Chapter Seven CHAPTER SEVEN Thunderstorms INTRODUCTION A thunderstorm is one of the more spectacular shows put on by nature Its potential dangers to the mariner are its sometimes torrential rain Which may reduce visibility to zero: sudden squalls of wind: its risk of interference with radio communications (known as “atmospherics" or "static"): and the possibility of damage to magnetic compasses if the ship is struck by lightning (a rare event) The causes of a thunderstorm's formation are such that in general it is more violent when the weather is relatively warm and humid In the Mediterranean for example, a violent thunderstorm may last for an hour or two creating serious difficulties for any ship trying to enter harbour and quite dangerous to a small vessel especially if she is under sail In temperate zones thunderstorms are not infrequent at any time of the year during the passage of a cold front due to cold air undercutting warmer air but that tends to be a squally occasion in any case CAUSES OFTHUNDERSTORMS The conditions necessary for the formation of thunderstorms are: (a) Cumulonimbus cloud with precipitation and the base below the 0°C isotherm (b) The lapse rate must exceed the saturated adiabatic to a height of at least 3,000 m (10,000 ft) above the cloud base thus facilitating vigorous convection (c)An adequate supply of moisture from below to facilitate this The latent heat released by condensation within the cloud boosts the upward convection In temperate latitudes these conditions are most likely to be found in cols and shallow depressions but the thunderstorms which result are not as frequent or violent as those which occur in the tropics Given the necessary conditions as above other favourable conditions are: (a) High surface temperature (b) little surface wind (c) “Trigger action", such as: (i) Horizontal convergence of surface air 51 Chapter Seven (ii) Orographic uplift of moist air (iii) Frontal uplift e.g a cold front (iv) Insolation over land (v) Advective heating or (vi) Any combination of the above HEAVY RAIN AND HAIL The formation of hail is described in Chapter 6: the very heavy nature of the rain which is also a frequent feature of thunderstorms is due to somewhat similar processes within the cumulonimbus cloud Hail and heavy rain although often present within the cumulonimbus cloud, not necessarily reach the ground on all occasions LIGHTNING AND THUNDER The intense activity within a Cb cloud results in the build up of tremendous electrical charges Scientific investigations have shown that the upper part of a thunder cloud is charged 'with positive electricity lower down the charge is negative Near the base there is often a small localised region which is positive The lightning flash is an electric spark on a gigantic scale, i.e., an enormous electrical discharge (estimated to be millions of volts) which takes place either within the thunder cloud, between two separate clouds, or between cloud and earth It renders the air white hot along its channel Danger from lightning The risk of a steel ship being struck by lightning is not very great because her masts and other prominent features, being part of her structure, are so perfectly earthed to the water in which she is floating Ashore, the risk to high buildings and other erections not fitted with lightning conductors and to trees is greater Thunder is the sound resulting from the instantaneous expansion and contraction of the air The rumbling effect which we hear is because the sound of the explosive report has to travel from different parts of the long lightning path to the observer and there may be echoes from the clouds Light travels very rapidly and can be treated as though it arrived instantaneously bur the sound travels at 335 metres per second 1,100 ft per second) The distance to a thunderstorm can be approximated by measuring the time in seconds between the arrival of the light and the arrival of the sound The distance in miles is found by dividing this figure by and the distance in 52 Chapter Seven kilometres is found by dividing the figure by When the lightning stroke takes place between cloud and earth, or across a clear space between two clouds, its main channel (and "branches") is directly visible to the eye and is called forked lightning, but when the channel is obscured by cloud, so that the emitted light is diffused, it is termed sheet lightning There are various theories as to the mechanism resulting in the separation of charges within a Cb cloud Most of them have good experimental support but it is thought that several of the charging processes operate together and in addition there are other electrical processes which are not yet completely understood TYPES OF THUNDERSTORMS Heat thunderstorms develop over land in warm, moist conditions accompanied by strong surface heating and convection Surface air flows in from all sides (thus to an observer positioned in advance of the storm it would appear to be travelling against the wind) In temperate latitudes they are most frequent in summer on late afternoons or evenings of warm sultry days with light winds Moumainous islands in the tropics are especially favourable Coastal thunderstorms can occur in any season by day or night They are most frequent in winter and are caused by a large lapse rare in Polar maritime air The final “strigger action" comes from the forced ascent of air crossing the coast from seaward, They are usually slight and dissipate rapidly on moving inland Frontal thunderstorms are more common in winter because there are more depressions An active cold front is unstable especially when there is a large temperature difference between the warm and cold air masses and if the frontal convergence is extremely marked as for example in a V shaped trough (see Line squall in Appendix 1) Warm front thunderstorms are uncommon and less active because the frontal slope is gradual They form at upper level which leaves little room for development of Cb cloud (Compare Figures 13.12 and 13.13) Occlusions sometimes produce thunderstorms, more often with cold occlusions than with warm ones 53 Chapter Seven GENERAL Thunderstorms over the ocean Except in the doldrums only the frontal type are experienced They are rare in high latitudes due to low temperatures and consequent lack of moisture At night it is possible, over land or sea for severe thunderstorms to develop given favourable conditions aloft but some initial "trigger action" is required to start the cloud formation Passage of a thunderstorm is associated with a sudden fall in temperature which can be of the order of l0oC or 11°C This is due to cold air from high level being dragged down to the surface by falling precipitation The precipitation is localised and may be very heavy The arrival of the cold air is associated with a sudden veer in wind direction with a fierce squall and there are violent squalls with large changes in wind direction as the storm passes The pressure generally rises at the forward edge of the storm and then there is a "wake low" at the rear of the storm QUESTIONS: (a) What are the three conditions essential for the development of a thunderstorm? (b) Name the pressure systems in which these conditions are most likely to be found in the middle-latitudes Given the essential conditions required in Question l(a), list other conditions which are favourable for thunderstorm development Name the three types of thunderstorms and discuss the conditions and time of day in which each is most likely to develop, also their seasons of greater frequency Write notes on the following: (a) Squalls in thunderstorms (b) Thunderstorms over the ocean (c)Thunderstorms at night (d) lightning and estimating its distance from the observer 54