StarEffects Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the StarEffects 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
36%
23204
19%
74659
11%
23158
46%
23158
22%
47863

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
20%
759
21%
2649
8%
757
37%
757
20%
1634
January11th to 20thNight
25%
735
18%
2484
8%
735
38%
735
16%
1678
January21st to endNight
23%
792
22%
2574
8%
792
40%
792
20%
1609
February1st to 10thNight
23%
720
17%
2340
8%
720
35%
720
17%
1545
February11th to 20thNight
14%
703
18%
2194
3%
700
33%
700
15%
1459
February21st to endNight
18%
592
23%
1776
8%
592
36%
592
17%
1188
March1st to 10thNight
24%
720
22%
2102
9%
720
40%
720
24%
1310
March11th to 20thNight
23%
720
21%
1980
9%
720
35%
720
21%
1333
March21st to endNight
24%
792
22%
2089
8%
792
35%
792
24%
1403
April1st to 10thNight
22%
720
18%
1800
4%
720
30%
720
21%
1247
April11th to 20thNight
29%
711
18%
1775
6%
711
35%
711
27%
1193
April21st to endNight
30%
720
19%
1800
5%
720
38%
720
26%
1225
May1st to 10thNight
35%
555
16%
1635
8%
555
40%
555
20%
1135
May11th to 20thNight
37%
354
18%
1416
12%
354
47%
354
15%
917
May21st to endNight
46%
388
15%
1551
15%
388
52%
388
18%
978
June1st to 10thNight
46%
360
19%
1440
19%
360
55%
360
21%
873
June11th to 20thNight
46%
360
15%
1440
14%
360
55%
360
21%
882
June21st to endNight
58%
360
17%
1440
20%
360
64%
360
26%
840
July1st to 10thNight
52%
350
13%
1400
16%
350
55%
350
18%
889
July11th to 20thNight
63%
348
18%
1392
21%
348
66%
348
26%
772
July21st to endNight
60%
486
15%
1573
14%
459
64%
459
25%
974
August1st to 10thNight
64%
537
17%
1600
13%
534
66%
534
29%
1024
August11th to 20thNight
55%
631
16%
1695
12%
631
57%
631
26%
1169
August21st to endNight
56%
792
18%
1980
8%
792
58%
792
32%
1322
September1st to 10thNight
50%
720
22%
1800
10%
720
54%
720
31%
1191
September11th to 20thNight
57%
720
30%
1800
21%
720
60%
720
36%
1138
September21st to endNight
49%
720
23%
2031
17%
720
55%
720
27%
1350
October1st to 10thNight
44%
720
18%
2160
14%
720
51%
720
25%
1406
October11th to 20thNight
41%
720
22%
2160
17%
720
51%
720
27%
1349
October21st to endNight
35%
792
25%
2396
15%
792
49%
792
26%
1502
November1st to 10thNight
41%
740
25%
2407
15%
740
56%
740
29%
1376
November11th to 20thNight
33%
760
24%
2474
14%
760
46%
760
23%
1533
November21st to endNight
33%
760
21%
2660
11%
760
46%
760
22%
1599
December1st to 10thNight
23%
760
17%
2660
7%
760
37%
760
16%
1766
December11th to 20thNight
28%
760
18%
3196
8%
759
42%
759
15%
2015
December21st to endNight
26%
827
11%
4790
8%
817
39%
817
11%
3039

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
16%6%25%5%35%5%6%8%39%
74724
Night
16%6%35%5%345%5%7%42%
75022
TransparencyDay
43%6%12%22%14%2
74435
Night
42%6%12%23%16%2
74845
SeeingDay
20%13%28%23%12%4
13767
Night
20%10%28%30%12%1
23215

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
12%8%35%5%45%6%6%7%39%
1517
11th to 20thDay
16%9%45%7%46%5%5%6%33%
1545
