Gorham Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gorham Observatory clear sky chart from 2005-11-05 to 2017-04-26.

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%
14419
18%
45578
9%
14388
42%
14388
21%
29679

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
19%
719
19%
2506
7%
716
36%
716
18%
1585
January16th to endNight
18%
768
19%
2688
5%
768
36%
768
16%
1712
February1st to 15thNight
17%
704
16%
2237
4%
701
32%
701
16%
1526
February16th to endNight
23%
631
21%
1893
8%
631
39%
631
19%
1266
March1st to 15thNight
19%
720
17%
2133
6%
720
32%
720
18%
1472
March16th to endNight
26%
768
23%
2066
8%
768
38%
768
24%
1362
April1st to 15thNight
26%
718
19%
1796
5%
718
34%
718
23%
1272
April16th to endNight
29%
537
17%
1593
7%
537
36%
537
21%
1082
May1st to 15thNight
32%
495
16%
1485
8%
495
40%
495
21%
1002
May16th to endNight
32%
381
10%
1407
9%
380
38%
380
13%
1003
June1st to 15thNight
34%
330
10%
1320
9%
330
42%
330
12%
916
June16th to endNight
50%
330
9%
1320
11%
330
54%
330
21%
800
July1st to 15thNight
54%
310
12%
1240
18%
310
57%
310
22%
763
July16th to endNight
55%
348
14%
1306
16%
326
59%
326
23%
767
August1st to 15thNight
53%
469
17%
1450
14%
467
56%
467
28%
918
August16th to endNight
50%
602
20%
1652
11%
602
54%
602
33%
1025
September1st to 15thNight
48%
660
20%
1725
11%
660
52%
660
35%
1095
September16th to endNight
48%
657
27%
1815
16%
657
56%
657
38%
1043
October1st to 15thNight
36%
660
22%
1854
11%
660
45%
660
26%
1214
October16th to endNight
25%
704
18%
2285
8%
704
38%
704
21%
1482
November1st to 15thNight
31%
700
22%
2277
11%
700
46%
700
23%
1425
November16th to endNight
28%
720
20%
2340
9%
720
42%
720
19%
1494
December1st to 15thNight
18%
720
18%
2502
5%
720
34%
720
17%
1656
December16th to endNight
17%
768
18%
2688
5%
768
34%
768
16%
1799

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
18%6%24424446%39%
46088
Night
18%6%24424446%40%
45748
Cloud CoverDay
18%6%2442445%6%39%
46098
Night
18%6%24424446%40%
45753
TransparencyDay
44%412%22%14%2
45776
Night
44%411%23%14%1
45578
SeeingDay
21%11%29%22%10%3
5323
Night
22%10%30%26%9%1
14420

