Optional theme: Extreme environments practice questions.

Northern Chile's Atacama Desert lies directly on the Pacific coast, yet many of its weather stations record no measurable rain in most years. Which combination of controls best explains this extreme aridity?

• Subsiding subtropical air, a cold offshore current and the Andean rain shadow
• Rising equatorial air, a warm offshore current and persistent onshore trade winds
• High altitude, permanent snow cover and a semi-permanent polar high
• Winter monsoon reversal, katabatic winds and a deep coastal fog layer

Answer: Subsiding subtropical air, a cold offshore current and the Andean rain shadow

Air that rises at the ITCZ subsides at about 30 degrees S, warming and drying, while the cold Humboldt Current chills the lowest air so that moisture is lost as coastal fog rather than rain. The Andes block moist air arriving from the east, so all three controls act in the same direction.

Saguaro cacti in the Sonoran Desert of Arizona, USA, keep their stomata closed during the day and open them at night. What is the main advantage of this adaptation?

• Water loss by transpiration is reduced because night air is cooler and more humid
• Light-dependent photosynthesis can continue throughout the hours of darkness
• Salts are prevented from accumulating in the water stored in the stem
• Oxygen uptake by the roots increases while the soil is at its coolest

Answer: Water loss by transpiration is reduced because night air is cooler and more humid

Gas exchange is shifted to the time of day when the saturation deficit is lowest, so carbon dioxide is taken in with the smallest possible water loss. Photosynthesis itself still needs light; only the uptake of carbon dioxide happens at night.

The table shows mean air temperatures for two stations, both at 63 degrees N and both 20 m above sea level. Which factor best explains the difference between them?

• The greater altitude of station Y
• The greater continentality of station Y
• The higher surface albedo at station X
• The rain-shadow position of station Y

Answer: The greater continentality of station Y

Latitude and altitude are identical, so the 31 degrees C gap in January must come from distance to the ocean. Water's high specific heat capacity keeps coastal winters mild, while continental interiors lose heat rapidly under clear, dry winter skies.

Crescent-shaped barchan dunes migrate slowly across the gravel plains of the northern Namib Desert. Which description of a barchan is correct?

• The horns point downwind and the steeper slip face is on the leeward side
• The horns point upwind and the steeper slip face is on the windward side
• The horns point downwind and the steeper slip face is on the windward side
• The horns point upwind and the dune carries no slip face at all

Answer: The horns point downwind and the steeper slip face is on the leeward side

Sand is driven up the gentle windward back until it avalanches down the lee face at about 33 degrees, which is why the steep side always faces away from the wind. The thin horns hold less sand, so they migrate faster than the thick centre and trail downwind.

The table shows the mean depth of the active layer measured at the end of summer at four periglacial sites. Which statement is supported by these data alone?

• The deepest active layer occurs at the coldest of the four sites
• Active layer depth is unrelated to summer air temperature
• Every site thaws to a depth of more than two metres in summer
• Active layer depth increases as mean July temperature rises

Answer: Active layer depth increases as mean July temperature rises

Depth rises from 45 cm to 120 cm as July temperature rises from 4 to 12 degrees C, so only the covariation may be stated; the deepest layer is at the warmest site and no site reaches 200 cm. A resource question is answered from the data given, not from what you expect to be true.

A glacially scoured outcrop has one side that is smooth, gently sloping and striated, while the opposite side is steep, craggy and stepped. Which landform is this?

• A drumlin of till deposited beneath moving ice
• An esker of sorted sand and gravel left by a meltwater stream
• A roche moutonnee of ice-moulded bedrock
• An arete separating two neighbouring corries

Answer: A roche moutonnee of ice-moulded bedrock

Ice abrades the up-glacier stoss slope smooth and striated, then meltwater refreezes on the lee side and plucks jointed blocks away, so the asymmetry records the direction of ice movement. A drumlin is depositional and has its blunt end facing up-glacier.