Consider the following explanation of convectional rainfall given in a VII grade textbook.
When air comes in contact with the hot surface of the earth, it gets heated, becomes light and rises in the form of air current. After the warm air current reaches the upper layers of the atmosphere, it expands adiabatically and loses heat. This leads to condensation and the formation of cumulus and cumulonimbus clouds which soon become heavy with condensed water vapour and cause rainfall.
On searching online, most easily found sources give the explanation on the same lines.
There are a few things I don't understand:
Which air is responsible for the rainfall? Is it just the moisture-laden air present above the oceans and the water itself that evaporates from water bodies or even the air present above land surface?
Obviously, all the air mass which receives sunlight whether above land or water becomes hotter and goes up but the air above oceans already has water-molecules while the air above the land is just the normal mixture of air with some percentage of oxygen, nitrogen etc molecules. Do the molecules from this normal mixture also combine at higher altitudes to form water molecules and add to the precipitation process?
How do we know that the expansion at higher altitudes is adiabatic and would precipitation in the form of rain not happen if it were not adiabatic and something else?
What exactly is meant when it is said that clouds become very heavy and can't hold on to water and therefore cause rain?
Any moisture-laden air, which may also be air above the land. Air above land has humidity as well.
The temperature drop is because the air expands as it rises. This is a key part of convection, so it is almost tautological or definitional. This is because the atmospheric pressure decreases as you go higher, so a blob of air with 1 atm of pressure will be surrounded by less pressure as it rises.
Steam or tiny water droplets can stay aloft as long as convective air currents lift them up faster than they can fall. Friction prevents water from falling too quickly. For steam or tiny droplets, the surface area per mass is large enough that they stay aloft. When the mass of a droplet gets too large due to condensation, this surface/mass ratio is too small, so there is not enough friction. Once the terminal velocity of a droplet becomes large enough, the rain falls.