Northern Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northern Skies Observatory clear sky chart from 2005-11-06 to 2024-01-27.

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
31%
22740
13%
74914
9%
22700
38%
22700
15%
53919

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 10thNight
19%
760
9%
2649
5%
757
24%
757
8%
2085
January11th to 20thNight
19%
760
8%
2660
4%
760
25%
760
8%
2086
January21st to endNight
21%
820
12%
2868
6%
820
29%
820
10%
2138
February1st to 10thNight
20%
720
10%
2397
5%
720
27%
720
9%
1884
February11th to 20thNight
20%
703
13%
2100
5%
700
30%
700
12%
1591
February21st to endNight
26%
592
15%
1776
8%
592
37%
592
14%
1317
March1st to 10thNight
24%
720
15%
2160
10%
720
32%
720
13%
1656
March11th to 20thNight
24%
720
14%
2102
7%
720
32%
720
15%
1561
March21st to endNight
27%
792
19%
2002
8%
792
34%
792
19%
1462
April1st to 10thNight
26%
720
16%
1800
5%
720
31%
720
17%
1362
April11th to 20thNight
28%
702
15%
1766
6%
702
33%
702
21%
1273
April21st to endNight
25%
540
17%
1620
8%
540
33%
540
17%
1144
May1st to 10thNight
24%
540
14%
1620
8%
540
30%
540
16%
1170
May11th to 20thNight
34%
367
18%
1429
15%
367
44%
367
18%
903
May21st to endNight
41%
389
13%
1551
12%
388
49%
388
19%
960
June1st to 10thNight
36%
360
12%
1440
12%
360
45%
360
15%
984
June11th to 20thNight
42%
360
12%
1440
13%
360
49%
360
18%
910
June21st to endNight
44%
360
10%
1440
12%
360
49%
360
17%
962
July1st to 10thNight
55%
350
12%
1400
18%
350
58%
350
20%
867
July11th to 20thNight
54%
348
14%
1392
19%
348
59%
348
23%
821
July21st to endNight
56%
394
15%
1486
18%
372
58%
372
22%
891
August1st to 10thNight
61%
380
17%
1444
19%
378
65%
378
25%
845
August11th to 20thNight
51%
554
17%
1618
13%
554
53%
554
28%
1043
August21st to endNight
52%
792
18%
1980
9%
792
54%
792
31%
1323
September1st to 10thNight
48%
720
15%
1800
7%
720
51%
720
32%
1182
September11th to 20thNight
50%
720
21%
1939
13%
720
54%
720
31%
1232
September21st to endNight
44%
720
21%
1980
15%
720
48%
720
28%
1293
October1st to 10thNight
39%
720
15%
1998
10%
720
45%
720
20%
1420
October11th to 20thNight
35%
720
15%
2247
10%
720
41%
720
19%
1568
October21st to endNight
25%
792
11%
2574
7%
792
31%
792
13%
1906
November1st to 10thNight
30%
740
16%
2407
11%
740
38%
740
16%
1685
November11th to 20thNight
28%
760
18%
2470
12%
760
37%
760
16%
1775
November21st to endNight
24%
760
12%
2588
9%
760
30%
760
12%
1944
December1st to 10thNight
15%
760
6%
2660
3%
760
19%
760
7%
2171
December11th to 20thNight
21%
760
8%
3259
5%
759
26%
759
6%
2592
December21st to endNight
18%
825
6%
4852
4%
817
24%
817
4%
3913

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%10%48%7%45%5%5%7%29%
74897
Night
16%10%48%7%45%5%5%6%29%
75146
TransparencyDay
54%5%11%19%10%1
74602
Night
54%5%11%20%9%1
74975
SeeingDay
25%14%28%20%10%3
13691
Night
23%12%25%29%11%1
22751

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 10thDay
13%12%7%14%9%45%6%5%5%19%
1517
11th to 20thDay
