Green Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Green Mountain Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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%
22754
11%
74987
7%
22711
32%
22711
13%
56059

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 10thNight
11%
760
6%
2649
3%
757
16%
757
5%
2224
January11th to 20thNight
12%
760
6%
2660
2%
760
17%
760
6%
2187
January21st to endNight
17%
835
9%
2919
4%
835
24%
835
8%
2282
February1st to 10thNight
13%
720
7%
2372
3%
720
19%
720
7%
1946
February11th to 20thNight
14%
703
9%
2126
3%
700
21%
700
9%
1687
February21st to endNight
18%
592
13%
1776
5%
592
28%
592
11%
1368
March1st to 10thNight
19%
720
12%
2160
8%
720
26%
720
12%
1677
March11th to 20thNight
20%
720
13%
2107
6%
720
28%
720
11%
1606
March21st to endNight
21%
792
17%
1980
5%
792
29%
792
17%
1492
April1st to 10thNight
22%
720
14%
1800
4%
720
27%
720
15%
1419
April11th to 20thNight
23%
711
12%
1775
5%
711
27%
711
16%
1375
April21st to endNight
24%
571
14%
1651
5%
571
30%
571
16%
1220
May1st to 10thNight
29%
496
14%
1576
10%
496
34%
496
15%
1136
May11th to 20thNight
30%
354
17%
1416
14%
354
41%
354
17%
916
May21st to endNight
36%
388
12%
1551
11%
388
46%
388
18%
991
June1st to 10thNight
31%
360
10%
1440
10%
360
39%
360
14%
1021
June11th to 20thNight
38%
360
11%
1440
13%
360
46%
360
18%
928
June21st to endNight
41%
360
9%
1440
13%
360
44%
360
15%
983
July1st to 10thNight
48%
350
12%
1400
16%
350
54%
350
21%
856
July11th to 20thNight
52%
348
13%
1392
19%
348
56%
348
21%
849
July21st to endNight
52%
394
14%
1486
18%
372
55%
372
21%
923
August1st to 10thNight
56%
358
17%
1422
18%
356
60%
356
23%
854
August11th to 20thNight
49%
589
16%
1653
10%
589
51%
589
26%
1128
August21st to endNight
47%
792
18%
1980
9%
792
49%
792
28%
1356
September1st to 10thNight
44%
720
13%
1800
6%
720
46%
720
29%
1223
September11th to 20thNight
46%
720
20%
1903
11%
720
51%
720
30%
1220
September21st to endNight
40%
720
20%
1980
14%
720
44%
720
25%
1339
October1st to 10thNight
32%
720
13%
2050
8%
720
37%
720
17%
1506
October11th to 20thNight
30%
720
13%
2209
9%
720
36%
720
15%
1644
October21st to endNight
19%
792
10%
2574
5%
792
27%
792
10%
2025
November1st to 10thNight
21%
740
13%
2407
7%
740
29%
740
13%
1771
November11th to 20thNight
21%
760
15%
2470
11%
760
27%
760
12%
1861
November21st to endNight
16%
760
9%
2645
5%
760
21%
760
8%
2105
December1st to 10thNight
12%
760
4%
2660
2%
760
15%
760
5%
2283
December11th to 20thNight
14%
760
7%
3259
3%
759
18%
759
5%
2641
December21st to endNight
12%
829
4%
4859
2%
817
16%
817
3%
4017

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%11%47%6%35%45%6%30%
74914
Night
19%12%5%8%6%34446%28%
75212
TransparencyDay
53%7%12%18%9%1
74619
Night
57%6%11%17%8%1
75041
SeeingDay
26%17%30%18%7%2
13640
Night
25%15%27%23%8%1
22762

