Oh OH!!! I know the answer to this one....
How to calculate cloud tops with wx radar...
(But you will have to depressurize the airplane)
Remove the radar display unit from the panel (although you may need to sneak a screwdriver on board because pilots aren't allowed to have screwdrivers).
Toss the unit out the window and start the timer...
When your unit disappears into the cloud tops, stop the timer...
Use the terminal velocity formula, v = the square root of ((2*m*g)/(ρ*A*C))
The rest is pretty academic.
(This really works by the way but I had to quick Google that last part)