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 – 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 – 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 (multicell storms).
Severe Thunderstorms:
Strong vertical wind shear causes up and down drafts to tilt and separate to different
regions. Thus, storms can be longer – lived and grow in strength.
Sometimes an inversion exists near the base of the cumulus cloud – 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.
e+
Atom
+
+
+
+
+
+
+
e-
+
-
Polarized atom in electric field
Sound travels ~ 1 mile every 5 sec (343m/s)
Light travels ~ 1 million miles in 5 sec! (3 X 108 m/s) (Let’s say, instantaneous)
After you see lightning, count # seconds until you hear thunder. Divide # sec by 5 and get
miles away to where strike was.
Tornadoes:
Rapidly rotating winds around a small intense area of low pressure. Spawned by severe
thunderstorms in general.
Stages:
1. Dust – whirl stage
• Dust – whirls at ground level
• Wall cloud forms – rotating clouds at base of storm
2. Organizing stage
• Increase in intensity, funnel cloud extends down partway to ground
3. Mature stage
• Funnel extends to ground, most damaging stage, nearly vertical
4. Shrinking stage
• Width decreases, funnel tilts
5. Decay stage
• Becomes rope-like, distorted and dissipates
All stages are completed usually in a few minutes and path extends several miles.
In order for a storm to spawn a tornado, the updraft must rotate. A rising spinning column
of air within the storm is a mesocyclone.
Conservation of angular momentum causes air to speed up as it rushes toward the center
of the low. Air goes down in the core of a tornado and up around the outside.
First observation is rotating clouds at the base of the storm. Then a wall cloud develops.
Then a funnel cloud emerges and grows toward the surface. Meanwhile, circulation of
dust and clouds can be seen at the surface. The funnel then forms to the surface.
Wind speeds can reach approximately 300 mph maximum.
Doppler Radar
Recall: weather radar
Transmitter
Pulsed microwaves (1-20cm wavelength) are sent toward cloud and some scatter off of
precipitation. Backscattered waves travel to detector and time elapsed is measured. Time
elapsed gives distance to cloud and intensity of scattered waves tells how much
precipitation (and what size). Intensity means the number of pulses entering the detector
per second.
Doppler radar uses Doppler shift and measures frequency change to get speed toward and
away from receiver.
observer
Higher frequency
Source of pulses
(moving to left)
V
observer
Lower frequency
Top view
The moving water drop shifts frequency of scattered waves.
Doppler radar reveals the mesocyclone inside storms and can detect tornadoes.