Astronomers by Necessity
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astronomers by Necessity clear sky chart from 2005-11-05 to 2017-09-10.

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
48%
15555
31%
46869
19%
15512
60%
15512
42%
24614

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
39%
897
32%
2327
13%
895
50%
895
43%
1306
January16th to endNight
38%
915
37%
2451
14%
915
53%
915
44%
1322
February1st to 15thNight
36%
704
30%
2112
14%
700
49%
700
37%
1142
February16th to endNight
26%
632
32%
1894
9%
632
43%
632
38%
1013
March1st to 15thNight
29%
597
29%
2037
13%
597
46%
597
31%
1096
March16th to endNight
30%
576
28%
2112
15%
576
49%
576
34%
1047
April1st to 15thNight
28%
539
26%
1947
11%
539
49%
539
34%
962
April16th to endNight
28%
540
30%
1699
13%
540
49%
540
40%
885
May1st to 15thNight
32%
540
25%
1620
13%
540
50%
540
35%
936
May16th to endNight
43%
562
26%
1684
17%
561
60%
561
44%
902
June1st to 15thNight
61%
540
23%
1620
19%
540
69%
540
54%
783
June16th to endNight
67%
540
28%
1620
29%
540
72%
540
49%
818
July1st to 15thNight
75%
510
25%
1530
26%
510
79%
510
46%
805
July16th to endNight
85%
564
34%
1594
39%
531
88%
531
58%
739
August1st to 15thNight
87%
539
36%
1643
42%
536
88%
536
59%
757
August16th to endNight
79%
573
30%
1925
36%
573
81%
573
41%
985
September1st to 15thNight
69%
525
24%
1925
27%
525
73%
525
34%
1028
September16th to endNight
72%
492
33%
1815
33%
492
77%
492
39%
897
October1st to 15thNight
60%
498
39%
1818
32%
498
72%
498
47%
815
October16th to endNight
53%
704
37%
2112
20%
704
64%
704
50%
1038
November1st to 15thNight
49%
808
32%
2208
15%
808
59%
808
49%
1166
November16th to endNight
39%
900
35%
2340
14%
900
51%
900
44%
1324
December1st to 15thNight
39%
900
34%
2340
14%
900
51%
900
42%
1347
December16th to endNight
36%
960
30%
2496
12%
960
48%
960
38%
1501

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%413324446%55%
47755
Night
7%313323347%60%
47065
Cloud CoverDay
8%413324446%55%
47753
Night
7%313323347%60%
47065
TransparencyDay
28%39%23%24%10%
47453
Night
25%39%22%25%13%
46870
SeeingDay
8%5%21%32%24%7%
5529
Night
9%6%31%35%15%1
15559

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
10%423312235%59%
1611
16th to endDay
10%313223446%58%
1782
February1st to 15thDay
9%323323345%58%
1750
16th to endDay
11%5%2232222460%
1580
March1st to 15thDay
13%5%23312247%52%
1922
16th to endDay
9%5%24313336%56%
2112
April1st to 15thDay
8%313324446%57%
1998
16th to endDay
9%42332433458%
2259
May1st to 15thDay
10%4245%23445%50%
2331
16th to endDay
7%5%245%25%5%6%7%47%
2430
June1st to 15thDay
5%212323448%60%
2340
16th to endDay
6%31242445%8%54%
2340
July1st to 15thDay
6%3145%35%46%9%48%
2340
16th to endDay
3114326%5%6%11%53%
2466
August1st to 15thDay
3213435%5%6%7%57%
2304
16th to endDay
6%3145%346%7%10%47%
2277
September1st to 15thDay
10%424425%5%5%6%48%
1922
16th to endDay
9%313424447%54%
1815
October1st to 15thDay
6%323223445%60%
1801
16th to endDay
6%31211222471%
1760
November1st to 15thDay
5%313324446%61%
1647
16th to endDay
8%32321332464%
1620
December1st to 15thDay
11%6%24323335%54%
1620
16th to endDay
12%6%13323346%52%
1728
January1st to 15thNight
10%413312235%61%
2327
16th to endNight
9%313212325%63%
2451
February1st to 15thNight
9%5%13312235%59%
2102
16th to endNight
10%322313336%59%
1894
March1st to 15thNight
12%313223345%55%
2037
16th to endNight
6%313323346%59%
2112
April1st to 15thNight
6%324333346%58%
1947
16th to endNight
7%313313336%61%
1699
May1st to 15thNight
8%42442435%7%53%
1619
16th to endNight
5%2133145%6%9%55%
1683
June1st to 15thNight
21122235%6%9%61%
1620
16th to endNight
3213313448%63%
1620
July1st to 15thNight
42123245%6%9%57%
1620
16th to endNight
211212345%13%61%
1702
August1st to 15thNight
211322349%66%
1653
16th to endNight
5%31332345%9%56%
1925
September1st to 15thNight
9%41343345%6%52%
1925
16th to endNight
6%21432335%7%57%
1815
October1st to 15thNight
6%212212237%65%
1818
16th to endNight
5%31222223469%
2112
November1st to 15thNight
4313223335%66%
2208
16th to endNight
9%413212335%61%
2340
December1st to 15thNight
10%413323346%57%
2340
16th to endNight
9%5%13424345%54%
2496

