Hurricanes grow stronger over warm waters and correspondingly lose
strength over cool waters. Hurricanes typically need an ocean
temperature of about 80º F, 26º C, to form. This page shows the
cooler waters following Hurricane Bonnie caused Hurricane Danielle
following in Bonnie's wake to lose strength and dissipate:

What Lies Beneath a Hurricane.
http://science.nasa.gov/headlines/y2000/ast11sep_1.htm

According to the graphic on this page, the temperature only had to be
reduced to about 75º F for this to occur.

So could we cover the expected hurricane path with chemicals that
produce a temperature reduction on mixing with water to reduce the
ocean temperature?
One of the most well-known chemicals with this property is ammonium
nitrate, NH4NO3, commonly used to make fertilizer. This temperature
reduction property also allows its use in instant cold packs.
According to this page 14 kg of ammonium nitrate could be used to
freeze 14 liters, 14 kg, of water:

Making ice without machinery
http://www.madsci.org/posts/archives...5573.Ch.r.html

The page calculates the amount of ammonium nitrate required to reduce
the temperature from 25º C to the freezing point but then notes an
additional amount of heat energy needs to be removed to induce the
phase change from liquid to solid.
If you only want to reduce the temperature from 25º C to 0º C, then
only about 1/4 the amount of NH4NO3 needed for freezing needs to be
used. And if you only need to cause a temperature reduction by about
3º C, the amount can be reduced further by a factor of 1/8th. So the
amount would be less than 1/30th that needed to induce freezing for
this low amount of temperature reduction.
There is a reason though why you might want to induce freezing. You
would want to keep the temperature reduced over the covered area for
some time so that the hurricane has time to dissipate. If the water
were frozen at the surface, then it would require some time for this to
melt. (BTW, the freezing point of seawater is only 2 degrees C less
than that of fresh water so this would require only minimally more
temperature reduction.)
The question is how much NH4NO3 would be required for this task? For
the freezing, about the same amount in weight as the water you wanted
to freeze. There are a couple of options for its placement. You could
try to freeze the surface water within the eye or you could freeze the
water in front of the hurricanes expected path.
I'll use an optimistic size of the eye as 10 km across, though for
some hurricanes the eye can be 3 to 4 times this size. So it would be
an area on the order of 100 square kilometers. How thick do you want
the ice? That depends on how quickly you would expect it to melt at the
26º C surrounding temperatures. I'll take as a guess for the thickness
of 1 cm. Then this is a volume of 10,000m x 10,000m x .01m = 1,000,000
m^3. This is 1,000,000 metric tons of water. Then it would require that
amount in weight of NH4NO3. The worldwide production of ammonium
nitrate is in the millions of tons per year so this would require a
significant proportion of that. But this is within the annual
production capacity of individual chemical plants:

Our Products - Terra Industries Inc.
http://www.terraindustries.com/our_products/intro.htm

So it is feasible if kept in storage until needed.
For transporting this amount, there are supertankers capable of
transporting hundreds of thousands of metric tons of crude oil. Less
than 10 would be sufficient to transport the required amount. The
ammonium nitrate would have to be sprayed at high speed to disperse it
over the required area.
It would require much less if you only wanted to decrease the
temperature 3 degrees C, perhaps only 30,000 metric tons for the same
volume of water. You would want this to be in very fine powder so would
rapidly mix with the water. A problem is the temperature would rapidly
rise from the surrounding water and air. What might be needed would be
some method of slow release to constantly keep the temperature lowered.
The packets containing the ammonium nitrate held within a slow release
fabric would also have to be buoyant so that the ammonium nitrate is
concentrated near the surface. However, the amount required might not
wind up to be significantly less than the freezing method because the
ammonium nitrate has to be continually supplied.
These were estimates for covering the surface water within the eye of
the hurricane. The eye is moving perhaps 10 km/hr and higher. At this
speed it would leave the covered area within an hour. Would this be
enough to dissipate the hurricane? Unknown.
The other possibility would be to cover the expected track ahead of
the hurricane. The front of the hurricane might be 100 km or more
across. For this to be feasible you would need a thinner region to
cover, say 100km by only 1 km. Then in this case the hurricane would
pass over this region even faster. But it is unknown which method,
covering the water within the eye or the water in front, would be more
effective in dissipating its strength.



Bob Clark