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 2018-10-08.

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%
17295
16%
50938
10%
17253
43%
17253
21%
35397

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
16%
975
11%
2716
4%
970
23%
970
13%
2136
January16th to endNight
18%
946
8%
2818
3%
946
23%
946
11%
2260
February1st to 15thNight
18%
761
10%
2425
3%
761
25%
761
11%
1854
February16th to endNight
23%
683
14%
2049
6%
683
30%
683
15%
1515
March1st to 15thNight
28%
780
15%
2265
10%
780
33%
780
18%
1625
March16th to endNight
33%
832
19%
2288
11%
832
40%
832
21%
1618
April1st to 15thNight
27%
613
13%
1960
8%
613
35%
613
18%
1382
April16th to endNight
36%
579
19%
1735
8%
579
46%
579
24%
1159
May1st to 15thNight
36%
585
14%
1755
8%
585
42%
585
21%
1231
May16th to endNight
50%
610
14%
1807
10%
609
55%
609
28%
1164
June1st to 15thNight
54%
585
15%
1560
11%
585
59%
585
32%
1014
June16th to endNight
59%
585
16%
1560
12%
585
63%
585
33%
996
July1st to 15thNight
68%
555
19%
1480
16%
555
70%
555
41%
879
July16th to endNight
70%
612
19%
1623
16%
579
73%
579
39%
950
August1st to 15thNight
67%
575
21%
1715
18%
572
69%
572
35%
1013
August16th to endNight
65%
621
21%
1918
18%
621
67%
621
33%
1123
September1st to 15thNight
60%
708
26%
2073
22%
708
64%
708
34%
1230
September16th to endNight
58%
777
25%
2145
19%
777
62%
777
34%
1329
October1st to 15thNight
46%
752
20%
2139
16%
752
50%
752
26%
1419
October16th to endNight
27%
768
17%
2421
11%
768
37%
768
18%
1662
November1st to 15thNight
35%
760
19%
2471
15%
760
42%
760
22%
1599
November16th to endNight
26%
780
15%
2535
10%
780
33%
780
15%
1827
December1st to 15thNight
19%
813
10%
2568
4%
813
25%
813
12%
1983
December16th to endNight
14%
1040
8%
2912
1%
1040
20%
1040
9%
2429

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
13%8%47%6%35%5%5%7%33%
52194
Night
14%8%36%5%3445%6%35%
51115
Cloud CoverDay
13%8%47%6%35%5%5%7%33%
52203
Night
14%8%36%5%3445%7%35%
51119
TransparencyDay
48%6%11%19%11%1
51888
Night
47%6%11%20%12%1
50938
SeeingDay
17%7%26%24%16%8%
7317
Night
18%7%28%29%13%1
17299

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%15%6%11%7%46%5%36%16%
1552
16th to endDay
14%17%8%10%8%44435%17%
1755
February1st to 15thDay
14%14%6%10%9%35%445%21%
1767
16th to endDay
16%12%6%10%8%35%5%5%5%20%
1710
March1st to 15thDay
15%10%5%8%7%35%446%27%
2014
16th to endDay
18%9%47%5%24445%31%
2288
April1st to 15thDay
19%10%5%6%5%25%445%29%
2327
16th to endDay
15%8%35%5%24447%37%
2500
May1st to 15thDay
18%8%36%5%345%5%6%31%
2526
16th to endDay
13%5%35%435%5%5%8%39%
2657
June1st to 15thDay
10%5%26%5%35%5%5%9%40%
2730
16th to endDay
9%5%35%6%36%6%8%9%35%
2730
July1st to 15thDay
6%425%435%5%5%9%45%
2730
16th to endDay
7%5%25%6%46%6%6%9%39%
2793
August1st to 15thDay
8%426%7%45%6%6%7%40%
2487
16th to endDay
11%6%25%6%346%7%7%38%
2634
September1st to 15thDay
13%426%5%346%6%7%39%
2204
16th to endDay
11%5%25%5%345%6%9%41%
2145
October1st to 15thDay
14%7%35%6%3445%6%37%
1990
16th to endDay
19%9%35%6%34446%32%
1800
November1st to 15thDay
15%7%36%5%3435%5%38%
1717
16th to endDay
17%12%47%6%35%45%5%28%
1755
December1st to 15thDay
15%12%7%10%6%4445%5%23%
1719
16th to endDay
19%16%8%11%7%34346%14%
1664
January1st to 15thNight
21%12%5%8%6%34446%22%
2716
16th to endNight
13%11%8%11%9%45%5%5%5%19%
2818
February1st to 15thNight
15%13%5%10%6%44345%26%
2423
16th to endNight
16%11%49%5%45%446%27%
2049
March1st to 15thNight
16%9%47%6%24446%34%
2253
16th to endNight
15%8%35%5%25%45%7%34%
2288
April1st to 15thNight
18%7%36%5%35%5%5%6%33%
1960
16th to endNight
15%8%35%42445%6%38%
1735
May1st to 15thNight
19%6%36%5%35%35%7%34%
1755
16th to endNight
12%5%2443436%7%42%
1805
June1st to 15thNight
9%5%15%5%3445%8%45%
1560
16th to endNight
7%425%5%345%7%9%43%
1560
July1st to 15thNight
5%42442445%7%54%
1560
16th to endNight
5%3244245%6%9%51%
1725
August1st to 15thNight
4415%5%35%5%6%9%49%
1726
16th to endNight
10%42443345%8%50%
1918
September1st to 15thNight
9%5%23423446%50%
2073
16th to endNight
11%4244235%5%7%47%
2145
October1st to 15thNight
13%7%35%434447%40%
2139
16th to endNight
19%8%36%5%345%35%35%
2421
November1st to 15thNight
14%8%35%5%3445%5%40%
2471
16th to endNight
18%10%5%7%5%3345%7%29%
2535
December1st to 15thNight
19%12%5%8%6%3435%6%23%
2568
16th to endNight
21%15%5%8%7%3445%5%16%
2912

