Blackwell
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blackwell clear sky chart from 2005-11-06 to 2024-01-14.

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
50%
23952
31%
74331
20%
23898
61%
23898
42%
38486

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 10thNight
39%
948
34%
2458
14%
946
51%
946
46%
1367
January11th to 20thNight
43%
911
34%
2383
15%
911
55%
911
45%
1306
January21st to endNight
38%
792
39%
2376
16%
792
58%
792
40%
1238
February1st to 10thNight
33%
720
34%
2160
12%
720
52%
720
40%
1155
February11th to 20thNight
31%
704
26%
2098
11%
700
44%
700
34%
1183
February21st to endNight
25%
592
34%
1776
9%
592
47%
592
37%
935
March1st to 10thNight
30%
714
27%
2154
10%
714
45%
714
33%
1241
March11th to 20thNight
34%
540
28%
1980
16%
540
49%
540
30%
1020
March21st to endNight
29%
594
30%
2178
15%
594
50%
594
38%
1026
April1st to 10thNight
27%
540
30%
1870
12%
540
50%
540
39%
918
April11th to 20thNight
26%
533
29%
1777
12%
533
48%
533
35%
938
April21st to endNight
30%
540
33%
1769
13%
540
57%
540
42%
874
May1st to 10thNight
35%
540
29%
1620
17%
540
53%
540
41%
863
May11th to 20thNight
46%
531
23%
1594
18%
531
55%
531
38%
903
May21st to endNight
43%
582
26%
1745
16%
582
58%
582
41%
961
June1st to 10thNight
63%
540
24%
1620
20%
540
72%
540
50%
812
June11th to 20thNight
57%
540
30%
1620
24%
540
69%
540
54%
767
June21st to endNight
69%
540
29%
1620
27%
540
75%
540
52%
802
July1st to 10thNight
80%
525
22%
1575
24%
525
81%
525
45%
843
July11th to 20thNight
87%
522
34%
1566
38%
522
88%
522
62%
708
July21st to endNight
90%
591
35%
1673
40%
558
92%
558
59%
773
August1st to 10thNight
88%
537
33%
1752
41%
534
90%
534
61%
730
August11th to 20thNight
79%
534
30%
1778
37%
534
80%
534
38%
928
August21st to endNight
83%
594
27%
1980
33%
594
85%
594
45%
973
September1st to 10thNight
77%
540
27%
1917
32%
540
79%
540
38%
990
September11th to 20thNight
71%
540
28%
1980
31%
540
74%
540
34%
1056
September21st to endNight
70%
540
33%
1980
33%
540
75%
540
41%
947
October1st to 10thNight
67%
540
36%
1980
34%
540
74%
540
47%
887
October11th to 20thNight
55%
540
40%
1980
27%
540
70%
540
43%
905
October21st to endNight
51%
787
37%
2371
20%
787
62%
787
46%
1196
November1st to 10thNight
54%
925
32%
2405
15%
925
62%
925
49%
1310
November11th to 20thNight
51%
950
32%
2470
14%
950
60%
950
52%
1291
November21st to endNight
42%
950
34%
2470
14%
950
53%
950
39%
1468
December1st to 10thNight
46%
950
36%
2470
17%
950
57%
950
45%
1395
December11th to 20thNight
36%
949
33%
2878
13%
948
49%
948
34%
1662
December21st to endNight
39%
1037
22%
4308
15%
1026
51%
1026
32%
2115

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
8%42442445%7%58%
74862
Night
7%42332345%7%60%
74920
TransparencyDay
26%5%10%22%25%13%
74587
Night
24%5%9%22%25%16%
74715
SeeingDay
9%7%24%31%23%6%
13886
Night
9%7%31%38%15%1
23965

