Hurricane Quadrants

There is a forecast for the outer extent of a storm's hurricane- and tropical-storm force wind fields, or the projected distance from center of the outer extent of each of these specific concentric circles, remembering that wind speeds generally decrease with distance from center. The forecast therefore is not for the entire extent or size of the tropical cyclone, but only for the size - expressed as the radius or extent from center in all directions - of its hurricane- and tropical storm-force wind fields, which constitute its most destructive parts.

Although hurricanes are roughly symmetrical, they are not exactly so, and the forecast has to attempt to anticipate these local asymmetries in a particular storm's wind field and how they might be expected to change over time.

In order to take the asymmetries into account, for the purpose of the forecast, and for analytical purposes, a hurricane is divided into four quadrants.

Diagram of concentric circles divided into four quadrants, with arrows indicating counterclockwise wind flow

Adapted from E. Holweg, TAFB/TPC, "Mariner's Guide..."

These are the quadrants of its circular wind field. Because the storm is divided into four sections, the wind field forecast, and everything having to do with the size of hurricanes, is expressed in terms of the distance from center (radius) of the winds in each of the storm's four quadrants, rather than as the distance across through center (diameter).

Size forecasts are issued for three of the storm's most important wind field thresholds: its hurricane-force or 64 kt (74 mph or higher) winds (inner swath), its 50 kt (58 mph or higher) winds (middle swath), and tropical storm-force or 34 kt (39 mph or higher) winds (outer swath). The 50 kt winds are included because of their importance to marine interests, as mariners find this wind speed to be an important safety threshold. The wind radii will not normally be the same for all four quadrants; if they were, the storm's wind field would be perfectly symmetrical around its axis, and this would be a very rare circumstance.

All of this information together, together with the eye diameter and the radius of the storm's maximum winds (innermost circle), is said to constitute, very broadly, the structure of the storm's wind field.

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