Answers to Formation of Deserts Lab
1. At the equator, where the Earth’s surface is mostly water, the sun’s rays heat the seas
strongly. The warm surface causes water to evaporate and the humid air to rise,
producing a low pressure belt. The rising air is like the rising air over a room heater – it
can only go up so far before it cools and lots of rain is produced. Over land this is where
the tropical rain forest are located.
The rising air also moves toward the poles as it cools and eventually stars to sink.
This generally happens near latitudes 30 N and 30 S. The sinking air compresses and
warms and causes high pressure. The air is already dry, And when it reaches the surface
it moves toward the equator or the poles, under the influence of the Earth’s rotation. The
warm dry air removes moisture from the surface, forming deserts. Air at the poles also is
sinking under high pressure and has a drying effect. There it is too cold to form a
traditional desert, but there is virtually no precipitation. Near 60 N and 60S there is
another low pressure belt as converging air from the high pressure zones produces low
pressure.
2. See picture in your textbook.
3a. The deserts of Eastern Washington and Oregon are rain shadow deserts. The moist
air coming in from the Pacific Ocean has to rise up over the Cascade Range. This
climbing air loses a lot of its moisture on the western side of the mountains. And then the
drier air compresses and warms as it moves down the eastern side. The warm dry air
removes moisture from the soil and forms the deserts.
3b-c. The deserts of these four states also are rain shadow deserts, but they also exist in
hot parts of the country, compounding the drying effect.
3d. The Atacama Desert is probably one of the driest places on Earth. It is high in the
Andes Mountains and is in a rain shadow. At the latitude of the Atacama, the Trade
Winds blow from the East and drop a lot of moisture on the tropical rain forest. Then the
dried air climbs the Andes and loses most of the rest of its moisture. The dry air
compresses and warms as it crosses the Atacama and it desiccates the land.
The Monte Desert is farther south and is affected y the southern westerlies. The
rain shadow is from the West and it is dry east of the Andes.
4a. The west coast should be drier (Kalahari Desert) because the Benguela Current is
carrying cold water from the Antarctic up along the coast of southwest Africa. On the
Indian Ocean side, the Agulhas Current is carrying warm equatorial water along the east
coast.
4b. The cold water works the same way cold currents do around the world. The water
does not evaporate very well and therefore moisture does not get added to the atmosphere
above it. The wind that blows inland over that current picks up little moisture. The
region is also very hot in the summer, and in August in Arizona and New Mexico there is
a localized version of the monsoon.
5. Central Asia would have moist air moving in from the Indian Ocean except that the
Himalayan Mountains and the Tibetan Plateau block it causing an immense rain shadow.
The central plains of the United States would be similar, except that here we do not have
a mountain range blocking the moist air from the Gulf of Mexico. Moisture can come
into the interior of the continent.
6. Some of this has been explained already. Low pressure usually means rising air that
cools and triggers precipitation. This is what typically happen when the barometer falls,
indicating clouds and rain to come. It is similar to rain shadows because the air is rising
and cooling. Similarly, with the monsoon, air masses are rising and cooling, producing
rain.
The reverse is true for sinking air and high pressure. The air is warming and this allows
it to hold more moisture, so there is a drying effect, no clouds, and fair weather. The
same thing happens on a larger scale at 30 N and 30 S. The sinking compresses, warms
and causes moisture to evaporate, creating deserts.
7a.
7b.
Large shallow roots network to absorb water quickly when it rains
Waxy coating to cut down evaporation
Needles instead of leaves to slow evaporation
Long tap roots
Nocturnal
Cold-blooded
Dormant
Large ears that act as radiators to loss excess heat
Using little or no water
Light coloration to absorb much less heat