A Met Guide For Beginners
by Tom Bradburyfrom Sailplane and Gliding
p172 - August/September 1987
Table of Contents
- Picking a Good Day
- Route Planning
- Variation of Thermal Strength
- Spacing of Thermals
- Sink between Thermals
- Variation of Lift with Height
- Looking for Lift
- Cloud Reading
- Cloud Streets
- Waves above Streets
- Avoiding Sink
- Spread out of Cumulus
- Warning Signs
- The Cycle of Spreadouts
- Spread out Situations
- Blue Thermal Days
- Top Cover of Cloud
- Thickening Pre-frontal Altostratus
- Fog and Low Stratus
- Haze Tops and Cloud Tops
- Use of Radio
These notes are for early cross-country pilots who (at present) are more interested in getting somewhere slowly than trying to win races.
1. Picking a good dayThe best conditions usually occur after the passage of a cold front when;
(a) mere is a ridge of high pressure moving across the country (or at least anticyclonically arrived isobars).
(b) The wind speed in tha 2000-5000ft range is less than 20kt (preferably near 10kts).
(c)The forecast Max temperature is at least 10 C higher than the dew point. (As a guide one may use the TV chart for the night Min and next day's Max temperatures; if the difference is 70"C or more then the cloudbase will probably become high enough.)
There is a useful rule relating cumulus base and the difference between the surface tempera- ture and dew point. While the temperature is ris- ing each degree C between the air temperature and the dew point is equivalent to about 400ft in the base of convective cloud. For example a difference of 10C should give a cloudbase of 4000ft. This rule is not valid once the temperature starts to fall.
2. Route planning
The wind. The wind at flying levels is best obtained from an aviation forecast but one can get an approximate guide from large scale fore- cast charts like these in the Telegraph. The Times alas no longer provides an adequate pic- ture.) Measure off a length of 300nm (this is 5" of latitude). Draw a line of this length at right angles to the isobars on the forecast chart and note the pressure difference between the ends. Multiply this by 2.5 and you have the wind speed at about 2000ft. This figure is strictly valid for latitude 52 north but it is close enough for most of the cen- tral and southern parts of England.)
Wind speed is usually critical for into wind legs. Although pundits can achieve an average air speed of 50-60kt on a good day, less experi- enced pilots will rarely exceed 30kt. This obliges slower pilots to avoid into wind legs unless the wind is very light. If headwinds are unavoidable the into wind leg is best attempted during the afternoon rather than in the morning.
Even with light winds the choice of track and TPs is influenced by wind direction because it is usually essential to keep clear of windward coasts. Unsoarable sea air tends to spread long distances inland across large flat areas (such as the Somerset levels and the regions round the Wash). There are rare occasions when the air is so dry and unstable that good thermals can be found right up to the windward coasts, but it seldom pays to bank on it. These areas are best crossed early in the day before inland convection has started to draw in damp sea air. (Fig. 3.)
There is an urge to get in the air and away down track as soon as possible. Resist this urge if you are only after Silver distance. Unless it is known that poor weather is approaching one can expect soaring conditions to become easier later in the day. The cloudbase usually rises to its Max in mid afternoon and thermals, though further apart, seem to be smoother and easier to work during the latter half of the day.
High ground warms up sooner than wide damp valleys and good thermals can be found over regions such as the Chitterns. Berkshire Downs, Cotswolds and the bigger hills of Wales a good two hours before any lift appears over low ground. On days of restricted convection this delay may be much longer. (Fig 4.)
4. Variation of thermal strengthIf occasions of cu-nlms are excluded the average rates of climb seem closely related to the height of cloudbase, or the top ot blue thermals. A survey carried out by the French showed that lift in knots was(approximately) 1.2 times the height of cloudbase in thousands of feet, minus 1 kt. Thus 2000ft produced a miserable 1.4kt. 4000ft gave 3.8kt and 6000ft 6.2kl. Almost every one finds stronger thermals than these during the course of a night but they nearly always have to stop and accept much weaker litt too. These figures are a useful guide for planning but no indication of absolute values.
