Questions: Climate Zones and Biomes

Question 1

The Atacama Desert in South America is one of the driest places on Earth, yet it lies at about 23°S — not at 30°S where Hadley cell descent peaks. What best explains its extreme aridity?

• A. It sits directly under the descending branch of the Hadley cell at 30° latitude
• B. The Andes Mountains create a rain shadow by blocking moisture from the Pacific
• C. The desert formed because South America's continental interior lacks ocean moisture
• D. Low solar intensity at that latitude suppresses evaporation and rainfall

Answer: (B)

The Atacama's exceptional dryness results from multiple stacking factors, but the primary mechanism here is the Andes rain shadow: the mountains force incoming Pacific air upward, precipitating its moisture on the windward side, leaving the lee side extremely dry. The cold Humboldt Current also suppresses evaporation further. This illustrates the key insight that while Hadley cell descent explains many subtropical deserts, geography (mountain ranges, ocean currents) produces significant deviations from the simple latitude pattern.

Question 2

London (51°N) has milder winters than Labrador, Canada, which sits at roughly the same latitude. What best explains this?

• A. London receives more direct solar radiation because it is slightly closer to the equator
• B. The Gulf Stream carries warm water northeastward and warms Western Europe far beyond what latitude alone predicts
• C. London is at lower elevation, so temperatures are warmer
• D. Continental interiors have warmer winters because land retains heat better than ocean

Answer: (B)

Both locations receive similar solar input at comparable latitudes. The Gulf Stream — a warm ocean current originating in the Gulf of Mexico — flows northeastward along the North Atlantic and transfers enormous heat to the atmosphere above, dramatically warming Western Europe. Labrador, on the continental side of the Atlantic, lacks this oceanic warming. Option D reverses the truth: continental interiors actually have colder winters and hotter summers than coastal locations (more extreme temperature ranges, not milder conditions).

Question 3

A desert climate is defined by low precipitation, so most deserts should be hot environments.

• T. True
• F. False

Answer: False

Deserts are defined by moisture deficit — low precipitation relative to evaporation potential — not by temperature. Cold deserts exist at high latitudes and elevations (Gobi Desert, Patagonian steppe, Antarctica) where cold air holds little moisture and precipitation is minimal. Antarctica is technically the world's largest desert. The misconception arises because familiar examples like the Sahara and Arabian Deserts are hot, but heat is not the defining criterion — aridity is.

Question 4

If global atmospheric circulation shifts so that the Intertropical Convergence Zone (ITCZ) permanently moves northward, the biomes in regions that formerly lay under the ITCZ would shift toward drier ecosystem types.

• T. True
• F. False

Answer: True

The ITCZ is the locus of convergence, uplift, and heavy rainfall that sustains tropical rainforests. If it moves north, regions previously under it lose the convergence-driven rainfall and experience reduced precipitation. Over time, vegetation would shift toward drier biome types — savanna, shrubland, or desert depending on the magnitude of change. This is exactly what paleoclimate records show: past orbital shifts that moved the ITCZ produced major biome migrations across Africa and South America.

Question 5

Why do subtropical deserts like the Sahara and Arabian Desert form at approximately 30° latitude rather than at the equator, even though both regions receive intense sunlight?

Model answer: At the equator, intense solar heating causes air to rise strongly in the ITCZ, producing convergence, uplift, cooling, and heavy rainfall that creates tropical rainforests. This rising air must descend somewhere — it does so at roughly 30° latitude as the descending branch of the Hadley cell. Descending air is compressed and warms adiabatically, reducing relative humidity and suppressing cloud formation and precipitation. It is the descending branch of circulation that creates subtropical deserts, not any deficit of solar energy.

The key is understanding that the Hadley cell is a circulation loop: air rises at the equator (releasing moisture as rain), flows poleward aloft, descends at ~30° (warming and drying as it compresses), and flows back toward the equator as surface winds. The desert-forming mechanism is dynamic — atmospheric circulation — not simply latitude or sunlight. This is why both the equator and 30° latitude receive strong sunlight but have opposite moisture regimes.