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%11%7%46%5%46%16%
1552
16th to endDay
15%17%8%10%8%44435%17%
1755
February1st to 15thDay
15%14%6%10%8%35%445%21%
1767
16th to endDay
16%13%5%10%8%36%45%6%20%
1710
March1st to 15thDay
15%10%5%8%7%25%446%27%
2024
16th to endDay
18%9%47%5%34446%31%
2287
April1st to 15thDay
19%10%5%6%5%35%45%5%29%
2327
16th to endDay
15%8%36%425%447%37%
2500
May1st to 15thDay
18%8%36%6%345%5%6%31%
2526
16th to endDay
13%6%25%4345%5%8%39%
2657
June1st to 15thDay
9%5%26%5%35%5%6%9%39%
2730
16th to endDay
9%5%35%6%36%6%8%9%34%
2730
July1st to 15thDay
6%425%435%5%5%9%45%
2730
16th to endDay
7%5%25%6%46%6%6%9%39%
2793
August1st to 15thDay
8%5%25%6%46%6%6%7%40%
2487
16th to endDay
10%6%35%6%35%5%7%8%38%
2634
September1st to 15thDay
12%426%5%35%6%6%8%39%
2204
16th to endDay
11%5%25%4345%6%9%41%
2145
October1st to 15thDay
13%7%35%6%3445%6%37%
1990
16th to endDay
19%8%36%6%35%446%32%
1800
November1st to 15thDay
15%6%37%5%3345%6%38%
1717
16th to endDay
16%12%47%6%35%446%28%
1755
December1st to 15thDay
15%12%7%11%6%35%45%5%23%
1719
16th to endDay
19%16%7%11%7%3435%6%14%
1664
January1st to 15thNight
21%12%5%8%6%24446%22%
2716
16th to endNight
13%11%8%11%9%45%5%5%5%19%
2818
February1st to 15thNight
15%13%6%9%6%44345%26%
2416
16th to endNight
17%10%49%5%45%446%27%
2049
March1st to 15thNight
16%9%47%6%24346%33%
2265
16th to endNight
15%8%35%5%3445%7%34%
2288
April1st to 15thNight
18%8%35%5%25%5%46%33%
1960
16th to endNight
15%8%25%4245%5%6%38%
1735
May1st to 15thNight
19%6%36%5%3446%7%34%
1754
16th to endNight
12%6%2443445%8%42%
1805
June1st to 15thNight
10%5%146%3445%8%45%
1560
16th to endNight
8%425%5%345%7%10%44%
1560
July1st to 15thNight
5%32442445%7%55%
1560
16th to endNight
43244245%6%10%51%
1725
August1st to 15thNight
5%315%5%35%46%9%48%
1726
16th to endNight
10%42442345%7%50%
1918
September1st to 15thNight
10%5%23423446%50%
2073
16th to endNight
11%5%244235%5%8%47%
2145
October1st to 15thNight
13%8%35%43345%7%40%
2139
16th to endNight
18%8%36%5%345%45%35%
2421
November1st to 15thNight
13%8%35%5%3445%5%40%
2471
16th to endNight
18%10%5%7%5%3445%6%29%
2535
December1st to 15thNight
19%12%5%8%6%3445%6%23%
2568
16th to endNight
21%14%6%8%6%45%45%6%16%
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
68%6%10%10%4
1553
16th to endDay
69%410%9%4
1755
February1st to 15thDay
64%6%7%14%6%
1769
16th to endDay
63%5%10%14%4
1710
March1st to 15thDay
57%5%11%16%7%1
2024
16th to endDay
53%410%20%8%1
2288
April1st to 15thDay
56%49%18%10%
2327
16th to endDay
46%6%11%18%15%2
2500
May1st to 15thDay
50%8%10%18%10%1
2526
16th to endDay
42%7%11%23%12%1
2661
June1st to 15thDay
38%7%12%23%15%1
2730
16th to endDay
41%9%13%19%15%1
2730
July1st to 15thDay
33%7%10%24%20%3
2581
16th to endDay
37%8%14%21%16%2
2632
August1st to 15thDay
39%7%13%24%13%1
2474
16th to endDay
40%8%11%22%15%2
2634
September1st to 15thDay
39%7%14%22%14%1
2204
16th to endDay
37%6%10%28%15%
2145
October1st to 15thDay
45%6%10%22%13%1
1990
16th to endDay
52%5%13%19%8%
1800
November1st to 15thDay
46%5%10%22%13%1
1717
16th to endDay
