St. Thomas
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Thomas clear sky chart from 2005-11-06 to 2024-01-22.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
36%
23090
15%
74537
10%
23039
42%
23039
18%
52308

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
15%
758
11%
2516
5%
757
23%
757
9%
1994
January11th to 20thNight
15%
760
6%
2470
3%
760
19%
760
6%
2053
January21st to endNight
20%
800
10%
2599
4%
800
26%
800
11%
2035
February1st to 10thNight
16%
720
7%
2340
2%
720
23%
720
9%
1857
February11th to 20thNight
21%
701
11%
2273
4%
699
29%
699
11%
1717
February21st to endNight
19%
592
12%
1776
4%
592
28%
592
12%
1378
March1st to 10thNight
25%
720
17%
2026
9%
720
33%
720
19%
1458
March11th to 20thNight
27%
720
18%
1980
8%
720
35%
720
21%
1399
March21st to endNight
31%
792
18%
2178
9%
792
38%
792
20%
1586
April1st to 10thNight
29%
720
15%
1913
6%
720
36%
720
22%
1360
April11th to 20thNight
33%
573
19%
1637
9%
573
41%
573
24%
1111
April21st to endNight
35%
540
18%
1620
7%
540
44%
540
22%
1092
May1st to 10thNight
41%
540
16%
1620
11%
540
44%
540
22%
1103
May11th to 20thNight
45%
531
18%
1593
11%
531
53%
531
25%
1056
May21st to endNight
54%
397
17%
1560
18%
397
60%
397
28%
864
June1st to 10thNight
53%
360
15%
1440
19%
360
60%
360
24%
822
June11th to 20thNight
52%
360
18%
1314
18%
360
60%
360
27%
771
June21st to endNight
53%
360
20%
1260
20%
360
62%
360
28%
733
July1st to 10thNight
67%
350
20%
1225
25%
350
70%
350
35%
654
July11th to 20thNight
62%
522
18%
1553
17%
522
65%
522
33%
919
July21st to endNight
64%
591
17%
1672
14%
558
67%
558
33%
989
August1st to 10thNight
68%
537
19%
1600
16%
534
70%
534
34%
956
August11th to 20thNight
69%
534
23%
1598
18%
534
72%
534
35%
945
August21st to endNight
61%
612
23%
1800
15%
612
66%
612
36%
1067
September1st to 10thNight
56%
720
19%
1844
11%
720
60%
720
33%
1194
September11th to 20thNight
63%
720
27%
1980
21%
720
66%
720
38%
1177
September21st to endNight
50%
720
20%
1980
15%
720
53%
720
28%
1314
October1st to 10thNight
46%
720
19%
2124
17%
720
50%
720
26%
1375
October11th to 20thNight
32%
720
16%
2160
11%
720
40%
720
20%
1507
October21st to endNight
27%
792
16%
2398
10%
792
34%
792
15%
1767
November1st to 10thNight
38%
740
21%
2406
16%
740
46%
740
22%
1535
November11th to 20thNight
23%
760
14%
2613
8%
760
33%
760
14%
1820
November21st to endNight
25%
760
9%
2660
6%
760
29%
760
11%
2000
December1st to 10thNight
18%
760
8%
2660
4%
760
24%
760
9%
2062
December11th to 20thNight
16%
760
6%
3268
2%
759
23%
759
7%
2503
December21st to endNight
14%
828
4%
4881
2%
817
20%
817
4%
4135

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
15%9%47%6%35%5%6%8%32%
75176
Night
16%10%47%6%345%5%7%34%
74744
TransparencyDay
50%7%13%19%11%1
74894
Night
50%6%12%19%11%1
74562
SeeingDay
18%10%27%25%14%6%
13839
Night
20%8%29%29%13%2
23104

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
17%14%5%11%9%5%6%5%45%19%
1645
11th to 20thDay
16%18%8%13%8%45%446%16%
