With the ominous satellite view at 1230 pm CDT today of an intensifying hurricane in the middle of the Gulf of Mexico…it has become the top news story of the weekend. I thought it would be good to write a blog on how a hurricane forms and how it works to produce such high wind, heavy rain, and storm surge.
1. Early development
Over warm ocean waters, where there is warm, humid air near the surface, thunderstorms sometimes form. In the case of tropical cyclone development, these thunderstorms are usually sparked by a tropical wave, or some kind of atmospheric wave moving through. Storms often tend to organize with each other a bit, as cool outflow boundaries create more storms. This forms a tropical disturbance. (The artwork is amateur…but makes the point. The colors represent radar colors of rain: green light, yellow moderate, red heavy).
An important characteristic of thunderstorms is that they release latent heat in the atmosphere. When water condenses from vapor to liquid, it releases heat, just like when you get out of the lake, pool, or shower and the water evaporates from liquid to vapor, it absorbs heat, making you feel cold even though it’s not cold. We show this type of process all the time in a skew-T, log-p diagram, or a graph of temperature and humidity with height in the atmosphere. Below is a typical one. Note the white dashed line, representing the air in a thunderstorm updraft, is 5-10 degrees C warmer than the undisturbed air around it. Since warm air at the same pressure is less dense than cool air, it accelerates upward through buoyancy. This is called instability. But, it shows that thunderstorms warm the upper atmosphere, and this is very important for hurricane formation.
If the storms are in a favorable environment (low shear, unstable air, plenty of humidity over the warm ocean water), they may intensify and organize, as shown below, into a tropical depression.
This is where the release of latent heat by thunderstorms converting water vapor to liquid water becomes very important. The pressure at the surface is caused by the weight of all the air above that point. If the air above a point gets warmed, it becomes less dense and lighter, so the pressure underneath it at the surface gets lower. Now we have a low pressure area. Air at low levels begins to flow from high pressure around the tropical depression toward low pressure in the tropical depression.
2. Development of a tropical cyclone
If the wind shear is low enough (we will get to that in a minute), and there is enough moisture and warm, unstable air above the warm water, this is where it starts becoming a true heat engine. The release of latent heat by storms lowers the pressure, causing convergence of air into the storm. The converging air can’t go into the ocean, it has to go up, creating more storms and releasing more latent heat. It is now a positive feedback loop, the pressure drops and pulls air in like a vacuum cleaner, air converges in, and a tropical storm forms.
The storms are still a bit disorganized in many tropical storms. But, the spin of the earth starts to take over here, too, keeping the converging air from filling the low pressure. The Coriolis force, caused by the spin of the earth, turns wind to the right in the Northern Hemisphere. Note the wind is blowing toward the low pressure in the center, but not directly towards it anymore. This is where the large counterclockwise circulation begins.
We mentioned wind shear earlier. Remember the main thing driving the low pressure in a tropical cyclone is latent heat released aloft by thunderstorms, causing the air to warm and get lighter, and lowering the pressure underneath. If there is little wind shear, the thunderstorms and latent heat can stay upright, keeping the lowest pressure near the center. If there is wind shear, the storms lean over, spreading out the effect of the latent heat and low pressure, not allowing pressures to drop as much near the surface.
Since the low pressure at the surface causes the high winds, and the convergence into the storm that forms more thunderstorms, wind shear tends to slow the positive feedback by keeping the low pressure area at the surface more spread out and less intense.
If the wind shear is low, and there is plenty of warm ocean water providing warm, humid air for storms, then the heat engine goes almost into automatic mode here, with the pressure continuing to drop near the center as big thunderstorms release huge amounts of latent heat. The pressure gradient becomes very large near the center of the storm, causing very strong winds, rotating counterclockwise because of the Coriolis force due to the spin of the earth. Yet, there is still just enough convergence, in addition to the unstable air, to keep storms going or intensifying even further.
