CMS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CMS Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
53%
15945
21%
46746
15%
15904
59%
15904
36%
26753

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
45%
898
19%
2506
12%
895
51%
895
35%
1486
January16th to endNight
45%
789
24%
2517
15%
789
57%
789
35%
1374
February1st to 15thNight
39%
698
16%
2288
10%
698
49%
698
28%
1367
February16th to endNight
40%
632
16%
2034
10%
632
48%
632
28%
1223
March1st to 15thNight
46%
720
21%
1989
10%
720
56%
720
40%
1120
March16th to endNight
46%
768
16%
2112
8%
768
52%
768
31%
1326
April1st to 15thNight
46%
545
15%
1804
11%
545
53%
545
26%
1096
April16th to endNight
45%
540
16%
1647
13%
540
47%
540
25%
1077
May1st to 15thNight
52%
539
16%
1532
11%
539
56%
539
31%
947
May16th to endNight
55%
563
13%
1498
11%
561
57%
561
28%
1001
June1st to 15thNight
53%
540
13%
1440
8%
540
56%
540
32%
923
June16th to endNight
70%
402
24%
1302
22%
402
74%
402
54%
578
July1st to 15thNight
78%
456
20%
1306
17%
456
82%
456
55%
633
July16th to endNight
76%
564
28%
1476
26%
531
79%
531
65%
673
August1st to 15thNight
79%
539
28%
1611
25%
536
81%
536
52%
772
August16th to endNight
75%
573
34%
1776
30%
573
78%
573
52%
823
September1st to 15thNight
69%
540
29%
1800
28%
540
75%
540
44%
893
September16th to endNight
66%
668
30%
1894
23%
668
70%
668
47%
1010
October1st to 15thNight
54%
660
23%
1980
16%
660
59%
660
32%
1220
October16th to endNight
54%
704
28%
2112
16%
704
64%
704
38%
1193
November1st to 15thNight
49%
847
25%
2394
17%
847
55%
847
39%
1368
November16th to endNight
48%
900
22%
2520
14%
900
55%
900
35%
1494
December1st to 15thNight
46%
900
16%
2520
9%
900
52%
900
34%
1503
December16th to endNight
46%
960
21%
2688
12%
960
55%
960
33%
1653

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
5%5%25%435%5%6%8%47%
48121
Night
45%25%42446%8%51%
46921
Cloud CoverDay
5%5%25%5%35%5%6%8%47%
48121
Night
5%5%25%42446%8%49%
46923
TransparencyDay
31%5%16%26%15%5%
47800
Night
30%417%28%14%4
46746
SeeingDay
8%10%29%33%14%3
6012
Night
6%5%24%44%16%2
15954

By Month

Percentage Cloud Cover NW (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
1325%335%49%10%50%
1433
16th to endDay
15%36%4345%6%9%51%
1718
February1st to 15thDay
15%47%5%25%5%5%9%46%
1575
16th to endDay
17%48%5%345%5%8%43%
1440
March1st to 15thDay
425%5%2449%12%47%
1968
16th to endDay
5%6%36%6%46%5%7%9%37%
2111
April1st to 15thDay
7%7%46%5%35%5%6%9%37%
2144
16th to endDay
9%9%37%6%36%6%5%8%32%
2310
May1st to 15thDay
9%7%35%435%5%6%7%41%
2418
16th to endDay
14%7%25%6%46%6%7%8%30%
2616
June1st to 15thDay
7%5%25%5%36%6%7%8%41%
2520
16th to endDay
1123246%7%9%59%
2651
July1st to 15thDay
2213335%6%8%8%53%
2568
16th to endDay
32123235%7%7%60%
2589
August1st to 15thDay
4314436%6%6%9%49%
2340
16th to endDay
33134345%5%7%58%
2426
September1st to 15thDay
7%31332435%6%59%
2160
16th to endDay
6%313423345%61%
1855
October1st to 15thDay
11%5%34335%5%5%8%43%
1650
16th to endDay
45%2433445%8%53%
1760
November1st to 15thDay
25%25%5%35%5%6%7%49%
1453
16th to endDay
26%345%243411%50%
1440
December1st to 15thDay
2328%5%246%6%11%45%
1440
16th to endDay
25%345%2447%9%50%
1536
January1st to 15thNight
5%36%425%5%7%8%49%
2506
16th to endNight
15%35%43445%9%53%
2517
February1st to 15thNight
15%37%7%36%5%5%9%45%
2277
16th to endNight
17%48%5%35%45%7%46%
2034
March1st to 15thNight
15%25%42447%11%51%
1989
16th to endNight
6%6%25%43447%9%44%
2109
April1st to 15thNight
6%7%344235%6%7%45%
1804
16th to endNight
8%9%36%6%3445%9%37%
1647
May1st to 15thNight
9%8%25%33445%6%46%
1531
16th to endNight
14%5%24435%47%10%36%
1498
June1st to 15thNight
7%415%5%35%5%6%9%45%
1440
16th to endNight
1112313348%66%
1302
July1st to 15thNight
1113245%6%12%60%
1386
16th to endNight
21121326%8%69%
1577
August1st to 15thNight
21123236%6%8%60%
1620
16th to endNight
321231335%7%64%
1776
September1st to 15thNight
6%42223347%61%
1800
16th to endNight
7%313222247%61%
1894
October1st to 15thNight
11%7%25%324346%47%
1980
16th to endNight
442333445%7%54%
2112
November1st to 15thNight
3425%4245%6%8%52%
2394
16th to endNight
28%25%5%245%6%9%48%
2520
December1st to 15thNight
15%37%5%345%6%10%47%
2520
16th to endNight
15%36%6%25%5%6%10%46%
2688

