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 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
31%
14995
19%
46793
9%
14962
42%
14962
20%
29926

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%
720
18%
2506
7%
716
32%
716
14%
1632
January16th to endNight
15%
768
21%
2532
4%
768
35%
768
16%
1590
February1st to 15thNight
12%
704
16%
2284
3%
701
29%
701
13%
1506
February16th to endNight
15%
631
25%
1893
5%
631
38%
631
16%
1211
March1st to 15thNight
18%
720
19%
2082
6%
720
33%
720
17%
1367
March16th to endNight
20%
768
21%
2039
7%
768
32%
768
19%
1346
April1st to 15thNight
21%
718
15%
1796
4%
718
28%
718
19%
1275
April16th to endNight
28%
720
18%
1800
4%
720
35%
720
25%
1204
May1st to 15thNight
33%
468
19%
1548
9%
468
44%
468
19%
1018
May16th to endNight
44%
374
15%
1497
12%
374
52%
374
19%
918
June1st to 15thNight
43%
360
15%
1440
14%
360
53%
360
20%
888
June16th to endNight
55%
360
16%
1440
15%
360
63%
360
27%
802
July1st to 15thNight
52%
340
13%
1360
17%
340
55%
340
21%
818
July16th to endNight
60%
397
14%
1438
13%
374
65%
374
27%
810
August1st to 15thNight
58%
554
18%
1624
12%
551
61%
551
31%
997
August16th to endNight
56%
764
21%
1910
11%
764
59%
764
33%
1236
September1st to 15thNight
43%
700
23%
1750
10%
700
49%
700
32%
1140
September16th to endNight
49%
657
25%
1784
18%
657
55%
657
31%
1142
October1st to 15thNight
39%
660
21%
1980
15%
660
49%
660
25%
1253
October16th to endNight
28%
704
22%
2148
9%
704
43%
704
23%
1331
November1st to 15thNight
29%
700
21%
2277
10%
700
45%
700
22%
1396
November16th to endNight
25%
720
19%
2457
9%
720
39%
720
16%
1553
December1st to 15thNight
18%
720
14%
2520
6%
720
29%
720
12%
1702
December16th to endNight
15%
768
17%
2688
2%
768
33%
768
12%
1791

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%35%5%5%7%36%
47837
Night
16%6%2442445%7%41%
46964
Cloud CoverDay
16%6%25%5%345%5%7%36%
47846
Night
16%5%2442445%7%41%
46970
TransparencyDay
45%5%11%21%13%1
47527
Night
42%412%23%14%1
46793
SeeingDay
22%14%25%20%11%4
5651
Night
21%15%26%24%10%1
14996

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%6%46%45%6%34%
1432
16th to endDay
13%9%36%5%45%46%7%32%
1689
February1st to 15thDay
15%8%47%6%35%45%6%31%
1578
16th to endDay
15%8%37%6%44445%35%
1580
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%32442445%7%41%
2098
16th to endDay
17%5%25%424346%43%
1843
October1st to 15thDay
18%4244345%5%7%40%
1650
16th to endDay
19%5%25%434445%37%
1724
November1st to 15thDay
16%8%24423336%44%
1581
16th to endDay
21%5%2442435%6%39%
1500
December1st to 15thDay
18%8%345%4445%9%31%
1440
16th to endDay
19%8%45%5%245%6%7%29%
1536
January1st to 15thNight
16%7%26%5%35%45%7%35%
2506
16th to endNight
12%8%35%5%2445%7%39%
2532
February1st to 15thNight
14%8%36%5%3345%6%37%
2282
16th to endNight
14%6%2442445%7%40%
1893
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%414323445%47%
1750
16th to endNight
15%5%24423346%47%
1784
October1st to 15thNight
19%424422346%45%
1980
16th to endNight
19%41342345%7%43%
2148
November1st to 15thNight
14%6%2442345%6%44%
2277
16th to endNight
18%4245%3445%8%37%
2457
December1st to 15thNight
21%5%25%6%345%6%7%31%
2520
16th to endNight
18%8%2443445%7%35%
2688

