Berkshire School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Berkshire School Observatory clear sky chart from 2005-11-06 to 2024-01-17.

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
27%
22969
16%
74749
8%
22925
37%
22925
17%
50751

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
14%
760
16%
2649
5%
757
29%
757
11%
1825
January11th to 20thNight
15%
747
13%
2611
4%
747
27%
747
11%
1895
January21st to endNight
15%
792
17%
2716
5%
792
30%
792
11%
1841
February1st to 10thNight
13%
720
13%
2326
3%
720
25%
720
11%
1684
February11th to 20thNight
10%
703
15%
2100
3%
700
25%
700
11%
1528
February21st to endNight
14%
592
18%
1776
7%
592
28%
592
11%
1290
March1st to 10thNight
15%
720
18%
2160
7%
720
32%
720
14%
1530
March11th to 20thNight
15%
720
16%
2098
5%
720
26%
720
14%
1495
March21st to endNight
18%
792
21%
1980
5%
792
30%
792
16%
1392
April1st to 10thNight
18%
720
16%
1800
3%
720
26%
720
16%
1353
April11th to 20thNight
22%
711
15%
1775
2%
711
29%
711
19%
1278
April21st to endNight
20%
633
18%
1713
5%
633
29%
633
19%
1215
May1st to 10thNight
21%
540
13%
1620
6%
540
27%
540
14%
1193
May11th to 20thNight
24%
379
19%
1441
11%
379
37%
379
16%
928
May21st to endNight
39%
388
14%
1551
13%
388
47%
388
18%
964
June1st to 10thNight
36%
360
16%
1440
12%
360
48%
360
17%
901
June11th to 20thNight
39%
360
12%
1440
12%
360
47%
360
18%
922
June21st to endNight
44%
360
14%
1440
14%
360
53%
360
22%
840
July1st to 10thNight
50%
350
15%
1400
18%
350
57%
350
25%
786
July11th to 20thNight
52%
348
14%
1392
16%
348
57%
348
22%
819
July21st to endNight
52%
394
18%
1486
18%
372
59%
372
23%
863
August1st to 10thNight
55%
380
18%
1444
16%
378
61%
378
27%
831
August11th to 20thNight
47%
706
15%
1770
7%
706
50%
706
29%
1215
August21st to endNight
50%
792
20%
1980
9%
792
54%
792
32%
1299
September1st to 10thNight
41%
720
19%
1800
7%
720
47%
720
30%
1213
September11th to 20thNight
45%
720
26%
1898
15%
720
52%
720
34%
1163
September21st to endNight
39%
720
24%
1980
16%
720
46%
720
27%
1294
October1st to 10thNight
35%
720
17%
2047
11%
720
41%
720
21%
1408
October11th to 20thNight
32%
720
18%
2160
10%
720
42%
720
22%
1405
October21st to endNight
23%
792
14%
2574
7%
792
34%
792
14%
1807
November1st to 10thNight
29%
740
22%
2407
11%
740
43%
740
21%
1505
November11th to 20thNight
23%
760
20%
2470
10%
760
37%
760
16%
1668
November21st to endNight
24%
760
17%
2563
7%
760
37%
760
15%
1701
December1st to 10thNight
15%
760
13%
2660
4%
760
27%
760
10%
1896
December11th to 20thNight
17%
760
12%
3253
3%
759
28%
759
8%
2298
December21st to endNight
12%
830
10%
4829
3%
817
26%
817
5%
3506

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%8%46%6%35%5%6%7%35%
74805
Night
16%8%36%6%35%5%5%7%36%
75010
TransparencyDay
48%6%12%21%12%2
74510
Night
47%6%12%21%12%1
74839
SeeingDay
26%17%28%19%8%2
13796
Night
24%16%28%23%8%1
22976

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
12%10%46%6%46%6%6%7%31%
1517
11th to 20thDay
