Unlike the two most popular sci-fi visual media franchises (Star Trek & Star Wars) Traveller is not afraid of planets that are inhospitable through having too much or not enough atmosphere. Of the dozen or so atmosphere types in the CT world generation system, exactly 3 can be breathed without assistance. It is far more common in Traveller for a world to face some kind of environmental factor that makes it hard for humans to live there. Place shapes culture, and Atmosphere is a big part of your Place. So then, in no order, here are a few thoughts on how a planet's atmosphere will affect adventurers who visit there, and how that planet's culture can be influenced.
The most important way that atmosphere comes into an adventure is reminding the players that they are not on Earth, they're on an alien world. Differences in temperature, sky color, mist or smoke or clouds of gasses, odd smells, all color and define the place they're in. Traveller provides a very wide palette of worlds, don't let every place seem like your home town.
Vacuum, Trace and Very Thin atmospheres have sharply defined shadows, as there is no atmospheric diffusion of light. Also, there will be bright star and moon light at night - no diffusion and there will rarely be any clouds. Denser atmospheres will harder to see through due to the abundance of molecules of translucent gases and vapors. Imagine a whole world shrouded in London's pea-soup fog.
Tainted atmospheres can have anything in them that makes breathing complicated, even too much oxygen. Our air is about 21% oxygen. Breathing excessive levels of oxygen (hyperoxia) can cause seizures and breathing difficulties. If the atmosphere code says tainted, then the PCs can't breath it without assistance. Decide on an effect, and enforce it.
The CT rules give no
guidance for how dense a Dense atmosphere can be - Venus' atmosphere is 92
times as dense as ours. Even if the temperature and atmospheric composition on
Venus were Earth-like, we still couldn't breathe there, we'd be crushed
by the enormous pressure. So here we have a world where the Vacc suit is
worn to keep the pressure out, not in! In Dense atmospheres, gas and smoke rounds from grenades or artillery shells will have their area coverage halved - the gaseous compounds will not disperse well. Dense atmospheres will hold smells really well. Dense atmospheres will cut weapon range, increasing the negative DM's at Long and Very Long range.
On the other hand, in Vacuum, Trace and Very Thin atmospheres, gas and smoke rounds will disperse almost instantly, their effective duration cut to 10% of normal. Weapon ranges will be extended owing to the reduction of wind resistance. It will also be harder to hear, as there is less matter (air) through which sound can propagate. The referee can emphasize the lack of sound - on Earth we're used to background noise, often of traffic; but on an airless world there would be silence. Silence can be unnerving for some people; and while there's no rules for the effects of silence, it is worth reminding the players of that fact.
In standard atmospheres, there can be harmless compounds in the air that affect the color of the sky, making for even multi-colored skies. There may be natural irritants, which don't qualify for the Tainted rating, but that may affect respiration. If your END is say, 11+, you're immune. Otherwise, have each PC roll 10+ to be immune, or the PC is forced to wear a filter mask to avoid coughing or sneezing fits.
Any atmosphere can be combined with very cold temperatures; if the hydrographic code is 4 or less, then the atmosphere is cold and dry, there will be very little snow or ice. Just cold.
Any atmosphere can be combined with high temperatures, which causes difficulty whether it is moist or dry. On such a planet, perhaps all the population lives near the poles where it's cooler.
On planets with thin atmospheres, there may be no trees, just shrubs and bushes.
Populations under pop code 5 are more likely to be collective in orientation, especially if they are on planets with difficult atmospheres. Cultures that live in sealed environs because of atmospheric difficulties will invariably produce communal behavior as a byproduct of living in close quarters - they want stability and order, and communal orientation will do that. Also there will be cooperative over competitive attitude, as well as a doing orientation. Everyone has to pitch in to ensure survival for all.
Cultures on worlds with Very Thin and or Tainted atmospheres that are
collective-oriented will insist on everyone carrying spare filters and
emergency masks. It will be expected that they will give aid (spares) to
anyone in need, friend or stranger. Also, a there can be a cultural
fear of being without one, which could be used against an NPC. Refusing
to share mask or filter would be a serious breach of hospitality.
The same culture may adopt a habit of covering the face while indoors
or in 'clean' air. Half of whole face masks, loose or rigid, can be
decorated and have styles. They might indicate something about the
status of the person wearing it. Only very close associates will allow
one another to see their uncovered faces. Makeup and such will not be
the norm, as it may affect mask seals. Eye contact would be of supreme importance in this culture, as the eyes would be the only visible part of the face.
In summary, there are many ways to emphasize how a planet is not like Earth; from sights, to sounds to smells to the ways humans interact with the environment. This is a science fiction game of travel and exploration, simple observations like the ones above can greatly help the players visualize the setting as an alien world.