Digital-SF Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Digital-SF Observatory clear sky chart from 2005-11-05 to 2018-11-30.

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
39%
16036
15%
51727
10%
16009
47%
16009
21%
33080

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
30%
777
7%
2716
5%
776
33%
776
8%
2155
January16th to endNight
35%
832
9%
2912
7%
832
38%
832
12%
2195
February1st to 15thNight
32%
758
11%
2486
7%
757
38%
757
13%
1825
February16th to endNight
36%
684
13%
2130
9%
684
42%
684
16%
1508
March1st to 15thNight
37%
780
11%
2295
7%
780
42%
780
16%
1656
March16th to endNight
34%
832
11%
2197
5%
832
39%
832
17%
1578
April1st to 15thNight
34%
779
12%
1949
4%
779
37%
779
21%
1393
April16th to endNight
34%
623
14%
1781
5%
623
42%
623
25%
1123
May1st to 15thNight
39%
389
20%
1559
18%
389
52%
389
24%
834
May16th to endNight
43%
407
11%
1624
11%
406
53%
406
21%
929
June1st to 15thNight
33%
390
11%
1560
6%
390
43%
390
24%
837
June16th to endNight
32%
390
11%
1560
7%
390
43%
390
28%
751
July1st to 15thNight
48%
370
18%
1480
14%
370
64%
370
52%
540
July16th to endNight
44%
408
26%
1544
19%
386
71%
386
57%
528
August1st to 15thNight
56%
429
31%
1573
24%
427
73%
427
60%
583
August16th to endNight
53%
748
32%
1990
16%
748
62%
748
56%
971
September1st to 15thNight
58%
780
30%
1950
13%
780
64%
780
58%
988
September16th to endNight
53%
776
26%
2112
16%
776
60%
776
40%
1191
October1st to 15thNight
47%
780
20%
2340
16%
780
54%
780
28%
1409
October16th to endNight
45%
832
17%
2613
14%
832
52%
832
21%
1713
November1st to 15thNight
35%
820
12%
2774
7%
820
42%
820
13%
1969
November16th to endNight
34%
840
9%
2940
5%
840
40%
840
12%
2158
December1st to 15thNight
36%
780
11%
2730
8%
780
41%
780
12%
2022
December16th to endNight
25%
832
9%
2912
3%
832
31%
832
9%
2224

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
6%6%35%5%35%5%6%8%44%
52597
Night
6%7%35%5%345%6%8%43%
51895
Cloud CoverDay
6%5%25%5%345%6%7%46%
52597
Night
7%6%35%5%345%6%8%45%
51899
TransparencyDay
34%11%15%21%13%3
52268
Night
35%12%16%21%11%2
51728
SeeingDay
10%22%32%23%9%2
6924
Night
9%14%29%28%14%2
16046

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 15thDay
11%11%5%8%8%45%6%7%8%22%
1552
16th to endDay
9%12%5%7%6%36%5%6%9%27%
1664
February1st to 15thDay
9%11%47%6%35%5%7%9%29%
1705
16th to endDay
7%9%48%8%35%6%7%9%29%
1629
March1st to 15thDay
9%11%5%9%7%35%6%6%7%25%
1992
16th to endDay
8%9%5%8%9%46%6%6%8%26%
2379
April1st to 15thDay
8%10%48%8%37%5%7%8%27%
2329
16th to endDay
5%7%35%5%35%5%7%8%40%
2453
May1st to 15thDay
5%5%26%436%5%7%9%42%
2721
16th to endDay
8%7%36%6%345%6%8%38%
2839
June1st to 15thDay
6%324435%6%6%9%44%
2730
16th to endDay
5%3145%345%6%8%51%
2730
July1st to 15thDay
1112324346%68%
2730
16th to endDay
211221223474%
2881
August1st to 15thDay
112213247%71%
2631
16th to endDay
11343345%7%64%
2564
September1st to 15thDay
222323346%67%
2340
16th to endDay
6%325%4345%6%7%50%
2174
October1st to 15thDay
9%6%25%5%3446%7%43%
1950
16th to endDay
10%7%37%5%3445%7%39%
1963
November1st to 15thDay
8%9%48%7%46%6%7%8%29%
1743
16th to endDay
10%13%48%6%35%5%6%8%27%
1674
December1st to 15thDay
8%11%5%9%7%45%5%5%7%28%
1560
16th to endDay
11%13%48%7%3446%7%27%
1664
January1st to 15thNight
10%10%5%8%7%46%6%6%9%24%
2716
16th to endNight
10%9%46%6%47%6%7%9%26%
2912
February1st to 15thNight
9%11%47%5%35%45%10%32%
2474
16th to endNight
6%9%45%6%35%47%7%38%
2130
March1st to 15thNight
8%8%47%5%35%5%7%8%33%
2295
16th to endNight
6%7%35%7%46%6%9%10%33%
2193
April1st to 15thNight
6%5%36%6%37%6%7%9%37%
1949
16th to endNight
5%5%25%5%25%5%7%12%42%
1781
May1st to 15thNight
5%4245%345%7%8%48%
1559
16th to endNight
5%5%245%36%6%7%8%43%
1624
June1st to 15thNight
4423435%5%8%7%50%
1560
16th to endNight
24134345%6%8%54%
1560
July1st to 15thNight
1111212346%74%
1560
16th to endNight
1111236%80%
1640
August1st to 15thNight
112447%75%
1581
16th to endNight
1112212348%70%
1990
September1st to 15thNight
211211247%73%
1950
16th to endNight
332432445%9%56%
2112
October1st to 15thNight
7%5%245%3345%6%49%
2340
16th to endNight
8%6%26%5%345%6%8%40%
2613
November1st to 15thNight
10%8%45%5%35%5%6%9%33%
2774
16th to endNight
8%11%47%6%35%5%6%7%31%
2940
December1st to 15thNight
8%10%5%7%6%45%5%6%8%30%
2730
16th to endNight
11%15%5%8%6%2435%7%27%
2912