21st to endDay
14%8%37%6%36%5%5%7%35%
1782
February1st to 10thDay
17%7%36%6%36%6%5%7%34%
1620
11th to 20thDay
15%7%46%5%45%45%8%37%
1654
21st to endDay
20%8%47%6%35%446%35%
1480
March1st to 10thDay
12%6%35%6%35%45%7%43%
1852
11th to 20thDay
16%6%36%7%45%5%6%8%36%
1980
21st to endDay
21%7%35%5%34447%37%
2265
April1st to 10thDay
16%8%36%5%3445%8%38%
2160
11th to 20thDay
16%7%25%43445%7%42%
2134
21st to endDay
18%426%5%35%5%6%8%39%
2160
May1st to 10thDay
18%6%35%6%36%5%5%7%36%
2339
11th to 20thDay
21%7%346%35%5%6%7%35%
2478
21st to endDay
15%6%25%6%35%5%6%8%39%
2725
June1st to 10thDay
13%5%25%5%45%6%6%10%38%
2520
11th to 20thDay
15%5%25%5%46%6%6%8%38%
2520
21st to endDay
11%4246%36%5%7%11%41%
2520
July1st to 10thDay
11%5%25%6%5%7%6%7%9%37%
2520
11th to 20thDay
9%426%5%46%6%7%9%42%
2520
21st to endDay
10%6%37%7%47%5%6%7%38%
2649
August1st to 10thDay
11%5%26%6%46%7%7%9%36%
2327
11th to 20thDay
13%6%35%5%347%7%8%40%
2186
21st to endDay
14%5%25%5%35%5%7%9%40%
2376
September1st to 10thDay
17%5%35%5%34446%44%
2160
11th to 20thDay
12%5%2442345%7%51%
2160
21st to endDay
18%8%35%5%34446%40%
1927
October1st to 10thDay
19%6%2343445%7%43%
1800
11th to 20thDay
18%5%25%5%3445%7%44%
1800
21st to endDay
20%7%244335%48%40%
1958
November1st to 10thDay
14%7%23423438%50%
1671
11th to 20thDay
18%6%1443345%7%44%
1701
21st to endDay
18%4344345%6%7%42%
1520
December1st to 10thDay
18%8%26%7%345%5%7%34%
1520
11th to 20thDay
22%5%25%3345%5%7%38%
1906
21st to endDay
19%8%37%6%35%6%6%7%30%
2772
January1st to 10thNight
15%6%35%5%3445%8%41%
2649
11th to 20thNight
18%8%35%5%445%5%7%36%
2484
21st to endNight
13%8%36%5%35%45%6%42%
2574
February1st to 10thNight
17%8%35%5%35%5%5%7%38%
2340
11th to 20thNight
17%6%37%6%25%447%38%
2192
21st to endNight
19%7%35%5%34446%39%
1776
March1st to 10thNight
15%5%25%4345%5%7%46%
2092
11th to 20thNight
17%8%25%434445%44%
1979
21st to endNight
18%7%345%3445%7%41%
2089
April1st to 10thNight
18%6%35%6%35%46%8%38%
1800
11th to 20thNight
15%6%35%5%2445%8%43%
1775
21st to endNight
20%5%245%2445%6%43%
1800
May1st to 10thNight
20%7%35%5%345%5%7%37%
1635
11th to 20thNight
18%6%36%6%35%5%5%7%38%
1415
21st to endNight
16%5%36%5%35%5%6%8%40%
1550
June1st to 10thNight
16%6%25%5%2445%8%43%
1440
11th to 20thNight
14%5%25%436%5%6%9%41%
1440
21st to endNight
12%425%435%5%6%9%45%
1440
July1st to 10thNight
11%6%26%7%46%6%6%8%38%
1440
11th to 20thNight
8%4245%446%7%10%44%
1440
21st to endNight
11%5%35%5%45%6%7%9%41%
1666
August1st to 10thNight
10%6%26%6%35%5%7%8%43%
1611
11th to 20thNight
12%6%27%7%35%6%5%7%39%
1695
21st to endNight
13%5%24435%6%6%8%43%
1980
September1st to 10thNight
15%5%35%5%3335%6%47%
1800
11th to 20thNight
13%4245%3345%7%51%
1800
21st to endNight
17%6%25%435%446%43%
2031
October1st to 10thNight
21%5%34433447%44%
2160
11th to 20thNight
17%6%24423446%47%
2160
21st to endNight
20%5%2443345%7%45%
2396
November1st to 10thNight
12%5%24435%45%6%52%
2407
11th to 20thNight
16%7%35%424446%45%
