Seven Sisters Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Seven Sisters Observatory clear sky chart from 2005-11-05 to 2017-04-25.

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
37%
15439
15%
45518
10%
15397
42%
15397
20%
31810

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
15%
900
11%
2506
4%
895
21%
895
13%
1983
January16th to endNight
19%
873
8%
2601
3%
873
23%
873
11%
2099
February1st to 15thNight
18%
701
10%
2235
3%
701
25%
701
11%
1705
February16th to endNight
24%
631
14%
1893
6%
631
32%
631
15%
1392
March1st to 15thNight
28%
720
14%
2091
9%
720
32%
720
17%
1516
March16th to endNight
33%
768
18%
2112
10%
768
40%
768
21%
1504
April1st to 15thNight
28%
566
13%
1809
8%
566
36%
566
19%
1264
April16th to endNight
36%
525
19%
1575
8%
525
46%
525
24%
1055
May1st to 15thNight
35%
495
14%
1485
8%
495
42%
495
20%
1049
May16th to endNight
52%
514
14%
1523
11%
513
56%
513
29%
968
June1st to 15thNight
52%
495
16%
1320
11%
495
57%
495
31%
874
June16th to endNight
60%
495
15%
1320
12%
495
63%
495
33%
849
July1st to 15thNight
67%
465
18%
1240
15%
465
69%
465
40%
743
July16th to endNight
69%
522
17%
1370
14%
489
72%
489
38%
806
August1st to 15thNight
65%
494
21%
1474
18%
491
68%
491
34%
876
August16th to endNight
66%
525
20%
1622
17%
525
68%
525
34%
942
September1st to 15thNight
60%
599
27%
1754
23%
599
63%
599
35%
1024
September16th to endNight
57%
657
23%
1815
18%
657
61%
657
32%
1136
October1st to 15thNight
47%
660
22%
1880
17%
660
52%
660
28%
1217
October16th to endNight
27%
704
15%
2219
9%
704
35%
704
17%
1538
November1st to 15thNight
35%
700
20%
2276
16%
700
42%
700
22%
1456
November16th to endNight
25%
720
14%
2340
10%
720
31%
720
14%
1719
December1st to 15thNight
19%
750
9%
2370
4%
750
24%
750
11%
1845
December16th to endNight
14%
960
8%
2688
1%
960
20%
960
9%
2250

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
14%8%47%6%35%5%5%7%32%
46117
Night
15%8%46%5%3445%6%35%
45695
Cloud CoverDay
13%8%47%6%35%5%5%7%32%
46126
Night
14%8%46%5%3445%6%35%
45699
TransparencyDay
49%6%11%19%11%1
45811
Night
48%6%11%20%12%1
45518
SeeingDay
16%6%26%24%16%8%
5259
Night
19%7%28%29%13%1
15443

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 15thDay
17%14%6%11%7%36%5%36%17%
1432
16th to endDay
15%17%9%10%8%45%435%15%
1620
February1st to 15thDay
13%14%7%10%9%35%445%21%
1627
16th to endDay
15%12%6%10%8%36%5%5%6%19%
1580
March1st to 15thDay
16%10%5%8%7%35%446%27%
1858
16th to endDay
18%9%5%7%5%2445%5%30%
2112
April1st to 15thDay
18%10%5%6%5%25%445%30%
2148
16th to endDay
15%7%35%5%24447%38%
2273
May1st to 15thDay
18%8%36%6%345%5%6%30%
2136
16th to endDay
14%5%35%435%5%47%40%
2238
June1st to 15thDay
10%6%37%5%35%5%6%9%37%
2310
16th to endDay
9%5%25%6%36%6%8%9%35%
2310
July1st to 15thDay
6%5%25%436%5%5%10%43%
2310
16th to endDay
7%5%246%46%7%6%9%39%
2387
August1st to 15thDay
9%425%7%35%6%6%7%40%
2145
16th to endDay
11%6%25%5%345%7%7%39%
2227
September1st to 15thDay
13%426%4346%5%7%41%
1865
16th to endDay
11%5%25%5%345%6%10%39%
1815
October1st to 15thDay
14%6%35%6%3445%7%39%
1749
16th to endDay
20%9%35%6%34446%31%
1650
November1st to 15thDay
14%7%36%5%3435%5%39%
1582
16th to endDay
18%12%47%6%35%45%5%26%
1620
December1st to 15thDay
15%12%7%10%6%45%45%5%22%
1587
16th to endDay
19%16%8%11%6%34346%14%
1536
January1st to 15thNight
22%12%5%8%6%23446%22%
2506
16th to endNight
13%11%8%11%9%45%45%5%19%
2601
February1st to 15thNight
15%13%5%9%6%44345%26%
2233
16th to endNight
15%10%49%5%45%45%6%28%
1893
March1st to 15thNight
16%10%37%6%24446%33%
2079
16th to endNight
15%8%35%6%35%45%7%34%
2112
April1st to 15thNight
18%7%35%5%35%5%5%6%34%
1809
16th to endNight
16%7%35%4245%5%6%38%
1575
May1st to 15thNight
19%6%36%5%35%36%7%32%
1485
16th to endNight
12%5%2443436%7%44%
1521
June1st to 15thNight
10%5%15%5%3445%7%44%
1320
16th to endNight
7%426%5%345%8%10%43%
1320
July1st to 15thNight
5%42442445%7%53%
1320
16th to endNight
5%3343245%6%9%50%
1472
August1st to 15thNight
4415%5%35%5%6%9%48%
1485
16th to endNight
11%423423347%51%
1622
September1st to 15thNight
10%5%23423346%52%
1754
16th to endNight
11%5%244235%5%7%45%
1815
October1st to 15thNight
12%6%35%424446%44%
1880
16th to endNight
19%8%37%4345%35%35%
2219
November1st to 15thNight
13%8%35%5%3445%5%41%
2276
16th to endNight
19%10%5%7%5%33446%28%
2340
December1st to 15thNight
20%12%5%8%6%34346%23%
2370
16th to endNight
22%15%6%8%6%3445%5%16%
2688

