Dive Atlas of the World. Jack JacksonЧитать онлайн книгу.
DIVES
Decompression dives are not recommended for recreational divers and most American recreational dive planners do not allow for them, although European dive tables do. There may be times when, for whatever reason, divers exceed the no-stop dive time limit at a given depth and then have to make stops on the ascent, long enough to let excess nitrogen diffuse out of their body tissues (decompression stops).
Different training agencies recommend different depths and times for these stops, though the deeper ones are more easily maintained in a swell. If the divers have not been very deep and not for too long, then one stop will be sufficient, usually at a depth of between 3m (10ft) and 6m (20ft). If the divers have been relatively deep or exceeded the no-stop time for longer, they will have to make additional stops at greater depths and then a longer one between 3m (10ft) and 6m (20ft). Special tables are available for diving at altitude or on Enriched Air Nitrox.
FINISHING A DIVE
Divers should finish all dives, whether decompression or not, with a five-minute safety stop at 3–6m (10–20ft). It can be difficult to hold a stop at 3m (10ft) in a swell. It is easier to hold 5m (16ft), which allows leeway if the swell causes you to ascend a little.
ALTITUDE AND FLYING AFTER DIVING
When diving at altitude, divers must use tables or computers designed for altitude diving. The reduced pressure in aeroplanes at height can cause large bubbles to form, causing decompression sickness in divers who fly before their body has had enough time to release most of the accumulated nitrogen. Even worse, high-flying aircraft cabins have been known to depressurize in flight. Divers intending to fly should allow at least 24 hours after diving.
CLIMATE CHANGE
Scientists disagree over the rate and likely extent of change resulting from global warming. Similar events have occurred throughout history and some glaciers are currently expanding. In Antarctica the Ross Ice Shelf has grown several kilometres (miles) in the last two decades. However, most glaciers are receding and large amounts of polar ice are melting. The resulting increase in sea levels threatens the existence of low-lying islands such as the Maldives and increases the risk of flooding on the lower sides of regions such as the UK.
THE EL NIÑO-SOUTHERN OSCILLATION PHENOMENON (ENSO) AND LA NIÑA
El Niño conditions can result in strange weather patterns in some diving areas. Warm water means that many sharks descend to deeper, colder water, but most importantly, animals that have symbiotic algae may expel them as in coral bleaching.
In normal years, the upwelling cold water in the trade-wind belts off the west coast of South America leads to rich fishing and causes the overlying air to cool below the temperature at which water vapour condenses (dew point), producing fog. However, sea-surface temperature changes in the equatorial Pacific sometimes produce a major climatic disturbance known as El Niño, Spanish for The Boy Child, because Peruvian fishermen noticed that it often began around Christmas. During an El Niño/Southern Oscillation Phenomenon, a weakening of the easterly trade winds in the Central Pacific means that warm surface water is no longer pushed west to allow for a cold, nutrient-rich upwelling off the coasts of Peru and Ecuador. The warmer sea-surface temperature transforms the coastal climate from arid to wet and causes the huge fish stocks, normally associated with the nutrient-rich cold water, to migrate away. The phenomenon is normally accompanied by a change in atmospheric circulation, called the Southern Oscillation. It is associated with changes in precipitation in regions of North America, Africa, and the western Pacific, droughts and bush fires in Australia and droughts in southeastern Asia, India and southern Africa. It is one of the main causes of change in the world’s climate, and the 1997/8 event was the worst on record. The sea-surface temperatures in most tropical seas were particularly high, resulting in large-scale coral bleaching, particularly in Bahrain and the Maldives. Nearly every region on earth felt El Niño’s effect in some way.
BLEACHING
Bleaching occurs when corals, anemones, clams and some other animals like sponges, expel their symbiotic algae (zooxanthellae) or the pigments of those algae. This is thought to occur due to higher temperatures and excess ultraviolet light penetration due to failed monsoons, very calm seas or lack of cloud cover. Some bleaching may be a seasonal event in the Indo-Pacific and the Caribbean, when full recovery is normal. If the water temperatures quickly return to normal then the animals recover, otherwise they die. Bleaching is most pronounced in water less than 15m (50ft) deep and particularly affects fast-growing species such as Acropora. Slower growing massive species like Porites also bleach, but are more likely to recover in a couple of months.
Bleaching was particularly far-reaching during the 1997/8 El Niño-Southern Oscillation Phenomenon, with areas such as Bahrain, the Maldives, Sri Lanka, Singapore, and parts of Madagascar and Tanzania being seriously affected. Scientists recently found that after having expelled one type of Zooxanthellae, some corals can take up other types that are better suited to the higher temperatures, thus enabling them to survive as long as temperatures do not get too high.
El Niño is called a warm event. La Niña, which means The Little Girl is called a cold event. (The phenomenon is also known as Viejo, the Spanish word for old.) The opposite of El Niño, with unusually cold surface temperatures in the Eastern Tropical Pacific, it usually, but not always, follows an El Niño and did so in 1998. The effects on global climate are the opposite to those of El Niño.
WEATHER, CURRENTS AND TIDES
In regions where there is a distinct summer and winter, many divers would normally avoid diving at offshore sites in winter. Some areas have distinct seasons of travelling storms of great violence that form over warm oceans when several thunderstorms release heat. These tropical cyclones are known as hurricanes in the North Atlantic and eastern North Pacific, and as typhoons in the western North Pacific. The winds of these systems revolve around a centre of low pressure, ‘the eye,’ in an anticlockwise direction in the northern hemisphere and in a clockwise direction in the southern hemisphere.
Tropical cyclones are a phenomenon of the tropical oceans. They originate in two distinct latitude zones, between 4° and 22° South and between 4° and 35° North. They are absent in the equatorial zone between 4° South and 4° North. Most tropical cyclones are spawned on the poleward side of the area known as the intertropical convergence zone (ITCZ).
Monsoon winds are primarily caused by the difference between temperatures over large landmasses and adjacent large oceans, notably Arabia, Asia, Australia, and the Indian subcontinent. Seasonal changes in temperature are large over land, but small over oceans. A monsoon wind blows from cold to warm regions and, in summer, from the sea towards land carrying humid air from the ocean. In winter it blows from the land toward the sea. As a consequence, where monsoons occur, one side of a landmass may get heavy rain and not be divable at one time of year and the opposite side of that landmass at another.
Some regions are known for consistently bad tropical cyclones or monsoons at certain times of year and the resorts shut down for that period. Regions where these events only occasionally cause problems, tend to stay open during the bad weather season, while offering cheaper rates. Divers who book resorts in these regions at this cheaper time of year should be aware that their holiday could be ruined.
Although not necessarily of tropical cyclone strength, bad weather can occur anywhere at any time of year. However, diving can be quite pleasant during inclement weather, if divers jump into the water and quickly descend below the swell. The real problems are in getting the boat out to the site and, worse, getting out of the water into a boat