Girard Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Girard Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
26%
14679
11%
46879
7%
14648
31%
14648
13%
35647

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
13%
720
6%
2506
3%
716
16%
716
5%
2135
January16th to endNight
15%
768
6%
2688
3%
768
19%
768
6%
2228
February1st to 15thNight
12%
704
6%
2302
2%
701
17%
701
6%
1964
February16th to endNight
18%
631
8%
1893
5%
631
23%
631
9%
1556
March1st to 15thNight
21%
720
12%
2160
8%
720
27%
720
12%
1695
March16th to endNight
26%
768
17%
2010
8%
768
33%
768
17%
1511
April1st to 15thNight
21%
718
13%
1796
4%
718
27%
718
15%
1427
April16th to endNight
24%
594
16%
1674
6%
594
32%
594
19%
1192
May1st to 15thNight
26%
447
15%
1527
10%
447
34%
447
16%
1065
May16th to endNight
31%
374
11%
1497
10%
374
38%
374
14%
1031
June1st to 15thNight
30%
360
9%
1440
8%
360
38%
360
12%
1026
June16th to endNight
41%
360
8%
1440
11%
360
46%
360
17%
953
July1st to 15thNight
46%
340
11%
1360
14%
340
50%
340
20%
854
July16th to endNight
50%
376
11%
1418
15%
354
52%
354
19%
881
August1st to 15thNight
50%
404
12%
1475
13%
402
52%
402
20%
965
August16th to endNight
51%
722
18%
1868
10%
722
54%
722
32%
1216
September1st to 15thNight
45%
720
12%
1820
7%
720
46%
720
27%
1274
September16th to endNight
44%
681
19%
1880
14%
681
47%
681
29%
1219
October1st to 15thNight
29%
660
16%
1901
9%
660
36%
660
17%
1397
October16th to endNight
21%
704
8%
2282
5%
704
26%
704
10%
1809
November1st to 15thNight
22%
700
13%
2277
9%
700
28%
700
11%
1759
November16th to endNight
21%
720
11%
2457
8%
720
25%
720
9%
1951
December1st to 15thNight
12%
720
4%
2520
1%
720
15%
720
4%
2198
December16th to endNight
8%
768
6%
2688
2%
768
12%
768
4%
2341

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
21%11%47%5%34446%27%
47995
Night
20%12%5%7%5%34345%27%
47049
Cloud CoverDay
20%11%47%5%34446%26%
48005
Night
19%12%5%8%6%34345%27%
47054
TransparencyDay
58%5%11%16%7%
47683
Night
59%5%10%16%7%
46879
SeeingDay
28%13%26%18%9%2
5676
Night
26%14%26%22%9%
14680

