Howdy class. And we finally made it to the ultimate hazard of Earth, our hurricanes. Yes, you heard that right. In this video, we will only be focusing on hurricanes. Hurricanes are the greatest storms we have here on Earth. They are called typhoons in the western Pacific and cyclones in the Indian Ocean. They are among the most destructive natural disasters and the majority of the US population lives within 75 kilometers of the coast, putting millions of people at risk to these monsters. Hurricanes typically form in late summer to early fall when the sea surface temperatures reach 27° C to provide enough heat and moisture for the storm. Warm ocean water is why hurricane formation over the South Atlantic and South Pacific are rare and anywhere pole word of 20° latitude. One thing I didn't know is that hurricanes do not actually develop within 5 degrees of the equator because the Corolis effect is too weak there. The Corolis effect is a force related to Earth's rotation that gives storms their spin. This makes most of the tropics the perfect place for these storms to form. This world map shows the regions where most hurricanes form as well as their principal months of occurrence and the tracks they most commonly follow. Again, notice that hurricanes do not form within 5 degrees of the equator due to the weak coralolis force. We also do not see much formation north and south of 20° latitude because the ocean waters are generally too cold. Here on this map, you can see that the Atlantic hurricane season is most active between August and October. In the Eastern Pacific, it can start a little earlier in June and last until October. The Western Pacific has a very long typhoon season from June to December. The Northern Indian Ocean cyclone season is also long between June to November. In the southern hemisphere, west of Australia, the season is between January and March. And east of Australia, the season is between January to March. So essentially, at all times on Earth, there is prime conditions somewhere for these destructive whirling tropical storms to form. The graph here shows the frequency of tropical storms and hurricanes from May 1st to December 31st in the Atlantic basin. It shows in orange the total number of storms expected over a span of a 100 years and in red is the total number of hurricanes. The period between late August in October is clearly the most active with the season peaking in September. Among the necessary ingredients for a hurricane to form is warm ocean temperatures above 27° which is 80° F. This map shows sea surface temperatures in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico on September 5th of 2017. This yellow to red line represents Hurricane Irma's track. You can see the hurricane strengthens from a category 3 to category 5 as it enters warmer and warmer waters. This was one of the worst hurricane seasons on record with Irma just one of many major hurricanes to make landfall that year. And we will talk about that season later. So, how do these hurricanes form typically? Well, in zones outside of the equator, cumulus clouds and precipitation generally occur in clusters and move westward along the surface trade winds. The cloud clusters move in association with weak troughs in sea level pressure that are called easterly waves or tropical waves due to their wavelike appearance on surface weather maps. Weather systems associated with these easterly waves are most frequently observed in the Caribbean basin. Similar events occur in the west central Pacific and off China's central east coast. These systems travel towards the west and are relatively predictable. We have gotten really good at forecasting. As the weak low pressure trough deepens, meaning the air pressure continues to drop, these cloud clusters begin to assume a more organized rotating cyclonic pattern and form a disturbance known as a tropical disturbance. Tropical disturbance can turn into a tropical depression and tropical depressions can then intensify and once they develop sustained wind speeds of at least 39 mph they are classified as a tropical storm and given a name. Keep in mind that this is based on sustained wind speed. Sustained wind speed is the wind speed averaged over a certain period of time, which on the Saphir Simpson scale that I'll show you soon that classifies hurricanes, this certain period of time that the winds are averaged over is one full minute. This means you could still have gusts which are instantaneous and not constant that exceed these speeds. Tropical storms can continue to intensify as well and once they develop sustained wind speeds of at least 74 miles hour, they are then classified as a tropical cyclone. Tropical cyclones in the western Atlantic and Eastern Pacific oceans get the label hurricane while the western Pacific is typhoon and the Indian Ocean is just cyclone. Hurricanes are efficient machines for drawing large quantities of excess heat away from the ocean surface and transporting them into the upper atmosphere. Once a cyclone is developed, it becomes self- sustaining. Vast quantities of heat energy are taken from the warm ocean below and transported up as latent heat. This energy is released as sensible heat when condensation occurs. This sensible heat provides the system with potential energy which is partially converted into kinetic energy causing the hurricane's violent winds. The warm core leads to upper level divergence at the