Oceanography Air – Sea Interaction

• On any given moment, nearly 2000 thunderstorms are occurring around the world.

• Capable of producing hail the size of baseballs.

• May trigger tornadoes and wind speeds up to 160 km/hr.

• May provide energy to produce hurricanes.

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 lifting

2)Frontal Thunderstorms – due to cold fronts.

Stages of a Thunderstorm

1) 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 electrical discharge produce by thunderstorms. Lightning is a giant spark.

•It appears to “flicker” because it is composed of a succession of multiple strokes that usually follow the exact same path of the initial stroke.

Lightning over Chick’s Beach, Virginia Beach.

•Several types of lightning are common:* In-cloud lightning extends from one charged region of a cloud to another.

* Cloud-to-cloud lightning extends between two clouds.

* Cloud-to-air lightning extends from a cloud to the air, not touching the ground.

* Cloud-to-ground lightning stretches from a cloud to the ground.

Formation of Lightning:

Stage 1:

Cumulonimbus clouds develop a separation of electric charge, with the tops of the clouds positively charged and the bottoms negatively charged.

Light weight particles have a positive (+)charge and float to the top of the cloud (like ice floats to the top of a glass). Negatively (-)charged particles sink to the bottom.

Stage 2:

A “step leader” on the bottom of the cloud form as it overcomes air`s resistance to the flow of electricity.

Rapidly rising air in a thunderstorm interacts with rapidly falling air within the thunderstorm to create separately positive and negative charged areas within the cloud (friction).

Stage 3:

Meanwhile, as the electrons approach the ground, the ground is becoming more and more positively charged due to the repulsion of electrons in the ground. This positively charged region moves up through any conducting objects on the ground -- houses, trees and people -- into the air. This is why it is very important to stay "low" during a storm or when lightning threatens.

Stage 4:

When the downward moving electrons meet the upward flowing positive regions at an altitude of a hundred meters or so, they form a complete circuit (See Figure) and the lightning begins.

Stage 5:

We now see the main act of lightning, the return stroke.

This extreme heating causes the air to expand at an explosive rate. The expansion creates a shock wave that turns into a booming sound wave, better known as thunder.

-Seconds after this girl walked away, lightning struck where she stood.

-The positive charge of her hair is attracted to the negative charge of a lightning cloud.

Oklahoma

Lightning strikes in the Kempsville Lakes area of Virginia Beach, off Baxter Road, around 3 a.m. July 12, 2005.

August 22, 2007: After a series of thunderstorms passed through Hampton Roads on Tuesday night, William Coyle (VA Lightning) photographed these anvil crawlers. In the foreground is the Westin Hotel & Residences at Virginia Beach's Town Center.

Facts about lightning:

•Lightning discharges can reach up to 200 million volts.

•Can reach temperatures up to 54,000° F. That is hotter 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 the United States is about 1 in 250,000. Approximately 75 – 150 people die each year due to lightning.

Why do we see lightning first, then hear the sound later? Light is faster than sound.

•Speed of light: 186,000 miles a second

•Speed of sound in air: At 21 degrees C (70°F): 344 meters per second or 1129 ft per second.

•Speed of sound in water: 1480 m/s or 4856 ft/s. More than 3000 miles per hour.

Calculating distance of lightning:

Foldable: Chapter 13.2, p. 334

Provide characteristics, steps of formation, and dangers of the following:

1) Severe thunderstorms

2) Lightning

3) Fury of Wind

4) Hail

5) Floods

6) Tornadoes

Pressure Systems• At Earth’s surface, rising air is associated with

low pressure and sinking air is associated with high pressure.

Weather Systems

• 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 SystemsLow-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

Cyclones

Cyclones are areas of low pressure. Cyclones usually exhibit nearly circular isobars.

•As air enters an area of low pressure from all directions, the Coriolis effect bends the direction of the wind to the right of its path.

•This creates a counterclockwise rotation around the low.

                                                  

Figure PC.6 Circulation within a low pressure system in the Northern

Hemisphere

Anticyclones

Anticyclones are areas of high pressure that exhibit nearly circular isobars.

•The Coriolis effect bends the air to the right of its path creating a clockwise rotation around 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 and highs exhibit a diverging, counterclockwise rotation. You can see this effect in the cloud pattern created by a cyclone in the Southern Hemisphere.

Tornadoes!!!

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.

Tornadoes• A tornado forms when wind speed and direction change

suddenly with height, a phenomenon known as wind shear.

Severe Weather

2. Updrafts in a thunderstorm lifts the “barrel” of air.

3. Air pressure drops and funnel forms and spins faster.

1. Wind Shear forms a horizontal “barrel” of air.

Tornado Formation

How are the rated?

Tornado Classification

Severe Weather

– The Fujita tornado intensity scale classifies tornadoes according to their path of destruction, wind speed, and duration.

– 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

– Tornadoes occur most frequently in a region called “Tornado Alley,” which extends from northern Texas through Oklahoma, Kansas, and Nebraska.

Most tornadoes form in the spring during the late afternoon and evening, when the seasons begin to change.

TornadoesTornado Safety

Severe Weather

– 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.

April 2008 – Suffolk, VA

Suffolk, VA 2008

Suffolk, VA April 2008

Suffolk, VA April 2008

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 describe a storm system in the Western Pacific.

Hurricane Isabel off coast of North Carolina from oil tanker – 2003

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.

1) Tropical Depression

•At first the storm system will appear as an unorganized system of thunderstorms.•Tropical depressions have wind speeds less than 38 mph.

Growth of a Hurricane: Storm Systems

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

Growth of a Hurricane

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 of Rodanthe on Hatteras Island. Storm surge from Hurricane Irene destroyed a 900-foot section of the 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 Katrina: August 29, 2005

Hurricane Names

According to the NHC, hurricanes were previously named after saints, like Hurricane Santa Ana, which struck Puerto Rico in 1825. Women's names are thought to have emerged before the end of the 19th century. Tropical storms in the Pacific were named for wives and girlfriends by Army and Navy meteorologists throughout the 1940's, as reported by the Wall Street Journal.

Retired names…

The WMO also handles retiring names and altering the lists for reasons of sensitivity or inappropriateness during committee meetings. Devastating hurricanes like Igor in 2010, 2008's Ike and Katrina in 2005, have all been retired as hurricane names due to the extreme damage caused in those years.

Time Lapse Videos of Hurricanes