Spenco Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spenco Observatory clear sky chart from 2005-11-05 to 2017-09-22.

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
40%
15761
21%
46891
14%
15718
48%
15718
24%
29685

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
23%
899
19%
2506
9%
895
32%
895
19%
1732
January16th to endNight
23%
956
17%
2564
6%
956
32%
956
20%
1806
February1st to 15thNight
20%
704
17%
2112
6%
701
30%
701
16%
1458
February16th to endNight
23%
631
18%
1893
7%
631
32%
631
18%
1277
March1st to 15thNight
24%
720
16%
2160
8%
720
31%
720
18%
1475
March16th to endNight
32%
630
15%
2013
9%
630
39%
630
17%
1366
April1st to 15thNight
35%
539
22%
1797
14%
539
49%
539
24%
1074
April16th to endNight
36%
540
20%
1792
13%
540
48%
540
21%
1121
May1st to 15thNight
41%
539
18%
1619
10%
539
48%
539
22%
1067
May16th to endNight
49%
561
19%
1614
11%
561
57%
561
24%
1032
June1st to 15thNight
55%
540
21%
1440
13%
540
61%
540
31%
861
June16th to endNight
63%
540
23%
1440
16%
540
67%
540
37%
840
July1st to 15thNight
63%
510
19%
1360
15%
510
65%
510
34%
836
July16th to endNight
64%
564
25%
1431
18%
531
73%
531
38%
832
August1st to 15thNight
72%
539
23%
1756
24%
536
75%
536
36%
922
August16th to endNight
71%
573
25%
1910
28%
573
74%
573
34%
1041
September1st to 15thNight
61%
561
27%
1821
29%
561
64%
561
32%
1036
September16th to endNight
58%
685
29%
1892
20%
685
64%
685
34%
1141
October1st to 15thNight
51%
660
31%
1858
23%
660
58%
660
34%
1092
October16th to endNight
42%
704
26%
2203
19%
704
51%
704
28%
1345
November1st to 15thNight
41%
700
29%
2276
23%
700
52%
700
31%
1274
November16th to endNight
30%
720
24%
2340
13%
720
43%
720
21%
1425
December1st to 15thNight
27%
786
16%
2406
10%
786
35%
786
16%
1678
December16th to endNight
20%
960
16%
2688
6%
960
29%
960
16%
1954

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
12%5%25%43445%7%41%
48000
Night
13%6%25%424447%44%
47069
TransparencyDay
40%6%12%23%14%2
47680
Night
39%5%11%23%17%3
46893
SeeingDay
13%12%18%22%21%11%
5796
Night
14%19%20%26%16%2
15765