top of the hurricane which feeds the vertical lifting within the eyewall and promotes the convergence at the surface. Once they pass over a large body of land or cooler waters, their energy sources cut off and they gradually weaken and die. Hurricanes are intense low pressure centers and as you move toward the center of a hurricane, the pressure will continue to decrease. A pressure gradient is the measure of how fast the pressure changes over a certain distance and the steeper pressure gradients result in stronger winds. This is because air will always move from high pressure to low pressure and the faster the pressure changes, the faster the air will move. As warm, moist air approaches the core of the storm, the air rises in a ring of cumulanimous clouds called the eyewall. The eyewall is where the greatest wind speeds and the heaviest rain occurs. The center of the storm is called the eye. And here the precipitation and wind speed decreases. Near the top of the hurricane, that's where we have air flowing outwards. Here's another profile of a hurricane. You can see that the tropical moisture is spiraling inwards, creating rainbands that pinwheel around the storm center. The eyewall is that zone where winds and rain are most intense. Air sinks in the eye and is warmed by compression. Air is outflowing at the top and is very important because it prevents the convergent flow at lower levels from filling in the storm. Again, it's an efficient, self-sustaining machine. The graph shown here has measurements of surface pressure in orange and wind speed in red during the passage of Cyclone Monty in Western Australia between February 29th and March 2nd in 2004. You can see where the pressure changes the quickest in that eyewall. The wind speeds pick up and then they off die off very suddenly in the eye of the storm where the lowest pressures are recorded. Then once the eye passes over, the pressure begins changing again. This time in the opposite direction and where it changes the quickest, the winds pick up in speed again. The amount of damage from a hurricane depends not just on the storm's intensity, but also the size and population density of the affected area and the shape of the ocean bottom near the shore, which is often controlled by the geology. The most important factor though is the strength of the storm. And we measured the intensity of hurricanes on a relative ranking scale from 1 to 5 called the Sapphir Simpson hurricane scale. Here is the Saphir Simpson scale. You can see each category and their associated central air pressures, winds, expected storm surges, and damage at landfall. You can see here that a category 1 has minimal damage while the strength increases progressively as you get to the category 5 which is catastrophic damage at landfall. This scale shown here is using the metric system. So the winds are in kilome hour and the storm surges in meters. Let me show you this scale using US customary units since I know that is what most of you if not all of you are more accustomed to including myself. So here you can see that category 1 starts at 74 miles per hour and typically has an air pressure greater than 980 millibars in the eye. These storms have storm surges typically between 4 and 5 ft and causes minimal damage. Storm surge of 4 to 5 ft means that everything along the shore that is less than 5t above sea level will be flooded. Category 2 starts at 96 mph to 110 mph. Keep in mind this is sustained wind. Wind gusts can still be much higher. Category 2 storms typically have an air pressure in the eye between 965 to 979 mibars. For reference, the average sea level pressure is around 1,13 millibars. So these storms are indeed low pressure systems. Category 2 storms can see storm surges from 6 to 8 feet and cause moderate damage at landfall. Major hurricanes start at a category 3 when wind speeds reach at least 111 mph. Category 3 is from 111 to 130 mph and they have an air pressure of 945 to 965 millibars. Typically, storm surge in category 3 storms are up to 9 to 12 ft and can cause extensive damage. This is the category of Katrina when it made landfall in Louisiana in 2004. Now, these storm surges that I've been mentioning are just expected storm surges because hurricanes are classified based on their sustained wind speeds. We can still see storm surges that exceed the expected level. For example, the highest ever recorded storm surge occurred during Hurricane Katrina, which produced a maximum storm surge of 27.8 ft in southern Mississippi. The highest storm surge noted in historical accounts was produced by cyclone Mahina in 1899, which was estimated to be almost 44 feet in Australia. But this wasn't recorded using modern methods of today. A category 5 is between 131 and 155 mph with an air pressure typically between 920 to 944 millibars and a storm surge of usually between 13 and 18 ft. Meaning everything less than 18 ft below sea level would be flooded and this causes extreme damage. An example of a category 4 storm would be the one that hit Galveston in 1900 and was the deadliest US hurricane in history, which I will talk about in a bit. Now, category 5 begins at 155 miles per hour and has an air pressure usually less than 920 millibars. The low lowest air pressure ever recorded on Earth was 870 millibars. This was recorded in October of 1979 by the US Air Weather Service 300 miles west of Guam in the Pacific Ocean in the eye of super typhoon Tip. And