Deep Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deep Sky Observatory clear sky chart from 2005-11-05 to 2017-09-23.

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
61%
16039
20%
47014
16%
15999
65%
15999
29%
29911

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
51%
717
19%
2327
15%
716
58%
716
27%
1432
January16th to endNight
48%
768
24%
2436
19%
768
57%
768
30%
1454
February1st to 15thNight
47%
701
23%
2103
16%
698
55%
698
32%
1275
February16th to endNight
50%
631
28%
1809
17%
631
61%
631
38%
1028
March1st to 15thNight
53%
720
30%
1980
18%
720
64%
720
42%
1105
March16th to endNight
58%
768
27%
2112
21%
768
63%
768
42%
1197
April1st to 15thNight
62%
718
23%
1976
19%
718
67%
718
44%
1096
April16th to endNight
61%
720
28%
1844
17%
720
67%
720
43%
1083
May1st to 15thNight
69%
617
27%
1697
19%
617
73%
617
42%
986
May16th to endNight
76%
561
21%
1682
17%
561
78%
561
34%
1038
June1st to 15thNight
68%
540
9%
1620
8%
540
68%
540
20%
1191
June16th to endNight
71%
540
7%
1620
6%
540
71%
540
20%
1219
July1st to 15thNight
71%
510
6%
1530
6%
510
71%
510
19%
1178
July16th to endNight
76%
564
6%
1595
7%
531
76%
531
20%
1199
August1st to 15thNight
76%
539
5%
1609
5%
536
76%
536
24%
1125
August16th to endNight
62%
744
5%
2076
5%
744
62%
744
18%
1571
September1st to 15thNight
61%
720
5%
1980
5%
720
61%
720
16%
1520
September16th to endNight
59%
689
11%
1903
11%
689
60%
689
18%
1416
October1st to 15thNight
67%
660
23%
1880
24%
660
69%
660
31%
1178
October16th to endNight
60%
704
25%
2112
24%
704
65%
704
31%
1286
November1st to 15thNight
63%
700
31%
2101
27%
700
69%
700
38%
1180
November16th to endNight
56%
720
30%
2186
25%
720
64%
720
34%
1275
December1st to 15thNight
57%
720
25%
2340
24%
720
62%
720
29%
1412
December16th to endNight
58%
768
22%
2496
22%
768
62%
768
31%
1467

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
7%5%37%7%47%6%6%8%34%
47872
Night
7%5%25%5%35%5%6%9%43%
47223
TransparencyDay
44%8%14%15%12%5%
47573
Night
35%8%16%19%14%5%
47026
SeeingDay
8%211%20%25%31%
5880
Night
8%215%33%28%10%
16048

By Month

Percentage Cloud Cover SE (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%6%35%6%35%6%6%9%37%
1611
16th to endDay
9%5%145%3446%8%45%
1785
February1st to 15thDay
10%5%26%5%3445%5%46%
1750
16th to endDay
6%5%26%5%45%45%6%47%
1661
March1st to 15thDay
5%2135%36%6%5%7%51%
1980
16th to endDay
7%325%5%36%5%5%7%47%
2111
April1st to 15thDay
5%4246%47%6%7%9%42%
1972
16th to endDay
5%425%5%46%6%6%8%43%
2115
May1st to 15thDay
4425%6%37%6%5%8%45%
2262
16th to endDay
5%437%9%5%8%6%6%8%32%
2431
June1st to 15thDay
11%8%5%9%9%5%7%6%7%8%20%
2340
16th to endDay
7%7%48%10%5%8%7%7%9%21%
2340
July1st to 15thDay
8%6%410%11%5%9%8%7%7%19%
2340
16th to endDay
8%6%49%10%8%9%7%6%9%19%
2465
August1st to 15thDay
5%5%39%10%6%8%8%8%10%22%
2340
16th to endDay
10%7%49%9%5%8%7%7%8%20%
2108
September1st to 15thDay
7%7%49%9%6%7%7%7%10%21%
1980
16th to endDay
9%9%410%9%5%7%7%6%9%19%
1894
October1st to 15thDay
7%6%26%8%46%6%6%8%35%
1749
16th to endDay
8%5%26%6%46%5%5%7%42%
1760
November1st to 15thDay
5%326%445%5%6%8%46%
1757
16th to endDay
7%5%26%5%35%5%5%7%43%
1773
December1st to 15thDay
8%6%37%5%46%45%7%39%
1620
16th to endDay
6%435%6%35%5%6%8%44%
1728
January1st to 15thNight
8%5%36%5%35%5%5%10%40%
2327
16th to endNight
10%5%235%35%45%9%44%
2436
February1st to 15thNight
10%5%25%5%2445%6%47%
2103
16th to endNight
5%325%42447%8%51%
1809
March1st to 15thNight
331443445%8%55%
1980
16th to endNight
5%324423449%55%
2112
April1st to 15thNight
42235%2445%8%55%
1976
16th to endNight
5%31343445%8%54%
1844
May1st to 15thNight
33135%25%45%8%55%
1697
16th to endNight
5%3145%36%6%6%11%44%
1682
June1st to 15thNight
7%6%38%7%46%6%8%11%27%
1620
16th to endNight
6%447%8%47%7%7%10%30%
1620
July1st to 15thNight
7%6%47%6%46%6%8%11%29%
1620
16th to endNight
4438%7%5%8%7%8%13%27%
1703
August1st to 15thNight
5%5%26%6%46%6%9%13%34%
1620
16th to endNight
9%6%37%6%45%6%7%12%29%
2076
September1st to 15thNight
6%7%38%7%5%7%6%8%10%26%
1980
16th to endNight
8%7%38%7%5%7%5%7%8%29%
1903
October1st to 15thNight
7%6%25%445%45%8%45%
1880
16th to endNight
9%6%2442445%8%46%
2112
November1st to 15thNight
6%42443445%8%51%
2101
16th to endNight
8%3245%3446%9%47%
2186
December1st to 15thNight
7%5%35%5%35%45%8%44%
2340
16th to endNight
5%425%5%35%5%7%8%46%
2496

