Mountain Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain Sky 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
47%
15882
26%
46936
15%
15837
58%
15837
38%
25683

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
38%
852
27%
2458
15%
847
52%
847
44%
1269
January16th to endNight
40%
768
33%
2496
19%
768
58%
768
43%
1203
February1st to 15thNight
33%
701
24%
2261
12%
699
49%
699
39%
1170
February16th to endNight
28%
632
33%
1894
12%
632
50%
632
42%
977
March1st to 15thNight
39%
720
27%
2061
10%
720
53%
720
42%
1103
March16th to endNight
37%
711
28%
2055
13%
711
51%
711
39%
1108
April1st to 15thNight
34%
539
24%
1798
11%
539
50%
539
31%
953
April16th to endNight
33%
540
25%
1800
13%
540
51%
540
29%
984
May1st to 15thNight
41%
540
21%
1681
13%
540
51%
540
28%
996
May16th to endNight
44%
563
21%
1497
10%
561
54%
561
36%
871
June1st to 15thNight
60%
540
26%
1440
18%
540
64%
540
48%
755
June16th to endNight
71%
540
25%
1440
21%
540
73%
540
48%
778
July1st to 15thNight
66%
510
14%
1360
12%
510
67%
510
25%
935
July16th to endNight
73%
564
13%
1540
13%
531
74%
531
23%
1040
August1st to 15thNight
69%
539
13%
1659
13%
536
69%
536
25%
1057
August16th to endNight
72%
573
20%
1910
21%
573
74%
573
27%
1173
September1st to 15thNight
67%
540
24%
1800
22%
540
74%
540
31%
1043
September16th to endNight
65%
541
35%
1844
27%
541
77%
541
44%
893
October1st to 15thNight
51%
660
31%
1980
19%
660
60%
660
42%
1059
October16th to endNight
52%
704
36%
2112
18%
704
66%
704
50%
1027
November1st to 15thNight
47%
845
33%
2245
15%
845
57%
845
52%
1140
November16th to endNight
45%
900
36%
2397
17%
900
58%
900
51%
1229
December1st to 15thNight
30%
900
23%
2520
7%
900
43%
900
42%
1337
December16th to endNight
31%
960
24%
2688
10%
960
42%
960
34%
1583

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
4424435%5%6%8%51%
47946
Night
341332345%7%59%
47121
TransparencyDay
26%10%14%24%19%4
47632
Night
22%8%15%26%21%5%
46937
SeeingDay
7%19%30%26%12%3
5766
Night
5%10%32%32%17%2
15885