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%345%23345%43%
1432
16th to endDay
15%7%244245%46%40%
1536
February1st to 15thDay
17%7%35%5%345%5%6%35%
1619
16th to endDay
13%9%46%5%33445%37%
1580
March1st to 15thDay
20%8%36%5%24345%35%
1817
16th to endDay
16%7%35%424345%42%
2157
April1st to 15thDay
18%7%34424446%39%
2152
16th to endDay
19%5%25%423446%40%
2279
May1st to 15thDay
23%6%46%423447%32%
2137
16th to endDay
20%5%35%4245%5%8%34%
2363
June1st to 15thDay
27%6%2442445%6%31%
2310
16th to endDay
19%5%35%425%5%5%8%35%
2310
July1st to 15thDay
13%5%25%5%36%5%6%9%36%
2310
16th to endDay
13%6%25%445%6%6%8%37%
2458
August1st to 15thDay
14%425%5%25%46%9%37%
2157
16th to endDay
11%41443445%8%44%
2195
September1st to 15thDay
14%423424447%46%
1904
16th to endDay
14%413313245%55%
1815
October1st to 15thDay
19%7%25%424335%41%
1775
16th to endDay
19%6%23423345%42%
1587
November1st to 15thDay
20%5%23313446%43%
1581
16th to endDay
21%5%25%42343443%
1620
December1st to 15thDay
21%7%345%23335%38%
1458
16th to endDay
23%11%3432335%434%
1536
January1st to 15thNight
15%9%2442445%6%40%
2506
16th to endNight
16%8%35%42345%5%39%
2688
February1st to 15thNight
13%9%36%6%36%5%45%35%
2236
16th to endNight
16%8%35%434345%39%
1893
March1st to 15thNight
19%8%345%34345%37%
2120
16th to endNight
17%7%23324336%44%
2066
April1st to 15thNight
18%7%34435%446%37%
1796
16th to endNight
21%6%24425%436%38%
1593
May1st to 15thNight
19%6%244245%6%7%36%
1485
16th to endNight
22%7%36%435%5%36%30%
1405
June1st to 15thNight
26%5%35%5%34446%29%
1320
16th to endNight
20%5%2342445%8%40%
1320
July1st to 15thNight
15%4245%46%5%6%8%37%
1320
16th to endNight
13%425%425%47%10%39%
1402
August1st to 15thNight
13%5%145%245%6%8%40%
1460
16th to endNight
13%42442345%8%45%
1652
September1st to 15thNight
15%423423436%50%
1725
16th to endNight
14%412212246%55%
1815
October1st to 15thNight
18%5%24433445%43%
1854
16th to endNight
21%6%24424346%39%
2285
November1st to 15thNight
18%5%2432445%5%44%
2277
16th to endNight
18%6%24325%345%42%
2340
December1st to 15thNight
21%8%34323335%38%
2502
16th to endNight
21%9%24424345%37%
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
15%8%245%23346%42%
1432
16th to endDay
15%8%344145%5%6%40%
1536
February1st to 15thDay
17%7%35%5%3445%6%35%
1619
16th to endDay
13%10%36%5%24346%37%
1580
March1st to 15thDay
20%8%36%5%23445%35%
1827
16th to endDay
16%7%35%423345%42%
2157
April1st to 15thDay
18%6%25%424446%39%
2152
16th to endDay
19%5%145%34346%40%
2279
May1st to 15thDay
23%5%36%424346%33%
2137
16th to endDay
20%5%25%4245%5%7%34%
2363
June1st to 15thDay
27%6%24424446%31%
2310
16th to endDay
19%5%24425%45%8%36%
2310
July1st to 15thDay
13%5%245%36%6%7%9%35%
2310
16th to endDay
13%5%24435%6%7%8%37%
2458
August1st to 15thDay
13%4245%35%5%6%9%38%
2157
16th to endDay
12%424335%5%5%8%45%
2195
September1st to 15thDay
14%324424447%46%
1904
16th to endDay
14%412313345%55%
1815
October1st to 15thDay
19%6%25%423435%41%
1775
16th to endDay
19%6%2343335%5%41%
1587
November1st to 15thDay
21%5%1331335%6%43%
1581
16th to endDay
21%5%25%32333444%
1620
December1st to 15thDay
21%8%35%42343439%
1458
16th to endDay
23%10%3432334434%
1536
January1st to 15thNight
15%8%2442445%6%40%
2506
16th to endNight
16%8%35%42445%5%40%
2688
February1st to 15thNight
13%9%26%5%36%445%35%
2229
16th to endNight
16%8%35%5%25%345%39%
1893
March1st to 15thNight
20%8%35%424445%36%
2133
16th to endNight
17%7%23414436%44%
2065
April1st to 15thNight
19%7%35%434445%38%
1796
16th to endNight
21%6%24324435%39%
1593
May1st to 15thNight
20%5%3342446%8%35%
1485
16th to endNight
22%7%46%436%435%30%
1405
June1st to 15thNight
26%5%35%5%25%35%6%29%
1320
16th to endNight
20%6%2342336%7%40%
1320
July1st to 15thNight
15%5%25%5%45%5%6%8%37%
1320
16th to endNight
13%425%5%245%8%9%38%
1402
August1st to 15thNight
13%5%145%345%6%8%40%
1460
16th to endNight
13%5%2442335%8%45%
1652
September1st to 15thNight
15%423423336%49%
1725
16th to endNight
14%412312336%55%
1815
October1st to 15thNight
17%6%2442435%5%43%
1854
16th to endNight
21%5%24423445%40%
2285
