Up on the summit of Mount Washington, Observers regularly experience hurricane force winds as they go about their daily duties. In fact, in the winter time, the summit is blasted by 74+ mph winds nearly every other day. But for much of the world, winds this high are fairly uncommon, and when they strike the coast, as a tropical cyclone bringing lightning, torrential downpours, and potentially hail and deadly storm surge, the results can be devastating.

 

Hurricane Season begins June 1st, and lasts until November 30th, but the majority of tropical cyclones form between the months of August, September, and October. The figure below shows the distribution of hurricane and tropical storm occurrences from May through December. Note the sharp uptick beginning around the first of August, that peaks somewhere around the 10th of September before tapering off to end the season.

 

 

A tropical cyclone is a general term given to an organized, rotating system of clouds and thunderstorms that originates over tropical or subtropical waters. Essentially, it is a low pressure system of which low level winds are rotating counterclockwise in the northern hemisphere or clockwise in the southern hemisphere.

 

Tropical Cyclone Classification:

Tropical Depression – Max sustained winds 38 mph or less (33 kts)

Tropical Storm – Max sustained winds of 39 to 73 mph (34 to 63 kts)

Hurricane – Max sustained winds of 74 mph or greater (64 kts +). Western North Pacific, hurricanes are called typhoons, in Indian Ocean and South Pacific Ocean, called cyclones.

Major Hurricane – tropical cyclone max sustained winds of 111 mph (96 kts) or higher, corresponding to Category 3, 4, or 5 on the Saffir-Simpson Hurricane Wind Scale.

 

The scale of a hurricane can reach 5-6 miles high and 300 to 400 miles wide and occasionally even bigger. These systems typically move around 10 to 15 miles an hour but have been known to move as fast as 40 mph. Tropical cyclones can cause a great deal of damage to property and substantial loss of life. However, they are also responsible for transporting heat and energy from near the equator to the cooler northern and southern reaches of the globe.

 

Generally speaking, there is a wide “belt” of low pressure that spans the equator. Immediately above the equator, winds blow from the northeast towards the equator. Immediately below the equator, winds blow from the south east. The figure below uses arrows to depict the direction of airflow over the Earth, in particular with respect to the Equator, denoted as a red line.

 

 

Within this belt of low pressure, the air and ocean are heated. Warming air begins to rise, and the moisture in the rising air condenses to form clouds, which eventually grow into storms. Much of the time these storms run their course and dissipate, however, on occasion, large groups cluster together and create a large area of warm, moist, rapidly rising air, which is responsible for developing an area of low pressure at the surface.

 

What causes normally routine thunderstorms to cluster together and form a tropical cyclone? To be honest, there are a few “ingredients” necessary to fulfill the recipe for a tropical cyclone. Some conditions that are conducive to tropical cyclone development include a source of warm, moist air which comes from the tropical oceans, surface winds blowing from different directions, converging and causing large parcels of warm, moist air to rise and form storm clouds, low wind shear, which allows the clouds to build to great height, and distance from the equator to allow the mass to spin or twist.

 

The Coriolis force, which is caused by Earth’s rotation helps to spin the rising column of air. The low pressure at the center intensifies as a result of the warm, moist air spinning and accelerating upwards and outwards. Eventually, a cylindrical wall, or “eye” takes shape. Inside the eye, skies are relatively cloudless, and winds are typically very calm. At the wall, winds are typically their strongest. Nearly 6 miles in the air, the tops of the storm clouds are spun outwards as the storm moves, spreading a thick layer of high clouds over the surrounding region. The figure below demonstrates the formation of a tropical cyclone. Pretty impressive!