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
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.
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.
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.
E.g. 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 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 super-cooled 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’ super-cooled 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.
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.
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 effect 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.
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