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
37%
15330
20%
47389
11%
15294
47%
15294
24%
30053

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
27%
718
19%
1796
5%
718
34%
718
29%
1197
April16th to endNight
37%
720
18%
1800
7%
720
42%
720
30%
1210
May1st to 15thNight
34%
533
16%
1613
8%
533
41%
533
21%
1103
May16th to endNight
49%
374
16%
1497
15%
374
54%
374
19%
922
June1st to 15thNight
54%
360
17%
1440
17%
360
61%
360
23%
859
June16th to endNight
57%
360
16%
1440
19%
360
61%
360
25%
840
July1st to 15thNight
57%
340
12%
1360
15%
340
58%
340
18%
869
July16th to endNight
65%
444
15%
1481
14%
417
69%
417
27%
882
August1st to 15thNight
62%
574
17%
1644
12%
571
64%
571
30%
1052
August16th to endNight
62%
764
18%
1910
10%
764
63%
764
34%
1248
September1st to 15thNight
55%
720
24%
1800
14%
720
60%
720
33%
1188
September16th to endNight
55%
717
26%
1945
20%
717
59%
717
33%
1236
October1st to 15thNight
45%
720
21%
2160
17%
720
52%
720
26%
1374
October16th to endNight
36%
768
28%
2317
16%
768
53%
768
31%
1373
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%
48362
Night
15%5%2442445%7%43%
47566
Cloud CoverDay
14%5%2443445%7%42%
48372
Night
15%5%2442445%7%44%
47571
TransparencyDay
39%6%12%22%16%2
48057
Night
39%5%11%22%17%2
47389
SeeingDay
17%8%25%25%16%6%
5916
Night
17%9%29%29%12%1
15332

