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 2019-06-10.

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
55%
18332
22%
53731
16%
18291
61%
18291
37%
30774

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
50%
1048
20%
2926
13%
1045
56%
1045
36%
1713
January16th to endNight
48%
921
23%
2937
15%
921
58%
921
36%
1591
February1st to 15thNight
41%
818
17%
2678
12%
818
50%
818
28%
1596
February16th to endNight
44%
736
16%
2369
12%
736
52%
736
27%
1435
March1st to 15thNight
48%
840
23%
2321
12%
840
58%
840
39%
1329
March16th to endNight
50%
896
16%
2464
8%
896
55%
896
32%
1529
April1st to 15thNight
46%
637
15%
2106
11%
637
53%
637
25%
1304
April16th to endNight
46%
624
16%
1904
14%
624
48%
624
25%
1228
May1st to 15thNight
54%
629
16%
1788
12%
629
57%
629
30%
1114
May16th to endNight
54%
659
13%
1754
11%
657
56%
657
27%
1181
June1st to 15thNight
55%
612
15%
1632
9%
612
59%
612
35%
1019
June16th to endNight
68%
436
23%
1411
20%
436
71%
436
51%
645
July1st to 15thNight
79%
496
22%
1421
19%
496
83%
496
57%
678
July16th to endNight
77%
612
27%
1609
25%
579
80%
579
65%
736
August1st to 15thNight
78%
575
28%
1719
26%
572
80%
572
53%
817
August16th to endNight
73%
621
33%
1925
29%
621
76%
621
51%
904
September1st to 15thNight
70%
585
31%
1950
30%
585
77%
585
46%
947
September16th to endNight
65%
763
30%
2161
22%
763
70%
763
45%
1174
October1st to 15thNight
54%
780
23%
2340
16%
780
60%
780
32%
1450
October16th to endNight
56%
832
29%
2496
18%
832
66%
832
40%
1383
November1st to 15thNight
51%
992
25%
2804
17%
992
57%
992
38%
1624
November16th to endNight
50%
1050
23%
2940
15%
1050
57%
1050
36%
1727
December1st to 15thNight
50%
1050
20%
2940
13%
1050
56%
1050
36%
1723
December16th to endNight
48%
1120
21%
3136
12%
1120
56%
1120
33%
1927

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%
54784
Night
45%25%42446%8%51%
53906
Cloud CoverDay
5%5%25%5%35%5%6%8%47%
54784
Night
5%5%25%42446%8%49%
53908
TransparencyDay
32%5%15%26%15%5%
54463
Night
30%417%28%14%4
53731
SeeingDay
9%9%30%33%14%3
8294
Night
7%5%23%44%16%2
18341

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%5%9%10%50%
1673
16th to endDay
15%36%5%345%6%8%49%
2004
February1st to 15thDay
15%37%6%25%5%5%10%45%
1845
16th to endDay
6%49%6%45%6%5%8%42%
1676
March1st to 15thDay
15%25%4245%9%11%48%
2296
16th to endDay
5%6%36%6%46%5%7%9%38%
2463
April1st to 15thDay
7%8%47%5%36%5%6%9%35%
2502
16th to endDay
9%9%37%6%36%6%5%8%33%
2658
May1st to 15thDay
10%7%35%5%35%5%6%7%40%
2822
16th to endDay
16%8%25%5%35%6%7%9%28%
3064
June1st to 15thDay
7%5%245%35%6%7%9%42%
2857
16th to endDay
2123246%7%9%57%
2872
July1st to 15thDay
2213335%6%8%8%53%
2782
16th to endDay
22124245%6%7%59%
2807
August1st to 15thDay
4214436%6%6%9%49%
2485
16th to endDay
33133345%5%7%57%
2629
September1st to 15thDay
6%31332435%6%59%
2340
16th to endDay
7%41342335%6%58%
2126
October1st to 15thDay
10%6%343345%5%8%43%
1950
16th to endDay
45%2433445%7%54%
2080
November1st to 15thDay
36%25%5%35%5%6%7%47%
1701
16th to endDay
26%245%243410%50%
1680
December1st to 15thDay
2327%5%246%6%10%48%
1680
16th to endDay
26%345%245%6%10%49%
1792
January1st to 15thNight
436%425%5%7%8%50%
2926
16th to endNight
15%35%42345%9%52%
2937
February1st to 15thNight
15%37%6%35%5%5%9%45%
2667
16th to endNight
16%38%6%45%5%5%7%45%
2369
March1st to 15thNight
15%25%5%2446%10%50%
2321
16th to endNight
5%6%25%4345%7%10%44%
2461
April1st to 15thNight
6%8%35%5%345%6%7%43%
2106
16th to endNight
8%9%36%5%3445%9%39%
1904
May1st to 15thNight
10%8%2433445%6%45%
1787
16th to endNight
15%5%24425%47%10%35%
1754
June1st to 15thNight
7%4145%35%5%5%9%47%
1632
16th to endNight
3113313348%63%
1411
July1st to 15thNight
1112245%6%11%62%
1501
16th to endNight
21121326%8%69%
1710
August1st to 15thNight
21123236%7%8%60%
1728
16th to endNight
3312312347%64%
1925
September1st to 15thNight
6%32223347%63%
1950
16th to endNight
8%413222247%59%
2161
October1st to 15thNight
11%7%35%32435%6%46%
2340
16th to endNight
442333445%7%56%
2496
November1st to 15thNight
3425%4245%6%9%51%
2804
16th to endNight
27%25%4245%6%9%49%
2940
December1st to 15thNight
15%26%5%2446%9%49%
2940
16th to endNight
16%36%5%25%5%7%10%46%
3136