Spacing of thermals. If the depth of convection is shallow thermals are close together. As ther- mals extend higher the spacing becomes wider. There seems to be no exact relationship between depth and spacing because late in the aftemoon the gaps between thermals continue to grow wider even though the depth of convection is no longer growing.
Sink between thermals. Early morning ther- mals usually produce weak lift with sink mainly confined to the immediate surroundings of the thermal. Later in the morning when convection is deeper and lift stronger the areas of sink often seem to extend much of the way across the gaps. When thermals become separated more widely (usually from mid to late afernoon) the inter- thermal sink is less troublesome although strong sink still occurs close to the best thermals. During the last hour or two of thermal activity the spacing is stroogly dependent on isolated hot spots such as sun facing ridges. In between these isolated areas the air can become very smooth with negligible sink.
Variation of lift with height. Over level ground thermal lift is almost always weak below about 1000ft and does not develop its best strength until 2000 is passed. If the thermal is feeding into a cumulus which is at least 1000ft deep the lift may show a further increase close to cloudbase. However, on days when the only clouds are very shallow cumulus the lift frequently decreases rapidty iust below cloudbase. On such days the cloud tops are restricted to a well marked stable layer. The cloud tops may protrude a small way into this stable layer due to the momentum behind the thermal. However, the rising airflow starts to spread out as it nears the inversion and as a result the lift ceases quite suddenly. Fig 5 shows the distribution of lift with height such days and why it is a waste of time to take the last few feet of the thermal. The same effect occurs when there are only blue thermals.
5. Looking for lift
Cloud reading. A major factor in the success of pundits is their ability to read clouds; it seems to be a skill best learn in youth.
(a) Active cumulus clouds usually have well defined flat, (sometimes slightly concave) bases and crisp bulging tops.
(b) The larger the cloud the harder it may be to find the lift. Sometimes such clouds have a slight step down in the base, or a region of slightly lower, rather ragged cloud. The best lift is frequently very close to this step.
(Fig 6.) Lacking such signs one may have to waste time searching round. The time will not be entirely wasted if one can establish a preferred location for the lift at that time of day
(c) The core of fhe thermal is often on the windward side or the sunny side of the cloud; if wind and sun are on the same side there is a good chance that the lift will also be on that side. Do not be too surprised if the core is actually in quite a different spot.
(d) Shallow clouds under a dry inversion some- times show a curled over hook lihe shape or, the top. (Fig 7.) This usually develops when there is a stronger wind above the inversion. The shear of wind takes the rising top and blows it over into the curling shape. iift is a]most always close under the windward side, with sink on the down shear side.
(e) Small clouds usually have a very brief life in the morning, but they are normally close enough for there to be working alternatives near by. The larger the clouds the longer their life cycles; when there are many large clouds (more than half cover) several will be slowly decaying without showing any clear signs of their weakness.
(f) If the lift is very strong (6 -10kt on an average) it is almost certain that there will also be very strong sink not far away. Unfortunately the reverse is not always true.
(g) While heading for a good looking cloud one may meet an unexpected surge of strong lift out in a cloudless gap. This is probably a vigorous young thermal about to form its own doud. These often produce much better climbs than the older clouds nearby.
Cloud streets. Streeting is common, even on blue thermal days. Streets generally form when the wind speed is over 15kt and may be wide- spread with strong winds. Streets are aligned along the wind direction (within a few degrees). This makes them invaluable for making progress into wind. (Fig 8.)
A single line of cloud may have formed from a local hot spot on the surface but the streets do not depend on irregularities in the surface tem- perature. Streeting occurs over the sea as well as over land, especially when fresh cold air sweeps out over a relatively warm sea on the western flank of a depression.
Streeting needs a stable layer to limit the depth of convection so that nearly all the cumulus tops are on the same level. The spacing between streets is usually about three times the depth of convection. If the tops are around 5000ft the streets are likely to be some three miles apart. If the inversion rises the spacing between streets increases, usually by the disappearance of weaker streets. (Not by all the lines edging further apart.)
