St. Croix Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St. Croix Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
21%
14979
11%
46244
5%
14934
27%
14934
13%
34756

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 15thNight
11%
750
8%
2506
2%
746
18%
746
7%
2032
January16th to endNight
11%
772
8%
2520
2%
772
19%
772
7%
2075
February1st to 15thNight
11%
704
5%
2286
2%
701
16%
701
7%
1900
February16th to endNight
15%
631
7%
2036
2%
631
22%
631
8%
1602
March1st to 15thNight
15%
720
13%
1983
4%
720
24%
720
12%
1528
March16th to endNight
16%
768
13%
2112
4%
768
24%
768
14%
1589
April1st to 15thNight
16%
545
12%
1802
5%
545
25%
545
11%
1332
April16th to endNight
17%
540
13%
1631
6%
540
24%
540
15%
1189
May1st to 15thNight
21%
540
9%
1524
4%
540
26%
540
14%
1180
May16th to endNight
22%
561
10%
1496
3%
560
27%
560
17%
1115
June1st to 15thNight
22%
531
6%
1431
2%
531
25%
531
16%
1102
June16th to endNight
31%
360
9%
1260
4%
360
36%
360
15%
914
July1st to 15thNight
37%
409
9%
1259
5%
409
42%
409
25%
804
July16th to endNight
33%
565
9%
1465
7%
532
36%
532
20%
1066
August1st to 15thNight
39%
528
12%
1575
8%
524
42%
524
22%
1093
August16th to endNight
39%
525
15%
1633
10%
525
43%
525
24%
1049
September1st to 15thNight
30%
495
17%
1650
9%
495
41%
495
22%
1033
September16th to endNight
34%
643
22%
1835
13%
643
41%
643
26%
1188
October1st to 15thNight
27%
660
18%
1980
7%
660
36%
660
20%
1348
October16th to endNight
17%
704
13%
2112
4%
704
25%
704
14%
1550
November1st to 15thNight
22%
728
12%
2420
8%
728
28%
728
13%
1781
November16th to endNight
20%
750
9%
2520
5%
750
25%
750
9%
1949
December1st to 15thNight
11%
750
7%
2520
1%
750
18%
750
7%
2052
December16th to endNight
8%
800
6%
2688
1%
800
14%
800
4%
2285

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
20%11%47%6%34446%27%
47728
Night
19%10%47%6%34446%26%
46417
TransparencyDay
58%5%10%17%7%
47419
Night
57%5%11%17%6%
46244
SeeingDay
29%16%30%16%5%1
7375
Night
26%15%31%20%5%
14979

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 15thDay
15%17%6%11%9%45%5%5%5%13%
1432
16th to endDay
16%13%6%9%10%6%6%44416%
1704
February1st to 15thDay
16%14%6%11%11%5%5%435%14%
1575
16th to endDay
11%17%8%9%6%3435%6%21%
1435
March1st to 15thDay
16%16%5%9%9%3333425%
1964
16th to endDay
16%15%7%8%5%2333329%
2112
April1st to 15thDay
20%13%47%6%35%345%25%
2147
16th to endDay
23%10%35%5%24346%29%
2328
May1st to 15thDay
24%12%37%42445%6%24%
2428
16th to endDay
22%11%36%5%2435%7%27%
2618
June1st to 15thDay
28%10%36%5%23446%23%
2538
16th to endDay
22%8%46%6%3435%7%27%
2700
July1st to 15thDay
16%6%34435%46%9%34%
2610
16th to endDay
18%8%46%5%46%45%8%26%
2601
August1st to 15thDay
20%7%25%6%25%5%5%7%31%
2288
16th to endDay
15%5%35%423448%40%
2217
September1st to 15thDay
21%6%245%34447%37%
1980
16th to endDay
18%6%24322347%44%
1790
October1st to 15thDay
21%7%24424337%37%
1650
16th to endDay
21%12%47%5%35%5%45%25%
1760
November1st to 15thDay
22%15%46%5%3343425%
1435
16th to endDay
19%15%5%8%6%345%45%19%
1440
December1st to 15thDay
22%14%8%11%7%25%345%14%
1440
16th to endDay
23%16%8%12%8%2432213%
1536
January1st to 15thNight
15%12%7%13%9%4445%6%16%
2506
16th to endNight
17%12%5%11%10%5%7%446%15%
2520
February1st to 15thNight
17%14%6%10%10%5%5%45%5%14%
2285
16th to endNight
13%16%6%10%8%35%34422%
2036
March1st to 15thNight
18%11%5%6%7%45%345%27%
1971
16th to endNight
17%13%5%8%5%23435%29%
2112
April1st to 15thNight
21%12%46%5%2435%6%26%
1802
16th to endNight
22%10%345%335%4431%
1631
May1st to 15thNight
24%10%36%5%34445%26%
1524
16th to endNight
18%8%25%5%35%5%6%8%29%
1494
June1st to 15thNight
26%8%36%42445%6%26%
1431
16th to endNight
19%7%36%5%35%5%47%29%
1260
July1st to 15thNight
14%4245%25%5%7%10%37%
1335
16th to endNight
15%6%37%6%35%5%7%8%30%
1565
August1st to 15thNight
17%7%35%435%5%5%7%36%
1587
16th to endNight
14%6%25%5%35%347%41%
1633
September1st to 15thNight
15%6%245%235%5%7%40%
1650
16th to endNight
16%6%25%423446%44%
1835
October1st to 15thNight
19%6%25%5%43446%36%
1980
16th to endNight
24%9%35%434446%29%
2112
November1st to 15thNight
22%12%46%424446%27%
2420
16th to endNight
18%13%5%7%7%45%435%23%
2520
December1st to 15thNight
21%12%5%10%8%45%445%17%
2520
16th to endNight
23%15%9%10%7%3443414%
2688

