Northstar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northstar Observatory clear sky chart from 2005-11-06 to 2024-01-14.

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
46%
23039
13%
74950
13%
22999
49%
22999
19%
51764

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
23%
760
5%
2646
5%
756
25%
756
6%
2255
January11th to 20thNight
30%
736
8%
2574
7%
736
33%
736
9%
2054
January21st to endNight
31%
792
6%
2772
6%
792
33%
792
9%
2211
February1st to 10thNight
30%
720
8%
2520
7%
720
35%
720
11%
1949
February11th to 20thNight
36%
704
9%
2166
8%
700
38%
700
15%
1630
February21st to endNight
38%
592
14%
1776
13%
592
42%
592
14%
1347
March1st to 10thNight
40%
720
10%
2160
9%
720
42%
720
15%
1655
March11th to 20thNight
41%
720
13%
2096
10%
720
44%
720
17%
1545
March21st to endNight
38%
792
8%
2023
4%
792
39%
792
15%
1604
April1st to 10thNight
48%
720
13%
1800
7%
720
49%
720
23%
1304
April11th to 20thNight
41%
633
16%
1701
9%
633
45%
633
23%
1197
April21st to endNight
49%
540
15%
1620
12%
540
52%
540
22%
1095
May1st to 10thNight
53%
432
16%
1512
17%
432
58%
432
27%
892
May11th to 20thNight
58%
354
18%
1416
27%
354
60%
354
23%
819
May21st to endNight
57%
388
11%
1498
15%
388
61%
388
21%
938
June1st to 10thNight
57%
360
10%
1260
12%
360
59%
360
23%
795
June11th to 20thNight
50%
360
10%
1260
8%
360
53%
360
22%
776
June21st to endNight
70%
360
15%
1260
18%
360
70%
360
35%
657
July1st to 10thNight
78%
350
18%
1330
24%
350
81%
350
53%
537
July11th to 20thNight
74%
348
23%
1392
36%
348
80%
348
53%
528
July21st to endNight
85%
394
30%
1488
47%
372
89%
372
62%
518
August1st to 10thNight
80%
358
30%
1424
42%
356
84%
356
52%
545
August11th to 20thNight
82%
477
31%
1545
32%
477
83%
477
55%
694
August21st to endNight
73%
715
31%
1903
21%
715
76%
715
52%
1012
September1st to 10thNight
70%
720
29%
1800
17%
720
73%
720
55%
979
September11th to 20thNight
65%
720
27%
1916
19%
720
67%
720
39%
1163
September21st to endNight
60%
720
24%
1980
19%
720
63%
720
36%
1195
October1st to 10thNight
57%
720
20%
2026
17%
720
60%
720
32%
1272
October11th to 20thNight
53%
720
18%
2281
19%
720
56%
720
24%
1473
October21st to endNight
44%
792
18%
2574
16%
792
49%
792
21%
1736
November1st to 10thNight
38%
740
9%
2405
8%
740
40%
740
13%
1833
November11th to 20thNight
33%
760
8%
2541
6%
760
35%
760
10%
1986
November21st to endNight
38%
908
7%
2808
5%
908
39%
908
13%
2168
December1st to 10thNight
35%
950
8%
2850
5%
950
37%
950
12%
2260
December11th to 20thNight
26%
949
3%
3476
2%
949
27%
949
5%
2995
December21st to endNight
21%
1015
3%
5151
2%
1007
23%
1007
4%
4147

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
14%9%36%5%3445%6%39%
74558
Night
15%10%36%5%345%5%6%36%
75197
TransparencyDay
45%8%13%19%13%3
74261
Night
47%8%12%18%11%4
75012
SeeingDay
17%12%24%25%17%5%
13739
Night
20%7%17%32%20%3
23052

