Severe Weather Thunderstorms, Lightning, Tornadoes & Hurricanes
Formation of Thunderstorms • On hot summer days, the surface of the Earth is heated by the sun (radiation). Energy is transferred from the Earth to the air just above the surface through the process of conduction.
Important Ingredients • The two most important ingredients for thunderstorm formation include: • Instability (unstable air due to being warmer less dense rises) • Moisture
Types of Thunderstorms • Orographic Thunderstorm • caused by air that is forced up by a mountain or hillside • Air Mass Thunderstorm • result of localized convection in an unstable air mass • Frontal Thunderstorm • occur along boundaries of weather fronts (e.g. cold front)
Squall Line • An intense line of thunderstorms that form right along or just ahead of a cold front
Supercell Thunderstorms • Occur when very strong updrafts are balanced by downdrafts • Storm can persist for many hours • Moist, unstable body of warm air may be forced to rise by approaching cold front
Supercell Thunderstorms • As warm air is forced up, the result is a strong, persistent updraft of warm moist air • Speeds in an updraft can reach up to 90 mph! • Water vapor condenses and forms cumulus clouds (latent heat released)….this is how thunderstorms grow!
Supercell Thunderstorms • At some point, condensation high in the cloud falls to the ground as rain. A cold downdraft forms as the rain, or hail, falls. • If conditions are right, some supercell thunderstorms will spawn tornadoes.
Thunderstorms in the U.S. • Which state has the highest number of thunderstorm days per year per 10,000 square miles? • Which state has the most lightning strikes per year per 10,000 square miles?
Thunderstorms in Florida • What makes this state such a thunderstorm and lightning hot spot? • Surrounded by water • Winds blow from the ocean over land • Ocean provides lots of moisture • Air flows come together over the peninsula • Moist air rises over Florida’s warm land surface
Classroom Thunderstorm!! Sounds of a Thunderstorm in the Classroom
Lightning • Happens when negative charges (electrons) in the bottom of the cloud are attracted to the positive charges (protons) in the ground
Lightning • The accumulation of electric charges has to be great enough to overcome the insulating properties of air. • When this happens, a stream of negative charges pours down towards a high point where positive charges have clustered due to the pull of the thunderhead.
Lightning • The connection is made and the protons rush up to meet the electrons. • We see lightning and hear thunder! • Bolt of lightning heats the air along its path causing it to expand rapidly; thunder is sound caused by rapidly expanding air! • Lightning Video!
Let’s Make Some Lightning! Materials • Styrofoam plate • Thumbtack • Pencil with new eraser • Aluminum pie pan • Small piece of wool fabric Procedure • Push the thumbtack through center of aluminum pie pan from the bottom. • Push the eraser end of the pencil into the thumbtack. (The pencil becomes a handle to lift the pan.) • Put Styrofoam plate upside-down on table. Rub the underside of the plate with the wool for one minute. • Pick up the pie pan using the pencil “handle,” and place it on top of the upside-down plate. • Touch the pie pan with your finger.
What’s Happening? • What happened when you touched the metal pie pan? • What caused that? • How do you think this experiment relates to the formation of lightning?
Tornadoes • Associated with large (supercell) thunderstorms • Often grow to over 40,000 feet • Column of warm humid air will begin to rise very quickly
Formation of a Tornado • How the column of air begins to rotate is not completely understood by scientists • One way the rotation appears to happen is when winds at 2 different altitudes blow at 2 different speeds creating wind shear
Formation of a Tornado • Example… • Wind at 1000 feet above the surface might blow at 5 mph • Wind at 5000 feet above the surface might blow at 25 mph • This causes horizontal rotating column of air
Formation of a Tornado • If column gets caught in a supercell updraft, updraft tightens the spin and speeds up • …think of an ice-skater or dancer and how they spin faster when arms are pulled close to their body • Funnel cloud is created!
Formation of a Tornado • The rain and hail in the thunderstorm cause the funnel to touch down creating a tornado.
Storm Chasers! • Check out some real video clips from actual storm chasers…Warning! DO NOT TRY THIS AT HOME!! • Tornado Video
Tornado Alley Activity • Read the directions associated with your map!
Hurricanes: How They Work • Huge storms! • Can be up to 600 miles across and have strong winds spiraling inward and upward at speeds of 75 to 200 mph. • Each hurricane lasts for over a week, moving 10-20 mph over the open ocean.
Hurricanes How They Work • Warm air at its center; the center of the storm is the calmest part • Called the eye and has only light winds and fair weather • Low level storm winds blow counterclockwise around eye in Northern Hemisphere
Hurricanes Are Important! • Help move heat from warm tropics to cooler temperate zones • Typically form between 5 to 15 degrees latitude • Thunder across the warm oceans of the world like the Atlantic, Gulf of Mexico, the Caribbean and the Western Pacific Ocean (typhoons), up to higher latitudes
When is Hurricane Season? • Hurricanes usually occur when the oceans have been warmed during the summer months • Hurricane season in the North Atlantic starts on June 1st and lasts until November 30th • Most hurricanes happen during the fall…why?
Storm Surge • As a hurricane’s winds spiral around the storm, they push water into a mount at the storm’s center • Mound of water becomes dangerous when the storm reaches land because it causes flooding along the coast • Water piles up, unable to escape anywhere but on land as the storm carries it landward
Storm Surge • If high tide happens to occur at the same time as a storm surge, the combination of the two is called storm tide • Water level may be 20 feet or more above normal!
SLOSH • Computer models allow forecasters to predict the amount of storm surge that will affect a coastal area • Model called SLOSH takes into account a storm’s strength, path, how the ocean shallows and the shape of the land and calculates how much storm surge a hurricane could cause!
How are Hurricanes Named? • Go here and find out…
How Much Does a Hurricane Weigh? • Every wonder about how much a hurricane actually weighs? • The answer might surprise you…click below to find out! • Hurricane and blue whales?
Hurricanes Affecting Wilmington • All hurricanes use the Saffir-Simpson scale: • Hurricane Diana- 1984 • Hurricane Gloria- 1985 • Hurricane Emily- 1993 • Hurricane Fran- 1996 • Hurricane Floyd- 1999 • Hurricane Ophelia- 2005 • Hurricane Irene- 2011