Ash Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ash Observatory 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
38%
15105
20%
46800
11%
15073
48%
15073
25%
29603

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
26%
718
21%
2506
10%
716
41%
716
20%
1576
January16th to endNight
23%
768
22%
2520
8%
768
42%
768
23%
1574
February1st to 15thNight
21%
704
18%
2252
6%
701
37%
701
21%
1437
February16th to endNight
21%
631
26%
1893
9%
631
41%
631
24%
1178
March1st to 15thNight
27%
720
20%
2099
9%
720
40%
720
26%
1323
March16th to endNight
26%
768
22%
2027
9%
768
36%
768
27%
1348
April1st to 15thNight
25%
718
18%
1796
5%
718
33%
718
29%
1205
April16th to endNight
34%
720
19%
1800
6%
720
39%
720
30%
1202
May1st to 15thNight
33%
528
17%
1608
8%
528
40%
528
20%
1101
May16th to endNight
48%
374
15%
1497
15%
374
53%
374
18%
943
June1st to 15thNight
50%
360
16%
1440
16%
360
57%
360
22%
866
June16th to endNight
60%
360
17%
1440
20%
360
65%
360
25%
833
July1st to 15thNight
57%
340
12%
1360
15%
340
60%
340
19%
857
July16th to endNight
63%
410
15%
1450
15%
386
68%
386
27%
832
August1st to 15thNight
65%
593
18%
1663
13%
590
66%
590
32%
1056
August16th to endNight
61%
764
19%
1910
11%
764
62%
764
33%
1266
September1st to 15thNight
55%
700
24%
1750
13%
700
59%
700
35%
1146
September16th to endNight
54%
657
25%
1779
19%
657
58%
657
34%
1126
October1st to 15thNight
45%
660
22%
1974
17%
660
53%
660
28%
1228
October16th to endNight
37%
704
26%
2148
15%
704
51%
704
31%
1265
November1st to 15thNight
36%
700
23%
2277
14%
700
50%
700
26%
1381
November16th to endNight
32%
720
20%
2403
12%
720
44%
720
22%
1509
December1st to 15thNight
27%
720
17%
2520
9%
720
39%
720
19%
1643
December16th to endNight
24%
768
21%
2688
7%
768
42%
768
19%
1708

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%5%2442445%7%42%
47832
Night
15%5%2442445%7%43%
46973
Cloud CoverDay
13%5%2443445%7%41%
47841
Night
14%6%2442445%7%43%
46979
TransparencyDay
40%6%12%22%16%2
47524
Night
40%5%11%22%17%2
46800
SeeingDay
17%8%24%25%16%5%
5740
Night
17%7%30%30%11%1
15108

