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UNSW : UNSW Atmosphere Study Guide
Atmosphere Study Guide 26 City dwellers of the future will experience more violent thunderstorms more often. And Mother Nature has nothing to do with it: our built environment is manufacturing its own weather. YOU’RE OUT IN your backyard on a sunny autumn day when you notice a shroud of high cloud has suddenly blocked out the Sun. On the horizon you see a dark, ominous mass of cloud over the city, almost certainly a thunderstorm. The wind has picked up too. As you head inside, the heavens open. Heavy rain floods the street. There’s a flash of lightning and five seconds later the thunder arrives. “About five kilometres away,” you think, remembering the calculation your grandfather taught you. That’s when the hail starts. Golf ball sized chunks of ice begin to blanket your backyard. And in less than an hour it’s all over. SEVERE STORMS You’ve just experienced a severe thunderstorm, which released more than 1015 joules of pent up energy. An equivalent amount of electricity could power all of the U.S . for about five hours. The National Severe Storms Laboratory (NSSL) run by the U.S . National Oceanic and Atmospheric Administration estimates that at any given moment there are 2,000 thunderstorms in progress around the world. When a thunderstorm contains tornadoes, hail over 18 mm in diameter or winds faster than 90 km/h, it’s classed as severe by the NSSL. Severe thunderstorms cause over two billion dollars worth of property damage and kill more than 100 people each year in the U.S . alone. The activity and the infrastructure in our cities is affecting the weather around us in three ways: 1. the concrete and asphalt heat up the ground relative to the countryside; 2. the buildings obstruct airflow; and 3. depending on the exact conditions, the particles in pollution can enhance or delay weather events. Even short term activities such as traffic jams can have an effect. What’s more, cities are set to explode, with the U.N. predicting the number of city dwellers will almost double to 6.4 billion by 2050, and changes in global climate are going to lead to stronger thunderstorms in regions that are already affected. 1. CITIES ARE HEATING UP Though they seem unpredictable, there is a simple recipe for thunderstorms. According to the NSSL, you need: a large temperature difference between the ground temperature and temperatures higher up in the atmosphere for an updraft of air; humid air at the Earth’s surface; and a trigger. Humidity provides the energy for the thunderstorm. When water vapour in a rising air mass cools and condenses, huge amounts of heat energy are released, expanding the air and resulting in large buoyant forces and vigorous updrafts high into the atmosphere. And finally, a trigger gives an upward push to the air to encourage the initial condensation. This could be anything that forces air to rise, such as a line of hills or the arrival of a cold front. Once this recipe is in place, the intensity of thunderstorms is influenced by the variation of wind with height, or ‘wind shear’, which dictates whether rain falls straight down back through the thunderstorm and disrupts it, and also by microscopic particles of dust and debris in the atmosphere, called aerosols, which cloud droplets form around. Our urban landscape is changing and experimenting with all these factors, and it’s having an effect. Back in the 1970s, the Metropolitan Meteorological Experiment in St Louis, Missouri found that the urban environment was found to be responsible for 45% more thunder, 83% more lightning and rainfall increases of between five and 25% immediately downwind of the city. There were also more hail days and greater hail intensity. Alex Ntelekos, a hydro-meteorologist who works in thunderstorm risk assessment with insurance broker Willis in London, agrees with the three main At any given moment there are 2,000 thunderstorms in progress around the world. The gathering storms WIKIMEDIA A severe storm over Sydney. Explain (article two)