L.17: Hurricanes

Hurricane

(Tropical Cyclones) Tropical cyclones are massive storms over warm oceans that can generate sustained winds of over 150 knots, as well as major amounts of rain and huge waves. Tropical cyclones are called hurricanes over the eastern Pacific Ocean and Atlantic Ocean, typhoons over Western Pacific Ocean, and simply cyclones over the Indian Ocean and in the Southern Hemisphere. EASTERN PACIFIC OCEAN AND ATLANTIC OCEAN = HURRICANES WESTERN PACIFIC OCEAN = TYPHOONS INDIAN OCEAN AND SOUTHERN HEMISPHERE = CYCLONES - winds of greater than 33 meters/second (or 64 knots, 74 miles/hour) - winds that circulate counterclockwise around the center of the low in the Northern Hemisphere, and clockwise in the Southern Hemisphere - size ranging from 100 to 1500 kilometers in diameter, with an average of about 600 kilometers (375 miles) Tropical cyclones usually originate over tropical oceans between about 10 and 20 degrees latitude. Hurricanes do not form over equatorial waters because the Coriolis force is too weak near the equator. Recall from Lesson 11 that the Coriolis force is a force caused by the rotation of the earth that influences the direction of wind flow. Tropical latitudes provide the warm sea-surface temperatures (over 80°F) that generate the development of a tropical cyclone. These warm oceans warm the air above them, creating instability, which you will see in a moment is the first step in the development of a hurricane. In most regions, tropical cyclones occur during the summer and fall (see the map below). Tropical cyclones tend to form when the intertropical convergence zone shifts toward the poles (for example, during the Northern Hemisphere's summer). In the North Atlantic, hurricane season runs from May to November. However, most hurricanes occur in late summer and early fall when ocean temperatures are at their warmest.

Tropical cyclones, regardless of region of the world, usually form between _______ degrees North or South Latitude.

10 to 20

A hurricane watch is released for an area how long in advance of potential landfall?

24-48 hours

Generally, a hurricane requires a minimum distance of ___________ miles from the equator.

300

On the Saffir-Simpson hurricane scale, a hurricane with winds in excess of 155 miles per hour (135 knots) and a central pressure of 910 millibars (26.87 inches) would be classified as a category __________ hurricane.

5

To be a real, true 'card-carrying' hurricane, the storm must have sustained winds of at least ______ miles per hour.

74

In general, a hurricane requires sea surface temperatures of at least _____ degrees Fahrenheit.

80

Hurricanes do not form:

Along the equator

___________ is the name of tropical storms with more than 74-mile-per-hour sustained winds that hit Australia.

Cyclone

Hurricanes that move into India are usually called ______________________ in this part of the world.

Cyclones

In order for a hurricane to form, the layer of warm water must extend from the ocean surface to a depth of at least 400 meters.

False

The Heat Engine and Organized Convection theories of hurricane development are effectively the same except for how they treat upper level winds.

False

True or false? Rain will fall continuously throughout a hurricane as it passes through a region.

False

Which of the following areas in the United States would most likely experience thunderstorms, hurricanes, and tornadoes during the course of one year?

Gulf coast states

Which would you not expect to observe as the eye of a hurricane passes directly over your area?

High winds

Hurricane Threats

Hurricane Ivan In 2004, Hurricane Ivan was a particularly unique storm that affected a large geographical area, including the Caribbean islands and the United States Gulf Coast from Louisiana to Florida, as well as areas further inland! It caused major flooding as it dissipated, and then regained strength as it moved off east over the Atlantic Ocean. Cyclone Nargis In May 2008, Cyclone Nargis made landfall in the impoverished country of Myanmar. It remains one of the deadliest storms in recorded history: over 85,000 people died. Hurricane Ike In September 2008, Hurricane Ike caused major damage to the city of Galveston, Texas. Many people chose not to heed evacuation warnings, and were in harm's way when the hurricane made landfall. Galveston was the site of one of the most damaging hurricanes in United States history in 1900. Over 8,000 people lost their lives due to that hurricane. Cyclone Yasi In February 2011, Cyclone Yasi hit the coast of Queensland, Australia, near the coastal cities of Cairns and Townsville. Hurricane Wilma Hurricane Wilma formed south of Jamaica in Mid-October. It drifted aimlessly until it picked up a northwest track. It then exploded in to a terrifying category 5 with maximum sustained winds of 185 miles per hour in an alarmingly short amount of time. Wilma's eye recorded the lowest pressure ever recorded in the Atlantic Ocean: 882 millibars. This storm was a monster and had South Florida in its sights from the beginning. It would slam in to Mexico. It picked up a strong trough that tossed it toward southwest Florida as a strong Category 3. It would make landfall in the morning of October 24th, 2005 and go through the "back door" over the Everglades. Wilma would intensify over the "River of Grass" and stay a strong category 3 as it hit Broward and Palm Beach Counties. Cost 50 billion dollars in damage.