Over England one may go as much as 50 miles under a good cloud street without turning but the crossing from one street to another has to be made through continuous heavy sink. Streets are much harder to follow on blue ther- mal days. On such a day an unusually prolonged spell of sink enoountered when flying up or downwind probably means that the track lies be- tween streets. Turn crosswind for a time.
Waves above streets (Fig 9.) Lee waves may develop above and at right angles to cloud streets. Such waves are not always marked by lenticular cloud. The first wave often occurs at the upwind end of a cloud street.
If, when flying along a cloud street, there is a stretch where the usual lift is replaced by sink and then there is a small zone of unusually strong and rough lift it is quite likely that the street is being influenced by the waves above. Waves have also been found parallel to streets of shallow cloud, the streets then seem to be act- ing as temporary hills.
6. Avoiding sink
The best instructors will tell you to "follow the energy", meaning to take a winding course under all the working clouds rather than heading out directly on track. A common problem is how best to dodge the decaying clouds. Clouds have a limited life and the small clouds tend to stop, working sooner than big ones, especially during the morning. Although the big clouds last longer they tend to leave a larger and more persistent area of sink.
When the moisture in a thermal condenses out as droplets of cloud there is a release of latent heat. This gives an added boost to the thermal. However once the lift ceases and the cloud starts to decay descent of air causes evaporation. Evaporatian removes all the heat previously released by condensation and this air becomes colder than its surroundings.
This cold mass produces heavy sink; the bigger the cloud has been the more extensive is the sink when the cloud decays.
The signs of decay are:
(a) Loss of sharpness in the cloud top: it starts to look fuzzy.
(b) The cloudbase ceases to be level.
(c) The cloud shadow changes from being solid to become a tattered area with holes. This is often the mosr reliable indicator if you are near cloudbase and heading for the next good lift
(d) Tall clouds which start to topple over in a wind shear usually decay. Never fly close under the over hanging part of such a cloud. Steer around an the upwind side it possible. The net loss in flying flve miles in relatively still air is often less than taking a direct course and going two miles through heavy sink.
(e) A cloud may be still be growing on the upwind side while decaying on the down- wind side. This is common with large clouds when there is an increase of wind speed with height. (Fig 10.)
As a shower advances downwind there is often a region of particulary strong lift under the lead- ing edge of the cloud. This can be used to gain or maintain sufficient height to fly around the end of the shower. It is usually wise to go around even the smallest shower. Flying straight through nearly always takes one into a large area of heavy sink.
Sometimes the lift continues right up to the shaft of precipitation (Fig 11). One may even make a climb with hail rattling off the canopy. but be prepared for very sudden and often, nasty sur- prises. Precipitation ncarly always changes ascending into descending air, often very sud- denly, sometimes within the space of a single tight circle.
Blue holes. A common problem in England is the short lived shower which dissolves to leave a blue hole. Although the cloud has vanished the sink may still persist; it pays to avoid flying under such a decayed shower, or across the stretch of ground upwind over which the shower has passed. Even when the sink has died out the cooling effect of the rain and the recently moist- ened ground inhibit thermals.
Defunct showers are only one of the reasons for blue holes: they may be the effect of an unsus- pected trough in a wave system higher up or due to preferential growth of big cumulus around the perimeter. When a group of cumulus clouds clumps together to produce an area of heavy cloud they may set up a wide area of surrounding sink which wipes out all the lesser cumuli which have not organised themselves in such a co- operative system. (Fig 12.) the development of a big cu-nim cell amongst a field of small cu fre- quently wipes out the tiddlers.
With so many reasons for blue holes it is wise to be cautious about setting out across one. The pilot of a Ventus recently set out into the blue from 3000ft. With tips to extend the span to 17 metres he was confident of reaching the other side. In fact he was on the grwnd seven miles downwind of the start.
A diversion of 30 degrees only adds a small amount to ones total distance; when going downwind even a 45 degrees change of heading is worthwhile. It is far better to take several short climbs at high level where the lift is good than to waste time scraping about low down where the lift is weak.
8. Spread out of cumulus
This ruins very many days which would other- wise have been magnificent. The main reasons are:
(a) A very unstable air mass which is too moist, and
(b) An inversion or stable layer which traps all the convection, beneath it.
