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West Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the West 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
31%
14797
16%
47051
8%
14765
41%
14765
19%
31482

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
25%
720
19%
2506
8%
716
39%
716
16%
1629
January16th to endNight
17%
768
17%
2688
5%
768
33%
768
12%
1789
February1st to 15thNight
20%
704
15%
2264
5%
701
32%
701
13%
1580
February16th to endNight
21%
631
18%
1893
7%
631
35%
631
16%
1308
March1st to 15thNight
20%
720
15%
2136
6%
720
33%
720
16%
1514
March16th to endNight
25%
768
21%
2058
7%
768
36%
768
22%
1365
April1st to 15thNight
24%
718
17%
1796
4%
718
32%
718
21%
1311
April16th to endNight
25%
546
16%
1626
6%
546
33%
546
19%
1114
May1st to 15thNight
28%
540
14%
1620
8%
540
35%
540
17%
1160
May16th to endNight
34%
392
11%
1514
10%
391
41%
391
13%
1055
June1st to 15thNight
32%
360
9%
1440
8%
360
40%
360
13%
1005
June16th to endNight
45%
360
9%
1440
11%
360
50%
360
20%
924
July1st to 15thNight
52%
340
11%
1360
18%
340
55%
340
21%
822
July16th to endNight
51%
376
13%
1418
14%
354
57%
354
21%
876
August1st to 15thNight
46%
494
12%
1565
8%
492
51%
492
22%
1014
August16th to endNight
51%
651
20%
1797
11%
651
54%
651
33%
1111
September1st to 15thNight
45%
720
18%
1876
8%
720
51%
720
33%
1197
September16th to endNight
47%
689
26%
1902
16%
689
54%
689
34%
1139
October1st to 15thNight
39%
660
20%
1863
10%
660
48%
660
24%
1229
October16th to endNight
25%
704
15%
2288
6%
704
36%
704
17%
1548
November1st to 15thNight
35%
700
21%
2277
13%
700
47%
700
21%
1482
November16th to endNight
30%
720
20%
2349
11%
720
41%
720
19%
1532
December1st to 15thNight
20%
747
15%
2678
4%
747
33%
747
14%
1857
December16th to endNight
21%
769
16%
2697
6%
769
34%
769
12%
1921

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
18%7%25%5%2445%6%36%
47851
Night
17%7%25%424446%38%
47221
TransparencyDay
47%5%11%21%12%1
47539
Night
45%5%11%22%13%1
47051
SeeingDay
24%12%26%21%11%3
5862
Night
22%11%28%27%9%
14797

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
15%7%345%35%46%7%35%
1432
16th to endDay
14%10%35%43445%5%38%
1536
February1st to 15thDay
15%10%46%6%35%5%45%31%
1594
16th to endDay
13%10%46%6%35%5%45%34%
1580
March1st to 15thDay
17%10%35%5%25%45%6%32%
1812
16th to endDay
14%7%36%5%24335%42%
2163
April1st to 15thDay
19%8%35%5%24345%36%
2152
16th to endDay
21%5%25%423445%38%
2334
May1st to 15thDay
27%7%26%5%24346%29%
2332
16th to endDay
21%6%345%35%5%5%7%32%
2610
June1st to 15thDay
28%6%3442445%6%29%
2520
16th to endDay
21%6%25%5%25%5%5%7%31%
2520
July1st to 15thDay
16%5%25%5%36%5%6%9%33%
2520
16th to endDay
15%425%6%36%6%7%8%33%
2654
August1st to 15thDay
16%5%26%6%35%5%6%7%33%
2373
16th to endDay
12%424425%46%8%44%
2402
September1st to 15thDay
14%424434446%45%
2083
16th to endDay
14%5%12323448%49%
1895
October1st to 15thDay
21%6%25%413345%39%
1766
16th to endDay
20%7%25%5%34447%35%
1584
November1st to 15thDay
20%6%1432346%6%38%
1581
16th to endDay
21%6%24424346%40%
1609
December1st to 15thDay
21%10%35%6%1345%5%32%
1278
16th to endDay
21%11%47%334445%29%
1521
January1st to 15thNight
15%9%34434446%39%
2506
16th to endNight
14%9%36%5%35%5%46%35%
2688
February1st to 15thNight
11%9%47%7%46%5%5%6%32%
2263
16th to endNight
13%10%36%5%25%445%38%
1893
March1st to 15thNight
17%10%35%5%25%445%34%
2123
16th to endNight
16%6%35%324435%44%
2058
April1st to 15thNight
19%7%35%5%25%435%36%
1796
16th to endNight
23%6%24425%435%37%
1626
May1st to 15thNight
21%7%25%435%445%35%
1620
16th to endNight
23%7%35%43445%6%30%
1512
June1st to 15thNight
25%7%34425%35%6%30%
1440
16th to endNight
19%6%2442345%10%36%
1440
July1st to 15thNight
14%4345%36%36%10%38%
1440
16th to endNight
14%4245%245%6%8%41%
1514
August1st to 15thNight
15%415%6%345%5%6%40%
1575
16th to endNight
12%5%2442236%8%47%
1797
September1st to 15thNight
15%414423246%49%
1876
16th to endNight
14%313323346%51%
1902
October1st to 15thNight
18%6%143235%5%6%42%
1863
16th to endNight
22%6%25%5%2335%7%35%
2288
November1st to 15thNight
18%6%2442435%6%40%
2277
16th to endNight
17%6%3442445%6%41%
2349
December1st to 15thNight
21%9%45%5%2434434%
2678
16th to endNight
19%11%46%435%35%5%31%
2697

