Thunderstorms and Squalls
Thunderstorms mau be generated along active cold fronts or over land on still, hot summer days. In both cases, strong convection currents are set up, and build large cumulonimbus clouds.
Raindrops in these large clouds grow so big that they disintegrate. This creates a charge of static electricity – which is eventually discharged to produce lightning and thunder.
Lightning is attracted to high points (such as a yacht’s mast), but is usually discharged quickly and safely through the mast and rigging to the sea. Very strong and rapidly shifting winds can be expected just in front of a thundercloud, as the cloud effectively ‘sucks’ air in from around it. Squalls generally pose a greater threat to small craft than the risk of lightning.
Line squalls may occur along the line of very active cold fronts, particularly if there is a marked kink in the isobars (i.e. shift in wind direction) along the line of the front. They are marked by a clearly-defined black ‘roll cloud’ , similar to that at the leading edge of a thunderstorm. The effect is similar: strong gusts of wind followed by heavy rain, but in extreme cases the turbulence may be enough to trigger waterspouts or tormadoes, representing a real danger to small craft that is best avoided by aiming to pass under the lightest part of the approaching roll cloud.