Tropical Cyclone Alley

Tropical Revolving Storms (TSRs) are referred to as cyclones in the Indian Ocean and South Pacific, typhoons in the Western Pacific and hurricanes in the Western Atlantic and Eastern Pacific Oceans. Most TSRs on average fall significantly to a pressure of 950 mb. The following is a powerful and safe rule for detecting Tropical Revolving Storms using a barometer. If you try to detect them by sensing a significant barometric pressure fall near its eye wall you would probably have to get within tens of nautical miles of the centre as the surface low pressure area is relatively small. This is too close for mariners to allow them to make safe re-routing decisions.

The rule is based on the fact that the normal sea level barometric pressure in the tropics, in the absence of a TRS, does not deviate much from its seasonal average. For example, seasonal average pressure can be 30.00 inHg or 1015.92 mb. The seasonal average pressure can be found in published Routeing Charts and Sailing Directions.

The Rule:

A fall of 5 mb below the seasonal average barometric pressure for the region indicates that a Tropical Revolving Storm is present..

The relationship between wind speed and distance from centre is given below:

Wind speed (kts) Distance from Centre (nm)
22 - 27 200
34 - 40 100
52 - 60 50

Ships need to avoid passing within 50 nm of the centre of a TRS.

Early Warning Sign

The diurnal variation in atmospheric pressure (daily atmospheric tide effect) is strong in the tropics. However, the diurnal variation disappears about 12 hours prior to the onset of the abovementioned fall below the seasonal average barometric pressure referred to in the Rule. This is an excellent early warning of the development of a TRS. The diurnal variation in atmospheric pressure for the region can be found in a climate atlas or Sailing Directions.

Nautical Notes on Tropical Revolving Storms

© R. J. Ellis 2014 All Rights Reserved. Use of this publication for educational purposes is permitted provided that appropriate acknowledgement or credit is given.