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%5%2321324459%
1611
16th to endDay
10%313213346%58%
1782
February1st to 15thDay
9%32332334459%
1750
16th to endDay
11%5%2331222460%
1580
March1st to 15thDay
13%5%23313247%52%
1922
16th to endDay
9%5%23313345%56%
2110
April1st to 15thDay
7%413323447%57%
1998
16th to endDay
8%5%2331433458%
2259
May1st to 15thDay
10%425%5%24445%50%
2331
16th to endDay
7%4245%35%5%5%7%47%
2430
June1st to 15thDay
5%312324448%59%
2340
16th to endDay
6%32242445%8%54%
2340
July1st to 15thDay
6%3145%36%46%10%48%
2340
16th to endDay
3213326%5%7%10%53%
2466
August1st to 15thDay
3213435%5%6%6%57%
2304
16th to endDay
6%3144446%7%10%47%
2277
September1st to 15thDay
10%4244345%46%48%
1922
16th to endDay
8%314423447%55%
1815
October1st to 15thDay
6%423313345%60%
1801
16th to endDay
6%2122223371%
1760
November1st to 15thDay
5%213324435%62%
1647
16th to endDay
8%31322233464%
1620
December1st to 15thDay
11%6%25%323235%54%
1620
16th to endDay
12%6%13324246%52%
1728
January1st to 15thNight
9%323312235%61%
2327
16th to endNight
9%313212335%63%
2451
February1st to 15thNight
9%5%13312235%59%
2102
16th to endNight
10%313323326%59%
1894
March1st to 15thNight
12%313223345%55%
2037
16th to endNight
6%413323346%58%
2112
April1st to 15thNight
5%32442335%6%58%
1947
16th to endNight
7%313323346%60%
1699
May1st to 15thNight
7%41442335%7%53%
1619
16th to endNight
5%2123245%7%9%55%
1683
June1st to 15thNight
21122348%9%61%
1620
16th to endNight
221331345%8%63%
1620
July1st to 15thNight
42123245%6%10%57%
1620
16th to endNight
21221346%13%60%
1702
August1st to 15thNight
21232235%9%66%
1653
16th to endNight
5%31431445%8%57%
1925
September1st to 15thNight
9%41343345%7%52%
1925
16th to endNight
6%314324348%57%
1815
October1st to 15thNight
6%212222237%65%
1818
16th to endNight
5%31222223469%
2112
November1st to 15thNight
431322333467%
2208
16th to endNight
9%413212335%61%
2340
December1st to 15thNight
10%413213346%57%
2340
16th to endNight
8%423424345%55%
2496

