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NAA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the NAA Observatory clear sky chart from 2005-11-05 to 2019-04-11.

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
37%
18173
18%
53340
11%
18131
44%
18131
23%
35841

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
22%
1050
15%
2926
4%
1045
31%
1045
19%
2130
January16th to endNight
21%
1120
13%
3091
4%
1120
29%
1120
16%
2342
February1st to 15thNight
21%
930
13%
2581
4%
930
28%
930
17%
1894
February16th to endNight
25%
735
17%
2205
8%
735
33%
735
17%
1565
March1st to 15thNight
27%
840
15%
2520
8%
840
32%
840
17%
1773
March16th to endNight
31%
725
16%
2346
11%
725
38%
725
18%
1618
April1st to 15thNight
29%
617
16%
2057
11%
617
38%
617
19%
1387
April16th to endNight
33%
579
18%
1862
11%
579
43%
579
22%
1257
May1st to 15thNight
38%
585
16%
1732
10%
585
45%
585
23%
1191
May16th to endNight
51%
610
17%
1625
9%
609
56%
609
32%
1049
June1st to 15thNight
52%
585
17%
1560
10%
585
57%
585
35%
980
June16th to endNight
50%
585
17%
1560
9%
585
54%
585
31%
1031
July1st to 15thNight
63%
555
19%
1480
13%
555
65%
555
40%
889
July16th to endNight
70%
612
22%
1545
17%
579
73%
579
43%
907
August1st to 15thNight
76%
575
22%
1683
21%
572
78%
572
42%
943
August16th to endNight
66%
621
20%
2070
21%
621
70%
621
34%
1160
September1st to 15thNight
63%
588
29%
1953
29%
588
68%
588
37%
1058
September16th to endNight
58%
777
26%
2145
20%
777
62%
777
35%
1302
October1st to 15thNight
45%
780
25%
2243
18%
780
51%
780
30%
1399
October16th to endNight
30%
832
20%
2642
12%
832
41%
832
23%
1704
November1st to 15thNight
30%
820
23%
2666
15%
820
41%
820
22%
1703
November16th to endNight
25%
882
17%
2772
8%
882
35%
882
20%
1873
December1st to 15thNight
24%
1050
14%
2940
5%
1050
31%
1050
17%
2165
December16th to endNight
18%
1120
11%
3136
3%
1120
25%
1120
12%
2521

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%8%36%5%3445%7%36%
53869
Night
14%8%36%5%24446%39%
53509
Cloud CoverDay
14%8%36%5%3445%7%36%
53878
Night
14%8%36%5%24446%39%
53514
TransparencyDay
46%6%11%21%12%1
53555
Night
45%5%11%22%14%2
53341
SeeingDay
16%8%28%25%14%5%
7988
Night
18%8%29%29%12%1
18177

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
16%14%48%6%35%346%26%
1672
16th to endDay
13%14%6%9%6%24446%25%
1834
February1st to 15thDay
17%14%5%7%6%36%435%25%
1940
16th to endDay
18%11%48%5%33446%30%
1840
March1st to 15thDay
16%13%37%6%34445%29%
2090
16th to endDay
18%9%37%6%35%45%6%30%
2592
April1st to 15thDay
19%9%47%5%2435%5%30%
2455
16th to endDay
19%7%25%5%24447%35%
2372
May1st to 15thDay
18%8%36%5%35%5%5%7%30%
2545
16th to endDay
14%5%25%5%35%5%6%6%39%
2841
June1st to 15thDay
12%5%24435%5%5%7%44%
2730
16th to endDay
10%425%6%35%6%7%9%36%
2730
July1st to 15thDay
7%4145%35%5%6%9%46%
2730
16th to endDay
7%425%5%36%6%7%9%40%
2879
August1st to 15thDay
7%5%25%5%35%5%7%10%41%
2518
16th to endDay
9%5%244346%6%9%42%
2484
September1st to 15thDay
12%414424446%49%
2324
16th to endDay
8%5%245%35%6%5%7%45%
2145
October1st to 15thDay
17%6%36%434336%40%
2044
16th to endDay
18%7%25%5%33447%37%
1933
November1st to 15thDay
16%8%36%414445%39%
1852
16th to endDay
18%8%47%5%23346%35%
1847
December1st to 15thDay
17%11%48%6%34345%29%
1680
16th to endDay
20%14%5%7%6%34435%23%
1792
January1st to 15thNight
16%11%47%5%2433435%
2926
16th to endNight
13%12%6%9%6%34345%29%
3091
February1st to 15thNight
17%13%5%6%5%2334433%
2579
16th to endNight
17%10%46%5%23445%33%
2205
March1st to 15thNight
15%10%46%5%35%43436%
2507
16th to endNight
17%8%36%5%34447%34%
2346
April1st to 15thNight
18%8%35%424447%35%
2057
16th to endNight
18%7%35%5%23346%39%
1862
May1st to 15thNight
17%6%15%5%345%6%7%36%
1732
16th to endNight
10%414435%46%9%45%
1624
June1st to 15thNight
9%42443445%6%49%
1560
16th to endNight
10%425%5%36%5%5%10%40%
1560
July1st to 15thNight
6%3145%1447%7%52%
1560
16th to endNight
5%2133245%8%9%51%
1641
August1st to 15thNight
331443446%10%53%
1692
16th to endNight
9%313424448%52%
2070
September1st to 15thNight
10%412323347%55%
1953
16th to endNight
8%42442445%7%50%
2145
October1st to 15thNight
14%5%25%433346%47%
2243
16th to endNight
19%7%35%423336%42%
2642
November1st to 15thNight
15%8%25%424335%43%
2666
16th to endNight
18%8%36%5%23246%37%
2772
December1st to 15thNight
19%9%37%5%24436%31%
2940
16th to endNight
22%15%5%8%6%3433423%
3136