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 10thDay
15%17%8%10%8%46%445%17%
1517
11th to 20thDay
17%13%6%12%7%46%445%22%
1520
21st to endDay
12%15%6%12%7%47%5%46%20%
1664
February1st to 10thDay
14%14%6%13%9%4446%6%19%
1584
11th to 20thDay
12%16%7%12%8%45%45%5%22%
1721
21st to endDay
15%15%7%12%8%45%33424%
1480
March1st to 10thDay
15%16%6%10%8%45%445%24%
1797
11th to 20thDay
18%14%6%9%7%3434427%
1849
21st to endDay
19%14%6%8%6%2443431%
2376
April1st to 10thDay
22%13%47%6%24446%29%
2160
11th to 20thDay
19%11%46%5%34446%34%
2134
21st to endDay
25%12%46%5%25%346%29%
2308
May1st to 10thDay
22%10%46%6%345%46%31%
2399
11th to 20thDay
18%9%35%4345%5%7%37%
2478
21st to endDay
18%7%35%6%345%7%8%34%
2725
June1st to 10thDay
25%6%35%5%35%5%6%9%27%
2520
11th to 20thDay
19%6%26%5%45%5%6%9%34%
2520
21st to endDay
21%8%36%7%36%5%7%7%29%
2520
July1st to 10thDay
15%6%26%6%35%5%6%8%39%
2520
11th to 20thDay
13%6%35%6%45%7%7%9%35%
2520
21st to endDay
16%5%25%6%45%6%7%9%35%
2744
August1st to 10thDay
13%6%36%5%46%6%6%9%37%
2506
11th to 20thDay
15%6%36%6%45%5%6%9%36%
2226
21st to endDay
13%7%35%6%445%6%8%39%
2376
September1st to 10thDay
18%6%2443335%7%45%
2160
11th to 20thDay
16%7%2342435%7%47%
2056
21st to endDay
16%8%35%435%6%6%7%39%
1980
October1st to 10thDay
25%10%46%5%34446%30%
1908
11th to 20thDay
18%12%36%6%3446%6%31%
1751
21st to endDay
27%14%46%6%3444424%
1782
November1st to 10thDay
18%13%48%6%35%446%31%
1671
11th to 20thDay
20%14%5%9%6%34445%25%
1710
21st to endDay
19%14%6%10%6%345%6%5%23%
1534
December1st to 10thDay
21%18%6%13%6%4434417%
1520
11th to 20thDay
20%16%6%10%6%3445%5%21%
1906
21st to endDay
23%16%6%11%8%4434415%
2772
January1st to 10thNight
16%18%8%15%9%45%44414%
2649
11th to 20thNight
16%16%8%12%8%46%44417%
2660
21st to endNight
12%15%7%12%8%5%6%5%45%21%
2919
February1st to 10thNight
16%15%7%14%9%4444419%
2372
11th to 20thNight
14%15%7%13%9%4444421%
2124
21st to endNight
14%16%7%10%7%35%445%25%
1776
March1st to 10thNight
16%18%6%9%7%3433426%
2150
11th to 20thNight
18%14%7%9%6%4334429%
2106
21st to endNight
20%14%48%5%33245%32%
1980
April1st to 10thNight
23%14%47%5%34445%27%
1800
11th to 20thNight
24%12%36%5%33335%31%
1775
21st to endNight
23%12%46%5%24445%32%
1651
May1st to 10thNight
25%10%35%5%245%5%7%30%
1576
11th to 20thNight
19%10%25%5%34447%36%
1415
21st to endNight
18%7%25%445%5%5%9%37%
1550
June1st to 10thNight
27%7%25%5%35%5%47%31%
1440
11th to 20thNight
20%5%35%6%245%6%9%36%
1440
21st to endNight
21%7%26%6%35%5%6%7%32%
1440
July1st to 10thNight
17%424436%7%5%9%40%
1440
11th to 20thNight
14%425%6%36%5%7%9%39%
1440
21st to endNight
15%6%245%35%5%7%9%38%
1574
August1st to 10thNight
13%6%25%6%35%5%6%8%42%
1432
11th to 20thNight
15%6%35%5%445%6%8%40%
1653
21st to endNight
15%6%35%4345%6%8%41%
1980
September1st to 10thNight
16%6%35%5%3435%7%44%
1800
11th to 20thNight
16%6%34424447%47%
1903
21st to endNight
18%7%25%44345%6%42%
1980
October1st to 10thNight
25%9%25%5%45%45%5%32%
2050
11th to 20thNight
23%11%36%6%3445%6%30%
2209
21st to endNight
28%14%36%5%34445%24%
2574
November1st to 10thNight
21%13%47%5%24445%30%
2407
11th to 20thNight
20%14%6%9%6%24345%27%
2470
21st to endNight
19%17%7%10%6%3445%5%21%
2645
December1st to 10thNight
20%22%8%11%7%45%33414%
2660
11th to 20thNight
23%15%6%10%7%4443419%
3288
21st to endNight
23%17%7%13%7%3443415%
4884

