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 2017-04-07.

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%
14682
20%
45437
11%
14646
47%
14646
24%
28838

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
24%
719
21%
2503
9%
716
41%
716
18%
1565
January16th to endNight
20%
768
22%
2496
7%
768
38%
768
21%
1572
February1st to 15thNight
18%
704
18%
2269
5%
701
33%
701
19%
1436
February16th to endNight
19%
631
25%
1893
8%
631
40%
631
22%
1195
March1st to 15thNight
25%
720
20%
2073
8%
720
40%
720
25%
1322
March16th to endNight
26%
768
21%
2043
9%
768
36%
768
27%
1367
April1st to 15thNight
25%
686
19%
1716
5%
686
33%
686
27%
1158
April16th to endNight
38%
660
19%
1650
7%
660
43%
660
31%
1091
May1st to 15thNight
35%
489
17%
1479
8%
489
42%
489
21%
1016
May16th to endNight
50%
342
15%
1369
15%
342
55%
342
20%
836
June1st to 15thNight
52%
330
17%
1320
18%
330
60%
330
21%
807
June16th to endNight
59%
330
16%
1320
20%
330
62%
330
25%
770
July1st to 15thNight
58%
310
13%
1240
16%
310
60%
310
19%
785
July16th to endNight
65%
410
14%
1363
13%
383
69%
383
27%
810
August1st to 15thNight
63%
526
18%
1506
12%
523
65%
523
32%
953
August16th to endNight
62%
700
17%
1750
9%
700
62%
700
33%
1149
September1st to 15thNight
57%
660
26%
1650
14%
660
61%
660
33%
1087
September16th to endNight
54%
657
24%
1782
18%
657
58%
657
31%
1155
October1st to 15thNight
44%
660
22%
1980
17%
660
52%
660
26%
1251
October16th to endNight
36%
704
26%
2126
15%
704
51%
704
30%
1270
November1st to 15thNight
35%
700
23%
2277
13%
700
49%
700
25%
1390
November16th to endNight
30%
720
21%
2424
11%
720
44%
720
21%
1511
December1st to 15thNight
25%
720
17%
2520
7%
720
39%
720
17%
1636
December16th to endNight
22%
768
21%
2688
7%
768
40%
768
18%
1706

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%5%2443445%7%42%
45798
Night
15%5%2442445%7%43%
45614
Cloud CoverDay
13%5%2443445%7%42%
45808
Night
15%5%2442445%7%44%
45619
TransparencyDay
39%6%12%22%16%2
45493
Night
40%5%11%22%16%2
45437
SeeingDay
17%8%25%25%16%6%
5094
Night
17%9%30%29%12%1
14684

