The Nether, with at least 4*4*4 space, any light level, immediately above a spawnable block.
Varies by proximity and difficulty
Ghasts normally float around The Nether with their eyes and mouth closed and periodically make high-pitched, cat or infant-like sounds. They open their red eyes and mouth when attacking. They make a chirp-like noise when shooting and scream pitifully when taking damage. The sounds that Ghasts make have a minimum intensity regardless of their distance, so they often sound closer when they are actually far away. Their Fireballs do not home in on the player, and can be deflected if the player hits them with an arrow or melee attack.
Unlike most other aggressive mobs, the Ghast does not have a pursuit mode - it does not attempt to draw closer to the player when at long range (and likewise is not particularly likely to float away). Compared to other mobs, the Ghast has a very long search range and can target a player up to 100 blocks away. When the player comes within about sixteen blocks, the Ghast will open fire and attempt to gain altitude.
The original sound of a Ghast was Daniel Rosenfeld's (C418) cat, but then he changed it to a sound found on 'FreeSound'.
One hit from a powerful bow can kill a Ghast.
Ghast's Fireballs will not destroy Cobblestone so you can guard a portal and make a wall where you can hide and jump out when you're ready to attack.
If you play in creative mode, you can spawn Ghasts in the overworld.
Ghasts are troubling mobs considering they can deactivate the Nether Portal and fly around.