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
19%7%49%5%23445%34%
1432
16th to endDay
13%9%46%6%245%46%37%
1656
February1st to 15thDay
15%9%5%6%42335%6%37%
1750
16th to endDay
12%8%37%6%36%5%6%5%33%
1580
March1st to 15thDay
20%7%36%433435%37%
1800
16th to endDay
17%8%35%5%345%6%7%32%
2219
April1st to 15thDay
14%5%24424447%43%
2151
16th to endDay
16%5%16%434447%39%
2166
May1st to 15thDay
17%6%35%5%25%45%9%34%
2331
16th to endDay
9%5%26%6%45%6%6%9%37%
2497
June1st to 15thDay
7%424335%6%6%8%47%
2520
16th to endDay
6%3245%35%6%8%8%44%
2520
July1st to 15thDay
9%5%25%5%35%5%7%9%40%
2520
16th to endDay
43235%46%6%7%10%45%
2639
August1st to 15thDay
7%316%435%5%5%9%45%
2190
16th to endDay
7%424335%5%6%10%46%
2292
September1st to 15thDay
11%5%25%5%345%5%7%44%
2127
16th to endDay
11%4245%33346%49%
1881
October1st to 15thDay
12%414333347%51%
1771
16th to endDay
15%414323346%50%
1669
November1st to 15thDay
10%5%343145%5%6%49%
1582
16th to endDay
20%5%25%423235%43%
1620
December1st to 15thDay
21%9%345%24335%35%
1551
16th to endDay
18%10%46%422346%36%
1536
January1st to 15thNight
17%8%36%423345%39%
2506
16th to endNight
15%9%36%5%24446%36%
2564
February1st to 15thNight
17%9%36%5%2434436%
2103
16th to endNight
14%7%46%435%5%35%39%
1893
March1st to 15thNight
16%8%25%434435%39%
2160
16th to endNight
17%7%25%435%447%36%
2013
April1st to 15thNight
12%6%3442445%6%46%
1797
16th to endNight
17%5%25%423348%42%
1792
May1st to 15thNight
15%5%345%35%347%41%
1619
16th to endNight
6%316%7%45%5%7%9%42%
1613
June1st to 15thNight
6%41432445%10%50%
1440
16th to endNight
4313325%5%7%11%50%
1440
July1st to 15thNight
8%42442346%9%50%
1440
16th to endNight
6%213435%5%6%9%52%
1528
August1st to 15thNight
5%21242446%10%54%
1767
16th to endNight
432443445%10%52%
1910
September1st to 15thNight
10%42431345%6%51%
1821
16th to endNight
10%5%2442345%5%52%
1892
October1st to 15thNight
10%414413446%54%
1858
16th to endNight
16%6%2432333449%
2203
November1st to 15thNight
12%414313346%52%
2276
16th to endNight
17%5%24424326%46%
2340
December1st to 15thNight
21%9%35%423446%34%
2406
16th to endNight
22%9%35%424445%34%
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
52%411%19%10%
1433
16th to endDay
46%5%12%27%7%
1656
February1st to 15thDay
46%411%24%10%1
1752
16th to endDay
50%6%12%18%11%1
1580
March1st to 15thDay
51%48%22%11%1
1800
16th to endDay
50%6%13%22%6%
2220
April1st to 15thDay
38%6%12%23%17%1
2151
16th to endDay
42%6%11%25%11%2
2166
May1st to 15thDay
47%7%12%21%10%
2331
16th to endDay
41%9%14%18%13%2
2501
June1st to 15thDay
31%7%13%25%18%2
2520
16th to endDay
32%8%13%20%20%3
2520
July1st to 15thDay
37%6%14%22%14%4
2371
16th to endDay
29%9%20%25%13%2
2473
August1st to 15thDay
33%8%13%24%17%2
2177
16th to endDay
32%7%14%21%18%5%
2292
September1st to 15thDay
36%7%9%22%18%5%
2127
16th to endDay
35%5%7%23%24%3
1881
October1st to 15thDay
32%410%26%21%5%
1771
16th to endDay
35%38%29%19%3
1669
November1st to 15thDay
33%5%9%22%20%7%
1582
16th to endDay
43%38%26%14%2
1620
December1st to 15thDay
51%412%20%10%1
1551
16th to endDay
50%412%21%9%1
1536
January1st to 15thNight
47%49%21%15%1
2506
16th to endNight
47%412%22%12%1
2564
February1st to 15thNight
50%39%24%10%1
2112
16th to endNight
46%5%12%22%12%
1893
March1st to 15thNight
47%5%10%19%13%4
2160
16th to endNight
47%5%14%20%10%1
2013
April1st to 15thNight
36%5%10%24%18%4
1797
16th to endNight
41%48%24%16%4
1792
May1st to 15thNight
42%5%12%23%14%
1620
16th to endNight
35%9%13%20%16%4
1615
June1st to 15thNight
29%6%14%24%22%2
1440
16th to endNight
25%8%15%22%22%5%
1440
July1st to 15thNight
31%6%12%26%16%6%
1360
16th to endNight
25%7%13%32%16%3
1431
August1st to 15thNight
24%7%17%23%23%3
1756
16th to endNight
27%6%13%25%20%5%
1910
September1st to 15thNight
31%5%8%26%21%6%
1821
16th to endNight
33%48%21%24%7%
1892
October1st to 15thNight
32%38%22%25%6%
1858
16th to endNight
39%36%26%17%6%
2203
November1st to 15thNight
32%39%22%25%7%
2276
16th to endNight
40%39%23%19%2
2340
December1st to 15thNight
50%410%19%12%1
2406
16th to endNight
52%411%19%10%1
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
35%11%34%10%7%1
90
16th to endDay
16%19%42%18%2
96
February1st to 15thDay
16%21%42%16%3
90
16th to endDay
19%18%39%16%6%
81
March1st to 15thDay
24%12%44%15%3
90
16th to endDay
21%18%27%23%8%
249
April1st to 15thDay
13%20%27%23%14%
270
16th to endDay
17%15%25%22%12%5%
270
May1st to 15thDay
18%17%24%22%12%4
288
16th to endDay
14%14%22%23%16%8%
397
June1st to 15thDay
9%13%13%23%27%13%
495
16th to endDay
6%14%19%29%21%8%
495
July1st to 15thDay
13%9%14%23%25%13%
475
16th to endDay
412%5%21%34%21%
499
August1st to 15thDay
12%6%7%20%28%24%
313
16th to endDay
11%6%7%20%35%18%
332
September1st to 15thDay
13%6%9%20%28%22%
315
16th to endDay
12%7%10%19%30%19%
278
October1st to 15thDay
13%7%19%18%27%12%
211
16th to endDay
14%332%25%21%3
96
November1st to 15thDay
48%24%33%18%10%
90
16th to endDay
33%8%22%24%10%1
90
December1st to 15thDay
15%11%36%24%7%4
90
16th to endDay
32%16%33%17%
96
January1st to 15thNight
17%25%29%22%5%
900
16th to endNight
18%23%29%24%3
956
February1st to 15thNight
19%27%27%22%2
704
16th to endNight
16%25%31%24%2
631
March1st to 15thNight
20%22%28%24%3
720
16th to endNight
17%22%23%26%9%1
630
April1st to 15thNight
13%22%26%27%10%
539
16th to endNight
16%20%21%28%12%
540
May1st to 15thNight
15%20%17%33%11%1
540
16th to endNight
7%17%17%32%23%2
563
June1st to 15thNight
8%18%11%33%25%2
540
16th to endNight
417%9%37%28%2
540
July1st to 15thNight
9%12%8%29%37%2
510
16th to endNight
5%16%6%26%37%8%
564
August1st to 15thNight
7%10%5%28%40%8%
539
16th to endNight
7%9%5%30%41%7%
573
September1st to 15thNight
11%10%9%26%35%6%
561
16th to endNight
11%10%11%24%32%8%
685
October1st to 15thNight
13%11%16%28%25%5%
660
16th to endNight
16%13%23%30%14%1
704
November1st to 15thNight
10%18%25%28%16%1
700
16th to endNight
19%22%24%22%10%
720
December1st to 15thNight
23%21%21%24%7%
786
16th to endNight
23%24%28%20%4
960
Page generated at 2017-09-23 05:43:47. Version ac04

Copyright 2017 A. Danko


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