Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Observatory clear sky chart from 2005-11-05 to 2017-10-31.

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
43%
15837
21%
47286
14%
15793
50%
15793
26%
30532

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
22%
900
16%
2506
7%
895
30%
895
18%
1828
January16th to endNight
24%
832
17%
2539
9%
832
32%
832
18%
1791
February1st to 15thNight
21%
704
15%
2112
5%
701
30%
701
17%
1494
February16th to endNight
20%
631
15%
1893
6%
631
28%
631
17%
1355
March1st to 15thNight
25%
720
16%
2135
9%
720
31%
720
19%
1483
March16th to endNight
33%
735
15%
2079
7%
735
40%
735
21%
1462
April1st to 15thNight
36%
539
21%
1797
13%
539
48%
539
27%
1083
April16th to endNight
42%
540
21%
1707
14%
540
51%
540
25%
1106
May1st to 15thNight
40%
540
17%
1620
10%
540
47%
540
22%
1108
May16th to endNight
58%
561
19%
1684
15%
561
61%
561
28%
1071
June1st to 15thNight
64%
540
20%
1489
16%
540
67%
540
38%
878
June16th to endNight
69%
540
21%
1440
16%
540
72%
540
41%
841
July1st to 15thNight
64%
510
17%
1360
14%
510
66%
510
33%
875
July16th to endNight
74%
564
23%
1525
21%
531
77%
531
39%
878
August1st to 15thNight
74%
539
25%
1630
24%
536
76%
536
38%
913
August16th to endNight
77%
573
25%
1910
27%
573
78%
573
38%
1003
September1st to 15thNight
67%
621
27%
1881
25%
621
70%
621
35%
1096
September16th to endNight
58%
717
30%
1980
24%
717
61%
717
35%
1198
October1st to 15thNight
56%
720
32%
1980
23%
720
62%
720
38%
1159
October16th to endNight
41%
768
26%
2432
20%
768
51%
768
29%
1476
November1st to 15thNight
42%
700
28%
2276
21%
700
53%
700
31%
1303
November16th to endNight
35%
720
22%
2340
16%
720
44%
720
22%
1525
December1st to 15thNight
29%
720
15%
2340
10%
720
35%
720
17%
1645
December16th to endNight
24%
903
15%
2631
7%
903
31%
903
16%
1961

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
13%6%35%5%3445%7%39%
48447
Night
13%6%35%5%2445%7%41%
47466
Cloud CoverDay
13%6%35%5%3445%7%39%
48456
Night
13%6%35%42445%7%41%
47471
TransparencyDay
43%5%10%22%15%2
48145
Night
41%5%10%22%16%3
47286
SeeingDay
15%6%22%24%19%10%
5868
Night
16%6%26%29%18%2
15839

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%9%39%5%23445%32%
1432
16th to endDay
13%9%47%6%3435%5%35%
1681
February1st to 15thDay
14%9%48%5%24347%34%
1750
16th to endDay
14%10%37%5%345%47%31%
1580
March1st to 15thDay
22%7%25%5%24335%36%
1814
16th to endDay
15%9%45%5%35%45%7%32%
2154
April1st to 15thDay
14%7%25%5%24445%40%
2151
16th to endDay
15%7%26%5%34446%38%
2253
May1st to 15thDay
19%7%36%5%345%5%5%32%
2331
16th to endDay
9%5%27%6%46%6%6%9%35%
2431
June1st to 15thDay
8%4246%45%5%6%8%44%
2467
16th to endDay
7%4245%36%6%7%10%42%
2520
July1st to 15thDay
12%7%25%6%35%5%5%9%35%
2520
16th to endDay
6%4246%36%6%7%10%40%
2534
August1st to 15thDay
6%425%6%46%5%6%9%43%
2317
16th to endDay
7%424435%6%7%9%43%
2292
September1st to 15thDay
12%6%25%5%2445%7%44%
2065
16th to endDay
11%435%5%33446%47%
1980
October1st to 15thDay
12%5%25%424345%49%
1980
16th to endDay
15%5%25%323446%45%
1786
November1st to 15thDay
11%5%24423447%48%
1582
16th to endDay
18%8%34423345%41%
1620
December1st to 15thDay
23%9%46%423335%32%
1620
16th to endDay
18%12%48%433436%30%
1587
January1st to 15thNight
17%8%47%6%43345%32%
2506
16th to endNight
16%10%46%5%2445%5%33%
2539
February1st to 15thNight
19%8%46%5%24446%33%
2110
16th to endNight
16%9%37%6%345%5%5%32%
1893
March1st to 15thNight
16%9%46%424445%37%
2122
16th to endNight
16%8%36%5%2445%7%35%
2079
April1st to 15thNight
13%5%35%42445%8%44%
1797
16th to endNight
14%6%26%5%35%447%41%
1707
May1st to 15thNight
14%6%36%5%25%5%5%6%37%
1620
16th to endNight
8%425%6%35%6%7%8%41%
1682
June1st to 15thNight
5%335%42445%9%51%
1489
16th to endNight
5%2133346%7%11%49%
1440
July1st to 15thNight
8%425%5%3435%9%47%
1440
16th to endNight
6%3135%25%5%5%10%50%
1631
August1st to 15thNight
5%31443346%10%51%
1641
16th to endNight
43144345%7%12%50%
1910
September1st to 15thNight
12%31431445%8%49%
1881
16th to endNight
11%5%34423337%50%
1980
October1st to 15thNight
8%3245%23347%52%
1980
16th to endNight
15%6%24323336%47%
2432
November1st to 15thNight
12%5%14423346%50%
2276
16th to endNight
17%7%35%423346%40%
2340
December1st to 15thNight
20%9%37%5%23345%32%
2340
16th to endNight
22%10%36%5%34446%28%
2631

