Big M Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big M 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
38%
16047
17%
46921
11%
16003
44%
16003
24%
31179

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
23%
900
13%
2506
5%
895
30%
895
18%
1821
January16th to endNight
22%
960
11%
2592
3%
960
28%
960
17%
1949
February1st to 15thNight
24%
877
13%
2288
4%
876
31%
876
21%
1652
February16th to endNight
27%
642
19%
1904
8%
642
36%
642
20%
1313
March1st to 15thNight
29%
675
15%
2115
9%
675
36%
675
20%
1405
March16th to endNight
29%
576
15%
2028
10%
576
36%
576
16%
1410
April1st to 15thNight
29%
539
16%
1797
11%
539
39%
539
19%
1204
April16th to endNight
37%
540
19%
1799
13%
540
46%
540
20%
1222
May1st to 15thNight
33%
540
16%
1470
8%
540
40%
540
23%
1050
May16th to endNight
48%
563
18%
1497
9%
561
54%
561
34%
950
June1st to 15thNight
53%
540
15%
1440
9%
540
56%
540
33%
923
June16th to endNight
52%
540
17%
1440
9%
540
56%
540
34%
915
July1st to 15thNight
66%
510
18%
1360
11%
510
68%
510
45%
769
July16th to endNight
69%
564
22%
1417
17%
531
71%
531
46%
800
August1st to 15thNight
71%
539
22%
1566
21%
536
73%
536
39%
901
August16th to endNight
65%
573
20%
1904
22%
573
67%
573
33%
1079
September1st to 15thNight
64%
540
27%
1800
28%
540
69%
540
37%
952
September16th to endNight
55%
629
26%
1857
18%
629
62%
629
35%
1081
October1st to 15thNight
44%
660
24%
1980
16%
660
51%
660
31%
1199
October16th to endNight
33%
704
18%
2181
11%
704
41%
704
23%
1406
November1st to 15thNight
34%
700
24%
2276
17%
700
44%
700
27%
1375
November16th to endNight
25%
876
16%
2496
8%
876
32%
876
21%
1733
December1st to 15thNight
21%
900
12%
2520
4%
900
27%
900
15%
1922
December16th to endNight
20%
960
11%
2688
3%
960
27%
960
12%
2148

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
14%7%36%5%3445%7%37%
47993
Night
13%7%35%42445%6%40%
47089
Cloud CoverDay
11%6%25%423346%47%
48003
Night
11%6%25%423345%50%
47094
Cloud CoverDay
14%7%36%5%3445%7%37%
48001
Night
13%7%35%5%2445%6%40%
47094
TransparencyDay
45%6%12%22%11%1
47678
Night
43%5%12%23%13%1
46922
SeeingDay
16%7%27%27%16%5%
6062
Night
17%7%30%31%12%1
16050

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
10%12%6%7%7%35%45%8%28%
1432
16th to endDay
11%15%6%10%7%44346%24%
1631
February1st to 15thDay
12%12%6%6%5%34446%32%
1575
16th to endDay
13%8%5%7%6%2435%6%35%
1569
March1st to 15thDay
15%9%46%433348%36%
1844
16th to endDay
18%8%46%5%3435%6%32%
2196
April1st to 15thDay
17%9%47%5%3445%6%30%
2151
16th to endDay
21%9%35%5%2435%5%34%
2160
May1st to 15thDay
17%9%36%6%3446%6%29%
2478
16th to endDay
15%5%26%6%25%5%6%8%35%
2617
June1st to 15thDay
14%5%25%5%345%6%7%39%
2520
16th to endDay
10%435%5%35%5%6%9%40%
2520
July1st to 15thDay
7%425%4346%7%8%45%
2520
16th to endDay
9%425%5%36%5%5%9%40%
2655
August1st to 15thDay
8%425%5%446%7%7%43%
2383
16th to endDay
13%5%25%5%2445%7%43%
2296
September1st to 15thDay
13%31333345%7%49%
2160
16th to endDay
13%5%35%434447%43%
1926
October1st to 15thDay
14%6%34424346%46%
1650
16th to endDay
18%7%25%424446%40%
1691
November1st to 15thDay
12%7%35%434447%41%
1582
16th to endDay
