Chapter 10 part 3
• Tornado : a rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground, aka cyclone or twister
• Funnel cloud: a tornado whose circulation has not reached the ground
• Apparently the clockwise rotating tornadoes are more rare than I thought and are usually in the form of waterspouts (except in the SH where most are clockwise rotating due to the opposite Coriolis force).
Tornado life cycle
• Dust-whirl stage – where dust swirling upward from the surface marks the tornado’s circulation on the ground and a short funnel often extends downward from the thunderstorm’s base
• Organizing stage – the tornado increases in intensity with an overall downward extent of the funnel
• Mature stage – damage normally is most severe as the funnel reaches its greatest width and is almost vertical
• Shrinking stage – there is an overall decrease in the funnel’s width, an increase in the funnel’s tilt, and a narrowing of the damage swath at the surface
• Decay stage – tornado stretched into the shape of a rope
Tornado Outbreak: When a large number of tornadoes forms over a particular region
• In May 2003, 516 tornadoes touched down in the US—the most in any month ever.
Tornado Occurrances
• The US has the most tornadoes in the world with 1000 on average per year with a record of 1424 in 1998
• Most occur in the central plains because it provides the proper atmospheric setting for the thunderstorms that spawn tornadoes
• Most occur from March to July, where May averages about 6 per day!
Tornado Winds
FIGURE 10.27 The total wind speed of a tornado is greater on one side than on the other. When facing an on-rushing tornado, the strongest winds will be on your left side. suction vortices : small rapidly rotating whirls perhaps 10m in diameter inside a tornado.

Seeking Shelter
• The pressure in the center of a tornado may be more than 100mb lower than its surroundings
• There is a momentary drop in outside pressure when the tornado is above the structure.
• Inside the house there is generally higher pressure and this can cause the roof to lift off of a house or a general explosion of the house.
• Tornado watch: tornadoes may develop within a specific area during a certain time period
• Tornado warning: a tornado has been spotted visually or on a radar screen. This warning is issued by the National Weather Service (NWS)
• Where should you take shelter during a tornado warning?
• Fujita scale: classifies tornadoes according to their rotational wind speed based on the damage done by the storm
Tornadic Thunderstorms
• Favorable Atmospheric Conditions
– Where the polar front jet stream and the cold, dry air cross the warm, humid surface air
• Supercell tornadoes: tornadoes that form with supercell thunderstorms (storms with strong vertical wind shear)
• FIGURE 10.31 Spinning vortex tubes created by wind shear.
FIGURE 10.32 The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm, producing a rotating air column that is oriented in the vertical plane.
Mesocyclone : a vertical column of cyclonically rotating air within a severe thunderstorm

FIGURE 10.34 A tornado-spawning thunderstorm over Oklahoma City on May
3, 1999, shows a hook echo: (a hook shape on a radar screen indicating the position of the mesocyclone) in its rainfall pattern on a Doppler radar screen. The colors red and orange represent the heaviest precipitation.
Wall cloud: the area of rotating clouds that lowers from the cumulonimbus from which a funnel cloud may come out of
Nonsupercell tornadoes: tornadoes that do not occur in association with a pre-existing wall cloud of a supercell
• Gustnadoes : a relatively weak tornado associated with a thunderstorm’s outflow.
It most often forms along the gust front
• Landspouts: relatively weak nonsupercell tornado that originates with a cumliform cloud in its growth stage and with a cloud that does not contain a midlevel mesocyclone. Its spin originates near the surface, they look like waterspouts but over land
FIGURE 10.37 (a) Along the boundary of converging winds, the air rises and condenses into a cumulus congestus cloud. At the surface the converging winds along the boundary create a region of counterclockwise spin. (b) As the cloud moves over the area of rotation, the updraft draws the spinning air up into the cloud, producing a nonsupercell tornado, or landspout . (Modified after Wakimoto and Wilson)

Severe Weather and Doppler Radar
• Doppler radar: a radar transmitter sends out microwave pulses and when this energy strikes an object, a small fraction is scattered back to the antenna.
– 1. Precipitation particles are detected
– 2. It can measure the speed at which precipitation is moving horizo ntally toward or away from the radar antenna to reveal the winds in a storm
• The TVS or Tornado vortex signature shows up as a region of rapidly changing wind directions toward or away from the rada r
–
– called a tornadic waterspout.
Fair weather waterspouts form the s ame way landspouts do (see figure 1 0.37), and the warm humid air near the wa ter helps create atmospheric instability
The waterspout does not draw water u p into its core, but it does spray water at the surface