Hurricanes Flashcards
Hurricane measuring
Saffir-Simpson Scale
what is a hurricane
A hurricane, formally known as a tropical cyclone, is a rapidly rotating storm system that has low atmospheric
pressure at its center. The word “tropical” refers to the parts of the Earth (the tropics) where hurricanes form and
spend most/all of their existence. “Cyclone” indicates that these storms are rotating
corilous effect
Note the right-hand, counter-
clockwise deflection of a moving
object in the Northern Hemisphere. In
the case of a tropical storm, each
moving air parcel in the storm is
deflected, causing the entire storm to
rotate and become a counter-
clockwise rotating cyclone in the
Northern hemisphere. The effect
works in reverse in the Southern
hemisphere, where storms rotate
clockwise. Importantly, the geometry
and physics of the Coriolis Effect
cause it to be smaller the closer a
location is to the Equator
Hurricane structure
The Coriolis Effect
deflection of the winds
flowing inward toward
the low-pressure
center creates the
characteristic spiral
structure of a
hurricane. Another
very distinctive feature
of a hurricane is the eye
- located at the very
center of the storm.
Recipe hurricane making
Warm Ocean Water - The minimum Sea Surface Temperature (SST) needed to support a hurricane is about 80 degrees Fahrenheit. At that temperature there is enough energy in the water to “fuel” the hurricane.
2. Pre-existing Disturbance - All hurricanes begin with some (perhaps seemingly insignificant) atmospheric disturbance. In fact, any of the many millions of thunderstorms the Earth experiences each year might, under the right conditions, grow to become a hurricane. Even a small area of minor horizontal wind shear can create the disturbance needed to”seed” a hurricane.
High Humidity - There needs to be sufficient water vapor to provide latent heat and water droplets upon
condensation. High humidity is usually present in the atmosphere above warm ocean water.
4. Low vertical wind shear - If wind direction or speed varies with elevation, the structure of the forming hurricane will
be distorted, and the fairly exacting mechanisms that sustain the hurricane are reduced in efficiency. For example,
the release of latent heat may be spread over larger areas, reducing the intensity of convection produced by the
released heat.
5. Away from the equator - Hurricanes depend on rotation to produce convergence as well as drawing in new moisture
and energy. The Coriolis Effect is too weak at Latitudes less than 5 degrees (North or South) to create rotation in a
tropical storm.
Tropical disturbance
A tropical disturbance is a thunderstorm or other atmospheric disruption in a tropical setting, particularly one that
occurs in an area favorable for storm growth. Disturbances are fairly minor weather events that most people would
consider entirely unexceptional. In fact, most disturbances do not develop into notable storms.
Tropical depression
A tropical disturbance may grow/develop into a tropical depression. While a tropical depression has a detectable low-
pressure center, it is irregularly shaped and does not show rotation. There are winds associated with this stage, but they
are in the modest 23-39 mph range.
Tropical storm
If a tropical depression develops further, it may become a tropical storm. A tropical storm has a distinct low-pressure
center and a fairly circular shape. The storm has existed long enough for the Coriolis Effect to initiate rotation. The
winds are significant, in the range of 40-73 mph. There is enough convergence, convection, and condensation to
generate significant amounts of precipitation. A tropical storm is a significant, potentially dangerous event; it is
officially named at this point.
Hurricane
A tropical storm that achieves wind speeds of 74 mph or more is classified as a hurricane. A hurricane has an eye,
eyewall, and spiral rain bands
Hurricane Movement
The movement of tropical storms is controlled by prevailing winds and other large scale atmospheric circulations. In
the north Atlantic, hurricanes tend to move westward and a bit north in the tropics. As they approach the mid-
latitudes, they start to move eastward. A typical travel speed of a hurricane system is 25 mph, but is also highly
variable. Storms that are either very-fast moving or very-slow moving present unique risks and demand particular
responses
There are several conditions that cause a tropical storm to dissipate
Tracking over cooler water - This removes the energy source that sustains the storm
Encountering significant wind shear (primarily vertical) - Wind shear distorts the structure of the storm and
reduces the efficiency of energy transfers within the storm.
Passing over land - crossing a land body separate the storm from its supply of energy and water vapor that
was provided by warm ocean water
The Saffir-Simpson Scale
The familiar “Category” numeric
scale for rating hurricanes is formally
referred to as the Saffir-Simpson
Scale. The Scale goes from
Category 1 to Category 5. Even a
Category 1 hurricane is a very serious
event. Category 5 storms are rare
and fearsome events. Only 34 have
ever been recorded in the
Atlantic/Caribbean/Gulf of Mexico
area, and only three of those hit the
USA as Category 5 storms.
The Saffir-Simpson generally assign
storms to categories according to
maximum wind speed, but the
atmospheric pressure at the storm
center is also considered. Wind
speeds and storm center pressure
are closely related.
