Layers of the atmosphere

Correct answer: C. It is the upper boundary of mesosphere and a very cold region

The governing concept is the thermal boundary between atmospheric layers. The mesopause is the upper boundary of the mesosphere and lies near the region where the temperature reaches one of its lowest values in the atmosphere. It separates the mesosphere from the thermosphere, where temperature begins to rise with altitude. Therefore option C is correct; the other choices confuse it with the troposphere, ozone layer or weather.

Correct answer: A. Entry of ozone-rich, dry air into the troposphere

Correct answer: A. Tropopause folding can carry stratospheric air downward across the tropopause. This air is generally drier and often has more ozone than the surrounding tropospheric air, so the fold can introduce relatively ozone-rich, dry air into the troposphere. The other options describe processes or conditions unrelated to tropopause folding.

Correct answer: C. Weak absorption of solar energy and radiative cooling in the mesosphere

C is correct. The mesopause, at the top of the mesosphere, is generally the coldest part of the atmosphere. The mesosphere absorbs relatively little solar energy, while radiative cooling—particularly by carbon dioxide—helps lower its temperature toward the mesopause.

Correct answer: C. The thermosphere is a temperature-defined layer, and the ionosphere is a region of ionization

The correct answer is C. The thermosphere is a major atmospheric layer identified by its temperature profile, which generally rises with altitude there. The ionosphere is identified by the presence of ionized particles and overlaps parts of more than one atmospheric layer. The two terms therefore describe different ways of classifying regions of the upper atmosphere.