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
16%14%48%6%3435%5%26%
1672
16th to endDay
14%14%6%10%6%23446%26%
1834
February1st to 15thDay
16%14%6%7%5%35%435%25%
1940
16th to endDay
18%11%48%5%33345%30%
1840
March1st to 15thDay
16%13%38%6%34445%29%
2100
16th to endDay
18%8%37%6%35%45%6%30%
2591
April1st to 15thDay
19%10%47%5%2445%5%30%
2455
16th to endDay
19%7%25%5%23447%35%
2372
May1st to 15thDay
18%8%36%5%35%5%5%7%29%
2545
16th to endDay
14%5%25%5%345%6%6%39%
2841
June1st to 15thDay
12%5%24435%5%5%7%44%
2730
16th to endDay
10%425%6%35%6%7%9%36%
2730
July1st to 15thDay
7%325%5%35%5%6%8%46%
2730
16th to endDay
7%425%6%46%6%7%10%40%
2879
August1st to 15thDay
7%5%25%5%35%6%7%9%41%
2518
16th to endDay
10%4245%35%5%6%9%42%
2484
September1st to 15thDay
12%424424446%48%
2324
16th to endDay
8%4245%35%5%5%7%46%
2145
October1st to 15thDay
17%6%35%434336%40%
2044
16th to endDay
18%8%26%43335%7%37%
1933
November1st to 15thDay
16%8%36%414445%39%
1852
16th to endDay
18%8%47%5%23346%35%
1847
December1st to 15thDay
16%11%5%8%5%35%346%28%
1680
16th to endDay
20%14%5%7%6%34435%23%
1792
January1st to 15thNight
16%11%47%5%23335%35%
2926
16th to endNight
13%11%7%9%6%25%445%29%
3091
February1st to 15thNight
16%13%5%6%5%2344433%
2572
16th to endNight
17%10%46%5%23445%33%
2205
March1st to 15thNight
16%10%46%5%3443436%
2520
16th to endNight
17%8%36%5%35%447%34%
2346
April1st to 15thNight
18%8%36%424447%35%
2057
16th to endNight
18%7%26%42335%5%39%
1862
May1st to 15thNight
17%6%15%5%345%5%7%36%
1731
16th to endNight
10%414435%45%9%45%
1624
June1st to 15thNight
10%42442436%6%49%
1560
16th to endNight
10%425%5%36%5%5%10%41%
1560
July1st to 15thNight
6%3145%1447%7%52%
1560
16th to endNight
5%2134245%8%9%51%
1641
August1st to 15thNight
332343447%9%53%
1692
16th to endNight
9%31442345%8%52%
2070
September1st to 15thNight
10%412323347%55%
1953
16th to endNight
8%414425%45%7%50%
2145
October1st to 15thNight
14%5%25%433246%47%
2243
16th to endNight
18%7%25%323346%42%
2642
November1st to 15thNight
15%7%25%324335%43%
2666
16th to endNight
18%8%36%423246%37%
2772
December1st to 15thNight
19%9%47%5%23436%31%
2940
16th to endNight
22%15%5%8%5%3433423%
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
59%412%14%8%
1673
16th to endDay
59%412%15%7%
1834
February1st to 15thDay
61%410%14%7%1
1942
16th to endDay
56%5%10%19%6%1
1840
March1st to 15thDay
57%5%9%20%6%
2100
16th to endDay
53%6%12%20%6%
2592
April1st to 15thDay
54%5%9%18%10%
2455
16th to endDay
48%6%12%16%12%3
2372
May1st to 15thDay
51%7%12%21%7%
2545
16th to endDay
42%6%11%20%15%2
2845
June1st to 15thDay
38%7%10%24%17%1
2730
16th to endDay
40%8%12%21%13%2
2730
July1st to 15thDay
34%7%13%25%15%3
2581
16th to endDay
36%8%15%22%14%2
2712
August1st to 15thDay
34%8%14%26%14%1
2503
16th to endDay
36%8%13%23%16%2
2484
September1st to 15thDay
34%6%10%24%18%5%
2324
16th to endDay
34%7%9%26%19%2
2145
October1st to 15thDay
44%5%11%23%13%2
2044
16th to endDay
46%5%12%21%12%1
1933
November1st to 15thDay
46%5%11%21%13%2
1852
16th to endDay
51%412%23%8%
1847
