Even after sunset, the air near the earth's surface continues to receive heat. This continued heating is mainly due to which of the following?

Introduction / Context:
During the day, the earth's surface is heated by solar radiation. After the sun sets, however, the air near the ground does not become cold immediately. Instead, it continues to be warmed for some time. The question asks you to identify the main mechanism that supplies this heat to the lower atmosphere after sunset. This concept is central in physical geography and climatology and helps explain daily temperature patterns.

Given Data / Assumptions:

• Solar radiation stops reaching the surface directly after sunset.
• The earth's surface has stored heat obtained during the day.
• The air near the surface is still in contact with the warmed ground.
• Several processes are possible: direct insolation, terrestrial radiation, conduction, and convection.

Concept / Approach:
Insolation refers to incoming solar radiation, which effectively stops at the surface when the sun is below the horizon. The earth's surface, having absorbed solar energy during the day, becomes a secondary source of heat. It emits long wave infrared radiation back into the atmosphere; this is called terrestrial radiation. The lower layers of the atmosphere absorb this long wave radiation and are warmed. Conduction and convection also play roles in redistributing heat near the surface, but the primary ongoing source of energy after sunset is the terrestrial radiation emitted by the warmed land and water surfaces rather than new energy arriving from the sun.

Step-by-Step Solution:
Step 1: Recognise that after sunset, there is no direct solar insolation reaching the earth's surface at that location.Step 2: Recall that the earth's surface absorbs solar energy during the day and becomes warmer.Step 3: After sunset, this warmed surface begins to emit long wave infrared radiation back toward the atmosphere, known as terrestrial radiation.Step 4: The air layers just above the surface absorb some of this terrestrial radiation and are heated from below.Step 5: Conclude that this process of terrestrial radiation is the main reason the air continues to receive heat after sunset.

Verification / Alternative check:
Daily temperature curves show that the maximum temperature often occurs in the afternoon, not at noon, and temperatures do not drop sharply at sunset. Instead, there is a gradual cooling as the surface loses heat by radiation. Meteorology and climatology texts emphasise that the earth acts as a heat source for the lower atmosphere by emitting terrestrial radiation, especially during the evening and night. While conduction and convection distribute this heat within the air, their energy ultimately comes from the surface radiation, confirming that terrestrial radiation is the fundamental mechanism indicated in the question.

Why Other Options Are Wrong:
Direct insolation from the sun, option A, is absent after sunset simply because the sun is no longer above the horizon. Conduction from the earth's core, option C, operates on geological timescales and contributes negligibly to daily atmospheric temperature changes. Convection currents from upper layers, option D, redistribute heat but are not the basic source of new energy after sunset; they depend on the heat supplied by the surface and its radiation. Only option B, terrestrial radiation emitted by the warmed earth, correctly identifies the main process that continues to heat the air near the surface after sunset.

Common Pitfalls:
Students sometimes overemphasise conduction and convection because these words are commonly associated with heat transfer in school experiments. However, in the atmosphere, radiation plays a dominant role, particularly in interactions between the surface and the air. Another pitfall is assuming that once the sun sets, no heating process occurs, which contradicts observed evening temperatures. To understand climate and weather, remember this sequence: the sun heats the surface by insolation, the surface heats the air mainly by terrestrial radiation, and conduction and convection then redistribute that heat within the atmosphere.

Final Answer:
Terrestrial radiation emitted by the warmed earth