By Month

Percentage Cloud Cover NW (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
27%18%6%7%6%35%46%414%
1519
11th to 20thDay
24%17%5%10%8%44435%16%
1464
21st to endDay
21%16%7%11%6%345%45%18%
1584
February1st to 10thDay
22%14%6%8%5%35%5%5%5%21%
1440
11th to 20thDay
17%17%5%8%5%35%5%5%5%25%
1679
21st to endDay
18%18%6%8%6%4444423%
1480
March1st to 10thDay
18%16%5%8%7%35%5%5%6%24%
1800
11th to 20thDay
18%13%48%5%46%5%5%6%25%
1863
21st to endDay
21%16%5%8%7%45%45%5%20%
2330
April1st to 10thDay
17%12%48%6%46%6%6%7%24%
2160
11th to 20thDay
13%10%48%7%45%6%5%6%31%
2210
21st to endDay
13%9%37%6%46%5%6%7%33%
2340
May1st to 10thDay
9%7%36%5%36%5%6%9%41%
2447
11th to 20thDay
11%7%37%7%35%6%6%9%36%
2477
21st to endDay
12%8%37%7%45%6%6%8%35%
2772
June1st to 10thDay
17%7%36%6%45%45%7%37%
2700
11th to 20thDay
15%6%36%6%445%6%8%36%
2700
21st to endDay
7%4145%35%5%5%7%56%
2700
July1st to 10thDay
4323324445%66%
2585
11th to 20thDay
421232345%6%70%
2520
21st to endDay
111122333479%
2741
August1st to 10thDay
3212422247%69%
2507
11th to 20thDay
4313323447%68%
2355
21st to endDay
442342446%6%59%
2449
September1st to 10thDay
5%413333446%65%
2160
11th to 20thDay
8%425%5%25%5%5%7%51%
2044
21st to endDay
9%6%26%5%345%6%8%46%
1980
October1st to 10thDay
14%8%35%5%34346%45%
1931
11th to 20thDay
16%8%35%434447%43%
1678
21st to endDay
20%10%37%42445%5%35%
1782
November1st to 10thDay
27%12%6%6%5%335%37%24%
1674
11th to 20thDay
23%14%5%10%5%35%5%4424%
1638
21st to endDay
20%18%5%9%7%4444421%
1349
December1st to 10thDay
20%15%8%9%6%45%5%45%20%
1330
11th to 20thDay
24%22%8%11%7%45%33310%
1682
21st to endDay
20%20%6%10%7%46%35%5%16%
2488
January1st to 10thNight
28%18%5%8%6%3444416%
2646
11th to 20thNight
24%13%6%9%7%34436%22%
2574
21st to endNight
19%15%6%8%8%4445%7%22%
2772
February1st to 10thNight
20%18%5%6%5%3445%5%26%
2520
11th to 20thNight
16%14%47%5%45%5%45%29%
2165
21st to endNight
14%14%46%7%45%5%5%7%28%
1776
March1st to 10thNight
16%12%48%6%45%5%5%5%28%
2160
11th to 20thNight
16%11%46%5%45%5%5%7%31%
2095
21st to endNight
16%10%48%8%46%6%6%7%25%
2023
April1st to 10thNight
11%9%36%6%46%7%7%8%33%
1800
11th to 20thNight
13%8%48%6%35%5%45%39%
1701
21st to endNight
11%8%37%6%45%5%7%6%39%
1620
May1st to 10thNight
8%6%25%5%45%7%6%8%44%
1512
11th to 20thNight
10%6%36%5%45%5%6%9%42%
1416
21st to endNight
12%6%36%6%35%5%7%9%38%
1498
June1st to 10thNight
13%5%27%5%35%6%6%8%41%
1260
11th to 20thNight
12%426%6%36%6%5%9%41%
1260
21st to endNight
6%4246%35%45%7%55%
1260
July1st to 10thNight
221241345%5%72%
1370
11th to 20thNight
3112323346%72%
1440
21st to endNight
1112223335%77%
1576
August1st to 10thNight
3313313246%72%
1432
11th to 20thNight
3213313346%70%
1545
21st to endNight
431332346%6%64%
1903
September1st to 10thNight
422332335%7%65%
1800
11th to 20thNight
6%425%5%3445%7%53%
1916
21st to endNight
12%5%2443445%8%51%
1980
October1st to 10thNight
12%5%26%5%35%447%48%
2026
11th to 20thNight
14%6%26%435%35%6%45%
2281
21st to endNight
18%7%36%5%3345%6%41%
2574
November1st to 10thNight
23%12%46%5%345%5%7%26%
2405
11th to 20thNight
22%13%5%6%6%35%5%45%26%
2541
21st to endNight
23%11%47%6%45%5%46%26%
2808
December1st to 10thNight
19%14%6%9%5%3445%6%26%
2850
11th to 20thNight
24%19%410%7%445%45%13%
3510
21st to endNight
20%16%5%10%8%5%6%6%45%15%
5182

