Split Rock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Split Rock Observatory clear sky chart from 2005-11-05 to 2017-03-31.

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
27%
14232
14%
45399
8%
14203
35%
14203
17%
31832

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
13%
720
7%
2506
2%
716
19%
716
7%
1988
January16th to endNight
18%
768
12%
2592
5%
768
26%
768
11%
1938
February1st to 15thNight
15%
702
10%
2287
3%
701
24%
701
11%
1711
February16th to endNight
18%
631
15%
1976
5%
631
30%
631
14%
1426
March1st to 15thNight
17%
720
14%
2058
6%
720
26%
720
16%
1503
March16th to endNight
28%
768
20%
2064
9%
768
37%
768
23%
1384
April1st to 15thNight
20%
658
17%
1646
5%
658
27%
658
20%
1196
April16th to endNight
30%
591
18%
1581
7%
591
37%
591
24%
1094
May1st to 15thNight
35%
340
20%
1330
16%
340
47%
340
20%
821
May16th to endNight
34%
342
15%
1369
10%
342
48%
342
21%
813
June1st to 15thNight
31%
330
10%
1320
7%
330
42%
330
15%
885
June16th to endNight
43%
330
9%
1320
11%
330
48%
330
19%
820
July1st to 15thNight
48%
310
16%
1240
22%
310
54%
310
30%
659
July16th to endNight
52%
348
13%
1306
11%
326
58%
326
28%
726
August1st to 15thNight
49%
458
16%
1439
11%
456
54%
456
30%
871
August16th to endNight
49%
627
20%
1677
9%
627
55%
627
37%
1018
September1st to 15thNight
42%
660
18%
1650
7%
660
47%
660
30%
1108
September16th to endNight
45%
657
22%
1806
13%
657
51%
657
31%
1150
October1st to 15thNight
31%
660
17%
1980
11%
660
39%
660
19%
1377
October16th to endNight
20%
704
10%
2164
4%
704
28%
704
12%
1658
November1st to 15thNight
26%
700
19%
2360
13%
700
36%
700
17%
1625
November16th to endNight
19%
720
11%
2520
7%
720
26%
720
10%
1922
December1st to 15thNight
16%
720
6%
2520
2%
720
21%
720
8%
1993
December16th to endNight
11%
768
9%
2688
3%
768
18%
768
8%
2146

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
17%9%37%6%34446%31%
45695
Night
16%9%46%5%3445%6%33%
45569
Cloud CoverDay
17%9%36%6%34446%31%
45705
Night
16%9%46%5%3445%6%33%
45574
TransparencyDay
52%5%11%18%10%1
45383
Night
50%5%12%19%10%1
45399
SeeingDay
22%13%30%20%9%3
4764
Night
22%13%31%23%8%
14235

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
20%16%6%8%6%23336%22%
1432
16th to endDay
15%15%6%10%8%35%335%21%
1630
February1st to 15thDay
14%15%6%10%7%35%445%22%
1575
16th to endDay
18%15%47%6%24346%27%
1496
March1st to 15thDay
15%12%5%8%8%45%44327%
1891
16th to endDay
14%8%46%43334440%
2155
April1st to 15thDay
22%10%45%433345%32%
1972
16th to endDay
19%8%35%434346%37%
2060
May1st to 15thDay
17%7%37%4245%47%34%
2290
16th to endDay
14%5%36%6%3445%7%37%
2394
June1st to 15thDay
21%8%36%5%35%46%7%28%
2310
16th to endDay
15%6%36%5%35%5%6%8%32%
2310
July1st to 15thDay
13%5%246%36%46%9%36%
2310
16th to endDay
11%6%36%6%46%5%6%8%35%
2458
August1st to 15thDay
12%7%38%6%35%5%5%7%35%
2179
16th to endDay
14%5%25%6%445%5%7%36%
2162
September1st to 15thDay
16%7%25%5%33347%39%
1980
16th to endDay
16%6%345%33446%42%
1820
October1st to 15thDay
22%9%27%5%33347%31%
1650
16th to endDay
27%11%36%6%45%335%23%
1708
November1st to 15thDay
15%8%38%435%6%45%34%
1497
16th to endDay
23%12%5%7%5%34346%22%
1440
December1st to 15thDay
22%16%5%10%6%3443318%
1440
16th to endDay
22%17%5%8%5%33336%19%
1536
January1st to 15thNight
19%13%6%10%7%45%44419%
2506
16th to endNight
12%15%5%9%7%34446%26%
2592
February1st to 15thNight
13%14%6%8%6%35%5%5%6%23%
2285
16th to endNight
14%12%5%6%6%3445%6%29%
1976
March1st to 15thNight
17%12%36%6%34346%31%
2046
16th to endNight
14%7%35%5%3434442%
2064
April1st to 15thNight
19%9%45%5%23346%35%
1646
16th to endNight
16%9%25%433346%39%
1581
May1st to 15thNight
18%6%35%32245%7%39%
1330
16th to endNight
12%6%3443345%8%42%
1367
June1st to 15thNight
16%7%36%6%3435%8%33%
1320
16th to endNight
13%5%35%5%35%5%7%7%38%
1320
July1st to 15thNight
8%324435%447%49%
1320
16th to endNight
8%2244345%7%9%47%
1402
August1st to 15thNight
9%6%25%43346%7%45%
1449
16th to endNight
8%5%15%423446%52%
1677
September1st to 15thNight
14%5%145%2445%6%43%
1650
16th to endNight
17%424313345%48%
1806
October1st to 15thNight
21%7%35%5%23347%34%
1980
16th to endNight
24%11%35%6%34446%25%
2164
November1st to 15thNight
16%10%36%5%345%5%6%33%
2360
16th to endNight
23%11%46%5%25%445%24%
2520
December1st to 15thNight
21%16%48%5%24446%21%
2520
16th to endNight
22%14%6%8%7%3434420%
2688

