To the southwest a warm front moves slowly toward us. 300 miles ahead of the front at ground level is the front at 25,000 feet identified by it's 'mares' tails' cloud, which point back toward the front and its sheets of cirro-stratus and alto-stratus. It will be raining within ten hours if the lower wind is backed from the upper cloud direction.

In good weather under an anticyclone cirrus forms in random patterns and directions. These clouds are not related to frontal systems and they come and go seemingly randomly. They don't 'point' in any single direction.

Aircraft condensation trails are made of ice crystal cloud and are formed by hot gas from the jet turbine cooling and condensing. Close up, the contrail is seen being rotored by the wing tip vortex. Contrails are formed in the moist airmass ahead of an approaching warm front or within an anticyclone's cirrus cloud where the engine exhust triggers condensation and ice crystalisation. Where a jet engine dissipates cloud it is called a 'dis-trail'.

Cirrocumulus layers can form in the stable conditions of high pressure. It usually comes and goes as the layer that it forms in gently sinks and rises. Cirro-cumulus of this sort is really sheet cloud in which parts are sinking and thus dissapearing. It foretells good weather.

On a sunny thermic day at Westbury a high-pressure inversion at about 8,000 feet is gently lifted into condensation by the warming of the underlying airmass. This photo shows a thin translucent alto stratus layer at the inversion. A cumulus pushes its head up into the inversion layer but cannot grow up beyond it.

Alto-cumulus is formed either by instability or by stability. The stable-air form starts as a sheet of alto-stratus, maybe from a decaying warm front. High pressure causes the cloud to decay leaving altocumulus lacunosus or 'mackeral skies'. This is good-weather anticyclonic cloud. In the photo the low-level fractus is moving from the north, the altocumulus sheet is from the west, thus good weather to come.

Warm frontal wave effects produce lens shaped or smooth cloud at the point where the wave reaches its peak. The lens clouds in this windy-day photo looking WNW from Westbury remained over several hours as standing wave lenticulars. This was on the leading edge of a warm front seen approaching from the west with its associated sheets of stable cloud.

Waves of air bouncing up and down can affect a sheet of AltoStratus cloud. In this photo the downward cycle of the wave is bringing the cloud sheet down below its dewpoint (condensation level) partially dissipating it and causing an attractive pattern. The wind is coming from WNW at sunset (directly ahead of the camera).

Verterbratus or rib-like bar clouds can form in a sheet of altocumulus when there is mid-level wave effects present. The undulating airflow causes the sheet of cloud to sink in a regular pattern across the wind direction. The photo shows a sheet of altostratus which was formed by the lifting effect of wave. The effect clearly looks rib-like.

Bars are formed at the horizontal interface of two different airmasses when the the airmasses have vastly different speeds. The effect is a rolling bar where the two masses touch and at right-angles to the wind. This can form on the front of a warm front at mid levels. This cloud may also be of a mild altostratus verterbratus type.

Wavebars formed at Westbury in a windy NW'ly in April 2005. These had real depth to them as is seen in this photo by Conway Shields

Waves of air bouncing up and down come off of the Welsh mountains and other hills and their low-level gust effect can be felt at Westbury when the wind is between West and NW. At westbury this can cause a gusty-wind surge-and-lull cycle. This Westbury photo from June 2004 shows an amazing array of wave bars and lenticular clouds causing lulls and strong periods. These bars mostly dissipated after sunset. Wind is WNW.

Nimbostratus is the thick layer of rain-bearing cloud associated with a warm front. The photo here shows lighter alto-stratus and the darker nimbostratus moving into Westbury from the west (the right of the image). It began to drizzle minutes after this photo was taken. The cloud is blocking the sun so it can be described as opacus cloud.

Nimbostratus eventually becomes a featureless grey raining sheet of cloud described as nebulosis (without features). The drizzle turns to heavier rain as the warm front arrives at ground level. The photo on the right shows this featureless sheet of nimbostratus raincloud. It this case it has attendant cumulus below it as it was occluding and becoming 'kata'.

Crepuscular Rays are not a weather phenomenon as such but a visual illusion formed by a gap appearing in a low and dark cloud sheet thru which the sun can stream.

In stable high pressure conditions there are one or more inversions where the air temperature doesn't get colder with altitude. This acts as a lid trapping the low-level atmosphere (car exhaust fumes, etc). In the picture - taken at a cloudbase of 4500 feet - the inversion is clearly seen. The grey area is the murky low-level air trapped under the 'lid'. The small thermic cumulus don't grow above the inversion lid.