Cyclone

INDIAN OCEAN AND SOUTHERN HEMISPHERE = CYCLONES

Tropical depression

If the winds increase to between 20 and 34 knots, and show circulation through a few closed isobars in their center, then a tropical depression has formed

CATEGORIZING HURRICANES

In the previous lesson, you learned about the Enhanced Fujita Scale as a means for categorizing the intensity of tornadoes. Hurricanes have their own categorization system: the Saffir-Simpson Hurricane Wind Scale. This scale categorizes hurricanes according to sustained wind speeds, with a Category 1 storm having the lowest wind speeds (approximately 74 to 95 miles per hour) and a Category 5 storm having the highest wind speeds (greater than 155 miles per hour). Note that the Saffir-Simpson scale was recently revised so that storm surge size is not included as part of the classification system. Naming Them Tropical cyclones occurring in all regions of the world are assigned names. These names help people distinguish between storms more easily than other identifying features, such as latitude/longitude coordinates or the dates that the storms originated. Alternating male and females names in alphabetical order are given to hurricanes. Different names are assigned to hurricanes in the eastern Pacific Ocean, the central Pacific Ocean, and the North Atlantic Ocean. Additionally, different sets of names are also used for typhoons and cyclones in other world oceans. Certain names may be retired if a storm is so destructive and devastating that meteorologists no longer want to repeat its name. This is intended to be respectful to those that were affected by such a devastating storm. Katrina is an example of a name that has been retired and will not be used in the naming of future storms.

Conditions for tropical cyclone development

Initially, a tropical disturbance will form as a mass of thunderstorms with little wind circulation. If the winds increase to between 20 and 34 knots, and show circulation through a few closed isobars in their center, then a tropical depression has formed. When these isobars are closely packed, and the wind has increased to between 35 and 64 knots, a tropical storm has formed. Finally, a hurricane is formed when winds exceed 64 knots (approximately 74 miles per hour). - Warm ocean waters (at least 80° F / 27° C) throughout a depth of about 150 feet (46 meters). - An atmosphere that cools quickly enough with height that it is potentially unstable to moist convection. - Relatively moist air near the middle section of the troposphere (16,000 feet / 4,900 meters). - In general, a minimum distance of at least 300 miles (480 kilometers) from the equator. - A pre-existing near-surface disturbance. - Low values (less than about 23 miles per hour / 37 kilometers per hour) of vertical wind shear between the surface and the upper troposphere. If you recall from previous lessons, vertical wind shear is the change in wind speed with height. A hurricane can survive for an extended period of time if it remains over warm water, which continues to act as fuel for the hurricane formation process. However, any of the following changes can weaken the storm, causing it to dissipate: - If the hurricane moves over cooler waters. - If the hurricane moves over much shallower waters. -- The storm's winds generate waves, and in shallow water the cooler waters from below are more easily brought to the surface. - If the hurricane moves over land. -- The storm loses its energy source of warm water, and friction with the land surface decreases the speed of surface winds. -And, if it moves into an area with strong vertical wind shear.

The region in where most hurricanes that form and hit the United State's Eastern Seaboard typically originate in what area?

North Atlantic Ocean

As a northward-moving hurricane passes to the east of an area, surface winds should change from:

Northeast to North to Northwest.

