Stormy weather is caused by low-pressure systems with ascending currents of air. Large volume of humid air is forced rapidly upwards causing intense activity and a revolving circulation of air. Frontal depressions cover large areas and produce strong winds, but smaller, more intense low-pressure systems, such as thunderstorms, tropical revolving storms, and tornadoes, produce even more severe storm weather.
When sailing and the wind suddenly changes direction by approximately 180°, look in the new downwind direction at the sky. Look for a dark grey, black, or greenish purple sky, with the sun becoming obscured then prepare for a violent squall, or seek immediate shelter. On open water you see an intense white line at the junction of sea and sky, this being the rain hitting the sea.
A thunderstorm is formed by intense localized area of low pressure that may produce strong winds and heavy rain or hail sometimes accompanied by thunder and lightning. Thunderstorms in tropical regions, are common, often occurring at night but are less frequent in temperate latitudes occurring often over large land masses. Thunderstorms at sea usually form along the line of a cold front.
[ Thunderstorms ] start when a large volume of air is forced upwards by being heated or being pushed upwards by a cold air front. Large convection currents form huge banks of cumulus cloud pushing upwards and then grow larger. When the system becomes active it causes cumulonimbus clouds to form. These are towering dark clouds that reach great heights with distinguishing anvil-shaped tops pointing in the storm’s direction of travel.
Higher cloud tops, the more intense the storm with ice crystals forming in the anvil top. When colliding within the cloud they create static electricity discharging to earth as lightning. Violent internal air currents create low rolls of cloud that form along the base of a storm that result in heavy rain or hail producing strong winds and wind shift. Behind the thunderstorm, the wind and temperature drop.
If at sea when a thunderstorm approaches get out of its path however, if the storm passes over, turn into it to get through it quicker.
Known by different names in different parts of the world, tropical revolving storms ( TRS ) are hurricanes in the Atlantic, in the western Pacific; typhoons, and the Indian Ocean; cyclones. Created in low latitudes they are discernible by enormous banks of cloud reaching to the limit of the atmosphere and indicate a very intense depression. The depression covers a smaller area than a frontal depression with the pressure gradient steeper, causing winds in excess of Force 12, with heavy and steep seas.
[ Winds spiral inwards ] towards the centre of the depression at all heights through the storm during a revolving storm, instead of gusting along the isobars at higher levels, as in a normal depression. Winds revolve clockwise in the southern hemisphere and anticlockwise in the northern hemisphere.
An "eye" is at the centre of the storm where the wind is temporarily calm, although the seas are rough and chaotic without the controlling influence of the wind. Clouds may clear for a short period of time but only briefly as the opposite cloud bank approaches, containing intense winds. Winds are strongest close to the eye and visibility is reduced, often to zero, with the air being filled with foam and spray.
Tropical storms mainly develop on the eastern side of oceans, maturing when they travel westward or north westward in the northern hemisphere, or westward or south-westward in the south. They move slowly at 10-15 knots in the early stages and later increase to 25 knots or more.
Nearing the [ western side of the ocean ] , they have a tendency to curve north or north-easterly in the northern hemisphere, or south or south-easterly in the southern hemisphere.
Tropical storms mostly occur in predictable seasons but can develop at other times. The origins and growth of tropical storms is monitored by weather forecasters via weather satellites with sailors receiving warnings by radio or Weatherfax.
Forecasting storms in tropical latitudes is done when a drop in barometric pressure of 5mb or more will herald an approaching storm. Other signs are large ocean swell, major changes in wind speed and direction with [ clouds ] moving from high cirrus through altostratus to heaped cumulus.
When in the path of a tropical storm, sailors should try to avoid being caught in the [ semi-circle ] , which is north of the centre in the northern hemisphere, and south of the centre in the southern hemisphere. A tropical storm is dangerous to small yachts so make for safe refuges, known as hurricane holes, found in areas in the vicinity to these storms, but be aware that harbours present as many dangers as being at sea.
Tornados are violent small scale disturbances and far more common over land than at sea, occurring on the eastern plains of North America. They form in hot, humid, thundery conditions and are created by severe convection currents occurring in large cumulonimbus clouds.
With a diameter of only a few hundred metres it rarely travels more than a few miles. In that distance it causes more destruction than almost any other natural occurrence. Winds at the centre may reach 200 knots, and the very low pressure is able to rip houses apart and throw cars in the air.
A type of tornado that is found at sea is a waterspout that forms under heavy cumulonimbus clouds containing strong convection currents. When forming, a waterspout has a funnel-shaped cloud extending from the cumulonimbus cloud towards the sea with a revolving motion causing a whirling mass of spray to rise from the sea.
If it continues to develop, the funnel end meets the spray cloud forming a spiralling column between cloud base and sea. The column’s top and bottom travel at different speeds and takes up a slanting position and finally breaks up.
A waterspout is a confined and short-term event but presents a danger to small craft. Waterspouts are is less severe than tornados, normally lasting less than 30 minutes, and covering an area approximately 30m in diameter. They move slowly and tend to be erratic being common in tropical latitudes.