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%7%245%24436%42%
1432
16th to endDay
14%7%24424346%45%
1701
February1st to 15thDay
16%6%25%3245%46%41%
1609
16th to endDay
13%8%36%4345%46%39%
1580
March1st to 15thDay
14%6%25%42445%6%42%
1850
16th to endDay
18%5%2442345%6%42%
2196
April1st to 15thDay
16%5%24423346%44%
2152
16th to endDay
15%42442445%7%43%
2160
May1st to 15thDay
17%6%25%5%25%5%6%6%37%
2352
16th to endDay
15%4245%3445%8%41%
2618
June1st to 15thDay
13%5%25%5%45%5%7%8%37%
2520
16th to endDay
10%326%5%35%5%7%10%39%
2520
July1st to 15thDay
8%5%35%5%35%5%6%10%38%
2520
16th to endDay
7%4246%37%6%6%11%38%
2617
August1st to 15thDay
11%426%5%35%6%5%8%40%
2274
16th to endDay
11%4245%3446%9%42%
2292
September1st to 15thDay
12%4244335%47%46%
2098
16th to endDay
14%5%25%5%34447%41%
1851
October1st to 15thDay
17%41432445%6%45%
1655
16th to endDay
17%5%23323435%48%
1724
November1st to 15thDay
16%6%13414346%47%
1581
16th to endDay
16%5%24423345%46%
1554
December1st to 15thDay
16%5%35%3245%6%7%40%
1440
16th to endDay
20%5%245%2445%6%38%
1536
January1st to 15thNight
17%5%244245%46%41%
2506
16th to endNight
16%6%25%324446%43%
2520
February1st to 15thNight
14%7%35%42435%6%41%
2251
16th to endNight
15%5%24323445%46%
1893
March1st to 15thNight
17%6%13323347%45%
2086
16th to endNight
16%7%245%33335%44%
2027
April1st to 15thNight
15%5%25%33345%6%43%
1796
16th to endNight
16%6%24423347%43%
1800
May1st to 15thNight
17%6%25%5%345%46%38%
1608
16th to endNight
14%5%35%5%35%5%5%7%39%
1495
June1st to 15thNight
14%5%2443445%8%42%
1440
16th to endNight
8%4244445%6%11%42%
1440
July1st to 15thNight
9%5%35%5%37%6%7%9%37%
1440
16th to endNight
7%425%6%345%6%9%43%
1548
August1st to 15thNight
8%5%24435%5%6%8%46%
1674
16th to endNight
11%5%25%4345%5%8%43%
1910
September1st to 15thNight
13%415%423436%49%
1750
16th to endNight
13%6%2442335%7%47%
1779
October1st to 15thNight
16%6%14423347%45%
1974
16th to endNight
16%413323246%50%
2148
November1st to 15thNight
14%6%25%32323449%
2277
16th to endNight
16%5%15%425%5%6%6%40%
2403
December1st to 15thNight
19%5%25%43445%6%38%
2520
16th to endNight
17%6%3442445%6%40%
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%7%245%24446%42%
1432
16th to endDay
14%6%24424346%44%
1701
February1st to 15thDay
15%6%35%4245%5%5%41%
1609
16th to endDay
12%8%36%43445%6%39%
1580
March1st to 15thDay
15%5%25%5%25%446%42%
1860
16th to endDay
18%5%2442345%6%41%
2195
April1st to 15thDay
16%5%3442335%6%44%
2152
16th to endDay
14%5%1442346%7%43%
2160
May1st to 15thDay
15%6%25%435%5%5%7%36%
2352
16th to endDay
14%425%5%3446%8%41%
2618
June1st to 15thDay
12%5%245%35%6%7%8%37%
2520
16th to endDay
10%425%436%5%8%10%39%
2520
July1st to 15thDay
8%5%26%6%35%5%7%9%38%
2520
16th to endDay
6%5%35%5%37%5%7%11%38%
2617
August1st to 15thDay
10%5%25%5%35%6%5%8%40%
2274
16th to endDay
11%5%245%25%46%10%41%
2292
September1st to 15thDay
12%4244345%47%46%
2098
16th to endDay
14%5%245%35%447%41%
1851
October1st to 15thDay
17%42332445%6%45%
1655
16th to endDay
16%5%23323436%47%
1724
November1st to 15thDay
16%6%13323346%47%
1581
16th to endDay
15%5%24424345%46%
1554
December1st to 15thDay
16%5%343245%5%7%40%
1440
16th to endDay
19%5%25%42436%6%38%
1536
January1st to 15thNight
17%5%25%42445%6%41%
2506
16th to endNight
16%6%25%324446%42%
2520
February1st to 15thNight
14%8%25%42345%6%41%
2244
16th to endNight
15%6%24424445%46%
1893
March1st to 15thNight
17%5%24324347%44%
2099
16th to endNight
16%6%345%24335%44%
2027
April1st to 15thNight
14%6%25%4235%5%6%43%
1796
16th to endNight
16%6%34433347%43%
1800
May1st to 15thNight
17%6%26%4345%47%37%
1608
16th to endNight
13%5%26%5%26%45%8%38%
1495
June1st to 15thNight
13%6%2443435%9%41%
1440
16th to endNight
7%5%245%345%6%10%43%
1440
July1st to 15thNight
8%5%345%37%6%7%8%36%
1440
16th to endNight
6%425%6%35%5%6%10%42%
1548
August1st to 15thNight
8%5%25%426%5%5%8%45%
1674
16th to endNight
11%5%25%435%5%5%8%43%
1910
September1st to 15thNight
13%415%423447%48%
1750
16th to endNight
13%6%24423347%47%
1779
October1st to 15thNight
16%5%23423347%45%
1974
16th to endNight
15%5%13323346%50%
2148
November1st to 15thNight