Forecasting hurricanes

Observation is the foundation for all hurricane forecasts. Throughout this course, you have learned about a variety of tools, maps, and methods that meteorologists use to observe and understand weather conditions around the world. Many of these methods and tools, including satellites, radar, and measurements from ocean buoys, contribute information for observing conditions leading to the formation of a tropical cyclone. Meteorologists also take into account historical data (such as the common paths taken by storms in previous years) and climate patterns, like El Niño and La Niña. Computer models take in these observations and perform millions of calculations to generate predictions of hurricane behavior and the general conditions of the atmosphere at a hurricane's location. The model results give guidance to the appropriate national centers and local offices and are used in the forecasting and warning process. After forecasters have examined the information and determine a potential path for the storm, a hurricane watch is issued 24 to 48 hours before the storm arrives. A hurricane warning is issued when it appears that the storm will strike the area within 24 hours.

Typhoons are what a hurricane is called in the:

Pacific Ocean

What is the name of the wind scale used to categorize hurricanes?

Saffir - Simpson

What is the name of the wind scale used to categorize hurricanes?

Saffir-Simpson

Eye Wall

Surrounding the eye is the eye wall. Within the eye wall are tall thunderstorms that produce heavy rains and usually the strongest winds associated with the hurricane. ... Away from the center you can see the alternating bands of clouds and cloud-free areas that trail away from the eye wall in a spiral shape (spiraling rain bands in the figure below). These bands are capable of producing strong rainstorms and wind, as well as tornadoes. However, sometimes there are gaps between the rain bands where no rain or wind is present. If you were to travel from the outer edge of a hurricane to its center, you would experience a progression from light rain and wind, to a dry and weak breeze, then back to increasingly heavier rainfall and stronger winds. This progression would repeat itself, with each period of rainfall and wind being more intense and long-lasting than the previous. A cross-section (like the figure above) of a hurricane reveals more information about its structure and movement. In a hurricane, air moves toward the center in a counterclockwise pattern in the northern hemisphere (clockwise in the southern hemisphere), and out the top in the opposite direction. Air sinks in the center of the storm, forming the eye that is mostly cloud-free. Strong updrafts, thick clouds, and heavy precipitation are present in the eye wall, and updrafts and downdrafts in the cloud and cloud-free bands, respectively. A surface pressure measurement will show that a hurricane is a low pressure center with surface pressure increasing continuously away from the center.

Which of the following changes can weaken a tropical cyclone, causing it to dissipate?

The tropical cyclone moves over cooler waters The tropical cyclone moves over land

2017 hurricane season

The 2017 hurricane season was particularly active and destructive. It has been classified as the seventh most active season on record since 1851 and the most active season since 2005. There were seventeen named storms and ten hurricanes, six of which were considered major hurricanes. Hurricanes Harvey, Irma, and Maria were three of the major hurricanes that made landfall (NOAA, 2017). Hurricane Harvey made landfall in Texas on August 25, 2017. The storm officially turned into a tropical storm east of the Lesser Antilles. The stormed moved northwest into the Gulf of Mexico where it was then classified as a Category 4 hurricane, only 56 hours before making landfall. Harvey produced wind speeds up to 130mph and brought up to 60 inches of rain in some areas (The Weather Company, 2017). Five days later, Harvey made a second landfall at the Texas-Louisiana border as a Tropical Storm. Harvey produced the most rainfall recorded by one single storm. The hurricane resulted in the need for 30,000 people to seek temporary shelter and caused approximately $75 billion dollars in damage (CNN, 2017). Hurricane Irma first made landfall in Cuba and the Caribbean Islands and then made landfall in southern Florida on September 10, 2017. Irma was classified as a Category 5 hurricane when it passed Cuba and the Caribbean Islands. It was then reclassified as a Category 4 before making landfall and then reclassified again to a Category 3 after making landfall (NASA, 2017). The hurricane stretched 650 miles wide and affected nine U.S. states. About 5.6 million people were included in the evacuation orders (CNN, 2017). Wind speeds sustained 130mph when first hitting Florida as a Category 4 storm. Irma resulted in dangerous storm surges, up to 10 feet higher than normal, from the Florida Keys to Tampa Bay. Storm surges were also experienced as far as Charleston, South Carolina (NASA, 2017). As Hurricane Jose weakened and traveled up the Atlantic coast of the United States, Hurricane Maria (reaching a category 5) battered the same region as Hurricane Irma had two weeks prior. In less than 18 hours, Maria transformed from a category 1 to category 5 storm shortly before making reaching Puerto Rico and Dominica. NASA meteorologists pointed out that Maria dropped incredible rainfall amounts -- similar to Harvey but occurring in just one day!