16%13%5%10%9%46%5%5%5%22%
1520
21st to endDay
10%13%6%9%11%47%6%47%24%
1632
February1st to 10thDay
14%13%6%11%9%6%7%445%21%
1559
11th to 20thDay
11%12%8%12%10%46%5%45%22%
1748
21st to endDay
13%13%6%13%9%5%5%5%5%5%23%
1480
March1st to 10thDay
13%13%7%10%8%5%5%445%26%
1797
11th to 20thDay
15%13%6%8%7%345%45%30%
1858
21st to endDay
17%12%6%10%8%34345%29%
2352
April1st to 10thDay
19%13%5%8%7%34345%29%
2160
11th to 20thDay
18%10%47%5%35%45%6%33%
2142
21st to endDay
24%9%5%8%6%25%5%5%5%26%
2340
May1st to 10thDay
22%10%47%6%34446%29%
2343
11th to 20thDay
19%8%36%5%445%5%6%35%
2478
21st to endDay
16%7%46%6%45%6%6%7%34%
2725
June1st to 10thDay
24%8%37%6%35%5%6%7%27%
2520
11th to 20thDay
17%7%36%5%45%5%6%7%35%
2520
21st to endDay
20%9%37%7%45%5%5%8%26%
2520
July1st to 10thDay
16%7%36%6%45%5%5%8%35%
2520
11th to 20thDay
14%5%36%7%5%6%6%7%9%32%
2520
21st to endDay
16%6%36%6%5%7%5%7%8%31%
2744
August1st to 10thDay
13%7%38%6%45%6%7%8%33%
2467
11th to 20thDay
14%8%36%7%45%6%7%9%32%
2278
21st to endDay
13%6%26%6%46%5%7%9%37%
2376
September1st to 10thDay
15%6%25%5%345%5%7%42%
2160
11th to 20thDay
15%6%35%333447%45%
2021
21st to endDay
15%7%35%5%35%46%9%37%
1980
October1st to 10thDay
24%10%37%43435%7%29%
1961
11th to 20thDay
16%10%37%5%35%5%6%6%32%
1711
21st to endDay
22%16%5%7%6%46%43424%
1782
November1st to 10thDay
14%11%46%6%45%45%8%32%
1671
11th to 20thDay
16%12%6%9%8%35%5%5%6%25%
1710
21st to endDay
18%12%5%11%7%4445%6%24%
1587
December1st to 10thDay
18%17%6%11%9%46%345%16%
1520
11th to 20thDay
21%13%6%9%5%34446%24%
1906
21st to endDay
22%15%5%13%8%35%43317%
2772
January1st to 10thNight
13%12%6%13%10%5%6%6%6%7%15%
2649
11th to 20thNight
14%13%5%11%8%5%7%6%5%6%19%
2660
21st to endNight
12%9%6%11%9%5%6%6%6%6%23%
2868
February1st to 10thNight
13%12%6%11%9%6%8%5%6%5%19%
2397
11th to 20thNight
10%10%6%10%9%5%8%6%6%6%23%
2098
21st to endNight
12%10%5%10%9%5%6%5%5%6%27%
1776
March1st to 10thNight
12%13%7%9%9%47%6%4425%
2150
11th to 20thNight
15%11%49%7%45%5%46%30%
2101
21st to endNight
16%11%47%6%445%45%33%
2002
April1st to 10thNight
20%10%48%7%445%46%29%
1800
11th to 20thNight
17%10%46%7%4345%7%34%
1766
21st to endNight
22%8%35%6%35%45%6%32%
1620
May1st to 10thNight
21%12%37%5%2345%7%30%
1620
11th to 20thNight
18%8%36%5%335%6%7%36%
1428
21st to endNight
17%6%25%6%35%6%6%9%37%
1550
June1st to 10thNight
21%7%35%6%45%45%7%32%
1440
11th to 20thNight
19%6%25%5%36%5%6%6%37%
1440
21st to endNight
19%6%35%5%45%6%6%8%32%
1440
July1st to 10thNight
15%5%36%6%35%5%6%8%38%
1440
11th to 20thNight
15%434335%6%6%10%40%
1440
21st to endNight
11%7%35%5%35%7%6%9%39%
1574
August1st to 10thNight
12%425%6%45%5%7%8%42%
1454
11th to 20thNight
13%8%346%245%5%9%41%
1618
21st to endNight
15%6%345%345%5%7%43%
1980
September1st to 10thNight
13%5%25%5%3445%8%46%
1800
11th to 20thNight
14%7%2443445%6%47%
1939
21st to endNight
14%7%244446%5%7%42%
1980
October1st to 10thNight
21%7%26%5%45%5%6%6%34%
1998
11th to 20thNight
16%10%37%6%5%446%6%33%
2247
21st to endNight