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
17%14%5%12%7%36%5%45%17%
1432
16th to endDay
15%17%9%10%8%44435%16%
1620
February1st to 15thDay
13%14%7%11%8%35%445%20%
1627
16th to endDay
15%12%6%10%8%36%5%5%6%19%
1580
March1st to 15thDay
16%10%5%8%7%25%446%27%
1868
16th to endDay
18%9%5%7%5%34446%30%
2111
April1st to 15thDay
18%10%5%6%435%35%5%30%
2148
16th to endDay
15%8%36%425%447%37%
2273
May1st to 15thDay
18%8%36%6%345%5%6%30%
2136
16th to endDay
13%6%25%4345%47%40%
2238
June1st to 15thDay
10%5%37%5%35%5%6%9%37%
2310
16th to endDay
9%5%25%6%36%6%8%9%35%
2310
July1st to 15thDay
6%5%25%436%5%5%10%43%
2310
16th to endDay
7%5%246%46%6%6%10%38%
2387
August1st to 15thDay
8%5%25%6%45%5%6%7%40%
2145
16th to endDay
10%6%35%5%345%8%8%39%
2227
September1st to 15thDay
13%426%4345%6%8%40%
1865
16th to endDay
12%5%25%4345%7%9%39%
1815
October1st to 15thDay
13%6%35%6%3445%7%39%
1749
16th to endDay
20%8%35%6%35%446%31%
1650
November1st to 15thDay
14%6%37%5%3345%6%39%
1582
16th to endDay
17%13%46%6%35%446%26%
1620
December1st to 15thDay
15%12%7%11%5%45%45%5%22%
1587
16th to endDay
19%16%8%11%6%3445%6%14%
1536
January1st to 15thNight
21%12%5%8%6%24446%22%
2506
16th to endNight
13%11%8%11%9%45%45%5%19%
2601
February1st to 15thNight
14%13%6%9%7%44345%26%
2226
16th to endNight
15%10%49%5%45%446%28%
1893
March1st to 15thNight
17%9%47%6%24446%32%
2091
16th to endNight
16%8%35%5%35%45%7%34%
2112
April1st to 15thNight
18%7%35%5%25%5%46%34%
1809
16th to endNight
16%8%25%4245%5%6%38%
1575
May1st to 15thNight
19%6%25%6%3446%7%32%
1484
16th to endNight
12%5%2443445%7%44%
1521
June1st to 15thNight
10%5%246%3445%8%44%
1320
16th to endNight
7%425%6%345%7%10%43%
1320
July1st to 15thNight
5%32442445%7%54%
1320
16th to endNight
44243245%6%10%50%
1472
August1st to 15thNight
4415%5%35%46%9%47%
1485
16th to endNight
11%413423347%51%
1622
September1st to 15thNight
10%423423346%52%
1754
16th to endNight
11%5%244235%6%7%45%
1815
October1st to 15thNight
11%7%35%433447%43%
1880
16th to endNight
19%8%36%5%345%35%35%
2219
November1st to 15thNight
13%8%35%43445%5%41%
2276
16th to endNight
19%10%5%7%6%24446%28%
2340
December1st to 15thNight
20%12%5%8%6%34346%23%
2370
16th to endNight
22%15%6%8%6%35%45%6%16%
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
68%5%11%11%4
1433
16th to endDay
71%49%9%4
1620
February1st to 15thDay
63%7%7%14%6%
1629
16th to endDay
63%6%10%14%4
1580
March1st to 15thDay
57%5%12%16%6%1
1868
16th to endDay
54%5%11%19%7%1
2112
April1st to 15thDay
54%49%19%11%
2148
16th to endDay
45%6%11%18%14%2
2273
May1st to 15thDay
52%8%10%18%9%
2136
16th to endDay
42%7%12%23%12%2
2242
June1st to 15thDay
41%7%12%22%13%1
2310
16th to endDay
41%9%13%18%17%1
2310
July1st to 15thDay
35%6%11%22%20%2
2161
16th to endDay
36%9%15%22%14%1
2226
August1st to 15thDay
38%7%12%24%13%1
2132
16th to endDay
39%8%11%21%16%2
2227
September1st to 15thDay
39%7%13%23%15%1
1865
16th to endDay
38%6%10%28%14%
1815
October1st to 15thDay
43%6%10%23%13%1
1749
16th to endDay
52%6%13%18%7%
1650
November1st to 15thDay
45%5%10%22%14%1
1582
16th to endDay