16%11%46%5%22235%38%
1461
December1st to 15thDay
16%13%48%5%35%345%27%
1440
16th to endDay
20%13%7%8%7%25%35%5%21%
1536
January1st to 15thNight
12%10%5%8%6%35%445%33%
2506
16th to endNight
13%11%5%10%6%3445%6%28%
2592
February1st to 15thNight
9%11%5%8%5%34445%35%
2285
16th to endNight
13%10%36%5%3345%6%37%
1904
March1st to 15thNight
16%9%35%323345%40%
2102
16th to endNight
18%8%35%5%35%446%32%
2028
April1st to 15thNight
19%8%35%434436%37%
1797
16th to endNight
19%7%3443445%5%37%
1799
May1st to 15thNight
18%5%35%43446%7%35%
1470
16th to endNight
11%43432445%9%45%
1496
June1st to 15thNight
12%5%2443446%7%45%
1440
16th to endNight
9%4244246%7%8%45%
1440
July1st to 15thNight
5%32431345%10%56%
1440
16th to endNight
5%2134245%6%10%54%
1513
August1st to 15thNight
6%32342446%9%52%
1575
16th to endNight
10%324324346%53%
1904
September1st to 15thNight
8%313323346%57%
1800
16th to endNight
8%4245%2345%7%50%
1857
October1st to 15thNight
13%5%23423445%49%
1980
16th to endNight
16%6%25%42335%6%42%
2181
November1st to 15thNight
11%7%35%323336%48%
2276
16th to endNight
17%8%45%5%23346%37%
2496
December1st to 15thNight
18%11%47%6%35%445%27%
2520
16th to endNight
21%13%5%7%6%35%44423%
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
7%9%37%6%3435%6%42%
1432
16th to endDay
9%13%49%6%33335%37%
1631
February1st to 15thDay
10%11%45%42323445%
1575
16th to endDay
9%8%36%5%23335%46%
1569
March1st to 15thDay
12%7%25%423336%48%
1854
16th to endDay
13%6%26%433345%45%
2196
April1st to 15thDay
14%7%25%423345%43%
2151
16th to endDay
17%9%245%2324443%
2160
May1st to 15thDay
14%7%35%5%3335%5%42%
2478
16th to endDay
13%5%25%5%2345%5%46%
2617
June1st to 15thDay
12%42443345%6%48%
2520
16th to endDay
8%4145%3345%7%51%
2520
July1st to 15thDay
6%3143345%6%7%53%
2520
16th to endDay
9%424424447%50%
2655
August1st to 15thDay
7%415%4345%7%6%49%
2383
16th to endDay
11%5%24423446%49%
2296
September1st to 15thDay
11%31233345%6%55%
2160
16th to endDay
11%424324337%50%
1926
October1st to 15thDay
14%5%2431333452%
1650
16th to endDay
15%6%24423435%47%
1691
November1st to 15thDay
9%6%24434345%52%
1582
16th to endDay
13%8%35%42223449%
1461
December1st to 15thDay
13%12%37%5%2323440%
1440
16th to endDay
15%12%47%5%2333436%
1536
January1st to 15thNight
8%8%47%6%2434446%
2506
16th to endNight
10%9%38%433346%41%
2592
February1st to 15thNight
7%10%37%42333447%
2278
16th to endNight
10%9%25%423346%47%
1904
March1st to 15thNight
12%8%25%32233452%
2115
16th to endNight
14%7%24424336%46%
2028
April1st to 15thNight
15%8%1442333448%
1797
16th to endNight
16%7%2442334446%
1799
May1st to 15thNight
15%425%413345%47%
1469
16th to endNight
10%324313347%55%
1496
June1st to 15thNight
10%5%24423446%52%
1440
16th to endNight
6%4134235%6%6%55%
1440
July1st to 15thNight
4213313348%63%
1440
16th to endNight
411231335%8%63%
1513
August1st to 15thNight
5%21232345%8%57%
1575
16th to endNight
9%313323246%59%
1904
September1st to 15thNight
6%313323345%62%
1800
16th to endNight
7%31442235%7%56%
1857
October1st to 15thNight
12%5%1332343455%
1980
16th to endNight
14%5%24323345%48%
2181
November1st to 15thNight