14%6%34413335%49%
2277
16th to endNight
16%5%24435%5%6%6%39%
2403
December1st to 15thNight
18%5%35%42445%5%38%
2520
16th to endNight
17%6%2442445%6%39%
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
42%413%21%13%4
1433
16th to endDay
40%311%25%15%3
1701
February1st to 15thDay
42%5%11%24%12%3
1612
16th to endDay
44%5%10%25%12%2
1580
March1st to 15thDay
42%5%13%23%14%2
1860
16th to endDay
42%5%9%21%17%3
2196
April1st to 15thDay
41%5%10%23%15%5%
2152
16th to endDay
39%5%10%17%22%5%
2160
May1st to 15thDay
44%8%10%18%15%2
2352
16th to endDay
40%7%13%23%13%1
2622
June1st to 15thDay
40%9%13%23%11%1
2520
16th to endDay
36%9%14%22%15%1
2520
July1st to 15thDay
39%7%12%21%15%2
2371
16th to endDay
35%8%15%24%13%1
2453
August1st to 15thDay
39%7%12%21%17%1
2261
16th to endDay
37%6%13%20%18%3
2292
September1st to 15thDay
36%6%11%20%22%2
2098
16th to endDay
41%49%17%24%1
1851
October1st to 15thDay
38%5%12%24%18%2
1655
16th to endDay
38%310%26%18%1
1724
November1st to 15thDay
39%411%22%18%3
1581
16th to endDay
39%49%27%16%2
1554
December1st to 15thDay
41%416%23%11%1
1440
16th to endDay
44%5%14%21%13%1
1536
January1st to 15thNight
42%5%11%20%16%3
2506
16th to endNight
41%410%25%15%1
2520
February1st to 15thNight
42%411%23%14%2
2252
16th to endNight
39%49%24%18%3
1893
March1st to 15thNight
39%413%23%16%3
2099
16th to endNight
43%211%20%17%3
2027
April1st to 15thNight
40%411%21%18%4
1796
16th to endNight
39%5%10%18%21%4
1800
May1st to 15thNight
46%7%8%18%17%2
1608
16th to endNight
43%8%13%16%15%2
1497
June1st to 15thNight
39%7%13%21%17%
1440
16th to endNight
34%8%13%26%15%1
1440
July1st to 15thNight
40%9%15%18%13%2
1360
16th to endNight
33%7%18%24%14%1
1450
August1st to 15thNight
34%8%13%22%20%1
1663
16th to endNight
37%7%12%20%19%2
1910
September1st to 15thNight
36%5%11%22%21%2
1750
16th to endNight
36%5%8%22%25%2
1779
October1st to 15thNight
38%411%23%17%3
1974
16th to endNight
35%310%28%19%2
2148
November1st to 15thNight
38%410%20%17%7%
2277
16th to endNight
42%5%10%23%16%2
2403
December1st to 15thNight
46%5%11%22%13%1
2520
16th to endNight
43%5%13%22%12%2
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
15%31%36%10%5%1
60
16th to endDay
24%23%28%23%1
91
February1st to 15thDay
25%27%29%15%2
126
16th to endDay
17%16%39%22%3
239
March1st to 15thDay
18%19%27%25%8%
270
16th to endDay
21%12%33%22%6%2
288
April1st to 15thDay
21%10%33%21%10%2
270
16th to endDay
19%8%28%25%15%2
270
May1st to 15thDay
29%14%23%18%11%3
288
16th to endDay
19%6%17%29%21%6%
331
June1st to 15thDay
15%6%24%27%19%6%
315
16th to endDay
13%5%24%32%19%4
315
July1st to 15thDay
15%120%33%22%5%
305
16th to endDay
7%216%32%30%10%
326
August1st to 15thDay
12%217%28%28%10%
313
16th to endDay
12%14%30%25%16%
332
September1st to 15thDay
12%8%22%22%21%12%
294
16th to endDay
20%317%28%14%15%
254
October1st to 15thDay
20%5%19%27%18%8%
255
16th to endDay
15%8%32%26%15%1
265
November1st to 15thDay
17%7%28%28%16%1
235
16th to endDay
18%10%37%17%13%1
174
December1st to 15thDay
21%23%25%15%15%
60
16th to endDay
35%10%37%14%1
64
January1st to 15thNight
19%13%36%26%4
720
16th to endNight
20%10%41%26%1
768
February1st to 15thNight
20%11%40%25%1
704
16th to endNight
19%13%41%22%2
631
March1st to 15thNight
20%12%36%25%5%
720
16th to endNight
19%8%41%22%7%1
768
April1st to 15thNight
18%12%39%24%4
718
16th to endNight
18%8%32%30%8%
720
May1st to 15thNight
19%7%34%29%8%
528
16th to endNight
15%324%40%15%1
375
June1st to 15thNight
16%222%42%15%
360
16th to endNight
8%222%51%14%
360
July1st to 15thNight
11%219%45%20%
340
16th to endNight
7%216%43%26%2
410
August1st to 15thNight
12%111%46%26%1
593
16th to endNight
14%214%35%28%3
764
September1st to 15thNight
16%119%37%22%2
700
16th to endNight
17%218%32%24%3
657
October1st to 15thNight
20%424%30%17%2
660
16th to endNight
19%8%31%29%10%
704
November1st to 15thNight
16%7%33%27%13%1
700
16th to endNight
18%10%33%26%10%
720
December1st to 15thNight
21%9%33%24%9%1
720
16th to endNight
19%13%36%27%3
768
Page generated at 2017-09-11 19:35:14. Version ac04

Copyright 2017 A. Danko


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