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Sea Height and Period
As a function of Windspeed, Duration and Fetch

Wind Speed Wind
Duration
6 hrs 12 hrs 18 hrs 25 hrs 35 hrs 45 hrs 55 hrs 70 hrs 80 hrs 90 hrs 100 hrs 120 hrs 140 hrs
22 kts

5.7@6
43

7.8@7.5
100
9.0@8
160
10@9
250
11@10
400
12@11
550
12@11.5
700
12@12
1000
12@12.5
1200
12@12.5
1400
12@13
1550
12@13
1950
12@13
2350
26 kts 7@6.6
48
10@8
110
12@9
170
13@10
280
14@11
410
15@12
600
16@13
800
16@13.5
1100
16@14
1350
17@14.5
1550
17.5@15
1850
17.5@15
2250
17.5@15.5
2600
30 kts 7.5@7.2
51
12@9
125
14@10
210
16@11
300
18@12
500
20@13
700
20@14
900
22@15
1200
22@16
1500
22@16
1750
23@16.5
2000
23@17
2500
23@17.5
3000
36 kts 11.6@8
60
16@10
140
19@11.5
235
22@13
360
25@14
540
27.5@15
800
29@16
1000
30@17.2
1400
30@18
1700
31@18.5
2000
31@19
2300
31@19.5
2900
31@20
3400
40 kts 14@8.8
64
19@11
150
23@12.5
260
26@14
400
29@15
590
32@16.2
880
34@17
1200
36@19
1500
37@19.5
1800
38@20
2200
39@21
2500
40@21
3100
41@22
3800
45 kts 16@9.3
70
23@12
170
27@13.5
285
31@15
425
35@16
630
39@18
950
41@18.5
1250
45@20
1600
45@21
2000
47@22
2300
49@22.5
2700
50@23
3600
50@24
4100
50 kts 19@10
75
27@12.5
180
31@14.5
300
37@16
450
43@17.5
700
46@19
1050
48@21
1350
54@22
1750
55@23
2100
58@23
2500
59@24
2900
60@25.5
3800
60@26.5
4250
55 kts 22.5@11
80
30@13
190
36@15
320
44@17
500
50@19
760
55@21
1150
59@22
1450
62@23
1900
65@24
2300
66@25
2600
69@26
3100
70@27
3900
70@28
4600
60 kts 25@11.5
83
35@14
200
42@16.5
350
50@18
510
56@20
800
67@22
1200
70@23.5
1500
75@25
2000
79@26
2450
80@28
2800
80@28
3250
82@30
4000
85@30
5000
65 kts 27.5@12
88
39@15
220
48@17
380
55@19
560
65@21
850
75@22
1250
80@25
1600
85@26.5
2100
90@28
2500
92@28.5
2950
95@30
3400
100@31
4200
100@33
5100
70 kts 30@13
91
43@16
235
55@18
395
62@20
600
71@22
880
82@25
1325
90@26
1600
98@29
2250
100@29.5
2600
104@30.5
3050
109@31
3600
115@32.5
4500
119@35
5600
75 kts 34@14
96
50@17
245
60@19
405
70@21
620
80@23
900
90@25.5
1400
99@27
1700
105@29
2300
110@31
2700
118@32
3150
120@33
3800
125@34
4800
130@36
6000
80 kts 37@14.5
100
54@17.5
255
65@20
425
72@22
640
85@23.5
975
100@26.5
1450
107@28
1800
119@30
2400
121@32
2850
133@33
3300
136@34
3900
140@35
4950
140@36.5
6100
85 kts 40@15
103
57@18
260
74@21
445
80@22
680
95@25
1000
109@27.5
1500
122@30
1900
133@32
2500
139@33.5
3000
140@35
3500
145@35.5
4050
155@37.5
5050
160@39.5
6500
90 kts 45@16
110
63@19
270
80@22
460
92@24
700
107@26.5
1100
120@29
1550
130@31.5
2000
140@33
2600
147@34.5
3100
155@36.5
3750
165@37
4250
170@40
5200
190@44
6800

These are theoretical values. Actual sea size and period can vary.
Blue cells are what is 'typically' experienced in a storm. The others are provided for informational purposes.

Instructions
If you want to try an alternative to using the wave models (a good idea) to estimate sea heights where the seas are still under the influence of a storms winds, use this table. Often the wave models have difficulty detecting and resolving seas in tight gradient/small fetch areas. This is particularly true in extratropical systems. Though NOAA produces a hurricane wave model product for the Southeast and Southwest US coasts, the rest of the globe remains unscanned.

To estimate significant sea height (highest 1/3 of all waves) and period within a storm, first obtain QuikSCAT imagery to confirm wind speeds and wind duration within a storm. (QuikSCAT imagery can be obtained from links provided off our maproom page under "Current Weather".) Capture this data for the life of the storm, or during it's peak. Then select the best combination of wind speed and wind duration (from the outer edges of the table) and navigate inward to the cell located at the intersection of those values. If the actual fetch area is less than the what is indicated in that cell, proceed one cell to the left (along the same row) until the actual fetch area best matches that found in a cell. Interpolate between cells as necessary.

