Cumulus Clues n Keep at least five nautical miles away from developing Cumulus/ large Cumulus. n Keep as far away as possible from Cumulonimbus because of hazards eg:- n turbulence; wind shear and gust fronts [which may exist quite some distance from the edge of the cloud], and n microbursts (a strong concentrated downburst of cold air).
Clumulus clues n Never try to outclimb a towering CU or developing CB. Their growth may exceed you climb rate and you may end up inside a storm cloud. n Avoid flying under the clear area below the anvil, because of the dangers presented by hail falling from the anvil.
DOWNBURST & MICROBURST n Wind squalls may also be generated by downbursts. n Concentrated, severe downdraughts are usually accompanied by a descending deluge of precipitation. n These induce an outward [horizontal] burst of damaging wind at the surface which on a smaller scale is known as a microburst.
n Outwardly curved rain shafts are a good sign of strong downburst or microburst winds and the steeper the angle the stronger the flow. n In the next slide, the rain curtain has a foot close to the ground. n Also note the rising rain or dust well to the right !
RAIN CURTAIN n Other clues of storm severity can be found in the rain curtain:- n If it is dark & smooth [as previous & next slide], very heavy rain is likely. n Rainfall in severe storms will become progressively heavier & sometimes mixed with hail. Weaker storms have patchy rainfall or short downpours. n Severe storms have multiple lightning bolts.
n Mammatus are rounded pouches or bulges which indicate descending pockets of small droplets or ice crystals from an anvil surface. n They can also be seen below middle level cloud. n They are associated with severe turbulence.
LENS SHAPED CLOUDS n When observed to the lee of a mountain range they indicate mountain wave activity and possible severe turbulence [especially below the cloud]. n Strong wind flow over ranges gives rise to downstream lee wave action. n Mountain waves can also occur without lens shaped clouds being present [in dry air].
TORNADO n The typical funnel shape of a tornado is formed when moist air condenses within the lower pressure of the rotating column of air. n Under relatively dry conditions, it may not form and the only evidence of a tornado at the surface may be indicated by a mass of debris [eg dust].
n A waterspout looks like a tornado, but occurs over water when cool unstable air passes over warmer waters. n Local topography etc allows local convergence of the air flow, which results in vigorous updraughts tightening up into spinning columns. n They mostly occur in late Summer & Autumn.
WALL CLOUD n A small cloud feature, particularly valuable is assessing a storms severe potential, beneath a rain free cloud base can be found toward the rear of the storm. n This localised cloud base lowering occurs at the site of the main, focused updraught into the system.
WALL CLOUD n As a storm becomes stronger and develops an organised inflow, its main updraught may begin to rotate slightly. n This can be seen as broad rotation of the cloud base beneath the main updraught or in the circular nature of the wall cloud. n In the Southern hemisphere viewed from a distance the rotation will be clockwise.
Supercell Visual Clues Visual characteristics can be used to determine supercells Visual characteristics can be used to determine supercells Anvils can indicate where the storm is moving and possible the strength of the updraft Anvils can indicate where the storm is moving and possible the strength of the updraft The direction of cirrues is being blown off is the general direction of movement The direction of cirrues is being blown off is the general direction of movement If the top is small, chance are the storm will be short- lived. If the top is small, chance are the storm will be short- lived. If the top appears dome-like and lasts for a fairly long period of time, the supercell is more than likely severe If the top appears dome-like and lasts for a fairly long period of time, the supercell is more than likely severe
AN ANVIL n Is the top of a thunderstorm cloud; n It can reach up to a height of 10-16 kilometres [approx 32 - 52,000 ft]; n May appear to be boiling,but more often has a fibrous, frozen appearance; n It is primarily composed of ice crystals; n The next slide shows a crisp thunderstorm anvil which is a good indicator of a strong updraught !
THE ANVIL n The anvil can indicate the age, strength & organisation of the thunderstorm. n Unevenness on the top indicates erratic growth, while n A diffuse edge suggests weak updraughts [a weaker system]. n The next slide is an example of a weak, fibrous anvil from a non severe storm.
n The top of the anvil is normally restricted by the tropopause and blown forward on strong winds aloft. n When the main updraught is very strong a portion of the anvil may push upward above the general level into the stratosphere. This feature is known as an overshooting top.
A SUPERCELL ¶ Maintains an intense steady state for many hours. · Is a dangerous cloud complex and accounts for most of the serious thunderstorm events. ¸ NOTE the high, crisp anvil in the next slide, which indicates a very strong, sustained updraught
Characteristics of severe storms n An overshooting top that is prominent and lasts for longer than several minutes. n a high anvil with a crisp edge; n a steep, almost vertical mass of boiling towers at the rear of the storm; n a tendency for the anvil to push back against the prevailing winds [a back- sheared anvil].