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 2017-09-20.

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
38%
14533
15%
46884
9%
14506
46%
14506
21%
30083

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%
717
8%
2506
6%
716
33%
716
9%
1959
January16th to endNight
35%
768
9%
2688
7%
768
39%
768
12%
2020
February1st to 15thNight
32%
698
9%
2291
6%
697
37%
697
13%
1696
February16th to endNight
37%
632
13%
1968
9%
632
43%
632
16%
1398
March1st to 15thNight
35%
720
10%
2118
6%
720
40%
720
14%
1558
March16th to endNight
33%
768
10%
2028
3%
768
38%
768
17%
1465
April1st to 15thNight
34%
719
13%
1799
5%
719
38%
719
22%
1269
April16th to endNight
33%
582
15%
1662
5%
582
41%
582
24%
1051
May1st to 15thNight
38%
359
20%
1439
18%
359
52%
359
25%
768
May16th to endNight
41%
375
11%
1496
11%
374
51%
374
20%
871
June1st to 15thNight
32%
360
10%
1440
5%
360
41%
360
23%
778
June16th to endNight
31%
360
12%
1440
6%
360
43%
360
28%
696
July1st to 15thNight
47%
340
16%
1360
12%
340
63%
340
52%
498
July16th to endNight
42%
376
26%
1416
20%
354
69%
354
56%
487
August1st to 15thNight
54%
402
30%
1474
21%
400
72%
400
58%
556
August16th to endNight
52%
690
32%
1836
16%
690
61%
690
55%
900
September1st to 15thNight
58%
720
29%
1800
12%
720
65%
720
58%
910
September16th to endNight
51%
675
27%
1836
17%
675
58%
675
39%
1049
October1st to 15thNight
45%
660
18%
1980
14%
660
51%
660
28%
1209
October16th to endNight
42%
704
15%
2211
10%
704
49%
704
19%
1498
November1st to 15thNight
36%
700
11%
2368
6%
700
42%
700
13%
1678
November16th to endNight
33%
720
10%
2520
5%
720
40%
720
13%
1840
December1st to 15thNight
34%
720
11%
2520
8%
720
39%
720
12%
1877
December16th to endNight
23%
768
8%
2688
3%
768
29%
768
8%
2052

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%36%5%35%5%6%8%43%
47977
Night
6%7%35%5%35%5%6%8%43%
47052
Cloud CoverDay
6%6%25%5%35%5%6%8%45%
47977
Night
7%6%35%5%345%6%8%44%
47056
TransparencyDay
35%11%15%21%13%3
47648
Night
35%12%16%21%11%2
46885
SeeingDay
11%22%33%23%8%1
5476
Night
9%15%29%28%13%2
14543

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
10%10%5%8%8%45%6%7%9%23%
1432
16th to endDay
8%11%5%7%5%36%5%7%9%28%
1536
February1st to 15thDay
10%12%47%6%36%5%7%9%26%
1570
16th to endDay
7%9%38%7%35%6%7%9%29%
1505
March1st to 15thDay
9%11%5%9%7%35%6%6%7%24%
1839
16th to endDay
8%9%5%9%9%47%6%6%8%24%
2196
April1st to 15thDay
7%9%48%7%37%5%7%9%27%
2149
16th to endDay
5%7%36%5%36%5%7%9%39%
2297
May1st to 15thDay
6%5%26%436%5%7%9%41%
2511
16th to endDay
8%6%36%6%35%5%6%8%37%
2615
June1st to 15thDay
6%325%5%35%5%6%9%44%
2520
16th to endDay
5%3145%3447%8%51%
2520
July1st to 15thDay
1112324346%68%
2520
16th to endDay
221221223473%
2657
August1st to 15thDay
112213347%69%
2478
16th to endDay
111343345%7%63%
2366
September1st to 15thDay
222323346%67%
2160
16th to endDay
6%325%5%35%5%6%7%49%
1890
October1st to 15thDay
9%6%26%5%345%6%8%42%
1650
16th to endDay
11%7%36%6%45%46%8%36%
1661
November1st to 15thDay
8%7%48%7%46%6%7%9%28%
1489
16th to endDay
10%13%48%6%35%5%6%8%27%
1440
December1st to 15thDay
8%12%5%9%7%45%45%7%28%
1440
16th to endDay
11%13%5%7%6%345%6%8%27%
1536
January1st to 15thNight
10%9%5%7%7%46%5%6%9%25%
2506
16th to endNight
10%9%46%6%47%6%8%9%27%
2688
February1st to 15thNight
9%12%47%5%35%35%10%31%
2279
16th to endNight
7%9%45%6%35%5%7%7%37%
1968
March1st to 15thNight
8%8%47%6%36%6%7%8%32%
2118
16th to endNight
6%7%35%7%45%6%9%10%32%
2024
April1st to 15thNight
6%436%6%47%6%6%9%38%
1799
16th to endNight
5%5%25%5%35%6%7%12%42%
1662
May1st to 15thNight
5%3245%345%7%8%48%
1439
16th to endNight
5%5%245%36%6%8%8%41%
1496
June1st to 15thNight
4423435%5%8%7%49%
1440
16th to endNight
24144245%6%8%54%
1440
July1st to 15thNight
1111212346%73%
1440
16th to endNight
1111136%80%
1512
August1st to 15thNight
11245%7%74%
1482
16th to endNight
111221235%8%69%
1836
September1st to 15thNight
2111211247%73%
1800
16th to endNight
332432445%9%55%
1836
October1st to 15thNight
7%425%5%3345%6%48%
1980
16th to endNight
8%7%26%5%346%7%9%36%
2211
November1st to 15thNight
10%8%35%6%45%5%7%8%33%
2368
16th to endNight
8%10%47%6%36%5%6%7%32%
2520
December1st to 15thNight
9%10%5%7%6%45%46%7%30%
2520
16th to endNight
11%15%5%8%5%2435%7%27%
2688