56%5%13%18%6%
1755
December1st to 15thDay
61%7%11%14%5%
1719
16th to endDay
70%5%12%7%3
1664
January1st to 15thNight
61%5%10%14%7%
2716
16th to endNight
63%6%13%11%4
2818
February1st to 15thNight
60%410%15%6%1
2425
16th to endNight
57%412%18%6%1
2049
March1st to 15thNight
51%5%10%18%11%1
2265
16th to endNight
48%5%10%22%11%1
2288
April1st to 15thNight
50%6%10%18%14%
1960
16th to endNight
44%411%18%17%3
1735
May1st to 15thNight
48%6%9%21%11%2
1755
16th to endNight
39%7%11%23%15%3
1807
June1st to 15thNight
36%7%11%27%16%
1560
16th to endNight
33%8%14%25%17%
1560
July1st to 15thNight
27%7%11%28%21%3
1480
16th to endNight
26%8%15%24%22%2
1623
August1st to 15thNight
30%7%13%30%16%2
1715
16th to endNight
32%6%11%25%19%3
1918
September1st to 15thNight
33%5%10%27%20%2
2073
16th to endNight
33%6%11%28%18%1
2145
October1st to 15thNight
42%5%10%22%17%
2139
16th to endNight
49%5%11%23%9%
2421
November1st to 15thNight
43%5%12%22%13%2
2471
16th to endNight
54%413%19%7%
2535
December1st to 15thNight
60%5%12%13%6%
2568
16th to endNight
67%6%10%11%4
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
20%11%40%20%41
135
16th to endDay
25%13%40%17%3
144
February1st to 15thDay
26%13%42%14%2
135
16th to endDay
32%9%41%13%2
120
March1st to 15thDay
26%13%30%22%7%
209
16th to endDay
23%8%32%25%9%1
352
April1st to 15thDay
25%10%31%24%7%1
330
16th to endDay
18%10%28%22%12%6%
319
May1st to 15thDay
22%8%30%18%15%4
348
16th to endDay
17%7%23%23%17%12%
413
June1st to 15thDay
12%5%27%31%17%5%
570
16th to endDay
12%5%27%30%16%5%
570
July1st to 15thDay
9%221%28%25%12%
550
16th to endDay
9%320%28%23%13%
509
August1st to 15thDay
9%115%28%24%20%
358
16th to endDay
15%217%21%25%16%
396
September1st to 15thDay
14%217%19%26%18%
375
16th to endDay
15%5%13%21%28%14%
374
October1st to 15thDay
18%6%25%25%14%8%
345
16th to endDay
24%10%30%27%6%
216
November1st to 15thDay
17%9%27%28%12%3
135
16th to endDay
21%18%39%15%5%
135
December1st to 15thDay
21%26%30%13%8%
135
16th to endDay
25%25%36%12%
144
January1st to 15thNight
27%11%36%19%5%
975
16th to endNight
17%10%39%26%5%
946
February1st to 15thNight
24%12%35%24%3
764
16th to endNight
23%11%33%26%4
683
March1st to 15thNight
22%11%29%29%6%
780
16th to endNight
19%9%32%28%9%
832
April1st to 15thNight
20%12%34%26%5%
613
16th to endNight
20%6%32%29%10%1
579
May1st to 15thNight
21%430%31%10%1
585
16th to endNight
16%223%34%19%2
611
June1st to 15thNight
12%223%45%16%
585
16th to endNight
8%220%43%23%1
585
July1st to 15thNight
8%115%42%28%4
555
16th to endNight
5%115%43%32%1
612
August1st to 15thNight
7%115%45%24%5%
575
16th to endNight
12%14%32%34%5%
621
September1st to 15thNight
12%317%32%28%6%
708
16th to endNight
13%416%30%28%6%
777
October1st to 15thNight
17%6%21%32%18%2
752
16th to endNight
23%10%33%23%7%
768
November1st to 15thNight
19%8%31%26%12%1
760
16th to endNight
25%11%31%23%7%
780
December1st to 15thNight
26%11%35%20%5%
813
16th to endNight
28%11%36%19%3
1040
Page generated at 2018-10-09 10:39:12. Version ac04

Copyright 2018 A. Danko


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