Explanation:
An air mass is a body of air with a relatively constant temperature and moisture content over a significant altitude. Air masses typically cover hundreds, thousands, or millions of square kilometers. A front is the boundary at which two air masses of different temperature and moisture content meet.
I believe the answer is a soil
i hope this helps you
from google:
"The Earth gets hotter as one travels towards the core, known as the geothermal gradient. Geothermal gradient is the amount the Earth's temperature increases with depth, indicating heat flowing from the Earth's warm interior to its surface. On average, the temperature increases by about 25°C for every kilometer of depth"