Why do hurricanes not form at or very close to the equator?

The Coriolis force is too weak

Tropical Cyclone

There are two theories of how hurricanes develop in tropical latitudes: the organized convection theory and the heat engine theory. The organized convection theory states that the energy for a hurricane comes from the following process: - First, surface convergence is present, which leads to the rising motion of warm, moist air. - This rising air condenses, releasing a large amount of latent heat: L = ± 2.5 MJ / kg - The heating of the air column causes the air column to stretch, producing high pressure and divergence aloft, and therefore lowering pressure at the surface. - Lower surface pressure increases the pressure gradient [recall the relationship between wind and pressure gradients from Lesson 11]. This generates stronger surface winds. - Stronger convergence also occurs at the storms center, enhancing convection, which continues the first step in the process--the rising motion of warm, moist air. - A chain reaction develops, and a tropical storm is born, which could develop into a hurricane. The heat engine theory compares the energy process in a hurricane to, as the name suggests, a heat engine. In a heat engine, heat is taken in at a high temperature, converted into work, then emitted at a low temperature. A hurricane is essentially a giant heat engine. - It takes in heat from the warm surface of the ocean and converts it to kinetic energy (energy for wind). - This energy is lost through the top of the hurricane through radiational cooling. - The strength of a hurricane is dependent upon the difference in air temperature between the tropopause and the surface, and the potential for moisture to evaporate from the ocean surface. Therefore, the warmer the ocean surface, the lower the minimum pressure of the storm, and the higher its wind speeds. This is the role of a tropical cyclone: take heat, stored in the ocean, and transfer it to the upper atmosphere where upper level winds carry that heat toward the poles. This heat transfer prevents the polar regions from being as cold as they potentially could be, and keeps the tropics from overheating. Simply put, hurricanes are a way for the earth to maintain the global heat and energy balance. To help you conceptualize the amount of energy generated by a hurricane, consider a rain rate of 5 centimeters per day over an area with a radius of 500 kilometers, typical for tropical storms and hurricanes. This storm may have a 7 day life cycle. The energy released by a hurricane of this scale is equivalent to a 50,000 megaton nuclear explosion. In other words, equal to the total explosive yield of the United States' & USSR's nuclear arsenals at the height of the Cold War! Regardless of the theory, the following conditions must be present in order for a hurricane to form: - Warm ocean waters (at least 80° F / 27° C) throughout a depth of about 150 feet (46 meters). - An atmosphere that cools quickly enough with height that it is potentially unstable to moist convection. - Relatively moist air near the middle section of the troposphere (16,000 feet / 4,900 meters). - In general, a minimum distance of at least 300 miles (480 kilometers) from the equator. - A pre-existing near-surface disturbance. - Low values (less than about 23 miles per hour / 37 kilometers per hour) of vertical wind shear between the surface and the upper troposphere. If you recall from previous lessons, vertical wind shear is the change in wind speed with height.

Heat Engine Theory

The heat engine theory compares the energy process in a hurricane to, as the name suggests, a heat engine. In a heat engine, heat is taken in at a high temperature, converted into work, then emitted at a low temperature. A hurricane is essentially a giant heat engine. - It takes in heat from the warm surface of the ocean and converts it to kinetic energy (energy for wind). - This energy is lost through the top of the hurricane through radiational cooling. - The strength of a hurricane is dependent upon the difference in air temperature between the tropopause and the surface, and the potential for moisture to evaporate from the ocean surface. Therefore, the warmer the ocean surface, the lower the minimum pressure of the storm, and the higher its wind speeds. This is the role of a tropical cyclone: take heat, stored in the ocean, and transfer it to the upper atmosphere where upper level winds carry that heat toward the poles. This heat transfer prevents the polar regions from being as cold as they potentially could be, and keeps the tropics from overheating. Simply put, hurricanes are a way for the earth to maintain the global heat and energy balance.