1710
21st to endDay
17%16%7%13%8%45%445%17%
1791
February1st to 10thDay
18%15%5%11%9%5%6%446%18%
1620
11th to 20thDay
17%12%7%12%8%5%6%445%21%
1573
21st to endDay
18%13%5%11%7%36%5%5%6%21%
1480
March1st to 10thDay
17%11%5%8%6%36%46%6%27%
1929
11th to 20thDay
14%12%5%10%7%3445%7%29%
1980
21st to endDay
20%11%48%6%44445%29%
2178
April1st to 10thDay
17%11%5%8%5%35%46%7%30%
2046
11th to 20thDay
17%10%46%6%345%5%8%32%
2287
21st to endDay
20%9%46%434446%34%
2340
May1st to 10thDay
17%7%37%6%35%5%6%8%33%
2340
11th to 20thDay
17%7%36%5%35%5%6%7%36%
2301
21st to endDay
11%5%25%5%35%6%6%8%42%
2716
June1st to 10thDay
13%5%25%6%35%5%7%8%41%
2520
11th to 20thDay
11%5%36%5%36%6%6%10%40%
2639
21st to endDay
11%5%25%5%345%8%9%43%
2700
July1st to 10thDay
9%424435%6%6%9%47%
2697
11th to 20thDay
11%5%25%5%45%6%7%10%40%
2350
21st to endDay
9%6%25%5%46%7%7%9%39%
2547
August1st to 10thDay
9%425%7%45%6%7%10%42%
2327
11th to 20thDay
9%6%26%6%5%7%6%7%10%37%
2303
21st to endDay
12%5%26%5%345%7%9%43%
2538
September1st to 10thDay
11%6%35%6%35%5%6%8%41%
2111
11th to 20thDay
10%5%25%5%45%5%6%8%45%
1980
21st to endDay
16%7%36%4345%7%7%37%
1980
October1st to 10thDay
16%7%36%6%245%5%8%36%
1836
11th to 20thDay
19%9%37%6%36%46%7%29%
1800
21st to endDay
24%10%48%6%35%5%5%6%24%
1958
November1st to 10thDay
14%9%36%7%36%6%5%9%34%
1672
11th to 20thDay
16%12%47%5%345%6%9%30%
1564
21st to endDay
20%15%6%7%6%46%5%5%6%20%
1520
December1st to 10thDay
17%16%6%9%8%35%45%7%20%
1520
11th to 20thDay
19%17%7%8%7%45%5%5%6%17%
1906
21st to endDay
29%18%7%9%6%35%45%5%10%
2772
January1st to 10thNight
19%13%7%11%7%45%5%45%21%
2516
11th to 20thNight
21%17%6%9%6%45%45%6%17%
2470
21st to endNight
15%13%7%12%7%35%45%6%22%
2599
February1st to 10thNight
18%15%6%8%7%445%5%6%21%
2340
11th to 20thNight
17%13%5%8%7%45%46%6%26%
2273
21st to endNight
19%12%48%7%46%6%5%5%24%
1776
March1st to 10thNight
16%8%49%8%445%5%8%31%
2016
11th to 20thNight
15%9%47%6%34447%35%
1979
21st to endNight
18%10%5%7%5%43445%34%
2178
April1st to 10thNight
18%9%45%5%35%45%7%35%
1913
11th to 20thNight
17%7%45%6%3445%7%37%
1637
21st to endNight
16%8%36%7%445%5%7%36%
1620
May1st to 10thNight
18%7%45%5%35%5%6%8%35%
1620
11th to 20thNight
13%6%36%5%46%5%5%7%40%
1592
21st to endNight
9%4344346%6%7%50%
1559
June1st to 10thNight
13%4245%25%6%6%8%45%
1440
11th to 20thNight
10%5%245%35%7%8%9%44%
1314
21st to endNight
11%424445%6%6%7%47%
1260
July1st to 10thNight
6%325%5%3435%8%56%
1260
11th to 20thNight
10%5%25%43446%9%50%
1605
21st to endNight
8%425%5%436%7%9%48%
1771
August1st to 10thNight
7%425%445%5%6%8%48%
1611
11th to 20thNight
7%425%5%35%5%6%9%50%
1598
21st to endNight
9%5%244345%6%10%49%
1800
September1st to 10thNight
9%7%26%6%3446%7%46%
1844
11th to 20thNight
10%425%4345%47%53%
1980
21st to endNight
16%5%35%5%35%5%5%6%42%
1980
October1st to 10thNight
14%6%35%5%345%5%7%43%
2124
11th to 20thNight
19%9%46%5%3445%6%36%
2160
21st to endNight
23%9%37%6%3436%6%29%
2398
November1st to 10thNight