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%35%5%8%9%47%
1672
16th to endDay
26%36%6%35%45%8%46%
2004
February1st to 15thDay
26%48%6%35%5%5%10%42%
1845
16th to endDay
17%38%5%35%6%8%9%40%
1676
March1st to 15thDay
24345%25%5%7%11%47%
2296
16th to endDay
6%5%36%6%36%6%7%10%38%
2464
April1st to 15thDay
7%7%37%6%35%5%6%8%37%
2502
16th to endDay
9%9%36%6%36%5%6%7%34%
2658
May1st to 15thDay
10%7%25%5%36%5%6%7%40%
2822
16th to endDay
16%6%26%436%7%6%9%29%
3064
June1st to 15thDay
7%5%245%35%6%7%9%41%
2857
16th to endDay
3212325%5%8%8%57%
2872
July1st to 15thDay
2213436%6%7%8%53%
2782
16th to endDay
22134245%6%7%59%
2807
August1st to 15thDay
4214435%6%7%8%50%
2485
16th to endDay
33134245%5%7%58%
2629
September1st to 15thDay
6%21322345%6%60%
2340
16th to endDay
7%414323446%58%
2126
October1st to 15thDay
10%7%2442445%7%44%
1950
16th to endDay
45%235%3335%7%54%
2080
November1st to 15thDay
26%35%5%35%5%5%8%47%
1701
16th to endDay
37%35%5%2246%9%48%
1680
December1st to 15thDay
2436%7%245%6%9%46%
1680
16th to endDay
36%25%5%2447%10%45%
1792
January1st to 15thNight
15%35%435%5%7%8%47%
2926
16th to endNight
26%26%42445%9%50%
2937
February1st to 15thNight
27%37%6%46%47%9%40%
2667
16th to endNight
27%47%7%45%45%8%42%
2369
March1st to 15thNight
25%25%4245%6%10%48%
2321
16th to endNight
6%6%26%5%35%46%12%40%
2464
April1st to 15thNight
8%7%35%5%345%5%7%41%
2106
16th to endNight
9%9%36%5%245%5%8%39%
1904
May1st to 15thNight
11%6%25%425%447%44%
1787
16th to endNight
14%6%245%246%7%10%34%
1753
June1st to 15thNight
7%425%5%45%46%8%46%
1632
16th to endNight
321232335%9%62%
1411
July1st to 15thNight
1122245%5%11%62%
1501
16th to endNight
21221245%8%68%
1710
August1st to 15thNight
21123246%6%8%59%
1728
16th to endNight
421222335%8%63%
1925
September1st to 15thNight
6%312323436%62%
1950
16th to endNight
9%313212347%58%
2161
October1st to 15thNight
11%7%35%32445%5%46%
2340
16th to endNight
442442445%7%54%
2496
November1st to 15thNight
46%25%42446%8%49%
2804
16th to endNight
47%25%425%46%8%47%
2940
December1st to 15thNight
25%27%6%25%5%7%9%45%
2940
16th to endNight
27%37%5%35%5%6%9%42%
3136

