How do thunderstorms form? There are 3 conditions:1) There must be abundant moisture in the lower atmosphere.2) Some mechanism must lift it so moisture can condense and release latent heat NOTE: Latent heat is the heat energy involved in the phase change of water. The heat then is "hidden" or stored in the water molecule until it is released during condensation. Latent heat released during condensation is an important source of energy to drive atmospheric systems like hurricanes and cumulus clouds.3) The atmosphere must have a “place” where the cloud can continue to develop. The atmosphere is unstable.
Types (let’s draw…):1) Air mass thunderstorms• Seabreeze• Landbreeze• Orographic lifting2)Frontal Thunderstorms – due to cold fronts.
Stages of a Thunderstorm1) Cumulus stage – air rises, creates updrafts, droplets condense, eventually as precipitation.2) Mature stage – water droplets that form at high, cool levels of the atmosphere. This cold air is dense and creates downdrafts/updrafts that produce the gusty winds.3) Dissipation stage – the production of downdrafts. Once moisture runs out, updrafts slow and eventually stop. Storm loses energy.
What is lightning?•A visible electricaldischarge produce bythunderstorms. Lightningis a giant spark.•It appears to “flicker”because it is composed ofa succession of multiplestrokes that usually followthe exact same path of theinitial stroke.
•Several types of lightning are common:* In-cloud lightning extends from one chargedregion of a cloud to another.* Cloud-to-cloud lightning extends between twoclouds.* Cloud-to-air lightning extends from a cloud tothe air, not touching the ground.* Cloud-to-ground lightning stretches from acloud to the ground.
Formation of Lightning:Stage 1:Cumulonimbus cloudsdevelop a separation ofelectric charge, with thetops of the cloudspositively charged andthe bottoms negativelycharged.Light weight particleshave a positive(+)charge and float tothe top of the cloud (likeice floats to the top of aglass). Negatively(-)charged particles sinkto the bottom.
Stage 2:A “step leader” onthe bottom of thecloud form as itovercomes air`sresistance to the flowof electricity.Rapidly rising air in athunderstorminteracts with rapidlyfalling air within thethunderstorm tocreate separatelypositive and negativecharged areas withinthe cloud (friction).
Stage 3:Meanwhile, as theelectrons approach theground, the ground isbecoming more and morepositively charged due tothe repulsion of electronsin the ground. Thispositively charged regionmoves up through anyconducting objects on theground -- houses, treesand people -- into the air.This is why it is veryimportant to stay "low"during a storm or whenlightning threatens.
Stage 4:When the downwardmoving electrons meetthe upward flowingpositive regions at analtitude of a hundredmeters or so, they form acomplete circuit (SeeFigure) and the lightningbegins.
Stage 5:We now see the mainact of lightning, thereturn stroke.This extreme heatingcauses the air toexpand at anexplosive rate. Theexpansion creates ashock wave that turnsinto a booming soundwave, better known asthunder.
-Seconds after this girl walkedaway, lightning struck whereshe stood.-The positive charge of herhair is attracted to the negativecharge of a lightning cloud.
Lightning strikes in the Kempsville Lakes area of VirginiaBeach, off Baxter Road, around 3 a.m. July 12, 2005.
August 22, 2007: After a series of thunderstorms passed throughHampton Roads on Tuesday night, William Coyle (VA Lightning)photographed these anvil crawlers. In the foreground is the WestinHotel & Residences at Virginia Beachs Town Center.
Facts about lightning:•Lightning discharges can reach up to 200 millionvolts.•Can reach temperatures up to 54,000° F. That ishotter than the surface of the sun!•Globally, there are about 100 – 125 flashes a second.That’s about 3 billion a year!•The chances of being struck by lightning in theUnited States is about 1 in 250,000. Approximately75 – 150 people die each year due to lightning.
Why do we see lightning first, then hear the sound later? Light is fasterthan sound.•Speed of light: 186,000 miles a second•Speed of sound in air: At 21 degrees C (70°F): 344 meters per second or1129 ft per second.•Speed of sound in water: 1480 m/s or 4856 ft/s. More than 3000 miles perhour. Calculating distance of lightning:
Foldable: Chapter 13.2, p. 334Provide characteristics, steps of formation, and dangers of the following:1) Severe thunderstorms2) Lightning3) Fury of Wind4) Hail5) Floods6) Tornadoes
Weather Systems Pressure Systems• At Earth’s surface, rising air is associated with low pressure and sinking air is associated with high pressure.• Rising or sinking air, combined with the Coriolis effect, results in the formation of rotating low- and high-pressure systems in the atmosphere.• Air in these systems moves in a general circular motion around either a high- or low- pressure center.
