Fireball Point of origin

Sage Advice: Fireball is described in Player’s Handbook p.241 or Player’s Basic Rules p.90
The Question reminds me that previous editions specify: A streak flashes from the pointing digit and, unless it impacts upon an obstacle prior to attaining the prescribed range. An early impact results in Fireball.

Blind fog

 Sage Advice: If you want to manage Advatages and Disadvantage rules you can find a good examples in Dungeon Master’s Guide p.239/234.
Advatages and Disadvantage rules are in Player’sHandbook p.173