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 2005-11-05 to 2017-10-31.

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
25%
15084
16%
47358
7%
15053
35%
15053
15%
32130

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
15%
720
15%
2506
6%
716
27%
716
9%
1780
January16th to endNight
11%
768
15%
2651
4%
768
26%
768
9%
1847
February1st to 15thNight
12%
704
12%
2222
2%
701
23%
701
10%
1628
February16th to endNight
13%
631
18%
1893
5%
631
29%
631
11%
1366
March1st to 15thNight
13%
720
14%
2160
5%
720
25%
720
10%
1554
March16th to endNight
17%
768
21%
1998
5%
768
29%
768
14%
1372
April1st to 15thNight
15%
718
14%
1796
2%
718
22%
718
14%
1353
April16th to endNight
21%
661
17%
1741
3%
661
31%
661
17%
1205
May1st to 15thNight
18%
496
17%
1576
6%
496
29%
496
13%
1084
May16th to endNight
28%
374
12%
1497
9%
374
36%
374
13%
966
June1st to 15thNight
28%
360
12%
1440
7%
360
39%
360
14%
954
June16th to endNight
38%
360
12%
1440
13%
360
47%
360
21%
831
July1st to 15thNight
44%
340
13%
1360
14%
340
52%
340
21%
794
July16th to endNight
46%
376
15%
1418
14%
354
55%
354
20%
821
August1st to 15thNight
46%
491
16%
1562
10%
489
52%
489
27%
939
August16th to endNight
48%
764
20%
1910
9%
764
52%
764
33%
1234
September1st to 15thNight
39%
720
20%
1806
9%
720
44%
720
29%
1187
September16th to endNight
45%
717
27%
1980
17%
717
52%
717
30%
1211
October1st to 15thNight
32%
720
20%
2085
13%
720
41%
720
21%
1384
October16th to endNight
24%
768
15%
2434
7%
768
36%
768
15%
1676
November1st to 15thNight
24%
700
19%
2277
10%
700
37%
700
16%
1491
November16th to endNight
20%
720
16%
2398
7%
720
32%
720
13%
1651
December1st to 15thNight
16%
720
13%
2520
4%
720
27%
720
9%
1787
December16th to endNight
11%
768
13%
2688
3%
768
23%
768
6%
2015

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%7%36%5%35%45%6%34%
48397
Night
16%7%36%5%3445%6%36%
47528
Cloud CoverDay
16%7%35%5%35%45%7%35%
48407
Night
16%7%35%5%3445%6%37%
47533
TransparencyDay
48%5%11%20%12%1
48085
Night
47%5%12%21%12%1
47358
SeeingDay
21%22%24%18%9%3
5931
Night
20%27%22%20%7%
15085