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
72%9%10%6%3
1519
11th to 20thDay
70%8%12%7%21
1464
21st to endDay
67%9%11%9%21
1584
February1st to 10thDay
65%8%12%10%41
1440
11th to 20thDay
61%9%13%11%6%
1689
21st to endDay
65%7%9%14%5%
1480
March1st to 10thDay
60%10%12%13%5%1
1800
11th to 20thDay
59%8%13%17%3
1863
21st to endDay
66%9%14%9%2
2330
April1st to 10thDay
57%9%16%15%3
2160
11th to 20thDay
51%8%14%17%9%1
2210
21st to endDay
49%10%14%18%8%1
2340
May1st to 10thDay
37%10%17%26%10%
2447
11th to 20thDay
43%9%16%23%9%
2480
21st to endDay
45%10%17%22%6%1
2773
June1st to 10thDay
45%8%14%22%10%
2700
11th to 20thDay
44%8%16%22%8%2
2700
21st to endDay
27%7%15%29%18%3
2700
July1st to 10thDay
20%5%11%27%25%12%
2516
11th to 20thDay
16%5%11%22%33%12%
2445
21st to endDay
10%36%23%44%14%
2589
August1st to 10thDay
16%49%24%33%13%
2490
11th to 20thDay
16%5%11%27%34%7%
2355
21st to endDay
23%6%11%28%27%5%
2449
September1st to 10thDay
21%6%11%30%23%9%
2160
11th to 20thDay
31%8%12%23%22%4
2044
21st to endDay
34%10%19%22%11%4
1980
October1st to 10thDay
41%6%13%29%11%
1931
11th to 20thDay
43%6%15%27%8%1
1678
21st to endDay
51%8%16%18%6%
1782
November1st to 10thDay
61%9%13%13%3
1674
11th to 20thDay
64%9%13%8%41
1638
21st to endDay
65%8%11%12%4
1349
December1st to 10thDay
66%8%10%12%3
1330
11th to 20thDay
77%8%8%6%1
1684
21st to endDay
66%10%11%9%4
2488
January1st to 10thNight
71%8%9%9%21
2646
11th to 20thNight
64%8%13%13%21
2574
21st to endNight
62%9%14%11%31
2772
February1st to 10thNight
60%8%14%12%5%1
2520
11th to 20thNight
56%7%17%13%7%
2166
21st to endNight
55%9%11%17%7%1
1776
March1st to 10thNight
56%9%13%15%7%1
2160
11th to 20thNight
52%7%13%18%9%
2096
21st to endNight
55%11%14%15%5%
2023
April1st to 10thNight
45%10%15%23%8%
1800
11th to 20thNight
47%7%11%21%13%1
1701
21st to endNight
44%8%13%23%11%1
1620
May1st to 10thNight
34%9%16%28%12%1
1512
11th to 20thNight
37%7%14%26%15%1
1416
21st to endNight
39%10%16%25%9%
1498
June1st to 10thNight
40%9%10%22%17%2
1260
11th to 20thNight
39%8%13%24%12%4
1260
21st to endNight
28%6%12%27%21%6%
1260
July1st to 10thNight
15%48%23%32%18%
1330
11th to 20thNight
14%37%21%31%23%
1392
21st to endNight
11%25%16%38%28%
1488
August1st to 10thNight
15%38%16%38%21%
1424
11th to 20thNight
16%49%22%36%13%
1545
21st to endNight
18%6%9%24%31%11%
1903
September1st to 10thNight
19%5%10%28%27%12%
1800
11th to 20thNight
28%8%10%22%24%8%
1916
21st to endNight
31%6%16%26%15%6%
1980
October1st to 10thNight
36%6%11%32%14%1
2026
11th to 20thNight
41%8%12%24%14%1
2281
21st to endNight
43%10%13%23%10%1
2574
November1st to 10thNight
56%9%16%14%41
2405
11th to 20thNight
61%8%14%10%5%2
2541
21st to endNight
58%8%15%13%5%
2808
December1st to 10thNight
61%8%13%14%4
2850
11th to 20thNight
71%9%10%8%1
3507
21st to endNight
63%13%12%8%3
5182

