Cahall Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cahall Observatory clear sky chart from 2005-11-05 to 2017-09-19.

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
28%
14929
18%
46838
8%
14896
40%
14896
21%
30397

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
14%
720
19%
2506
5%
716
32%
716
17%
1597
January16th to endNight
14%
768
19%
2688
3%
768
34%
768
14%
1746
February1st to 15thNight
15%
704
17%
2179
4%
701
30%
701
16%
1473
February16th to endNight
16%
631
24%
1893
7%
631
37%
631
20%
1244
March1st to 15thNight
18%
720
17%
2145
6%
720
32%
720
18%
1469
March16th to endNight
21%
768
23%
2046
7%
768
35%
768
23%
1386
April1st to 15thNight
21%
718
18%
1796
5%
718
29%
718
23%
1273
April16th to endNight
25%
582
19%
1662
5%
582
34%
582
22%
1133
May1st to 15thNight
25%
540
15%
1620
7%
540
32%
540
20%
1116
May16th to endNight
28%
439
11%
1561
8%
438
34%
438
19%
1037
June1st to 15thNight
33%
360
11%
1440
8%
360
42%
360
16%
961
June16th to endNight
41%
360
11%
1440
13%
360
48%
360
23%
854
July1st to 15thNight
44%
340
12%
1360
14%
340
50%
340
24%
795
July16th to endNight
48%
376
16%
1418
16%
354
54%
354
22%
874
August1st to 15thNight
46%
527
16%
1597
11%
524
50%
524
28%
984
August16th to endNight
56%
711
22%
1857
13%
711
59%
711
36%
1166
September1st to 15thNight
48%
720
20%
1872
10%
720
53%
720
35%
1201
September16th to endNight
47%
673
26%
1858
17%
673
54%
673
35%
1118
October1st to 15thNight
33%
660
24%
1832
11%
660
45%
660
27%
1194
October16th to endNight
24%
704
16%
2288
6%
704
37%
704
21%
1464
November1st to 15thNight
27%
700
19%
2277
9%
700
42%
700
21%
1446
November16th to endNight
25%
720
20%
2340
8%
720
40%
720
19%
1473
December1st to 15thNight
19%
720
18%
2475
5%
720
35%
720
16%
1612
December16th to endNight
14%
768
19%
2688
4%
768
32%
768
16%
1781

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%2442445%7%39%
47988
Night
16%6%25%424446%40%
47009
Cloud CoverDay
17%6%2442445%7%40%
47998
Night
17%6%25%424446%40%
47014
TransparencyDay
43%411%23%15%2
47677
Night
44%411%22%15%2
46838
SeeingDay
20%11%31%24%10%2
5945
Night
20%11%33%25%7%
14929

