The supporting force exerted by a fluid on an object immersed in it is called

buoyant force

Explanation:

The buoyant force is the pressure that is exerted on a immersed object by the surrounding fluid.  

The buoyant force acts on the surface of the solid object even if the object is immersed in the fluid partially or completely. If the object is immersed in a fluid, the net force is given by the formula:

where Patm is the atmospheric pressure at the top of the object

pgh = pressure of the part  in the fluid

F(bottom) is the upward force on the object

The net force is the same as the wight of the displaced object.

The principle is known as the Archimedes principle as the buoyant force is equals the weight of the fluid displaced by the object.  

answer: the exterior temperature of commercial jet planes as they leave the stratosphere and return to the ground increases steadily.

explanation:

the lower most layer of atmosphere is troposphere which extends from 0 to 12 km above the surface of the earth. the temperature initially decreases as distance is increased from the earth's surface. commercial jets fly in lower part of the stratosphere. the stratosphere extends upto 50 km. the temperature in stratosphere starts rising again due to presence of ozone which traps the uv rays.

thus, when the planes leave the stratosphere and return to the ground, the exterior temperature increases steadily.