(c) The arrival of extra moisture near the inver- sion level, often from a very weak old front which has temporarily lost all its cloud due to subsidence.
It usually needs a depth of at least 2000ft from cloudbase to the inversion for spread out to become extensive. Each thermal takes up more moisture and spreads it out under the inversion adding to that already present until a solid layer of cloud is formed.
When such an overcast area appears one should try and stay high using any scraps of lift under darker patches of cloud. Until the sun breaks through there will be few if any thermals rising off the ground.
(1)The morning starts cloudless and visibility is often very good.
(2) The first cumulus forms unusually early and the cloudbase is low. (If the first cu have a high cloudbase there is much less threat of spread out.)
(3) Some of the first clouds may shoot up as narrow columns with no proper bases. (The base decays before the top has finished rising.)
FIGURE 13 (4) A lenticular cap of cloud may appear just above the top of a growing cumulus. This has the latin term "pileus". The formation shows that as the top of the cu ascends it pushes up some of the moist layer air above. This push is just enough to cool the upper layer below condensation point; it shows that the layer was nearly saturated at that level before the cu formed. Pileus is an almost infallible sign of subsiquent spread out.(Fig 13.)
The cycle of spreading out. When an almost total layer of strato-cu has formed thermals become very sparse or totally absent. Lacking a continued supply of moisture from below, the layer may break up in an hour or two. This allows the sun to set off more thermals so that the pro- cess is repeated. With a really thick layer the cycle is so slow that no wothhwhile clearance develops until evening.
Two things can act to disperse such a sheet Further subsidence may bring the inversion too low for a tall cloud cover to develop, or the arrival of drier air may result in the cloudbase litting up to within a few hundred feet of the inversion when the sheet will disperse. The two processes can occur together to bring about a rapid improve- ment in soaring conditions. The extra subsidence may be found near the axis of an advancing ridge, (which is one reason why ridges often give the best soaring weather in summer).
Spread out situations. The problem is most troublesome near to windward coasts especially when the air over this country has come round the perimeter of an Atlantic anticyuclone and arrived over us from the north or north-west.
9. Blue Thermal Days
Competition pilots have to set off on blue day but they have the advantage of many other gliders to find and mark the thermals. It is much harder for a beginner to succeed when there is no other glider in sight.
The most important factor, (after the wind speed) is the height of the inversion. With only 3000 ft between ground level and the inversion unaccompanied cross-country flying is very dif- ficult. If the convective layer extends up to 4000ft it is probably worth a try. With 5000ft to work in the prospects become quite good.
Possible thermal sources are towns, sun fac- ing ridges, and areas of higher ground which are relatively dry.
Regions to avoid if possible are wide damp valleys. These may be devoid of thermals except where there is a large town. Even when some thermals do develop they are often weaker and do not go up as far as those over the high ground. The lack of thermals is due to the abundance of lush vegetation and the generally moist ground. So much of the sun's energy is wasted just evaporating the water that not enough is left to produce good thermals. (See also fhe last issue, "Blue Skies" by John Williamson, p126.)
Slopes. these were the first resort of early soar- ing pilots and are now the last resort of most cross-country pilots. Windward slopes may save the day when all else has failed. Ridges work best when there is no high ground upwind. Upwind ridges may set off lee waves; if these are out of phase with your ridge the lift may be damped out. Notice that rapid alterations of lift and sink may be due as much to thermals breaking away from the slope as to the mean flow of air uphill. Ther- mals often come off from one area like a stream of bubbles and one may need to head back into wind several times before finally escaping.
10. Top cover of cloud
The arrival of a layer of cirrus nearly always reduces the strength of the sun. If the lower air is already full of active thermals the top cover tends to make thermals rather smoother and less strong. However. if it is early in the day, or there is a low inversion, (when the full power of the sun is needed to produce any thermals at all), then the cirrus often stops thermals completely. On such days a gap in the cirrus may allow a nanow zone of thermals to develop when most of the area has gone dead.
