What are the external factors in the tourism system?

I agree completely with the above answer, and would only
add a couple of other external factors to consider as far as tourism
goes:

1)  Currency Rates -
This ties in to the above post in the reference to the strength of other countries'
economies, but to name a specific example, the value of the Euro dropped significantly
when a European Union bailout of Greece was announced.  This affected American tourism
because it immediately became more expensive for all Europeans to travel to the US, no
matter what internal factors in the US were at play.

2) 
World Security - The airlines went into a tailspin (pardon
the bad pun) right after the 9/11 attacks when people in general viewed airline travel
as unsafe.  No matter how irrational that may have been, it affected tourism revenue and
the tourism industry itself had nothing to do with it.  A similar thing happened after
the subway attacks in Spain and London, and people avoided those countries for a
while.