Chapter 15: Thunderstorms and Tornadoes
Thunderstorms are storms containing lightning and thunder!
All thunderstorms, ordinary or air-mass thunderstorms, and severe thunderstorms have 3
stages:
1) Cumulus โ Warm, humid air rises in a conditionally unstable environment. A
cumulus cloud forms that penetrates across the freezing line and water (ice) droplets
grow.
2) Mature โ Falling precipitation creates a downdraft. Entrainment of cold, dry air from
outside the cloud adds to the downdraft. Cloud top reaches stable stratosphere and
anvil forms.
Charge separation occurs with + ions moving from warmer falling hail to lighter ice
crystals. Top of cloud becomes +, bottom - , ground induces + charge. Lightning
forms. Heavy rain, hail, gusting wind in strong up and down drafts.
3) Dissipating - Downdrafts dominate, moisture supply cut off, thunderstorm dies out.
Downdrafts may trigger another storm by causing humid surface air to lift up (multi-
cell storms).
Severe Thunderstorms:
Strong vertical wind shear causes up and down drafts to tilt and separate to different
regions. Thus, storms can be longer โ lived and grow in strength.
Sometimes an inversion exists near the base of the cumulus cloud โ a shelf cloud is an
indication of the inversion. This inhibits smaller storms and allows massive
thunderstorms to form.
Lightning and Thunder
Lightning is an electrical discharge. A column of electrons (-) extends from cloud to
ground. The electrons come from the breakdown of air molecules caused by the high
electric field. Once the column reaches the ground (closed circuit) the electrons quickly
dump. At this point, the + ions move upward in the path created by the electrons and
make a return stroke.
Thunder comes from the shock wave caused by the heating of the air column. It is a
sonic boom.
Electric fields are most intense near sharp objects.
