What is the climate in the underdark?

[Hawksteel]

Is it meant to be hot, temperate, humid or cold?

[GalacticAxekick]

Like the surface, it varies.

There are underdark deserts and forests, tropics and tundra, rivers and oceans.

[sengmeng]

As was stated, it varies, but as per some of the Forgotten Realms tie-in novels, it is generally warmer than the surface above it, due primarily to magical emanations from certain crystals.

[Green Traveller]

Adding up to what others said, real life cave systems also vary based on their geography.

Typically, the deeper you go, the warmer it gets. Go deep enough, and even metals starts boiling (read: Russia's Kola Well, the deepest hole on Earth)

Subterranean Cave systems have many variations that affects temperature.

If the cave system is well ventilated (lots of openings, entrances & airshafts, etc) like in a man-made coalmine, the ambient temperature inside almost matches with the temperature outside.

Caves in mountainous areas are more likely to be warmer due to geothermal activity being closer to the surface (steam vents, lava)

Caves located next to bodies of water are obviously going to colder, especially when situated next to an underground aquifer.

[Mechalich]

While there is significant temperature variation in the underdark, the range is generally from mild to hot. There are few areas where the underdark is legitimately cold and ice and snow are not a significant feature of the environment. The other thing is that whatever the regional temperature happens to be, it's going to be pretty constant, without any appreciable seasonality.

[Hawksteel]

Thank you everyone.