Thickening pre-frontal altostratus. Such cloud almost atways has a disastrous effect on thermals, stopping them very quickly. Note the "almost": there are occasions when the air is so unstable that even the arrival af this grey sheet of cloud does not completely kill on all thermals and on rare occasions one may still find lift (usually smooth and weak) persisting almost up to the time when the rain starts.
11. Fog and low stratus
These are signg of very stable conditions at the lowest level; it is useful to know about them when route planning. Some of the sun's heat is wasted in evaporating the fog before any thermals can develop. Even when the fog has been burnt off the area is apt to be lacking in decent thermais for many hours. In summer sea fog or low stratus often moves in again from the coast on blue days when sea breezes begin. Although the sun may continue to burn off the stratus as it comes inland the air will probably never develop useful ther- mals until it has spent three of four hours over warm ground. Even then the lift is likely to be shallow and weak. The boundary often shows up as a marked change of visibility. When easterly winds develop over England the effect of North sea stratus can spread from the Wash to the Cotswolds by mid afternoon. (Fig 14.)
Most of our summer haze comes from the coo- tinent when winds over the UK are between ENE and SE. It is usually trapped beneath an anticyclonic inversion. The chief effect of haze is to delay the start of thermals in the morning, and to cut them off earlier in the evening. It is notice- able that thermals become weaker if one flies into the haze from an area of good visibility. Few long cross countries have been achieved in really hazy weather.
Some of the haze particles are hygroscopic, that is they tend to absorb moisture by accelerat- ing the condensation of water vapour. This makes the visibility worse in regions of high humidity, especially in the layer within two or three hundred feet of the cloudbase. Since gliders often fly in this layer the collision risk is increased.
Hot weather and summer haze often go together. The restricted visibility makes it next to impossible to see clouds ahead well enough. It thunderstorms break out (as they often do after a hot hazy spell). one cannot see the distant thun- derheads until one climbs above the haze layer. Instead the storm's approach is marked by thick- ening gloom where the cloud shadow falls on the haze.
Haze tops and cloud tops. Strong thermals often reach the inversion with enough momen- tum to penetrate a short distance into the stable layer. On blue days it may be worth accepting the reduced lift at the top in order to get above the inversion for a briet time. The great improvemen( in visibility allows one to see any small cu tops in the distance and may reveal those active areas of convection previously hidden from sight.
Big cumulus can grow through a haze layer and extend high up to levels where visibilty is almost infinite. The haze layer seems hardly affected by this deep convection; it remains at its original level. A cloud climb is particularly satisfy- ing on such days but brings navigation problems; it may be impossible to make out any ground features when looking down through the haze.
Use of radio. There are three useful plain language broadcasts of airfield weather reports. They are updated every half hour. Reception is often difficult at very low level except near the transmitters.
The frequencies are:
- London North 126.6 MHz
- London South 128.6 MHz
- London Main 135.375 MHZ
A similar VOLMET broadcast consistlng chiefly of RAF airfields is broadcast on 4722 and 11200 kHz. This can usually be heard on the ground but needs an HF receiver tuned to the upper sideband. Ordinary short wave receivers are inadequate unless they have a BFO (Beat Frequency Oscillator).
AIRMETThe new telephone AlRMET service gives three regional forecasts and are available between 0600 and 2300. me numbers are:
Southern England 0898 500 436
Northern England and Wales 0898 500 435
Scotland and Northern Ireland 0898 500 434
There is an equivalent night service from specified Met offices from 2000 to 0600.
The numbers are:
Heathrow 01 745 3103
Manchester WC 061 429 0927
Glasgow WC 041 221 6113
Forecast Period Outlook Windvalid available to time
1600 0600-7400 2000 0900
1200 1200-2000 0200 1500
1800 1800-0200 0800 2100These forecasts are not cheap. The BT rates are 66p for 3 min at the cheap period and 1.01 pounds at peak and standard rate time, plus VAT! The dura- tion of the forecast may take 4-8 min depending on the complexity of the weather situation and it could wdl cost over 2 pounds at peak times. Clubs without routine forecasts would do well to make one call and pin up the forecast for all to see.
Assembled by Larry Bogan - Sept 1997