Re: Just when you thought it was safe
Originally Posted by
Jem
->
It seems the answer is blown in the wind. What about Tornado's? are they as bad and as dangerous?
Much smaller measured in metres rather than kilometres for cyclones - I think they are caused purely by the right wind conditions. I saw a very small one form on Sydney harbour, scatter a yacht race, capsize a couple of boats, come ashore, rip the roof off part of a house very close to the water and dissipate. (that was in the 60s)
About a year ago one hit Kiama near where I live it took a week for them to decide it was a tornado not just a severe storm cell. The damage was confined to a very narrow strip a few houses away were undamaged
http://www.illawarramercury.com.au/s...ermath/?cs=300
When I was looking for the details about the Kiama Tornado I found this:
For a cyclone/hurricane/typhoon to form several preconditions must be met:
- Warm ocean waters (of at least 26.5°C) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m). Warm waters are necessary to fuel the heat engine of the tropical cyclone.
- An atmosphere which cools fast enough with height (is "unstable" enough) such that it encourages thunderstorm activity. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development.
- Relatively moist layers near the mid-troposphere (5 km). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity.
- A minimum distance of around 500 km from the equator. Some of the earth's spin (Coriolis force) is needed to maintain the low pressure of the system. (Systems can form closer to the equator but it's a rare event)
- A pre-existing disturbance near the surface with sufficient spin (vorticity) and inflow (convergence). Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organised system with sizeable spin and low level inflow.
- Little change in the wind with height (low vertical wind shear, i.e. less than 40 km/h from surface to tropopause). Large values of wind shear tend to disrupt the organisation of the thunderstorms that are important to the inner part of a cyclone.
Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favourable conditions do not develop.