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
21%8%39%433345%32%
1432
16th to endDay
13%9%47%5%3435%6%35%
1681
February1st to 15thDay
15%9%48%5%23346%34%
1750
16th to endDay
14%9%47%6%25%5%5%7%31%
1580
March1st to 15thDay
23%7%25%5%24335%36%
1824
16th to endDay
15%9%445%45%45%7%33%
2153
April1st to 15thDay
14%7%25%5%24446%40%
2151
16th to endDay
16%6%35%6%24446%38%
2253
May1st to 15thDay
20%7%26%5%345%5%5%33%
2331
16th to endDay
10%5%26%6%37%5%6%8%36%
2431
June1st to 15thDay
9%4245%445%6%8%44%
2467
16th to endDay
7%424446%6%6%10%42%
2520
July1st to 15thDay
13%6%35%5%35%5%5%10%36%
2520
16th to endDay
6%4246%26%6%7%10%41%
2534
August1st to 15thDay
7%425%6%46%5%5%8%44%
2317
16th to endDay
7%4245%35%6%7%8%44%
2292
September1st to 15thDay
12%6%35%5%2445%7%43%
2065
16th to endDay
11%425%5%3345%6%48%
1980
October1st to 15thDay
12%5%24423445%49%
1980
16th to endDay
15%425%323446%45%
1786
November1st to 15thDay
12%5%2441345%7%48%
1582
16th to endDay
19%8%34324245%41%
1620
December1st to 15thDay
24%9%47%423335%32%
1620
16th to endDay
18%12%38%434436%30%
1587
January1st to 15thNight
17%9%47%7%33345%32%
2506
16th to endNight
16%10%45%5%2445%5%33%
2539
February1st to 15thNight
19%7%46%5%34345%33%
2103
16th to endNight
16%9%47%5%345%5%6%32%
1893
March1st to 15thNight
17%9%35%5%24345%37%
2135
16th to endNight
17%7%36%424447%35%
2079
April1st to 15thNight
13%6%25%32445%8%44%
1797
16th to endNight
14%5%25%434447%41%
1707
May1st to 15thNight
15%6%36%5%25%5%5%6%36%
1619
16th to endNight
8%425%6%35%6%6%9%41%
1682
June1st to 15thNight
5%325%4245%5%8%51%
1489
16th to endNight
6%2134247%7%11%49%
1440
July1st to 15thNight
8%425%5%24448%47%
1440
16th to endNight
6%313425%5%5%10%50%
1631
August1st to 15thNight
5%31433447%10%51%
1641
16th to endNight
43134245%7%11%51%
1910
September1st to 15thNight
12%31332345%8%49%
1881
16th to endNight
11%5%24422337%51%
1980
October1st to 15thNight
8%3245%33347%52%
1980
16th to endNight
16%5%25%422336%47%
2432
November1st to 15thNight
13%424423346%50%
2276
16th to endNight
17%8%35%5%23245%40%
2340
December1st to 15thNight
20%9%36%5%24346%32%
2340
16th to endNight
22%11%36%5%33446%29%
2631

