North Ashburnham Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Ashburnham Observatory clear sky chart from 2005-11-05 to 2017-09-16.

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
22%
14861
18%
46809
6%
14829
35%
14829
18%
30217

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
10%
720
20%
2506
4%
716
30%
716
12%
1608
January16th to endNight
11%
768
19%
2688
3%
768
32%
768
12%
1712
February1st to 15thNight
9%
704
16%
2204
2%
701
25%
701
12%
1467
February16th to endNight
11%
631
21%
1893
4%
631
31%
631
14%
1236
March1st to 15thNight
12%
720
17%
2160
5%
720
29%
720
14%
1459
March16th to endNight
17%
768
22%
2018
5%
768
31%
768
19%
1345
April1st to 15thNight
16%
718
18%
1796
2%
718
25%
718
19%
1261
April16th to endNight
20%
606
18%
1686
3%
606
31%
606
20%
1135
May1st to 15thNight
19%
540
15%
1620
5%
540
30%
540
17%
1121
May16th to endNight
22%
380
12%
1502
7%
379
33%
379
14%
992
June1st to 15thNight
24%
360
13%
1440
7%
360
37%
360
14%
951
June16th to endNight
33%
360
11%
1440
10%
360
43%
360
20%
877
July1st to 15thNight
39%
340
13%
1360
13%
340
48%
340
22%
823
July16th to endNight
43%
376
15%
1418
13%
354
52%
354
25%
808
August1st to 15thNight
39%
476
16%
1547
9%
474
46%
474
26%
927
August16th to endNight
45%
744
21%
1890
9%
744
49%
744
34%
1214
September1st to 15thNight
33%
720
21%
1845
8%
720
41%
720
33%
1178
September16th to endNight
39%
658
26%
1818
14%
658
49%
658
34%
1081
October1st to 15thNight
26%
660
22%
1859
9%
660
40%
660
23%
1213
October16th to endNight
21%
704
16%
2279
5%
704
35%
704
17%
1478
November1st to 15thNight
20%
700
21%
2277
7%
700
38%
700
19%
1426
November16th to endNight
18%
720
20%
2355
7%
720
33%
720
16%
1495
December1st to 15thNight
13%
720
16%
2520
3%
720
29%
720
13%
1640
December16th to endNight
11%
768
19%
2688
4%
768
29%
768
11%
1770

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%6%25%5%3445%6%37%
47935
Night
16%7%25%42445%6%40%
46979
Cloud CoverDay
16%6%25%5%3445%7%37%
47945
Night
15%6%25%42445%6%40%
46984
TransparencyDay
45%411%21%14%2
47623
Night
43%412%22%14%2
46809
SeeingDay
22%15%31%19%8%2
5826
Night
22%17%32%19%6%
14861

