Yes I know it's caustic valley, but to simplify it for someone who hasn't played, I figured Desert sums up what it is. No it's not technically a desert, but i find it fits the best for those who have never walked it. Yes it's technically an arid/volcano. But there is no lava, and the volcano is simply making heat. I'll add in a side note for you however.
To be more clear, Caustic Valley is a "hot map"- that is, the ambient heat level is enough that a 'Mech constantly has an exceptional burden on it's heat sinks above and beyond the baseline. Frozen City is the opposite, while River City and Forest Colony are "heat neutral".
In addition, heat levels vary with terrain. Walk into a lake, and if you have leg-mounted heat sinks, your 'Mech will cool better- the deeper the water on you, the more it helps. Stop on top of a vent spewing hot gases and smoke on Caustic, and you'll heat up even more than the map normally causes- some places can leave an overheating 'Mech cooling so slowly that a shutdown can leave you frozen in place for a small eternity (on the other hand, there's shallow lakes and even deeper pools that'll help mitigate the heat load Caustic puts on every 'Mech).