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
14345%36%5%8%10%47%
1432
16th to endDay
26%36%6%35%45%8%48%
1718
February1st to 15thDay
26%48%5%35%5%410%42%
1575
16th to endDay
28%47%435%5%8%10%41%
1440
March1st to 15thDay
14345%25%5%7%12%47%
1968
16th to endDay
7%5%36%6%36%5%8%11%36%
2112
April1st to 15thDay
6%7%36%6%35%5%6%8%38%
2144
16th to endDay
8%9%37%6%36%5%6%7%33%
2310
May1st to 15thDay
9%6%25%5%25%5%6%7%43%
2418
16th to endDay
15%6%26%436%7%6%9%31%
2616
June1st to 15thDay
7%5%25%5%35%6%7%9%40%
2520
16th to endDay
2112225%5%8%9%58%
2651
July1st to 15thDay
2213426%6%7%8%53%
2568
16th to endDay
22134245%6%7%59%
2589
August1st to 15thDay
4314435%6%7%8%50%
2340
16th to endDay
32134245%5%7%58%
2426
September1st to 15thDay
7%21332345%6%60%
2160
16th to endDay
6%314323445%60%
1855
October1st to 15thDay
11%7%2442446%7%44%
1650
16th to endDay
45%235%3335%7%52%
1760
November1st to 15thDay
25%35%5%35%5%6%8%49%
1453
16th to endDay
26%35%5%2246%10%48%
1440
December1st to 15thDay
2436%7%25%6%6%9%43%
1440
16th to endDay
36%25%5%2437%10%47%
1536
January1st to 15thNight
15%36%446%5%7%8%45%
2506
16th to endNight
26%26%42445%8%50%
2517
February1st to 15thNight
27%47%6%35%47%9%41%
2277
16th to endNight
28%47%6%35%45%8%42%
2034
March1st to 15thNight
25%25%4245%6%11%48%
1989
16th to endNight
6%6%25%5%35%46%11%40%
2112
April1st to 15thNight
8%7%35%425%5%6%7%43%
1804
16th to endNight
9%10%37%5%345%5%8%37%
1647
May1st to 15thNight
10%6%25%425%447%45%
1531
16th to endNight
14%5%245%346%7%10%35%
1497
June1st to 15thNight
8%425%5%45%46%8%44%
1440
16th to endNight
221232235%9%64%
1302
July1st to 15thNight
1122245%6%11%60%
1386
16th to endNight
21121235%8%69%
1577
August1st to 15thNight
21123246%6%8%59%
1620
16th to endNight
321222335%8%63%
1776
September1st to 15thNight
7%312323446%61%
1800
16th to endNight
7%323212347%60%
1894
October1st to 15thNight
12%7%25%32345%6%47%
1980
16th to endNight
44245%25%5%5%7%52%
2112
November1st to 15thNight
46%25%4245%5%8%51%
2394
16th to endNight
47%26%425%5%6%8%46%
2520
December1st to 15thNight
25%27%6%35%5%7%9%43%
2520
16th to endNight
27%47%5%25%5%6%10%42%
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
27%6%26%27%9%2
1433
16th to endDay
30%5%20%28%13%2
1718
February1st to 15thDay
36%5%23%25%9%
1575
16th to endDay
36%6%18%23%11%3
1440
March1st to 15thDay
26%5%25%31%11%
1968
16th to endDay
38%7%20%26%6%
2112
April1st to 15thDay
41%5%19%23%8%
2144
16th to endDay
47%5%13%26%7%
2310
May1st to 15thDay
37%415%30%10%
2418
16th to endDay
46%7%18%19%7%
2620
June1st to 15thDay
36%5%18%24%12%2
2520
16th to endDay
18%415%28%21%12%
2651
July1st to 15thDay
23%5%10%28%20%12%
2419
16th to endDay
20%36%20%28%19%
2427
August1st to 15thDay
26%5%8%25%20%12%
2325
16th to endDay
23%47%22%28%14%
2426
September1st to 15thDay
24%37%24%29%9%
2160
16th to endDay
24%310%29%26%5%
1855
October1st to 15thDay
36%5%16%27%12%2
1650
16th to endDay
29%414%31%15%4
1760
November1st to 15thDay
31%6%14%24%20%3
1453
16th to endDay
29%420%29%13%1
1440