17%10%48%7%46%6%5%7%27%
1488
21st to endDay
12%8%5%7%8%47%6%5%7%32%
1638
February1st to 10thDay
17%11%48%8%35%6%46%27%
1632
11th to 20thDay
14%9%48%7%45%5%5%6%33%
1748
21st to endDay
17%12%5%7%7%3445%5%30%
1480
March1st to 10thDay
13%9%5%8%6%4445%7%34%
1797
11th to 20thDay
16%9%48%5%35%45%6%32%
1857
21st to endDay
19%10%47%6%3445%6%34%
2376
April1st to 10thDay
17%9%47%5%35%446%34%
2160
11th to 20thDay
14%8%36%6%445%46%39%
2134
21st to endDay
20%7%47%6%2445%6%34%
2245
May1st to 10thDay
22%9%46%6%345%5%6%31%
2340
11th to 20thDay
19%6%25%5%35%5%5%7%38%
2468
21st to endDay
14%6%45%5%35%5%6%8%38%
2725
June1st to 10thDay
17%6%35%6%46%6%6%9%33%
2520
11th to 20thDay
17%7%35%5%36%5%6%9%34%
2520
21st to endDay
16%7%37%6%36%5%6%9%33%
2520
July1st to 10thDay
12%6%46%6%36%5%6%8%38%
2520
11th to 20thDay
11%6%35%6%35%5%7%10%39%
2520
21st to endDay
12%6%35%6%46%6%7%8%37%
2744
August1st to 10thDay
11%6%26%6%36%6%8%9%37%
2467
11th to 20thDay
13%6%36%6%45%7%8%8%34%
2126
21st to endDay
13%6%35%5%35%6%5%8%41%
2376
September1st to 10thDay
12%6%36%6%35%5%6%7%39%
2160
11th to 20thDay
14%6%35%5%3445%6%47%
2061
21st to endDay
17%6%24435%5%6%8%39%
1980
October1st to 10thDay
23%7%36%5%33345%37%
1910
11th to 20thDay
15%7%37%5%345%6%6%40%
1800
21st to endDay
24%10%36%5%35%45%5%30%
1782
November1st to 10thDay
12%9%35%5%34346%45%
1671
11th to 20thDay
18%8%45%5%25%5%5%6%37%
1710
21st to endDay
17%9%36%5%35%5%5%6%36%
1615
December1st to 10thDay
19%9%6%7%6%35%5%5%6%29%
1520
11th to 20thDay
20%10%46%5%2445%6%35%
1906
21st to endDay
20%13%8%9%7%34446%22%
2772
January1st to 10thNight
14%10%48%7%45%46%5%34%
2649
11th to 20thNight
18%9%37%8%36%5%5%7%28%
2611
21st to endNight
11%10%47%7%46%46%7%33%
2716
February1st to 10thNight
18%8%46%7%45%6%5%6%30%
2326
11th to 20thNight
14%9%48%7%36%6%6%7%29%
2098
21st to endNight
18%8%47%8%346%5%5%32%
1776
March1st to 10thNight
16%9%37%6%35%5%5%7%34%
2150
11th to 20thNight
15%9%46%8%45%5%5%6%33%
2097
21st to endNight
17%8%37%6%34446%38%
1980
April1st to 10thNight
16%9%5%8%6%445%6%6%31%
1800
11th to 20thNight
18%9%36%6%3445%5%38%
1775
21st to endNight
20%9%36%5%33447%36%
1713
May1st to 10thNight
20%10%47%7%35%5%5%5%30%
1620
11th to 20thNight
17%9%35%5%3445%7%38%
1440
21st to endNight
16%5%26%6%345%7%6%40%
1550
June1st to 10thNight
17%7%35%4345%5%7%40%
1440
11th to 20thNight
16%7%26%6%25%6%6%10%34%
1440
21st to endNight
14%5%26%5%3446%8%43%
1440
July1st to 10thNight
14%425%4345%6%7%46%
1440
11th to 20thNight
14%425%5%36%5%7%10%39%
1440
21st to endNight
11%5%25%5%46%5%5%7%45%
1574
August1st to 10thNight
9%5%35%5%346%6%9%45%
1454
11th to 20thNight
10%7%36%6%45%7%7%8%37%
1770
21st to endNight
14%425%5%35%45%8%45%
1980
September1st to 10thNight
14%6%25%5%35%5%5%7%43%
1800
11th to 20thNight
14%5%23424336%53%
1898
21st to endNight
15%8%25%42445%6%45%
1980
October1st to 10thNight
20%8%35%5%34446%39%
2047
11th to 20thNight
15%7%25%5%45%5%6%7%40%
2160
21st to endNight
22%10%35%6%3435%6%33%
2574
