[Expectant_Philosopher]

Can we inject cold water into the winds of a storm to cause a tornado to dissipate?  I remember seeing a waterspout when I was younger and noticed how the white tornado dissipated as soon as it hit land.  Was this due to a change in density?  Could we change the density of a tornado by injecting it with cold water?  Could we line up water jets like they use for snow machines in a ring around a city and blast cold water into the storm to cause the tornado to lose energy? 

[Expectant_Philosopher]

The string of water jets could be automated with strong up drafts triggering the cold water torrents.  Any company or individual attempting such an endeavor would gain immediate notoriety.  If successful constant publicity at every tornado instance.  Failure would build goodwill that the individual was concerned enough for citizens to try to mitigate the impact of tornados.

[CliffordK]

I presume it would take a lot of water. 

I tried to look up information about tornadoes crossing rivers, lakes, and reservoirs, where they naturally could suck up a lot of water, but didn't find much.

Keep in mind that a tornado is just one part of a larger storm system that includes the meeting of cool polar air, warm tropical air, warm ground beneath, and cool clouds above. 

The individual cyclones can appear, travel for a bit, disappear, then reappear a little further on.  So, dissipating a single cyclone without dissipating the storm system may be ineffective.

[Expectant_Philosopher]

Surface area of dispersion rather than volume of water is key.  The target is the warm dry updraft.  Snow machine water jets atomize the water to finest particles they can so that freezing temperatures instantly freeze them into snow.  In the warmer climate near where tornados form the machines are not going to get the freezing temperatures unless you incorporate a refrigeration unit, but that isn't needed, or wanted.  You need liquid water to be most effective.  The atomized water blown into the air, would act like those misters you see at amusement parks and cities to cool people off from the heat of the day.  The dispersion of the water would rapidly cool the warm air being sucked up into the tornado, slowing its rise and causing the temperatures of the downdraft and the updraft to equalize.  A tornado that crosses a body of water may suck up water but it is not necessarily atomized, a water spout that you would think was made up entirely of water sucked up from a lake...is not made up entirely of water sucked up from the lake.  Land tornados that cross water are not in contact with the surface of the water very long, and there is little or no atomizing of the water on those contacts.  The movement of atomized water particles would be governed by the wind directions, if directed into the updraft, the surface area of dispersion in the updraft would rapidly cool the entire updraft.  The lore of being safe from tornados if you live near water may be due to the variability of the state of the body of water at the time of the storm - in an atomized state they tend to defeat tornados.  Over time a number of tornados do cross bodies of water, but the number of cases of personal witness of a tornado dissipating on approach to water builds the lore.  Depending on the lore will not keep you safe, but the lore indicates a symptom of science that can affect tornados.

[ScientificSorcerer]

I imagine that inorder for this to work you'll probably need to get pretty close to a tornado, with some sort of supercharged steam blaster mounted on a firetruck tank thing that has a storm radar which will take you across the country chasing tornadoes nomadic-ally and heroically too especially after that huge tornado that wrecked everything a few weeks ago :) are you man enough to pilot it to save the town? it might suck you up. but if you invented it you would have a legacy and an epic story to tell one day if you dont die