Tropical depressions, tropical storms and hurricanes are just a few of the weather phenomena our Bay News 9 Weather Experts explain in the following question-and-answer session.

• What is a tropical depression? What is a tropical storm?
• How are hurricanes ranked?
• What does an average hurricane season mean?
• What year was the most active? What year was the least active?
• Why have there been so many more storms recently?
• Do I need to open my windows when a hurricane approaches?
• Why are hurricanes named?
• How and why are names retired?
• Does El Niño affect hurricanes?
• What is UTC and Z time?

---

What is a Tropical Depression? What is a Tropical Storm?

Both are tropical cyclones. A tropical depression has winds up to 38 mph.

A tropical storm has wind speeds of 39 to 73 mph. Both have a center and a closed wind circulation.

• Return to top

---

How are hurricanes ranked?

A tropical cyclone becomes a hurricane when sustained winds reach 74 mph.

According to the Saffir-Simpson scale, a category 1 hurricane has winds of 74-95 mph.

A category 2 hurricane has winds of 96-110 mph.

A category 3 hurricane has winds of 111-130 mph.

A category 4 hurricane has winds of 131-155 mph.

A category 5 hurricane has winds of 156 mph and greater. Category 3, 4 and 5 hurricanes are also classified as major hurricanes or intense hurricanes.

• Return to top

---

What does an average hurricane season mean?

An average hurricane season brings 10.6 tropical storms. Six of those become hurricanes and two become major hurricanes, meaning category 3 or greater.

The average is based on data from 1968 to 2003. Officially, the Atlantic hurricane season is from June 1 to November 30, although storms can form outside this time period.

• Return to top

---

What year was the most active? What year was the least active?

Until recently, 1933 had the most named storms on record with 21. In 2005, that record was broken when the National Hurricane Center identified 28 storms. Since all of the traditional names had been used for 2005, the last six named storms were called "Alpha," "Beta," "Gamma," "Delta," "Epsilon," and "Zeta," the first six letters of the Greek alphabet.

1933 is now second, and 1995 is third with 19 tropical storms. 2005 also had the most hurricanes in one season with 15. The least number of tropical storms happened in 1983 when just four storms formed. In 1982, just two hurricanes formed, making it the year with the least amount of hurricanes since 1968.

• Return to top

---

Why have there been so many more storms recently?

The right atmospheric and oceanic conditions are in place for hurricanes to form. The sea-surface temperatures are above normal in the region of the Atlantic Ocean and the Caribbean where most hurricanes form.

Hurricanes need water temperatures of at least 80 degrees, which can typically be found in these areas. In fact, water temperatures in the mid to upper 80s have been occurring.

There is also low wind shear. When the wind shear is high, it can prevent storms from forming or prevent hurricanes from strengthening. The easterly winds from Africa are also favorable, allowing tropical storms and hurricanes to track great distances from east to west across the Atlantic.

Researchers have also identified cycles for hurricanes and say we are now in one of the decades in which above-normal activity should be expected.

• Return to top

---

Do I need to open my windows when a hurricane approaches?

That's a question we get every hurricane season. The answer is a resounding no. It is a myth that opening windows will help equalize pressure in your house when a hurricane approaches.

Your windows should be boarded up with plywood or shutters. Leaving your windows open will just bring a lot of rain into your house and flying debris could fly into your home, too. Don't waste time taping your windows either. It won't help prevent hurricane damage. It's just another myth.

• Return to top

---

Why are hurricanes named?

A tropical cyclone is given a name when it becomes a tropical storm. It's much easier to remember the name of a storm than try to track it by using latitude and longitude. It also helps prevent confusion when there is more than one tropical storm or hurricane occurring at the same time.

In 1953, the U.S. Weather Bureau began assigning women's names to tropical storms. In 1979, men's names were included on the list. The names are in alphabetical order, excluding the letters Q, U, X, Y and Z.

Today, the list includes names of English, Spanish and French origin because these languages are most commonly used by the countries in the Atlantic Basin. There are six lists of names. Each list is used in rotation every six years.

• Return to top

---

How and why are names retired?

A name is retired when the storm caused so many deaths or so much destruction that it would be confusing or insensitive to use the name again. The World Meteorological Organization is in charge of retiring hurricane names and choosing new names.

The headline-making hurricanes of 2004 -- Charley, Frances, Ivan and Jeanne -- have all been retired. They will be replaced by Colin, Fiona, Igor, and Julia when the list is used again this year.

• Return to top

---

Does El Niño affect hurricanes?

It can. In years with an El Niño, there are typically fewer tropical storms and hurricanes because vertical shear increases during El Niño years. The vertical shear can prevent tropical cyclones from forming and can prevent storms from intensifying.

El Niño is a warming of the equatorial Pacific Ocean waters, which usually occurs every three to seven years and affects weather patterns around the world.

La Niña is the opposite of El Niño and is characterized by cooler than normal ocean waters in the tropical Pacific. In years with La Niña, researchers have found that there is an increased number of hurricanes and an increased chance that the United States and Caribbean will experience hurricanes.

• Return to top

---

What is UTC and Z time?

If you're trying to read a weather map or satellite or radar picture, you'll probably come across these time stamps. UTC stands for Universal Time Coordinated. Z stands for Zulu time. They refer to the time at Prime Meridian, which is zero degrees longitude. The time is on a 24-hour clock.

To convert to our time zone, subtract five hours for Eastern Standard Time and subtract four hours for Eastern Daylight Time. For example, 12Z or 12 UTC is 7 a.m. in our time zone when we are in standard time. It would be 8 a.m. if we are in Daylight Saving Time. Using UTC or Z time allows anyone in the world to view the same maps and images and convert it to their own time zone.

• Return to top