Ask Mr Brain...all will be explained

PUBLISHED : Monday, 10 May, 1999, 12:00am
UPDATED : Monday, 10 May, 1999, 12:00am

Why is it that many of the world's deserts are located on the west sides of continents? You find them in the western United States and along the west coasts of South America and Africa.

It's certainly true when you look at the world map that a number of deserts lie on the west coasts of continents. But they exist in other places as well.

A more consistent trend finds them straddling the tropics of Cancer and Capricorn. This is due in part to warm, water-laden air that rises from equatorial regions and releases rains in the tropics as it moves towards the Poles. Later it descends, minus its moisture, in the sub-tropical latitudes - perpetuating the aridity.

In one of nature's paradoxes, deserts can exist next to oceans. Examples would be the fog-banked deserts of South America and Africa, which owe their existence largely to cold ocean currents that flow from Antarctic regions towards the Equator.

The Peru Current streams along South America's west coast off the Atacama Peruvian deserts. The Benguela Current sweeps the Namib desert on the southwestern edge of Africa.

The cool water of these currents brushing against warm coastal air throws off blankets of fog, which trap moisture at the surface level and prevent the moist air from rising to form rain clouds.

Such fog can quench aridity, but winds along the western fringes of South America and Africa flow parallel to the coast. These southerly winds hold fog near the shore so that little moisture reaches the desert realms.

What causes a sonic boom when an aircraft flies faster than the speed of sound? A thunder-like sonic boom is the shock wave produced by an object moving faster than the speed of sound, that is 344 metres per second or 1,234 kilometres per hour, at 20 degrees Celsius.

When an aircraft, or other object, travels at the speed of sound, it catches up with its own noise. At faster speeds, the aircraft pushes the sound waves ahead of itself faster than the noise would usually travel - much in the way a boat produces a bow wave when it moves through water.

The piled-up sound - pressurised air - takes the form of a violent shock wave which is heard as a thundering sonic boom when the pressure is released.

Aircraft generate two sonic booms, at the nose and at the tail. Usually the two are heard as one boom by people on the ground. However, Nasa says vehicles the size and mass of the space shuttles produce two easily distinguishable distinct booms.

Sonic booms result in an increase in the air pressure. At normal flying altitudes, it is usually less than a kilogram more than the 10,310 kilograms per square metre of normal atmospheric pressure.

At present Nasa is conducting a series of test flights in the Mojave Desert to measure the characteristics of sonic booms in a bid to develop quieter and more environmentally-friendly aircraft.