These systems are less frequent but often provide superb opportunities for photography, as well-developed lines of cumulonimbus generation persist over favourable areas. In Wales, these often form in a N-S convergence line along its border with England. Clear skies and well-defined cloud structures are the order of the day -such storms are slow-moving and often they have developed by mid-morning and then last into the evening, with regeneration taking place repeatedly.

3) Unstable airflows from SW, W or NW associated with Atlantic depressions, all year around

Atlantic depressions are a strong feature of Welsh weather, occurring all year round, but most vigorously October-March, when winds can gust to over 100mph on occasion. Heavy showers occur following the passage of the cold front, feeding in off the Irish Sea. Squally winds and rattling lentil-sized hail are the usual mixture in with the rain but on occasion things get a bit more impressive, with cumulonimbus development along well-defined squall-lines, mammatus cloud on the undersides of anvils and plenty of lightning. A SW force 5 wind seems to generate good conditions, particularly in the Spring and Autumn: in winter a Nor-westerly sometimes brings thundery snow showers. Clear air and excellent cloud structures are typical, the showers fast-moving but frequent.

PLANNING THE CHASE: KNOWING YOUR PATCH AND GETTING THE ESSENTIAL DATA

Following and photographing storms in the Welsh hills is a challenging occuptation. Good vantage points combined with safe places to stop are not that common. There are very few roads where you can simply stop the minute you see something good! The best plan is to get to know the layout of your usual chase area on non-storm days, noting places where you can safely stop and that command extensive views. Make mental notes of these as they will be important target points on a stormy day!

Basic TV and radio forecasts will alert you to the possibilities. Look out for "rain, followed by brighter, showery weather" for instance. But you will have to pin down the activity a bit more for results.

For storm formation, convection is required - the process by which warm, moist masses (or parcels) of air are able to rise up into cold, drier air. In such conditions the water vapour condenses out as water droplets or ice crystals - clouds. As these particles start to fall back earthwards due to gravity, precipitation in the form of rain or hail occurs. The vertical transport of heat and moisture sets up an updraght, while the gusty winds that accompany the rain from a convective cloud are known as downdraughts. The stronger the updraught, the longer the water and ice particles will be kept in the cloud and the heavier the consequent shower will be. But the important thing is to work out where - and when - active convection leading to shower or storm development will occur.

(L): A good sign that serious convection is underway is when ordinary cumulus clouds start to take on a "towering" appearance. Such development, if it continues unchecked, can lead to cumulonimbus formation - the classic thundercloud!

Data can readily be found on the Internet. The BBC and Met Office weather sites give the basics. Firstly, learn how to interpret synoptic charts. Then, there are more specialised data sources that are important to check when forecasting convective activity. See my LINKS page to access essential forecasting data. Two important datasets of particular interest to storm-chasers are the CAPE and the Lifted Index:

CAPE (Convectively Available Potential Energy)

This is measured in Joules per Kilogram of air (J/Kg) and defines how much energy is available or released by a rising parcel of air between 1000hpa and 500hpa. It is an important tool in forecasting the likelihood of thunderstorms. Values of more than zero are indicative of instability (necessary for convection). Other factors have to be taken into account though! If CAPE values exceed 1000J/Kg then severe thunderstorms are probable, but even values of >100 can produce results!

LI (Lifted Index)

The Lifted Index is a value calculated by subtracting an air parcel's temperature from the temperature of the surrounding air into which it has risen, giving a value in ºC. If the Lifted Index is less than zero (eg. -2), it indicates the parcel is warmer than the air into which it has risen. Hot air rises: therefore a negative Lifted Index shows that the air parcel still has buoyancy. In such unstable conditions, showers and thunderstorms are more likely. Very low Lifted Index values (-5 or less) are rare over the UK and are indicative of conditions in which severe thunderstorms may develop.

Both CAPE and Lifted Index values are plotted as overlays on maps of the UK. They are generally updated every 6 hours and the updates must be checked as the new plots will be based on more recent data!

Rainfall radar is useful when combined with satellite images and sferics plots - the latter being plots of the location and rate of lightning strikes. These are updated more frequently, at least hourly, and can allow you to pinpoint where a storm currently is and, importantly, where it is heading. Ideally the thing to look for is an isolated heavy storm with clear areas nearby, permitting good chances of photographing cloud structure.

CHASE TACTICS FOR PHOTOGRAPHERS

If storm cells are fast-moving (99% of the time this means that they are bowling along on a strong SW round to NW airflow), then don't try to keep up with them (by driving like a maniac and risking your and other folk's lives) but try to meet them head-on. The Mid-Wales coast allows superb extensive views both seaward and back towards the mountains in many areas, so that it is a natural choice in such setups. You can photograph an approaching storm, move a little to the N or S to avoid the core and then retrace your steps to photograph it again as it retreats away - which is a prime time for mammatus-clouds, backlit by the sun (hopefully).

Because the best storms in photographic terms are usually fast-movers in a brisk westerly-quadrant wind, what was a fantastic sight when you stopped the car may have disappeared by the time you have set up your tripod, changed lenses, taken light readings etc. Learn to take photographs by hand without getting camera-shake! Use medium-fast film like 100-200 ASA so that the exposures are fast and take several shots if something looks really good - it improves the chance of getting one really good one. If a storm is slow-moving, that's when you can take your time with photography.

Finally, if lightning bolts are crashing all around you, stay in your vehicle! Remember that the best chances for good photographs are not in the middle of a storm but to one side of it. The precipitation cores of big storms are also dangerous due to high winds and standing water on the roads and are places to avoid - or, if passing through them, places for extremely cautious driving, especially bearing in mind that some other road-users may not understand or appreciate the hazards.

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