This is the 971st most frequent Russian word.
упал
"Fell" (past tense of "fall").
Here, 'упал' indicates an object (the ball) falling due to gravity.
Мяч упал на землю.
The ball fell to the ground.
In this context, 'упала' describes a drop or decline in a measurable quantity (temperature).
Температура ночью сильно упала.
The temperature dropped significantly at night.
'Упал' refers to a person (he) losing balance or footing and falling down.