The chlorine and ammonia gasses share the planet's atmosphere with other gasses, therefore they do not normally react violently to each other...However, if an ammonia fog cloud and a chlorine fog cloud vaporizing from the snowfall the night before were pure enough, they could produce a reaction and ignite. That has very rarely happened over the past 27 million years though. But when it does happen, it results in a midair, ground shaking, blast, or series of multiple blasts, some being loud enough to shatter eardrums and destroy sound recording equipment up to 12 kilometers (7 miles) away.
https://www.youtube.com/watch?v=lIijFSN35x8 loud as it can go, but still louder beyond comprehension (Be careful of your ears though. It's a loud video as it is).
Some of those rarely occurring blasts could have been heard as far as 480 kilometers (300 miles) away had there been anyone on the planet to hear them. At 8 kilometers (5 miles) away, you could feel the shock wave rattle through your body...That is in spite of the planet's present barometric pressure being at about 320 millibars.
There have also been occasions, again a very occurrence over the past 27 million years, when a meteor would strike a morning ammonia fog cloud...The heat generated by the meteor would be well above the flash point of the ammonia cloud required to ignite it and blow it up, also resulting in the same kind of midair, ground shaking, blast wave.
These very rare occurrences, however, happen on an average of only once, at any given location on the planet, every twenty thousand years or so.

If there had been a visiting expedition team at this distance from this blast, the shock wave would have shattered ear drums and sinuses, broken bones, and blown their spacesuits off of their bodies into shreds. The team and their space craft would have been hurtled hundreds of yards...The space craft, depending on origin, make and model, would have likely been literally ripped to pieces.
If a scout detail was as far as 480 kilometers (300 miles) away, they would hear the distant sound of the blast 25 minutes later...
https://www.youtube.com/watch?v=lIijFSN35x8 with sound turned down so you can barely hear it is what the scout detail would hear.
Conversation of the scout detail in their language;
Scout #1 "Hear that?!"
Scout #2 "Yea!".
Scout #1 "What could it be?"
Scout #2 "I don't know...Thunder or something exploded".
Scout leader "Whatever it was, and as far away as it sounded, it had to be big".
Scout #1 "Sounded like it happened again and again...One explosion after another".
Scout leader "It sounded like it came from the direction where the ship is. Radio them to see if they know what happened".

Not knowing...the mother ship 300 miles away is a scattered spread of torn metal...the mother ship crew are all dead...and they are a stranded scout detail with no way back off the planet, it would not be long before the scout detail would be radioing;
"Scout to Mother Ship. Did you hear that noise over your way?..."
"...Mother Ship. Hello?"
"Scout to Mother Ship. Can you hear me?...Scout to Mother Ship, respond please".
No reply.
"Mother Ship, what is your present situation? We are not hearing you".
Scout #1 "We're getting nothing"
Scout leader "Keep trying".
Scout #2 "Mother Ship. Are you receiving us?...Hello...Mother Ship".

Fortunately, there was no one on the planet at the time of this explosion.

The odds of there being a gas explosion somewhere on the planet during a 24 hour mission there are about 7,300,000 to 1.
As for being in the same geographical location where a blast occurs, the odds are even slimmer than that... Chances of winning the lottery are greater.
I can imagine some poor sap continually playing the lottery on the planet he (or she) was from, and never hitting the odds just right to win anything...But hitting the less than 1 odd in a greater than 7 million odds of THIS.

This one does not seem to have been sparked by a meteor plunging into an ammonia cloud...Because of the planet having a chlorinated atmosphere, the telltale greyish brown smoke trail would have been visible.