Hi Chris,
The relationship between wake turbulence and the timing between departures is dependant on the aircraft types involved in the scenario. In the United Kingdom at least, there is a 3 or 4 minute wake turbulent departure published in CAP 493 - Manual of Air Traffic Services (Part 1).
http://www.caa.co.uk/docs/33/CAP493Part1.pdf
I start my stopwatch as soon as the preceding departure rotates but from speaking to a number of my colleagues, there are only a handful of people that do this; the rest rely on ATC or some internal clock!
Eg: B747 departs full-length followed by an A320 departing from the same point with an optimistic take-off roll of 25 seconds. In the UK, CAP493 states a requirement of 2 mins wake turbulence therefore I would not start my take-off roll until 1:35 seconds after the B747 rotates, therefore I should have at least 2 mins.
On a couple of occasions I have been instructed to “roll now” with insufficient time remaining between the preceding departure and the next arrival were the wake turbulence spacing would have been unachievable. As for crosswinds and its effect on wake turbulent separation times, I am of the opinion that this would make life for our ATC friends a little too complicated for a number of reasons (including: SIDs, Missed approaches, Emergency turns etc) and in the absence of an approved procedure I just stick to the timings as listed in CAP 493.
One last thing to think about is the actual wind, when I used to operate out of Cardiff and on more than one occasion too, I have noticed the wind socks pointing at each other and nope my eyes are working just fine! I have noticed this at a few airfields as well but it is rather rare… I must be bored to notice these things!
Regards
C & B