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 15thDay
70%5%11%10%1
1433
16th to endDay
69%5%7%12%4
1704
February1st to 15thDay
71%5%8%10%3
1578
16th to endDay
63%411%14%5%
1435
March1st to 15thDay
64%38%14%8%1
1975
16th to endDay
59%28%18%9%
2112
April1st to 15thDay
62%46%14%11%1
2147
16th to endDay
55%49%17%11%1
2328
May1st to 15thDay
60%49%17%7%
2428
16th to endDay
57%6%10%18%6%1
2622
June1st to 15thDay
62%410%14%6%
2538
16th to endDay
56%6%11%16%8%
2700
July1st to 15thDay
45%7%16%20%10%
2458
16th to endDay
54%7%12%19%4
2438
August1st to 15thDay
52%5%14%17%9%
2275
16th to endDay
40%6%15%26%9%
2217
September1st to 15thDay
47%412%22%11%1
1980
16th to endDay
41%310%27%16%
1790
October1st to 15thDay
46%5%11%25%11%
1650
16th to endDay
59%5%10%19%5%
1760
November1st to 15thDay
62%411%14%7%
1435
16th to endDay
65%6%12%11%4
1440
December1st to 15thDay
72%39%12%1
1440
16th to endDay
76%46%8%4
1536
January1st to 15thNight
67%5%11%12%2
2506
16th to endNight
67%7%8%13%2
2520
February1st to 15thNight
70%410%10%3
2286
16th to endNight
65%412%12%5%
2036
March1st to 15thNight
59%410%17%7%
1983
16th to endNight
57%39%19%9%
2112
April1st to 15thNight
58%47%16%10%2
1802
16th to endNight
55%39%16%12%1
1631
May1st to 15thNight
59%5%10%17%5%
1524
16th to endNight
52%7%13%19%6%1
1496
June1st to 15thNight
58%413%16%6%
1431
16th to endNight
52%8%11%19%6%
1260
July1st to 15thNight
38%8%20%21%9%
1259
16th to endNight
48%8%17%18%6%
1465
August1st to 15thNight
47%5%15%23%7%
1575
16th to endNight
42%5%17%25%9%
1633
September1st to 15thNight
42%413%27%11%1
1650
16th to endNight
41%410%29%13%
1835
October1st to 15thNight
47%411%25%10%
1980
16th to endNight
55%410%20%8%
2112
November1st to 15thNight
56%413%17%6%
2420
16th to endNight
62%5%12%14%4
2520
December1st to 15thNight
68%5%10%13%2
2520
16th to endNight
73%5%7%10%2
2688

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 15thDay
27%20%32%18%
240
16th to endDay
27%24%32%11%3
256
February1st to 15thDay
26%20%36%13%2
236
16th to endDay
23%27%27%16%4
212
March1st to 15thDay
27%29%25%12%4
270
16th to endDay
31%25%25%9%5%1
288
April1st to 15thDay
32%24%30%9%1
270
16th to endDay
29%22%29%13%5%
270
May1st to 15thDay
36%18%24%14%41
384
16th to endDay
34%13%32%15%3
519
June1st to 15thDay
37%14%27%15%4
495
16th to endDay
29%11%33%17%42
495
July1st to 15thDay
22%7%35%24%8%1
475
16th to endDay
26%9%30%22%8%1
462
August1st to 15thDay
23%6%34%24%9%2
297
16th to endDay
21%7%33%20%13%4
268
September1st to 15thDay
23%12%33%21%9%
255
16th to endDay
22%14%28%22%7%4
247
October1st to 15thDay
27%8%34%15%9%4
225
16th to endDay
34%23%22%13%5%1
240
November1st to 15thDay
33%13%25%18%7%2
235
16th to endDay
31%20%26%13%6%1
240
December1st to 15thDay
35%21%28%12%1
240
16th to endDay
34%25%28%8%2
256
January1st to 15thNight
27%21%33%16%1
750
16th to endNight
28%22%32%14%2
772
February1st to 15thNight
30%19%34%14%1
704
16th to endNight
27%23%29%15%4
631
March1st to 15thNight
27%22%30%16%3
720
16th to endNight
29%17%30%17%5%
768
April1st to 15thNight
31%16%29%17%4
545
16th to endNight
28%17%32%17%4
540
May1st to 15thNight
31%14%30%18%5%
540
16th to endNight
23%13%38%22%2
561
June1st to 15thNight
30%11%32%21%3
531
16th to endNight
25%7%30%29%6%
360
July1st to 15thNight
15%438%32%8%
409
16th to endNight
18%6%36%30%9%
565
August1st to 15thNight
20%6%28%32%10%1
528
16th to endNight
17%7%30%33%11%
525
September1st to 15thNight
19%7%37%25%9%
495
16th to endNight
20%9%31%26%10%
643
October1st to 15thNight
24%12%31%20%8%1
660
16th to endNight
31%17%30%16%4
704
November1st to 15thNight
30%13%27%19%6%1
728
16th to endNight
27%21%25%20%5%
750
December1st to 15thNight
33%20%29%13%2
750
16th to endNight
33%22%28%12%2
800
Page generated at 2017-08-13 15:16:19. Version ac04

Copyright 2017 A. Danko


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