December1st to 15thDay
57%412%17%7%
1680
16th to endDay
64%410%13%6%
1792
January1st to 15thNight
52%411%18%12%
2926
16th to endNight
56%412%16%9%
3091
February1st to 15thNight
54%411%18%10%
2581
16th to endNight
53%311%20%9%1
2205
March1st to 15thNight
51%410%19%10%2
2520
16th to endNight
49%5%12%19%11%2
2346
April1st to 15thNight
48%410%21%13%1
2057
16th to endNight
46%5%8%20%13%5%
1862
May1st to 15thNight
45%6%12%24%11%
1732
16th to endNight
34%8%14%20%19%2
1626
June1st to 15thNight
34%6%11%26%19%3
1560
16th to endNight
38%7%12%22%18%1
1560
July1st to 15thNight
28%6%12%27%19%4
1480
16th to endNight
25%7%15%27%18%4
1545
August1st to 15thNight
25%7%16%29%19%2
1683
16th to endNight
30%6%13%27%18%4
2070
September1st to 15thNight
28%5%10%26%24%4
1953
16th to endNight
31%6%9%29%19%2
2145
October1st to 15thNight
37%49%26%19%2
2243
16th to endNight
44%311%23%15%1
2642
November1st to 15thNight
43%38%22%17%3
2666
16th to endNight
48%313%23%10%
2772
December1st to 15thNight
54%410%19%9%
2940
16th to endNight
65%38%14%6%
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
30%10%29%26%3
180
16th to endDay
19%14%40%20%5%
192
February1st to 15thDay
31%18%36%11%1
180
16th to endDay
30%13%36%13%5%1
159
March1st to 15thDay
32%16%32%16%2
180
16th to endDay
27%8%30%23%8%2
362
April1st to 15thDay
25%14%29%20%10%
372
16th to endDay
19%11%27%25%12%3
319
May1st to 15thDay
20%10%31%22%10%4
367
16th to endDay
16%9%30%23%15%5%
597
June1st to 15thDay
13%6%29%32%14%4
570
16th to endDay
12%7%33%28%13%5%
570
July1st to 15thDay
8%428%26%20%11%
550
16th to endDay
7%228%30%20%10%
595
August1st to 15thDay
7%218%32%23%15%
452
16th to endDay
8%418%28%27%12%
396
September1st to 15thDay
12%316%31%25%10%
375
16th to endDay
8%5%19%30%28%8%
374
October1st to 15thDay
23%8%24%27%12%3
274
16th to endDay
22%16%35%19%5%
192
November1st to 15thDay
24%12%24%22%12%4
180
16th to endDay
27%12%36%18%5%
180
December1st to 15thDay
17%22%36%15%6%1
180
16th to endDay
25%19%44%7%2
192
January1st to 15thNight
21%12%37%20%6%
1050
16th to endNight
19%12%35%29%2
1120
February1st to 15thNight
24%10%35%24%4
933
16th to endNight
22%13%33%24%5%
735
March1st to 15thNight
21%11%35%24%6%
840
16th to endNight
20%8%33%26%10%
725
April1st to 15thNight
22%11%32%26%6%
617
16th to endNight
21%7%32%26%10%1
579
May1st to 15thNight
20%430%29%12%
585
16th to endNight
11%426%37%18%
611
June1st to 15thNight
11%227%43%14%
585
16th to endNight
10%327%37%18%1
585
July1st to 15thNight
7%118%43%26%2
555
16th to endNight
5%115%44%29%3
612
August1st to 15thNight
419%48%31%4
575
16th to endNight
10%15%37%30%4
621
September1st to 15thNight
11%316%33%29%6%
588
16th to endNight
10%5%18%34%27%3
777
October1st to 15thNight
17%7%25%32%16%1
780
16th to endNight
22%12%31%24%9%
832
November1st to 15thNight
20%10%35%23%9%1
820
16th to endNight
22%11%34%24%7%
882
December1st to 15thNight
26%10%33%24%5%
1050
16th to endNight
29%11%33%22%3
1120
Page generated at 2019-04-12 18:23:22. Version ac04

Copyright 2019 A. Danko


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