Dvorak Technique - Details of The Method

Details of The Method

Dvorak T-Number and Corresponding Intensity
T-Number 1-min Winds Category Min. Pressure
Atlantic NW Pacific
1.0 - 1.5 25 29 46 TD ---- ----
2.0 30 35 56 TD 1009 1000
2.5 35 40 65 TS 1005 997
3.0 45 52 83 TS 1000 991
3.5 55 63 102 TS 994 984
4.0 65 75 120 Cat 1 987 976
4.5 77 89 143 Cat 1–2 979 966
5.0 90 104 167 Cat 2–3 970 954
5.5 102 117 189 Cat 3 960 941
6.0 115 132 213 Cat 4 948 927
6.5 127 146 235 Cat 4 935 914
7.0 140 161 260 Cat 5 921 898
7.5 155 178 287 Cat 5 906 879
8.0 170 196 315 Cat 5 890 858

In a developing cyclone, the technique takes advantage of the fact that cyclones of similar intensity tend to have certain characteristic features, and as they strengthen, they tend to change in appearance in a predictable manner. The structure and organization of the tropical cyclone are tracked over 24 hours to determine if the storm has weakened, maintained its intensity, or strengthened. Various central cloud and banding features are compared with templates that show typical storm patterns and their associated intensity. If infrared satellite imagery is available for a cyclone with a visible eye pattern, then the technique utilizes the difference between the temperature of the warm eye and the surrounding cold cloud tops to determine intensity (colder cloud tops generally indicate a more intense storm). In each case a "T-number" and a Current Intensity (CI) value are assigned to the storm. These measurements range between 1 (minimum intensity) and 8 (maximum intensity). The T-number and CI value are the same except for weakening storms, in which case the CI is higher. For weakening systems, the CI is held as the tropical cyclone intensity for 12 hours, though research from the National Hurricane Center indicates that six hours is more reasonable. The table at right shows the approximate surface wind speed and sea level pressure that corresponds to a given T-number. The amount a tropical cyclone can change in strength per 24 hour period is limited to 2.5 T-numbers per day.

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