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
15%8%34314445%42%
1432
16th to endDay
16%5%25%5%2435%5%42%
1536
February1st to 15thDay
18%7%24435%5%46%37%
1681
16th to endDay
14%8%24424445%42%
1580
March1st to 15thDay
19%7%35%423448%35%
1803
16th to endDay
16%8%35%413446%42%
2175
April1st to 15thDay
17%6%35%5%3435%5%38%
2152
16th to endDay
18%5%1442336%7%42%
2297
May1st to 15thDay
24%9%35%423435%33%
2332
16th to endDay
17%5%2443445%7%40%
2563
June1st to 15thDay
24%5%3442445%7%32%
2520
16th to endDay
14%5%245%35%6%7%9%35%
2520
July1st to 15thDay
11%435%6%35%6%7%8%36%
2520
16th to endDay
11%5%25%5%36%6%7%9%36%
2654
August1st to 15thDay
12%6%25%435%5%6%9%38%
2340
16th to endDay
11%415%435%5%6%8%43%
2345
September1st to 15thDay
13%4235%2436%7%46%
2088
16th to endDay
14%5%13323336%49%
1848
October1st to 15thDay
21%6%24323235%43%
1796
16th to endDay
16%6%2442345%7%42%
1584
November1st to 15thDay
22%6%2432234442%
1581
16th to endDay
16%6%24425%446%41%
1620
December1st to 15thDay
17%8%25%5%33436%39%
1485
16th to endDay
23%9%24425%335%34%
1536
January1st to 15thNight
16%8%35%424345%40%
2506
16th to endNight
16%9%35%5%33345%39%
2688
February1st to 15thNight
15%8%26%435%445%37%
2178
16th to endNight
16%8%25%42344442%
1893
March1st to 15thNight
17%8%35%5%34446%37%
2132
16th to endNight
16%9%35%42333443%
2046
April1st to 15thNight
15%8%36%434446%36%
1796
16th to endNight
20%6%25%424345%39%
1662
May1st to 15thNight
21%7%26%42335%6%34%
1620
16th to endNight
19%5%35%5%2446%8%34%
1559
June1st to 15thNight
22%6%35%43445%7%33%
1440
16th to endNight
15%435%335%46%8%40%
1440
July1st to 15thNight
11%5%26%5%35%5%6%8%40%
1440
16th to endNight
11%5%36%6%37%45%9%36%
1514
August1st to 15thNight
7%5%26%5%445%6%8%42%
1608
16th to endNight
12%4135%235%5%8%46%
1857
September1st to 15thNight
13%3144345%6%6%45%
1872
16th to endNight
14%5%1431335%6%50%
1858
October1st to 15thNight
20%5%23323436%45%
1832
16th to endNight
18%5%15%434446%40%
2288
November1st to 15thNight
18%6%25%423345%43%
2277
16th to endNight
17%6%3442345%6%40%
2340
December1st to 15thNight
19%7%35%5%24445%39%
2475
16th to endNight
18%9%25%423446%36%
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
16%9%24414446%41%
1432
16th to endDay
16%6%245%2345%5%43%
1536
February1st to 15thDay
19%6%34425%446%37%
1681
16th to endDay
15%8%2433445%5%41%
1580
March1st to 15thDay
20%8%25%423447%35%
1813
16th to endDay
17%8%25%413446%41%
2175
April1st to 15thDay
18%7%35%43335%5%38%
2152
16th to endDay
19%42442335%7%42%
2297
May1st to 15thDay
26%8%34423436%33%
2332
16th to endDay
18%5%2442445%7%40%
2563
June1st to 15thDay
25%5%3442445%8%32%
2520
16th to endDay
15%5%245%35%5%6%9%36%
2520
July1st to 15thDay
13%5%25%5%35%6%6%8%36%
2520
16th to endDay
12%425%5%35%6%7%8%37%
2654
August1st to 15thDay
12%5%25%5%35%5%5%8%39%
2340
16th to endDay
12%4145%25%46%8%43%
2345
September1st to 15thDay
14%4145%2345%6%47%
2088
16th to endDay
14%5%14323336%50%
1848
October1st to 15thDay
21%6%2442324444%
1796
16th to endDay
17%6%2332335%7%42%
1584
November1st to 15thDay
23%6%1422333442%
1581
16th to endDay
17%6%24334447%41%
1620
December1st to 15thDay
17%7%245%24346%39%
1485
16th to endDay
23%9%24434335%35%
1536
January1st to 15thNight
16%8%25%424336%41%
2506
16th to endNight
17%9%35%43333440%
2688
February1st to 15thNight
15%8%26%5%24445%38%
2171
16th to endNight
16%7%35%42434442%
1893
March1st to 15thNight
17%8%345%35%346%36%
2145
16th to endNight
17%8%35%43333443%
2045
April1st to 15thNight
16%8%36%5%24346%36%
1796
16th to endNight
20%6%245%34335%39%
1662
May1st to 15thNight
22%6%26%43335%6%34%
1620
16th to endNight
19%5%35%5%25%45%8%34%
1559
June1st to 15thNight
22%6%34435%35%6%34%
1440
16th to endNight
15%434335%36%8%40%
1440
July1st to 15thNight
11%5%25%5%245%6%8%40%
1440
16th to endNight
11%5%35%6%36%5%6%8%36%
1514
August1st to 15thNight
8%5%25%6%346%6%8%42%
1608
16th to endNight
12%4134335%5%8%46%
1857
September1st to 15thNight
14%324435%5%5%6%45%
1872
16th to endNight
15%5%2421335%6%50%
1858
October1st to 15thNight
20%42432343445%
1832
16th to endNight
18%5%145%33446%40%
