By which mode of heat transfer does energy from the sun predominantly reach the Earth's surface through space?

Introduction / Context:
Understanding solar heat gain is essential in building physics, HVAC load calculations, and climate science. The mechanism by which solar energy travels from the sun to Earth explains why glazing, shading, and surface absorptivity are critical in thermal design.

Given Data / Assumptions:

• Space between the sun and Earth is essentially a vacuum.
• No material medium is required for electromagnetic wave propagation.
• Atmosphere modifies the spectrum via absorption and scattering, but the transport through space is primarily wave-based.

Concept / Approach:
Conduction and convection require a material medium; neither can occur across the vacuum of space. Thermal radiation, the emission of electromagnetic waves (primarily in the shortwave solar spectrum), transfers energy across a vacuum at the speed of light. When these waves strike Earth’s atmosphere and surfaces, a portion is absorbed and converted to internal energy, warming the planet.

Step-by-Step Reasoning:

Verification / Alternative check:
Solar constant (~1361 W/m^2 outside the atmosphere) represents radiant energy flux; building load software explicitly models solar radiation, not conduction or convection from the sun.

Why Other Options Are Wrong:

• (a) and (e) Conduction cannot cross a vacuum.
• (b) Convection occurs within fluids like air but cannot carry energy from the sun through space.
• (d) Phase change transport is irrelevant in interplanetary vacuum conditions.

Common Pitfalls:
Assuming convection dominates because we feel “hot air”; that sensation is terrestrial heat transfer after radiation is absorbed.

Final Answer:
Thermal radiation (electromagnetic waves)