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
41%7%8%19%18%4
1612
16th to endDay
35%48%21%20%9%
1785
February1st to 15thDay
38%47%16%20%13%
1752
16th to endDay
35%5%9%17%20%11%
1661
March1st to 15thDay
30%6%10%24%20%8%
1980
16th to endDay
33%5%9%20%21%8%
2112
April1st to 15thDay
34%7%14%19%18%5%
1972
16th to endDay
36%6%10%16%22%8%
2115
May1st to 15thDay
34%6%11%19%19%8%
2262
16th to endDay
45%8%13%18%11%1
2435
June1st to 15thDay
58%9%16%12%3
2340
16th to endDay
54%12%18%12%1
2340
July1st to 15thDay
59%12%19%7%1
2201
16th to endDay
56%12%20%9%1
2310
August1st to 15thDay
49%14%26%8%
2327
16th to endDay
56%15%19%7%1
2108
September1st to 15thDay
54%13%20%9%1
1980
16th to endDay
56%11%16%10%31
1894
October1st to 15thDay
41%8%11%16%14%6%
1749
16th to endDay
39%5%10%13%21%10%
1760
November1st to 15thDay
33%6%9%20%19%10%
1757
16th to endDay
37%6%9%17%16%12%
1773
December1st to 15thDay
43%412%17%16%5%
1620
16th to endDay
34%6%12%21%17%7%
1728
January1st to 15thNight
38%5%13%22%15%5%
2327
16th to endNight
36%49%23%17%7%
2436
February1st to 15thNight
35%48%17%20%12%
2112
16th to endNight
28%6%13%22%21%8%
1809
March1st to 15thNight
26%411%30%20%7%
1980
16th to endNight
26%5%13%25%20%8%
2112
April1st to 15thNight
24%6%16%26%19%6%
1976
16th to endNight
26%6%13%24%23%6%
1844
May1st to 15thNight
26%6%13%27%19%6%
1698
16th to endNight
30%10%18%23%16%
1684
June1st to 15thNight
45%12%22%15%4
1620
16th to endNight
44%15%23%12%4
1620
July1st to 15thNight
45%16%25%10%2
1530
16th to endNight
44%15%27%12%
1595
August1st to 15thNight
36%16%37%9%
1609
16th to endNight
44%17%27%9%1
2076
September1st to 15thNight
47%16%22%10%2
1980
16th to endNight
48%12%18%15%41
1903
October1st to 15thNight
35%7%14%17%19%5%
1880
16th to endNight
34%6%10%15%23%9%
2112
November1st to 15thNight
29%410%24%21%8%
2101
16th to endNight
32%5%9%22%18%11%
2186
December1st to 15thNight
36%5%12%24%14%6%
2340
16th to endNight
33%6%15%20%17%7%
2496

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%340%18%12%6%
90
16th to endDay
13%10%34%21%16%3
96
February1st to 15thDay
7%12%24%31%15%8%
90
16th to endDay
11%6%25%40%16%
162
March1st to 15thDay
5%326%37%22%4
270
16th to endDay
8%321%28%31%6%
288
April1st to 15thDay
3220%33%28%10%
270
16th to endDay
7%212%29%27%20%
270
May1st to 15thDay
5%10%23%36%23%
288
16th to endDay
6%16%17%33%34%
331
June1st to 15thDay
16%28%11%20%40%
315
16th to endDay
8%27%20%61%
315
July1st to 15thDay
7%5%16%68%
305
16th to endDay
10%210%17%59%
326
August1st to 15thDay
7%26%20%61%
313
16th to endDay
11%119%18%56%
332
September1st to 15thDay
10%2310%22%49%
315
16th to endDay
11%18%34%43%
283
October1st to 15thDay
10%17%29%34%17%
255
16th to endDay
11%115%28%31%12%
272
November1st to 15thDay
6%212%33%30%14%
265
16th to endDay
8%218%34%30%4
243
December1st to 15thDay
6%234%43%13%
90
16th to endDay
5%314%31%37%8%
96
January1st to 15thNight
10%5%23%40%18%1
720
16th to endNight
11%429%39%13%
768
February1st to 15thNight
12%5%25%41%13%
704
16th to endNight
7%428%44%13%
631
March1st to 15thNight
4430%43%15%
720
16th to endNight
6%325%46%17%
768
April1st to 15thNight
5%223%46%20%1
718
16th to endNight
7%221%43%23%
720
May1st to 15thNight
5%114%48%27%2
618
16th to endNight
5%16%37%41%7%
563
June1st to 15thNight
8%1423%48%14%
540
16th to endNight
8%212%44%31%
540
July1st to 15thNight
8%27%46%35%
510
16th to endNight
7%27%43%39%
564
August1st to 15thNight
7%16%45%39%
539
16th to endNight
13%5%15%34%29%
744
September1st to 15thNight
10%15%16%39%27%
720
16th to endNight
12%1323%42%16%
689
October1st to 15thNight
12%7%35%37%6%
660
16th to endNight
12%115%36%29%4
704
November1st to 15thNight
7%117%43%26%3
700
16th to endNight
10%122%43%20%1
720
December1st to 15thNight
10%419%41%23%
720
16th to endNight
6%322%38%26%2
768
Page generated at 2017-09-24 09:15:53. Version ac04

Copyright 2017 A. Danko


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