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
55%39%19%11%
1433
16th to endDay
48%410%25%8%1
1681
February1st to 15thDay
50%310%24%9%1
1752
16th to endDay
51%5%12%21%9%
1580
March1st to 15thDay
51%310%20%12%1
1824
16th to endDay
50%6%10%22%8%
2154
April1st to 15thDay
43%5%9%23%16%
2151
16th to endDay
45%5%9%22%14%3
2253
May1st to 15thDay
49%7%10%19%11%
2331
16th to endDay
43%8%12%18%14%1
2435
June1st to 15thDay
35%7%11%24%18%2
2467
16th to endDay
34%8%13%21%19%3
2520
July1st to 15thDay
44%6%12%18%14%4
2371
16th to endDay
34%9%15%23%14%2
2377
August1st to 15thDay
36%7%11%23%19%2
2304
16th to endDay
34%7%12%21%18%4
2292
September1st to 15thDay
38%6%10%19%19%5%
2065
16th to endDay
37%48%22%23%2
1980
October1st to 15thDay
36%47%26%22%3
1980
16th to endDay
39%48%27%18%2
1786
November1st to 15thDay
36%38%25%19%7%
1582
16th to endDay
45%28%23%17%1
1620
December1st to 15thDay
55%39%20%9%1
1620
16th to endDay
55%411%18%7%1
1587
January1st to 15thNight
54%49%18%12%1
2506
16th to endNight
51%5%10%22%9%1
2539
February1st to 15thNight
52%410%23%7%1
2112
16th to endNight
52%5%12%19%9%
1893
March1st to 15thNight
49%49%21%12%2
2135
16th to endNight
48%6%14%21%8%1
2079
April1st to 15thNight
38%411%22%20%3
1797
16th to endNight
42%5%9%23%13%6%
1707
May1st to 15thNight
45%6%10%23%12%1
1620
16th to endNight
36%8%12%20%16%4
1684
June1st to 15thNight
28%7%12%26%20%4
1489
16th to endNight
26%7%17%21%20%6%
1440
July1st to 15thNight
36%6%10%23%18%5%
1360
16th to endNight
28%6%13%29%18%3
1525
August1st to 15thNight
27%7%14%26%20%4
1630
16th to endNight
24%7%14%28%20%4
1910
September1st to 15thNight
31%5%9%24%21%7%
1881
16th to endNight
33%46%24%25%5%
1980
October1st to 15thNight
31%49%24%25%5%
1980
16th to endNight
38%37%26%19%3
2432
November1st to 15thNight
35%29%22%23%6%
2276
16th to endNight
45%38%22%18%1
2340
December1st to 15thNight
54%310%20%9%1
2340
16th to endNight
56%410%17%8%1
2631

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
34%15%28%13%7%
90
16th to endDay
10%31%44%12%1
96
February1st to 15thDay
17%31%36%12%2
90
16th to endDay
19%16%44%14%4
81
March1st to 15thDay
27%14%33%21%2
114
16th to endDay
19%13%37%18%9%1
288
April1st to 15thDay
20%11%26%27%11%2
270
16th to endDay
19%8%30%20%15%5%
270
May1st to 15thDay
26%8%26%21%11%6%
288
16th to endDay
16%6%21%25%16%13%
331
June1st to 15thDay
9%417%33%23%11%
442
16th to endDay
9%5%24%29%21%9%
495
July1st to 15thDay
17%318%27%21%11%
475
16th to endDay
6%18%29%30%22%
394
August1st to 15thDay
12%112%21%23%28%
313
16th to endDay
10%11%26%32%18%
332
September1st to 15thDay
15%113%22%30%16%
315
16th to endDay
15%114%19%33%15%
314
October1st to 15thDay
13%422%31%18%9%
315
16th to endDay
16%9%34%25%12%1
189
November1st to 15thDay
11%8%26%26%17%8%
90
16th to endDay
32%12%26%23%5%
90
December1st to 15thDay
20%14%31%25%5%3
90
16th to endDay
33%13%42%9%1
96
January1st to 15thNight
22%13%36%22%4
900
16th to endNight
22%13%35%26%2
832
February1st to 15thNight
24%10%36%25%1
704
16th to endNight
21%12%40%23%1
631
March1st to 15thNight
21%11%34%26%5%
720
16th to endNight
19%8%33%28%8%1
735
April1st to 15thNight
15%8%36%27%11%
539
16th to endNight
16%5%30%35%11%1
540
May1st to 15thNight
17%5%28%31%16%
540
16th to endNight
11%218%37%23%7%
563
June1st to 15thNight
6%119%41%28%3
540
16th to endNight
5%116%41%32%2
540
July1st to 15thNight
11%12%35%35%4
510
16th to endNight
7%9%31%40%10%
564
August1st to 15thNight
7%8%35%41%6%
539
16th to endNight
5%8%34%46%5%
573
September1st to 15thNight
13%210%30%36%6%
621
16th to endNight
13%416%23%37%5%
717
October1st to 15thNight
11%421%30%27%5%
720
16th to endNight
19%5%29%30%14%1
768
November1st to 15thNight
14%7%30%26%19%1
700
16th to endNight
21%9%29%28%9%
720
December1st to 15thNight
26%8%29%26%8%
720
16th to endNight
29%10%30%24%5%
903
Page generated at 2017-11-01 17:23:35. Version ac04

Copyright 2017 A. Danko


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