphere at different temperatures. Thunder lasts longer than lightning because of the time it takes for the sound to travel from different parts of the flash.

You can roughly estimate how far away a thunderstorm is by measuring the interval between the lightning flash and the start of the thunder. If you count the time in seconds and then divide by three, you will have the approximate distance in kilometres. Thunder is rarely heard at a distance of more than 20 km.

Tropical Storms (Cyclones)

A tropical cyclone is a low pressure system over tropical or sub-tropical waters, with convection (i.e. thunderstorm activity) and winds at low levels, circulating either anti-clockwise (in the northern hemisphere) or clockwise (in the southern hemisphere). The terms hurricane and typhoon are regionally-specific names for a strong tropical cyclone.

Tropical storms (cyclones or hurricanes) can devastate large areas of lands and they tend to cause more damage in LEDC’s due to a lack of funding to help limit damage.

Impact and Response to Hazards

LEDC’s respond differently to MEDC’s during hazards due to a lack of money which could help limit damage. LEDC’s often rely on MEDC’s to provide funding or aid during hazards to enable them to rebuild the country.

However, some LEDC’s are better prepared as due to their geographical location, they are often more prone to regular hazards so have therefore put plans in place.

Economic relief and recovery after a natural disaster

This video examines the balance sheet of the recovery of Honduras after Hurricane Mitch. The hurricane threatened the long term economic stability of Honduras by destroying the banana plantations the country relied upon. International relief was often in the form of loans, which has added to the country's existing debts.