2474
21st to endNight
15%5%25%43445%8%44%
2660
December1st to 10thNight
19%8%25%5%346%5%7%36%
2660
11th to 20thNight
18%7%25%5%35%5%5%6%39%
3288
21st to endNight
19%8%35%6%35%5%6%7%34%
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
43%5%14%23%12%3
1518
11th to 20thDay
52%6%13%18%11%2
1545
21st to endDay
48%5%12%21%12%2
1782
February1st to 10thDay
49%7%13%20%11%1
1620
11th to 20thDay
46%5%10%25%12%2
1663
21st to endDay
52%410%20%10%3
1480
March1st to 10thDay
41%5%12%26%13%2
1855
11th to 20thDay
46%6%11%19%14%4
1980
21st to endDay
48%49%19%17%3
2265
April1st to 10thDay
45%5%10%21%14%5%
2160
11th to 20thDay
42%5%11%22%16%5%
2134
21st to endDay
43%5%11%20%16%5%
2160
May1st to 10thDay
46%8%9%19%15%4
2339
11th to 20thDay
47%7%10%20%13%3
2482
21st to endDay
42%10%12%22%13%2
2725
June1st to 10thDay
41%8%14%23%14%2
2520
11th to 20thDay
42%8%13%23%11%3
2520
21st to endDay
36%8%14%26%15%1
2520
July1st to 10thDay
41%8%12%22%16%2
2450
11th to 20thDay
34%9%14%25%16%2
2445
21st to endDay
42%7%11%21%15%3
2501
August1st to 10thDay
40%10%12%23%15%
2314
11th to 20thDay
38%8%14%23%15%1
2186
21st to endDay
38%8%12%22%17%3
2376
September1st to 10thDay
41%7%10%20%18%4
2160
11th to 20thDay
33%5%10%25%23%4
2160
21st to endDay
45%5%8%18%21%2
1927
October1st to 10thDay
41%6%12%24%17%1
1800
11th to 20thDay
41%412%24%18%2
1800
21st to endDay
43%5%11%23%16%2
1958
November1st to 10thDay
35%414%25%19%3
1671
11th to 20thDay
40%5%11%23%19%2
1701
21st to endDay
40%6%13%30%9%2
1520
December1st to 10thDay
50%6%15%19%10%1
1520
11th to 20thDay
46%416%26%8%1
1906
21st to endDay
51%6%15%19%8%
2772
January1st to 10thNight
42%413%24%14%3
2649
11th to 20thNight
48%5%14%21%10%2
2484
21st to endNight
42%412%25%15%2
2574
February1st to 10thNight
46%5%14%24%10%2
2340
11th to 20thNight
47%410%20%16%3
2194
21st to endNight
46%410%21%16%3
1776
March1st to 10thNight
38%5%12%25%17%3
2102
11th to 20thNight
43%5%10%22%16%3
1980
21st to endNight
44%410%19%18%4
2089
April1st to 10thNight
44%5%11%21%15%4
1800
11th to 20thNight
39%7%11%20%20%3
1775
21st to endNight
41%6%11%21%17%5%
1800
May1st to 10thNight
46%7%9%19%14%3
1635
11th to 20thNight
46%8%10%15%19%3
1416
21st to endNight
41%9%13%19%15%3
1551
June1st to 10thNight
40%7%11%22%19%2
1440
11th to 20thNight
38%9%13%24%14%2
1440
21st to endNight
34%9%14%28%14%1
1440
July1st to 10thNight
42%10%13%18%14%3
1400
11th to 20thNight
32%8%16%26%15%2
1392
21st to endNight
36%10%16%21%16%2
1573
August1st to 10thNight
37%8%14%23%16%1
1600
11th to 20thNight
42%9%13%18%17%2
1695
21st to endNight
36%8%14%20%19%3
1980
September1st to 10thNight
39%6%11%22%18%4
1800
11th to 20thNight
34%6%8%24%25%3
1800
21st to endNight
42%6%8%19%22%2
2031
October1st to 10thNight
43%5%11%24%16%2
2160
11th to 20thNight
39%5%10%29%15%2
2160
21st to endNight
40%410%26%18%2
2396
November1st to 10thNight
33%5%12%26%19%4
2407
11th to 20thNight
41%49%25%17%4
2474
21st to endNight
39%5%13%28%14%1
2660