In the early evening the air gets cold and sinks causing an overnight low-level inversion. On the slope this is called Katabatic Flow or 'mountain wind' as the air sinks back down the slope often negating the on-slope lift close to the hill. In this photo a farm fire flattens out in the sinking inversion above it. When you see this happening you know it is time to pack the wing away.

Ground fog only forms between 4mph and 7mph on cold nights when the sky is clear or with no low-level cloud, thus allowing super-cooling of the air just above the surface. If the wind picks up past 7mph the fog rises off the ground completely into low-level stratus and the sun breaks it up In the photo the advection fog-bank persisted in a lee wind from the south.

Jetstreams are the interfaces between polar and tropical airmass cells and they are occasionally marked by high level cirrus bands in various appearances. This photo shows a jetstream cirrus band which moved at visibly high speed (150mph or more) over Mere in the Summer of 2004. When cirrus moves visibly it is travelling at least 100mph and a jetstream is near.

Floccus are tufts of mid- or high-level cloud that give the impression of a 'flock' of sheep. Floccus are formed in unstable mid-level conditions and presage possible thundery conditions. As the day moves on they often grow into the larger hooked-tail Altocumulus Uncinus showers which are already forming in this photo below the main floccus sheet. The result is shown in the photo below...

...Two hours later and the floccus his set off cumulus uncinus which precipitate water or ice tails (sometimes called virga) which fall many thousands of feet. A huge mass of wispy uncinus ice-showers fall into the layer below it and is blown upwind to the left causing a hooked effect due to change of wind speed and direction. No rain reached the ground.

The light-coloured cloud is afternoon Cumulonimbus moving to the left. Behind the Cb the air is heavily seeded with instability and moisture - an ideal breeding ground for new showers. This photo shows two cumulus fractus clouds with rain falling as virga. Normally rain only falls from such thin clouds over the ocean where the airmass is extra wet.

Wave effects become significant when the wind is very strong. The photo shows a 'street' effect from wave in 40mph surface winds. The wave peaks are producing standing wave lenticulars under which is a bar of altocumulus which also showed lenticular effects at times. A double wavebar was formed not long after the photo was taken.

This photo shows a large cumulus-like wave cloud with a smaller one in front of it. While it looks like a sculpted cumulus the whole cloud was actually a standing wave lenticular and it formed and vanished and reformed over a period of minutes always in the same place over Trowbridge. This is a very deep lentie for the UK with a guesstimated depth of maybe 3000 feet.

When the wave effect is deep through an airmass of varying levels of humidity lenticulars can form a cloud effect known as a pile of plates. This is where lenticulars are formed one above the other with a noticable gap between. We don't get the dramatic plates lenties that occur abroad but this photo shows two lenticulars in standing wave and both have lower and higher plates.

Cumulus that form in mainly high-pressure systems create gentle 'fair weather' cumulus clouds which have a distinctly flattened appearence. These are called humilis because of this modest appearance compared with other types of heaped clouds. They are flattened by coming up against the high-pressure inversion.

Cumulus form when the sun heats the ground. Here is a post cold-front airmass with its sharp skies and cumulus. The wind gets stronger with altitude breaking up the thermals. In these conditions they tend to form into lines or streets of cumulus mediocrus. Were the wind to drop cumulus congestus would possibly form.

On a sunny, breezy day vigorous cumulus can form into lines downwind of the thermal source. This causes the ground to heat along these streets and thus maintain the cloud street. The wind increases with height often with a lowish inversion above keeping the tops from going upward. A cloud street of this sort gives continuous lift underneath it making for an excellent XC opportunity. The photo shows a street as seen from 2000 feet.

On a sunny day with an unstable airmass the cumulus grow so quickly that a small and hot source can 'rocket' upward causing a tall, thin cloud. These cumulus will be rising extremely quickly with sharp edges. They quickly outclimb their heat sources with the tops dropping down quickly. A larger cumulus congestus can form once the initial upward surge is over.

On a sunny day with an unstable airmass the cumulus congestus will form. On this day there was a strong wind at mid levels so the congestus has formed more into streets than into towering cumulus. This particular congestus mediocris has a clean, flat base showing that it is a powerful thermal and would make for good flying.

When the airmass is unstable small cumulus quickly grow into towering cumulus, cumulus congestus. Congestus are the largest cumulus we should really thermal near as the next stage is Cumulonimbus. Congestus can grow above 15,000 feet and can be rough. It is not always easy to distinguish between a large Cu-Cong and its change to a Cu -Nimb. This congestus rained on us ten minutes later technically becoming a cumulonimbus.