Percentage Cloud Cover SE (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
20%16%5%8%6%23345%23%
1432
16th to endDay
14%15%6%11%7%4443422%
1630
February1st to 15thDay
14%15%6%10%7%35%445%22%
1575
16th to endDay
18%15%5%7%6%3334427%
1496
March1st to 15thDay
15%12%5%8%8%45%43327%
1902
16th to endDay
14%9%35%5%24435%41%
2154
April1st to 15thDay
22%10%35%434345%33%
1972
16th to endDay
19%9%34424445%37%
2060
May1st to 15thDay
17%7%46%5%245%47%34%
2290
16th to endDay
14%6%25%7%35%35%6%38%
2394
June1st to 15thDay
21%7%36%5%25%5%5%8%28%
2310
16th to endDay
15%6%36%6%35%5%6%7%33%
2310
July1st to 15thDay
12%6%35%6%36%5%6%8%36%
2310
16th to endDay
11%5%36%6%46%5%6%8%34%
2458
August1st to 15thDay
13%6%36%7%345%5%8%34%
2179
16th to endDay
14%5%25%6%45%5%5%7%36%
2162
September1st to 15thDay
16%6%25%5%2335%6%39%
1980
16th to endDay
17%6%25%433346%41%
1820
October1st to 15thDay
21%8%37%5%33346%31%
1650
16th to endDay
27%10%36%6%44335%24%
1708
November1st to 15thDay
15%8%47%5%35%6%5%5%34%
1497
16th to endDay
23%13%48%5%34346%22%
1440
December1st to 15thDay
22%16%5%9%6%3443318%
1440
16th to endDay
22%17%6%8%5%33335%19%
1536
January1st to 15thNight
19%13%6%10%7%34445%18%
2506
16th to endNight
13%15%5%9%6%3435%5%26%
2592
February1st to 15thNight
12%14%5%9%6%35%5%5%6%23%
2278
16th to endNight
14%12%47%6%3445%7%29%
1976
March1st to 15thNight
18%11%46%6%34336%31%
2058
16th to endNight
14%7%35%5%35%33442%
2064
April1st to 15thNight
19%9%45%423446%35%
1646
16th to endNight
16%8%25%5%33347%39%
1581
May1st to 15thNight
18%6%35%32345%6%39%
1330
16th to endNight
12%6%345%3445%7%42%
1367
June1st to 15thNight
16%7%45%6%3435%8%33%
1320
16th to endNight
13%425%5%25%5%7%7%38%
1320
July1st to 15thNight
8%324435%45%7%50%
1320
16th to endNight
8%324435%47%9%47%
1402
August1st to 15thNight
8%6%25%5%3346%7%45%
1449
16th to endNight
8%5%25%424348%50%
1677
September1st to 15thNight
15%5%1443445%6%43%
1650
16th to endNight
17%424323345%48%
1806
October1st to 15thNight
21%7%25%5%2435%6%34%
1980
16th to endNight
24%10%36%5%3445%6%25%
2164
November1st to 15thNight
15%9%46%4345%5%5%34%
2360
16th to endNight
22%12%46%5%35%45%5%24%
2520
December1st to 15thNight
21%16%5%7%6%34446%21%
2520
16th to endNight
23%14%5%9%6%2434420%
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
64%49%13%7%1
1433
16th to endDay
65%47%11%8%1
1630
February1st to 15thDay
64%410%14%5%
1577
16th to endDay
60%311%12%11%1
1496
March1st to 15thDay
61%410%15%6%1
1902
16th to endDay
46%39%19%15%5%
2155
April1st to 15thDay
53%47%17%13%2
1972
16th to endDay
48%38%18%17%3
2060
May1st to 15thDay
48%5%10%17%15%2
2290
16th to endDay
45%6%11%20%13%1
2398
June1st to 15thDay
53%7%11%18%9%
2310
16th to endDay
47%6%12%19%12%1
2310
July1st to 15thDay
40%6%13%27%10%
2161
16th to endDay
45%7%14%22%9%
2291
August1st to 15thDay
45%6%15%21%10%
2165
16th to endDay
44%6%15%23%9%
2162
September1st to 15thDay
44%412%25%11%
1980
16th to endDay
43%5%10%30%10%
1820
October1st to 15thDay
52%5%10%19%10%1
1650
16th to endDay
63%5%9%15%5%
1708
November1st to 15thDay
49%6%16%17%9%
1497
