Hurricane Season: A Primer

The ingredients for a hurricane include a pre-existing weather disturbance, warm tropical oceans, moisture, and relatively light winds aloft.

How do hurricanes form? (Credit:
How do hurricanes form? (Credit:

If the right conditions persist long enough, they can combine to produce the violent winds, incredible waves, torrential rains, storm surges, and floods we associate with this phenomenon.

For a hurricane to form, the ocean water must be warmer than 81degrees. The heat and moisture from this warm water is ultimately the source of energy for hurricanes.

Hurricanes evolve through a life cycle of stages from birth to death. A tropical disturbance in time can grow to a more intense stage by attaining a sustained wind speed of 74 mph. Hurricanes can often live for a long period of time—as much as two to three weeks. They may initiate as a cluster of thunderstorms over the tropical ocean waters.

Once a disturbance has become a tropical depression, the amount of time it takes to achieve the next stage, tropical storm, can take as little as half a day to as much as a couple of days. It may not happen at all. The same may occur for the amount of time a tropical storm needs to intensify into a hurricane. Atmospheric and oceanic conditions play major roles in determining these events.

Hurricanes are huge, really enormous in size. The average hurricane is 200-400 miles across. Big ones will be 550-plus miles.

The relative strength of a hurricane is measured on a scale based on its greatest wind speed. This scale is named the Saffir-Simpson Scale for the men who invented it. This scale ranges from categories 1 to 5, with category 1 hurricanes being the weakest, and 5s the most intense. Hurricanes strong enough to be considered intense start at category 3 or with sustained winds exceeding 111 mph.

This photo is a composite of three days' views (Aug. 23, 24, and 25, 1992) of Hurricane Andrew as it slowly moved across south Florida from east to west. (Credit: NASA)
This photo is a composite of three days’ views (Aug. 23, 24, and 25, 1992) of Hurricane Andrew as it slowly moved across south Florida from east to west. (Credit: NASA)

There have only been two category 5 hurricanes that made landfall on the mainland U.S. (Florida Keys 1935 and Camille 1969). Recent intense hurricanes to make landfall on the United States were Opal in 1995 and Fran in 1996.

On average ten tropical storms develop over the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico each storm season. Many of these remain over the ocean. Six of these storms become hurricanes each year.

In an average three-year period, roughly five hurricanes strike the United States coastline, killing approximately 50 to 100 people anywhere from Texas to Maine. Of these, two are typically “major” or “intense” hurricanes, with winds greater than 110 mph.

Researchers continue to investigate possible interactions between hurricane frequency and El Niño. El Niño is a phenomenon where ocean surface temperatures become warmer than normal in the equatorial Pacific. In general, warm El Niño events are characterized by more tropical storms and hurricanes in the eastern Pacific and a decrease in the Atlantic, Gulf of Mexico, and the Caribbean Sea.

Under the right atmospheric conditions, hurricanes can sustain themselves for as long as a couple of weeks. Upon reaching cooler water or land, hurricanes rapidly lose intensity.

Hurricanes don’t occur suddenly, like in the movies. It takes days and weeks for hurricanes to build from tropical depression, to tropical storm, and finally to hurricane. There is plenty of warning before a hurricane hits.

Hurricanes normally travel slowly averaging 10-20 miles an hour, though on rare occasions they can move along as fast as 70 mph or creep along at two or three.

Hurricanes do not travel in straight lines. They take curving paths, often looping and backtracking, and zig-zagging.

Hurricanes can have tremendous amounts of rain or very little.

Hurricanes have an eye, the center of the storm. The eye can be from five to 120 miles across with most being 20-40. The eye can be eerily calm with clear skies, fooling people into thinking the storm is over. However, once the eye passes over, the other half of the storm is still left to endure, with sudden ferocious winds coming from the opposite direction.

Threat Map for Hurricane season 2013 (Source:
Threat Map for Hurricane season 2013 (Source:

The sustained winds of a hurricane (74 to over 190 mph) are destructive and cause severe damage. However, hurricanes often spawn numerous tornadoes which also cause much of the damage. Flying debris can be a bigger hazard than the wind itself.

Hurricanes are tropical in nature but are not restricted to tropical areas, the coast, or the summer. Some of the worst and most damaging hurricanes have hit the Carolinas and northward in September.

The majority of hurricanes occur during late August and September.

Note: This is Part 3 of a 4-part series on Hurricanes and the RVer

Part 1: Hurricane Primer for RVers

Part 2: Are You Prepared for This Year’s Hurricane Season?