Cells indicate significant sea height (in ft), significant sea period (in secs) with minimum fetch area (in nautical miles) listed underneath. The cell should be read as follows:

5.7@6
43

"5.7 ft @ 6 secs and requires 43 nautical miles of continuous fetch at the defined speed and duration."

Example: Assume you are looking to determine significant seas and peak period for 50 kt winds blowing for 25 hours. Using the table, we find such winds generate seas of 37 ft with a period of 16 secs, but require a fetch area of at least 450 nautical miles. If the fetch area you are considering is less that 450 nmiles (over the 25 hour timespan), then move one cell to the left. If the actual fetch area being considered is equal to 300 nmiles, then the seas generated were really 31 ft @ 14.5 secs. If not, then continue to the left until the desired fetch parameters are obtained. Interpolate between cells as-required to provide higher fidelity.

 

Other Considerations
Virtual Fetch creates both increases in Duration and Fetch. In such instances, move one or more cells to the right as-required.

Fully Developed Seas: For a given wind speed, as duration increases, at some point sea size ceases to increase and only period increases. And then even period goes steady state. When this happens the sea is considered to be "fully developed". That is, for a given wind speed, eventually a maximum sea size and period is reached. No matter how much larger the fetch area becomes, seas cannot increase in size or period without adding more wind velocity. A good example can be seen in the table on the 22 kt row in the 100-140 hr duration columns. Notice there is no increase in either sea size or period after 100 hrs. This sea is considered fully developed. Conversely, at higher wind velocities a 'fully developed sea' can never be reached, mainly because of the physical limitations imposed by our planet. That is, there is either not enough open ocean available to accommodate the fetch requirements or it is impossible for a storm to evolve such that it meets the windspeed and fetch requirements. For example, look at the 90 kt row in the 140 hr column. Even here seas are estimated at 190 ft @ 44 secs and still increasing in size and period. But to generate such a swell, a storm would have to stretch from New Zealand to California and produce 90 kts winds nonstop over the entire 6800 nmile fetch area for nearly 6 days - a meteorological impossibility.

Other Sources: The table above is not one of a kind. A simple query using you favorite search engine will likely link you other sources, all with differing data. To provide some contrast and breadth to the learning experience, we've provided another table below.

This one is organized a bit different. Its two axis's are Wind Speed (left) and Fetch Length (top). The internal cells identify significant sea height (in ft), period (in secs) with minimum time (in hrs) listed underneath. The cell should be read as follows:

5.7@6
43

"5.7 ft @ 6 secs and requires 43 hours of continuous fetch at the defined speed and length."

 

Wind Speed
Fetch Length (nmiles)
50
75
100
150
200
300
400
500
600
700
800
900
20 kts
8@4.5
13
8@5
14
8@5.5
17
8@6
22
8@6.5
28
9@7.5
38
9@7.5
46
9@8.5
56
-
-
-
-
25 kts
9@4.5
9
11@5.5
12
12@6
16
12@6.5
20
12@7.3
26
13@8
34
14@8.5
42
14@9.2
50
14@10
56
-
-
-
30 kts
10@4.5
8.5
14@6
12
15@6.3
14
16@7.2
19
18@8
24
18@8.8
32
18@9.4
38
19@10
46
20@10.5
52
20@11
58
-
-
35 kts
15@5.6
8
17@6.2
10
20@7
13
22@7.8
18
23@8.4
22
24@9.5
30
25@10
36
25@10.5
42
25@11
48
25@11.7
54
26@12.3
60
-
40 kts
18@6
7
21@6.5
10
23@7.2
12
25@8
16
28@9
20
30@10
28
31@10.5
34
32@11.4
40
33@12
46
34@12.5
51
34@13
56
35@13.5
61
45 kts
20@6.5
7
25@7
10
27@ 7.7
12
31@8.5
16
33@9.5
19
36@10.5
26
38@11.4
32
40@12
38
40@12.5
43
41@13
48
42@13.5
54
42@14
58
50 kts
23@6.9
6.5
27@7.5
9
30@8
11
35@9
15
38@9.5
18
43@11
25
45@11.8
30
46@12.5
36
49@13
41
50@13.8
46
50@14.3
51
50@14.8
56
55 kts
27@7.2
6.5
29@7.5
8.5
33 @ 8.2
10
40@9.4
14.5
43@10
18
48@11.3
24
53@12.3
29
55@13.2
35
55@13.8
39
58@14.3
44
60@14.9
49
60@15.4
53
60 kts
25@7.5
6
30@8
8
37@8.5
10
45@9.5
14
50@10.5
17
57@12.5
22
60@13
28
62@13.5
33
65@14
38
67@15
42
67@15.5
47
70@16
50

The big difference one notices about this table as compared to the first one is how much shorter the significant sea periods are. For example: 40 kt winds for 20 hrs covering 200 nmiles produce 28 ft seas @ 9 secs, where the first table indicates 23 ft seas @ 12.5 secs over 260 nmiles (18 hrs). The second table appears to error on the high side for seas but the low side for period. Based on lot's of real-time buoy reading experience, the first table appears to be closer to the truth. But herein lies the problem with the any of the tables, that none are completely right for all conditions. If anything, the Wavewatch III wavemodel appears to be the best simulation constructed to date, and that is why we rely on it the most (for estimating sea heights). Still ,the tables have their uses, especially in more extreme conditions where the models tend to fall apart most.

4702

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