On this fairly high-pressure day cumulus congestus grew up to a mid-level inversion layer, they then overdeveloped (spread out against the lid). The resulting layer of stratocumulus cloud began to sink back down showing mamma 'pouches' which seem to spread out like ripples on water.

This is a young and active Cumulonimbus. It has quickly rocketed up through a layer of mid-level alto-cu and is producing heavy rain. The cloud grew in the wake of a previous cumulonimbus shower cloud that seeded the air behind it with its rain. The Cb is moving to the right showing a negative wind gradient - the wind was stronger at lower levels on this occasion. The upward disruption of mid-levels has also caused lato-cumulus to form.

This massive thundercloud was moving to the right and had twin anvils at its rear. Normally the anvil (or incus) is ice crystals but here the lower anvil is a water anvil.This sort of Cb is very dangerous to fly within 10 miles of in light aircraft. It was made up of very many cells. The anvils were at the rear of the cloud as there was a negative wind gradient (less windy with height).

On a sunny day when the airmass is unstable strong cumulus continue upward to become large and precipitation bearing monsters. In winter they grow to maybe 20,000 to 25,000 feet because the tropopause ceiling is lower. This classic photo of a Cumulonimbus formed after a cold front and it spread out into the classic incus anvil top when it reached the winter tropopause. This Cu-nimb will be producing heavy rain, hail and strong gusty winds.

This is a summer cumulonimbus incus in a mature stage of its life. Its ice crystal anvil is spread high and wide against the high summer tropopause maybe up above 30,000 feet. It gave rain, hail and lightning. It was made of many cells with the newest, most-active cell pushing up through the tropopause lid. Low level turbulence-cloud and gust-front clouds obscure this preditor from unwary pilots. Vertical winds can exceed 5000 fpm.

In the winter when the air is at medium-cold temperatures the snow dropping within a cumulonimbus melts and is taken up into the cloud again turning into frozen rain-droplets - hail. It hits other super-cooled water droplets on the way and grows in to larger rime ice. When it gets heavy enough to escape the updrafts in the front of the cumulonimbusin falls out on us as hail precipitation.

Mamma is seen on the underside of mature cumulonimbus clouds. It is not always accompanied by precipitation as the cloud is often decaying. The cloudy air is cold and heavy and descending rapidly out of the back of the Cb. If this air reaches the ground you will get a rough, vertical and horizontal gust front. Photo Helena Svensson

When air pushes up quickly into a higher moist airmass it can push that air quickly into condensation, forming a wispy pileus 'cap' cloud temporarily above it as it bubbles upward. In the photo the lower (most active) quickly ascending cumulus bubble has formed a cap which it will quickly pust up past. Cap cloud signifies very unstable air so expect thermals to be ascending violently at low level too.

When air pushes up dramatically into a very moist airmass it can form pileus caps but when conditions are right these pileus caps extend into more widespread blankets of wispy cloud. This lovely photo shows this effect nicely as two cumulus congestus in a cloud street push upward into very unstable mid-level moist air. This degree of capping is pretty uncommon in the UK.

This is of the rear of an active Cumulonimbus crossing Westbury in a strong wind. Some minutes previously there was just heavy rain here, then the cold air and rain pulled the cloud down into mamma bulges. Finally the photo shows the back of the Cb dropping, cloud and all in a massive downdraught. The full-size picture shows this sequence.

A Funnel Cloud is a rotating cloud spout. It becomes a Tornado when it reaches the ground. Funnel clouds are created within Cumulonimbus clouds, by wind shear. On 1st August 2005 this funnel cloud dropped from a fast ascending cumulus congestus (almost a cu-nimb but with no rain). It hung in the sky above Melksham and Trowbridge for maybe 20 minutes before dissapearing. Like the tornado below it was forned in light winds which completely changed direction with altitude causing shearing in buildups.

On 16 Oct 2004 a funnel cloud dropped from a small Cb base and became an 'F0' tornado when it touched down 1km SSW of Westbury town causing minor damage. It had two spouts rotating anti-clockwise around each other and lasted 8 minutes. No real rain came from the cu-nimb cloud which spawned it and was part of a line of CBs out of the Bristol Channel - the south-west's very own 'tornado-alley'.

Lightning occurs when water, ice and air moves up and down violently in a powerful cumulonimbus cloud causing static electricity. Most lightning is intra-cloud (so-called sheet) lightning. Cloud-to-ground lightning occurs when downward moving stepped-leaders meet upward reaching streamers and a path of least resistance is made. Then the lightning bolt occurs building from the ground upward. Flickering lightning is when multiple lightning strikes occur sequencially down the same channel.