Hurricanes Blog 2 - How do hurricanes form?

This is the 2nd in a series of 5 blog posts on hurricanes. You can click here for the first one. This blog will concentrate on the atmospheric dynamics behind hurricane formation and intensification.

Most hurricanes begin as "tropical disturbances", or organized groups of thunderstorms in the tropics that last for at least 12-24 hours. To understand how these groups of thunderstorms organize into a tropical depression or tropical storm, we must first examine three neat atmospheric processes: latent heat, surface pressure, and wind balancing.

1. Latent heat

It takes energy to evaporate water. Think about when you get out of a warm swimming pool on a warm day. The water temperature may be 87 degrees, and the air temeprature may be 87 degrees, but if the wind is blowing when you get out of the pool, it feels cold to you until the water dries off your skin. Same when you get out of the shower. The energy used to evaporate the water is called latent heat, and it has to come from somewhere, so it comes from the cooling of the water on your skin. That energy is now in that water vapor though, so if it condenses back to liquid water, the energy will be released in the form of heat. This is called latent heat. Latent heat is released when water is condensed from water vapor into liquid water.

So, when a thunderstorm forms and air parcels rise through the atmosphere, they may initially cool, but once their dewpoint is reached and water starts condensing, especially in a moist tropical atmosphere, latent heat is released, warming the atmosphere. Often, the release of latent heat by normal thunderstorms in tropical regions may warm the upper levels of the atmosphere by 5 to 10 degrees F. In hurricanes, it can be more than 20 degrees F, as we will see below.

2. Surface pressure

The air pressure exerted by the atmosphere at the ground (or ocean) surface is basically equal to the weight of all air molecules above that point. That's why pressure decreases with height, and baseballs go a lot farther when hit by the Colorado Rockies at home than by the LA Dodgers.

The density of the air is equal to the mass (weight) of each cubic foot of air. Cold air is more dense than warm air, so when the air warms aloft, it lowers the air density, and therefore it lowers the weight of the air in each cubic foot. Reducing the weight of the air at any level above the surface lowers the surface pressure, as less weight is pushing down on the ground/ocean.

(Krishnamurti 2012). This is a cross-section (east is to the right, up is up) of temperature (compared to normal) through Hurricane Katrina in 2005. These temperature departures are in degrees C. Near the center of the storm, temperatures aloft are 25 degrees F above normal, and 15-20 degrees warmer than they are just 100 miles to the east or west. It is this very warm air aloft in a hurricane that causes the low pressure at the surface.

3. Wind balance

There are several forces that push air around in the atmosphere to produce wind. The three main ones involved in a tropical storm are the pressure gradient (air tends to flow from high to low pressure), the Coriolis force (air is accelerated toward the right in the Northern Hemisphere), and the centrifugal force (air gets thrown outward from the center of circulation).

So, if an organized area of thunderstorms in the tropics produces enough warm air aloft, surface pressures can drop, and a surface low can form. Air initially tries to flow toward the low pressure, but it is deflected to the right by the Coriolis force, as shown below.

Initially, the air flows directly toward the low pressure in the center. But, with time, the Coriolis force deflects the wind to the right. In a tropical storm, the wind still blows with a component toward the center, but it is deflected to the right enough to cause a counterclockwise rotation. In most non-tropical weather systems, the winds become completely in balance with the Coriolis force, known as geostrophic balance.

4. Bringing it all together - the feedback loop

If an area of low pressure forms due to the warming aloft produced by a tropical disturbance, and there is not too much wind shear (winds aloft to tilt the storms with height, causing the heat and associated surface low pressure to be spread out), wind will start blowing in toward the low pressure in a counterclockwise fashion, (see the diagram above). If the ocean water is warm enough (usually over about 80 F), the air converging into the low pressure area can cause more and bigger storms to form, br bringing in a larger volume of moist air from around the initial storms, and because when air converges near the ocean surface, it tends to rise (it can't go down into the water). These bigger storms then release more latent heat, further lowering the pressure at the surface, forming a tropical depression, and if the water is warm enough, eventually a tropical storm. This is the feedback loop getting going.

The bigger storms caused by the pooling of moisture and lifting due to convergence release more heat aloft and produce lower surface pressures. The lower surface pressures lead to a stronger pressure gradient, and therefore stronger winds. The stronger winds then lead to more convergence of moisture, more big storms, more heat release, and even lower pressures. Once the winds get strong (well into tropical storm range, at least 50 or 60 mph), waves in the ocean get bigger, and researchers suggest that warm sea spray gets blown into the air, providing an additional source of warmth aloft as it is carried upward in the storms around the center. If the water is warm enough and the feedback loop continues, a hurricane will be present.

(learnnc.org)

The low pressure in the center of a hurricane is always strongest at the surface, and decreases in intensity with height, because as you go up, there is less warm air above you to cause low pressure. Once a hurricane becomes strong enough and the low pressure at the surface gets deep enough, air starts sinking due to the extreme low pressure at the surface relative to the pressure aloft. This sinking air actually dries up some of the clouds in the middle, producing the eye of the hurricane. Also, in organized tropical storms and hurricanes, the thunderstorms tend to orient in spiral bands around the center, with areas of descending air between them, as shown above.

The picture below is of Hurricane Ivan in 2004, and was taken from the International Space Station at an oblique angle. Notice the spiral bands and the taller thunderstorms near the eye.

(NASA, boston.com)