Astro-chalet
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro-chalet clear sky chart from 2005-11-05 to 2018-02-22.

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
30%
15655
19%
48914
9%
15622
42%
15622
20%
31237

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
17%
780
18%
2716
7%
776
32%
776
14%
1749
January16th to endNight
15%
832
21%
2743
4%
832
35%
832
16%
1723
February1st to 15thNight
11%
764
16%
2479
2%
761
28%
761
12%
1650
February16th to endNight
14%
659
25%
1976
5%
659
37%
659
15%
1279
March1st to 15thNight
18%
720
19%
2082
6%
720
33%
720
17%
1367
March16th to endNight
20%
768
21%
2039
7%
768
33%
768
19%
1346
April1st to 15thNight
21%
718
15%
1796
4%
718
27%
718
19%
1275
April16th to endNight
27%
720
18%
1800
4%
720
34%
720
25%
1204
May1st to 15thNight
33%
468
19%
1548
9%
468
43%
468
19%
1018
May16th to endNight
43%
374
15%
1497
12%
374
51%
374
19%
918
June1st to 15thNight
43%
360
15%
1440
14%
360
53%
360
20%
888
June16th to endNight
54%
360
16%
1440
16%
360
62%
360
27%
802
July1st to 15thNight
51%
340
13%
1360
17%
340
54%
340
21%
818
July16th to endNight
58%
397
14%
1438
13%
374
63%
374
27%
810
August1st to 15thNight
56%
554
18%
1624
12%
551
59%
551
31%
997
August16th to endNight
56%
764
21%
1910
11%
764
58%
764
33%
1236
September1st to 15thNight
44%
720
23%
1800
10%
720
50%
720
32%
1176
September16th to endNight
51%
717
27%
1947
19%
717
57%
717
33%
1221
October1st to 15thNight
40%
720
20%
2160
14%
720
49%
720
25%
1372
October16th to endNight
29%
768
24%
2343
11%
768
44%
768
24%
1434
November1st to 15thNight
30%
760
20%
2472
10%
760
44%
760
21%
1530
November16th to endNight
26%
780
21%
2662
10%
780
41%
780
18%
1650
December1st to 15thNight
18%
780
14%
2730
5%
780
29%
780
12%
1852
December16th to endNight
14%
832
17%
2912
2%
832
33%
832
13%
1922

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
16%6%25%5%345%5%7%36%
49341
Night
16%6%2442445%7%41%
49085
Cloud CoverDay
16%6%25%5%345%5%7%37%
49350
Night
16%5%2442445%7%42%
49091
TransparencyDay
45%5%11%21%13%1
49031
Night
41%412%23%14%1
48914
SeeingDay
23%14%25%20%11%4
6141
Night
21%15%26%24%10%1
15656

