Table of Contents Formation of Precipitation 3



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Tornado


Rapidly whirling, funneled-shaped cloud (called a vortex) that reaches down from a storm cloud to touch Earth’s surface. Since air is invisible, the vortex that we see is water droplets, dust, and debris sucked up from the ground.

Caution: Take shelter in a basement or in the center of a building immediately when tornado warnings are issued for your area. Stay away from windows.



  • Usually brief, touching the ground approximately 15 minutes or less

  • Wind speeds may reach 480 km/hr (230 mph)

  • Counterclockwise in both the Northern and Southern Hemispheres

  • Stages of Tornado Formation:

    • Overshooting top

    • Wall cloud descends below base of cloud and slowly rotates

    • Funnel cloud descends from wall cloud

    • Funnel cloud touches ground

  • Tornado Watch – tornadoes are possibly in your area

  • Tornado Warning – tornadoes have been seen in the sky or on weather radar

  • Occur most often in the US

  • Approx. 800 tornadoes a year

  • Tornado Alley - Warm humid air mass moves north from Gulf of Mexico and meets a cold dry mass that is moving south from Canada; SD, Iowa, Nebraska, Kansas, Missouri, Oklahoma, TX, NM, and Arkansas

  • Enhanced Fujita Scale- Measuring intensities of tornadoes based on a scale developed by Dr. Theodore Fujita at the University of Chicago in the 1960s

    • EF0 Gale 65-85mph

    • EF1 Weak 86-110 mph

    • EF2 Strong 111-135 mph

    • EF3 Severe 136-185mph

    • EF4 Devastating 186-200 mph

    • EF5 Incredible over 200 mph

  • The deadliest tornado on record in the United States was the tri-state tornado outbreak of March 18, 1925. Several tornadoes demolished portions of Missouri, Illinois, and Indiana, killing 747 people and injuring over 2000 more.


Homework Assignment #2





  1. What should you do if you’re caught outside during a lightning storm?


  1. Why do you see lightning before you hear thunder?


  1. What is the difference between a severe thunderstorm watch and a severe thunderstorm warning?


  1. In what direction do tornadoes spin in NJ?


  1. What is the difference between a tornado watch and a tornado warning?


  1. Where is tornado valley?

Tropical Storms and Hurricanes


  • Tropical storms form over warm ocean waters. They begin as a tropical disturbance which is nothing more than a cluster of big thunderstorms. When this growing storm begins to rotate, it becomes a tropical depression. When wind speeds reach 40 miles per hour it is called a tropical storm.

  • Hurricanes are tropical storms that have winds of 119 km/hr

  • Usually occur between June and November in the eastern US

  • Hurricanes that occur in western US are called typhoons.

  • Size can be 220-700 km in diameter

  • Eye – center of a hurricane

  • Storm Surge - Hurricane pulls up ocean water an average of 1 cm per 1 mb of air pressure drop; Regions along shores and coasts are inundated with water being pushed ahead of the hurricane

  • Stages of Hurricane Development:

    1. Tropical Disturbance

Mass of thunderstorms begin to organize

Light wind circulation



    1. Tropical Depression

Wind speed 20-34 knots

Central low pressure developing with rotation of thunderstorms



    1. Tropical Storm

35-64 knots

Strong central low pressure

Increasing wind speeds

Forward movement across oceans



    1. Hurricane

64 knots or more

Well developed central low pressure; eye may be visible

Moving to west along with global winds (NE or SE Trades)

Can move up to 50 knots over open ocean



Highest wind speed on the forward traveling side

  • Saffir Simpson Scale – measures hurricane intensity by comparing wind speed and air pressure

    • Tropical Storm – 39-73 mph winds

    • Category 1 – 74-95 mph

    • Category 2 – 96-110 mph

    • Category 3 – 111-130 mph

    • Category 4 – 131-155 mph

    • Category 5 – 156 mph and up

  • Naming Hurricanes:

    • Initial tracking of hurricane was by latitude and longitude

    • WWII – use of military terms to identify individual hurricanes (Alpha, Bravo, Tango…)

    • Early 1950s – use of female names

    • Late 1970s – Male and female names are used; alternate male/female name for Pacific or Atlantic hurricanes


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