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
44%7%11%21%13%1
1433
16th to endDay
45%413%22%13%1
1536
February1st to 15thDay
52%6%10%20%10%
1597
16th to endDay
50%411%16%13%3
1580
March1st to 15thDay
52%410%17%12%2
1822
16th to endDay
44%48%17%19%4
2163
April1st to 15thDay
48%46%20%15%3
2152
16th to endDay
46%5%9%19%13%3
2334
May1st to 15thDay
55%47%17%12%1
2332
16th to endDay
49%6%10%19%12%1
2614
June1st to 15thDay
55%5%12%17%8%
2520
16th to endDay
51%7%10%18%11%
2520
July1st to 15thDay
44%7%13%23%10%
2371
16th to endDay
44%8%13%23%9%
2487
August1st to 15thDay
46%7%13%24%7%
2359
16th to endDay
34%6%15%26%16%1
2402
September1st to 15thDay
38%5%11%27%16%1
2083
16th to endDay
34%49%29%20%1
1895
October1st to 15thDay
46%410%23%13%1
1766
16th to endDay
48%411%21%12%1
1584
November1st to 15thDay
42%5%14%22%12%1
1581
16th to endDay
45%414%19%14%1
1609
December1st to 15thDay
52%5%11%19%10%
1278
16th to endDay
55%5%10%15%11%1
1521
January1st to 15thNight
44%6%12%23%11%1
2506
16th to endNight
47%5%9%19%14%2
2688
February1st to 15thNight
49%6%12%18%11%1
2264
16th to endNight
47%6%10%19%13%2
1893
March1st to 15thNight
50%49%20%11%2
2136
16th to endNight
41%410%18%19%4
2058
April1st to 15thNight
50%47%18%17%1
1796
16th to endNight
48%411%18%13%3
1626
May1st to 15thNight
50%6%7%17%14%2
1620
16th to endNight
52%6%11%18%8%1
1514
June1st to 15thNight
53%6%11%19%8%1
1440
16th to endNight
43%7%14%22%11%1
1440
July1st to 15thNight
39%7%16%25%10%
1360
16th to endNight
39%9%16%23%11%
1418
August1st to 15thNight
42%7%14%25%9%
1565
16th to endNight
34%6%14%28%15%
1797
September1st to 15thNight
36%5%11%30%15%
1876
16th to endNight
32%5%10%32%17%1
1902
October1st to 15thNight
42%5%12%22%14%1
1863
16th to endNight
47%5%10%24%10%1
2288
November1st to 15thNight
43%5%12%23%13%1
2277
16th to endNight
43%6%12%22%14%1
2349
December1st to 15thNight
52%410%21%10%
2678
16th to endNight
53%5%10%18%11%
2697

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
23%18%28%25%5%
60
16th to endDay
21%25%32%15%4
64
February1st to 15thDay
28%15%30%20%4
202
16th to endDay
17%24%28%22%6%
239
March1st to 15thDay
28%18%27%20%4
270
16th to endDay
22%23%25%15%9%3
288
April1st to 15thDay
31%12%24%19%8%3
270
16th to endDay
27%19%26%12%7%5%
270
May1st to 15thDay
38%12%23%14%7%3
288
16th to endDay
26%12%31%22%6%1
331
June1st to 15thDay
27%12%28%22%7%1
315
16th to endDay
22%9%35%20%10%1
315
July1st to 15thDay
18%6%32%23%13%5%
305
16th to endDay
15%5%30%29%13%4
326
August1st to 15thDay
17%7%28%23%16%6%
313
16th to endDay
15%10%17%30%18%7%
332
September1st to 15thDay
19%8%22%20%20%7%
315
16th to endDay
19%9%20%21%15%15%
283
October1st to 15thDay
23%12%20%18%19%5%
248
16th to endDay
25%12%30%20%10%
240
November1st to 15thDay
25%10%26%22%12%1
235
16th to endDay
34%14%19%21%9%
229
December1st to 15thDay
25%16%36%15%6%
60
16th to endDay
39%17%28%15%
64
January1st to 15thNight
21%14%32%27%4
720
16th to endNight
22%16%33%23%3
768
February1st to 15thNight
18%18%34%26%3
704
16th to endNight
22%16%31%23%5%
631
March1st to 15thNight
26%15%32%20%4
720
16th to endNight
21%16%30%23%7%1
768
April1st to 15thNight
24%14%32%23%5%
718
16th to endNight
27%14%29%21%7%
546
May1st to 15thNight
26%10%27%26%8%
540
16th to endNight
25%7%26%31%8%
392
June1st to 15thNight
27%6%26%31%7%
360
16th to endNight
19%225%38%13%
360
July1st to 15thNight
17%121%38%21%
340
16th to endNight
14%123%38%20%
376
August1st to 15thNight
17%322%37%18%1
494
16th to endNight
14%423%33%21%2
651
September1st to 15thNight
16%326%30%19%2
720
16th to endNight
16%5%23%28%21%3
689
October1st to 15thNight
23%8%24%27%15%1
660
16th to endNight
27%11%30%24%5%
704
November1st to 15thNight
22%10%28%29%8%1
700
16th to endNight
21%13%26%26%10%1
720
December1st to 15thNight
27%14%31%21%5%
747
16th to endNight
28%15%26%25%3
769
Page generated at 2017-09-24 09:15:50. Version ac04

Copyright 2017 A. Danko


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