Showdown
by Jeff
Image text: The tumbleweed then tried to roll off into the sunset, but due to the Old West's placement north of the subtropical ridge, the prevailing winds were in the wrong direction.
Hope everyone had a safe and happy Labor Day holiday. Now let's dive into this Old West themed xkcd.
First and foremost, this is a reference to the cliche of a Old West showdown in which two combatants face off against each other in an empty desert clearing. While the two combatants are staring each other down, a tumbleweed "tumbles" past them. Tumbleweeds are as the name indicates a group of weeds that get caught together and are pushed by the wind. They are usually used in movies and TV to indicate an abandoned or empty place.
Back to the comic, the tumbleweed tumbles between the two gun fighters and shoots both of the duelers.
The image text references the fact that in the Western genre of movies, the hero will ride off "into the sunset", which means the hero only appears to be riding into the sun as he or she rides of directly towards it. In this case, the tumbleweed can not "roll off into the sunset" because in the Western USA, where most Westerns are set, the jet stream or "prevailing winds" blow from west to east.
So, the tumbleweed would have to roll off into the rising sun. But, that does not work as well with the Western genre.

September 7th, 2010
Not rolling into the sunset? That would make it a weird Western movie. It just can’t be – the wind should be forced to change its direction or else…
September 8th, 2010
I thought: “how hard can that be? Just change the earth’s rotational motion.”
But then I realised that this will probably shift the poles, so we have to re-define north and south …
So let’s just go to the mall and buy all the hair (no, not hare) dryers we can find.
September 8th, 2010
Hollywood’s been known to use a wind machine, from time to time.
September 25th, 2010
A tumbleweed is usually a single plant whose stem detaches when it’s mature. The tumbling action distributes its seeds. A common tumbleweed species is called Russian Thistle.
It’s possible that the guns coming out of the tumbleweed refers to the Russian Mafia, but what do I know?
December 14th, 2010
[apparently] Nothing much in this comic, just a spoof on the usual “one guy shoots other after long wait with tumbleweed tumbling past between them and”.
In this comic, neither guys shoot, instead, the TUMBLEWEED itself shoots both guys.
The idea of something as insignificant as tumbleweed actually killing supposedly main characters is kind of funny in itself.
April 17th, 2011
But are duels not traditionally fought with “pistols at dawn”? If so, the tumbleweed would roll off into the sun, and so it would look appropriate as long as they filmed it on a short enough timescale.
August 9th, 2011
I thought a lot of the Old West shootouts (rather than formal European duels) took place “At High noon”.