10%6%2431232457%
2276
16th to endNight
12%6%25%422335%50%
2496
December1st to 15thNight
16%9%36%42343440%
2520
16th to endNight
18%10%36%5%2333338%
2688

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
10%11%5%8%6%35%45%8%28%
1432
16th to endDay
11%14%5%12%7%45%346%24%
1631
February1st to 15thDay
12%12%7%7%5%2445%5%32%
1575
16th to endDay
13%9%46%6%3445%7%35%
1569
March1st to 15thDay
15%9%46%434347%36%
1854
16th to endDay
18%8%37%5%34446%32%
2194
April1st to 15thDay
17%9%47%6%2445%6%30%
2151
16th to endDay
21%8%36%434345%33%
2160
May1st to 15thDay
17%9%36%6%445%6%6%28%
2478
16th to endDay
14%5%35%6%35%5%6%8%35%
2617
June1st to 15thDay
14%425%5%345%5%8%39%
2520
16th to endDay
9%435%5%35%5%6%9%40%
2520
July1st to 15thDay
7%424435%6%6%9%45%
2520
16th to endDay
9%435%6%35%6%6%9%39%
2655
August1st to 15thDay
7%425%5%45%5%7%8%42%
2383
16th to endDay
12%5%245%335%5%7%42%
2296
September1st to 15thDay
13%31242345%8%48%
2160
16th to endDay
12%5%35%5%24447%43%
1926
October1st to 15thDay
15%6%24423436%46%
1650
16th to endDay
17%7%25%424346%39%
1691
November1st to 15thDay
12%7%3443445%7%41%
1582
16th to endDay
17%10%46%5%22335%38%
1461
December1st to 15thDay
16%13%5%9%5%34445%28%
1440
16th to endDay
20%12%7%8%7%34445%21%
1536
January1st to 15thNight
12%10%5%8%6%35%446%32%
2506
16th to endNight
12%11%5%9%6%4445%6%28%
2592
February1st to 15thNight
9%11%5%8%5%35%445%35%
2278
16th to endNight
13%9%36%5%24346%37%
1904
March1st to 15thNight
15%9%45%422346%40%
2115
16th to endNight
18%8%36%5%35%446%32%
2028
April1st to 15thNight
18%8%35%5%33446%36%
1797
16th to endNight
19%7%3443445%6%36%
1799
May1st to 15thNight
17%6%36%42446%8%35%
1469
16th to endNight
10%425%32445%10%46%
1496
June1st to 15thNight
11%425%42446%6%45%
1440
16th to endNight
9%5%244245%7%9%45%
1440
July1st to 15thNight
5%31432345%9%56%
1440
16th to endNight
5%21242446%9%54%
1513
August1st to 15thNight
6%323325%46%9%52%
1575
16th to endNight
10%324323347%52%
1904
September1st to 15thNight
8%31332335%6%57%
1800
16th to endNight
8%4145%2345%8%50%
1857
October1st to 15thNight
13%5%2342345%5%49%
1980
16th to endNight
16%6%25%5%23446%42%
2181
November1st to 15thNight
11%7%25%323346%48%
2276
16th to endNight
16%8%36%5%2335%6%37%
2496
December1st to 15thNight
19%10%5%7%6%25%5%45%27%
2520
16th to endNight
21%13%47%5%35%44423%
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
54%415%15%9%
1433
16th to endDay
60%413%14%6%
1631
February1st to 15thDay
53%411%21%7%
1576
16th to endDay
48%6%11%23%9%1
1569
March1st to 15thDay
48%5%15%21%8%1
1854
16th to endDay
52%6%11%22%6%1
2196
April1st to 15thDay
52%5%10%22%9%
2151
16th to endDay
52%410%19%11%1
2160
May1st to 15thDay
53%6%11%20%7%
2478
16th to endDay
45%7%12%20%13%1
2621
June1st to 15thDay
41%7%12%22%14%
2520
16th to endDay
38%8%13%23%14%1
2520
July1st to 15thDay
34%8%15%26%13%1
2371
16th to endDay
39%8%14%21%13%2
2488
August1st to 15thDay
36%8%12%27%14%1
2368
16th to endDay
38%6%12%26%13%2
2296
September1st to 15thDay
33%411%29%19%1
2160
16th to endDay
39%5%11%28%13%
1926
October1st to 15thDay
39%5%11%26%16%
1650
16th to endDay
45%5%13%23%10%1
1691
