Description: Hurricane Bertha, 1st 2008 Atlantic basin hurricane (July 2008)

Images and text on Bertha, July 2008

Hurricanes are referred to by different labels, depending on where they occur. They are called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons if they occur in the Northwest Pacific Ocean, west of an imaginary line called the International Date Line. Near Australia and in the Indian Ocean, they are referred to as tropical cyclones.

Hurricanes are most common during the summer and early fall. In the Atlantic and the Northeast Pacific, for example, August and September are the peak hurricane months. Typhoons occur throughout the year in the Northwest Pacific but are most frequent in summer. In the North Indian Ocean, tropical cyclones strike in May and November. In the South Indian Ocean, the South Pacific Ocean, and off the coast of Australia, the hurricane season runs from December to March. Approximately 85 hurricanes, typhoons, and tropical cyclones occur in a year throughout the world. In the rest of this article, the term hurricane refers to all such storms.

This series of images chronicles the development and decay of the first hurricane of the 2008 Atlantic hurricane season, Hurricane Bertha. The images were taken by the Tropical Rainfall Measuring Mission (TRMM) satellite, which uses microwave and radar sensors to see both the swirling clouds and bands of rain that define the structure of the storm.

TRMM captured the first image, top left, on July 4, a day after Bertha had become a tropical storm. As shown in the large image, which covers a broader area, the storm was immediately west of the Canary Islands off the coast of West Africa. The cluster of thunderstorms had just started to take a circular shape. While the spots of red indicate there were regions of heavy rain, little organization was evident.

Bertha strengthened as it slowly moved west over the warm waters of the Atlantic Ocean. When TRMM captured the next image on July 7 at 17:03 UTC (1:03 p.m. EDT), top right, the storm was approaching its maximum intensity. Between July 7 and July 8, Bertha grew from a tropical storm with winds of 100 kilometers per hour (63 miles per hour or 55 knots) to a major hurricane (Category 3 storm) with winds of 195 km/hr (120 mph or 105 knots). At the time of the image, Bertha was intensifying from a Category 1 to a Category 3 hurricane, according to the National Hurricane Center. Not surprisingly, the storm was much more organized on July 7 than it was on July 4. A circular band of dark red defined the northern edge of the eyewall, where rain and winds were most intense. Other bands of heavy rain (yellow and green) circled the center of the storm.

The eye was larger and cloud-free when TRMM captured its next image of the storm on July 9, lower left. The eyewall was crisply defined in an arc of dark red, indicating extremely intense rain. The heavy rain in the eyewall is a good indication that a lot of heat was being released into the storm’s core, driving its circulation. This circulation was evident in the surrounding rain field, which curves around the storm in distinctive bands. When TRMM captured this image at 23:20 UTC (7:20 p.m. EDT) on July 9th, Bertha had regained Category 2 intensity with winds estimated at 170 km/hr (104 mph or 90 knots) by the National Hurricane Center.

The final image in the series was taken on July 13 at 16:20 UTC (12:20 p.m. EDT). At this stage, the storm was almost completely devoid of rain near its center. Instead, only the remains of the large outer eyewall (as evidenced by the ring of light to moderate rain shown in blue and green, respectively), wrapped around the eastern side of the storm. This is typically where the strongest winds reside, which at this time were estimated at 100 km/hr (63 mph or 55 knots) by the National Hurricane Center.

Hurricanes are a normal result of pre-existing conditions in weather patterns that occur over the ocean. The hurricane’s motion is in the counterclockwise direction (in the northern hemisphere) as a result of the Coriolis effect. As the storm system moves over warm ocean water, heat and water vapor rise up in the center of the storm. It is the warm air rising and cooler air sinking that maintains and strengthens the rotation of the storm. In order to gain enough strength to become a hurricane, the ocean water needs to be approximately 27 degree Celsius (80 degrees Fahrenheit). Areas most likely to have water temperatures this high are located in the Caribbean, also known as Hurricane Alley. When a hurricane approaches the East coast of the United States, global wind patterns above 30 degrees longitude help to push the storm northward along the east coast.

(information adapted from the NASA earth Observatory as well as World Book at NASA)

Images of Sea Surface Temperature (Celsius), July 2008