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UNSW : UNSW Atmosphere Study Guide
Atmosphere Study Guide 27 ways that our cities alter thunderstorm dynamics. The first of these is the ‘urban heat island’ effect (UHI). The concrete and asphalt in urban areas heat up faster during the day than forests or grasslands, and using energy also releases heat. This leads to a ground temperature difference of up to three degrees Celsius compared to the surrounding countryside, Ntelekos says. But temperatures higher up in the atmosphere are unaffected – the large temperature difference means that air is more buoyant than the air above it and will rise in an ‘updraft’ until it reaches air of the same density. This can be enough to trigger rain or thunderstorms that wouldn’t have otherwise occurred. 2. CITIES OBSTRUCT AIRFLOW “Approcahing air masses will be warmed and lifted when they come close to the city,” explains Ntelekos. “The air gets saturated and you can get more rain.” Another influence on thunderstorms is the buildings and infrastructure, or ‘urban canopy layer’, which alters the surface wind flow. Wind flowing over a city encounters obstruction at ground level as it moves over and around buildings, which slows its progress. But more air continues to arrive, and has a damming effect. This forces some of the air to rise over the obstruction, the same way that water flows over rocks in a stream. If the air is humid enough and condensation occurs, then this can also trigger a thunderstorm that otherwise might not have occurred. 3. CITIES HAVE MORE POLLUTION Pollution also affects thunderstorms, though this effect is the most complicated and least understood, according to Ntelekos. Pollution can either enhance or suppress activity, says Daniel Rosenfeld, a cloud physicist at the Hebrew University of Jerusalem. If aerosol concentrations are higher, then a larger number of smaller cloud droplets form. These smaller droplets take longer to coalesce into raindrops, which delays the onset of rain, but it can also extend the lifetime of the storm. In some cases, the longer lifetime leads to the transport of more liquid water high into the cloud, where it freezes, releasing additional heat and further invigorating the thunderstorm, says Rosenfeld. For example, in storms, if humidity is over 100%, then condensation will occur on aerosols, releasing energy in the process. But if humidity is under 100%, evaporation will occur, which absorbs energy and weakens storms. Due to cities, land surface is predicted to warm faster than the ocean. In the future, the temperature difference between land and the atmosphere will be greater than it is today, which is conducive to stronger storms. Balancing the fact that there will be greater temperature differences above cities is that less wind shear is expected in warmer climates, according to meteorologist Jeff Trapp, from the University of Purdue in Indiana. Wind shear prolongs the storm’s lifetime by stopping rain falling back through the storm’s updraft. But the increase in available energy will outweigh the reduction in wind shear, says Trapp. A warmer climate will see an increase in the number of days where conditions are ripe for severe thunderstorm development. Trapp’s research has concentrated on the U.S ., where models predict these effects will be concentrated around the densely populated areas of the Gulf of Mexico and the Atlantic coast. As cities grow and climate warms there is strong evidence that areas already prone to thunderstorms will see bigger, stronger storms, leading to greater property losses. There may not be much we can individually do to limit humanity’s influence on thunderstorms, but it’s looking as if teaching your grandchildren how to time the gap between lightning and thunder will be even more relevant in the years ahead. – Aaron Cook Explain (article two) As cities grow there is strong evidence we will see bigger, stronger storms. iSTOCKPHOTO:WIKIMEDIA Storm clouds roll in over Swifts Creek in Victoria.