By Month

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 10thDay
7%424323347%60%
1707
11th to 20thDay
9%423332346%62%
1669
21st to endDay
9%5%13323446%59%
1980
February1st to 10thDay
9%423323347%60%
1800
11th to 20thDay
9%6%35%424435%56%
1749
21st to endDay
10%6%34313245%60%
1480
March1st to 10thDay
10%6%24435%45%7%49%
1804
11th to 20thDay
11%5%24423445%56%
1980
21st to endDay
7%433424446%59%
2178
April1st to 10thDay
7%32342345%7%59%
2085
11th to 20thDay
9%5%24433445%56%
2134
21st to endDay
8%423423345%61%
2187
May1st to 10thDay
9%433333447%56%
2340
11th to 20thDay
11%5%244245%5%7%52%
2302
21st to endDay
10%5%245%35%5%6%8%49%
2531
June1st to 10thDay
4324324448%63%
2340
11th to 20thDay
43223335%6%8%61%
2340
21st to endDay
6%32342445%8%58%
2340
July1st to 10thDay
7%314336%6%8%11%47%
2340
11th to 20thDay
3213335%45%8%62%
2340
21st to endDay
3212435%5%6%10%58%
2546
August1st to 10thDay
12123345%5%8%67%
2172
11th to 20thDay
5%425%6%35%6%7%10%48%
2123
21st to endDay
4424435%6%7%9%53%
2376
September1st to 10thDay
10%424435%5%5%7%51%
2041
11th to 20thDay
10%5%24445%45%9%49%
1980
21st to endDay
10%324433446%56%
1980
October1st to 10thDay
441342345%7%63%
1980
11th to 20thDay
9%323222335%67%
1972
21st to endDay
12%41332223466%
1967
November1st to 10thDay
8%414333346%63%
1673
11th to 20thDay
4413323447%65%
1710
21st to endDay
10%433323335%59%
1710
December1st to 10thDay
12%5%14323347%57%
1710
11th to 20thDay
8%5%244345%46%56%
2161
21st to endDay
10%5%24322335%63%
3135
January1st to 10thNight
8%5%24323346%62%
2458
11th to 20thNight
9%422313345%63%
2383
21st to endNight
8%423323435%62%
2376
February1st to 10thNight
9%5%34422336%61%
2160
11th to 20thNight
11%5%23334446%55%
2098
21st to endNight
8%6%23322336%61%
1776
March1st to 10thNight
9%6%25%4345%5%6%51%
2154
11th to 20thNight
11%5%24333345%58%
1980
21st to endNight
7%31332345%7%61%
2178
April1st to 10thNight
5%42342445%7%61%
1870
11th to 20thNight
9%4132245%5%8%57%
1777
21st to endNight
8%323222246%65%
1769
May1st to 10thNight
7%42432335%8%59%
1620
11th to 20thNight
7%323335%47%10%53%
1593
21st to endNight
9%41332346%12%53%
1745
June1st to 10thNight
221232448%11%60%
1620
11th to 20thNight
32123245%6%10%63%
1620
21st to endNight
421222346%7%65%
1620
July1st to 10thNight
4313425%47%12%54%
1620
11th to 20thNight
21322345%8%68%
1620
21st to endNight
11122246%8%11%62%
1772
August1st to 10thNight
11222335%10%70%
1762
11th to 20thNight
44235%345%6%9%54%
1778
21st to endNight
331333446%8%62%
1980
September1st to 10thNight
6%32443446%9%57%
1917
11th to 20thNight
7%5%25%43446%8%52%
1980
21st to endNight
7%41332345%8%60%
1980
October1st to 10thNight
421332345%8%64%
1980
11th to 20thNight
9%312322235%67%
1980
21st to endNight
9%313212335%67%
2371
November1st to 10thNight
8%423322236%65%
2405
11th to 20thNight
5%213323447%66%
2470
21st to endNight
12%423423345%56%
2470
December1st to 10thNight
9%423312447%61%
2470
11th to 20thNight
9%5%245%23446%55%
3036
21st to endNight
8%6%24422435%60%
4532

