Last Thursday (19-10-00) was a day of unusual weather. A small low pressure system was moving slowly southeastwards across Port Phillip Bay and directing a south and southwest airflow across Geelong. The extreme instability of the atmosphere saw several rotating funnel clouds develop and descend over Batesford and our western suburbs.
Funnel clouds can easily develop into tornadoes if they descend to ground level. There the temperature is much warmer and causes massive updrafts which fuel extremely strong winds. They are visible rotating swirls of water vapour attached to a lowered cumulonimbus wall cloud, which may even be rotating itself.
Hamlyn Heights funnel 19-10-2000 (Please click to enlarge)
The people who took the photos are fortunate they are still around to tell us about their experience. A relatively small rotating funnel such as those seen at several hundred feet altitude can generate winds of over 250 kilometres per hour when they reach the ground and the damage in an urban area can be horrendous.
Between 12.30 pm and 2 pm a number of funnels were seen. They began to develop, then dissipate and redevelop regularly over Batesford, Hamlyn Heights and Herne Hill. The only reason they did not develop into tornadoes was that the wall cloud from which they descended was only rotating slowly and was relatively small in size.
The great instability of the atmosphere over west Geelong on that day can be judged from the fact that, while the temperature on the ground was about 19 degrees C, it was minus 23 degrees at an altitude of 5000 metres. Normally on such a day you would only expect it to be around minus 11 degrees at that altitude.
Thus, with a 42 degree temperature differential in 5000 metres, what else could the lower atmosphere do than to be in turmoil?