"Glade" might be the word you're looking for.
There are also a number of similar terms.
A meadow or pasture.
It is called a plain.
It is a prarie
Pampas
A thick forest is more densely packed, and able to keep heat in better. A grassy field is more open and exposed to the general air, and will thus lose heat more easily.
Swarm
A grassy plain with few trees is called a Savanna
field?
lawn
Pasture.
The grassy plains are called "Llanos". In other areas of South America they may be called "pampas".
The forest
It depends on the wildlife in the area. If there is a grassy egg breaker hidden in the dirt it will most definitely be broken.
Plains
It is a prarie
Fertile
Something
track and field got its name because when you run you on a track. the field part is because you compete in the other competions on a field a grassy area.
If the Sun is out, the grassy field would FEEL warmer, because you are getting warmed by the Sun. If it is Cloudy or at night, the thick forest would hold in more heat than the open Field.