There’s an entire database full of statistics on natural disaster deaths and economic costs run by the Centre for Research on the Epidemiology of Disasters (CRED) stretching back to the 1900s.
Of this database, hurricanes (that’s “willy-willys” if you’re Australian) are responsible for thousands of deaths worldwide and billions of dollars worth of damage every year. The most disastrous one to date was the 1970 Bhola cyclone, which killed anywhere between 250,000 and 500,000 people in Bangladesh.
Thankfully, the total number of recorded deaths related to natural disasters has been in decline. Hurricane Patricia, which made landfall in Mexico very recently as the most powerful hurricane ever recorded with wind-speeds of up to 200mph, which resulted in 6 recorded deaths.
As increasing evidence points to climate change resulting in more intense hurricanes, and with hurricanes like Patricia likely becoming more regular in the future, it’s worth learning just how these forces of nature work.
To make a hurricane (given the cool name “cyclogenesis”), a number of conditions need to be met.
For starters, hurricanes form above waters with a surface temperature above 27C, which allows for deep convection, and makes the atmosphere unstable enough for hurricanes to form. Because the main energy source of a hurricane is from the release of latent heat (the energy absorbed or emitted by a substance changing state – like water vapour condensing) from ocean evaporation and cloud formation, hurricanes can only form and gather strength over warm oceans.
This is why they form in tropical waters, and between the months of May and September.
Hurricanes won’t form near the equator, where the Coriolis force is near zero. The Coriolis force (caused by the Earth’s constant easterly rotation) imparts rotation onto the air as it flows to the centre of the low pressure system. Most hurricanes form at 5 – 10 latitude above or below the equator, where the Coriolis force is sufficient to cause the air to spin. When combined with the relative vorticity of the air (defined as how much air wants to rotate) it helps air parcels develop into hurricanes.
Hurricanes won’t form underneath jet streams, where the vertical wind shear (or change of wind speed with height) is too high and “blows” apart the cyclone by moving the hurricane’s “warm core” and drying out the middle parts of the troposphere.
Jet Streams can help hurricanes to form, though. On its own, the massive amounts of latent heat released from convective cells aren’t enough to develop into a hurricane.
But when a trough (a strong southern bend) in the jet stream or in westerlies (winds which predominantly flow west to east) are the same size as the tropical storm, it can steer the system into an area where the air in the upper atmosphere diverges, which encourages air to flow into the system at the surface. This helps to spin up the cyclone by producing very high wind speeds at the surface, and allowing a low pressure system to deepen.
A distinctive feature of hurricanes is that they have what is known as “warm cores”, which are caused by 100-200 cumulonimbus clouds releasing latent heat into the centre, and allow the core to be 10-18C warmer than the surrounding environment. Without this warm core, there would be a reduced inflow of new heat and moisture in the system, which would lead to a reduced convection and latent heat release, and would likely prevent a full blown hurricane forming.
This enhancement of a storm system by convection is known as Conditional Instability of the Second Kind. A major driver of the CISK is the temperature differences between the ocean surface and the upper troposphere, which can go from 27C to -73C.
Air spirals into the surface low and rises adiabatically (where a fluid changes volume and temperature without losing heat energy to the surroundings). It then sinks outside of the storm (as shown in the picture below). This circulation of air is known as a Carnot cycle – and is the most efficient way of converting a given amount of heat energy into work.
(Credit: North Carolina University)
This is important to know, because the adiabatic warming of sinking air helps to enhance the high temperatures in a hurricane and help it maintain its energy. Without sinking air in the eye, the low pressure systems that lead to hurricane development couldn’t form.
Together with continuous inflows of heat and moisture at the surface, removal of air higher up, a small amount of friction at the sea surface, and latent heat releases from condensing water vapour, these help to maintain a hurricane’s intensity.
If one of these systems changes – like moving over land or cooler ocean waters then the storm will die. Increased friction from land surfaces can give a boost to surface winds coming together, which can cause tornadoes to form when combined with strong vertical wind shears in convective cells, but the removal of the energy source of warm ocean waters kills the system within a day or two of it making landfall.
Hurricanes develop from an initial low pressure area, which under the right conditions, can grow into tropical storms. As air spirals into the centre, it gains latent heat through evaporation at the ocean level. As it nears the centre, it rises in convection within the eye wall, or in spiral convective bands. As it cools and condenses into clouds, it releases the latent heat it gained from the ocean. This produces sinking air in lower levels and rising air at the top, which helps to draw in more heat, air and moisture at the surface, producing a hurricane.
(Bonus Round for you Hurricane Nerds: How the Hurricane Eye forms is a pretty interesting topic. You can read it here. We go into a maths-light explanation of the physics of a hurricane eye, and talk about how science still doesn’t know how the hurricane eye forms).