According to huygens's principle, what is the explanation for boundary behaviors?

The Huygens–Fresnel principle says that every point in the wavefront is also a source of spherical "wavelets", and these wavelets interfere with each other, and the sum of these spherical wavelets are the new wavefront (this also happens when the wave hits a interface, and mostly in these cases is usefull this principle). So, for example, when a plane wave hits a surface, in that moment the wave can "change" a spherical wave now (you can see this when a plane wave impacts a single slit). This also explains why a beam changes of direction when it hits a surface and is diffracted.

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.