November1st to 15thDay
42%5%10%26%12%2
1582
16th to endDay
49%319%21%5%
1461
December1st to 15thDay
58%5%12%16%6%
1440
16th to endDay
65%411%11%7%
1536
January1st to 15thNight
52%5%11%19%10%
2506
16th to endNight
55%415%15%8%
2592
February1st to 15thNight
50%412%21%10%1
2288
16th to endNight
47%5%12%23%8%2
1904
March1st to 15thNight
46%412%20%13%1
2115
16th to endNight
52%412%21%7%1
2028
April1st to 15thNight
48%310%22%13%1
1797
16th to endNight
47%5%9%19%13%3
1799
May1st to 15thNight
46%7%12%20%12%
1470
16th to endNight
33%7%15%24%17%1
1498
June1st to 15thNight
35%6%12%25%18%1
1440
16th to endNight
33%8%12%24%18%3
1440
July1st to 15thNight
24%5%13%31%20%3
1360
16th to endNight
23%6%18%29%19%2
1417
August1st to 15thNight
27%7%12%30%20%1
1566
16th to endNight
31%5%12%28%16%4
1904
September1st to 15thNight
27%5%10%26%28%1
1800
16th to endNight
31%5%10%33%17%1
1857
October1st to 15thNight
35%410%25%21%2
1980
16th to endNight
42%5%13%24%13%1
2181
November1st to 15thNight
38%411%24%18%2
2276
16th to endNight
48%413%23%9%
2496
December1st to 15thNight
58%410%18%6%
2520
16th to endNight
64%411%12%6%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
21%7%40%25%5%
90
16th to endDay
27%8%33%21%6%3
96
February1st to 15thDay
20%17%41%11%10%
90
16th to endDay
24%14%29%23%6%1
81
March1st to 15thDay
22%12%40%20%32
90
16th to endDay
26%10%32%18%11%2
180
April1st to 15thDay
22%9%32%25%8%
270
16th to endDay
24%10%27%22%13%2
270
May1st to 15thDay
20%11%31%24%10%2
435
16th to endDay
17%10%29%24%14%4
517
June1st to 15thDay
16%6%26%29%18%3
495
16th to endDay
11%6%31%29%14%6%
495
July1st to 15thDay
7%323%31%21%12%
475
16th to endDay
11%225%29%22%8%
515
August1st to 15thDay
8%121%35%24%8%
493
16th to endDay
13%5%15%30%21%13%
336
September1st to 15thDay
12%220%31%23%9%
315
16th to endDay
14%5%19%30%23%6%
267
October1st to 15thDay
11%8%31%24%22%2
90
16th to endDay
21%6%38%28%5%
96
November1st to 15thDay
7%11%27%27%18%6%
90
16th to endDay
36%8%32%14%7%
90
December1st to 15thDay
24%11%40%17%5%1
90
16th to endDay
31%18%35%11%3
96
January1st to 15thNight
17%12%38%24%7%
900
16th to endNight
18%9%37%28%4
960
February1st to 15thNight
16%10%39%26%6%
880
16th to endNight
20%10%35%25%7%
642
March1st to 15thNight
22%9%31%29%6%
675
16th to endNight
22%7%34%26%8%
576
April1st to 15thNight
22%10%33%28%5%
539
16th to endNight
22%7%29%31%8%1
540
May1st to 15thNight
19%5%35%29%8%1
540
16th to endNight
13%229%38%14%
563
June1st to 15thNight
14%324%43%13%
540
16th to endNight
10%228%40%15%1
540
July1st to 15thNight
6%119%42%28%2
510
16th to endNight
7%16%43%29%2
564
August1st to 15thNight
7%13%46%29%3
539
16th to endNight
11%16%41%26%4
573
September1st to 15thNight
10%316%37%25%6%
540
16th to endNight
10%324%37%21%2
629
October1st to 15thNight
16%5%29%30%17%1
660
16th to endNight
20%8%33%28%8%
704
November1st to 15thNight
15%10%36%26%10%1
700
16th to endNight
20%9%38%22%7%
876
December1st to 15thNight
24%9%37%22%5%
900
16th to endNight
27%8%34%24%4
960
Page generated at 2017-09-24 08:15:54. Version ac04

Copyright 2017 A. Danko


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