Weathe by Malcolm RileyWaterspouts

Some mariners when yacht racing use a technique of sailing into the area near a large cumulus cloud where shower activity is occurring to gain advantage from the windier conditions.
The rain falling from the cloud is usually associated with a down draft of cold air often with wind speeds much in excess of the wind in the surrounding area. This cold air reaches the surface of the water and spreads out in all directions.
If the cloud is moving (and they usually are) the strongest winds are a combination of the cloud movement and the downdraft… in front of the cloud and in the direction that it is moving. While this ‘extra’ wind can and does provide faster sailing conditions it does come with some risk.
The winds generated by showers and storms may be completely different to the forecast ‘wind of the day’ in both strength and direction.
In the 2001 Sydney to Hobart some of the yachts encountered showery weather and a couple of vessels had close encounters with waterspouts that caused some damage to the yachts.
Cumulus (the puffy) clouds are often small with little vertical extent. However, trough lines, areas of wind convergence or the heating of the day can cause the cloud to grow and they can grow rapidly. The vertical extent of the cloud is an indication of the amount of energy stored within the cloud.
If the cloud continues to grow it can become a cumulonimbus or thunder cloud.
As the cloud grows the updrafts and downdrafts within the cloud will get stronger and extend further from the cloud. The down draft extends down from the cloud and spreads out as the wind previously mentioned. The updraft flows up into the cloud from the surrounding area.
In conditions where there is significant vertical wind shear (see Afloat April and May 2010) a horizontal rotor may form. One indicator of a horizontal rotor is a roll cloud like the morning glory in far north Queensland.
Showers and storms produce downdrafts that are different from the ‘wind of the day’.If a rotor spinning in a horizontal plane moves into the area where there is an updraft, the updraft ‘picks up’ the rotor and it is transformed into spinning in a vertical plane. A curious thing occurs in this process and it is an example of conservation of angular momentum.
A good conceptual example what happens in this situation is watching an ice skater spin. They start off with a rather slow rotation with their arms extended horizontally, as they bring their arms inwards towards their body their speed of rotation increases.
This is not a true conservation of momentum due to the friction of the ice. However, when a skater does the same spin in mid-air it is a true conservation of angular momentum. In this case the forces are so large that many struggle to bring in their arms as they are not strong enough.
Horizontal rotor is turned to the vertical by the updraft.The rotor turning vertically starts off the spin of the column of air. Air then rushes in from all directions to rise into the column. The conservation of angular momentum will then make the air in that column spin at incredible speeds. Water is picked up from the surface of the ocean and the resulting column of spinning air and water is a waterspout.
Large cumulus clouds and cumulonimbus clouds have a life of about 30 minutes over the ocean. They are constantly forming and decaying. This means that waterspouts usually have duration of around 15 to 20 minutes.
However, storms over land can grow into supercell storms that have a different structure to storms over the ocean. These supercell storms can persist for many hours, generate tornados (land-based waterspouts) that can persist for over three hours and leave a damage trail of 350 kilometres.
During the day waterspouts are usually visible although they can be hidden by rainfall. At night they will be more difficult to see. If lightning is visible the cloud type that can generate waterspouts is in your area. When there is lightning around as well as waterspouts there is the potential for strong wind gusts (even many kilometres away from the storm), hail and heavy rain.

Waterspouts have been a long time problem for mariners. 

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.

 

Check out the following:

Footage of a series of powerful waterspouts near the coastal suburb of Terrigal, on Australia’s New South Wales coast. http://www.youtube.com/watch?v=V37nSc_pTY4
Conservation of angular momentum spin. Natalia Kanounnikova set the Guinness World Record for fastest spin on ice skates at Rockefeller Center, NYC. New world record is 308rpm as measured with a Sport Tach. http://www.youtube.com/watch?v=AQLtcEAG9v0