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
15%5%24334446%44%
2152
16th to endDay
15%42442445%7%44%
2160
May1st to 15thDay
16%6%35%5%25%45%7%37%
2348
16th to endDay
14%4244345%7%8%39%
2618
June1st to 15thDay
11%5%25%435%6%6%10%38%
2520
16th to endDay
10%4245%35%5%6%10%42%
2520
July1st to 15thDay
8%425%6%35%5%7%10%40%
2520
16th to endDay
6%325%6%47%5%8%11%38%
2583
August1st to 15thDay
10%5%25%6%345%7%9%39%
2292
16th to endDay
10%4145%245%7%9%43%
2292
September1st to 15thDay
12%5%15%42445%7%46%
2160
16th to endDay
14%5%15%5%3445%7%41%
2013
October1st to 15thDay
16%6%14423446%44%
1800
16th to endDay
16%41432335%5%48%
1905
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
15%6%2442435%6%44%
1796
16th to endNight
17%5%2332445%7%43%
1800
May1st to 15thNight
17%6%36%5%24446%38%
1613
16th to endNight
14%6%25%5%3435%8%40%
1495
June1st to 15thNight
13%5%245%25%35%9%43%
1440
16th to endNight
9%4344345%6%10%42%
1440
July1st to 15thNight
8%5%25%7%47%6%6%9%36%
1440
16th to endNight
6%326%425%6%8%10%41%
1582
August1st to 15thNight
9%4146%35%5%5%7%45%
1655
16th to endNight
11%42442445%8%46%
1910
September1st to 15thNight
12%5%24323447%48%
1800
16th to endNight
13%6%24414448%45%
1945
October1st to 15thNight
18%5%145%22346%44%
2160
16th to endNight
15%413423336%52%
2317
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
15%5%24324446%44%
2152
16th to endDay
15%5%2432435%7%44%
2160
May1st to 15thDay
16%6%244345%5%7%38%
2348
16th to endDay
14%414425%5%6%9%39%
2618
June1st to 15thDay
11%425%435%5%7%10%38%
2520
16th to endDay
10%4245%35%5%6%10%42%
2520
July1st to 15thDay
9%425%6%35%5%7%9%41%
2520
16th to endDay
6%3246%5%6%6%8%11%39%
2583
August1st to 15thDay
10%5%25%5%345%7%8%40%
2292
16th to endDay
10%4245%245%6%8%44%
2292
September1st to 15thDay
12%5%24434447%46%
2160
16th to endDay
15%425%5%34447%42%
2013
October1st to 15thDay
17%5%24323445%45%
1800
16th to endDay
16%41442335%5%48%
1905
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%2442445%7%44%
1796
16th to endNight
18%424323446%44%
1800
May1st to 15thNight
18%6%26%5%2345%6%38%
1613
16th to endNight
15%5%245%3436%7%41%
1495
June1st to 15thNight
13%5%245%25%448%43%
1440
16th to endNight
10%334435%5%6%10%43%
1440
July1st to 15thNight
8%5%25%7%46%6%6%9%36%
1440
16th to endNight
7%225%6%25%5%9%9%42%
1582
August1st to 15thNight
9%415%6%345%6%7%45%
1655
16th to endNight
11%4244245%5%8%46%
1910
September1st to 15thNight
13%5%25%32445%6%48%
1800
16th to endNight
13%6%245%23447%46%
1945
October1st to 15thNight
18%5%14423346%44%
2160
16th to endNight
15%413323336%52%
2317
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
40%5%11%22%15%4
2152
16th to endDay
39%5%10%17%21%5%
2160
May1st to 15thDay
43%7%12%19%15%2
2348
16th to endDay
38%9%14%21%13%1
2622
June1st to 15thDay
38%9%14%24%11%1
2520
16th to endDay
34%8%16%21%17%1
2520
July1st to 15thDay
37%7%14%21%15%3
2371
16th to endDay
33%10%15%25%13%2
2422
August1st to 15thDay
37%8%14%21%16%1
2279
16th to endDay
35%7%13%21%17%3
2292
September1st to 15thDay
36%6%11%22%20%2
2160
16th to endDay
40%5%9%19%23%2
2013
October1st to 15thDay
39%5%12%21%18%2
1800
16th to endDay
37%48%27%19%1
1905
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
40%412%20%18%4
1796
16th to endNight
39%6%10%18%19%5%
1800
May1st to 15thNight
46%6%10%16%17%2
1613
16th to endNight
41%7%14%16%16%2
1497
June1st to 15thNight
38%6%13%23%17%
1440
16th to endNight
34%8%13%24%16%2
1440
July1st to 15thNight
42%9%15%17%12%3
1360
16th to endNight
33%10%17%22%14%1
1481
August1st to 15thNight
35%9%13%21%19%1
1644
16th to endNight
34%8%13%21%19%2
1910
September1st to 15thNight
36%7%9%22%21%3
1800
16th to endNight
37%48%22%23%3
1945
October1st to 15thNight
40%5%10%22%17%2
2160
16th to endNight
34%49%30%18%2
2317
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
19%14%34%19%8%3
270
16th to endDay
21%7%28%25%12%4
270
May1st to 15thDay
28%12%26%18%11%1
288
16th to endDay
18%420%28%21%7%
331
June1st to 15thDay
14%426%26%21%6%
315
16th to endDay
11%426%32%20%4
315
July1st to 15thDay
13%118%32%24%8%
305
16th to endDay
7%113%34%33%10%
326
August1st to 15thDay
14%115%30%25%13%
313
16th to endDay
10%111%33%25%17%
332
September1st to 15thDay
14%422%22%20%15%
315
16th to endDay
19%314%29%20%12%
314
October1st to 15thDay
21%319%29%16%9%
315
16th to endDay
14%12%29%25%14%3
332
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
18%12%37%25%5%
718
16th to endNight
20%6%30%30%11%
720
May1st to 15thNight
20%6%32%32%7%
533
16th to endNight
14%5%21%37%20%
375
June1st to 15thNight
14%220%43%18%
360
16th to endNight
8%123%48%18%
360
July1st to 15thNight
11%219%44%21%
340
16th to endNight
8%214%40%30%2
444
August1st to 15thNight
11%14%42%28%2
574
16th to endNight
13%114%36%29%3
764
September1st to 15thNight
15%218%35%25%2
720
16th to endNight
16%216%33%25%4
717
October1st to 15thNight
21%421%30%17%3
720
16th to endNight
18%9%32%27%10%1
768
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 2017-11-01 17:23:40. Version ac04

Copyright 2017 A. Danko


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