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
30%25%19%29%12%
1612
16th to endDay
27%36%17%31%14%
1782
February1st to 15thDay
28%25%19%27%16%
1751
16th to endDay
30%1419%26%17%
1580
March1st to 15thDay
33%38%16%25%12%
1922
16th to endDay
31%28%23%22%11%
2112
April1st to 15thDay
27%310%23%19%15%
1998
16th to endDay
30%36%18%26%13%
2259
May1st to 15thDay
36%48%19%20%10%
2331
16th to endDay
34%5%13%20%21%4
2434
June1st to 15thDay
23%412%26%26%5%
2340
16th to endDay
27%312%28%20%7%
2340
July1st to 15thDay
30%5%16%28%17%2
2201
16th to endDay
21%5%16%34%19%2
2311
August1st to 15thDay
23%6%13%27%23%4
2290
16th to endDay
30%7%13%27%19%2
2277
September1st to 15thDay
34%5%10%21%21%6%
1922
16th to endDay
29%37%20%26%12%
1815
October1st to 15thDay
26%28%25%23%13%
1801
16th to endDay
19%2420%33%19%
1760
November1st to 15thDay
23%36%26%26%12%
1647
16th to endDay
25%2418%28%19%
1620
December1st to 15thDay
34%2419%27%11%
1620
16th to endDay
33%25%23%25%8%
1728
January1st to 15thNight
27%15%17%30%17%
2327
16th to endNight
24%25%17%31%18%
2451
February1st to 15thNight
29%27%14%26%20%
2112
16th to endNight
28%15%18%25%19%
1894
March1st to 15thNight
31%28%17%22%17%
2037
16th to endNight
26%310%22%26%10%
2112
April1st to 15thNight
26%310%20%20%17%
1947
16th to endNight
25%311%18%27%13%
1699
May1st to 15thNight
31%5%10%19%22%11%
1620
16th to endNight
23%5%17%23%22%5%
1685
June1st to 15thNight
16%5%15%32%23%6%
1620
16th to endNight
20%411%29%25%8%
1620
July1st to 15thNight
21%6%11%32%22%5%
1530
16th to endNight
14%415%34%26%4
1594
August1st to 15thNight
15%312%30%31%6%
1643
16th to endNight
24%5%13%26%24%5%
1925
September1st to 15thNight
30%49%22%24%7%
1925
16th to endNight
25%47%21%24%15%
1815
October1st to 15thNight
21%110%26%24%15%
1818
16th to endNight
20%3417%30%24%
2112
November1st to 15thNight
22%36%25%24%18%
2208
16th to endNight
26%2417%28%20%
2340
December1st to 15thNight
28%26%21%26%13%
2340
16th to endNight
32%36%19%25%13%
2496

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
13%15%35%25%8%1
90
16th to endDay
9%8%44%28%9%
96
February1st to 15thDay
7%13%36%37%4
90
16th to endDay
12%453%23%6%
81
March1st to 15thDay
22%13%36%23%4
90
16th to endDay
7%14%33%33%9%2
96
April1st to 15thDay
12%18%36%23%7%
117
16th to endDay
11%15%42%21%9%
369
May1st to 15thDay
12%12%38%25%8%2
468
16th to endDay
11%7%34%28%13%3
517
June1st to 15thDay
7%118%45%23%3
495
16th to endDay
5%114%39%30%8%
495
July1st to 15thDay
5%7%42%35%8%
475
16th to endDay
3334%40%17%
515
August1st to 15thDay
4231%46%14%
457
16th to endDay
10%8%35%31%13%
317
September1st to 15thDay
3311%55%26%
119
16th to endDay
6%121%48%22%
90
October1st to 15thDay
5%8%33%43%8%
90
16th to endDay
9%420%38%25%2
96
November1st to 15thDay
14%334%32%11%4
90
16th to endDay
17%21%33%14%11%2
90
December1st to 15thDay
18%5%31%35%5%3
90
16th to endDay
9%15%45%25%4
96
January1st to 15thNight
12%10%33%34%8%
900
16th to endNight
12%10%36%32%8%
915
February1st to 15thNight
11%11%38%32%5%
704
16th to endNight
12%12%45%25%2
632
March1st to 15thNight
13%12%42%26%5%
597
16th to endNight
7%12%48%26%5%
576
April1st to 15thNight
7%14%48%27%2
539
16th to endNight
8%11%50%22%6%
540
May1st to 15thNight
11%7%45%27%6%1
540
16th to endNight
6%341%32%15%
563
June1st to 15thNight
2129%47%17%1
540
16th to endNight
420%51%20%1
540
July1st to 15thNight
411%52%28%1
510
16th to endNight
26%54%34%2
564
August1st to 15thNight
2348%42%2
539
16th to endNight
7%5%42%38%5%
573
September1st to 15thNight
10%19%43%32%3
525
16th to endNight
7%13%43%31%2
492
October1st to 15thNight
7%127%39%20%2
498
16th to endNight
8%5%31%37%15%1
704
November1st to 15thNight
7%5%35%35%15%1
808
16th to endNight
12%8%37%30%10%
900
December1st to 15thNight
13%9%35%34%7%
900
16th to endNight
12%10%38%30%8%
960
Page generated at 2017-09-11 18:35:25. Version ac04

Copyright 2017 A. Danko


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