The past few days have seen a major increase in thunderstorm activity over eastern Australia. Thunderstorms are storms that are accompanied by lightning and thunder within cumulonimbus clouds. The name cumulonimbus rises from the Latin cumulus meaning heap and nimbus meaning rain-bearing. These are the tower-like, vertical clouds which are formed by water vapor carried by powerful updrafts (upward air currents). Associated with these tall vertical clouds, which in places like Darwin can reach up to 20km in height, we usually find low altitude cumulus cloud which grow horizontally rather than vertically. When found individually, cumulus clouds are puffy or cotton-like. The combination of these two clouds, however, give rise to the classic mushroom-like shape of the thunderstorm clouds. Well-developed cumulonimbus clouds are characterized by flat, anvil-like tops which are more commonly found in the tropics. Thunderstorms can happen alone, in clusters or along a cold front in squall lines: - Multicells, are several individual cells grouped together with new cells forming as the old ones decay. - A supercell is a very big thunderstorm which can sustain itself over several hours. - A squall line can form as far as 300km ahead of a front with a line of thunderstorms that can extend for up to 500km. Although thunderstorms are more common in the tropics, it is in the humid sub-tropics were we find the most severe. Even though Darwin sees the highest annual average number of thunder days in Australia with about 80 days a year, Sydney sees more severe ones. Along the east costs, the incidence of thunder days is high along the Great Dividing Range (GDR) and adjacent areas (40-60 days a year). The GDR provides the lifting mechanism, taking the high humidity from the ground level to the upper levels of the atmosphere. In this case, winds close to the ground are forced upwards as they hit the mountains, leading to the condensation of the water vapor and hence the formation of the clouds. Currently, conditions are ripe along eastern Australia for thunderstorms to develop. A high over the Tasman has been directing hot and humid northerly winds bringing a hot and humid air mass throughout the eastern seaboard. This has been feeding moisture to a near stationary trough in eastern Australia which combined with the GDR the right breeding ground for these clouds to form. Over the next few days, easing conditions will prevail over NSW and VIC, with the focus of these thunderstorms turning north towards QLD.
Posted on: Sat, 06 Dec 2014 02:37:27 +0000
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