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
19%20%7%13%7%45%33312%
1432
16th to endDay
12%16%6%12%7%5%7%5%5%5%15%
1536
February1st to 15thDay
13%17%8%14%7%5%6%43314%
1558
16th to endDay
14%16%8%11%9%45%33318%
1580
March1st to 15thDay
18%14%6%9%5%3433423%
1790
16th to endDay
20%13%5%8%5%3222431%
2211
April1st to 15thDay
23%14%47%423245%27%
2152
16th to endDay
23%10%25%5%24445%32%
2286
May1st to 15thDay
23%7%37%5%34446%29%
2433
16th to endDay
22%7%35%5%25%45%7%29%
2618
June1st to 15thDay
31%7%1442445%6%26%
2520
16th to endDay
23%6%25%5%35%5%5%8%28%
2520
July1st to 15thDay
17%5%35%6%25%6%6%8%31%
2520
16th to endDay
17%6%26%5%36%5%7%8%29%
2654
August1st to 15thDay
20%6%36%6%45%5%6%8%27%
2463
16th to endDay
15%6%245%335%5%7%39%
2333
September1st to 15thDay
21%6%25%323336%38%
2139
16th to endDay
19%5%33423448%41%
1873
October1st to 15thDay
26%9%35%5%24345%29%
1727
16th to endDay
28%16%36%42433321%
1590
November1st to 15thDay
20%15%36%423445%27%
1581
16th to endDay
19%18%6%9%42323323%
1503
December1st to 15thDay
23%20%7%8%6%3423314%
1440
16th to endDay
24%22%9%11%5%2432311%
1536
January1st to 15thNight
17%18%8%13%7%45%43314%
2506
16th to endNight
12%18%7%11%8%5%6%33318%
2688
February1st to 15thNight
14%16%8%16%8%4433415%
2301
16th to endNight
14%18%8%11%7%3432321%
1893
March1st to 15thNight
20%15%5%7%6%2333426%
2147
16th to endNight
17%15%5%6%42433433%
2010
April1st to 15thNight
24%13%5%6%5%3333427%
1796
16th to endNight
21%8%35%422336%36%
1674
May1st to 15thNight
22%7%26%425%346%33%
1527
16th to endNight
21%6%345%35%45%7%32%
1495
June1st to 15thNight
30%7%2432345%7%26%
1440
16th to endNight
18%6%24425%5%7%8%34%
1440
July1st to 15thNight
14%5%36%5%36%47%9%34%
1440
16th to endNight
14%425%5%346%7%6%38%
1514
August1st to 15thNight
16%5%345%3446%8%36%
1485
16th to endNight
14%5%24424348%45%
1868
September1st to 15thNight
20%5%25%433445%41%
1820
16th to endNight
20%41332335%6%45%
1880
October1st to 15thNight
23%9%26%5%23345%32%
1901
16th to endNight
30%13%25%5%2443423%
2282
November1st to 15thNight
22%13%46%5%35%346%24%
2277
16th to endNight
21%16%6%9%5%2333324%
2457
December1st to 15thNight
23%21%8%10%5%3433313%
2520
16th to endNight
22%22%8%9%6%3422313%
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
18%18%8%13%9%35%42312%
1432
16th to endDay
12%15%7%11%9%47%5%45%16%
1536
February1st to 15thDay
12%16%9%14%9%5%6%43313%
1558
16th to endDay
13%17%8%13%9%45%33317%
1580
March1st to 15thDay
16%17%5%9%7%35%43423%
1800
16th to endDay
20%13%5%8%5%3222430%
2211
April1st to 15thDay
21%16%5%7%423335%26%
2152
16th to endDay
22%11%35%424436%31%
2286
May1st to 15thDay
23%9%36%5%24346%29%
2433
16th to endDay
23%7%36%5%3445%7%28%
2618
June1st to 15thDay
31%6%2442445%6%26%
2520
16th to endDay
24%6%25%5%345%5%7%28%
2520
July1st to 15thDay
18%5%36%5%36%5%6%8%31%
2520
16th to endDay
18%7%25%6%36%5%5%8%29%
2654
August1st to 15thDay
21%7%26%6%45%5%5%8%26%
2463
16th to endDay
16%6%25%5%245%5%8%37%
2333
September1st to 15thDay
22%6%25%323446%38%
2139
16th to endDay
19%5%34314348%41%
1873
October1st to 15thDay
26%11%25%423445%28%
1727
16th to endDay
27%18%36%43433321%
1590
November1st to 15thDay
20%16%46%3235%46%25%
1581
16th to endDay
18%18%7%8%5%2424323%
1503
December1st to 15thDay
21%20%8%9%6%2433414%
1440
16th to endDay
21%22%9%11%6%3422311%
1536
January1st to 15thNight
15%17%8%13%7%36%44314%
2506
16th to endNight
11%16%8%12%8%5%7%5%3416%
2688
February1st to 15thNight
13%16%8%15%10%45%33315%
2294
16th to endNight
14%17%8%11%7%35%33420%
1893
March1st to 15thNight
20%14%5%8%6%3433425%
2160
16th to endNight
16%15%6%7%5%2323433%
2009
April1st to 15thNight
22%14%46%6%3333427%
1796
16th to endNight
22%8%35%423336%36%
1674
May1st to 15thNight
22%8%26%5%24346%33%
1527
16th to endNight
20%7%25%6%26%36%5%32%
1495
June1st to 15thNight
29%7%25%32345%7%26%
1440
16th to endNight
18%6%245%25%5%7%8%33%
1440
July1st to 15thNight
14%5%35%6%25%5%7%8%34%
1440
16th to endNight
13%5%246%26%6%6%7%38%
1514
August1st to 15thNight
17%5%25%3345%6%8%36%
1485
16th to endNight
13%6%14423449%44%
1868
September1st to 15thNight
20%6%14434345%41%
1820
16th to endNight