Part 4: The One-Eyed Monster: Storm Surge & Saffir-Simpson Scale

Worth Pondering…

Two things Florida can teach the other 49 states: how to make a good margarita and how to deal with the aftermath of a hurricane.
—Tom Feeney

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NOAA Predicts Near-Normal 2012 Hurricane Season

Conditions in the atmosphere and the ocean favor a near-normal hurricane season in the Atlantic Basin this season, NOAA announced from Miami at its Atlantic Oceanographic and Meteorological Laboratory, and home to the Hurricane Research Division.

Hurricane Irene in 2011 was a reminder that tropical systems can affect the Northeast and of the threat of inland flooding. (Credit: NOAA)
Hurricane Irene in 2011 was a reminder that tropical systems can affect the Northeast and of the threat of inland flooding. (Credit: NOAA)

For the entire six-month season, which began June 1, NOAA’s Climate Prediction Center says there’s a 70 percent chance of nine to 15 named storms (with top winds of 39 mph or higher), of which four to eight will strengthen to a hurricane (with top winds of 74 mph or higher) and of those one to three will become major hurricanes (with top winds of 111 mph or higher, ranking Category 3, 4, or 5).

Based on the period 1981-2010, an average season produces 12 named storms with six hurricanes, including three major hurricanes.

“NOAA’s outlook predicts a less active season compared to recent years,” said NOAA Administrator Jane Lubchenco, Ph.D.

“But regardless of the outlook, it’s vital for anyone living or vacationing in hurricane-prone locations to be prepared. We have a stark reminder this year with the 20th anniversary of Hurricane Andrew.”

Andrew, the Category 5 hurricane that devastated South Florida on August 24, 1992, was the first storm in a late-starting season that produced only six named storms.

Favoring storm development in 2012: the continuation of the overall conditions associated with the Atlantic high-activity era that began in 1995, in addition to near-average sea surface temperatures across much of the tropical Atlantic Ocean and Caribbean Sea, known as the Main Development Region.

August 24, 2012 will be the 20th anniversary of Hurricane Andrew's devastating landfall in South Florida. (Credit: NOAA)
August 24, 2012 will be the 20th anniversary of Hurricane Andrew’s devastating landfall in South Florida. (Credit: NOAA)

Two factors now in place that can limit storm development, if they persist, are: strong wind shear, which is hostile to hurricane formation in the Main Development Region, and cooler sea surface temperatures in the far eastern Atlantic.

“Another potentially competing climate factor would be El Niño if it develops by late summer to early fall. In that case, conditions could be less conducive for hurricane formation and intensification during the peak months (August-October) of the season, possibly shifting the activity toward the lower end of the predicted range,” said Gerry Bell, Ph.D., lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center.

“NOAA’s improvement in monitoring and predicting hurricanes has been remarkable over the decades since Andrew, in large part because of our sustained commitment to research and better technology. But more work remains to unlock the secrets of hurricanes, especially in the area of rapid intensification and weakening of storms,” said Lubchenco.

“We’re stepping up to meet this challenge through our Hurricane Forecast Improvement Project, which has already demonstrated exciting early progress toward improving storm intensity forecasts.”

Lubchenco added that more accurate forecasts about a storm’s intensity at landfall and extending the forecast period beyond five days will help America become a more Weather-Ready Nation.

The seasonal outlook does not predict how many storms will hit land. Forecasts for individual storms and their impacts are provided by NOAA’s National Hurricane Center, which continuously monitors the tropics for storm development and tracking throughout the season using an array of tools including satellites, advance computer modeling, hurricane hunter aircraft, and land- and ocean-based observations sources such as radars and buoys.

“Every hurricane season we ask families, communities, and businesses to ensure they are prepared,” said Tim Manning, FEMA deputy administrator for protection and national preparedness.

“Being prepared includes developing a family emergency plan, putting an emergency kit together or updating your existing kit, keeping important papers and valuables in a safe place, and getting involved to ensure your community is ready.”

Hurricane Wilma in 2005. Hurricane Wilma was a category 5 hurricane and one of the costliest storms in history. Hurricane Wilma's highest sustained winds was at 185 mph. Lowest pressure point measured at 882 mbar. (Credit:
Hurricane Wilma in 2005. Hurricane Wilma was a category 5 hurricane and one of the costliest storms in history. Hurricane Wilma’s highest sustained winds was at 185 mph. Lowest pressure point measured at 882 mbar. (Credit:

NOAA’s outlook for the Eastern Pacific basin is for a near-normal hurricane season and the Central Pacific basin is expected to have a below-normal season. NOAA will issue an updated seasonal outlook for the Atlantic hurricane season in early August, just prior to the historical peak of the season.

NOAA’s mission is to understand and predict changes in the Earth’s environment, from the depths of the ocean to the surface of the sun, and to conserve and manage our coastal and marine resources.

Worth Pondering…

Anyone who says they’re not afraid at the time of a hurricane is either a fool or a liar, or a little bit of both.
—Anderson Cooper

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