this typhoon had sustained winds of 190 miles per hour. That's sustained winds. Gusts exceeded this. It was the largest and most intense tropical cyclone ever recorded. Typhoon Tip impacted the Caroline Islands, the Philippines, the Korean Peninsula, Japan, Northeast China, Soviet Far East, and even Alaska, causing 99 total fatalities. Category 5 storms typically have storm surges over 20 feet and cause catastrophic damage at landfall. Hurricane damage is most devastating in the coastal zone and the deadliest hazard of the storm is storm surge. A storm surge is a dome of water 65 to 80 km wide that sweeps across the coast near the point where the eye makes landfall. This can inflict immense damage on low-lying coastal areas. In the northern hemisphere, storm surge is most intense on the right side of the eye where winds are blowing towards the shore. The left side of the wall has winds blowing away from the shore, so the storm surge isn't as big of a threat there. Here's an example of how destructive storm surge can be. This is a photo of Crystal Beach here in Texas on September 16th, 2008 after Hurricane Ike came ashore. At landfall, the storm had sustained winds of 105 mph. It was the extraordinary storm surge that caused most of the damage shown here. Wind damage is the most obvious type of hurricane damage, and it affects a much larger area than storm sur. Hurricanes commonly produce tornadoes as well as a secondary threat to their immense power. Heavy rains and inland flooding are also problems that it can affect areas hundreds of kilometers from the coast several days after the hurricane has even passed. An example of this would be Hurricane Floyd in 1999, which caused the largest peace time evacuation in US history. Luckily, we have gotten pretty good at detecting and tracking hurricanes to predict locations of impact and can issue warnings and evacuation sometimes several days before its arrival. But prior to weather satellites, few storm warnings were given to the population. A grave example happened in Galveston in 1900 where a huge storm struck the barrier island with little to no warning, killing over 8,000 people. So, let's focus on Atlantic hurricanes because they impact us the most. Hurricanes have wind speeds from 74 mph up to 200 mph. If the speeds are less than 74 mph, remember, we call them a tropical storm. And storms don't get their names until they hit tropical storm status at 39 mph. These storms can extend over 400 miles in diameter. Typhoon Tip, which I briefly mentioned, had the lowest pressure and the highest wind speed, was also the largest storm, and it had a wind diameter of 1,380 miles. It was about the size of Australia. So, these storms can impact large areas of land and millions of people at one time. Atlantic hurricanes typically form in the warm tropical waters off the west coast of Africa. Then they move westward across the Atlantic due to the global wind patterns. They usually threaten the Caribbean, Gulf Coast, and the entire east coast of the United States. These storms are actually an important way that the sun's energy is transported from the equator towards the poles. They occur most commonly in late summer and early fall when the sea surface temperatures are the greatest and they rotate counterclockwise in the northern hemisphere around an area of low pressure called the eye due to the corololis effect. Here's a chart showing the occurrence of hurricanes and tropical storms by month. Again, you can see here that the season peaked right around September 10th. We have seen some recent increases in the Atlantic hurricane season. The average number of named storms from 1970 to 1994 was about 8.6 per year. The average number of storms since 1995 is now over 13 a year. The average number of hurricanes between 1970 and 94 was five. Since then, it is now 7.7 each year. The average number of major hurricanes, which remember are category 3 or higher between the 70s and 94 was about 1 and a half per year. We are currently now seeing around 3.6 per year. Major hurricanes between 1979 and 2017 were up 15% in the last half of that period. Here's a chart showing the number of tropical storms and hurricanes in the Atlantic between 1950 and 2016. Notice the year 2005 is the highest on this graph. That was the deadly year of Katrina. 2005 was one of the most active seasons in history until its record was broken just a few years ago in 2020. We saw seven major hurricanes in 2005 which is tied with 2020. Some notorious storms from 2005 include four category fives with Katrina, which weakened to a category 3 before hitting Louisiana, but it was a category 5 at its strongest. Hurricane Emily, Rita, and Wilma. And Wilma was the most intense Atlantic hurricane on record. Here is a chart showing US hurricanes ranked by death toll. The most recent one on this list, number two, Hurricane Maria, killed 2,975 people, most in Puerto Rico. Now, this one doesn't get as much attention because it impacted Puerto Rico and not the mainland United States. And also, Hurricane Harvey was another notable storm that year that gets a lot of attention because that one did hit the mainland. But Maria was actually much, much worse. And the island of Puerto Rico has never been the same since. Now, you can also see number one at this list is Galveastston and at number four is Katrina. We'll be talking about both of those historic storms in the next video. I'll see you there.