Olin,

On Wed, 7 Sep 2005 14:40:48 -0400, Olin Lathrop wrote:

> Howard Winter wrote:
> > The wind direction is affected by the rotation of
> > the Earth (Coriolis effect) but the presence and strength of the wind
> > isn't.
> 
> I don't think it's that clear.  If one side of the planet always faced the
> sun, would there be much wind?  I think probably a lot less.  

Winds would there, but very different!  If the Earth was a smooth, homogeneous solid sphere, the point closest 
(with the best aspect) to the Sun would be heated most, and would heat the air above it, with decreasing 
heating in all directions away from this perihelion point.  The hotter air would rise, to be replaced by 
cooler air flowing in from all sides, which on the surface would be felt as wind.  Convection would then set 
in, with "ring doughnut" shaped flow which would settle at steady speeds when all the temperatures had 
stabilised.

However, apart from rotating, the Earth is far from a smooth homogeneous solid sphere - it's covered with 
(mostly) water, and land which varies in colour, material, and altitude.  All of these things mean that the 
heating of the surface, and of the air above it, is very uneven, and thus winds of all sorts are created for 
various reasons.  Fohn, Chinook and Mistrale winds, for example, are all created by particular surface 
profiles (mountains, mainly!).  On-shore and off-shore breezes are due to the different heating properties of 
water and land.  If rotation was the main cause of wind, the poles would be almost calm, but I understand the 
Antarctic regularly records wind speeds of 200mph.

> Also think of
> the trade winds and other wind belts which blow predominantly in a east or
> west direction.  
> It sounds quite plausible that both the strength and
> presence (really the same thing since presence is strength not = 0) of this
> wind are directly related to the earth's rotation.  It's not so clear that
> the power behind the wind comes from that, and most of it probably doesn't,
> but I don't think it can be dismissed so easily.

Actually the Trade Winds start blowing North and South, caused by the convection I mentioned above, and are 
turned East and West by the Coriolis effect - the winds describe curved paths away from the equator.  I accept 
that the direction is caused by Earth-spin, but the presence and speed is down to solar heating.
 
> > Tides are caused by the gravitational pull of the Sun and the Moon on
> > the water - the rotation of the Earth contributes friction, which may
> > help or hinder the flow of water, but it doesn't cause it (except to
> > the extent that the apparent motion of the Sun is caused by it).
> 
> Yeah, but that's exactly the effect the does cause tides, or at least causes
> them to go up and down (which is where the power is derived from).  The
> rotation of the earth doesn't in itself cause friction.  That happens
> because things deform or move due to the changing pull of the external
> gravitational field, which is all powered by the inertia of the earth
> rotating.

No!!!  While tidal effects caused by the Sun's gravity can be said to be because of the Earth's rotation (and 
thus the apparent movement of the Sun), the major cause of tides is the Moon, as can be seen during a Neap 
tide, when the Moon and Sun are at rightangles in relation to us, the high tide is below the Moon.  The Moon 
of course orbits us on its own with no reference to the Earth's rotation.  If tides were all due to the 
Earth's rotation/Sun, there would be tides at the same time each day, some time after noon (to account for the 
lag caused by friction and inertia) and 12 hours later.  But tides are much more complex, and are affected to 
some extent by the rest of the Solar System - to get even reasonably accurate prediction/modelling of tides 
you have to take account of the Moon, Sun, Mars, Venus, and Jupiter at least, otherwise the reality soon 
wanders away from the prediction/model.
 
> Think of the reverse.  Let's say you make a float that goes up and down on
> the ocean surface due to the tides, then hook it up to a generator to
> extract power from that up and down motion.  Explain where that power comes
> from if not from the kinetic energy stored in the rotating mass of the
> planet? 

It comes from the potential energy of the water being pulled towards the various celestial bodies, so forming 
a higher column as water flows in from surrounding areas, and then lowering again as the external influence 
"moves away".  In one way it's similar to a "storm surge", where an area of low air pressure inside (say) a 
hurricane causes a hump in the water below it, but in this case the extra upwards force comes from added 
gravity above, rather than reduced air pressure.

> By extracting the power from the float, you are making the planet slow down ever so slightly.

Well you are extracting the power from the water, which depending on the direction of flow towards your 
collection point may in fact cause pro-spin friction between water and Earth, so having the opposite effect!  
The Americas form quite a barrier to water trying not to flow with the Earth's rotation due to its inertia, so 
the water "piles up" against the East coast, "pushing" against the coast so tending to slow down the rotation, 
so if you extracted energy all down the coast, there would be less "push" so less tendency to slow.

(I'm fairly confident in most of the above, but less so in this last paragraph! :-)

Cheers,


Howard Winter
St.Albans, England


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