57%5%12%18%5%
1620
December1st to 15thDay
62%7%11%14%4
1587
16th to endDay
71%5%11%7%3
1536
January1st to 15thNight
61%5%10%14%7%
2506
16th to endNight
64%6%12%11%3
2601
February1st to 15thNight
60%5%11%16%5%1
2235
16th to endNight
56%413%18%6%1
1893
March1st to 15thNight
52%6%11%18%9%1
2091
16th to endNight
48%6%11%21%10%1
2112
April1st to 15thNight
48%6%10%19%14%
1809
16th to endNight
44%5%11%18%17%2
1575
May1st to 15thNight
49%6%10%20%10%1
1485
16th to endNight
38%6%11%24%15%3
1523
June1st to 15thNight
38%7%11%26%15%
1320
16th to endNight
34%8%15%23%17%
1320
July1st to 15thNight
29%6%11%27%21%3
1240
16th to endNight
27%8%15%24%20%3
1370
August1st to 15thNight
30%8%13%28%17%2
1474
16th to endNight
32%5%12%25%19%4
1622
September1st to 15thNight
32%410%28%20%2
1754
16th to endNight
34%6%11%27%17%1
1815
October1st to 15thNight
40%5%11%23%19%
1880
16th to endNight
50%5%11%23%8%
2219
November1st to 15thNight
42%5%11%22%14%2
2276
16th to endNight
56%412%18%7%
2340
December1st to 15thNight
61%5%12%14%5%
2370
16th to endNight
68%6%10%10%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%13%43%20%31
90
16th to endDay
26%12%37%18%5%
96
February1st to 15thDay
21%12%48%15%2
90
16th to endDay
22%11%43%18%31
81
March1st to 15thDay
30%12%29%20%6%
158
16th to endDay
22%8%31%25%10%1
288
April1st to 15thDay
23%8%32%25%8%1
270
16th to endDay
17%10%29%22%12%8%
250
May1st to 15thDay
23%6%29%19%15%5%
228
16th to endDay
18%6%21%22%18%13%
282
June1st to 15thDay
13%5%28%30%15%5%
420
16th to endDay
12%427%30%18%7%
420
July1st to 15thDay
10%321%26%26%12%
400
16th to endDay
8%220%31%22%13%
373
August1st to 15thDay
11%13%29%23%22%
253
16th to endDay
15%116%19%27%20%
268
September1st to 15thDay
12%217%18%30%18%
255
16th to endDay
16%5%12%23%29%12%
254
October1st to 15thDay
15%5%25%28%16%9%
255
16th to endDay
26%8%32%26%6%
162
November1st to 15thDay
7%13%26%33%14%4
90
16th to endDay
25%14%37%14%7%
90
December1st to 15thDay
17%26%34%15%5%
90
16th to endDay
25%30%36%8%
96
January1st to 15thNight
28%11%37%17%5%
900
16th to endNight
17%11%37%26%6%
873
February1st to 15thNight
23%13%35%24%3
704
16th to endNight
21%11%33%27%4
631
March1st to 15thNight
23%11%28%28%6%
720
16th to endNight
19%8%32%28%9%
768
April1st to 15thNight
19%12%34%26%6%
566
16th to endNight
20%6%33%28%9%1
525
May1st to 15thNight
22%428%32%10%1
495
16th to endNight
15%223%35%20%2
515
June1st to 15thNight
12%224%44%15%
495
16th to endNight
7%220%42%24%2
495
July1st to 15thNight
8%115%43%28%3
465
16th to endNight
5%116%44%29%1
522
August1st to 15thNight
7%116%45%23%5%
494
16th to endNight
13%12%31%35%5%
525
September1st to 15thNight
12%318%32%28%5%
599
16th to endNight
13%417%30%28%5%
657
October1st to 15thNight
15%6%22%33%19%2
660
16th to endNight
24%10%33%23%7%
704
November1st to 15thNight
18%9%32%25%13%1
700
16th to endNight
26%10%30%23%7%
720
December1st to 15thNight
27%12%35%20%5%
750
16th to endNight
29%11%35%19%3
960
Page generated at 2017-04-26 17:21:48. Version ac04

Copyright 2017 A. Danko


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