16th to endDay
62%5%10%12%6%1
1440
December1st to 15thDay
69%5%12%10%3
1440
16th to endDay
66%5%10%10%6%
1536
January1st to 15thNight
66%5%11%11%4
2506
16th to endNight
59%5%10%16%8%
2592
February1st to 15thNight
57%6%14%15%5%
2287
16th to endNight
52%5%11%18%10%
1976
March1st to 15thNight
54%411%19%9%
2058
16th to endNight
43%410%21%16%2
2064
April1st to 15thNight
49%38%20%15%2
1646
16th to endNight
45%48%20%18%2
1581
May1st to 15thNight
43%410%21%15%3
1330
16th to endNight
40%5%12%22%16%2
1369
June1st to 15thNight
48%6%13%18%11%
1320
16th to endNight
41%7%13%22%13%
1320
July1st to 15thNight
31%5%14%31%16%1
1240
16th to endNight
31%8%19%25%14%
1306
August1st to 15thNight
35%6%17%26%13%
1439
16th to endNight
31%5%16%32%12%
1677
September1st to 15thNight
39%6%14%28%10%
1650
16th to endNight
38%412%34%9%
1806
October1st to 15thNight
49%5%12%20%12%
1980
16th to endNight
58%6%13%15%6%
2164
November1st to 15thNight
49%5%13%17%11%1
2360
16th to endNight
59%6%11%16%4
2520
December1st to 15thNight
64%6%13%12%3
2520
16th to endNight
66%5%10%11%6%
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
33%26%18%15%6%
60
16th to endDay
45%12%31%8%2
80
February1st to 15thDay
34%27%33%4
90
16th to endDay
36%16%31%13%2
155
March1st to 15thDay
25%18%32%19%4
270
16th to endDay
24%16%34%13%8%1
286
April1st to 15thDay
26%22%24%16%9%
210
16th to endDay
22%16%28%17%10%4
210
May1st to 15thDay
21%14%31%18%7%6%
228
16th to endDay
15%13%37%19%12%1
267
June1st to 15thDay
25%11%30%21%9%1
255
16th to endDay
13%10%34%26%11%3
255
July1st to 15thDay
13%6%30%30%15%4
245
16th to endDay
14%8%37%22%12%5%
270
August1st to 15thDay
15%5%28%31%11%7%
253
16th to endDay
15%229%28%15%8%
268
September1st to 15thDay
18%7%25%20%15%11%
255
16th to endDay
19%9%28%22%11%7%
254
October1st to 15thDay
23%16%27%17%11%2
255
16th to endDay
33%16%29%17%2
263
November1st to 15thDay
19%9%37%22%9%1
151
16th to endDay
28%16%35%11%6%1
60
December1st to 15thDay
25%31%25%16%1
60
16th to endDay
40%26%31%1
64
January1st to 15thNight
28%18%31%15%4
720
16th to endNight
22%16%36%19%4
768
February1st to 15thNight
20%20%38%17%2
704
16th to endNight
20%19%38%17%3
631
March1st to 15thNight
26%16%34%20%2
720
16th to endNight
19%15%32%22%9%1
768
April1st to 15thNight
24%17%33%20%3
658
16th to endNight
20%16%29%23%9%
591
May1st to 15thNight
20%8%29%30%8%1
340
16th to endNight
14%7%38%27%11%
343
June1st to 15thNight
19%11%30%29%8%
330
16th to endNight
15%429%37%11%
330
July1st to 15thNight
7%5%30%37%18%
310
16th to endNight
8%231%42%14%
348
August1st to 15thNight
13%226%35%19%2
458
16th to endNight
10%430%32%19%2
627
September1st to 15thNight
16%6%29%26%18%2
660
16th to endNight
20%8%22%26%19%2
657
October1st to 15thNight
26%11%27%22%11%1
660
16th to endNight
30%13%30%19%6%
704
November1st to 15thNight
22%14%31%20%9%1
700
16th to endNight
32%16%26%17%7%
720
December1st to 15thNight
29%18%30%17%3
720
16th to endNight
32%14%34%15%2
768
Page generated at 2017-04-01 08:23:16. Version ac04

Copyright 2017 A. Danko


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