November1st to 15thNight
18%5%24333445%44%
2277
16th to endNight
18%5%24334345%42%
2340
December1st to 15thNight
22%8%35%32433439%
2502
16th to endNight
21%9%24423445%37%
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
43%310%24%15%2
1433
16th to endDay
42%213%26%13%1
1536
February1st to 15thDay
47%412%22%12%1
1622
16th to endDay
48%210%21%13%3
1580
March1st to 15thDay
50%212%17%13%2
1827
16th to endDay
43%37%21%18%5%
2157
April1st to 15thDay
45%38%20%17%3
2152
16th to endDay
43%310%21%17%3
2279
May1st to 15thDay
51%28%16%18%2
2137
16th to endDay
46%5%13%20%10%2
2367
June1st to 15thDay
51%413%20%8%1
2310
16th to endDay
45%412%22%13%1
2310
July1st to 15thDay
40%5%15%26%11%
2161
16th to endDay
40%6%16%23%12%1
2291
August1st to 15thDay
40%5%18%23%10%
2143
16th to endDay
34%415%27%15%1
2195
September1st to 15thDay
36%410%28%19%
1904
16th to endDay
32%48%29%23%2
1815
October1st to 15thDay
45%29%24%15%2
1775
16th to endDay
43%410%26%13%1
1587
November1st to 15thDay
41%414%24%14%1
1581
16th to endDay
44%313%19%17%2
1620
December1st to 15thDay
48%311%20%14%
1458
16th to endDay
52%410%20%11%1
1536
January1st to 15thNight
43%411%23%14%2
2506
16th to endNight
46%39%22%15%2
2688
February1st to 15thNight
49%311%20%13%1
2237
16th to endNight
47%310%22%13%2
1893
March1st to 15thNight
50%38%21%13%2
2133
16th to endNight
41%38%19%19%6%
2066
April1st to 15thNight
47%36%20%18%2
1796
16th to endNight
48%38%20%14%4
1593
May1st to 15thNight
46%410%19%15%3
1485
16th to endNight
55%5%10%17%9%1
1407
June1st to 15thNight
54%5%10%19%8%1
1320
16th to endNight
42%5%14%22%13%1
1320
July1st to 15thNight
41%6%15%24%11%
1240
16th to endNight
38%6%15%27%11%
1306
August1st to 15thNight
39%5%14%26%12%1
1450
16th to endNight
36%413%28%16%
1652
September1st to 15thNight
36%411%30%16%
1725
16th to endNight
30%38%32%22%1
1815
October1st to 15thNight
42%310%24%16%2
1854
16th to endNight
45%310%26%12%1
2285
November1st to 15thNight
42%312%23%16%2
2277
16th to endNight
44%312%21%15%1
2340
December1st to 15thNight
49%211%23%12%
2502
16th to endNight
50%311%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%21%26%30%1
60
16th to endDay
17%23%35%20%3
64
February1st to 15thDay
26%13%36%20%2
223
16th to endDay
18%17%34%22%6%
239
March1st to 15thDay
27%16%33%17%31
270
16th to endDay
22%18%33%13%9%3
288
April1st to 15thDay
27%12%31%18%7%1
270
16th to endDay
24%12%35%16%8%2
254
May1st to 15thDay
31%9%30%17%7%3
228
16th to endDay
23%10%32%22%7%2
267
June1st to 15thDay
23%16%27%22%8%1
255
16th to endDay
20%5%34%25%12%1
255
July1st to 15thDay
16%428%28%15%6%
245
16th to endDay
15%330%31%15%4
270
August1st to 15thDay
16%5%23%30%19%3
253
16th to endDay
15%5%19%30%19%10%
268
September1st to 15thDay
14%5%24%28%18%8%
255
16th to endDay
17%7%30%19%11%13%
254
October1st to 15thDay
20%13%20%24%14%6%
248
16th to endDay
23%12%31%21%10%
240
November1st to 15thDay
23%11%26%25%10%2
235
16th to endDay
26%12%27%23%8%2
240
December1st to 15thDay
19%20%38%17%3
78
16th to endDay
37%25%26%9%1
64
January1st to 15thNight
20%16%37%20%4
720
16th to endNight
22%16%34%22%3
768
February1st to 15thNight
20%16%38%22%2
704
16th to endNight
22%12%34%24%5%
631
March1st to 15thNight
25%13%37%20%3
720
16th to endNight
23%13%32%23%6%
768
April1st to 15thNight
23%10%34%25%4
718
16th to endNight
24%11%29%24%9%
537
May1st to 15thNight
23%8%29%27%10%1
495
16th to endNight
27%7%25%28%9%1
381
June1st to 15thNight
29%5%20%31%12%
330
16th to endNight
23%118%40%16%
330
July1st to 15thNight
16%119%39%20%1
310
16th to endNight
15%17%44%19%1
348
August1st to 15thNight
13%119%41%21%1
469
16th to endNight
14%222%38%19%3
602
September1st to 15thNight
16%224%33%21%1
660
16th to endNight
17%425%28%19%5%
657
October1st to 15thNight
20%9%26%25%16%1
660
16th to endNight
25%10%32%24%6%
704
November1st to 15thNight
23%9%31%26%7%1
700
16th to endNight
21%16%27%25%8%
720
December1st to 15thNight
26%12%36%19%4
720
16th to endNight
27%13%37%18%2
768
Page generated at 2017-04-27 17:48:47. Version ac04

Copyright 2017 A. Danko


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