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
30%19%16%19%12%4
180
11th to 20thDay
32%16%19%18%8%7%
166
21st to endDay
31%12%28%18%11%
176
February1st to 10thDay
32%18%29%11%9%
160
11th to 20thDay
29%19%23%19%8%1
372
21st to endDay
26%18%23%23%10%1
346
March1st to 10thDay
26%22%22%18%9%3
420
11th to 20thDay
23%15%21%23%17%1
420
21st to endDay
27%19%24%19%9%1
462
April1st to 10thDay
24%15%29%21%8%3
420
11th to 20thDay
19%23%27%17%11%4
412
21st to endDay
18%16%26%26%12%3
420
May1st to 10thDay
13%13%27%31%14%3
423
11th to 20thDay
17%9%25%25%19%6%
448
21st to endDay
12%11%19%30%20%8%
534
June1st to 10thDay
18%10%21%25%20%5%
630
11th to 20thDay
16%14%30%22%15%3
630
21st to endDay
7%8%24%28%27%6%
630
July1st to 10thDay
6%6%23%35%28%3
511
11th to 20thDay
5%9%28%35%20%4
444
21st to endDay
37%24%35%24%8%
487
August1st to 10thDay
6%5%23%32%25%8%
447
11th to 20thDay
3321%36%26%11%
437
21st to endDay
6%8%27%29%21%9%
495
September1st to 10thDay
9%8%27%27%21%8%
450
11th to 20thDay
10%13%24%24%22%7%
450
21st to endDay
12%14%25%24%20%5%
450
October1st to 10thDay
16%8%26%26%16%6%
450
11th to 20thDay
19%14%22%23%14%8%
328
21st to endDay
31%13%22%21%10%3
297
November1st to 10thDay
35%13%17%25%10%
270
11th to 20thDay
35%11%19%18%16%2
236
21st to endDay
28%13%18%23%16%1
180
December1st to 10thDay
28%17%21%16%12%6%
180
11th to 20thDay
38%11%26%17%6%2
180
21st to endDay
36%14%19%20%11%2
198
January1st to 10thNight
39%9%17%23%9%2
760
11th to 20thNight
30%10%16%28%13%3
736
21st to endNight
28%6%19%31%15%2
792
February1st to 10thNight
31%9%21%28%9%2
720
11th to 20thNight
27%9%18%31%13%2
704
21st to endNight
21%11%21%28%17%2
592
March1st to 10thNight
22%9%20%30%16%1
720
11th to 20thNight
23%11%16%31%18%1
720
21st to endNight
21%9%22%33%12%2
792
April1st to 10thNight
16%7%20%36%18%1
720
11th to 20thNight
16%9%24%33%15%2
633
21st to endNight
14%8%22%34%20%2
540
May1st to 10thNight
10%5%22%33%26%3
432
11th to 20thNight
12%5%17%28%30%8%
354
21st to endNight
14%416%30%30%5%
389
June1st to 10thNight
14%6%16%36%24%4
360
11th to 20thNight
13%8%23%32%22%3
360
21st to endNight
6%412%40%32%6%
360
July1st to 10thNight
4212%40%40%3
350
11th to 20thNight
3315%47%31%2
348
21st to endNight
229%43%41%5%
394
August1st to 10thNight
2310%37%41%8%
358
11th to 20thNight
418%37%40%10%
477
21st to endNight
5%414%35%38%4
715
September1st to 10thNight
6%215%39%31%6%
720
11th to 20thNight
8%5%15%29%35%8%
720
21st to endNight
13%5%17%33%26%6%
720
October1st to 10thNight
15%6%13%39%26%2
720
11th to 20thNight
18%6%15%32%25%3
720
21st to endNight
22%8%20%30%17%4
792
November1st to 10thNight
31%8%16%33%13%
740
11th to 20thNight
31%9%18%27%13%1
760
21st to endNight
29%9%16%27%16%2
908
December1st to 10thNight
27%8%18%31%15%1
950
11th to 20thNight
35%8%17%29%9%
950
21st to endNight
36%11%17%27%8%
1026
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