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
14%6%245%34446%43%
1434
16th to endDay
14%7%24426%246%44%
1728
February1st to 15thDay
15%6%243345%5%5%42%
1592
16th to endDay
13%7%35%5%2346%5%41%
1580
March1st to 15thDay
15%425%5%25%35%6%43%
1873
16th to endDay
18%5%24435%445%42%
2181
April1st to 15thDay
16%5%24434446%43%
2048
16th to endDay
14%42432445%6%46%
1980
May1st to 15thDay
15%5%35%5%25%5%6%7%37%
2151
16th to endDay
13%424435%5%7%8%41%
2394
June1st to 15thDay
11%5%24435%6%7%10%37%
2310
16th to endDay
10%4245%35%5%6%9%42%
2310
July1st to 15thDay
8%425%5%35%5%6%10%40%
2310
16th to endDay
5%215%6%48%5%8%11%38%
2393
August1st to 15thDay
9%5%25%6%345%7%8%40%
2101
16th to endDay
10%4145%245%7%9%43%
2100
September1st to 15thDay
12%5%14425%45%7%46%
1980
16th to endDay
15%5%25%5%2435%7%41%
1846
October1st to 15thDay
16%5%13423446%45%
1650
16th to endDay
16%41432335%5%47%
1746
November1st to 15thDay
15%5%24424445%46%
1581
16th to endDay
17%424423346%46%
1534
December1st to 15thDay
17%425%5%245%6%6%41%
1440
16th to endDay
18%5%34415%5%5%6%38%
1536
January1st to 15thNight
17%5%25%424446%42%
2503
16th to endNight
16%6%23423346%43%
2496
February1st to 15thNight
14%6%45%41445%6%41%
2268
16th to endNight
15%6%13324446%46%
1893
March1st to 15thNight
16%5%1442335%6%45%
2061
16th to endNight
17%5%25%433336%44%
2043
April1st to 15thNight
16%6%2442435%6%43%
1716
16th to endNight
17%5%2332445%6%45%
1650
May1st to 15thNight
16%6%36%5%2445%6%38%
1479
16th to endNight
13%6%25%5%3435%8%41%
1367
June1st to 15thNight
14%5%25%5%25%35%9%41%
1320
16th to endNight
9%4344445%5%10%43%
1320
July1st to 15thNight
8%5%25%7%47%6%6%9%37%
1320
16th to endNight
6%426%5%25%6%8%10%41%
1464
August1st to 15thNight
8%4146%345%5%8%46%
1517
16th to endNight
11%5%2442445%7%46%
1750
September1st to 15thNight
13%6%24323447%48%
1650
16th to endNight
14%6%24424448%44%
1782
October1st to 15thNight
17%5%135%2235%6%44%
1980
16th to endNight
16%413423336%51%
2126
November1st to 15thNight
13%6%35%41334449%
2277
16th to endNight
16%5%2442445%6%42%
2424
December1st to 15thNight
18%5%25%5%34446%39%
2520
16th to endNight
18%5%244235%5%7%40%
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
14%6%25%5%34445%43%
1434
16th to endDay
15%6%24425%346%44%
1728
February1st to 15thDay
15%6%243345%45%43%
1592
16th to endDay
13%7%35%43346%6%41%
1580
March1st to 15thDay
15%5%25%5%25%347%43%
1884
16th to endDay
17%5%2443445%5%42%
2180
April1st to 15thDay
16%5%24424446%43%
2048
16th to endDay
14%5%2432445%7%45%
1980
May1st to 15thDay
15%6%244345%5%8%37%
2151
16th to endDay
13%424425%5%6%9%41%
2394
June1st to 15thDay
12%424435%5%7%10%37%
2310
16th to endDay
11%3245%35%5%6%9%43%
2310
July1st to 15thDay
9%425%5%35%5%6%9%41%
2310
16th to endDay
5%3246%5%6%6%8%11%39%
2393
August1st to 15thDay
9%5%25%5%345%7%8%40%
2101
16th to endDay
10%4245%245%7%8%44%
2100
September1st to 15thDay
12%5%24434447%46%
1980
16th to endDay
16%5%25%5%34347%41%
1846
October1st to 15thDay
17%5%23323446%46%
1650
16th to endDay
16%41442335%5%47%
1746
November1st to 15thDay
16%5%14323445%47%
1581
16th to endDay
17%424423346%46%
1534
December1st to 15thDay
17%425%5%2446%6%41%
1440
16th to endDay
19%6%2442445%7%38%
1536
January1st to 15thNight
17%5%25%42435%6%42%
2503
16th to endNight
16%6%24424346%44%
2496
February1st to 15thNight
14%6%4441445%6%42%
2261
16th to endNight
16%6%13324346%46%
1893
March1st to 15thNight
16%5%1442335%6%45%
2073
16th to endNight
17%5%25%424336%43%
2043
April1st to 15thNight
16%5%24424447%43%
1716
16th to endNight
17%42432345%6%45%
1650
May1st to 15thNight
17%6%26%5%2345%6%38%
1479
16th to endNight
13%5%25%5%3436%7%42%
1367
June1st to 15thNight
14%5%345%25%449%41%
1320
16th to endNight
10%434435%5%6%9%44%
1320
July1st to 15thNight
8%5%25%7%36%6%6%9%36%
1320
16th to endNight
7%225%5%25%5%9%10%41%
1464
August1st to 15thNight
9%315%6%345%5%7%46%
1517
16th to endNight
11%4244245%47%47%
1750
September1st to 15thNight
13%5%25%32445%6%48%
1650
16th to endNight
14%6%245%23347%44%
1782
October1st to 15thNight
18%5%1441335%6%44%
1980
16th to endNight
16%413323336%51%
2126
November1st to 15thNight