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
14%7%38%6%46%6%6%5%30%
1432
16th to endDay
14%7%45%8%37%5%5%6%30%
1572
February1st to 15thDay
15%10%48%8%46%5%5%5%26%
1637
16th to endDay
14%9%48%7%25%445%32%
1580
March1st to 15thDay
16%8%48%5%3445%7%30%
1790
16th to endDay
17%8%46%5%34446%35%
2223
April1st to 15thDay
17%9%46%5%35%435%35%
2152
16th to endDay
16%6%36%5%2345%6%39%
2217
May1st to 15thDay
21%8%35%5%3445%5%31%
2385
16th to endDay
16%6%35%5%3446%7%35%
2618
June1st to 15thDay
21%6%245%35%5%5%8%30%
2520
16th to endDay
15%6%35%5%35%6%6%9%31%
2520
July1st to 15thDay
11%6%36%6%26%5%6%8%35%
2520
16th to endDay
11%5%35%5%36%6%6%8%36%
2654
August1st to 15thDay
14%6%25%6%35%5%7%8%34%
2373
16th to endDay
11%5%25%435%5%6%7%40%
2292
September1st to 15thDay
13%6%36%5%45%5%5%6%39%
2154
16th to endDay
14%5%1442346%6%46%
1980
October1st to 15thDay
20%8%35%5%23345%38%
1873
16th to endDay
19%7%27%5%2445%434%
1788
November1st to 15thDay
15%8%35%424447%39%
1581
16th to endDay
17%7%35%5%245%46%36%
1560
December1st to 15thDay
18%8%5%6%6%3445%6%31%
1440
16th to endDay
18%12%6%7%5%3445%5%24%
1536
January1st to 15thNight
16%8%37%7%35%45%6%30%
2506
16th to endNight
14%10%37%6%35%45%6%31%
2651
February1st to 15thNight
14%8%46%7%36%7%5%6%28%
2221
16th to endNight
15%9%36%7%345%5%6%32%
1893
March1st to 15thNight
17%8%35%6%3445%6%32%
2147
16th to endNight
16%6%36%5%33446%39%
1998
April1st to 15thNight
18%9%47%6%34345%33%
1796
16th to endNight
17%7%25%433446%39%
1741
May1st to 15thNight
18%9%36%5%2444436%
1576
16th to endNight
17%6%25%5%35%5%6%7%35%
1495
June1st to 15thNight
19%7%25%4246%5%6%35%
1440
16th to endNight
13%425%5%2346%8%43%
1440
July1st to 15thNight
12%4145%25%5%5%7%43%
1440
16th to endNight
12%4245%36%45%8%41%
1514
August1st to 15thNight
10%425%4246%6%8%44%
1572
16th to endNight
11%415%42445%7%45%
1910
September1st to 15thNight
14%4245%24446%46%
1806
16th to endNight
11%6%23324345%51%
1980
October1st to 15thNight
16%6%25%423446%41%
2085
16th to endNight
19%8%25%5%34345%35%
2434
November1st to 15thNight
16%8%35%424246%40%
2277
16th to endNight
15%8%46%6%26%5%5%6%32%
2398
December1st to 15thNight
20%9%36%6%35%446%29%
2520
16th to endNight
19%10%47%6%35%5%5%6%25%
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
14%7%46%6%36%6%5%6%32%
1432
16th to endDay
14%7%36%6%46%46%7%31%
1572
February1st to 15thDay
15%9%47%7%46%5%5%6%27%
1637
16th to endDay
14%9%48%6%25%445%33%
1580
March1st to 15thDay
16%8%37%6%34447%31%
1800
16th to endDay
16%9%35%6%33445%36%
2223
April1st to 15thDay
18%8%35%6%3443436%
2152
16th to endDay
16%6%26%5%335%46%40%
2217
May1st to 15thDay
22%6%35%6%34446%31%
2385
16th to endDay
16%6%25%5%25%5%6%7%35%
2618
June1st to 15thDay
22%5%245%45%5%5%8%30%
2520
16th to endDay
15%6%25%6%35%5%6%9%32%
2520
July1st to 15thDay
12%5%36%6%35%5%6%8%36%
2520
16th to endDay
12%5%35%6%45%5%6%8%37%
2654
August1st to 15thDay
13%6%246%35%5%7%9%34%
2373
16th to endDay
12%5%25%5%35%5%6%7%40%
2292
September1st to 15thDay
13%6%35%5%35%5%5%6%38%
2154
16th to endDay
14%42442436%7%45%
1980
October1st to 15thDay
19%8%25%423345%40%
1873
16th to endDay
18%7%26%5%345%5%5%35%
1788
November1st to 15thDay
15%8%26%32345%7%39%
1581
16th to endDay
18%6%35%5%24346%37%
1560
December1st to 15thDay
18%7%46%6%245%5%5%32%
1440
16th to endDay
18%12%5%6%5%3445%6%26%
1536
January1st to 15thNight
16%8%37%6%35%35%6%32%
2506
16th to endNight
14%9%46%6%45%5%5%5%33%
2651
February1st to 15thNight
14%7%47%6%36%6%6%6%30%
2214
16th to endNight
15%8%26%7%445%5%5%34%
1893
March1st to 15thNight
18%8%36%5%3445%7%33%
2160
16th to endNight
16%6%35%5%23346%40%
1997
April1st to 15thNight
17%9%47%6%34335%34%
1796
16th to endNight
17%7%25%5%34345%40%
1741
May1st to 15thNight
18%8%36%5%24445%36%
1576
16th to endNight
17%5%25%425%5%6%7%35%
1495
June1st to 15thNight
20%7%24425%5%5%6%35%
1440
16th to endNight
13%425%43435%8%43%
1440
July1st to 15thNight
12%315%5%25%5%6%8%43%
1440
16th to endNight
12%5%245%35%5%5%8%42%
1514
August1st to 15thNight
10%425%4246%5%9%44%
1572
16th to endNight
11%415%5%25%35%7%46%
1910
September1st to 15thNight
14%425%423446%46%
1806
16th to endNight
11%5%24414346%51%
1980
October1st to 15thNight
