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Analyzing Tropical Systems and How They Affect You
Table of Contents
Introduction ............................................................................................................................................................................................... 2
Tropical Storm Characteristics and Life Cycles .................................................................................................................................... 3
Analyzing Tropical Storms ...................................................................................................................................................................... 6
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Introduction
Low-pressure systems affect mariners in a wide variety of ways. They come in two varieties: extra-tropical and tropical. Extra-tropical systems are often found
north of the subtropical high-pressure systems in the Northern Hemisphere (south of the high-pressure systems in the Southern Hemisphere). Tropical systems
usually originate in the tropical regions but their most common locations are not their defining characteristics. The two varieties differ substantially in how they
are formed and develop, their size, speed, presence or absence of fronts and other factors. In addition, different requirements are necessary to create each type of
low-pressure system as well as what “ingredients” are necessary for their full development.
Here are some of the basic differences:
Tropicals
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Tropical storms (or tropical cyclones) are warm storms. They feed on warm air and energy from warm water. The warmer and moister the air, the more
the storm intensifies, assuming that upper atmosphere wind shear is not an inhibiting factor.
Tropicals generally are smaller than extra-tropical storms – only 100 nm to 400 nm wide. With severe tropical cyclones or hurricanes, the only hope for
the sailor may be to route around them. With large extra-tropicals, the size of the system may eliminate that option.
The strongest winds are closest to the center of the system. They may have a very distinct eye as they develop.
They do not have fronts associated with them.
For an animation of Hurricane Isabel, click here. Note: this is a large file, best viewed with a high-speed connection.
Extra-tropicals
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Extra-tropical systems are a mixture of warm and cold air. They intensify when warm, moist air mixes with cold, dry air. The greater the temperature
contrast, the stronger the storm.
As a result, there can be occasions when a warm, moist tropical storm recurves to the north and mixes with an extra-tropical system containing cold,
dry air. The added warm, moist air from the tropical system increases the temperature contrast in the new combined system and the whole system
acquires extra-tropical characteristics.
Extra-tropical systems are often 500 to 1,000 nm across.
The strongest winds are often located away from the center of the storm, especially as the extra-tropical system develops into a more advanced stage.
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Tropical Storm Characteristics and Life Cycles
ACTION
Determine if conditions
exist for the formation of
tropical low pressure
systems
EXPLANATION
For tropical low-pressure systems to fully develop, there must be:
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A deep layer of warm ocean water with temperatures over 80 degrees Fahrenheit. In colder temperatures, tropicals either
do not develop, or they begin to fall apart.
A heat source and water vapor at the system's base.
Converging surface winds. Winds will begin to swirl in a counter-clockwise direction in the Northern Hemisphere.
An unstable atmosphere. Air must be able to rise as it heats up at the surface, and fall when it cools higher up in the
atmosphere. The warm and humid atmosphere must go up to at least 15-20,000 feet.
No wind shear disrupting the vertical convective forces. Wind shear at higher altitudes would limit the development of
tropical systems by “ripping them apart” when they get high enough to encounter the wind shear. The presence of strong
wind currents, such as the jet stream, would serve to limit or stop the formation of a tropical system.
High pressure at 25-30,000 feet.
The presence of all of these factors serves to build huge thunderstorms and increase the intensity of the tropical system.
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ACTION
Look for conditions
associated with the initial
stage development of a
tropical low-pressure
system.
EXPLANATION
Tropical storms develop in stages: tropical low-pressure system, tropical depression, tropical storm (also called tropical cyclones in
some parts of the world), and hurricane or typhoon. “Super typhoons” can also develop in parts of the Pacific if their center
pressures fall below 920 mb.
Initial stages of a tropical low-pressure system include:
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Disturbed area with clusters of thunderstorms – In the Atlantic, as an example, the clusters could be along a tropical wave
that originates in northwest Africa or along an old cold front that can occur anywhere from Cape Hatteras to the Bahamas
into the Gulf of Mexico.
Associated upper level low – In the North Atlantic, when in conjunction with the thunderstorms, the upper level low could
take place anywhere west of 40 degrees W and from the Gulf Stream south.
