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Hurricanes

Learning Objectives   


Hurricanes start as upper atmospheric disturbances,  troughs of low pressure coming off of Africa.  They end as centers of low pressure with winds in excess of 74 mph and an average size of 500 miles in diameter.  These storm systems are called Hurricanes in the Atlantic Ocean and the Eastern Pacific.  They are called typhoons in the Western Pacific (across the International dateline) and cyclones in the Indian Ocean.
 
 

Recipe for a Hurricane

Ocean water that is at least 79 degrees to a depth of 200 ft

High humidity and high temperature

Converging surface winds due to pressure wave or trough of low pressure


Winds aloft and at surface in relatively same direction



Formation of a hurricane:

In the tropics the ocean is warm and the air is warm.  Evaporation of water into the air condenses into clouds and thunderstorms.  If there is no disturbance in the upper atmosphere then only ordinary thunderstorms result and they quickly dissipate.  If, however, there is a trough of low pressure or a remnant center of low pressure then as the air rises and forms clouds it slowly begins to rotate.  Air moving into this counterclockwise rotation rises, forming clouds.  As more air moves in and more clouds form the mass increases which increases the rotation.  With increased rotation more air is pulled into the system and cloud mass increases.  Increased cloud mass increases rotation, increases air flow, increasing cloud mass.  Hurricanes are feed-back systems.
 

 

Image from: http://hurricanes.noaa.gov/pdf/hurricanebook.pdf

Tropical Depression: wind speed less than 39 mph.
Tropical Storm: wind speeds between 39 and 74 mph
Hurricane: wind speeds in excess of 74 mph.
Using the recipe above:

The ocean water must be 79 degrees to a depth of 200 ft.  If the water is cooler evaporation is not as efficient.  If the depth of warm water is less than 200 ft then cold water from below will kill the developing hurricane.

High humidity and high temperature.  Necessary for evaporation and cloud development.

Converging surface winds due to low pressure.  This allows for air to be pulled into center low.

Winds aloft and at surface must be in same direction.  If the winds aloft are blowing at an angle to surface winds the top will shear off of the system and the hurricane will break apart.


Structure of a hurricane:

Hurricanes are actually composed of bands of squall line thunderstorms closely wrapped around a low pressure center.  There are up and down drafts within the hurricane.
 

 

Image from: http://hurricanes.noaa.gov/pdf/hurricanebook.pdf

Eye: The center of the hurricane is called the eye, this is where the lowest pressures will be experienced.  In the eye there is upward flow along the outside but in the center of the eye air is descending producing very calm conditions.  In well developed eyes the Sun can be seen.  The average eye is approximately 25 miles in diameter.

 

Image from: http://hurricanes.noaa.gov/pdf/hurricanebook.pdf
 

Eye Wall: clouds along the outside of the eye in the center of the hurricane.  The highest wind speeds are experienced in the eye wall.  The tighter wrapped the eye is, the stronger the winds in the eye wall.  The photo below was taken in the eye of a hurricane.

 

Photo from: NOAA
 

Rain Bands: lines of thunderstorms that make up the hurricane.  The further from the eye the more widely spaced the rain bands become.


Experiencing a hurricane:

As the hurricane approaches the weather is actually quite nice.  This is due to descending bands of air on the outer edges of the storm. How many times have you heard storm chasers remark "it's hard to believe a hurricane is approaching because the weather is so clear?"

As the outer edge of the hurricane moves over your area you will feel the winds increase and rain begins.  Rain and wind occurs in discreet bands, between bands there is no rain and the wind subsides.

As the hurricane moves closer the wind and rain increases, the time interval between rain bands decreases until there seems to be no period when the rain and wind ceases.

The strongest winds will be as the eye wall approaches.

As the eye moves over your position the wind and rain stops and there is calm.  This length of time this calm lasts depends on the size of the eye and the speed of the hurricane.

Once the eye passes over, you will be in the highest winds again (the eye wall) but they will be blowing in the opposite direction.

As the hurricane moves on the rain and wind will steadily decrease.  The outer edges of the storm are characterized by descending air, it is usually very clear and calm after the storm passes.


Effects of a hurricane:

Wind: winds will be strongest in the eye wall.  Wind does not usually produce the most damage in a hurricane although it has been suggested that tornadoes embedded within the cloud mass can produce severe damage.

The Saffir-Simpson Scale is a 1-5 rating of hurricane strength.

Storm Surge: storm surge is the most destructive effect of a hurricane and usually responsible for the most deaths.
The low pressure in the eye causes the ocean surface to be pulled upward in a "bulge".  The bulge is over a large area and only rises a few feet.  As the bulge moves ashore however its effects are exaggerated by the sloping seabed and causes the sea level to rise quickly.  Storm surges can reach heights of 18 - 20 ft in strong hurricanes.  This means that the sea surface rises 18 - 20 ft.  In coastal areas this causes extreme flood hazards.

 

Image from: http://hurricanes.noaa.gov/pdf/hurricanebook.pdf
 


Movement of a hurricane:

Hurricanes move across the ocean very slowly, usually less than 20 mph.  Their direction is determined by global wind patterns, other atmospheric systems (high and low pressure systems), ocean currents and isobaric surfaces that produce divergence and convergence aloft.  Prediction of hurricane movement has greatly improved over the last ten years.


End of a hurricane:

Hurricanes dissipate when they move ashore or move into areas of cold sea water.

The heat from the air and sea in the Tropics is what fuels the hurricane.  The heat continually causes warm air to rise, adding to cloud mass and rotation. Once the warm air and warm water are removed the hurricane rains itself out and slowly dissipates.

Hurricanes will often move into the area south of Hampton Roads and dissipate but we will still receive large amount of rain because it acts as a low pressure cyclone bringing rain to our area.



 

Hurricane Names