Thunderstorms at Sea- How They Form and What to Do

by Ken Batt, Bureau of Meteorology, Sydney on 28 Jul 2006 Cruising or Racing, thunderstorms are a major threat to yachts as Meterologist Kenn Batt explains.

'I still remember very vividly a situation that occurred whilst racing on Wild Oats during the 1993 Southern Cross Cup Series off Sydney Heads. It went like this- we were racing in the Manly Circle when an ugly, green-tinged thunderstorm moved towards us from the west.

'The call went up to prepare the boat/crew for severe wind gusts. This we did but….. we ended up with 2 to 3 cm diameter hail falling on us whilst the wind went all around us!! It was still very uncomfortable to say the least! Oh to have been the navigator at the time! But the worst ordeal was experienced by our skipper, Roger 'Hicko' Hickman.

'Since Wild Oats had a metal (titanium) steering system, every time he put his hand on the tiller he kept on receiving severe electrical shocks from the lightning that was hitting the water and being conducted through the water and into our steering system. It was probably a blessing that there wasn’t much wind around in so much as 'hands on steering' had to be kept to the absolute minimum! Not a nice experience to say the least.'

So what are thunderstorms you may ask?

Thunderstorms are relatively small meteorological phenomena, only about a few kilometres across, and very short-lived. In most cases each individual thunderstorm lasts a matter of minutes, sometimes extending to a few hours. Frequently more than one thunderstorm occurs at the same time or in quick succession and this extends the duration of the event.

Thunderstorms are in themselves not associated with any particular pressure system which you could see on a weather map, but may often form close to a front or a low because of the favourable meteorological conditions for thunderstorm development which these systems can create. The rising moist air that forms thunderstorms, occurs only over a small area, but rises very fast - about 20 knots in a normal thunderstorm, but up to about 100 knots in a very severe thunderstorm.

Thunderstorms by definition always produce thunder and lightning, and may also produce rain, strong wind gusts, hail, waterspouts and tornadoes.

Tornadoes are 'tubes' of extremely strong winds, which appear in the familiar 'funnel' shape from the base of some of thunderstorm clouds. They can live for up to 30 minutes or so, and the extreme wind strengths can cause devastating damage.

Waterspouts, the maritime equivalent to the land tornado, are meteorologically speaking quite different in origin from tornadoes- for a start they need not be associated with thunderstorms but can be produced from ordinary cumulus clouds. However, they should be given a wide berth at sea, as they can also be very dangerous.

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The rainfall from thunderstorms starts and stops quite suddenly and is fairly intense while it lasts. The strong winds take the form of sudden sharp shifts in wind speed direction - squalls or gusts (possibly greater than 40% more than the 10 minute average speed) - which may not last more than a few minutes. The most severe form of these thunderstorm downdrafts are known as downbursts or microbursts, they have been observed first hand by the author at sea with speeds of around 70 to 80 knots which lasted only several minutes and DID A LOT OF DAMAGE!

How do thunderstorms develop?

If we have simultaneously, a high moisture content of the air (high relative humidity close to the earth’s surface), an unstable atmosphere and a lifting mechanism (a cold front, trough of low pressure, topographical barrier, just to name a few) to lift the moist air into the unstable environment- we should witness a thunderstorm event.

The simplest type of thunderstorm is a single 'cell'. The life cycle of a cell has three main stages:

In the 'towering cumulus' stage the thunderstorm or storm is not much more than a large cumulus cloud in which the air is all rising - called the 'updraft'. No rain or anvil cloud has yet formed. The upper edge of the cloud usually appears sharp and can often be seen growing by the second.

The 'mature stage' still has an updraft which has now reached the highest level it can (often the tropopause, the 'lid' to the lowest 10 to 20 km of the atmosphere, called the troposphere), and the cloud consequently has started to spread out at the top into the anvil shape; it usually has a soft feathery appearance since it is high enough to be composed of ice crystals rather than water droplets.

This is the fully developed cumulonimbus cloud. Although the thunderstorm proper is quite small as mentioned above, the spreading anvil cloud can cover an enormous area. It is the anvil clouds which make thunderstorms visible on satellite pictures, showing up as big blobs.

A second significant feature is that the storm has probably produced rain which falls out of the cloud base and is associated with descending air called the downdraft. It is this downdraft which when it reaches the ground and spreads out horizontally generates the sudden gusty winds nearly always associated with thunderstorms.

The downdraft is formed largely by evaporation of rain in the air below the storm, which creates a cool blob of air which sinks to the earth’s surface.

Although the downward acceleration of this cooled air is very small, the downdraft generally originates thousands of feet up in the storm, and even with only a couple of degrees of cooling, by the time the downdraft air reaches the ground it is moving fast enough to produce strong gusty winds. This also explains why the wind gusts of thunderstorms are cool and fairly moist.

The downdraft is often called the thunderstorm outflow, and the edge of it - the 'gust front' - is a favourite spot for new cells to develop, with air forced upwards as the gust front moves along. The downdraft can sometimes be clearly seen on radar or satellite imagery as a clear area of cloud-free air around a large thunderstorm cell - the sinking air dissipates any cloud in the surrounding area.

In the 'dissipating' stage of a cell, a weak updraft still exists at higher levels, but the storm consists mostly of downdraft, which has spread out sufficiently to cut off the warm moist air feeding the updraft, and since this is what sustains the storm, the cell quickly degenerates.

All three stages together take only about 30 to 50 minutes to occur. However, even non-severe thunderstorms are usually composed of more than one cell, and may last considerably longer as a result.

Thunderstorms are most common over the land in the afternoons in the late spring and summer months, but can occur in any month or at any time of day, especially over the sea.

How do thunderstorms move?

Thunderstorms usually DO NOT move with the surface wind direction and speed. Each storm cell moves with an average of the winds through the depth of the storm cloud. These average winds are known as the steering winds or more precisely, the pressure-weighted mean wind. Since the wind may, and usually does, change in both direction and speed with height, this average may be quite different from the surface wind velocity.

In the Australian East Coast for example, this steering wind on average occurs at a height of around 14000 to 16000 feet or 4 to 5 km. On most days, this wind is a westerly. This would mean that if thunderstorms formed on the Ranges to the west of Sydney or Brisbane they would move towards the coast.

Since many thunderstorms are in fact formed from a number of different cells, the formation and degeneration of cells can influence the direction in which the storm complex moves.

Thunderstorms almost never circle around and move over the same spot again, but fr