23rd March 2006 - 01:11 PM
Dust devils are similar to tornadoes. Yet unlike a tornado which always spin counterclockwise, dust devils can spin in either direction. The idea of the Coriolis force and the rising of warm air does not apply to these. Why does water content always make cyclones spin counterclockwise?
23rd March 2006 - 10:15 PM
I think the answer is that dust storms are small compared to cyclones and hurricanes .. the latter cover a significant range of lattitude (hence Coriolis force) whereas dust storms are too small for the effect to have any influence on their direction of rotation. http://en.wikipedia.org/wiki/Coriolis_effe...htubs.2Ftoilets
I have heard (elsewhere) that bathtubs drain either way however careful you are (even in England).
I'd say probably nothing to do with water vapour.
23rd March 2006 - 11:59 PM
This morning, before posting, to test if my sink always drains counterclockwise, I did two experiments. The sink is symmetrical rectangular, stainless steel. The first was to fill the sink and then pull the plug. Sure enough, counterclockwise, in spite of the symmetry. The second experiment, I swirled the water clockwise and then pulled the plug. It drained clockwise. I was hoping it would resist and try to go the other way. But it actually drained a little faster.
The same is sort of true of water based cyclones. The condensation of water will pull a vacuum within the air. This creates a directional wind force, that supposedly works in conjunction with the Coriolis force. Together that will make water based cyclones always spin counterclockwise in the northern hemisphere.
The dust devils lack water to pull a vacuum. The dual spin orientation is strange because, as the air rises, it should begin to expand like a dry high pressure system going into a lower pressure system. One would expect a clockwise spin yet both spin directions are observed to occur. Like you said, the small size, maybe up to a 1000ft tall, is not influenced by the Coriolis force.