Percentage Cloud Cover SW (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
11%11%6%8%8%45%6%5%7%22%
1552
16th to endDay
9%12%47%5%35%5%6%9%29%
1664
February1st to 15thDay
9%10%47%5%35%6%7%8%30%
1705
16th to endDay
7%8%36%7%46%6%7%9%32%
1629
March1st to 15thDay
10%9%48%6%46%6%6%8%28%
1992
16th to endDay
8%8%38%8%36%6%6%8%30%
2379
April1st to 15thDay
8%8%37%7%36%6%7%8%31%
2329
16th to endDay
46%25%5%35%6%7%8%43%
2453
May1st to 15thDay
5%325%5%36%5%7%9%45%
2721
16th to endDay
7%5%35%5%35%6%6%7%41%
2839
June1st to 15thDay
6%3245%35%5%7%8%48%
2730
16th to endDay
4213425%5%6%8%54%
2730
July1st to 15thDay
1112223346%70%
2730
16th to endDay
21222223476%
2881
August1st to 15thDay
111222346%73%
2631
16th to endDay
1112324447%65%
2564
September1st to 15thDay
222312347%69%
2340
16th to endDay
5%3144236%5%7%53%
2174
October1st to 15thDay
8%5%25%5%35%5%6%7%46%
1950
16th to endDay
10%5%25%6%345%5%7%41%
1963
November1st to 15thDay
8%8%37%8%36%7%7%8%30%
1743
16th to endDay
10%11%47%6%46%5%5%8%27%
1674
December1st to 15thDay
9%11%48%7%35%6%6%6%29%
1560
16th to endDay
12%12%5%7%6%245%5%8%28%
1664
January1st to 15thNight
10%11%6%8%8%45%6%6%9%23%
2716
16th to endNight
10%10%48%7%46%6%6%9%25%
2912
February1st to 15thNight
9%10%47%6%3446%9%34%
2474
16th to endNight
7%8%35%5%35%6%7%8%38%
2130
March1st to 15thNight
9%8%36%5%35%5%6%9%34%
2295
16th to endNight
6%7%35%5%36%6%7%10%37%
2197
April1st to 15thNight
6%5%25%6%46%6%6%9%40%
1949
16th to endNight
5%424445%5%6%10%47%
1781
May1st to 15thNight
5%3245%245%6%7%52%
1559
16th to endNight
5%4145%45%5%7%8%46%
1624
June1st to 15thNight
4313335%5%6%9%51%
1560
16th to endNight
23234245%6%7%57%
1560
July1st to 15thNight
111112337%75%
1560
16th to endNight
111123481%
1640
August1st to 15thNight
1225%7%77%
1581
16th to endNight
112212347%72%
1990
September1st to 15thNight
211212236%75%
1950
16th to endNight
321333345%8%58%
2112
October1st to 15thNight
7%415%42445%7%52%
2340
16th to endNight
8%5%25%5%35%5%7%8%42%
2613
November1st to 15thNight
11%6%26%5%35%5%7%8%35%
2774
16th to endNight
9%10%5%7%6%35%5%5%8%33%
2940
December1st to 15thNight
9%10%47%7%45%5%5%7%32%
2730
16th to endNight
12%14%5%8%5%445%5%6%28%
2912