November1st to 10thNight
14%8%35%5%3335%6%44%
2407
11th to 20thNight
18%6%46%5%36%45%6%36%
2470
21st to endNight
13%8%46%7%36%5%6%6%36%
2563
December1st to 10thNight
19%10%48%6%45%5%5%7%28%
2660
11th to 20thNight
19%9%47%7%45%5%5%6%30%
3288
21st to endNight
20%11%7%9%6%34446%25%
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
50%6%10%23%11%1
1518
11th to 20thDay
56%7%12%16%7%1
1488
21st to endDay
51%6%13%18%11%2
1638
February1st to 10thDay
57%6%11%15%10%1
1632
11th to 20thDay
52%6%8%20%13%2
1757
21st to endDay
56%48%18%12%1
1480
March1st to 10thDay
49%5%12%19%12%2
1800
11th to 20thDay
52%5%10%19%12%2
1857
21st to endDay
52%46%17%16%4
2376
April1st to 10thDay
51%5%8%22%12%2
2160
11th to 20thDay
46%5%9%22%16%2
2134
21st to endDay
50%49%19%13%4
2245
May1st to 10thDay
55%5%8%19%12%2
2340
11th to 20thDay
44%5%10%24%15%2
2472
21st to endDay
43%7%14%22%12%2
2725
June1st to 10thDay
47%6%15%21%10%1
2520
11th to 20thDay
47%5%14%22%11%1
2520
21st to endDay
47%6%14%22%11%1
2520
July1st to 10thDay
43%6%13%22%15%1
2450
11th to 20thDay
39%6%16%27%12%
2445
21st to endDay
42%6%16%20%14%2
2591
August1st to 10thDay
42%6%16%24%12%1
2453
11th to 20thDay
44%7%16%24%9%1
2126
21st to endDay
40%6%13%25%15%1
2376
September1st to 10thDay
43%6%11%25%13%1
2160
11th to 20thDay
39%5%10%24%22%2
2061
21st to endDay
41%6%11%24%15%2
1980
October1st to 10thDay
51%48%22%13%2
1910
11th to 20thDay
43%5%14%24%12%1
1800
21st to endDay
56%6%10%15%11%2
1782
November1st to 10thDay
40%6%14%24%14%3
1671
11th to 20thDay
47%6%10%21%15%2
1710
21st to endDay
48%5%13%23%9%2
1615
December1st to 10thDay
56%5%13%15%10%
1520
11th to 20thDay
51%6%13%21%9%1
1906
21st to endDay
62%5%9%14%8%
2772
January1st to 10thNight
51%5%10%19%12%2
2649
11th to 20thNight
55%6%14%16%8%2
2611
21st to endNight
48%6%10%20%14%2
2716
February1st to 10thNight
55%5%12%17%9%2
2326
11th to 20thNight
50%8%11%16%13%2
2100
21st to endNight
53%6%8%20%10%3
1776
March1st to 10thNight
49%6%10%21%12%2
2160
11th to 20thNight
51%6%10%19%11%2
2098
21st to endNight
47%49%19%17%4
1980
April1st to 10thNight
54%5%9%20%11%2
1800
11th to 20thNight
49%5%10%19%17%1
1775
21st to endNight
49%410%20%13%4
1713
May1st to 10thNight
56%5%8%17%13%2
1620
11th to 20thNight
47%5%10%21%15%2
1441
21st to endNight
43%7%17%20%12%1
1551
June1st to 10thNight
44%7%12%26%11%1
1440
11th to 20thNight
46%6%15%22%10%
1440
21st to endNight
38%6%17%26%13%
1440
July1st to 10thNight
35%6%16%27%14%2
1400
11th to 20thNight
39%7%17%27%10%
1392
21st to endNight
37%6%17%24%14%2
1486
August1st to 10thNight
35%7%18%29%11%1
1444
11th to 20thNight
40%8%18%24%9%1
1770
21st to endNight
39%5%12%28%16%1
1980
September1st to 10thNight
40%6%12%26%15%1
1800
11th to 20thNight
34%412%26%23%1
1898
21st to endNight
40%5%9%26%19%1
1980
October1st to 10thNight
46%6%11%22%13%2
2047
11th to 20thNight
42%5%15%25%12%1
2160
21st to endNight
52%412%19%11%1
2574
November1st to 10thNight
42%411%22%18%3
2407
11th to 20thNight
47%6%11%18%14%3
2470