20%42341335%6%44%
1880
October1st to 15thNight
23%10%25%423345%33%
1901
16th to endNight
29%13%35%5%24335%22%
2282
November1st to 15thNight
19%14%47%6%34445%25%
2277
16th to endNight
19%16%6%11%6%2333323%
2457
December1st to 15thNight
21%22%8%10%6%3432313%
2520
16th to endNight
20%22%8%11%8%2423212%
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
77%37%8%2
1433
16th to endDay
67%6%10%10%3
1536
February1st to 15thDay
75%46%11%1
1561
16th to endDay
71%47%8%6%
1580
March1st to 15thDay
65%49%13%5%1
1800
16th to endDay
59%38%18%9%1
2211
April1st to 15thDay
61%37%16%9%1
2152
16th to endDay
54%48%17%11%3
2286
May1st to 15thDay
55%410%17%10%
2433
16th to endDay
54%7%11%17%8%
2622
June1st to 15thDay
57%6%12%16%6%
2520
16th to endDay
53%7%11%20%7%
2520
July1st to 15thDay
46%8%15%22%6%
2371
16th to endDay
50%8%14%20%5%
2487
August1st to 15thDay
53%7%16%17%4
2449
16th to endDay
43%6%15%23%10%
2333
September1st to 15thDay
46%5%12%24%10%
2139
16th to endDay
41%5%13%27%10%
1873
October1st to 15thDay
57%6%12%16%6%
1727
16th to endDay
67%311%12%4
1590
November1st to 15thDay
58%7%10%14%7%1
1581
16th to endDay
65%5%9%12%6%
1503
December1st to 15thDay
75%5%8%8%2
1440
16th to endDay
80%35%6%4
1536
January1st to 15thNight
73%5%8%8%2
2506
16th to endNight
70%5%7%11%4
2688
February1st to 15thNight
73%47%11%2
2302
16th to endNight
68%47%10%8%
1893
March1st to 15thNight
63%47%17%5%1
2160
16th to endNight
56%38%16%13%2
2010
April1st to 15thNight
62%37%14%12%
1796
16th to endNight
50%49%18%14%2
1674
May1st to 15thNight
51%5%7%17%15%2
1527
16th to endNight
51%7%11%19%8%1
1497
June1st to 15thNight
56%5%12%19%6%
1440
16th to endNight
44%9%13%21%9%
1440
July1st to 15thNight
42%7%16%24%9%
1360
16th to endNight
41%8%16%24%9%
1418
August1st to 15thNight
43%8%18%22%7%
1475
16th to endNight
37%6%17%26%11%
1868
September1st to 15thNight
44%5%13%25%9%
1820
16th to endNight
39%5%13%28%12%
1880
October1st to 15thNight
53%5%9%20%9%
1901
16th to endNight
63%6%9%14%5%
2282
November1st to 15thNight
61%5%10%14%6%2
2277
16th to endNight
66%49%14%5%
2457
December1st to 15thNight
76%5%6%8%1
2520
16th to endNight
78%36%7%3
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
28%15%38%10%8%
60
16th to endDay
35%20%29%9%31
64
February1st to 15thDay
28%29%26%13%2
122
16th to endDay
30%19%28%16%3
239
March1st to 15thDay
31%17%28%17%3
270
16th to endDay
29%17%26%12%10%3
288
April1st to 15thDay
33%17%23%19%41
270
16th to endDay
30%15%27%14%7%4
270
May1st to 15thDay
34%13%26%12%8%4
288
16th to endDay
23%17%30%20%7%1
331
June1st to 15thDay
37%8%28%17%8%
315
16th to endDay
25%10%36%20%5%1
315
July1st to 15thDay
21%9%31%26%8%2
305
16th to endDay
19%5%30%31%11%1
326
August1st to 15thDay
26%7%24%22%15%3
313
16th to endDay
21%9%20%21%18%8%
332
September1st to 15thDay
22%7%24%20%18%6%
315
16th to endDay
17%12%20%23%17%10%
275
October1st to 15thDay
32%14%19%18%12%3
255
16th to endDay
41%14%19%18%5%
241
November1st to 15thDay
30%14%30%15%7%2
235
16th to endDay
30%21%21%14%8%2
123
December1st to 15thDay
43%15%21%16%11
60
16th to endDay
43%28%25%3
64
January1st to 15thNight
25%22%30%16%3
720
16th to endNight
22%21%29%20%5%
768
February1st to 15thNight
23%20%33%19%3
704
16th to endNight
23%19%31%20%4
631
March1st to 15thNight
31%15%26%22%3
720
16th to endNight
25%18%25%20%9%
768
April1st to 15thNight
30%15%27%22%4
718
16th to endNight
27%17%27%18%9%
594
May1st to 15thNight
26%11%29%22%9%
447
16th to endNight
24%10%27%29%7%
375
June1st to 15thNight
35%5%22%26%9%
360
16th to endNight
18%6%26%34%12%
360
July1st to 15thNight
15%328%32%18%
340
16th to endNight
14%423%39%17%
376
August1st to 15thNight
19%5%18%33%21%1
404
16th to endNight
16%5%22%31%22%2
722
September1st to 15thNight
23%5%21%33%14%1
720
16th to endNight
23%6%24%23%20%1
681
October1st to 15thNight
28%11%24%21%12%1
660
16th to endNight
37%13%23%19%5%
704
November1st to 15thNight
26%16%28%19%7%1
700
16th to endNight
29%23%20%15%8%1
720
December1st to 15thNight
34%22%26%12%4
720
16th to endNight
32%20%26%15%3
768
Page generated at 2017-09-22 07:35:37. Version ac04

Copyright 2017 A. Danko


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