14%6%24423235%49%
2277
16th to endNight
17%5%1442445%6%42%
2424
December1st to 15thNight
18%5%25%5%34446%39%
2520
16th to endNight
18%5%24424447%40%
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
42%5%13%20%13%4
1435
16th to endDay
41%5%11%24%14%3
1728
February1st to 15thDay
42%5%11%25%12%2
1594
16th to endDay
42%5%12%26%12%2
1580
March1st to 15thDay
41%5%12%24%15%1
1884
16th to endDay
43%5%9%20%18%3
2181
April1st to 15thDay
41%5%11%21%15%4
2048
16th to endDay
37%5%10%16%23%6%
1980
May1st to 15thDay
43%7%13%19%14%2
2151
16th to endDay
37%8%15%21%14%2
2398
June1st to 15thDay
39%9%14%24%11%
2310
16th to endDay
35%8%16%20%17%1
2310
July1st to 15thDay
37%7%14%22%15%3
2161
16th to endDay
33%10%15%25%13%2
2232
August1st to 15thDay
37%8%14%22%16%1
2088
16th to endDay
35%7%13%22%17%4
2100
September1st to 15thDay
36%6%12%20%20%3
1980
16th to endDay
41%49%18%23%2
1846
October1st to 15thDay
38%5%13%23%18%2
1650
16th to endDay
38%49%26%18%1
1746
November1st to 15thDay
39%412%21%18%3
1581
16th to endDay
39%410%27%15%2
1534
December1st to 15thDay
40%7%14%23%11%1
1440
16th to endDay
44%6%14%21%12%1
1536
January1st to 15thNight
42%5%12%20%16%3
2503
16th to endNight
41%411%25%15%1
2496
February1st to 15thNight
42%412%23%14%2
2269
16th to endNight
39%49%22%19%3
1893
March1st to 15thNight
39%413%24%15%2
2073
16th to endNight
42%411%20%17%3
2043
April1st to 15thNight
41%412%19%18%4
1716
16th to endNight
37%6%9%18%21%6%
1650
May1st to 15thNight
46%7%10%17%16%2
1479
16th to endNight
40%7%15%16%16%3
1369
June1st to 15thNight
40%6%13%21%16%
1320
16th to endNight
34%8%12%25%16%1
1320
July1st to 15thNight
41%9%15%17%12%2
1240
16th to endNight
32%10%17%23%14%1
1363
August1st to 15thNight
34%8%14%21%19%1
1506
16th to endNight
34%8%12%21%19%2
1750
September1st to 15thNight
36%7%10%22%21%3
1650
16th to endNight
38%47%20%23%3
1782
October1st to 15thNight
39%5%11%23%17%2
1980
16th to endNight
35%49%29%19%2
2126
November1st to 15thNight
39%49%21%17%7%
2277
16th to endNight
40%5%11%23%16%2
2424
December1st to 15thNight
45%5%11%22%13%1
2520
16th to endNight
42%6%13%23%12%1
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
22%21%42%9%11
61
16th to endDay
22%23%35%16%1
96
February1st to 15thDay
20%27%30%19%1
108
16th to endDay
16%18%41%20%1
239
March1st to 15thDay
17%17%31%25%7%
270
16th to endDay
21%11%38%18%6%2
288
April1st to 15thDay
21%16%34%15%8%3
235
16th to endDay
18%8%26%25%14%5%
210
May1st to 15thDay
28%7%25%21%14%2
228
16th to endDay
14%5%18%26%24%9%
267
June1st to 15thDay
15%424%27%22%6%
255
16th to endDay
12%223%33%22%5%
255
July1st to 15thDay
14%115%30%28%10%
245
16th to endDay
5%112%35%33%11%
270
August1st to 15thDay
12%113%33%26%13%
253
16th to endDay
10%18%32%26%21%
268
September1st to 15thDay
14%319%21%21%18%
255
16th to endDay
21%214%30%17%12%
254
October1st to 15thDay
21%320%30%15%8%
255
16th to endDay
15%11%31%26%13%1
269
November1st to 15thDay
17%8%32%20%19%1
235
16th to endDay
18%14%34%19%12%
154
December1st to 15thDay
20%20%33%11%13%1
60
16th to endDay
37%10%32%17%1
64
January1st to 15thNight
19%15%36%22%5%
720
16th to endNight
21%14%40%22%1
768
February1st to 15thNight
17%16%43%20%1
704
16th to endNight
19%19%38%20%2
631
March1st to 15thNight
19%14%37%24%5%
720
16th to endNight
20%9%40%22%7%
768
April1st to 15thNight
19%12%38%24%4
686
16th to endNight
19%7%31%31%11%
660
May1st to 15thNight
19%5%31%34%8%
489
16th to endNight
12%5%22%38%20%
343
June1st to 15thNight
15%220%42%18%
330
16th to endNight
8%120%49%19%
330
July1st to 15thNight
11%218%45%21%
310
16th to endNight
7%215%41%30%2
410
August1st to 15thNight
10%13%44%28%1
526
16th to endNight
14%114%35%30%4
700
September1st to 15thNight
16%216%35%25%3
660
16th to endNight
17%316%33%24%3
657
October1st to 15thNight
21%422%30%17%3
660
16th to endNight
19%9%31%28%10%
704
November1st to 15thNight
16%9%33%25%13%1
700
16th to endNight
18%14%31%26%8%
720
December1st to 15thNight
21%13%34%21%9%
720
16th to endNight
20%14%38%24%3
768
Page generated at 2017-04-08 05:35:18. Version ac04

Copyright 2017 A. Danko


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