December1st to 10thNight
48%5%13%20%13%1
2660
11th to 20thNight
45%6%13%21%13%2
3288
21st to endNight
47%7%15%22%9%
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
24%24%30%14%6%1
180
11th to 20thDay
30%22%31%14%2
202
21st to endDay
22%25%34%13%42
264
February1st to 10thDay
29%23%31%15%3
240
11th to 20thDay
27%23%35%14%1
321
21st to endDay
31%17%32%19%1
346
March1st to 10thDay
22%21%33%21%3
420
11th to 20thDay
23%25%32%15%41
420
21st to endDay
28%19%29%17%6%2
462
April1st to 10thDay
24%20%32%16%7%2
420
11th to 20thDay
22%19%28%19%9%4
412
21st to endDay
27%15%33%17%5%2
420
May1st to 10thDay
28%14%26%19%10%3
423
11th to 20thDay
23%14%32%20%9%3
447
21st to endDay
19%6%25%27%16%6%
485
June1st to 10thDay
16%11%32%24%14%4
450
11th to 20thDay
20%8%31%23%15%3
450
21st to endDay
14%10%29%28%15%5%
450
July1st to 10thDay
14%422%28%22%10%
445
11th to 20thDay
8%424%33%24%8%
444
21st to endDay
12%429%32%18%6%
488
August1st to 10thDay
13%421%30%22%11%
446
11th to 20thDay
15%418%31%21%10%
439
21st to endDay
13%423%31%17%12%
495
September1st to 10thDay
20%7%20%22%21%9%
450
11th to 20thDay
14%5%21%28%20%12%
450
21st to endDay
23%12%21%21%18%5%
450
October1st to 10thDay
25%5%24%24%16%7%
450
11th to 20thDay
19%13%28%24%14%2
450
21st to endDay
23%12%32%22%10%1
484
November1st to 10thDay
17%13%26%26%15%3
365
11th to 20thDay
19%16%32%24%8%
361
21st to endDay
21%20%33%18%7%1
180
December1st to 10thDay
26%20%32%14%7%1
180
11th to 20thDay
26%23%31%15%41
180
21st to endDay
27%20%26%19%6%2
198
January1st to 10thNight
21%15%40%21%4
760
11th to 20thNight
25%15%30%26%4
735
21st to endNight
17%17%37%28%2
792
February1st to 10thNight
23%16%32%28%2
720
11th to 20thNight
24%20%37%18%2
703
21st to endNight
25%17%35%20%3
592
March1st to 10thNight
22%16%34%25%4
720
11th to 20thNight
25%17%32%20%6%1
720
21st to endNight
22%15%35%23%5%
792
April1st to 10thNight
24%18%32%22%5%
720
11th to 20thNight
17%14%35%26%8%
711
21st to endNight
24%13%29%27%8%
720
May1st to 10thNight
25%9%26%30%10%
555
11th to 20thNight
19%8%29%33%9%1
354
21st to endNight
16%5%24%36%17%1
389
June1st to 10thNight
17%5%23%35%20%
360
11th to 20thNight
16%5%24%41%14%1
360
21st to endNight
11%322%49%15%1
360
July1st to 10thNight
11%321%42%21%3
350
11th to 20thNight
10%219%41%27%1
348
21st to endNight
12%316%43%26%1
486
August1st to 10thNight
12%311%42%28%3
537
11th to 20thNight
15%216%34%31%3
631
21st to endNight
15%318%34%28%2
792
September1st to 10thNight
16%321%33%23%3
720
11th to 20thNight
14%417%35%26%5%
720
21st to endNight
21%320%33%21%2
720
October1st to 10thNight
23%6%22%28%17%4
720
11th to 20thNight
21%9%23%33%13%1
720
21st to endNight
23%10%27%30%10%
792
November1st to 10thNight
16%8%30%33%13%1
740
11th to 20thNight
20%14%29%29%8%1
760
21st to endNight
18%13%31%31%7%
760
December1st to 10thNight
26%13%32%22%6%1
760
11th to 20thNight
21%13%33%28%6%
760
21st to endNight
23%11%33%29%3
836
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