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
14%6%46%5%36%446%35%
1552
16th to endDay
13%8%36%5%45%46%7%32%
1830
February1st to 15thDay
16%8%47%6%35%45%6%31%
1713
16th to endDay
16%8%37%5%43445%35%
1644
March1st to 15thDay
16%7%26%5%35%6%5%7%33%
1865
16th to endDay
17%7%35%5%24336%37%
2184
April1st to 15thDay
18%8%25%42345%5%37%
2152
16th to endDay
16%6%25%5%24446%41%
2160
May1st to 15thDay
22%6%25%5%3445%6%33%
2412
16th to endDay
18%6%35%5%45%5%5%8%33%
2618
June1st to 15thDay
16%6%25%5%35%6%6%9%32%
2520
16th to endDay
14%5%25%6%35%6%6%8%34%
2520
July1st to 15thDay
13%426%6%36%5%6%9%34%
2520
16th to endDay
11%5%25%5%46%6%7%8%35%
2630
August1st to 15thDay
16%5%25%6%35%6%7%8%33%
2313
16th to endDay
15%6%2432445%7%41%
2292
September1st to 15thDay
17%4245%2345%7%41%
2160
16th to endDay
16%5%25%42435%6%44%
2010
October1st to 15thDay
19%5%24434447%40%
1800
16th to endDay
18%5%25%434446%39%
1881
November1st to 15thDay
17%8%24323336%43%
1716
16th to endDay
20%5%2432435%6%42%
1625
December1st to 15thDay
18%8%345%3445%9%32%
1560
16th to endDay
18%8%46%5%245%6%7%29%
1664
January1st to 15thNight
16%6%25%5%35%45%7%37%
2716
16th to endNight
12%9%35%5%2445%7%40%
2743
February1st to 15thNight
15%8%36%5%3345%6%36%
2477
16th to endNight
15%6%2442445%7%40%
1976
March1st to 15thNight
18%5%3442445%6%42%
2069
16th to endNight
16%7%2432435%6%42%
2039
April1st to 15thNight
17%6%25%6%35%45%6%36%
1796
16th to endNight
18%424423336%45%
1800
May1st to 15thNight
20%6%2442445%5%39%
1548
16th to endNight
16%6%2342436%7%40%
1495
June1st to 15thNight
18%42342435%8%40%
1440
16th to endNight
11%32442446%9%46%
1440
July1st to 15thNight
12%5%245%2446%8%41%
1440
16th to endNight
10%3235%335%6%11%44%
1535
August1st to 15thNight
12%4244235%5%8%44%
1635
16th to endNight
12%5%1442445%7%46%
1910
September1st to 15thNight
16%414334445%47%
1800
16th to endNight
14%424423446%48%
1947
October1st to 15thNight
20%424322346%44%
2160
16th to endNight
19%41342345%7%45%
2343
November1st to 15thNight
14%6%2442445%6%43%
2472
16th to endNight
17%42443445%8%39%
2662
December1st to 15thNight
20%6%35%5%345%6%7%30%
2730
16th to endNight
18%8%2443445%7%36%
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
15%6%36%6%35%45%5%36%
1552
16th to endDay
14%7%36%5%45%5%6%7%33%
1830
February1st to 15thDay
16%7%46%7%3446%7%32%
1713
16th to endDay
16%7%46%6%3445%7%34%
1644
March1st to 15thDay
16%6%25%5%46%5%5%7%33%
1875
16th to endDay
17%7%35%5%34346%38%
2183
April1st to 15thDay
18%7%25%42445%6%38%
2152
16th to endDay
16%5%245%2345%7%41%
2160
May1st to 15thDay
21%5%25%5%35%5%5%5%33%
2412
16th to endDay
18%5%345%345%6%7%34%
2618
June1st to 15thDay
16%5%246%45%6%7%8%32%
2520
16th to endDay
14%425%5%45%5%7%8%35%
2520
July1st to 15thDay
11%6%25%7%36%6%5%10%34%
2520
16th to endDay
11%5%25%6%45%6%8%9%35%
2630
August1st to 15thDay
15%5%25%6%45%6%6%8%34%
2313
16th to endDay
15%5%2442445%8%41%
2292
September1st to 15thDay
16%425%5%2445%7%41%
2160
16th to endDay
15%5%245%235%5%5%45%
2010
October1st to 15thDay
19%6%14424447%40%
1800
16th to endDay
19%5%15%33445%7%39%
1881
November1st to 15thDay
17%7%34432345%44%
1716
16th to endDay
19%42432435%6%43%
1625
December1st to 15thDay
18%7%344445%5%7%33%
1560
16th to endDay
18%8%35%5%25%47%6%32%
1664
January1st to 15thNight
16%6%25%5%3445%7%38%
2716
16th to endNight
13%9%3442445%6%41%
2743
February1st to 15thNight
14%8%36%5%2445%7%37%
2470
16th to endNight
16%5%24415%45%7%42%
1976
March1st to 15thNight
18%5%24423445%42%
2082
16th to endNight
17%6%2432445%7%42%
2039
April1st to 15thNight
17%6%25%5%25%45%7%37%
1796
16th to endNight
18%42442435%5%44%
1800
May1st to 15thNight
19%6%2433345%6%39%
1548
16th to endNight
16%6%334245%46%42%
1495
June1st to 15thNight
18%5%2343436%7%41%
1440
16th to endNight
10%4133336%5%10%45%
1440
July1st to 15thNight
12%425%43446%9%41%
1440
16th to endNight
10%3234345%6%10%44%
1535
August1st to 15thNight
12%5%144345%5%8%44%
1635
16th to endNight
12%5%2342445%8%45%
1910
September1st to 15thNight
15%5%24423336%47%
1800
16th to endNight
14%424422446%49%
1947
October1st to 15thNight
20%414323346%45%
2160
16th to endNight
18%41331345%6%46%