Tropical low-pressure systems develop into tropical depressions when:
Look for conditions that
cause a tropical lowpressure system to develop
into a tropical depression
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Thunderstorms organize, and an area of surface low pressure forms.
Thunderstorms converge towards the center of lower pressure. In the Northern Hemisphere, the thunderstorms spiral in a
counter-clockwise direction. In the Southern Hemisphere, the thunderstorms spiral in a clockwise direction.
Sustained winds are less than 35 MPH.
When these conditions are met, the tropical low is classified as a tropical depression.
In order for the tropical depression to continue to develop and become a tropical storm:
Look for conditions that
cause a tropical depression
to develop into a tropical
storm
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The inflow of thunderstorms and warm humid air must increase.
The pressure must continue to fall, and sustained wind speeds must exceed 35 MPH.
At that point, in the North Atlantic, a tropical storm is named.
• Although naming protocols vary in other parts of the world, in the N. Atlantic and both the Eastern and Central North
Pacific, each storm is named alphabetically beginning with “A” for the first storm of the new calendar year.
• In the Western North Pacific, the alphabet is not restarted at the beginning of the new year. Rather, the alphabetical process
proceeds through “Z” and restarts at “A” without regard to calendar years.
• Some parts of the world refer to tropical storms as “tropical cyclones.”
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ACTION
Look for conditions that
cause a tropical storm to
develop into a hurricane
EXPLANATION
Tropical storms typically develop into a full scale hurricane when:
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Center pressure continues to drop with clouds building as warm, humid air continues to flow into the system.
An “eye” forms at the center of the low-pressure system (although an eye does not always form).
Sustained winds reach 74 MPH. Tropical systems then become classified as hurricanes in the North Atlantic and Eastern
Pacific.
Notes about a hurricane’s speed of travel:
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Over warm water tropical systems generally travel at relatively low speeds.
As they reach land or colder water, they can greatly increase the speed at which they travel while diminishing in overall
strength.
A tropical depression system can lose some or all of its strength if:
Determine if conditions
exist for tropical systems to
diminish in strength
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It hits land, since it loses its primary source of energy – the warm ocean. Also, the increased friction over the land surface
slows the inflow of energy into the convective system.
Dry air is introduced, instead of the moist air of the ocean, so fewer clouds are created.
Cooler air replaces the warmer air.
Strong wind shear tears the vertical structure of the storm apart.
When tropical storms and hurricanes move into colder regions:
Determine the storm's
behavior as it moves into a
colder region
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They lose a great deal of their concentrated strength.
They may also gain speed over the ground as the systems move forward.
o In the North Atlantic, as an example, wind speeds may diminish, but the systems can speed up as they pass south
of New England and recurve toward Europe.
o Hurricanes that had been traveling at 5 to 10 knots with over 75 to 150 knot winds may now have wind speeds
less than 60 knots but travel at 30 knots or more.
As a result, tropicals can adopt extra-tropical qualities.
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Analyzing Tropical Storms
ACTION
Identify a hurricane
on an IR satellite
image
EXPLANATION
From a satellite, hurricanes have a signature set of characteristics.
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In the Northern Hemisphere they spin in a counter-clockwise direction (reversed in the Southern Hemisphere).
There are feeder bands but no fronts. That is because tropical systems are comprised of all one type of air mass – warm,
moist air −- whereas fronts are the boundary areas between distinctly different cold and warm air masses.
Sometimes distinct eye may be visible; at other times it may be more difficult to discern.
This is a static IR satellite image of Hurricane Isabel. For an animation showing Hurricane Isabel, click here.
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ACTION
Use visible light
satellite images to
identify a tropical
storm hurricane
EXPLANATION
As in all visible light satellite images, the source of the light is the sun. If you see shading of gray in the cloud structure, it is a result
of different light levels, and not temperature or height variations other than those height levels that may create some degree of shading.
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Feeder bands or lower level clouds may be more prominent on visible light images than they are on IR images. The warmer
temperatures of the lower clouds tend to make lower level clouds look less prominent on IR images.
By toggling back and forth between visible light and IR images taken at the same time, you can attain a level of
understanding of both height (temperature) and presence of different types of cloud cover in different locations.
