It’s a question that get’s asked a lot and it can get really confusing when a tornado forms over water like one did Thursday afternoon over Mobile Bay.
The most obvious answer is that spouts are weaker (winds generally less than 50 mph) while a tornado is much stronger (65-200+ mph). But that doesn’t explain the why, which can be answered by how the two develop.
Waterspouts are spawned by low-based cumulus clouds that generally have no lighting (think typical rain cloud). Since these pop-up showers are most common in the summertime, waterspouts are also more common in the summer.
Also, since these clouds only climb to around 20,000 ft. they have weak updrafts which can only create a weak circulation. Waterspouts are short-lived and generally dissipate when they reach land.
Sidebar: The “water” you see in a spout is not water being sucked up from the ocean. It’s actually condensation from water droplets (like a cloud). The spray at the base of a spout generally is only carried up a few yards.
Tornadoes are formed in a much different environment. A twister is spawned from a supercell cumulonimbus cloud that can reach 40,000+ ft. in height. These are severe storms with tremendous updrafts that create violent circulations in the atmosphere.
These storms are far more likely in spring and fall when cold fronts crash into our muggy Gulf air. When a tornado happens to form over water it can move inland and create far more damage than a waterspout.
These are the “general” rules, but as you can imagine they are some grey areas where the line between a strong waterspout and a weak tornado can become blurred. I hope though, that this answers some of the questions about waterspout versus tornado.