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
58%15%11%10%31
1552
16th to endDay
51%13%16%14%4
1664
February1st to 15thDay
49%16%16%13%3
1705
16th to endDay
43%18%17%14%6%
1629
March1st to 15thDay
50%15%14%13%5%
1992
16th to endDay
49%15%17%14%2
2379
April1st to 15thDay
48%13%17%15%4
2329
16th to endDay
33%15%20%21%8%
2453
May1st to 15thDay
31%12%18%26%10%1
2721
16th to endDay
37%13%20%20%7%
2843
June1st to 15thDay
30%12%17%25%11%2
2730
16th to endDay
23%8%19%29%16%2
2730
July1st to 15thDay
15%6%10%24%31%11%
2581
16th to endDay
12%47%23%33%18%
2714
August1st to 15thDay
12%5%10%31%29%10%
2614
16th to endDay
16%7%12%28%29%5%
2564
September1st to 15thDay
16%6%11%28%29%7%
2340
16th to endDay
27%11%16%23%15%5%
2174
October1st to 15thDay
34%11%15%24%12%
1950
16th to endDay
39%11%15%22%11%
1963
November1st to 15thDay
47%16%14%14%5%1
1743
16th to endDay
53%14%14%13%4
1674
December1st to 15thDay
53%11%13%16%4
1560
16th to endDay
52%14%14%13%5%
1664
January1st to 15thNight
56%14%13%10%4
2716
16th to endNight
51%12%16%14%3
2912
February1st to 15thNight
46%14%17%16%4
2486
16th to endNight
40%15%16%20%6%
2130
March1st to 15thNight
42%15%16%19%6%
2295
16th to endNight
37%17%21%20%3
2197
April1st to 15thNight
36%16%19%21%6%
1949
16th to endNight
29%13%23%23%8%1
1781
May1st to 15thNight
28%9%17%31%11%1
1560
16th to endNight
30%12%22%26%7%
1624
June1st to 15thNight
26%11%20%28%11%1
1560
16th to endNight
23%8%20%31%14%2
1560
July1st to 15thNight
10%5%15%30%27%11%
1480
16th to endNight
7%410%24%33%19%
1544
August1st to 15thNight
7%5%13%31%30%11%
1573
16th to endNight
11%6%13%32%28%6%
1990
September1st to 15thNight
11%6%13%29%31%7%
1950
16th to endNight
21%9%18%29%15%6%
2112
October1st to 15thNight
30%10%14%28%15%1
2340
16th to endNight
37%12%13%23%13%
2613
November1st to 15thNight
42%15%16%17%6%1
2774
16th to endNight
47%14%15%15%7%
2940
December1st to 15thNight
49%13%14%18%4
2730
16th to endNight
55%11%15%11%5%
2912

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
18%12%14%32%14%7%
90
16th to endDay
22%16%27%18%11%3
96
February1st to 15thDay
17%22%37%17%41
275
16th to endDay
18%25%31%18%4
271
March1st to 15thDay
16%33%26%17%3
330
16th to endDay
19%33%29%13%4
352
April1st to 15thDay
15%34%28%14%42
329
16th to endDay
11%33%32%12%6%3
318
May1st to 15thDay
14%21%33%23%5%1
348
16th to endDay
8%17%33%25%12%1
394
June1st to 15thDay
9%24%36%21%8%
375
16th to endDay
8%26%36%18%9%1
375
July1st to 15thDay
216%36%34%8%
365
16th to endDay
217%36%30%11%1
391
August1st to 15thDay
311%33%32%14%4
359
16th to endDay
214%32%35%12%3
396
September1st to 15thDay
319%30%28%14%2
375
16th to endDay
7%18%32%29%12%
375
October1st to 15thDay
13%22%33%20%9%1
375
16th to endDay
15%15%22%27%12%6%
283
November1st to 15thDay
20%26%27%19%4
148
16th to endDay
16%21%27%18%12%4
118
December1st to 15thDay
12%16%30%18%11%11%
90
16th to endDay
14%14%30%29%11%
96
January1st to 15thNight
14%17%21%27%15%2
780
16th to endNight
16%12%23%30%13%3
832
February1st to 15thNight
12%15%29%29%11%1
764
16th to endNight
10%18%26%28%13%1
684
March1st to 15thNight
13%14%28%31%10%1
780
16th to endNight
8%21%30%27%10%1
832
April1st to 15thNight
8%18%33%26%10%1
779
16th to endNight
9%19%33%24%11%2
623
May1st to 15thNight
7%17%31%28%13%1
390
16th to endNight
6%11%32%34%12%1
407
June1st to 15thNight
6%16%40%27%9%
390
16th to endNight
5%19%40%22%11%1
390
July1st to 15thNight
112%36%39%10%
370
16th to endNight
13%40%31%11%1
408
August1st to 15thNight
7%34%33%18%5%
429
16th to endNight
210%31%30%19%5%
748
September1st to 15thNight
310%25%28%24%7%
780
16th to endNight
411%26%28%22%5%
776
October1st to 15thNight
8%14%25%29%18%3
780
16th to endNight
11%13%24%27%17%5%
832
November1st to 15thNight
14%15%28%29%11%
820
16th to endNight
15%15%25%24%14%4
840
December1st to 15thNight
13%11%29%28%14%2
780
16th to endNight
17%16%29%29%5%
832
Page generated at 2018-12-01 08:35:35. Version ac04

Copyright 2018 A. Danko


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