Page:Lawhead columbia 0054D 12326.pdf/125

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represents a total reflection (a perfectly white surface). To get an idea of the relative values at play here, consider the following table.[1]


Surface Albedo
Equatorial oceans at noon 0.05
Dense forest 0.05-0.10
Forest 0.14-0.20
Modern city 0.14-0.18
Green crops 0.15-0.25
Grassland 0.16-0.20
Sand 0.18-0.28
Polar oceans with sea ice 0.6
Old snow 0.4-0.6
Fresh snow 0.75-0.95
Clouds 0.40-0.9
Spherical water droplet with low angle of incidence[2] 0.99


Taking albedo into account will clearly affect the outcome of the model we’ve been working with. We were implicitly treating the Earth as if it were a perfect absorber—an object with albedo 0—which would explain why our final result was so far off base. Let’s see how our result changes when we jettison this assumption. We will stick with the simplification we’ve been working with all along so far and give a single average albedo value for the Earth as a whole, a value which is generally referred to as the “planetary albedo.” More nuanced energy


  1. Adapted from Ricklefs (1993)
  2. This explains why, in practice, the albedo of large bodies of water (e.g. oceans or very large lakes) is somewhat higher than the listed value. Choppy water has a layer of foam (whitecap) on top of it, which has an albedo value that’s much closer to the value for a water droplet than to the value for calm water. The value of the oceans as a whole, then, is somewhere between the values of a water droplet and calm water. This is an example of the sort of small space-scale difficulty that causes problems for the more sophisticated general circulation model, discussed in more detail in Chapter Six.

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