21st to endNight
46%6%16%23%8%
2563
December1st to 10thNight
55%5%13%18%9%
2660
11th to 20thNight
53%8%13%14%11%1
3288
21st to endNight
59%7%9%17%9%
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
32%24%29%10%41
180
11th to 20thDay
38%27%25%9%11
172
21st to endDay
25%32%28%12%31
200
February1st to 10thDay
43%29%23%42
252
11th to 20thDay
36%27%27%9%2
416
21st to endDay
33%25%28%12%2
346
March1st to 10thDay
31%23%32%13%1
420
11th to 20thDay
32%24%29%13%21
420
21st to endDay
37%21%26%9%42
462
April1st to 10thDay
34%22%26%11%5%1
420
11th to 20thDay
29%27%24%12%6%2
412
21st to endDay
31%20%28%15%5%1
420
May1st to 10thDay
35%21%25%13%5%2
423
11th to 20thDay
24%19%33%19%41
447
21st to endDay
20%12%32%22%12%2
485
June1st to 10thDay
20%14%32%21%12%1
450
11th to 20thDay
21%13%33%22%10%1
450
21st to endDay
19%16%32%20%12%2
450
July1st to 10thDay
15%9%34%27%13%3
445
11th to 20thDay
15%6%30%33%14%3
444
21st to endDay
14%11%34%27%12%2
488
August1st to 10thDay
15%9%25%33%13%4
446
11th to 20thDay
15%8%29%27%13%8%
439
21st to endDay
15%11%30%26%13%6%
495
September1st to 10thDay
20%9%22%26%16%6%
450
11th to 20thDay
18%13%26%20%14%9%
450
21st to endDay
22%11%32%18%12%5%
450
October1st to 10thDay
30%10%26%17%12%4
450
11th to 20thDay
24%15%28%24%7%1
450
21st to endDay
35%15%25%18%6%1
396
November1st to 10thDay
24%18%26%22%10%1
365
11th to 20thDay
34%20%25%15%5%1
370
21st to endDay
31%21%26%19%21
275
December1st to 10thDay
28%29%28%11%4
180
11th to 20thDay
42%23%26%8%2
180
21st to endDay
31%23%31%14%2
198
January1st to 10thNight
27%23%30%17%3
760
11th to 20thNight
32%20%30%16%2
747
21st to endNight
25%23%30%18%3
792
February1st to 10thNight
29%21%30%18%2
720
11th to 20thNight
28%25%32%14%1
703
21st to endNight
32%23%27%14%3
592
March1st to 10thNight
28%22%31%16%2
720
11th to 20thNight
27%25%30%14%4
720
21st to endNight
28%22%26%17%6%
792
April1st to 10thNight
28%20%29%19%4
720
11th to 20thNight
26%20%28%20%6%
711
21st to endNight
27%19%30%19%41
633
May1st to 10thNight
29%12%31%21%6%
540
11th to 20thNight
23%16%33%24%4
379
21st to endNight
20%6%28%30%14%1
389
June1st to 10thNight
22%8%28%30%11%1
360
11th to 20thNight
20%8%26%31%14%
360
21st to endNight
15%8%26%39%12%1
360
July1st to 10thNight
17%323%37%19%1
350
11th to 20thNight
16%421%44%15%1
348
21st to endNight
13%6%22%40%18%
394
August1st to 10thNight
11%323%46%17%1
380
11th to 20thNight
15%324%33%22%2
706
21st to endNight
16%5%22%36%19%2
792
September1st to 10thNight
17%7%27%31%17%2
720
11th to 20thNight
17%8%24%29%17%5%
720
21st to endNight
21%9%24%30%13%2
720
October1st to 10thNight
25%11%22%28%12%2
720
11th to 20thNight
21%16%27%27%8%1
720
21st to endNight
30%17%27%21%5%
792
November1st to 10thNight
22%17%27%25%8%1
740
11th to 20thNight
26%19%27%20%6%1
760
21st to endNight
22%20%30%22%5%1
760
December1st to 10thNight
29%23%28%16%4
760
11th to 20thNight
31%21%28%17%3
760
21st to endNight
32%21%31%14%2
836
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