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 2018-02-14.

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%
15767
11%
48566
5%
15722
27%
15722
13%
36477

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
10%
825
7%
2716
2%
821
16%
821
7%
2205
January16th to endNight
12%
838
8%
2730
3%
838
19%
838
7%
2236
February1st to 15thNight
10%
760
6%
2466
2%
757
16%
757
7%
2045
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%
539
12%
1608
8%
535
42%
535
22%
1115
August16th to endNight
38%
573
16%
1782
10%
573
44%
573
24%
1146
September1st to 15thNight
31%
540
17%
1800
10%
540
41%
540
23%
1108
September16th to endNight
34%
702
22%
2002
13%
702
41%
702
25%
1303
October1st to 15thNight
28%
720
18%
2160
7%
720
36%
720
22%
1449
October16th to endNight
18%
768
15%
2304
5%
768
28%
768
15%
1665
November1st to 15thNight
22%
802
12%
2627
7%
802
27%
802
13%
1942
November16th to endNight
19%
825
10%
2730
4%
825
25%
825
10%
2112
December1st to 15thNight
11%
825
7%
2730
1%
825
18%
825
6%
2246
December16th to endNight
7%
880
6%
2912
1%
880
13%
880
4%
2484

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%
49510
Night
18%10%47%6%34446%27%
48739
TransparencyDay
57%5%10%17%7%
49201
Night
57%5%11%17%6%
48566
SeeingDay
29%17%30%16%5%1
7982
Night
26%15%31%20%5%
15767

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%
1552
16th to endDay
16%13%6%10%9%6%6%44416%
1846
February1st to 15thDay
17%14%6%11%10%5%4435%14%
1696
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%
2340
16th to endDay
15%5%35%42345%8%41%
2420
September1st to 15thDay
20%6%245%34447%37%
2160
16th to endDay
17%6%24422347%43%
1953
October1st to 15thDay
20%7%24424337%38%
1800
16th to endDay
20%11%47%5%35%445%28%
1920
November1st to 15thDay
21%15%46%5%33435%25%
1558
16th to endDay
19%15%5%8%6%345%45%20%
1560
December1st to 15thDay
23%15%8%11%6%25%345%14%
1560
16th to endDay
22%15%7%12%9%25%33313%
1664
January1st to 15thNight
15%12%6%13%9%4445%6%16%
2716
16th to endNight
17%12%5%10%10%5%6%5%45%16%
2730
February1st to 15thNight
18%13%6%11%10%5%5%45%5%14%
2465
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
16%7%35%435%5%5%7%35%
1620
16th to endNight
14%6%25%5%35%347%41%
1782
September1st to 15thNight
14%6%245%2345%7%42%
1800
16th to endNight
15%6%25%5%23446%42%
2002
October1st to 15thNight
18%5%25%5%43446%37%
2160
16th to endNight
23%8%35%434446%31%
2304
November1st to 15thNight
21%12%36%5%24446%27%
2627
16th to endNight
17%13%48%7%45%435%24%
2730
December1st to 15thNight
22%13%5%10%8%45%44416%
2730
16th to endNight
22%15%8%10%8%3443313%
2912

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
69%5%11%11%1
1553
16th to endDay
69%5%7%12%4
1846
February1st to 15thDay
71%48%10%4
1699
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%
2327
16th to endDay
40%6%15%26%10%
2420
September1st to 15thDay
45%413%23%11%1
2160
16th to endDay
42%411%25%16%
1953
October1st to 15thDay
45%5%11%26%11%
1800
16th to endDay
56%5%10%21%5%
1920
November1st to 15thDay
61%5%10%14%7%
1558
16th to endDay
64%6%12%12%4
1560
December1st to 15thDay
72%39%12%2
1560
16th to endDay
75%46%9%4
1664
January1st to 15thNight
66%5%11%12%3
2716
16th to endNight
67%6%8%13%2
2730
February1st to 15thNight
70%410%10%3
2466
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%
1608
16th to endNight
42%5%16%24%10%
1782
September1st to 15thNight
40%413%28%11%1
1800
16th to endNight
41%411%28%12%
2002
October1st to 15thNight
46%411%26%10%
2160
16th to endNight
53%411%21%9%
2304
November1st to 15thNight
56%5%13%17%6%
2627
16th to endNight
61%5%13%14%4
2730
December1st to 15thNight
69%5%10%12%2
2730
16th to endNight
74%5%6%10%3
2912

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
28%21%32%17%
285
16th to endDay
28%25%32%10%2
304
February1st to 15thDay
31%20%34%12%2
277
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%7%34%23%8%2
313
16th to endDay
20%8%33%20%12%3
332
September1st to 15thDay
21%13%34%21%8%
315
16th to endDay
22%12%29%22%9%3
304
October1st to 15thDay
24%10%35%15%9%4
270
16th to endDay
30%25%21%14%6%1
288
November1st to 15thDay
31%15%26%17%7%2
280
16th to endDay
30%21%28%12%5%1
285
December1st to 15thDay
38%21%26%11%1
285
16th to endDay
34%27%26%8%2
304
January1st to 15thNight
27%22%33%15%1
825
16th to endNight
28%22%32%14%2
838
February1st to 15thNight
31%19%34%14%1
760
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
539
16th to endNight
17%7%31%32%11%
573
September1st to 15thNight
18%9%36%25%9%
540
16th to endNight
20%9%30%27%10%
702
October1st to 15thNight
22%12%32%21%8%1
720
16th to endNight
29%17%30%16%5%
768
November1st to 15thNight
30%14%28%19%6%1
802
16th to endNight
27%20%26%19%4
825
December1st to 15thNight
35%19%28%13%2
825
16th to endNight
33%22%28%12%2
880
Page generated at 2018-02-15 11:58:39. Version ac04

Copyright 2018 A. Danko


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