14%8%35%5%35%46%7%40%
2406
11th to 20thNight
17%10%36%6%46%5%6%7%31%
2613
21st to endNight
22%12%47%6%35%5%5%7%25%
2660
December1st to 10thNight
20%13%5%9%6%35%5%5%7%22%
2660
11th to 20thNight
16%18%6%8%6%445%5%6%22%
3288
21st to endNight
28%18%6%9%7%3443414%
4884

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
68%6%11%11%4
1646
11th to 20thDay
70%7%10%9%5%1
1710
21st to endDay
70%6%8%10%6%1
1791
February1st to 10thDay
69%5%11%10%41
1620
11th to 20thDay
66%410%14%6%
1582
21st to endDay
63%5%11%14%6%1
1480
March1st to 10thDay
57%5%14%16%7%1
1932
11th to 20thDay
55%5%11%18%10%2
1980
21st to endDay
58%49%17%11%1
2178
April1st to 10thDay
54%411%19%11%1
2046
11th to 20thDay
50%6%11%19%13%1
2287
21st to endDay
51%5%12%18%12%2
2340
May1st to 10thDay
47%7%13%19%12%2
2340
11th to 20thDay
47%7%11%19%14%2
2305
21st to endDay
38%7%15%23%15%2
2716
June1st to 10thDay
40%7%12%25%13%2
2520
11th to 20thDay
39%7%13%22%16%2
2639
21st to endDay
36%8%14%25%16%1
2700
July1st to 10thDay
32%8%13%26%18%3
2623
11th to 20thDay
37%7%16%25%14%2
2280
21st to endDay
37%9%16%22%15%2
2405
August1st to 10thDay
35%9%16%28%12%1
2314
11th to 20thDay
40%10%16%23%11%1
2303
21st to endDay
38%7%15%24%14%2
2538
September1st to 10thDay
39%8%16%23%13%1
2111
11th to 20thDay
34%7%14%29%15%
1980
21st to endDay
43%7%14%23%11%1
1980
October1st to 10thDay
45%6%14%20%13%1
1836
11th to 20thDay
53%7%14%16%9%1
1800
21st to endDay
61%7%12%14%6%
1958
November1st to 10thDay
47%8%13%22%8%2
1672
11th to 20thDay
52%7%16%14%10%1
1564
21st to endDay
65%6%13%13%3
1520
December1st to 10thDay
64%7%12%13%3
1520
11th to 20thDay
67%8%13%8%3
1906
21st to endDay
76%8%10%5%1
2772
January1st to 10thNight
65%6%11%12%7%
2516
11th to 20thNight
70%7%10%8%5%
2470
21st to endNight
63%6%11%14%5%1
2599
February1st to 10thNight
64%7%11%13%6%
2340
11th to 20thNight
60%5%12%16%7%
2275
21st to endNight
60%6%9%17%6%1
1776
March1st to 10thNight
52%5%12%21%10%1
2026
11th to 20thNight
50%411%20%12%2
1980
21st to endNight
53%5%8%20%12%3
2178
April1st to 10thNight
49%5%11%19%14%2
1913
11th to 20thNight
46%410%20%17%2
1637
21st to endNight
46%5%10%19%16%3
1620
May1st to 10thNight
47%6%9%21%13%4
1620
11th to 20thNight
44%5%10%21%15%4
1593
21st to endNight
32%8%13%26%18%3
1560
June1st to 10thNight
35%8%10%28%15%3
1440
11th to 20thNight
33%10%13%23%19%2
1314
21st to endNight
35%6%12%26%19%1
1260
July1st to 10thNight
28%7%8%31%22%5%
1225
11th to 20thNight
32%7%11%29%17%4
1553
21st to endNight
32%7%15%23%19%3
1672
August1st to 10thNight
32%7%13%32%14%1
1600
11th to 20thNight
30%8%13%32%15%2
1598
21st to endNight
31%7%14%29%17%2
1800
September1st to 10thNight
37%6%13%27%15%2
1844
11th to 20thNight
31%5%12%32%18%1
1980
21st to endNight
41%6%13%25%13%1
1980
October1st to 10thNight
41%6%11%24%17%1
2124
11th to 20thNight
49%5%12%20%12%1
2160
21st to endNight
55%6%11%17%9%1
2398
November1st to 10thNight
41%6%13%25%14%1
2406
11th to 20thNight
52%6%13%19%9%1
2613
21st to endNight