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%37%6%35%5%5%5%35%
1432
16th to endDay
13%7%36%5%45%5%6%7%33%
1689
February1st to 15thDay
15%6%46%7%3446%7%32%
1578
16th to endDay
16%7%46%6%35%45%7%34%
1580
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
17%4145%2445%7%42%
2098
16th to endDay
17%5%244235%45%44%
1843
October1st to 15thDay
18%5%1442445%7%40%
1650
16th to endDay
20%6%15%435%45%6%38%
1724
November1st to 15thDay
17%7%34432345%45%
1581
16th to endDay
21%42432445%6%41%
1500
December1st to 15thDay
19%7%344445%6%7%33%
1440
16th to endDay
19%8%35%5%2447%6%31%
1536
January1st to 15thNight
16%6%25%5%3445%6%37%
2506
16th to endNight
13%8%345%2445%6%41%
2532
February1st to 15thNight
13%8%36%5%2445%7%38%
2275
16th to endNight
15%5%24415%45%7%43%
1893
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
16%5%14423336%48%
1750
16th to endNight
15%4245%22436%47%
1784
October1st to 15thNight
20%414323346%45%
1980
16th to endNight
19%41332345%6%45%
2148
November1st to 15thNight
14%5%35%42335%6%45%
2277
16th to endNight
18%5%24434447%39%
2457
December1st to 15thNight
20%5%26%5%245%6%7%32%
2520
16th to endNight
19%7%25%42445%7%36%
2688

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
49%411%23%8%2
1433
16th to endDay
49%412%21%11%
1689
February1st to 15thDay
51%411%20%10%1
1580
16th to endDay
49%410%24%10%1
1580
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%20%20%1
2098
16th to endDay
41%27%23%21%2
1843
October1st to 15thDay
41%5%14%23%14%1
1650
16th to endDay
46%311%23%13%1
1724
November1st to 15thDay
42%312%21%16%3
1581
16th to endDay
44%411%24%14%1
1500
December1st to 15thDay
49%417%18%9%1
1440
16th to endDay
52%5%14%17%9%
1536
January1st to 15thNight
46%413%21%11%2
2506
16th to endNight
42%313%26%12%1
2532
February1st to 15thNight
45%5%13%21%11%1
2284
16th to endNight
41%49%27%15%2
1893
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
1750
16th to endNight
38%36%24%24%2
1784
October1st to 15thNight
40%410%26%15%2
1980
16th to endNight
39%413%28%14%
2148
November1st to 15thNight
39%313%23%15%4
2277
16th to endNight
44%313%24%13%1
2457
December1st to 15thNight
49%414%19%10%
2520
16th to endNight
48%415%22%9%
2688

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
40%21%25%8%5%
60
16th to endDay
24%28%26%17%2
89
February1st to 15thDay
26%38%29%5%
93
16th to endDay
23%25%33%15%2
239
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
18%14%21%17%17%10%
294
16th to endDay
20%7%20%20%16%14%
254
October1st to 15thDay
25%5%23%24%15%5%
255
16th to endDay
23%18%29%21%7%
265
November1st to 15thDay
22%17%25%20%11%2
235
16th to endDay
25%25%27%17%3
120
December1st to 15thDay
43%20%15%8%10%3
60
16th to endDay
43%21%28%6%
64
January1st to 15thNight
23%22%32%18%3
720
16th to endNight
21%24%33%18%1
768
February1st to 15thNight
24%23%36%15%1
704
16th to endNight
26%24%31%16%2
631
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%23%28%20%2
700
16th to endNight
18%5%18%29%23%3
657
October1st to 15thNight
24%8%21%28%14%2
660
16th to endNight
24%16%27%24%7%
704
November1st to 15thNight
19%17%28%22%11%1
700
16th to endNight
22%19%26%23%6%1
720
December1st to 15thNight
26%20%29%17%5%
720
16th to endNight
25%23%31%16%2
768
Page generated at 2017-09-11 19:35:26. Version ac04

Copyright 2017 A. Danko


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