2343
November1st to 15thNight
14%5%2442445%6%44%
2472
16th to endNight
17%5%2443445%7%41%
2662
December1st to 15thNight
20%6%26%5%345%5%7%32%
2730
16th to endNight
18%7%25%42445%7%37%
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
48%411%23%9%2
1553
16th to endDay
49%412%21%11%
1830
February1st to 15thDay
51%411%19%11%1
1715
16th to endDay
49%410%24%9%1
1644
March1st to 15thDay
48%5%14%19%10%1
1875
16th to endDay
48%310%20%15%1
2184
April1st to 15thDay
47%49%22%12%3
2152
16th to endDay
41%48%22%18%3
2160
May1st to 15thDay
51%5%9%19%13%1
2412
16th to endDay
48%6%12%17%13%1
2622
June1st to 15thDay
46%7%13%21%10%
2520
16th to endDay
43%7%13%23%10%1
2520
July1st to 15thDay
45%6%11%21%14%1
2371
16th to endDay
41%6%12%24%12%1
2465
August1st to 15thDay
46%6%11%20%14%
2300
16th to endDay
40%5%9%24%18%1
2292
September1st to 15thDay
41%5%10%19%20%1
2160
16th to endDay
39%37%24%22%2
2010
October1st to 15thDay
42%5%13%21%14%1
1800
16th to endDay
45%311%24%14%1
1881
November1st to 15thDay
43%312%21%15%3
1716
16th to endDay
42%311%26%14%1
1625
December1st to 15thDay
49%416%18%10%
1560
16th to endDay
51%5%14%16%10%
1664
January1st to 15thNight
45%413%21%12%2
2716
16th to endNight
43%313%26%12%1
2743
February1st to 15thNight
46%5%13%21%11%1
2479
16th to endNight
42%49%26%14%2
1976
March1st to 15thNight
43%313%23%13%2
2082
16th to endNight
41%412%23%15%2
2039
April1st to 15thNight
46%49%21%15%2
1796
16th to endNight
40%5%9%21%20%2
1800
May1st to 15thNight
45%410%20%16%1
1548
16th to endNight
42%5%15%18%14%2
1497
June1st to 15thNight
41%6%12%25%14%
1440
16th to endNight
32%7%15%31%12%
1440
July1st to 15thNight
40%6%13%24%14%1
1360
16th to endNight
32%7%19%22%14%2
1438
August1st to 15thNight
36%5%15%25%16%1
1624
16th to endNight
36%5%11%23%21%2
1910
September1st to 15thNight
37%411%25%19%1
1800
16th to endNight
36%37%24%25%2
1947
October1st to 15thNight
40%410%26%14%2
2160
16th to endNight
38%413%29%14%
2343
November1st to 15thNight
40%413%23%14%4
2472
16th to endNight
42%312%25%13%1
2662
December1st to 15thNight
50%414%19%10%
2730
16th to endNight
47%415%22%10%
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
38%22%25%8%4
90
16th to endDay
24%25%33%14%2
134
February1st to 15thDay
31%36%28%3
138
16th to endDay
25%25%32%14%2
265
March1st to 15thDay
24%24%26%21%2
270
16th to endDay
22%28%26%15%5%2
288
April1st to 15thDay
26%17%30%17%7%1
270
16th to endDay
21%14%34%14%11%4
270
May1st to 15thDay
36%14%22%13%9%3
288
16th to endDay
21%9%27%25%11%3
331
June1st to 15thDay
23%14%22%24%13%2
315
16th to endDay
21%10%32%20%12%2
315
July1st to 15thDay
18%6%24%27%18%4
305
16th to endDay
14%6%24%29%18%5%
326
August1st to 15thDay
18%7%18%28%19%8%
313
16th to endDay
15%420%29%17%12%
332
September1st to 15thDay
17%14%21%17%18%10%
315
16th to endDay
17%8%19%21%17%14%
314
October1st to 15thDay
26%5%22%22%15%6%
315
16th to endDay
23%18%27%20%9%
326
November1st to 15thDay
24%16%27%20%10%1
280
16th to endDay
26%27%26%15%3
155
December1st to 15thDay
37%18%22%11%7%2
90
16th to endDay
37%26%30%6%
96
January1st to 15thNight
23%23%31%17%3
780
16th to endNight
21%24%34%18%1
832
February1st to 15thNight
25%23%35%15%1
764
16th to endNight
26%24%30%15%2
659
March1st to 15thNight
26%22%29%17%3
720
16th to endNight
21%22%30%17%6%1
768
April1st to 15thNight
23%17%34%20%4
718
16th to endNight
22%15%30%22%9%
720
May1st to 15thNight
24%10%26%28%8%
468
16th to endNight
19%7%21%36%14%1
375
June1st to 15thNight
21%6%21%31%17%
360
16th to endNight
12%423%43%15%
360
July1st to 15thNight
15%5%18%37%21%
340
16th to endNight
12%316%40%24%2
397
August1st to 15thNight
17%214%38%24%3
554
16th to endNight
15%413%34%28%3
764
September1st to 15thNight
17%8%22%29%20%2
720
16th to endNight
17%6%18%29%23%5%
717
October1st to 15thNight
24%8%21%27%15%2
720
16th to endNight
23%16%26%24%7%
768
November1st to 15thNight
20%17%28%22%10%1
760
16th to endNight
22%19%26%24%6%1
780
December1st to 15thNight
26%20%29%17%6%
780
16th to endNight
24%24%31%16%2
832
Page generated at 2018-02-23 11:21:28. Version ac04

Copyright 2018 A. Danko


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