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%35%7%6%8%23%
1432
16th to endDay
9%12%47%5%36%46%9%28%
1536
February1st to 15thDay
10%10%47%5%45%6%7%8%28%
1570
16th to endDay
7%8%36%7%45%6%7%9%32%
1505
March1st to 15thDay
10%10%48%6%46%6%6%8%26%
1839
16th to endDay
8%8%38%8%36%7%6%7%29%
2196
April1st to 15thDay
7%8%37%7%36%6%7%9%31%
2149
16th to endDay
46%25%5%35%6%7%8%42%
2297
May1st to 15thDay
5%4245%36%5%7%9%44%
2511
16th to endDay
7%5%35%5%35%6%7%8%40%
2615
June1st to 15thDay
6%3245%35%5%7%8%47%
2520
16th to endDay
5%314425%5%6%8%53%
2520
July1st to 15thDay
1112223346%69%
2520
16th to endDay
21222223474%
2657
August1st to 15thDay
111222446%71%
2478
16th to endDay
1113324447%65%
2366
September1st to 15thDay
222312347%69%
2160
16th to endDay
5%3144236%5%7%52%
1890
October1st to 15thDay
8%5%25%5%245%6%7%45%
1650
16th to endDay
11%6%26%6%35%6%6%7%38%
1661
November1st to 15thDay
9%6%36%8%37%7%7%9%29%
1489
16th to endDay
11%11%47%7%46%5%5%8%26%
1440
December1st to 15thDay
9%12%5%8%7%345%6%6%29%
1440
16th to endDay
12%12%5%7%6%245%5%8%29%
1536
January1st to 15thNight
10%11%6%7%8%45%6%6%9%24%
2506
16th to endNight
10%10%48%7%46%6%6%9%26%
2688
February1st to 15thNight
10%11%47%6%3446%8%32%
2279
16th to endNight
7%8%35%5%26%6%7%8%38%
1968
March1st to 15thNight
9%8%46%6%45%6%6%9%33%
2118
16th to endNight
6%7%35%6%36%6%7%10%36%
2028
April1st to 15thNight
6%5%25%6%45%6%5%9%42%
1799
16th to endNight
5%414445%5%6%10%46%
1662
May1st to 15thNight
5%2245%245%6%7%52%
1439
16th to endNight
5%415%5%45%5%7%8%44%
1496
June1st to 15thNight
4313435%5%6%9%50%
1440
16th to endNight
23234245%6%7%56%
1440
July1st to 15thNight
111112337%74%
1440
16th to endNight
111123481%
1512
August1st to 15thNight
11225%7%76%
1482
16th to endNight
112212348%71%
1836
September1st to 15thNight
211212236%75%
1800
16th to endNight
421332345%9%57%
1836
October1st to 15thNight
7%415%5%3436%6%50%
1980
16th to endNight
8%5%26%6%35%6%7%9%38%
2211
November1st to 15thNight
11%5%36%5%35%5%7%8%36%
2368
16th to endNight
9%10%5%7%7%35%5%5%7%33%
2520
December1st to 15thNight
10%10%46%7%35%5%5%7%32%
2520
16th to endNight
12%14%5%8%5%3445%6%28%
2688