2288
November1st to 15thNight
18%6%25%424345%43%
2277
16th to endNight
17%6%35%42335%6%41%
2340
December1st to 15thNight
19%7%25%42334439%
2475
16th to endNight
18%9%26%424446%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
42%410%25%12%3
1433
16th to endDay
42%311%27%13%1
1536
February1st to 15thDay
47%312%21%13%1
1684
16th to endDay
43%312%23%14%2
1580
March1st to 15thDay
48%312%22%10%3
1813
16th to endDay
43%39%22%15%4
2175
April1st to 15thDay
46%29%21%17%2
2152
16th to endDay
40%39%24%17%3
2297
May1st to 15thDay
52%37%18%15%2
2332
16th to endDay
41%5%14%23%13%2
2567
June1st to 15thDay
48%6%12%20%10%
2520
16th to endDay
41%5%13%22%14%1
2520
July1st to 15thDay
40%6%10%23%15%2
2371
16th to endDay
40%6%13%23%11%3
2487
August1st to 15thDay
40%5%13%24%14%1
2327
16th to endDay
35%5%11%26%19%1
2345
September1st to 15thDay
35%5%10%23%23%1
2088
16th to endDay
36%36%27%24%2
1848
October1st to 15thDay
42%38%24%16%3
1796
16th to endDay
41%311%24%15%2
1584
November1st to 15thDay
45%310%24%14%1
1581
16th to endDay
43%410%25%13%1
1620
December1st to 15thDay
46%315%18%16%
1485
16th to endDay
53%313%18%10%
1536
January1st to 15thNight
45%312%22%14%1
2506
16th to endNight
47%39%21%15%2
2688
February1st to 15thNight
48%311%21%12%1
2179
16th to endNight
44%39%22%17%2
1893
March1st to 15thNight
48%39%20%13%3
2145
16th to endNight
45%27%19%20%4
2046
April1st to 15thNight
47%47%20%17%2
1796
16th to endNight
47%37%20%16%3
1662
May1st to 15thNight
50%310%17%14%3
1620
16th to endNight
47%5%15%19%9%2
1561
June1st to 15thNight
50%5%13%19%10%
1440
16th to endNight
40%5%13%23%16%1
1440
July1st to 15thNight
38%6%13%26%14%
1360
16th to endNight
41%6%12%27%9%2
1418
August1st to 15thNight
37%6%15%23%15%2
1597
16th to endNight
34%5%11%28%19%
1857
September1st to 15thNight
36%412%25%19%1
1872
16th to endNight
34%37%25%26%2
1858
October1st to 15thNight
42%310%23%17%3
1832
16th to endNight
43%413%22%14%1
2288
November1st to 15thNight
44%311%22%15%2
2277
16th to endNight
43%312%23%14%2
2340
December1st to 15thNight
47%313%22%12%
2475
16th to endNight
48%312%21%12%
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
20%28%35%13%3
60
16th to endDay
23%21%42%12%
64
February1st to 15thDay
24%17%42%14%2
242
16th to endDay
13%23%42%16%4
239
March1st to 15thDay
22%22%36%13%4
270
16th to endDay
20%20%38%11%8%
288
April1st to 15thDay
22%14%35%20%7%
270
16th to endDay
21%9%36%25%42
270
May1st to 15thDay
32%14%26%18%5%3
288
16th to endDay
20%6%35%28%7%1
331
June1st to 15thDay
26%7%31%24%8%1
315
16th to endDay
13%6%40%29%8%2
315
July1st to 15thDay
19%226%34%15%1
305
16th to endDay
13%323%41%15%2
326
August1st to 15thDay
14%328%35%15%2
313
16th to endDay
14%321%33%21%5%
332
September1st to 15thDay
15%7%24%28%17%4
315
16th to endDay
20%425%24%13%13%
266
October1st to 15thDay
24%9%24%22%14%5%
252
16th to endDay
17%13%37%24%7%
240
November1st to 15thDay
25%12%30%17%11%2
235
16th to endDay
18%20%33%15%10%1
240
December1st to 15thDay
22%20%37%14%5%
105
16th to endDay
26%18%42%9%3
64
January1st to 15thNight
21%19%40%15%3
720
16th to endNight
23%18%36%17%3
768
February1st to 15thNight
21%18%39%19%1
704
16th to endNight
22%18%38%17%3
631
March1st to 15thNight
22%17%37%19%3
720
16th to endNight
22%15%35%20%6%
768
April1st to 15thNight
21%13%39%21%4
718
16th to endNight
22%11%36%23%5%
582
May1st to 15thNight
25%7%36%21%7%
540
16th to endNight
20%7%37%26%7%
439
June1st to 15thNight
24%5%29%32%7%
360
16th to endNight
14%433%37%10%
360
July1st to 15thNight
12%231%42%11%
340
16th to endNight
12%128%46%10%
376
August1st to 15thNight
10%329%43%12%
527
16th to endNight
15%218%40%19%2
711
September1st to 15thNight
14%228%38%14%1
720
16th to endNight
18%325%28%20%4
673
October1st to 15thNight
22%8%28%25%11%2
660
16th to endNight
21%13%37%20%6%
704
November1st to 15thNight
20%14%31%22%10%1
700
16th to endNight
20%18%29%22%7%1
720
December1st to 15thNight
22%17%36%18%4
720
16th to endNight
24%18%38%15%2
768
Page generated at 2017-09-20 04:49:51. Version ac04

Copyright 2017 A. Danko


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