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 10thDay
25%37%21%28%16%
1708
11th to 20thDay
26%35%18%31%17%
1669
21st to endDay
27%37%16%32%15%
1980
February1st to 10thDay
26%47%17%27%19%
1800
11th to 20thDay
33%35%21%22%16%
1759
21st to endDay
29%25%12%29%23%
1480
March1st to 10thDay
34%47%15%26%14%
1804
11th to 20thDay
31%47%17%25%15%
1980
21st to endDay
27%5%8%21%23%17%
2178
April1st to 10thDay
25%5%10%18%23%20%
2085
11th to 20thDay
31%48%14%24%19%
2134
21st to endDay
27%48%18%27%16%
2187
May1st to 10thDay
30%5%7%20%23%15%
2340
11th to 20thDay
32%6%11%20%21%9%
2306
21st to endDay
33%6%12%22%20%6%
2531
June1st to 10thDay
19%7%15%30%22%7%
2340
11th to 20thDay
20%6%11%29%25%9%
2340
21st to endDay
24%6%12%24%24%10%
2340
July1st to 10thDay
27%11%19%27%14%2
2276
11th to 20thDay
19%6%13%32%24%6%
2271
21st to endDay
17%7%18%38%19%2
2405
August1st to 10thDay
15%5%14%33%28%4
2158
11th to 20thDay
29%10%16%27%16%4
2123
21st to endDay
25%9%14%25%21%6%
2376
September1st to 10thDay
31%7%9%21%22%10%
2041
11th to 20thDay
33%7%10%21%22%7%
1980
21st to endDay
28%49%19%26%13%
1980
October1st to 10thDay
21%6%12%24%24%13%
1980
11th to 20thDay
23%46%16%29%23%
1972
21st to endDay
25%35%18%30%19%
1967
November1st to 10thDay
24%47%22%27%17%
1673
11th to 20thDay
20%39%22%28%17%
1710
21st to endDay
29%36%19%27%17%
1710
December1st to 10thDay
29%47%19%30%12%
1710
11th to 20thDay
30%48%14%25%18%
2159
21st to endDay
27%2419%30%19%
3135
January1st to 10thNight
25%25%18%30%19%
2458
11th to 20thNight
25%25%20%29%19%
2383
21st to endNight
25%46%15%27%23%
2376
February1st to 10thNight
28%27%16%26%21%
2160
11th to 20thNight
31%46%18%21%20%
2098
21st to endNight
27%35%14%26%25%
1776
March1st to 10thNight
32%47%15%24%16%
2154
11th to 20thNight
30%48%19%22%18%
1980
21st to endNight
22%5%12%19%24%18%
2178
April1st to 10thNight
23%5%10%22%18%22%
1870
11th to 20thNight
27%5%9%17%22%20%
1777
21st to endNight
22%49%22%29%14%
1769
May1st to 10thNight
24%6%10%19%24%17%
1620
11th to 20thNight
25%9%15%20%21%10%
1594
21st to endNight
25%7%15%27%19%7%
1746
June1st to 10thNight
17%7%18%31%21%6%
1620
11th to 20thNight
16%6%13%31%26%9%
1620
21st to endNight
17%6%12%29%24%11%
1620
July1st to 10thNight
22%8%18%32%17%4
1575
11th to 20thNight
13%413%29%30%11%
1566
21st to endNight
11%8%16%35%28%3
1673
August1st to 10thNight
11%5%13%33%31%7%
1752
11th to 20thNight
25%7%12%29%21%6%
1778
21st to endNight
19%7%15%28%23%7%
1980
September1st to 10thNight
25%7%11%25%21%12%
1917
11th to 20thNight
29%6%13%23%22%7%
1980
21st to endNight
24%48%24%25%16%
1980
October1st to 10thNight
18%5%12%24%24%16%
1980
11th to 20thNight
22%37%17%26%25%
1980
21st to endNight
22%36%19%29%21%
2371
November1st to 10thNight
24%38%21%25%20%
2405
11th to 20thNight
19%48%21%29%20%
2470
21st to endNight
31%36%15%28%18%
2470
December1st to 10thNight
25%37%20%27%17%
2470
11th to 20thNight
31%35%18%25%18%
3037
21st to endNight
28%3418%29%19%
4532

