AI2 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AI2 Observatory clear sky chart from 2005-11-05 to 2018-01-12.

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%
15496
20%
48371
12%
15464
47%
15464
23%
30786

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
29%
765
19%
2662
11%
761
40%
761
19%
1691
January16th to endNight
28%
768
21%
2547
11%
768
42%
768
21%
1593
February1st to 15thNight
25%
704
17%
2269
8%
701
38%
701
18%
1495
February16th to endNight
25%
631
25%
1893
11%
631
43%
631
22%
1197
March1st to 15thNight
27%
720
19%
2102
9%
720
39%
720
22%
1386
March16th to endNight
29%
768
22%
2026
9%
768
39%
768
24%
1339
April1st to 15thNight
29%
718
17%
1796
7%
718
33%
718
24%
1239
April16th to endNight
35%
720
18%
1800
6%
720
41%
720
26%
1207
May1st to 15thNight
33%
477
18%
1557
9%
477
43%
477
17%
1027
May16th to endNight
45%
374
15%
1497
13%
374
53%
374
17%
926
June1st to 15thNight
45%
360
15%
1440
16%
360
52%
360
19%
893
June16th to endNight
56%
360
16%
1440
20%
360
63%
360
23%
802
July1st to 15thNight
54%
340
13%
1360
17%
340
56%
340
18%
835
July16th to endNight
56%
379
16%
1421
18%
357
64%
357
24%
792
August1st to 15thNight
58%
571
18%
1641
14%
568
60%
568
28%
1039
August16th to endNight
60%
764
21%
1910
12%
764
62%
764
33%
1262
September1st to 15thNight
53%
720
23%
1800
12%
720
57%
720
33%
1173
September16th to endNight
58%
717
28%
1938
21%
717
63%
717
33%
1199
October1st to 15thNight
46%
720
21%
2160
20%
720
51%
720
26%
1361
October16th to endNight
40%
768
25%
2358
17%
768
52%
768
28%
1430
November1st to 15thNight
39%
760
20%
2472
15%
760
48%
760
24%
1515
November16th to endNight
34%
780
22%
2640
15%
780
46%
780
22%
1628
December1st to 15thNight
29%
780
15%
2730
8%
780
39%
780
15%
1855
December16th to endNight
25%
832
18%
2912
7%
832
39%
832
17%
1902

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
15%5%25%5%35%45%7%38%
48972
Night
16%5%2442445%7%42%
48542
Cloud CoverDay
15%5%25%5%35%45%7%39%
48981
Night
16%5%2442445%7%42%
48548
TransparencyDay
43%5%11%22%15%1
48661
Night
41%412%23%16%2
48371
SeeingDay
18%12%18%25%18%7%
6050
Night
16%16%20%32%12%1
15497

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
13%6%36%6%445%45%40%
1520
16th to endDay
14%6%45%5%345%5%7%37%
1674
February1st to 15thDay
15%6%36%5%35%46%7%35%
1592
16th to endDay
14%6%36%5%35%45%5%38%
1580
March1st to 15thDay
16%7%25%5%35%5%5%7%35%
1847
16th to endDay
17%7%24424346%40%
2196
April1st to 15thDay
18%7%25%424447%39%
2152
16th to endDay
15%425%5%335%6%6%41%
2160
May1st to 15thDay
21%5%245%25%45%6%35%
2405
16th to endDay
16%6%245%25%5%6%9%34%
2618
June1st to 15thDay
15%5%25%436%5%7%9%34%
2520
16th to endDay
14%425%6%35%46%8%38%
2520
July1st to 15thDay
11%5%25%6%36%5%7%10%35%
2520
16th to endDay
10%425%6%45%6%7%9%36%
2648
August1st to 15thDay
14%5%246%45%7%6%7%35%
2295
16th to endDay
14%424425%347%44%
2292
September1st to 15thDay
15%4245%34447%43%
2160
16th to endDay
14%415%3245%46%45%
2019
October1st to 15thDay
18%5%15%32335%7%42%
1800
16th to endDay
19%414335%346%42%
1866
November1st to 15thDay
18%7%24423335%44%
1716
16th to endDay
18%5%13324347%45%
1648
December1st to 15thDay
17%7%36%32445%7%36%
1560
16th to endDay
20%7%35%4245%5%6%34%
1664
January1st to 15thNight
16%6%25%434446%40%
2662
16th to endNight
15%8%3442445%6%41%
2547
February1st to 15thNight
13%8%36%5%34446%39%
2268
16th to endNight
15%5%35%32445%5%43%
1893
March1st to 15thNight
17%5%24324346%42%
2089
16th to endNight
16%6%25%323446%43%
2026
April1st to 15thNight
17%5%25%5%24447%39%
1796
16th to endNight
18%42432345%7%43%
1800
May1st to 15thNight
19%7%2433445%6%37%
1557
16th to endNight
17%5%15%3345%47%41%
1495
June1st to 15thNight
17%5%2342345%7%42%
1440
16th to endNight
10%4135%245%6%9%45%
1440
July1st to 15thNight
12%325%6%36%46%9%40%
1440
16th to endNight
9%423425%5%6%11%44%
1517
August1st to 15thNight
12%4245%235%6%8%43%
1652
16th to endNight
12%5%245%3445%9%42%
1910
September1st to 15thNight
14%5%14424346%47%
1800
16th to endNight
12%5%14422447%49%
1938
October1st to 15thNight
18%5%23323346%45%
2160
16th to endNight
19%5%13213346%47%
2358
November1st to 15thNight
14%6%24324445%46%
2472
16th to endNight
17%5%3442345%6%42%
2640
December1st to 15thNight
19%6%35%435%5%46%34%
2730
16th to endNight
18%7%24434446%38%
2912

