. Temperatures above normal at sea level,
generally 26°C (79°F) or more in the lower layers.
l Moisture above normal at all levels.
l Westerlies greater than average, north of the
latitude of the seasonal maximum.
. Easterlies weaker than average in a wide zone.
l Easterlies decreasing with height.
. The latitude of the subtropical ridge is higher
than normal above the 500-hPa level.
l Evidence of a fracture in a trough aloft (at 200
. Long waves that are slowly progressive.
. A zone of heavy convection is present, indicating
the absence of the trade inversion.
l An increasing surface pressure gradient north
and west of the suspect area.
. The disturbance has relative motion toward the
upper ridge at or above 400 hPa.
Other Indications of Development
Other indications of development are sea swell and
tide observations. Swells associated with a tropical
storm will have a frequency less than average and an
amplitude greater than average. The normal swell
frequency is 8 per minute in the Atlantic and 14 per
minute in the Gulf of Mexico. Hurricane winds set up
swells with a frequency that can decrease to four per
minute. The period of the swell will also be much longer
Swells will approach the observer
approximately from the direction in which the storm is
located. The swell height is an indication of the storms
intensity, especially when the swells have not
encountered shallow water before reaching shore.
Abnormally high tides along broad coastlines and
along shores of partially enclosed water bodies are also
a good indication of storm development. The highest
tides will normally be found to the right of the storm
path, looking downstream.
Meteorological satellites have greatly aided in the
detection of tropical disturbances, especially during the
early life cycle.
It is important for meteorologists,
analysts, and forecasters to be able to effectively
interpret these pictures to extract their maximum
benefit. Through proper interpretation of satellite data,
determination of size, movement, extent of coverage,
and an approximation of surface wind speed and
direction can be made. Satellites have become the most
important method of detection of disturbances.
Aircraft reconnaissance also plays an important role
in tropical cyclone detection throughout the North
Atlantic, Caribbean Sea, and the Gulf of Mexico. This
has become even more important when used in
conjunction with the data from satellites. Areas of
suspicious cloud structure can be investigated by
aircraft whose crews include trained meteorologists.
These reconnaissance flights can provide on-station
data, either high- or low-level, that would not be
Another method of detecting tropical disturbances
is through the use of radar. Present radar ranges extend
about 300 miles and can scan an area approximately
300,000 square miles.
The Navy and NOAA also maintain several
different types of moored METOC buoys that report
various meteorological and oceanographic elements.
These stations send out measured data automatically or
The problems of formation, detection, and location
are in reality a single three-in-one problem. One is
dependent on the other.
In the case of satellite pictures, reconnaissance, and
radar detection, the location is fairly certain, barring
navigational errors. If detection is made through the
analysis, the exact location is more difficult to ascertain
in the incipient stage of the storm, especially when the
analysis is diffuse. The exact location should be decided
upon only after the most intensive study of the data. The
analyst should be prepared to revise his or her decision
in the face of developments which are more conclusive.
Only when easterlies extend vertically to 25,000
feet or more at the latitude of the vortex is intensification
possible. This most frequently occurs when the
subtropical ridge lies poleward of its normal position for
the season. The following characteristics are
indications of intensification:
l Movement is less than 13 knots.