In the structure of the atmosphere, which layer is known for the fact that the usual decrease in temperature with increasing altitude is absent or largely reversed?

Introduction / Context:
Earth atmosphere is divided into several layers based on temperature variation with height. Normally in the lowest layer, temperature decreases as altitude increases. However, there is a particular layer where this pattern does not continue and temperature may remain almost constant or even increase with height. This question tests basic climatology and atmospheric science by asking you to identify that special layer.

Given Data / Assumptions:

• We are dealing with the vertical structure of the atmosphere.
• The question mentions a layer where decrease in temperature with altitude is absent.
• Options are Troposphere, Ionosphere, Stratosphere, and Mesosphere.
• We assume standard textbook definitions for each layer.

Concept / Approach:
In the troposphere, temperature generally decreases with height. Above the troposphere lies the stratosphere. In the lower part of the stratosphere, temperature stops decreasing and in the upper stratosphere it actually increases with altitude due to absorption of ultraviolet radiation by ozone. Therefore, the normal lapse rate of temperature decrease is absent there. The ionosphere is not a single temperature layer but a region defined by ionization, and the mesosphere is again a zone where temperature decreases with height before rising in the thermosphere.

Step-by-Step Solution:
Step 1: Recall that the lowest layer is the troposphere, where temperature usually decreases with height. Step 2: Remember that above the tropopause lies the stratosphere, which contains the ozone layer. Step 3: Understand that in the stratosphere temperature becomes nearly constant and then increases with height because ozone absorbs ultraviolet radiation and warms the layer. Step 4: Compare this to the mesosphere, where temperature once again declines with altitude. Step 5: Select Stratosphere as the correct answer because it is the layer where decrease in temperature is absent or reversed.

Verification / Alternative check:
A quick way to verify is to remember the standard temperature profile diagram of the atmosphere that shows temperature versus altitude. The graph usually shows a decrease in the troposphere, an increase in the stratosphere, another decrease in the mesosphere, and then an increase in the thermosphere. This alternating pattern makes the stratosphere stand out as the first region above the surface where the normal cooling with height does not occur.

Why Other Options Are Wrong:
Troposphere: Here the normal environmental lapse rate operates, so temperature clearly decreases with height.
Ionosphere: This is a functional region including parts of the mesosphere and thermosphere and is defined by ionization levels, not directly by a single temperature pattern.
Mesosphere: In this layer, temperature drops with height and reaches very low values at the mesopause, so the normal decrease is definitely present.

Common Pitfalls:
Students sometimes confuse the stratosphere with the ionosphere because both are higher layers. Another pitfall is thinking only about the ozone layer without remembering its effect on temperature. To avoid confusion, remember the rule: in the stratosphere ozone absorbs ultraviolet rays, warming the air and preventing the usual temperature decrease with altitude.

Final Answer:
The atmospheric layer where the normal decrease of temperature with altitude is absent is the Stratosphere.