23%12%47%6%36%46%6%24%
2574
November1st to 10thNight
16%11%48%7%45%5%5%6%31%
2407
11th to 20thNight
16%10%6%9%7%4445%7%29%
2470
21st to endNight
16%11%6%12%8%45%45%6%24%
2588
December1st to 10thNight
19%15%6%11%8%5%7%5%4415%
2660
11th to 20thNight
20%13%6%11%8%45%5%5%5%18%
3288
21st to endNight
22%14%6%15%7%45%44416%
4884

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 10thDay
65%48%16%6%
1518
11th to 20thDay
63%410%13%7%2
1520
21st to endDay
59%410%16%9%1
1632
February1st to 10thDay
65%48%15%7%2
1559
11th to 20thDay
64%5%9%15%7%1
1757
21st to endDay
63%48%12%10%2
1480
March1st to 10thDay
61%310%15%9%3
1800
11th to 20thDay
56%410%14%13%3
1858
21st to endDay
57%46%17%12%3
2352
April1st to 10thDay
58%37%18%10%3
2160
11th to 20thDay
52%310%20%14%1
2142
21st to endDay
59%49%17%9%3
2340
May1st to 10thDay
56%37%21%11%2
2343
11th to 20thDay
49%5%9%20%15%2
2482
21st to endDay
48%6%13%21%11%2
2725
June1st to 10thDay
56%412%19%8%
2520
11th to 20thDay
46%6%9%23%14%2
2520
21st to endDay
55%5%11%19%8%1
2520
July1st to 10thDay
44%5%13%24%13%
2450
11th to 20thDay
46%7%13%25%8%
2445
21st to endDay
47%7%13%23%10%
2591
August1st to 10thDay
46%6%16%22%9%
2453
11th to 20thDay
46%6%16%23%9%
2278
21st to endDay
42%6%14%24%13%1
2376
September1st to 10thDay
39%5%11%29%14%1
2160
11th to 20thDay
39%49%31%15%1
2021
21st to endDay
43%5%15%23%12%2
1980
October1st to 10thDay
55%310%20%11%1
1961
11th to 20thDay
49%5%14%24%6%1
1711
21st to endDay
65%49%13%7%2
1782
November1st to 10thDay
49%5%13%20%11%1
1671
11th to 20thDay
58%6%11%13%10%2
1710
21st to endDay
60%416%16%4
1587
December1st to 10thDay
70%310%12%4
1520
11th to 20thDay
61%48%19%8%1
1906
21st to endDay
73%46%10%7%
2772
January1st to 10thNight
64%6%10%14%6%
2649
11th to 20thNight
64%5%11%13%5%
2660
21st to endNight
59%5%9%17%9%1
2868
February1st to 10thNight
65%49%13%8%1
2397
11th to 20thNight
59%48%18%9%1
2100
21st to endNight
57%410%18%10%2
1776
March1st to 10thNight
60%47%18%9%2
2160
11th to 20thNight
56%410%18%11%2
2102
21st to endNight
51%48%20%14%3
2002
April1st to 10thNight
56%48%19%12%2
1800
11th to 20thNight
49%49%24%13%
1766
21st to endNight
51%5%10%18%12%3
1620
May1st to 10thNight
56%37%19%14%2
1620
11th to 20thNight
45%5%10%21%16%2
1429
21st to endNight
44%5%16%23%11%1
1551
June1st to 10thNight
51%7%10%22%10%
1440
11th to 20thNight
46%5%13%22%11%2
1440
21st to endNight
48%7%13%23%10%
1440
July1st to 10thNight
42%7%17%21%13%1
1400
11th to 20thNight
38%7%17%28%10%
1392
21st to endNight
39%8%16%26%11%1
1486
August1st to 10thNight
38%6%17%30%9%
1444
11th to 20thNight
39%7%17%25%12%
1618
21st to endNight
39%6%14%28%12%
1980
September1st to 10thNight
36%7%15%30%11%
1800
11th to 20thNight
39%412%32%13%1
1939
21st to endNight
40%5%14%26%12%2
1980
October1st to 10thNight
50%5%13%21%11%1
1998
11th to 20thNight
51%414%23%7%
2247
21st to endNight
58%5%12%15%9%1
2574
November1st to 10thNight
52%5%11%21%9%1
2407
11th to 20thNight
54%5%10%20%9%2
2470
21st to endNight
