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Thunderstorms Spring 2016 Kyle Imhoff Atmospheric Stability A Matter of Stability • bubbles of air vs. the atmospheric environment • bubbles change temperature by either – dry adiabatic lapse rate (9.8C/1km) (5.5F/1,000ft) – moist adiabatic lapse rate (5C/1km) (3.5F/1,000ft) • Environment lapse rate varies – Stability - a measure of whether an air bubble moves when prodded (heated or forced lift) • unstable - moves and never returns • stable - may move, but returns to same place Atmospheric Stability Stable Air Characteristics – clear or stratiform clouds – generally light surface winds – smoke, haze, smog accumulate – Inversions are common (morning fog) Unstable Air Characteristics – clear or cumuliform clouds – generally gusty winds – lack of haze – Thunderstorms can develop if enough moisture is available Thunderstorms Thunderstorms always form in an unstable air mass - and are associated with Cumulonimbus Clouds (Cb) Most thunderstorms occur in the spring and summer (with a maximum in frequency around the solstice - June 22) The majority of thunderstorms occur between 2-8pm due to daytime heating Thunderstorm Evolution Initial Stage – Cumulus to Mature – Strongest Vertical Motions (25 m/sec) – Cloud grows from 15,000-35,000 ft – Electrification occurs – Downdraft starts – Heaviest rain and hail – Life span: 20 minutes Mature Stage – Mature to Dissipating – Weaker Vertical Motions (5-10 m/sec) – Cloud reaches maximum height spreads out into Anvil – Downdraft spreads far – Lighter rain – Less Electrification Life Cycle of a Thunderstorm Where Storms Form Formation Zones – Differences in Temperature and Moisture • • • • • • Clear/Cloudy regions Sea/Lake Breeze Mountain vs Valley Edge of Fog areas Dry line - West U.S. Large River Valley T-storm Types Types of T-Storms – Air mass, single cell – Clusters – Supercell (HP or LP) – Squall Lines – Derechoes – Mesoscale Convective Complexes (MCC’s) Squall Line Squall Line/Air Mass Thunderstorms Effects The Effects of Thunderstorms: – Gust Front • Straight-line winds – Derecho • Microbursts – Tornadoes • Mini and Maxi – Fujita Scale – Flash Flooding – Hail • pea to softball sized Severe Thunderstorms Four main ingredients: – Instability – Moisture – Lifting Mechanism – Wind speed changes direction/speed with height (called wind shear) Forecasting Thunderstorms Storm Prediction Center – Issues watches, outlooks, discussions regarding potential severe thunderstorms and tornadoes NWS Forecast Offices – Issue warnings on severe thunderstorms and tornadoes Summary Thunderstorms come in many shapes and sizes – all produced by convection Key ingredients are instability, moisture, and a lifting mechanism Very difficult to predict timing and location of thunderstorms Important due to their potential socioeconomic impacts