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%5%25%28%10%2
1673
16th to endDay
32%419%27%12%1
2004
February1st to 15thDay
35%423%25%9%
1845
16th to endDay
36%7%17%23%13%2
1676
March1st to 15thDay
27%5%22%31%12%1
2296
16th to endDay
38%6%20%26%6%
2464
April1st to 15thDay
43%5%19%22%8%
2502
16th to endDay
46%5%12%26%8%
2658
May1st to 15thDay
40%415%29%9%
2822
16th to endDay
47%7%18%18%7%
3068
June1st to 15thDay
35%5%18%23%14%2
2857
16th to endDay
19%414%28%21%11%
2872
July1st to 15thDay
23%5%9%28%20%12%
2633
16th to endDay
20%37%21%28%18%
2645
August1st to 15thDay
26%5%8%26%20%11%
2470
16th to endDay
23%47%23%27%14%
2629
September1st to 15thDay
24%37%25%30%9%
2340
16th to endDay
26%310%29%24%5%
2126
October1st to 15thDay
36%414%26%15%2
1950
16th to endDay
28%414%33%15%4
2080
November1st to 15thDay
33%5%14%24%18%2
1701
16th to endDay
30%419%30%13%1
1680
December1st to 15thDay
29%5%24%25%12%1
1680
16th to endDay
31%5%19%30%11%2
1792
January1st to 15thNight
29%5%23%27%11%2
2926
16th to endNight
29%322%28%13%2
2937
February1st to 15thNight
35%5%21%28%6%1
2678
16th to endNight
38%419%23%12%2
2369
March1st to 15thNight
28%420%31%13%1
2321
16th to endNight
35%420%29%8%
2464
April1st to 15thNight
38%6%20%25%7%
2106
16th to endNight
42%416%28%7%
1904
May1st to 15thNight
37%417%29%10%
1788
16th to endNight
42%6%22%20%7%
1754
June1st to 15thNight
32%6%20%24%13%2
1632
16th to endNight
18%312%34%21%9%
1411
July1st to 15thNight
14%5%11%31%24%11%
1421
16th to endNight
12%39%22%32%20%
1609
August1st to 15thNight
17%411%27%24%14%
1719
16th to endNight
18%39%25%25%16%
1925
September1st to 15thNight
22%38%25%29%10%
1950
16th to endNight
25%39%30%25%5%
2161
October1st to 15thNight
37%412%31%11%3
2340
16th to endNight
27%413%33%16%3
2496
November1st to 15thNight
30%416%30%15%2
2804
16th to endNight
31%420%27%14%1
2940
December1st to 15thNight
30%5%23%28%11%
2940
16th to endNight
34%5%17%29%10%2
3136

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
3118%43%24%8%
180
16th to endDay
46%29%45%7%5%
192
February1st to 15thDay
318%41%31%5%
180
16th to endDay
6%13%35%30%13%1
159
March1st to 15thDay
5%20%36%30%6%
376
16th to endDay
11%18%35%25%7%1
416
April1st to 15thDay
12%18%38%24%5%
389
16th to endDay
21%15%31%21%8%
378
May1st to 15thDay
14%8%27%30%15%2
509
16th to endDay
21%6%18%29%19%4
677
June1st to 15thDay
9%9%25%34%18%3
616
16th to endDay
48%30%35%19%2
570
July1st to 15thDay
36%28%31%25%5%
550
16th to endDay
26%31%38%18%2
522
August1st to 15thDay
8%5%34%36%11%3
359
16th to endDay
46%32%37%14%4
396
September1st to 15thDay
7%6%35%33%12%4
375
16th to endDay
11%10%32%29%13%2
346
October1st to 15thDay
12%10%29%34%11%2
180
16th to endDay
413%24%32%16%7%
192
November1st to 15thDay
6%8%28%37%12%6%
180
16th to endDay
12%10%28%40%6%1
180
December1st to 15thDay
3325%51%12%3
180
16th to endDay
7%428%47%11%1
192
January1st to 15thNight
38%28%42%13%3
1050
16th to endNight
46%32%44%9%2
921
February1st to 15thNight
48%34%40%9%2
824
16th to endNight
5%7%30%43%12%1
736
March1st to 15thNight
38%30%43%12%1
840
16th to endNight
7%7%25%45%12%1
896
April1st to 15thNight
10%6%26%45%9%
637
16th to endNight
14%6%22%39%15%
624
May1st to 15thNight
14%5%18%39%21%1
630
16th to endNight
16%414%38%23%2
659
June1st to 15thNight
7%6%22%39%22%1
612
16th to endNight
5%317%47%26%
436
July1st to 15thNight
2113%47%33%1
496
16th to endNight
2113%50%29%1
612
August1st to 15thNight
3110%51%32%1
575
16th to endNight
4212%47%30%2
621
September1st to 15thNight
7%313%48%23%3
585
16th to endNight
9%315%44%22%5%
763
October1st to 15thNight
15%118%44%16%3
780
16th to endNight
6%326%44%15%3
832
November1st to 15thNight
6%5%28%46%10%1
992
16th to endNight
7%7%26%49%9%1
1050
December1st to 15thNight
38%27%48%10%1
1050
16th to endNight
5%5%29%47%10%2
1120
Page generated at 2019-06-10 17:29:45. Version ac04

Copyright 2019 A. Danko


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