60%5%13%17%5%
2588
December1st to 10thNight
70%49%13%3
2660
11th to 20thNight
67%5%8%12%8%
3288
21st to endNight
72%47%14%3
4884

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 10thDay
22%18%30%19%10%1
180
11th to 20thDay
32%19%26%16%5%2
180
21st to endDay
22%21%35%13%7%2
188
February1st to 10thDay
38%22%29%8%3
224
11th to 20thDay
30%19%32%15%4
416
21st to endDay
28%19%29%21%31
346
March1st to 10thDay
31%14%31%19%4
420
11th to 20thDay
32%16%31%14%7%1
420
21st to endDay
29%19%26%15%8%3
462
April1st to 10thDay
34%14%25%19%6%2
420
11th to 20thDay
32%19%25%14%8%2
412
21st to endDay
36%15%26%11%9%3
420
May1st to 10thDay
35%16%25%17%6%2
423
11th to 20thDay
24%15%34%18%7%2
447
21st to endDay
19%17%33%20%10%1
485
June1st to 10thDay
27%15%31%16%9%2
450
11th to 20thDay
21%15%29%22%9%4
450
21st to endDay
23%13%33%21%9%2
450
July1st to 10thDay
19%9%28%28%12%3
445
11th to 20thDay
17%6%33%29%11%4
444
21st to endDay
18%8%35%24%12%3
488
August1st to 10thDay
15%9%28%27%18%4
446
11th to 20thDay
17%7%29%26%15%7%
439
21st to endDay
17%11%26%23%16%6%
495
September1st to 10thDay
16%10%26%24%17%8%
450
11th to 20thDay
18%11%24%19%17%10%
450
21st to endDay
18%11%26%26%12%8%
450
October1st to 10thDay
28%11%20%20%13%6%
450
11th to 20thDay
24%12%28%22%10%3
405
21st to endDay
36%12%23%22%8%
396
November1st to 10thDay
27%14%24%22%13%1
365
11th to 20thDay
24%20%25%21%8%1
370
21st to endDay
32%15%24%21%8%
247
December1st to 10thDay
26%17%32%17%7%2
180
11th to 20thDay
38%16%28%14%4
180
21st to endDay
36%14%26%19%6%
198
January1st to 10thNight
22%15%30%27%5%
760
11th to 20thNight
26%16%27%27%4
760
21st to endNight
18%18%30%28%5%
820
February1st to 10thNight
21%17%31%27%5%
720
11th to 20thNight
21%17%32%25%4
703
21st to endNight
21%15%29%29%5%
592
March1st to 10thNight
25%15%28%30%3
720
11th to 20thNight
25%16%27%24%8%
720
21st to endNight
27%14%25%25%9%1
792
April1st to 10thNight
29%14%25%27%6%
720
11th to 20thNight
23%19%23%25%10%
702
21st to endNight
28%14%28%25%6%
540
May1st to 10thNight
28%11%29%24%8%
540
11th to 20thNight
20%8%29%33%8%1
367
21st to endNight
17%9%25%35%12%1
389
June1st to 10thNight
25%426%35%10%
360
11th to 20thNight
20%8%21%35%16%
360
21st to endNight
21%323%39%12%1
360
July1st to 10thNight
17%217%46%16%2
350
11th to 20thNight
17%318%41%19%2
348
21st to endNight
13%5%18%42%21%1
394
August1st to 10thNight
14%314%38%31%
380
11th to 20thNight
17%319%37%22%3
554
21st to endNight
18%416%35%24%3
792
September1st to 10thNight
16%5%22%36%19%2
720
11th to 20thNight
18%9%18%30%22%4
720
21st to endNight
18%8%24%29%19%2
720
October1st to 10thNight
24%10%19%28%17%2
720
11th to 20thNight
23%10%24%28%13%1
720
21st to endNight
29%11%25%27%7%
792
November1st to 10thNight
24%12%24%28%10%1
740
11th to 20thNight
22%13%28%26%9%2
760
21st to endNight
25%15%26%26%8%
760
December1st to 10thNight
28%15%29%23%41
760
11th to 20thNight
28%16%24%28%5%
760
21st to endNight
31%14%24%26%4
836
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