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
14%6%35%6%35%5%4440%
1520
16th to endDay
13%6%45%6%245%5%7%37%
1674
February1st to 15thDay
15%7%36%435%5%5%6%36%
1592
16th to endDay
15%6%36%6%35%45%5%37%
1580
March1st to 15thDay
16%7%25%6%35%45%8%34%
1857
16th to endDay
17%7%24434446%40%
2195
April1st to 15thDay
18%7%35%424347%39%
2152
16th to endDay
15%425%5%3346%6%42%
2160
May1st to 15thDay
21%5%245%25%45%6%35%
2405
16th to endDay
17%6%24435%5%6%9%35%
2618
June1st to 15thDay
15%425%436%5%6%10%33%
2520
16th to endDay
13%5%25%5%35%46%8%39%
2520
July1st to 15thDay
11%5%25%7%45%5%7%10%35%
2520
16th to endDay
10%5%25%5%36%5%7%9%37%
2648
August1st to 15thDay
14%6%25%5%46%6%6%7%34%
2295
16th to endDay
14%425%435%346%44%
2292
September1st to 15thDay
15%425%435%447%43%
2160
16th to endDay
14%415%325%45%6%45%
2019
October1st to 15thDay
17%5%25%42345%6%42%
1800
16th to endDay
19%41443435%6%42%
1866
November1st to 15thDay
17%7%24423335%44%
1716
16th to endDay
17%5%2341335%6%45%
1648
December1st to 15thDay
18%7%35%43436%7%36%
1560
16th to endDay
19%7%35%4245%5%6%34%
1664
January1st to 15thNight
16%6%35%434446%40%
2662
16th to endNight
16%8%34424446%41%
2547
February1st to 15thNight
13%8%36%5%34446%39%
2261
16th to endNight
15%5%3432445%5%44%
1893
March1st to 15thNight
18%5%24424346%42%
2102
16th to endNight
16%6%25%423437%43%
2026
April1st to 15thNight
17%5%25%5%2445%6%39%
1796
16th to endNight
18%5%2332335%6%44%
1800
May1st to 15thNight
19%7%2443445%6%38%
1557
16th to endNight
16%5%23425%447%42%
1495
June1st to 15thNight
17%5%2442345%7%41%
1440
16th to endNight
10%4244245%7%9%44%
1440
July1st to 15thNight
11%425%6%35%5%6%9%39%
1440
16th to endNight
9%423425%6%6%11%43%
1517
August1st to 15thNight
12%4245%245%6%7%44%
1652
16th to endNight
13%5%245%3445%9%42%
1910
September1st to 15thNight
15%5%1442335%5%47%
1800
16th to endNight
12%5%24423437%49%
1938
October1st to 15thNight
18%6%23323346%45%
2160
16th to endNight
18%5%13213346%48%
2358
November1st to 15thNight
15%5%24324446%46%
2472
16th to endNight
17%5%3442345%6%42%
2640
December1st to 15thNight
19%6%26%5%35%45%6%34%
2730
16th to endNight
18%7%24434446%38%
2912