Pressure SystemsHigh-Pressure Systems– In a high-pressure system, air sinks, so that when it reaches Earth’s surface it spreads away from the center.– The Coriolis effect causes the overall circulation around a high-pressure center to move in a clockwise direction in the northern hemisphere.– High-pressure systems rotate in a counterclockwise direction in the southern hemisphere.
Pressure SystemsLow-Pressure Systems– A wave cyclone, one of the main producers of inclement weather in the middle latitudes, usually begins along a stationary front.– Part of the front moves south as a cold front and another part of the front moves north as a warm front.– This sets up a counterclockwise or cyclonic circulation that can form into a fully developed low-pressure system.
Pressure Systems Low-Pressure Systems– In a low-pressure systems, air rises, causing an inward net flow toward the center and then upward.– In contrast to air in a high- pressure system, air in a low- pressure system in the northern hemisphere moves in a counterclockwise direction.– This movement is reversed in the southern hemisphere.
Cyclones and Anticyclones CyclonesCyclones are areas oflow pressure. Cyclonesusually exhibit nearlycircular isobars.•As air enters an area oflow pressure from alldirections, the Corioliseffect bends the directionof the wind to the right ofits path.•This creates acounterclockwise Figure PC.6 Circulation within a low pressure system in the Northernrotation around the low. Hemisphere
AnticyclonesAnticyclones areareas of highpressure that exhibitnearly circularisobars.•The Coriolis effectbends the air to theright of its pathcreating aclockwise rotationaround the high. Figure PC.7 Circulation within a high pressure system in the Northern Hemisphere
Because the Coriolis effect works in the opposite direction in the Southern Hemisphere,circulation around lows are clockwise and inward toward the center at the surface andhighs exhibit a diverging, counterclockwise rotation. You can see this effect in the cloudpattern created by a cyclone in the Southern Hemisphere.
Tornadoes • A tornado is a violent, whirling column of air in contact with the ground. • Before a tornado reaches the ground, it is called a funnel cloud.• Tornadoes are often associated with thunderstorms.• The air in a tornado is made visible by dust and debris drawn into the swirling column, or by the condensation of water vapor into a visible cloud.
Severe Weather Tornadoes• A tornado forms when wind speed and direction change suddenly with height, a phenomenon known as wind shear.
Tornado Formation1. Wind Shear 2. Updrafts in a 3. Air pressureforms a horizontal thunderstorm lifts drops and“barrel” of air. the “barrel” of air. funnel forms and spins faster.
Severe Weather How are the rated?– The Fujita tornado intensity scale classifies tornadoes according to their path of destruction, wind speed, and duration.Tornado Classification– The scale ranges from F0, which is characterized by winds of up to 118 km/h, to the violent F5, which can pack winds of more than 500 km/h.
Tornado Distribution Most tornadoes form in the spring during the late afternoon and evening, when the seasons begin to change.– Tornadoes occur most frequently in a region called “Tornado Alley,” which extends from northern Texas through Oklahoma, Kansas, and Nebraska.
Severe Weather TornadoesTornado Safety– In the United States, an average of 80 deaths and 1500 injuries result from tornadoes each year.– The National Weather Service issues tornado watches and warnings before a tornado actually strikes.– The agency stresses that despite advanced tracking systems, advance warnings may not be possible.– Signs of an approaching or developing tornado include the presence of dark, greenish skies, a towering wall of clouds, large hailstones, and a loud, roaring noise similar to that of a freight train.
Hurricanes Hurricane Isabel (2003). Viewed from the International Space Station
What Is a Hurricane?• A hurricane is an intense tropical storm system with powerful winds that exceed 74 mph. A hurricane is composed of strong thunderstorms with well defined surface circulation.• Hurricanes have an average size of 500 miles in diameter. Hurricane Floyd – September 14, 1999
Hurricane or Typhoon?• These storm systems can have two different names depending on their location.• The term hurricane is used when the storm system is in the Atlantic or Eastern Pacific.• Typhoon is used to Hurricane Isabel off coast of North Carolina from describe a storm oil tanker – 2003 system in the Western Pacific.
