B.2.4—Earth’s albedo variation

Earth’s albedo is variable

Earth’s average albedo is not a universal fixed property. It changes with the surfaces and clouds that are illuminated and with the angle at which radiation arrives.

Daily variation

Cloud cover changes with weather and the position of the Sun changes during the day. Both alter the fraction of incident radiation reflected by a region.

Latitude and incidence angle

At different latitudes, sunlight arrives at different angles to the surface normal. The effective surface and reflected fraction therefore vary; snow, ice, ocean, land and clouds also contribute different albedos.

Climate link

If snow or ice melts, a darker surface may reflect less and absorb more incoming energy. This is a consequence of changing albedo, not a change in the definition of albedo.

How it is assessed

The evidence uses a multiple-choice selection of the three correct dependencies and a related question about local solar intensity and location/cloud cover.

Command terms

State / Explain

What earns marks

State all three syllabus dependencies: Earth’s albedo varies daily, with cloud formation, and with latitude. Explain that changing illumination angle and surface/cloud cover changes the reflected fraction.

Watch for

Treating Earth’s albedo as a fixed constant or omitting cloud formation and latitude.

Representative question

[Maximum number: 1]

A student makes three statements about Earth's albedo.

I. It varies daily.
II. It depends on latitude.
III. It depends on cloud formation.

Which of the statements are correct?

I and II only

I and III only

II and III only

I, II and III

Start with the energy balance

For a planet at steady average temperature, absorbed incoming radiant power equals emitted outgoing radiant power. Albedo controls the reflected fraction; emissivity controls thermal emission relative to a black body.

Average incoming solar energy

The solar constant S is an intensity on a surface perpendicular to the rays. A spherical planet averages the intercepted power over four times the projected area, giving $S/4$ ; with albedo a, the simple globally averaged absorbed intensity is $(1-a)S/4$ .

Atmospheric mechanism

Earth emits infrared radiation. Greenhouse molecules absorb selected wavelengths through molecular resonance or energy-level transitions, then re-emit in all directions, including back toward the surface.

Human enhancement

The natural greenhouse effect supports a habitable surface temperature. Human-driven increases in greenhouse-gas concentration augment the effect; fossil-fuel burning is a primary cause of this enhanced greenhouse effect.

B.2.4—Earth’s albedo variation

Explain Why Earth’s Albedo Varies

B.2.4 Exam Analysis

Assessment in practice

Synthesize B.2 Greenhouse Effect

Question 1