By Month

Percentage Cloud Cover SW (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
1313325%45%8%59%
1489
16th to endDay
142432235%9%61%
1728
February1st to 15thDay
1425%32345%9%56%
1600
16th to endDay
43442345%9%56%
1580
March1st to 15thDay
15%24425%449%56%
1896
16th to endDay
35%24435%45%8%52%
2169
April1st to 15thDay
26%344245%5%9%52%
2150
16th to endDay
47%26%5%35%46%8%45%
2160
May1st to 15thDay
6%6%36%6%36%6%6%8%41%
2266
16th to endDay
45%245%36%5%6%8%47%
2616
June1st to 15thDay
3314425%6%7%9%50%
2520
16th to endDay
22134345%7%8%55%
2520
July1st to 15thDay
7%425%7%47%7%7%8%37%
2520
16th to endDay
6%426%7%5%6%8%8%8%34%
2518
August1st to 15thDay
8%5%36%8%46%6%7%7%35%
2290
16th to endDay
6%5%25%5%36%6%8%9%40%
2292
September1st to 15thDay
6%4246%35%6%7%8%44%
2160
16th to endDay
43134245%46%60%
1924
October1st to 15thDay
6%21342335%7%58%
1650
16th to endDay
2312323336%66%
1760
November1st to 15thDay
1212222345%70%
1602
16th to endDay
1313222236%71%
1560
December1st to 15thDay
15%23322349%61%
1440
16th to endDay
1334425%46%11%52%
1536
January1st to 15thNight
1412213346%67%
2458
16th to endNight
1412113236%69%
2496
February1st to 15thNight
1414312347%64%
2251
16th to endNight
15%13323247%64%
1894
March1st to 15thNight
1323323447%62%
2061
16th to endNight
241332445%5%62%
2055
April1st to 15thNight
342332445%7%58%
1798
16th to endNight
5%6%2432435%5%56%
1800
May1st to 15thNight
6%6%245%2445%7%50%
1680
16th to endNight
7%413324447%55%
1497
June1st to 15thNight
22123235%7%6%63%
1440
16th to endNight
21123246%5%7%61%
1440
July1st to 15thNight
7%426%6%35%6%7%9%39%
1440
16th to endNight
6%3345%47%6%9%11%37%
1647
August1st to 15thNight
5%4245%46%6%7%9%43%
1668
16th to endNight
5%4245%446%8%10%43%
1910
September1st to 15thNight
7%314325%5%8%8%49%
1800
16th to endNight
431221335%6%65%
1844
October1st to 15thNight
6%312322446%62%
1980
16th to endNight
3312212436%68%
2112
November1st to 15thNight
2212212447%68%
2245
16th to endNight
2312212336%71%
2397
December1st to 15thNight
2422322247%66%
2520
16th to endNight
15%2442345%7%57%
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
23%8%19%30%17%
1490
16th to endDay
22%7%17%32%16%3
1728
February1st to 15thDay
23%10%20%30%12%2
1601
16th to endDay
24%8%13%30%20%2
1580
March1st to 15thDay
23%7%15%27%23%2
1896
16th to endDay
27%9%12%29%18%3
2169
April1st to 15thDay
26%10%12%27%19%3
2150
16th to endDay
29%11%11%24%19%2
2160
May1st to 15thDay
34%13%12%22%16%1
2266
16th to endDay
29%11%14%20%19%5%
2620
June1st to 15thDay
25%10%13%17%22%11%
2520
16th to endDay
21%8%10%20%25%12%
2520
July1st to 15thDay
33%15%14%15%14%6%
2371
16th to endDay
37%15%19%17%7%1
2362
August1st to 15thDay
38%15%13%14%12%4
2275
16th to endDay
30%13%14%22%14%3
2292
September1st to 15thDay
29%12%15%19%16%6%
2160
16th to endDay
20%8%11%23%30%4
1924
October1st to 15thDay
24%9%9%27%24%5%
1650
16th to endDay
17%6%11%32%27%4
1760
November1st to 15thDay
14%7%14%25%33%5%
1602
16th to endDay
16%411%35%24%6%
1560
December1st to 15thDay
22%9%22%30%14%1
1440
16th to endDay
27%11%21%25%11%3
1536
January1st to 15thNight
19%7%19%30%21%2
2458
16th to endNight
17%7%14%34%22%4
2496
February1st to 15thNight
22%7%21%30%15%3
2261
16th to endNight
19%7%14%30%25%3
1894
March1st to 15thNight
19%8%15%28%25%2
2061
16th to endNight
21%7%12%31%24%2
2055
April1st to 15thNight
24%8%12%28%21%4
1798
16th to endNight
26%8%13%26%21%3
1800
May1st to 15thNight
32%7%13%24%18%4
1681
16th to endNight
25%8%17%25%16%7%
1498
June1st to 15thNight
17%8%13%22%20%17%
1440
16th to endNight
18%8%12%21%23%15%
1440
July1st to 15thNight
38%12%14%16%10%7%
1360
16th to endNight
34%17%21%17%6%2
1540
August1st to 15thNight
33%13%18%16%11%6%
1659
16th to endNight
30%14%17%21%12%4
1910
September1st to 15thNight
27%12%16%19%17%7%
1800
16th to endNight
19%7%10%27%29%5%
1844
October1st to 15thNight
21%7%11%27%25%6%
1980
16th to endNight
15%6%9%28%34%5%
2112
November1st to 15thNight
14%6%14%27%31%6%
2245
16th to endNight
16%5%9%35%27%6%
2397
December1st to 15thNight
20%6%21%28%20%1
2520
16th to endNight
25%10%17%27%15%2
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
10%15%34%32%6%1
90
16th to endDay
412%44%34%4
96
February1st to 15thDay
6%20%50%21%2
90
16th to endDay
428%37%22%7%
81
March1st to 15thDay
5%36%34%19%3
186
16th to endDay
7%43%36%10%1
288
April1st to 15thDay
10%39%30%13%31
269
16th to endDay
13%45%28%10%1
270
May1st to 15thDay
9%28%35%19%6%
342
16th to endDay
7%20%31%25%11%3
517
June1st to 15thDay
414%27%31%18%4
495
16th to endDay
38%27%32%22%5%
495
July1st to 15thDay
11%7%20%30%24%6%
475
16th to endDay
9%419%36%23%6%
377
August1st to 15thDay
11%9%28%34%14%1
315
16th to endDay
8%9%33%34%10%3
332
September1st to 15thDay
6%13%35%27%13%3
315
16th to endDay
7%14%37%25%13%2
181
October1st to 15thDay
5%15%27%28%15%6%
90
16th to endDay
7%16%28%27%17%3
96
November1st to 15thDay
416%23%37%15%2
90
16th to endDay
433%38%21%2
90
December1st to 15thDay
431%47%15%1
90
16th to endDay
5%20%46%23%3
96
January1st to 15thNight
312%42%29%9%2
852
16th to endNight
311%41%31%10%2
768
February1st to 15thNight
39%50%29%6%
704
16th to endNight
314%50%26%4
632
March1st to 15thNight
313%40%32%10%
720
16th to endNight
315%40%31%8%
711
April1st to 15thNight
6%19%38%28%7%
539
16th to endNight
8%18%35%27%8%
540
May1st to 15thNight
7%12%32%32%12%1
540
16th to endNight
7%11%31%30%15%2
563
June1st to 15thNight
38%23%41%20%3
540
16th to endNight
2316%42%31%3
540
July1st to 15thNight
8%110%33%40%5%
510
16th to endNight
6%6%28%49%8%
564
August1st to 15thNight
7%19%35%38%7%
539
16th to endNight
5%111%36%38%6%
573
September1st to 15thNight
7%114%39%29%7%
540
16th to endNight
5%422%36%25%6%
541
October1st to 15thNight
6%7%29%34%16%3
660
16th to endNight
46%32%35%17%3
704
November1st to 15thNight
310%35%34%13%1
845
16th to endNight
5%13%35%32%12%2
900
December1st to 15thNight
414%47%25%6%
900
16th to endNight
414%46%25%9%
960
Page generated at 2017-09-23 04:43:40. Version ac04

Copyright 2017 A. Danko


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