Water Level A Few Metres example essay topic

Waterspouts When a tornado forms or passes over a water surface, it is termed a waterspout. Like tornadoes, they may assume many shapes and often occur in series or families. Measurements of their forward speeds are scarce, but estimates vary from a few kilometres an hour to as high as 64 to 80 kilometres per hour. Contrary to popular opinion, a waterspout does not "suck up" water to great heights, though it may lift the water level a few metres. The main visible cloud consists mostly of freshwater clouds produced by condensation of water vapour; however, a sheath of spray often rotates around the lower portion of the vortex tube.

One of the largest and most famous waterspouts, observed near Massachusetts on Aug. 19, 1896, was witnessed by thousands of vacationers and several scientists. Its height was estimated to be 1,095 metres and its width 256 metres at the crest, 43 metres at centre, and 73 metres at the base. The spray surrounding the vortex tube near the water surface was about 200 metres wide and 120 metres high. The spout lasted 35 minutes, disappearing and reappearing three times. Most waterspouts are smaller, with much shorter lives. This exceptional spout is an example of one that apparently was spawned by thunderstorm-squall conditions, similar to those that produce tornadoes over land.

There are few authentic cases of large ships ever being destroyed by a spout, although spouts are a dangerous hazard to small vessels. A few intense waterspouts have caused deaths when they moved inland over populated areas. The belief that firing a cannonball or other projectile into a spout can "break it up" has no scientific foundation. Whirlwinds In the general sense, a whirlwind is any rotating mass of air or atmospheric vortex. The term is, however, commonly restricted to atmospheric systems smaller than a tornado but larger than eddies of microscale turbulence. A whirlwind is usually named after the visible phenomenon associated with it; thus there are dust whirls, or dust devils; sand whirls, or sand pillars; and fire, smoke, and even snow whirls, or spouts.

In contrast to the pendant form of the tornado funnel, a dust or sand devil develops from the ground upward, usually under hot, clear-sky conditions. The whirl shape is normally that of a cylindrical column or an inverted cone. The axis of rotation is usually vertical, but it may be inclined. The direction of rotation may be either clockwise or counterclockwise. Vortices with a horizontal axis of rotation are sometimes called rolls, or rotors.

Dust and sand whirls are not nearly as violent as tornadoes, although jackrabbits have been lifted by the more intense vortices. The whirls measure in diameter from several centimetres to a few hundred metres, and visible heights from a few metres to at least 1,500 metres. This is probably not the upper limit, for sailplane pilots have used their spirally ascending currents to soar to above 4,570 metres. Like tornadoes and waterspouts, dust and sand devils often appear in groups or series.

Eleven of them were simultaneously sighted in Ethiopia, and in the Mojave Desert in eastern California a series of smaller whirls followed in the wake of a larger primary vortex. Such secondaries are sometimes referred to in India as "dancing devils". Fire whirlwinds are a problem to the forest rangers who must cope with them. A historical example of a great fire vortex, produced by war, is that which formed over Hamburg following a massive aerial bombing during the night of July 27-28, 1943. The fire fueled a storm of counterclockwise rotation 1.6 to three kilometres in diameter and nearly five kilometres high, with winds estimated to be more than 160 kilometres per hour in some sections.