1. Thunderstorms
Spring 2016
Kyle Imhoff

2. 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

3. 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

4. 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

5. 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

6. Life Cycle of a Thunderstorm

7. 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

8. T-storm Types Types of T-Storms Air mass, single cell Clusters
Supercell (HP or LP)
Squall Lines
Derechoes
Mesoscale Convective Complexes (MCC’s)

9. Squall Line

10. Squall Line/Air Mass Thunderstorms

11. Effects The Effects of Thunderstorms: Gust Front Tornadoes
Straight-line winds
Derecho
Microbursts
Tornadoes
Mini and Maxi
Fujita Scale
Flash Flooding
Hail
pea to softball sized

12. Severe Thunderstorms Four main ingredients: Instability Moisture
Lifting Mechanism
Wind speed changes direction/speed with height (called wind shear)

13. 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

14. 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