HURRICANE's
While talking about "Hurricane"
Nicholas, the hurricane expert on The Weather Channel said: If the
hurricane strengthens, it will move more north. If it remains weak, it
will tend to go more to the west. I thought "steering currents" in the
atmosphere determine which way a hurricane moves. How can this be?
By Chris Cappella, USATODAY.com
Steering winds
hard to forecast
Winds from around 15,000 to 40,000
feet above the ocean are a key to whether a hurricane will hit the
USA.While forecasters have plenty of data from over land, where weather
balloons are launched twice a day to measure upper-air winds, and the
temperatures and air pressures that help determine where and how hard
the winds blow. But, hurricanes hitting the USA's East Coast come from
over the Atlantic Ocean, with weather balloon readings from only one
station - Bermuda. During the 1997 hurricane season, forecasters began
receiving upper air measurements from a new Gulfstream jet that is able to take wide-ranging
measurements from over the ocean around hurricanes. During 1997 and
1998 data from the jet was fed into computer models. Early indictions
are that this information is improving forecasts
How hurricanes first formed
From late spring to
early fall, weather conditions come together to form swirling tropical
cyclones over the Atlantic and Pacific oceans. These
develop from areas of low air pressure and thunderstorms over the warm
seawater. The thunderstorms give off heat
that warms the atmosphere. Air rises and
the barometric pressure falls even more. As
the air pressure drops, winds increase, and a tropical depression may
form. When steady winds reach 39 miles an
hour, the cyclone is called a tropical storm and it gets a name. If winds reach a speed of 74 miles an hour
inside the tropical cyclone, we call it a hurricane.
Saffir-Simpson
Scale
Hurricane intensities are ranked by their
sustained wind speed
using the Saffir-Simpson scale of potential damage.
Category 1 74-95 mph
wind Minimal Damage
Category
2 96-110
mph wind Moderate Damage
Category
3
111-130 mph wind Extensive Damage
Category
4
131-155 mph wind Extreme Damage
Category
5
Above 155 mph wind Catastrophic Damage
Hurricane: Long
before Europeans settled in the Caribbean Islands, some people believe
the island natives called their god of evil ”Huracan,” and this became
their word for tropical cyclones. The Spanish adopted it and the word
“hurricane” appeared in English around 1560.
Tropical cyclones often pick up
speed after making landfall.
This is not always the case,
however. Some tropical cyclones slow down over land,
especially at lower latitudes such as in Texas and Mexico.
In 1994 Tropical Storm Alberto moved over Georgia and Alabama
and slowed to a snail’s pace. As it
crawled over the southeastern United States, heavy rain led to
disastrous flooding.
One thing to
remember: friction with land is associated with wind speed and not the
speed of motion of the entire weather system. Tropical
cyclones can pick up speed after landfall, but the winds within the
system will usually weaken. A good example
of this is Hurricane Isabel. After making
landfall on the North Carolina coast, Isabel encountered faster
steering winds as it became assimilated into a mid-latitude weather
system. The system picked up considerable
speed over land while the winds within the system generally slowed.
A big factor in
whether or not the tropical cyclone moves faster or slower after
landfall is its latitude. Atlantic storms typically move with the
steering currents in the tropics from east to west.
If they turn northward, they are influenced more by the
mid-latitude westerly winds. The transition from being steered by the
“trade winds” to being steered by the ”westerlies” is what causes many
hurricanes and tropical storms to head toward the U.S. coast only to
turn away from land and back out to sea before they make landfall.
These strong west winds in the upper levels often (but not always) help
speed up the tropical cyclone. This increase in speed would occur
whether it made landfall or not, simply because the storm is moving
into a higher latitude. Again, this is a
general statement, and steering wind currents can vary in strength and
direction both in the tropics as well as in the mid-latitudes.
Hurricanes move
east to west and most
US weather systems move west to east.
Non-tropical
weather systems (low pressure areas, gales and storms) generally move
from west to east while tropical cyclones (tropical depressions,
tropical storms and hurricanes) generally move from east to west. The
answer to this is simple, they each move with the "steering" current
they are embedded in. The steering current basically pushes them along.
In non-tropical areas steering winds blow from west to east and hence
so to do weather systems. In tropical areas steering winds blow from
east to west and hence so to do tropical weather systems. The boundary
that separates steering from west to east from steering from east to
west is the subtropical ridge of high pressure, typically located near
30 degrees north latitude (farther south in winter and farther north in
summer). South of this ridge of high pressure we find "trade winds"
(blowing from east to west), north of this high pressure ridge we find
"westerlies" (blowing from west to east).
Frequently tropical cyclones will
move generally toward the west but
also move north of about 30 degrees north latitude. When this happens
we see the cyclone "re-curve" and begin moving toward the east in the
same direction as non-tropical weather systems do in those locations.
