Professional Pilot, May 2019

PROFESSIONAL PILOT May 2019 73 All storms follow the same formative pattern Energy that builds up near the surface is released to rise into the atmosphere As it rises it expands and cools condensing into cloud droplets the moisture it carries The condensation releases heat energy that allows the updraft to continue higher and higher so long as it remains warmer than the surrounding environment Eventually the air cools and dries to a point where it is denser than the air rising from beneath it The colder air aloft begins to descend as a downdraft carrying precipitation with it In most cases weak winds aloft ensure the downdraft falls through the updraft disrupting both and preventing the formation of a circulation cell Those types of storm are called air mass storms They may produce a lighting stroke or 2 but normally go from formation to dissipation in 30 minutes or so Nearly all storms that occur in the tropics including those that form as part of a tropical cyclone fall into this category More dangerous storms are those that develop into an area of windshear aloft The gradually increasing winds with altitude tilt the updraft meaning that when the downdraft begins it descends adjacent to the updraft This is what creates a convective circulation When the downdraft reaches the surface it spreads out with the gust front acting as a wedge to enhance the lifting of warm and humid air into the cloud Storms in which the circulation cell is established can maintain their strength for an hour or more Air rising in a storm normally has some rotation around its axis due to the physics of air spiraling in about a low the rising air creates a smallscale surface low beneath the storm But the most severe thunderstorms are those in which the storm updraft is spun about by winds from different directions as it ascends creating a mesocyclone Storms with this sort of rotation are supercells and usually are accompanied by extreme dangers to aviation Geography Thunderstorms are most frequently encountered in places on earth where warm humid air is forced to ascend This can be along the intertropical convergence zone where tropical air from the northern and southern hemispheres flows together near the equator It can also be along a coastline where the daily sea breeze circulation brings routine storms onshore in the day and offshore at night It can be where air is forced to flow uphill such as the front range of a mountain chain Or it can be where cold fronts sweep across the landscape with regularity Some of the most storm prone places are Florida and Colorado in the US equatorial Africa Indonesia and the Amazon basin The strongest supercells are frequent visitors to the central US plains where humid air from the Gulf of Mexico runs up against cooler and drier air pushing in from the west Finally a key ingredient in thunderstorms is something that keeps a lid on convective energy until it reaches a critical threshold after which the air rises explosively Most frequently this lid is supplied by a low level temperature inversion that often sets up overnight Just 1 3 C warming at a few thousand feet AGL is sufficient This inversion keeps the warming surface air in place with all of its growing energy until eventually either something comes along to push it through the inversion and into the colder free atmosphere above or it attains enough energy to penetrate a weak spot in the inversion Once a convective current is established through the cap the rate at which it ascends will be tied to the difference in density between it and the surrounding environment The greater the temperature difference with the atmosphere being colder the more explosively the updraft will rise In the most favorable conditions a storm can go from cap bust to mature storm in a few minutes Whats worse is that developing storms often do not contain precipitation droplets that are large enough to be seen on radar As a result the storm may not show up on your scope or show only as an area of light precipitation right up until the point at which it cracks its first Mature but dissipating airmass thunderstorm that produced a classic anvil head as it reached the temperature inversion at the top of the troposphere Most storms are airmass storms that die out as quickly as they form but can present widespread dangers to aircraft Typical anvil head of a supercell thunderstorm Supercells are the largest and strongest of thunderstorms and may cover several square miles of sky Frequently they can be seen and avoided but occasionally they are embedded in thick cloud cover Photo courtesy NOAA