Eventually, there was enough gas to create an atmosphere. This is a simple question of density. Helium is less dense than the air around it. You can do the simple experiment with water and cooking oil. Put them in a glass, and the oil will always float above the water because it is less dense.
Tornadoes can form at any time, day or night.
The important thing is to have the right conditions. First of all, tornadoes can only form from one type of cloud. Only the most advanced and largest clouds can make tornadoes. Plus, there must be a high level of instability in the atmosphere, and a high dew point. The height of the tropopause depends on a number of factors, including the surface temperature.