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 10thDay
13%11%39%30%7%1
270
11th to 20thDay
14%16%27%29%11%2
249
21st to endDay
9%17%40%25%9%1
264
February1st to 10thDay
16%9%38%30%7%
240
11th to 20thDay
19%14%38%22%6%1
240
21st to endDay
16%17%44%19%41
198
March1st to 10thDay
13%15%39%29%4
240
11th to 20thDay
18%13%36%25%8%1
240
21st to endDay
9%16%43%27%5%
264
April1st to 10thDay
8%15%48%23%6%
345
11th to 20thDay
11%17%46%21%31
412
21st to endDay
8%18%42%24%7%1
447
May1st to 10thDay
11%11%36%33%7%
603
11th to 20thDay
12%6%32%32%14%4
625
21st to endDay
12%5%30%37%14%1
679
June1st to 10thDay
4216%49%25%4
630
11th to 20thDay
3120%50%23%3
630
21st to endDay
5%114%33%39%8%
630
July1st to 10thDay
7%15%31%44%12%
621
11th to 20thDay
2433%47%13%
618
21st to endDay
25%25%50%18%
684
August1st to 10thDay
21329%50%16%
470
11th to 20thDay
6%15%33%42%13%
437
21st to endDay
416%32%42%15%
495
September1st to 10thDay
9%25%25%40%20%
331
11th to 20thDay
6%27%26%43%16%
270
21st to endDay
13%28%26%36%14%
270
October1st to 10thDay
3219%37%32%7%
270
11th to 20thDay
10%7%30%31%19%3
270
21st to endDay
18%8%23%29%18%4
297
November1st to 10thDay
13%6%30%33%16%3
270
11th to 20thDay
10%14%28%31%14%4
270
21st to endDay
13%18%34%21%12%2
270
December1st to 10thDay
17%9%30%31%10%1
270
11th to 20thDay
7%15%40%30%9%
270
21st to endDay
15%12%38%27%8%
297
January1st to 10thNight
11%8%39%32%9%
950
11th to 20thNight
12%10%33%36%8%
911
21st to endNight
10%10%38%34%7%
792
February1st to 10thNight
13%10%42%29%7%
720
11th to 20thNight
14%11%41%29%5%
704
21st to endNight
13%13%48%23%3
592
March1st to 10thNight
13%10%44%29%4
714
11th to 20thNight
12%12%39%31%6%
540
21st to endNight
8%13%48%28%3
594
April1st to 10thNight
6%10%54%25%4
540
11th to 20thNight
9%11%51%27%1
533
21st to endNight
8%10%50%26%5%
540
May1st to 10thNight
7%7%48%33%5%
540
11th to 20thNight
8%5%39%37%10%1
531
21st to endNight
9%340%38%10%
584
June1st to 10thNight
3328%50%16%
540
11th to 20thNight
3134%53%9%
540
21st to endNight
4221%47%24%1
540
July1st to 10thNight
5%8%49%35%3
525
11th to 20thNight
36%52%37%1
522
21st to endNight
1454%39%1
591
August1st to 10thNight
15%47%45%1
537
11th to 20thNight
5%16%46%40%3
534
21st to endNight
416%44%43%3
594
September1st to 10thNight
5%18%44%39%3
540
11th to 20thNight
9%29%44%35%2
540
21st to endNight
9%115%45%30%1
540
October1st to 10thNight
5%222%51%19%1
540
11th to 20thNight
10%6%26%42%16%1
540
21st to endNight
10%9%28%37%15%2
787
November1st to 10thNight
9%6%28%41%16%1
925
11th to 20thNight
6%7%32%41%12%1
950
21st to endNight
14%10%31%33%11%1
950
December1st to 10thNight
12%8%31%39%9%1
950
11th to 20thNight
13%11%37%32%7%
950
21st to endNight
10%9%38%34%9%
1045
Page generated at 2024-01-11 12:31:57. Version ac07

Copyright 2024 Attilla Danko


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