To illustrate the warm air aloft in the hurricane as the main cause of the low pressure, take a look at the graphs below. The top-left one shows dropsonde data from a hurricane aircraft, and top-right shows a numerical simulation of a hurricane. In both cases, we can see air that is about 15 degrees C (24 degrees F) warmer than the surrounding environment. This warm air is much lighter than the air around it, causing the pressure at the surface to drop significantly, and become much lower than the air around it. This type of focused, upper air warmth gives the hurricane its low pressure and intense pressure gradient from high to low pressure, and the strong winds.
The chart above shows computer model expected temperatures around Hurricane Ida at 7 am Sunday, at 500 mb (about 18,000 ft). Note, in the center, temperatures are much warmer, 34 F near the eye. Temperatures drop off quickly to the mid to upper 20s 100 miles out from the center, and near 20 further out.
4. Hurricane Effects
The high winds are caused by the huge change in pressure over a short distance. The air tries to rush from high to low pressure, but the Coriolis (spin of the earth) turns it right until a rough equilibrium is reached where the wind is going fast enough that the Coriolis force and the pressure gradient force are in balance. Here is an example of surface pressure isobars (lines of equal pressure) and winds (colors) from Ida’s expected location Sunday afternoon. Note how, over areas with normal weather at that time, the pressure only changes about 2 mb (0.06″) over the distance from Birmingham to Meridian, MS, or about 100 miles. In the hurricane, the pressure changes from 980 mb near the center at Morgan City, LA to 1000 mb about 50 miles away. That is 20 mb per 50 miles, while over Alabama it is 1 mb per 50 miles. This causes very high winds. Note the wind flags are flowing counterclockwise but also have a convergent component.
The biggest danger in a hurricane is often storm surge. The strong wind pushing the surface of the water clearly makes huge waves. But, with a long-term push of the water in one direction, the entire surface of the water will no longer be flat, but get higher where the wind is pushing it, and lower where the wind is pulling it. The storm surge can cause water to be up to 15-20 feet above normal tide in some landfalling hurricanes. Depending on how flat the land is, this can push water a few 100 feet inland, or miles inland (as it did in Mississippi in Katrina). A smaller version of this happens on The Great Lakes, Lake Okeechobee, and even in the sloughs along a lake during a large thunderstorm (water rose 4″ in Bluff Creek during a 65 mph wind storm out in the river in May 2020).
Wind is strong and can destroy structures. But the destructive power of water can be much larger, because water weighs 1,000 times as much air. Wind of 115 mph has a force of 28 pounds per square foot, while water moving 11 mph has a force of 270 pounds per square foot. So, water surging in from a hurricane can do a lot of damage, and often does more damage than the wind even though few structures are fully submerged and get the full effect.
Tornadoes often form on the north and east side of a hurricane, too. As the hurricane moves inland, friction with the ground slows the wind greatly, and the Coriolis force drops quickly. So, air starts blowing in toward the center of the hurricane and starts to fill in the low pressure. But, this creates large wind shear between the ground and just a few 100 feet above the ground, where friction is not as much of a factor, and this wind shear, along with the unstable air around hurricanes, often produces favorable conditions for tornadoes.
Hurricane Danny in August 1985 came inland with 90 mph winds…not weak, but not a super strong hurricane. However, as it weakened and moved through the Southeast, it caused 34 tornadoes in Alabama alone, with 13 significant (F2+) tornadoes across the Southeast US. I recall JB Elliott, on NOAA Weather Radio that day, saying “if you see a threatening storm approaching, take shelter, because tornadoes are popping up sometimes faster than we can issue the warnings.” Some were killed.
Inland flooding is another big threat with a hurricane, especially if it moves slowly. Hurricane Harvey produced catastrophic flooding in Houston, TX in 2017, with some areas getting over 50″ of rainfall.
We will have a complete update on Hurricane Ida on this blog this evening, so stay tuned.
Dr. Tim Coleman
Coleman Knupp and Dice LLC