Organized Convection Theory

The organized convection theory states that the energy for a hurricane comes from the following process: - First, surface convergence is present, which leads to the rising motion of warm, moist air. - This rising air condenses, releasing a large amount of latent heat: L = ± 2.5 MJ / kg - The heating of the air column causes the air column to stretch, producing high pressure and divergence aloft, and therefore lowering pressure at the surface. - Lower surface pressure increases the pressure gradient [recall the relationship between wind and pressure gradients from Lesson 11]. This generates stronger surface winds. - Stronger convergence also occurs at the storms center, enhancing convection, which continues the first step in the process--the rising motion of warm, moist air. - A chain reaction develops, and a tropical storm is born, which could develop into a hurricane.

Which of the statements is false about hurricanes?

They can occur between the dates of June 1st and November 30th

Which statement below is not correct concerning hurricanes?

They may strengthen as they make landfall.

Tropical versus midlatitude cyclone

Tropical cyclones and midlatitude cyclones are large weather systems with the capability of producing major weather events, these systems are quite different in their formation and structure. Tropical Cyclones A warm core of low pressure Storm weakens with height Sinking air in the center (eye) Isobars are more dense and circular Smaller in area No fronts associated with storm Midlatitude Cyclones A cold core Storm intensifies with height Strongest winds are aloft in the jet stream Isobars are more course and less circular Larger in area Associated with cold and warm fronts Question: What property do both midlatitude and tropical cyclones share? Answer: Rotation! They are both rotating air masses of low pressure.

What is the first stage of a hurricane?

Tropical disturbance

Hurricanes have strong divergence at their tops.

True

One property that both midlatitude and tropical cyclones share is rotation.

True

Tropical cyclones are usually stronger than mid-latitude cyclones

True

Tropical cyclones are usually stronger than mid-latitude cyclones.

True

Typhoon

WESTERN PACIFIC OCEAN = TYPHOONS

Tropical storm

When these isobars are closely packed, and the wind has increased to between 35 and 64 knots, a tropical storm has formed.

Eye

When viewed from above (such as in a satellite image) we can clearly see the center, or eye, of the hurricane. The eye, which is usually approximately 20 to 40 miles in diameter, is a calmer, generally clear area of sinking air and light winds that usually do not exceed 15 miles per hour. People who may be less informed about the structure of a hurricane may think that the storm has passed when the eye passes over their area, however, the storm has only moved halfway through the region. The storm will strike again, this time with winds traveling in the opposite direction of those that have already passed.

The main difference between a hurricane and a tropical storm is that:

Winds speeds are greater in a hurricane

Hurricane names are retired when:

a hurricane of a name causes significant death and destruction.

In a hurricane, the eye wall represents:

a zone of intense thunderstorms around the center

Typically, as you move closer to the eye of the storm, the pressure will ________________________.

drop

Most hurricanes have fronts

false

A hurricane warning:

gives the percent chance of a hurricane's center passing within 65 miles of a community.

An intense storm of tropical origin that forms over the Pacific Ocean adjacent to the west coast of Mexico would be called a:

hurricane

Which of the following only forms over water?

hurricanes

The strongest winds in a hurricane are found:

in the eye wall

Pressure at the center of a hurricane is __________ than the surroundings at the surface and __________ than the surroundings aloft.

lower;higher

The vertical structure of the hurricane shows an upper-level __________ of air, and a surface __________ of air.

outflow ; inflow

Which method below describes how scientists have tried to modify hurricanes?

seeding the hurricanes with silver iodide

At the periphery of a hurricane, the air is __________, and several kilometers above the surface, in the eye, the air is __________.

sinking; sinking

Which of the following is not an atmospheric condition conducive to the formation of hurricanes?

strong upper-level winds

The role of tropical cyclones is to:

take heat stored in the ocean, and transfer it to the upper atmosphere where upper level winds carry that heat toward the poles.

Storms that form in the tropics are given names when:

they reach tropical storm strength.

The first three stages of a developing hurricane are (from first stage to third stage):

tropical disturbance, tropical depression, and tropical storm.