After all, a steering current will move tropical and non-tropical
weather systems about the same, if they are about the same depth.
Unfortunately the world is not so
simple and we have changing winds and
steering currents in the east-west direction as well as in the
north-south direction. It is for this reason we see rather wild and
highly varied tracks to tropical cyclones. Steering currents are
constantly changing in speed and direction due to continuously varying
atmospheric weather patterns.”
Although
it hasn’t happened yet, it is possible for hurricane to
strike the U.S. west coast.
There are
two factors that make it fairly unlikely. As
you may know, hurricanes tend to move toward the west after they
form. In the Atlantic, this brings them toward the
East Coast of the U.S. In the Pacific,
this movement brings them away from our shores. Another
reason is that hurricanes thrive on warm ocean water.
Along the East Coast of the United States, the Gulf
Stream provides plenty of water at temperatures of 80 degrees or more
during the summer and early fall. But
on the West Coast, ocean temperatures rarely get above the low
70s. That’s just too cool to allow a hurricane to
maintain its strength.
Charley (August, 2004) was
a Category four storm with sustained 145 mph winds. Yet, now, in the
aftermath, it is being said that the highest gust only touched 127, far
below what it was supposed to have been. Does this mean the storm
wasn't really as powerful as first thought?
Since surface
observations of a tropical cyclone at
sea are impossible, the winds are estimated. But it's usually a pretty
good guess. Hurricane Hunters drop instruments from airplanes above the
storm to measure mid and upper level winds and barometric pressure, and
formulas are used to derive the surface wind speed from those readings.
Specific barometric pressure ranges correspond to specific wind speed
ranges. Also, observations from satellite can be used to estimate the
wind, since the storms exhibit certain characteristics with particular
wind speeds.
Clocked wind speeds on land are
less than the wind speeds of a tropical
cyclone at sea for several reasons. First of all, the wind speed posted
for a tropical cyclone at sea represents the forecasted maximum
sustained wind over an open ocean ten meters (33 ft) above the surface.
In other words, this is the highest wind you would find in some part of
the center of the tropical storm or hurricane over water, not over
land. Winds below 33 feet, where most surface measurements are taken,
will not be as strong. Also, as a hurricane or tropical storm makes
landfall, the center of maximum wind rarely passes over an anemometer,
and even if it did, the wind-measuring instrument would probably be
destroyed by the wind, so you wouldn’t get a maximum reading anyway.
Portable wind instruments aren't substantial enough to carry into the
eye, which would be a difficult and foolhardy endeavor. Besides, the
hurricane loses strength as soon as it hits land, so only anemometers
located on the beaches would give a maximum reading. In that case, wind
gusts at the coast would be close to the maximum sustained winds of the
cyclone while it was at sea.
One other element to consider is
that the wind speeds of a hurricane
are often revised upward or downward after-the-fact for the record
books. Once meteorologists get a look at the damage, they are able to
get a pretty good idea of what sort of wind speeds actually occurred.
This is the same way they determine the F strength of tornadoes--by
surveying the damage. This is why Andrew was eventually raised to a
Category 5 hurricane--up from a Category 4--which it was classified as
for years.
If you were
standing on a
California beach and the waves were coming in toward you, then if you
were standing on a beach in China would the waves be going away from
you and heading for California? Or is there a place in the middle of
the ocean where the waves split heading toward all the coasts? What
makes the waves head towards the land?
All ocean
surface waves
(except Tsunamis) are caused by the wind. Ocean
waves move in the same direction the wind is blowing. But waves have a
very long lifetime, and usually only dissipate when they hit a coast.
So as waves move away from the wind area that generates them they are
called "swells." Swells can move thousands
of miles away from the area where the wind generated them.
For example, swells that routinely affect Southern California in
summer come from the south. When we track
them we find they have been formed by strong winter storms in the
Southern Hemisphere. That means that they
have traveled more than 3500 miles to the California coast, a trip that
takes about one week.
So at any
given time waves
are being generated locally by the local wind and swells are moving
across oceans from distant wind generation areas. Those swells move
along what are called "great-circle" tracks. Those
are the shortest distance between two points on a sphere, or in this
case, Earth. As swells move away from the
wind generation area that made them they "decay," or get smaller and
smaller as they spread out. In Southern
California when you see swells this summer from the south they could be
from a hurricane or tropical storm off the coast of Mexico or from a
winter storm in the Southern Hemisphere (remember when it is summer in
the northern hemisphere it is winter in the southern hemisphere). When you see swells of 8-10 feet in California
that have traveled 3500 miles or so, you can be sure that they were
very large (30-40 feet) waves in the area where winds generated them.
At any moment, waves and
swells at a location (such as China or California) are the result of
any wind that is pointing toward that location and generates swells.
So waves and swells in China may be generated
from completely different winds or storms than those in California.
Go to South East U.S. map