Cumulus Spread Out
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CUMULUS SPREAD OUT The ruin of many a good day Tom Bradbury from SAILPLANE & GLIDING HE EXASPERATING FEATURE of most way from 2000 to 5300 feet and was only 0.7° Critical factors These two soundings show spread out days is that they share many at the base of the inversion. up the difference between widely scattered Tfair weather indicators, so it can be hard cu and a total spread out. Scattered cu exist to distinguish between good and bad days. • The condensation level (marked CL) was in a dry atmosphere with more than 5°C sepa- low, near 955 mb or about 1500 feet. This ration between air temperature and dew point Cases of spread out Spread out is due suggests a cloud depth of some 3800 feet. under a weak inversion. Spread out is likely mainly to excessive moisture in the atmos- Anything more than 2000 feet depth of cloud with a very marked inversion, 2° or less sepa- phere but it is strongly influenced by the de- favours persistent spread out. An inversion at ration between dew point and air temperature velopment of an inversion. The problem is 7000–8000 feet with a big depth of cloud un- with a cloud depth of at least 2000 feet. The caused by very slow evaporation of older der it tends to give so much spread out that inversion is important because it concentrates masses of cloud. cross–country flying is nearly impossible. On all the lifted moisture at much the same level this occasion very little surface heating was just under the inversion. If the inversion is • In a dry atmosphere thermals carry mois- needed to start convection. The extra energy destroyed the moisture is spread out over ture up from low level to form clouds. When from release of latent heat is shown by the greater depth and a complete layer of cloud the thermal ends evaporation into the shaded area. The larger this shaded area the takes longer to form. surrounding dry air disperses the cloud more energy is available for forming cloud. If and cooling makes the sink stronger. Thus the shaded area grows wider with height the The development of spread out Figure 3 scattered cumulus are often separated by early morning cu tend to shoot up like rockets illustrates a typical sequence of events from areas of strong sink in clear air. until they hit the inversion. The first clouds early morning to midday. Time runs from left • If the air aloft is already moist it takes much may not have the energy to go so far; these to right. Letters mark stages of development. longer for evaporation to dissolve the de- tend to slow down and become tilted over if caying patches of cloud. The sky begins there is a stronger wind aloft. A shows lots of scruffy bits of cu which form to fill up with bits of cloud at many levels. soon after sunrise. This early appearance Sink is weaker under an overcast of spread Figure 2 shows a sounding made at the same shows little heat is needed to start convection out stratocu but this is offset by a lack of time to illustrate the difference which often and suggests the air is too moist, thus giving sunshine to set off fresh thermals. occurs over a distance of 300 or 400 miles. a low cloudbase. An early warning It is often a bad sign if • The air was much drier, the separation be- B shows columns of cu rocketing up in the cumulus start to form soon after the sun has tween air temperature and dew point was moist unstable air. These have not yet enough risen. It means that the air is so moist that at least 10°C between 1000 and 5000 feet. energy to reach the inversion so they slow little heat is needed to form cloud which al- down and become tilted over by the wind. most always has a low base. Tall cu with a • The inversion (which was just below 5000 low cloudbase often precedes spread out. feet asl) was much less marked; the tem- C At this stage the cu have become stronger perature only rose 1° in the next 900 feet. and their ascent pushes up some of the moist Figure 1 shows a temperature sounding on a Much more heat was needed to start con- air aloft. This results in lenticular cloud caps day of extensive spread out. The tephigram vection and the condensation level was called pileus. They are smooth because they shows some factors which nearly always pro- much higher than in Figure 1. are not part of a bubbly cumulus; they are duce a spread out layer soon after convec- rather like bow waves ahead of a blunt nosed tion starts: Since the air was drier at the surface the con- barge. The pileus tend to stay in the moist densation level was much higher, being nearly zone and the cumulus may build through them. • A marked inversion (this day at 832 mb, 4000 feet asl. Much heat was needed before Pileus are a reliable sign of spread out later nearly 5300 msl). Above this level the tem- any thermals reached this level so cumulus on but they do not appear on every occasion. perature rose some 6.5° in just over 700 feet. formed much later. The depth of cloud was much smaller too. The small shaded area D shows the cumulus big enough to carry • The separation between dew point and air above CL on Figure 2 shows very little extra moisture up to the inversion where it spread temperature was less than 2°C nearly all the energy was released by condensation. out horizontally. Figure 1 -5 0 5 Figure 2 -5 0 5 10 10 800 800 dew point temp SALR CL 15 15 900 900 CL DALR 1000 1000 mb mb 6 free flight 2/94 overshoot spread out time pileus mamma H W ABCDE FG Figure 3 E shows a cu top overshooting the inversion pen after the task has been set and before A continuous sheet of cloud off Ireland or the and pushing a dome into the warmer and any satellite picture is available to show what west coast of Scotland often indicates cu usually very dry air aloft. At this stage there is is happening. It does not always need an old spread out over England the next day when usually lots of lift under the cloud and climbs front; a weak trough may trigger off a wide the wind is northwesterly. The cloud sheet can be made which bring you out into clear band of spread out. may disperse over the cold land during the air above the inversion. The lift of such a cu night but it will usually form again a few hours which has its width “W” equal or greater than Going round or pushing through? after cu have developed. its height “H” is usually at least 15 minutes An active cloud edge offers the chance of a and may be longer if there is a steady feed of fairly fast diversion round the spread out; un- On days when the air is moving slowly from new thermals into the cloudbase. fortunately it does not often go far enough the west an area of spread out over Ireland though it may take you to a rift which heads in during the afternoon gives warning that the F shows the column of cumulus narrowing. the direction you want to go. The active edge same may occur over England the next day. The chimney carrying moisture upwards is no is often a good place to take a climb under (Since moisture is a major reason for spread longer supported by buttresses of younger one of the stronger cu. out Ireland seems to suffer from it even more cells as at E. At this stage the overshooting than England.) For Europeans the arrival of top probably starts to collapse. The descend- Lift under the gloom The soundings on spread out over England is a warning of prob- ing air sinks down and appears on the under- spread out days are almost always very un- lems for them the next day. However, a long side of the cloud sheet like a set of upside– stable beneath the inversion and need little land track often dries up the air enough to down cumuli. These are termed mammatus extra heat to set off more thermals. These are break up a stratocu sheet. The North Sea clouds. Mammatus is an indication of the col- apt to be much weaker than those formed coasts from the Low Countries round to Den- lapse of powerful cu which have overshot the under a sunny sky. The weakness is some- mark suffer from stratocu at least as much as inversion and are now falling back. Mamma times compensated for by the thermals being the UK but the cloud sheet usually breaks up usually have a short life; they change their larger and much smoother. Sink does not over Eastern Europe. shape and size rapidly. Thunderstorms often entirely vanish but is often much less than produce an area of mamma on the inactive between scattered cumulus. As a result one Shelter effect The stratocu sheet often side. It is often a good idea to avoid the re- can go quite long distances under an unwel- breaks up on crossing high ground if the cloud gion underneath mammatus cloud as the sink coming sky. On such days it can be extremely top is not more than 2000 feet above the hills.