December1st to 15thDay
32%6%26%24%9%
1440
16th to endDay
30%5%21%30%10%2
1536
January1st to 15thNight
29%6%24%26%10%1
2506
16th to endNight
29%421%28%13%2
2517
February1st to 15thNight
35%5%21%28%6%1
2288
16th to endNight
37%420%23%11%2
2034
March1st to 15thNight
27%422%31%12%
1989
16th to endNight
36%5%20%29%7%
2112
April1st to 15thNight
36%6%19%26%8%
1804
16th to endNight
43%417%27%6%
1647
May1st to 15thNight
37%416%29%11%
1532
16th to endNight
40%6%22%19%8%
1498
June1st to 15thNight
34%7%20%23%12%1
1440
16th to endNight
16%212%34%22%10%
1302
July1st to 15thNight
15%5%12%32%22%11%
1306
16th to endNight
12%38%22%31%22%
1476
August1st to 15thNight
18%410%26%25%14%
1611
16th to endNight
17%39%26%26%16%
1776
September1st to 15thNight
23%39%24%27%11%
1800
16th to endNight
23%39%30%27%4
1894
October1st to 15thNight
36%413%31%9%3
1980
16th to endNight
29%413%32%16%3
2112
November1st to 15thNight
28%417%29%16%2
2394
16th to endNight
32%420%26%13%2
2520
December1st to 15thNight
31%5%25%27%8%
2520
16th to endNight
34%5%18%29%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
318%41%23%13%
90
16th to endDay
4226%45%12%9%
96
February1st to 15thDay
322%37%30%6%
90
16th to endDay
9%19%30%29%7%2
81
March1st to 15thDay
326%37%27%5%
258
16th to endDay
12%22%36%25%3
288
April1st to 15thDay
9%19%39%24%6%
269
16th to endDay
21%16%33%21%5%
270
May1st to 15thDay
13%10%27%29%15%2
375
16th to endDay
18%7%19%30%20%3
517
June1st to 15thDay
9%9%25%35%18%2
495
16th to endDay
28%30%37%17%3
495
July1st to 15thDay
35%27%31%25%6%
475
16th to endDay
26%29%37%20%2
448
August1st to 15thDay
9%5%33%36%11%2
315
16th to endDay
45%30%38%15%4
332
September1st to 15thDay
8%7%36%32%11%3
315
16th to endDay
10%11%32%29%14%2
251
October1st to 15thDay
14%10%25%33%12%4
90
16th to endDay
6%10%22%37%17%5%
96
November1st to 15thDay
5%8%22%31%18%13%
90
16th to endDay
13%8%31%41%5%
90
December1st to 15thDay
5%327%52%10%1
90
16th to endDay
7%221%47%18%2
96
January1st to 15thNight
310%31%40%11%3
900
16th to endNight
46%33%41%10%2
789
February1st to 15thNight
47%34%40%10%2
704
16th to endNight
5%7%31%41%12%1
632
March1st to 15thNight
48%32%40%13%1
720
16th to endNight
8%7%26%44%11%
768
April1st to 15thNight
10%6%26%45%10%
545
16th to endNight
14%6%24%38%15%1
540
May1st to 15thNight
13%5%20%38%21%1
540
16th to endNight
16%415%38%23%1
563
June1st to 15thNight
8%6%22%40%21%
540
16th to endNight
3318%48%26%
402
July1st to 15thNight
2112%47%33%1
456
16th to endNight
3113%49%30%1
564
August1st to 15thNight
3110%51%32%
539
16th to endNight
4212%47%30%2
573
September1st to 15thNight
7%313%48%22%3
540
16th to endNight
7%315%44%22%5%
668
October1st to 15thNight
15%117%44%17%3
660
16th to endNight
6%427%44%13%3
704
November1st to 15thNight
6%6%29%44%10%2
847
16th to endNight
7%7%26%48%9%1
900
December1st to 15thNight
38%29%45%11%1
900
16th to endNight
5%5%29%46%11%2
960
Page generated at 2017-09-22 05:35:30. Version ac04

Copyright 2017 A. Danko


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