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
45%411%24%10%3
1521
16th to endDay
45%413%23%12%1
1674
February1st to 15thDay
45%5%12%20%13%1
1595
16th to endDay
46%412%24%10%1
1580
March1st to 15thDay
47%414%20%10%1
1857
16th to endDay
45%310%21%17%2
2196
April1st to 15thDay
44%410%22%13%4
2152
16th to endDay
40%410%20%20%3
2160
May1st to 15thDay
48%49%20%15%2
2405
16th to endDay
44%6%13%20%13%1
2622
June1st to 15thDay
44%7%14%22%10%
2520
16th to endDay
41%6%13%25%11%1
2520
July1st to 15thDay
43%6%11%22%15%1
2371
16th to endDay
39%6%12%24%13%1
2481
August1st to 15thDay
44%6%12%19%17%
2282
16th to endDay
39%48%25%20%2
2292
September1st to 15thDay
40%410%21%21%1
2160
16th to endDay
39%36%24%23%2
2019
October1st to 15thDay
42%413%21%16%1
1800
16th to endDay
41%310%27%15%1
1866
November1st to 15thDay
43%213%20%17%3
1716
16th to endDay
39%311%28%15%1
1648
December1st to 15thDay
46%5%16%19%10%1
1560
16th to endDay
49%5%15%17%11%
1664
January1st to 15thNight
43%413%21%13%3
2662
16th to endNight
43%312%25%13%1
2547
February1st to 15thNight
45%412%21%12%2
2269
16th to endNight
41%39%25%17%2
1893
March1st to 15thNight
43%413%22%13%2
2102
16th to endNight
41%410%22%18%2
2026
April1st to 15thNight
44%410%21%16%2
1796
16th to endNight
40%49%20%22%3
1800
May1st to 15thNight
45%5%10%19%17%2
1557
16th to endNight
41%7%15%17%15%3
1497
June1st to 15thNight
42%6%12%22%16%
1440
16th to endNight
33%6%14%31%13%
1440
July1st to 15thNight
41%7%12%22%14%1
1360
16th to endNight
32%7%17%25%14%2
1421
August1st to 15thNight
35%7%13%23%17%2
1641
16th to endNight
37%5%10%22%21%2
1910
September1st to 15thNight
38%310%23%21%1
1800
16th to endNight
34%47%23%27%2
1938
October1st to 15thNight
41%311%25%16%2
2160
16th to endNight
37%412%29%16%1
2358
November1st to 15thNight
39%412%21%17%4
2472
16th to endNight
41%311%26%14%1
2640
December1st to 15thNight
49%413%20%10%1
2730
16th to endNight
46%414%21%11%
2912

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
19%14%32%23%8%1
82
16th to endDay
20%8%26%31%13%
87
February1st to 15thDay
17%11%40%26%3
109
16th to endDay
17%15%27%33%6%
239
March1st to 15thDay
17%24%21%27%8%
270
16th to endDay
18%17%25%24%10%3
288
April1st to 15thDay
18%16%23%24%13%2
270
16th to endDay
17%13%23%20%17%7%
270
May1st to 15thDay
32%18%14%17%12%4
288
16th to endDay
18%13%14%25%19%8%
331
June1st to 15thDay
16%13%19%21%21%7%
315
16th to endDay
18%14%16%26%19%5%
315
July1st to 15thDay
17%7%11%26%26%9%
305
16th to endDay
13%10%10%23%29%13%
326
August1st to 15thDay
17%7%9%24%26%14%
313
16th to endDay
12%5%5%26%28%20%
332
September1st to 15thDay
13%7%17%22%21%17%
315
16th to endDay
16%7%10%21%23%21%
314
October1st to 15thDay
20%7%11%27%23%10%
315
16th to endDay
16%10%27%27%16%2
322
November1st to 15thDay
19%12%19%27%17%2
280
16th to endDay
15%17%26%26%12%1
178
December1st to 15thDay
21%16%25%24%10%2
90
16th to endDay
31%9%31%26%2
96
January1st to 15thNight
15%20%29%29%6%
765
16th to endNight
17%19%27%31%3
768
February1st to 15thNight
15%21%29%31%2
704
16th to endNight
16%20%31%28%3
631
March1st to 15thNight
20%20%27%26%4
720
16th to endNight
16%19%26%27%8%
768
April1st to 15thNight
16%21%28%27%5%
718
16th to endNight
15%21%21%29%11%
720
May1st to 15thNight
22%19%18%28%9%
477
16th to endNight
16%18%11%39%13%1
375
June1st to 15thNight
17%19%10%34%17%
360
16th to endNight
8%17%9%48%15%
360
July1st to 15thNight
12%12%9%42%22%1
340
16th to endNight
8%17%8%39%22%1
379
August1st to 15thNight
15%10%5%40%25%1
571
16th to endNight
13%9%6%35%32%3
764
September1st to 15thNight
16%9%12%36%22%2
720
16th to endNight
15%9%8%33%27%5%
717
October1st to 15thNight
18%10%15%33%19%2
720
16th to endNight
19%10%25%33%11%
768
November1st to 15thNight
15%16%22%30%13%1
760
16th to endNight
15%18%23%31%9%
780
December1st to 15thNight
22%18%21%29%7%1
780
16th to endNight
19%19%28%29%3
832
Page generated at 2018-01-12 22:01:35. Version ac04

Copyright 2018 A. Danko


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