Heat and cold can kill you here. The temperature where you stand comes from real climate data for that latitude, that altitude and that month. The server checks it every three seconds.
Between 0 C and 30 C nothing happens. Most of the inhabited world stays inside that range all year, so about seventy per cent of the map never sees this system do anything.
Outside that range you take damage. About four per cent of the map is cold enough to kill you: Siberia, inland Canada, the Antarctic coast. About six per cent is hot enough: the Sahara, Arabia, the Lut, inland Australia.
What helps
Armour changes how hot or cold you feel. A full leather set makes you feel about ten degrees warmer, which is worth one whole band. A full metal set is worth about four degrees, because metal traps heat and insulates badly.
Being warmer helps in the cold and hurts in the heat. The same leather set that saves you in Siberia will kill you in the Sahel.
Reading it
/climate shows the temperature where you stand and what the month is doing to it. /climate units c and /climate units f switch between Celsius and Fahrenheit. The server default is Celsius. There is also a gauge on your screen.