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 10thDay
68%6%11%12%4
1518
11th to 20thDay
63%7%11%11%7%1
1520
21st to endDay
63%6%13%10%7%1
1664
February1st to 10thDay
63%8%9%13%5%2
1584
11th to 20thDay
64%410%15%6%
1730
21st to endDay
64%5%10%10%10%1
1480
March1st to 10thDay
62%412%13%7%2
1800
11th to 20thDay
61%410%12%11%2
1849
21st to endDay
57%49%16%11%3
2376
April1st to 10thDay
57%49%19%9%2
2160
11th to 20thDay
52%5%9%18%15%1
2134
21st to endDay
57%5%10%16%10%2
2308
May1st to 10thDay
55%5%8%19%12%2
2399
11th to 20thDay
47%6%10%19%16%2
2482
21st to endDay
46%9%15%19%10%1
2725
June1st to 10thDay
52%8%14%16%9%
2520
11th to 20thDay
46%7%11%21%13%2
2520
21st to endDay
52%8%12%18%9%1
2520
July1st to 10thDay
41%7%14%25%12%1
2450
11th to 20thDay
42%10%14%25%9%
2445
21st to endDay
43%10%15%22%9%
2591
August1st to 10thDay
41%10%17%23%8%
2492
11th to 20thDay
44%7%17%23%9%
2226
21st to endDay
41%8%13%25%12%1
2376
September1st to 10thDay
39%7%14%28%12%1
2160
11th to 20thDay
38%5%12%29%15%1
2056
21st to endDay
42%8%17%21%10%2
1980
October1st to 10thDay
55%6%10%19%9%1
1908
11th to 20thDay
51%8%16%19%6%
1751
21st to endDay
64%6%9%13%7%1
1782
November1st to 10thDay
54%7%13%17%8%
1671
11th to 20thDay
60%6%13%12%8%2
1710
21st to endDay
61%7%16%14%3
1534
December1st to 10thDay
70%7%10%10%2
1520
11th to 20thDay
65%5%12%13%5%
1906
21st to endDay
69%9%12%7%3
2772
January1st to 10thNight
72%5%11%8%3
2649
11th to 20thNight
70%6%10%10%4
2660
21st to endNight
63%6%11%12%7%1
2919
February1st to 10thNight
68%6%10%9%6%1
2372
11th to 20thNight
65%49%14%7%1
2126
21st to endNight
61%5%10%15%7%1
1776
March1st to 10thNight
63%310%15%8%1
2160
11th to 20thNight
60%5%8%17%8%2
2107
21st to endNight
55%49%17%12%3
1980
April1st to 10thNight
60%5%8%17%10%1
1800
11th to 20thNight
57%6%8%17%12%1
1775
21st to endNight
55%5%10%15%11%3
1651
May1st to 10thNight
53%6%8%16%14%2
1576
11th to 20thNight
47%5%10%20%15%2
1416
21st to endNight
45%7%15%19%13%1
1551
June1st to 10thNight
53%7%11%18%9%1
1440
11th to 20thNight
45%7%14%21%10%2
1440
21st to endNight
48%9%13%20%9%1
1440
July1st to 10thNight
38%10%16%21%14%1
1400
11th to 20thNight
38%10%16%25%11%
1392
21st to endNight
39%10%15%24%11%1
1486
August1st to 10thNight
37%9%17%27%10%
1422
11th to 20thNight
42%8%15%24%11%1
1653
21st to endNight
40%8%14%26%12%
1980
September1st to 10thNight
40%7%15%27%11%1
1800
11th to 20thNight
39%5%14%30%12%
1903
21st to endNight
42%7%13%24%12%2
1980
October1st to 10thNight
53%6%11%19%10%1
2050
11th to 20thNight
55%7%12%20%6%
2209
21st to endNight
63%6%10%13%7%1
2574
November1st to 10thNight
56%6%12%17%8%1
2407
11th to 20thNight
59%6%11%15%7%2
2470
21st to endNight
64%7%12%13%3
2645
December1st to 10thNight
75%5%9%9%1
2660
11th to 20thNight
68%6%10%11%5%
3288
21st to endNight
72%7%9%11%2
4884