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
56%15%11%10%31
1432
16th to endDay
51%13%16%14%4
1536
February1st to 15thDay
51%15%16%13%2
1570
16th to endDay
43%18%17%14%5%
1505
March1st to 15thDay
51%15%14%12%5%
1839
16th to endDay
50%15%17%14%2
2196
April1st to 15thDay
47%12%18%16%4
2149
16th to endDay
35%15%20%21%6%
2297
May1st to 15thDay
31%11%18%25%10%1
2511
16th to endDay
38%14%20%19%6%
2619
June1st to 15thDay
31%12%17%25%11%2
2520
16th to endDay
24%8%19%29%16%2
2520
July1st to 15thDay
15%6%10%24%30%11%
2371
16th to endDay
13%48%25%31%16%
2490
August1st to 15thDay
12%5%11%31%28%10%
2461
16th to endDay
16%7%12%28%28%5%
2366
September1st to 15thDay
17%6%11%26%30%8%
2160
16th to endDay
27%11%16%22%16%6%
1890
October1st to 15thDay
34%12%16%24%11%
1650
16th to endDay
42%11%15%20%8%
1661
November1st to 15thDay
47%16%14%15%31
1489
16th to endDay
52%14%14%14%4
1440
December1st to 15thDay
53%11%13%16%5%
1440
16th to endDay
51%14%15%14%5%
1536
January1st to 15thNight
54%14%13%11%4
2506
16th to endNight
51%12%17%14%31
2688
February1st to 15thNight
47%14%17%15%4
2291
16th to endNight
41%14%16%20%5%
1968
March1st to 15thNight
43%16%16%16%6%
2118
16th to endNight
38%17%21%19%2
2028
April1st to 15thNight
35%15%19%22%7%
1799
16th to endNight
29%14%23%24%7%
1662
May1st to 15thNight
28%8%17%32%12%1
1440
16th to endNight
31%12%23%24%6%
1496
June1st to 15thNight
27%11%20%28%10%1
1440
16th to endNight
23%8%20%30%14%2
1440
July1st to 15thNight
10%416%30%26%11%
1360
16th to endNight
7%410%26%32%18%
1416
August1st to 15thNight
7%6%14%31%29%11%
1474
16th to endNight
11%7%13%32%27%6%
1836
September1st to 15thNight
11%5%13%27%32%8%
1800
16th to endNight
22%9%18%27%15%6%
1836
October1st to 15thNight
31%10%15%27%13%1
1980
16th to endNight
39%13%15%22%9%
2211
November1st to 15thNight
42%15%16%18%5%1
2368
16th to endNight
47%13%15%15%7%
2520
December1st to 15thNight
49%14%14%17%4
2520
16th to endNight
55%11%16%11%4
2688

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%10%11%35%15%10%
60
16th to endDay
23%12%23%20%15%4
64
February1st to 15thDay
20%17%37%18%4
230
16th to endDay
18%25%32%17%4
225
March1st to 15thDay
17%37%28%14%2
270
16th to endDay
20%34%29%11%4
288
April1st to 15thDay
13%33%30%14%5%2
269
16th to endDay
12%35%33%10%43
270
May1st to 15thDay
17%22%31%23%31
288
16th to endDay
9%19%33%24%12%1
330
June1st to 15thDay
8%22%38%21%7%
315
16th to endDay
7%24%37%19%9%
315
July1st to 15thDay
115%39%34%8%
305
16th to endDay
218%37%29%10%
327
August1st to 15thDay
411%36%31%11%4
315
16th to endDay
214%33%34%12%3
332
September1st to 15thDay
420%30%26%14%3
315
16th to endDay
8%19%34%30%8%
272
October1st to 15thDay
14%22%35%18%7%1
255
16th to endDay
19%15%26%31%5%2
173
November1st to 15thDay
14%21%35%24%4
74
16th to endDay
18%23%20%16%16%5%
60
December1st to 15thDay
15%21%41%18%3
60
16th to endDay
9%14%29%31%15%
64
January1st to 15thNight
14%18%21%28%14%2
720
16th to endNight
16%12%23%29%14%3
768
February1st to 15thNight
13%13%28%30%12%1
704
16th to endNight
11%17%25%28%14%1
632
March1st to 15thNight
12%15%29%31%9%1
720
16th to endNight
9%21%30%28%9%1
768
April1st to 15thNight
8%18%33%25%11%1
719
16th to endNight
8%19%34%23%10%2
582
May1st to 15thNight
7%18%31%27%12%1
360
16th to endNight
6%12%34%32%12%1
375
June1st to 15thNight
6%16%41%26%9%
360
16th to endNight
5%19%40%21%11%1
360
July1st to 15thNight
210%37%39%9%
340
16th to endNight
113%42%31%9%1
376
August1st to 15thNight
7%36%33%16%4
402
16th to endNight
211%31%30%19%5%
690
September1st to 15thNight
311%24%28%24%7%
720
16th to endNight
5%12%27%28%20%5%
675
October1st to 15thNight
8%14%27%27%18%4
660
16th to endNight
11%14%25%29%14%4
704
November1st to 15thNight
13%15%29%30%9%1
700
16th to endNight
15%15%26%24%14%4
720
December1st to 15thNight
14%11%30%29%12%
720
16th to endNight
18%16%29%28%6%
768
Page generated at 2017-09-21 04:18:18. Version ac04

Copyright 2017 A. Danko


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