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ACTION
Use surface analysis
maps to identify a
tropical storm or
hurricane
EXPLANATION
Surface forecasts will begin to indicate the expectation of a tropical storm or hurricane as soon as the meteorologists know it will affect
that particular time frame.
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Early signs may include tropical waves that are becoming exaggerated, perhaps looking like they will close their circulation
as the isobars take increasingly sharp turns.
You may have 96 or 48 hours of forewarning. If the conditions are sufficiently odd, catching the meteorologists unaware,
you may have less. Late season hurricanes can have increased erratic behavior patterns leading to a reduced warning time.
Tropical storms and hurricanes can look relatively small on a large surface analysis. Don’t let their size confuse you
regarding their potential ferocity.
Here are the Surface Analysis maps that were valid at times before and after the corresponding satellite images were taken. The
Surface Analyses show how quickly the hurricane falls apart when it hits land.
For an animation showing a series of surface analysis maps, click here.
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ACTION
Use radar to “see” a
hurricane
EXPLANATION
Radar will show the rainfall that is associated with a hurricane.
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Although you may not be able to deduce what the wind is doing and which way it will shift, radar may be your only form of
visibility in heavy rains.
Even in less severe situations than hurricanes, radar may be the best way to guide your way through the heaviest downpours.
This is the NOAA radar image of the Washington DC area during Hurricane Isabel.
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ACTION
Determine the
oceanographic
effects of hurricanes
and tropical storms
EXPLANATION
Tropical storms and hurricanes often have exaggerated effects on oceanographic conditions:
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Significant storm activity can raise or drop water levels in bays or other enclosed areas. Storms of any type can push water in
front of them.
• Conversely, water can also be driven down out of a bay or sound by winds blowing the water seaward.
• Tides can become exaggerated. Times may be close to those listed on tide tables, but currents and heights may be completely
distorted.
• As the water moves into shallower areas, the waves can become taller and steeper.
• When at sea, a rapidly moving tropical system can have a quickly shifting wind pattern. As the wind direction changes
quickly, the steep breaking seas can come at you from a wild variety of directions.
• If the winds are strong enough, the seas can quickly build. If they are building and shifting quickly, the sea state can become
extremely confused, making safe navigation more a problem of dealing with the waves than it is dealing with the wind.
Here is a series of strip charts showing the tidal change due to storm surge, wind speeds / directions and pressure changes as Hurricane
Isabel approached and went through the Norfolk, VA area.
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ACTION
Routing Decisions Evasive Action
EXPLANATION
When confronted with a tropical storm or hurricane, the best course of action is avoidance. If avoidance is not possible:
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Steer away from the leading right hand quadrant (Northern Hemisphere, or leading left hand quadrant in the Southern
Hemisphere). In those quadrants, the speed of the system moving across the water is added to the speed of the wind in the
system, creating stronger winds and more confused seas.
o For example, in the Northern Hemisphere if the system is moving to the west at 15 knots and the winds around the
system are generally 80 knots, the winds in the leading right-hand quadrant will generally be 95 knots, while the
winds in the trailing left-hand quadrant will generally be 65 knots.
If you know the track the storm will take, take a heading that is perpendicular to the expected track, to put some distance
between you and the storm. While tropicals are generally comparatively small systems, their worst conditions can cover a
couple of hundred miles or more.
Here is a wind field illustration of Hurricane Isabel, showing the wind speeds for the various sectors of the system. Note how quickly
the surface friction over the land reduces the expected wind speeds over land:
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ACTION
EXPLANATION
Here is a list of some tropical storm sites with more information on surface pressure forecasts and analyses:
Forecast data
http://www.nhc.noaa.gov/
http://www.nhc.noaa.gov/index.shtml
http://www.hurricanehunters.com/
http://www.aoml.noaa.gov/hrd/
http://www.nhc.noaa.gov/aboutrsmc.shtml
http://www.wmo.int/pages/index_en.html
Additional
Resources
Further detailed information about hurricanes can either be acquired from “Weather For Sailors” or the “Mariner’s Guide to
Hurricanes,” from the National Weather Service available here as an Adobe Acrobat file.
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©2010 L. Roberts and B. Biewenga
(203) 389-4440
Last Modified: April 14, 2010
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