Diving Physics - Physical Phenomena of Interest To Divers

Physical Phenomena of Interest To Divers

The physical phenomena found in large bodies of water are:

• Effects of weather such as wind, which causes waves, and changes of temperature and atmospheric pressure on and in the water.

Even moderately high winds can prevent diving because of the increased risk of becoming lost at sea or injured. Low water temperatures make it necessary for divers to wear diving suits and can cause problems such as freezing of diving regulators.

• Haloclines, or strong, vertical salinity gradients.

For instance where fresh water enters the sea, the fresh water floats over the denser saline water and may not mix immediately. Sometimes visual effects, such as shimmering and reflection, occur at the boundary between the layers, because the refractive indices differ.

• Ocean currents can transport water over thousands of kilometres, and may bring water with different temperature and salinity into a region.

Some ocean currents have a huge effect on local climate, for instance the warm water of the North Atlantic drift moderates the climate of the north west coast of Europe.

• Thermoclines, or sudden changes in temperature.

Where the air temperature is higher than the water temperature, shallow water may be warmed by the air and the sunlight but deeper water remains cold resulting in a lowering of temperature as the diver descends. This temperature change may be concentrated over a small vertical interval, when it is called a thermocline.

Where cold, fresh water enters a warmer sea the fresh water may float over the denser saline water, so the temperature rises as the diver descends.

In lakes exposed to geothermal activity, the temperature of the deeper water may be warmer than the surface water. This will usually lead to convection currents.

Water at near-freezing temperatures is less dense than slightly warmer water - maximum density of water is at about 4°C - so when near freezing, water may be slightly warmer at depth than at the surface.

• Tidal currents and changes in sea level caused by gravitational forces and the earth's rotation.

Some dive sites can only be dived safely at slack water when the tidal cycle reverses and the current slows. Strong currents can cause problems for divers. Buoyancy control can be difficult when a strong current meets a vertical surface. Divers consume more breathing gas when swimming against currents. Divers on the surface can be separated from their boat cover by currents.

On the other hand, drift diving is only possible when there is a reasonable current.