17%6%25%424345%42%
2085
16th to endNight
19%7%245%34446%36%
2434
November1st to 15thNight
16%8%35%324445%41%
2277
16th to endNight
15%8%46%5%25%5%5%6%33%
2398
December1st to 15thNight
19%9%36%5%35%45%6%30%
2520
16th to endNight
19%9%36%6%45%5%5%6%27%
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
51%5%11%21%9%1
1433
16th to endDay
52%6%13%18%8%
1572
February1st to 15thDay
57%6%10%15%9%
1640
16th to endDay
55%38%19%12%1
1580
March1st to 15thDay
52%413%18%9%1
1800
16th to endDay
50%48%18%15%3
2223
April1st to 15thDay
52%38%21%12%2
2152
16th to endDay
44%48%22%16%3
2217
May1st to 15thDay
53%47%20%11%2
2385
16th to endDay
46%7%12%20%11%1
2622
June1st to 15thDay
51%6%13%19%8%
2520
16th to endDay
47%6%14%19%11%
2520
July1st to 15thDay
45%5%11%22%13%1
2371
16th to endDay
42%6%17%20%12%1
2487
August1st to 15thDay
44%6%14%22%10%
2359
16th to endDay
40%6%11%25%15%
2292
September1st to 15thDay
44%5%10%21%15%1
2154
16th to endDay
36%49%24%22%1
1980
October1st to 15thDay
48%48%23%13%2
1873
16th to endDay
51%5%12%18%11%1
1788
November1st to 15thDay
44%5%14%20%13%1
1581
16th to endDay
49%411%21%12%1
1560
December1st to 15thDay
53%414%18%7%
1440
16th to endDay
59%6%11%14%7%
1536
January1st to 15thNight
53%5%11%17%10%1
2506
16th to endNight
52%5%10%19%10%1
2651
February1st to 15thNight
53%7%12%15%9%
2222
16th to endNight
51%5%9%19%11%2
1893
March1st to 15thNight
52%5%12%19%10%1
2160
16th to endNight
46%310%20%15%4
1998
April1st to 15thNight
54%39%18%12%1
1796
16th to endNight
46%49%20%16%2
1741
May1st to 15thNight
50%48%19%14%2
1576
16th to endNight
45%7%17%18%10%1
1497
June1st to 15thNight
48%6%12%22%9%
1440
16th to endNight
38%418%26%11%
1440
July1st to 15thNight
36%7%14%28%11%
1360
16th to endNight
38%5%17%25%12%1
1418
August1st to 15thNight
34%7%18%27%11%1
1562
16th to endNight
36%5%13%29%15%
1910
September1st to 15thNight
38%412%25%17%
1806
16th to endNight
34%48%27%24%1
1980
October1st to 15thNight
43%411%21%16%2
2085
16th to endNight
49%414%21%10%
2434
November1st to 15thNight
46%311%20%15%2
2277
16th to endNight
49%5%14%19%9%1
2398
December1st to 15thNight
54%414%17%8%
2520
16th to endNight
58%6%9%16%8%
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
25%35%26%6%5%1
60
16th to endDay
24%30%31%14%
70
February1st to 15thDay
27%35%26%7%3
153
16th to endDay
20%37%25%12%2
239
March1st to 15thDay
29%31%25%12%1
270
16th to endDay
27%29%23%11%42
288
April1st to 15thDay
26%28%24%12%6%
270
16th to endDay
18%27%25%16%7%4
270
May1st to 15thDay
34%25%17%12%6%3
288
16th to endDay
17%21%29%21%9%
331
June1st to 15thDay
22%21%25%17%11%1
315
16th to endDay
17%22%27%18%13%
315
July1st to 15thDay
14%14%33%23%11%2
305
16th to endDay
14%16%24%27%13%3
326
August1st to 15thDay
19%14%20%25%12%7%
313
16th to endDay
11%12%21%26%16%11%
332
September1st to 15thDay
16%15%20%24%13%9%
315
16th to endDay
15%11%25%16%17%12%
314
October1st to 15thDay
21%13%22%24%13%4
315
16th to endDay
24%22%24%22%6%
303
November1st to 15thDay
20%23%21%22%10%
235
16th to endDay
22%28%23%16%6%2
180
December1st to 15thDay
25%33%23%15%3
60
16th to endDay
37%26%29%6%
64
January1st to 15thNight
20%36%24%16%2
720
16th to endNight
23%36%24%14%1
768
February1st to 15thNight
19%35%26%15%1
704
16th to endNight
23%37%23%13%2
631
March1st to 15thNight
26%35%22%13%2
720
16th to endNight
21%36%20%14%6%
768
April1st to 15thNight
22%33%24%16%3
718
16th to endNight
21%31%22%16%6%1
661
May1st to 15thNight
22%27%26%17%5%
496
16th to endNight
18%22%26%22%9%
375
June1st to 15thNight
20%22%24%22%9%
360
16th to endNight
12%20%23%33%9%
360
July1st to 15thNight
13%14%22%32%16%
340
16th to endNight
11%20%15%35%17%
376
August1st to 15thNight
12%12%23%34%16%1
491
16th to endNight
11%11%21%30%22%3
764
September1st to 15thNight
15%14%25%26%15%1
720
16th to endNight
14%15%19%26%17%6%
717
October1st to 15thNight
20%19%21%26%10%1
720
16th to endNight
23%25%23%21%5%
768
November1st to 15thNight
20%30%20%20%6%1
700
16th to endNight
18%33%24%16%5%1
720
December1st to 15thNight
27%32%20%14%4
720
16th to endNight
26%37%21%11%1
768
Page generated at 2017-11-01 17:23:29. Version ac04

Copyright 2017 A. Danko


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