How Do Hurricanes Form?• First, the water temperature must be at least 80 degrees to a depth of 150 ft.• Next, the evaporation of water into the air forms clouds or thunderstorms.• If the atmosphere has no disturbance, but has light winds and low pressure, the clouds that form will start to slowly rotate counterclockwise.
Growth of a Hurricane: Storm Systems1) Tropical Depression•At first the storm systemwill appear as anunorganized system ofthunderstorms.•Tropical depressionshave wind speeds lessthan 38 mph.
2. Tropical Storm - The tropical depression then becomes more organized. - A tropical storm has wind speeds from 39-73 mph.
3. Hurricane• The tropical storm then gains strength and energy until it is highly organized. Normally at this point the eye of the hurricane forms.• Once again the winds of a hurricane are a minimum of 74 mph. Hurricane Katrina - 2005
Structure of a Hurricane• The eye is described as the relatively calm center of a hurricane that is more than half surrounded by a wall cloud.• The eye wall is an organized band of clouds immediately surrounding the center of the hurricane. The eye wall holds the highest wind speeds.
Movement of a Hurricane• Hurricanes move across the ocean slowly at about 20 mph.• The direction a hurricane travels is determined by global wind patterns and ocean currents, high and low pressure systems, and ocean currents.
Effects of a Hurricane • Storm surge is the abnormal rise in sea level that accompanies a hurricane. • The low pressure of the eye causes the sea to be pulled into a “bulge.” Storm surges can reach 18-20 ft in strong hurricanes.
Storm Surge • Storm surge is the most destructive effect of a hurricane and responsible for the most deaths.
National Park Service Ranger Jeff Goad looks at the destruction to N.C. 12 on the north edge ofRodanthe on Hatteras Island. Storm surge from Hurricane Irene destroyed a 900-foot section ofthe road, stranding residents and workers and cutting off tourist access to the area.
How Are Hurricanes Rated?• The Saffir-Simpson scale is a 1-5 rating of hurricane strength.• In this scale 1 is the weakest and 5 is the strongest.• Also, a “major” hurricane is classified as a category 3 or higher.
How Do Hurricanes Compare on the Saffir-Simpson Scale?• Category1: no real damage to building structures. Damage primarily to unanchored mobile homes, shrubbery, and trees. Also, some coastal flooding and minor pier damage.• Category 2:some roofing material, door, and window damage. Considerable damage to vegetation, mobile homes, etc. Flooding damages piers and small crafts.
How Do Hurricanes Compare on the Saffir-Simpson Scale?• Category 3:some structural damage to small residences and utility. Mobile homes are destroyed. Flooding near the coast destroys smaller structures with larger structures damaged by floating debris. Terrain may be flooded well inland.• Category 4:some complete roof structure failure on small residences. Major erosion of beach areas.Terrain may be flooded well inland.• Category 5:complete roof failure on many residences and industrial buildings. Some complete building failures with small utility buildings blown over or away. Flooding causes major damage to lower floors of all structures near the shoreline. Massive evacuation of residential areas may be required.
Hurricane Season• The official season for the Atlantic Basin (Atlantic ocean, Caribbean sea, and gulf of Mexico) is from June 1st to November 30th.
Emergency Notification: 1. Hurricane Watch• A hurricane watch is an announcement given to specific coastal areas that a hurricane poses a possible threat, generally within 36 hours.
2. Hurricane Warning• This is a warning that sustained winds of 74 mph or higher are expected in a specific coastal area within 24 hours or less.• A hurricane warnings are usually in effect even after the strong winds have decreased due to the dangerously high water and waves.
Hurricane NamesAccording to the NHC, hurricanes were previously named after saints, like Hurricane Santa Ana,which struck Puerto Rico in 1825. Womens names are thought to have emerged before the end of the19th century. Tropical storms in the Pacific were named for wives and girlfriends by Army and Navymeteorologists throughout the 1940s, as reported by the Wall Street Journal.
Retired names…The WMO also handles retiring names andaltering the lists for reasons of sensitivity orinappropriateness during committee meetings.Devastating hurricanes like Igor in 2010,2008s Ike and Katrina in 2005, have all beenretired as hurricane names due to the extremedamage caused in those years.