Paper 3 · June 2023 · Settlement and Urban Geography
On Fig. 5.1 the isotherms bend outward where the river crosses the city, marking a tongue of cooler air along the channel. Why is the river corridor cooler than the built up area around it?
AWater reflects almost all incoming solar radiation, so the river has a far higher albedo than any built surface in the city.
BThe river channel lies well below the general level of the city, so cold air draining into it is compressed and cools further.
CFlowing water generates no friction with the air above it, so no sensible heat can be transferred from the surface at all.
DOpen water has a high specific heat capacity so it warms slowly, and evaporation from its surface consumes latent heat, cooling the air above.
Explanation
Water needs a great deal of energy to raise its temperature, so it warms far more slowly than concrete or tarmac, and energy that would otherwise heat the air is instead consumed as latent heat when water evaporates. Water in fact has a low albedo at high sun angles, and compression of sinking air warms it rather than cooling it.
Derived from ZIMSEC Advanced Level Geography 6037/3 Paper 3 Practical Test, June 2023 session, Q5