The Clear Nights Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Clear Nights 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
50%
16136
24%
48798
16%
16094
58%
16094
28%
30020

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
27%
780
23%
2522
10%
776
44%
776
21%
1546
January16th to endNight
28%
832
26%
2652
13%
832
43%
832
23%
1646
February1st to 15thNight
33%
759
25%
2279
12%
757
47%
757
25%
1410
February16th to endNight
26%
631
25%
1893
10%
631
41%
631
25%
1155
March1st to 15thNight
35%
720
25%
2050
12%
720
46%
720
29%
1261
March16th to endNight
41%
768
23%
2112
12%
768
49%
768
32%
1284
April1st to 15thNight
44%
662
25%
1920
17%
662
52%
662
33%
1119
April16th to endNight
51%
540
27%
1698
17%
540
61%
540
33%
958
May1st to 15thNight
53%
540
22%
1620
14%
540
59%
540
31%
988
May16th to endNight
63%
561
22%
1682
17%
561
67%
561
32%
1036
June1st to 15thNight
71%
540
18%
1620
16%
540
73%
540
31%
1021
June16th to endNight
77%
540
17%
1620
15%
540
78%
540
33%
994
July1st to 15thNight
74%
510
13%
1530
12%
510
75%
510
27%
1020
July16th to endNight
76%
564
15%
1595
14%
531
77%
531
29%
1031
August1st to 15thNight
74%
539
17%
1636
18%
536
74%
536
26%
1090
August16th to endNight
67%
573
18%
1910
20%
573
68%
573
22%
1270
September1st to 15thNight
65%
720
19%
1980
18%
720
66%
720
27%
1339
September16th to endNight
62%
717
25%
1980
20%
717
66%
717
29%
1293
October1st to 15thNight
58%
720
30%
1992
24%
720
64%
720
37%
1185
October16th to endNight
53%
768
36%
2304
27%
768
66%
768
38%
1251
November1st to 15thNight
53%
760
30%
2429
26%
760
63%
760
35%
1308
November16th to endNight
42%
780
29%
2535
20%
780
57%
780
30%
1454
December1st to 15thNight
38%
780
25%
2535
16%
780
51%
780
24%
1553
December16th to endNight
33%
832
21%
2704
14%
832
42%
832
19%
1808

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%245%35%5%5%7%42%
49270
Night
12%42442445%7%46%
48998
TransparencyDay
39%7%11%18%18%5%
48971
Night
35%6%10%19%20%7%
48801
SeeingDay
14%7%18%22%22%13%
5909
Night
14%7%20%29%23%4
16139

By Month

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
16%6%33423346%45%
1746
16th to endDay
17%6%23333445%46%
1923
February1st to 15thDay
16%6%24423446%43%
1883
16th to endDay
17%6%1442434445%
1580
March1st to 15thDay
15%425%423347%45%
1908
16th to endDay
15%42443445%6%44%
2111
April1st to 15thDay
12%5%34424446%47%
2037
16th to endDay
10%32442435%7%50%
2260
May1st to 15thDay
10%5%245%35%45%7%44%
2331
16th to endDay
9%4245%35%5%6%10%41%
2431
June1st to 15thDay
9%425%6%48%7%7%9%34%
2340
16th to endDay
5%3245%37%6%7%11%42%
2340
July1st to 15thDay
10%436%7%46%7%8%11%29%
2340
16th to endDay
6%5%26%7%48%7%9%11%29%
2465
August1st to 15thDay
10%436%7%5%8%8%8%10%27%
2310
16th to endDay
14%6%27%7%37%6%6%8%28%
2280
September1st to 15thDay
14%425%6%36%5%6%7%35%
1980
16th to endDay
17%5%14324445%45%
1980
October1st to 15thDay
15%413312336%53%
1967
16th to endDay
11%313323335%58%
1920
November1st to 15thDay
12%313324445%54%
1756
16th to endDay
13%424434345%50%
1755
December1st to 15thDay
17%6%24423346%43%
1755
16th to endDay
23%6%35%423345%36%
1872
January1st to 15thNight
15%6%24423345%46%
2522
16th to endNight
17%5%25%5%22335%45%
2652
February1st to 15thNight
15%4345%23345%46%
2271
16th to endNight
15%424424436%48%
1893
March1st to 15thNight
12%5%23424445%50%
2050
16th to endNight
12%413314447%50%
2112
April1st to 15thNight
12%42231345%8%51%
1920
16th to endNight
8%41443346%7%51%
1698
May1st to 15thNight
9%315%5%35%5%5%8%46%
1619
16th to endNight
7%3245%35%5%5%8%47%
1682
June1st to 15thNight
7%4245%35%5%7%11%42%
1620
16th to endNight
43145%35%6%8%12%42%
1620
July1st to 15thNight
8%4235%45%7%9%12%36%
1620
16th to endNight
5%3245%37%8%9%11%37%
1703
August1st to 15thNight
6%325%7%47%6%8%11%37%
1647
16th to endNight
12%5%246%45%6%7%10%34%
1910
September1st to 15thNight
9%425%6%26%6%6%9%39%
1980
16th to endNight
12%4245%3345%7%46%
1980
October1st to 15thNight
14%41332335%5%53%
1992
16th to endNight
9%213323335%59%
2304
November1st to 15thNight
10%323313245%58%
2429
16th to endNight
12%424323346%53%
2535
December1st to 15thNight
15%6%24414346%46%
2535
16th to endNight
19%6%35%43435%6%37%
2704