59%6%17%14%41
2660
December1st to 10thNight
62%7%13%14%4
2660
11th to 20thNight
63%6%14%12%5%
3288
21st to endNight
75%6%11%6%2
4884

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
23%18%34%18%7%1
240
11th to 20thDay
24%14%42%13%5%2
270
21st to endDay
25%15%31%24%41
267
February1st to 10thDay
27%16%40%16%1
240
11th to 20thDay
23%20%38%18%1
240
21st to endDay
23%27%36%13%1
198
March1st to 10thDay
24%12%35%22%5%1
372
11th to 20thDay
18%16%35%25%6%1
420
21st to endDay
29%14%28%21%7%1
462
April1st to 10thDay
20%14%34%23%9%1
420
11th to 20thDay
23%17%27%18%11%4
412
21st to endDay
24%13%30%23%8%3
420
May1st to 10thDay
23%12%26%24%12%3
423
11th to 20thDay
18%8%26%26%16%5%
447
21st to endDay
13%5%24%32%18%9%
485
June1st to 10thDay
14%5%31%31%14%5%
450
11th to 20thDay
11%8%24%33%18%6%
569
21st to endDay
12%430%32%18%4
630
July1st to 10thDay
9%419%32%25%11%
621
11th to 20thDay
14%423%28%21%11%
453
21st to endDay
10%319%34%26%9%
488
August1st to 10thDay
9%318%30%25%15%
446
11th to 20thDay
10%416%30%26%14%
439
21st to endDay
12%418%28%26%11%
495
September1st to 10thDay
14%320%30%22%11%
450
11th to 20thDay
8%321%23%30%14%
450
21st to endDay
16%8%26%25%16%8%
450
October1st to 10thDay
20%8%24%21%20%7%
450
11th to 20thDay
19%15%34%23%8%2
450
21st to endDay
28%12%34%20%6%1
473
November1st to 10thDay
18%20%33%18%8%3
270
11th to 20thDay
20%22%30%19%6%3
201
21st to endDay
26%16%36%21%2
180
December1st to 10thDay
27%18%29%18%6%2
180
11th to 20thDay
26%22%33%14%4
180
21st to endDay
28%19%33%18%21
198
January1st to 10thNight
23%16%37%17%6%1
760
11th to 20thNight
30%13%31%22%4
760
21st to endNight
20%11%37%26%6%
800
February1st to 10thNight
26%11%37%23%3
720
11th to 20thNight
23%14%33%25%41
703
21st to endNight
24%12%38%22%3
592
March1st to 10thNight
20%10%36%28%7%
720
11th to 20thNight
20%10%38%24%7%1
720
21st to endNight
23%8%31%28%9%1
792
April1st to 10thNight
23%11%32%28%6%
720
11th to 20thNight
21%9%32%26%11%1
573
21st to endNight
20%6%31%33%9%
540
May1st to 10thNight
21%5%26%34%14%1
540
11th to 20thNight
15%229%39%13%2
531
21st to endNight
13%325%37%22%2
399
June1st to 10thNight
14%226%42%16%1
360
11th to 20thNight
12%327%42%17%
360
21st to endNight
12%324%43%18%1
360
July1st to 10thNight
5%119%43%28%4
350
11th to 20thNight
13%215%39%27%4
522
21st to endNight
10%117%45%25%2
591
August1st to 10thNight
9%113%40%31%5%
537
11th to 20thNight
7%213%39%35%4
534
21st to endNight
11%118%38%27%5%
612
September1st to 10thNight
12%419%34%26%6%
720
11th to 20thNight
11%317%35%30%5%
720
21st to endNight
17%422%33%18%6%
720
October1st to 10thNight
18%5%24%32%18%3
720
11th to 20thNight
23%11%27%26%12%1
720
21st to endNight
26%9%32%27%6%
792
November1st to 10thNight
17%11%27%29%14%1
740
11th to 20thNight
22%12%37%20%6%2
760
21st to endNight
28%11%30%24%7%
760
December1st to 10thNight
28%15%33%18%5%1
760
11th to 20thNight
24%16%36%19%4
760
21st to endNight
30%12%34%21%3
836
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