Thunderstorms Conditions required: 1. Conditional instability 2. Trigger Mechanism (eg. front, sea-breeze front, mountains, localized zones of excess surface heating, dry ground)

1. heating within boundary layer air trapped here due to stable layer aloft increasing heat/moisture within BL 2.External trigger mechanism forces parcel to rise to the lifting condensation level (LCL) Clouds form and temperature follows MALR 3.Parcel may reach level of free convection (LFC). Parcel accelerates under own buoyancy. Warmer than its surroundings. 4.Saturated parcel will continue to rise to LOC

CAPE Convective available potential energy (shaded area in thermodynamic diagram on previous slide) Used by meteorologists to estimate the potential intensity of thunderstorms The Severe Storm Environment 1.High surface dew point 2.Cold air aloft (increases conditional instability) 3.Statically-stable layer capping the boundary layer 4.Strong winds aloft (favours tornado development)

5.Wind shear in low levels (allows for long-lasting storms) 6.Dry air at mid-levels (increases downdraft velocities)

Tornado Development 1.Pre-storm conditions: Horizontal shaft of rotating air at altitude of wind shift (generally S winds near surface and W winds aloft) 2. If capping is breached and violent convection occurs, the rotating column is tilted toward the vertical

What is a ‘supercell’ ? Defined by mid-level rotation (mesocyclone) Highest vorticity near updraft core Supercells form under the following conditions: High CAPE, capping layer, cold air aloft, large wind shear

Tornadogenesis 1.Mesocyclone 5-20 km wide develops 2.Vortex stretching: Lower portion of mesocyclone narrows in strong updrafts 3.Wind speed increases here due to conservation of angular momentum 4.Narrow funnel develops: visible due to adiabatic cooling associated with pressure droppage

The Fujita Scale F-0: Light damage. Winds up to 116 km/h F-1: Moderate damage. Winds 116 to 180 km/h F-2: Considerable damage. Winds 180 to 253 km/h F-3: Severe damage. Winds 253 to 332 km/h F-4: Devastating damage. Winds 332 to 418 km/h F-5: Incredible damage. Winds above 418 km/h F-0 and F-1 tornadoes are considered "weak" F-2 and F-3 are "strong" F-4 and F-5 are "violent"

Lightning Source of lightning: the cumulonimbus cloud Collisions between supercooled cloud particles and graupel (and hail) causes cloud to become charged Most of the base of the cumulonimbus cloud becomes negatively charged – the rest becomes positively charged (positive electric dipole) Net transfer of positive ions from warmer object to colder object (hailstone gets negatively charged & fall toward bottom - ice crystals get + charge) Many theories exist: open area of research

Four types of cloud- ground lightning Most common

Intracloud Discharges Cloud to Ground Discharges - death and destruction of property - disruption of power and communication - ignition of forest fires - Lightning is an excellent source of soil nitrogen!

Cloud-ground lightning 90% induced by negatively charged leaders 10% induced by positively charged leaders Sometimes, there are ground to cloud leaders Negative cloud-ground lightning Leaders branch toward the ground at about 200 km/s, with a current of Amperes The return stroke produces the bright flash

Potential difference between lower portion of negatively-charged leader and ground ~10,000,000 + V As the leader nears the ground, the electric potential breaks the threshold breakdown strength of air An upward-moving discharge is emitted from the Earth to meet with the leader

The return stroke lasts about 100 microseconds, and carries a charge of 30 kiloAmperes Produces the main flash The temperature along the channel heats to 30,000 + K, creating an expanding high pressure channel Produces thunder shockwaves

NESW Storm Position