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
41%48%15%25%5%
1747
16th to endDay
39%37%17%22%9%
1923
February1st to 15thDay
41%47%19%21%5%
1885
16th to endDay
40%46%18%21%6%
1580
March1st to 15thDay
39%49%18%20%7%
1908
16th to endDay
39%6%9%21%18%4
2112
April1st to 15thDay
37%38%21%20%7%
2037
16th to endDay
32%6%9%21%24%6%
2260
May1st to 15thDay
38%6%12%18%20%4
2331
16th to endDay
35%8%13%25%14%2
2435
June1st to 15thDay
42%10%15%21%8%1
2340
16th to endDay
32%11%16%22%14%2
2340
July1st to 15thDay
43%12%19%16%6%2
2201
16th to endDay
41%13%15%19%7%1
2310
August1st to 15thDay
46%11%15%17%7%
2297
16th to endDay
51%10%13%13%10%
2280
September1st to 15thDay
44%9%12%15%15%2
1980
16th to endDay
40%6%10%19%19%4
1980
October1st to 15thDay
32%410%19%25%6%
1967
16th to endDay
29%36%15%32%12%
1920
November1st to 15thDay
32%47%15%28%11%
1756
16th to endDay
36%46%15%27%9%
1755
December1st to 15thDay
42%5%7%19%17%7%
1755
16th to endDay
50%47%17%13%6%
1872
January1st to 15thNight
40%46%18%24%6%
2522
16th to endNight
40%35%17%22%10%
2652
February1st to 15thNight
39%35%19%21%11%
2280
16th to endNight
38%36%18%24%8%
1893
March1st to 15thNight
35%5%8%20%20%9%
2050
16th to endNight
33%410%21%22%6%
2112
April1st to 15thNight
31%48%24%21%9%
1920
16th to endNight
29%6%11%21%21%10%
1698
May1st to 15thNight
34%7%13%19%22%4
1620
16th to endNight
32%9%12%26%15%4
1684
June1st to 15thNight
34%8%17%28%11%
1620
16th to endNight
31%9%19%22%14%2
1620
July1st to 15thNight
34%13%21%18%7%3
1530
16th to endNight
33%12%23%19%9%2
1595
August1st to 15thNight
37%11%17%22%9%1
1636
16th to endNight
40%10%17%17%12%1
1910
September1st to 15thNight
38%10%13%16%16%4
1980
16th to endNight
37%8%10%19%19%5%
1980
October1st to 15thNight
31%49%19%24%9%
1992
16th to endNight
27%35%14%32%15%
2304
November1st to 15thNight
29%37%16%29%13%
2429
16th to endNight
32%46%17%26%12%
2535
December1st to 15thNight
39%47%20%20%8%
2535
16th to endNight
47%47%17%17%5%
2704

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
19%20%31%21%5%
135
16th to endDay
13%34%31%11%7%1
144
February1st to 15thDay
20%22%40%14%2
130
16th to endDay
20%12%49%16%1
81
March1st to 15thDay
16%16%26%28%10%1
198
16th to endDay
17%10%29%26%15%
288
April1st to 15thDay
15%13%25%22%18%3
270
16th to endDay
13%5%22%28%23%6%
270
May1st to 15thDay
12%9%28%17%17%15%
288
16th to endDay
12%418%23%25%15%
331
June1st to 15thDay
10%11%20%30%26%
315
16th to endDay
10%10%25%22%30%
315
July1st to 15thDay
7%7%19%32%31%
305
16th to endDay
5%17%15%38%31%
326
August1st to 15thDay
14%6%26%29%23%
313
16th to endDay
15%15%22%33%21%
332
September1st to 15thDay
16%112%18%32%19%
315
16th to endDay
16%111%28%28%13%
314
October1st to 15thDay
23%215%25%24%8%
315
16th to endDay
10%9%20%26%25%7%
336
November1st to 15thDay
24%8%19%25%17%4
174
16th to endDay
8%12%26%31%16%5%
135
December1st to 15thDay
24%14%27%22%11%
135
16th to endDay
33%13%28%20%2
144
January1st to 15thNight
21%16%27%25%8%
780
16th to endNight
19%14%31%27%6%
832
February1st to 15thNight
19%13%28%33%5%
760
16th to endNight
16%16%36%27%3
631
March1st to 15thNight
15%12%31%30%8%
720
16th to endNight
15%11%27%29%14%
768
April1st to 15thNight
15%10%27%28%17%
662
16th to endNight
10%8%23%31%24%1
540
May1st to 15thNight
9%424%33%23%4
540
16th to endNight
9%214%30%35%6%
563
June1st to 15thNight
8%18%33%37%10%
540
16th to endNight
5%8%29%42%12%
540
July1st to 15thNight
8%6%24%45%14%
510
16th to endNight
5%6%23%48%15%
564
August1st to 15thNight
8%1324%49%12%
539
16th to endNight
14%15%23%42%11%
573
September1st to 15thNight
10%10%25%42%10%
720
16th to endNight
15%110%31%36%4
717
October1st to 15thNight
17%118%30%25%6%
720
16th to endNight
10%7%22%30%25%3
768
November1st to 15thNight
11%7%22%32%22%2
760
16th to endNight
13%12%25%30%16%1
780
December1st to 15thNight
17%10%27%29%13%1
780
16th to endNight
23%12%27%29%7%
832
Page generated at 2018-02-15 10:58:48. Version ac04

Copyright 2018 A. Danko


The Clear Nights Observatory Clear Sky Chart |Charts Home