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 10thDay
27%26%27%16%3
180
11th to 20thDay
33%24%28%13%11
180
21st to endDay
22%29%34%11%4
196
February1st to 10thDay
33%37%24%5%1
224
11th to 20thDay
32%25%31%8%3
389
21st to endDay
31%23%34%10%2
346
March1st to 10thDay
34%20%28%17%2
420
11th to 20thDay
31%24%26%16%3
420
21st to endDay
34%20%26%12%5%2
462
April1st to 10thDay
36%18%26%14%5%1
420
11th to 20thDay
33%21%23%15%6%2
412
21st to endDay
34%20%26%13%5%2
420
May1st to 10thDay
33%17%26%14%7%2
423
11th to 20thDay
22%17%35%19%5%2
447
21st to endDay
21%16%34%21%7%1
485
June1st to 10thDay
31%15%32%17%5%1
450
11th to 20thDay
23%13%35%20%7%2
450
21st to endDay
24%15%34%20%7%1
450
July1st to 10thDay
18%10%32%27%11%2
445
11th to 20thDay
14%11%36%25%12%1
444
21st to endDay
17%7%38%29%7%1
488
August1st to 10thDay
15%11%31%27%14%2
446
11th to 20thDay
18%7%32%25%14%5%
439
21st to endDay
14%15%30%25%12%4
495
September1st to 10thDay
18%14%30%20%12%6%
450
11th to 20thDay
17%15%23%21%17%7%
450
21st to endDay
19%13%31%20%12%5%
450
October1st to 10thDay
28%15%26%15%13%4
450
11th to 20thDay
26%19%27%20%7%1
426
21st to endDay
38%17%25%18%3
396
November1st to 10thDay
31%18%27%19%5%1
365
11th to 20thDay
31%25%24%14%41
370
21st to endDay
34%18%32%15%1
194
December1st to 10thDay
31%22%31%13%4
180
11th to 20thDay
32%28%29%10%1
180
21st to endDay
33%25%29%10%3
198
January1st to 10thNight
26%23%33%16%2
760
11th to 20thNight
28%23%30%16%4
760
21st to endNight
19%20%34%22%4
835
February1st to 10thNight
25%18%36%19%2
720
11th to 20thNight
23%22%36%17%2
703
21st to endNight
27%20%30%19%4
592
March1st to 10thNight
27%17%30%24%2
720
11th to 20thNight
27%22%25%21%5%
720
21st to endNight
30%16%27%19%7%1
792
April1st to 10thNight
30%16%27%22%4
720
11th to 20thNight
32%18%24%19%8%1
711
21st to endNight
30%15%27%22%6%
571
May1st to 10thNight
30%13%24%25%8%
496
11th to 20thNight
21%9%34%26%8%1
354
21st to endNight
21%8%29%31%10%1
389
June1st to 10thNight
28%8%28%27%9%1
360
11th to 20thNight
21%7%26%35%11%
360
21st to endNight
22%5%25%37%9%1
360
July1st to 10thNight
14%5%27%40%13%1
350
11th to 20thNight
13%326%39%18%1
348
21st to endNight
15%5%22%46%12%
394
August1st to 10thNight
16%418%42%19%1
358
11th to 20thNight
16%5%22%36%19%3
589
21st to endNight
16%6%24%32%20%2
792
September1st to 10thNight
18%9%24%32%16%2
720
11th to 20thNight
20%8%21%30%19%3
720
21st to endNight
19%10%27%25%16%2
720
October1st to 10thNight
29%10%23%22%15%1
720
11th to 20thNight
28%13%24%25%9%1
720
21st to endNight
35%13%28%20%4
792
November1st to 10thNight
28%17%29%17%8%1
740
11th to 20thNight
26%20%28%17%8%1
760
21st to endNight
29%21%28%16%5%1
760
December1st to 10thNight
30%24%26%16%3
760
11th to 20thNight
28%22%28%19%3
760
21st to endNight
32%21%30%14%3
836
Page generated at 2024-01-28 05:26:34. Version ac07

Copyright 2024 Attilla Danko


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