I know that aircraft generate wake turbulence and wingtip vortices, and the strength of those phenomena are related to the size of the aircraft, but how powerful are wingtip vortices in terms a non-pilot would understand?
For example, if a human were standing inside of a wingtip vortex would it have the strength to knock them off their feet? If the wingtip vortex were turned vertical like a tornado would it be an EF1, or an EF5?
If we're talking about large aircraft the answer is Powerful enough that you want to stay out of them.
I refer you to this delightful FAA video in which a Cessna (looks like a 172) was flown into the wake of a C-5 Galaxy. The Cessna was rolled through 360 degrees by the vortex.
NASA research has shown that the rotational speed of wake vortices can exceed 150 miles per hour (130 knots).
While the Enhanced Fujita Scale rates tornadoes based on damage a 150 mile per hour wind speed is in the range estimated for an EF3 tornado ("Severe Damage", defined as Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations are badly damaged.) - This should reduce your desire to be anywhere near the wake vortex of a large aircraft.
If we're talking about smaller aircraft (say two Cessnas flying in trail, and the trailing aircraft somehow gets below the lead) it's strong enough to be unpleasant - the wake from the leading aircraft will shake and rock the wings of the trailing aircraft, but it probably won't flip the trailing aircraft over.
Many pilots are familiar with encountering their own wake at some point in flight training: It can happen when practicing steep turns in calm air.
If the weather and timing of the turn are "just right" and you produced a circle rather than an oval yo will hit the descending wake from your outside wing as you roll level at the 360 degree mark. This produces a slight "bump" at the end of the maneuver (visible in this video).
Depending on the size of the aircraft, very powerful. One way of thinking about his would be that the velocities in the wake are directly related to the lift produced. So, heavier the aircraft, larger the lift and greater the velocities.
A human being inside a vortex of a large would feel like he's inside a tornado. For all practical purposes, it is a mini-tornado.
The maximum vortex strength occurs when the aircraft is heavy, slow and clean (with low wingspan). Wind speeds upto 240-300 kmph has been recorded in certain cases.
This is the main reason for having separation intervals between two aircraft taking off, especially after a 'heavy' aircraft. The best examples of what happens when someone is inside these wingtip vortices comes from incidents where (smaller) aircraft gets caught up in the wake of (larger) aircraft and it doesn't make for very happy reading.
...experienced an uncommanded 52° roll to the left, in conjunction with an 8° nose-down pitching motion. Immediately after, the aircraft rolled through wings level to a 21° right bank angle. The aircraft also experienced an altitude loss of 300 to 400 ft in the 9 to 15-second period during which the crew regained control of the aircraft.
From these incidents, it is pretty clear that getting into the vortices, especially the large one is a very bad idea.