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
13%7%35%5%35%5%5%6%36%
1432
16th to endDay
12%7%26%5%35%46%7%39%
1536
February1st to 15thDay
15%6%36%6%46%6%5%7%31%
1653
16th to endDay
14%9%45%5%34445%38%
1580
March1st to 15thDay
18%9%246%34347%34%
1790
16th to endDay
17%6%35%5%24445%41%
2202
April1st to 15thDay
17%9%36%5%24445%35%
2152
16th to endDay
17%6%25%43445%6%38%
2273
May1st to 15thDay
23%8%37%42345%5%30%
2336
16th to endDay
19%6%25%5%3435%8%36%
2618
June1st to 15thDay
24%6%3343445%8%31%
2520
16th to endDay
16%5%26%6%36%5%6%8%31%
2520
July1st to 15thDay
14%5%36%5%36%5%6%8%34%
2520
16th to endDay
13%6%25%5%35%5%6%7%36%
2654
August1st to 15thDay
14%425%6%45%5%6%8%35%
2391
16th to endDay
11%425%6%35%46%8%41%
2309
September1st to 15thDay
14%5%25%5%3445%6%42%
2115
16th to endDay
14%5%24422335%50%
1815
October1st to 15thDay
20%6%25%5%24345%39%
1768
16th to endDay
15%7%26%5%2436%6%39%
1592
November1st to 15thDay
17%7%35%323345%42%
1581
16th to endDay
18%6%25%433335%43%
1602
December1st to 15thDay
18%6%35%5%35%445%37%
1440
16th to endDay
20%9%34425%5%5%6%32%
1536
January1st to 15thNight
14%8%35%5%2445%5%39%
2506
16th to endNight
13%9%36%5%25%446%38%
2688
February1st to 15thNight
13%8%35%5%35%5%5%6%37%
2203
16th to endNight
14%8%35%32445%7%38%
1893
March1st to 15thNight
17%7%34425%5%5%6%37%
2147
16th to endNight
14%7%35%424446%42%
2018
April1st to 15thNight
17%7%25%5%345%5%5%36%
1796
16th to endNight
20%6%35%334246%40%
1686
May1st to 15thNight
19%7%45%5%35%35%5%34%
1620
16th to endNight
19%6%36%5%2445%8%34%
1500
June1st to 15thNight
23%6%35%424446%34%
1440
16th to endNight
15%5%234245%6%6%42%
1440
July1st to 15thNight
12%425%5%36%46%8%40%
1440
16th to endNight
12%4245%35%45%9%42%
1514
August1st to 15thNight
10%425%5%246%6%6%44%
1557
16th to endNight
13%5%145%3336%8%44%
1890
September1st to 15thNight
13%5%24323447%47%
1845
16th to endNight
13%424423346%52%
1818
October1st to 15thNight
19%6%24424345%43%
1859
16th to endNight
17%7%35%42445%7%38%
2279
November1st to 15thNight
16%5%35%42444444%
2277
16th to endNight
16%6%35%5%24445%40%
2355
December1st to 15thNight
18%8%34424446%37%
2520
16th to endNight
18%9%25%3245%5%7%35%
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
13%7%27%5%26%46%6%37%
1432
16th to endDay
12%7%15%5%35%5%5%6%40%
1536
February1st to 15thDay
15%6%37%5%46%6%5%8%30%
1653
16th to endDay
14%9%35%6%34345%38%
1580
March1st to 15thDay
18%9%25%6%34447%34%
1800
16th to endDay
16%6%35%434436%40%
2202
April1st to 15thDay
16%8%36%7%35%34436%
2152
16th to endDay
16%6%25%5%2445%7%38%
2273
May1st to 15thDay
24%8%35%6%3345%5%30%
2336
16th to endDay
18%6%25%6%2435%8%35%
2618
June1st to 15thDay
24%6%2443445%8%31%
2520
16th to endDay
15%436%6%36%6%6%8%32%
2520
July1st to 15thDay
13%5%36%5%36%5%6%8%34%
2520
16th to endDay
13%5%25%6%45%5%6%7%36%
2654
August1st to 15thDay
13%5%26%5%45%5%6%8%35%
2391
16th to endDay
11%5%25%6%25%5%6%8%40%
2309
September1st to 15thDay
13%5%245%345%47%42%
2115
16th to endDay
13%5%24323346%50%
1815
October1st to 15thDay
20%5%345%34345%39%
1768
16th to endDay
15%6%16%6%3345%6%39%
1592
November1st to 15thDay
18%6%25%42344443%
1581
16th to endDay
17%5%345%33336%43%
1602
December1st to 15thDay
17%6%36%435%446%37%
1440
16th to endDay
20%8%34425%5%5%7%32%
1536
January1st to 15thNight
14%8%36%424446%40%
2506
16th to endNight
13%9%35%5%35%5%46%38%
2688
February1st to 15thNight
13%8%36%5%35%45%5%38%
2196
16th to endNight
15%7%44425%446%39%
1893
March1st to 15thNight
17%7%35%425%5%5%6%36%
2160
16th to endNight
14%6%35%424446%42%
2017
April1st to 15thNight
16%7%36%6%2346%5%37%
1796
16th to endNight
20%5%25%5%23336%40%
1686
May1st to 15thNight
19%8%35%43445%6%34%
1620
16th to endNight
19%6%35%5%3445%8%33%
1500
June1st to 15thNight
23%6%35%424446%35%
1440
16th to endNight
15%5%244245%5%7%41%
1440
July1st to 15thNight
12%5%25%435%5%5%9%40%
1440
16th to endNight
12%3245%35%5%5%8%42%
1514
August1st to 15thNight
10%425%5%35%5%6%6%43%
1557
16th to endNight
12%41442336%9%44%
1890
September1st to 15thNight
13%42443345%6%47%
1845
16th to endNight
13%324423336%51%
1818
October1st to 15thNight
18%6%23423436%42%
1859
16th to endNight
17%7%3432445%7%38%
2279
November1st to 15thNight
15%6%35%424345%44%
2277
16th to endNight
16%7%25%5%3445%5%40%
2355
December1st to 15thNight
18%8%344235%5%5%37%
2520
16th to endNight
18%9%25%5%235%5%7%36%
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
46%410%24%11%3
1433
16th to endDay
40%415%25%13%1
1536
February1st to 15thDay
50%5%12%19%12%
1656
16th to endDay
47%310%19%15%3
1580
March1st to 15thDay
50%211%18%13%2
1800
16th to endDay
45%29%20%16%5%
2202
April1st to 15thDay
48%39%20%14%2
2152
16th to endDay
44%38%21%17%3
2273
May1st to 15thDay
53%37%19%13%2
2336
16th to endDay
46%5%13%20%12%1
2622
June1st to 15thDay
50%5%12%20%10%
2520
16th to endDay
47%6%11%20%12%
2520
July1st to 15thDay
44%6%11%22%13%1
2371
16th to endDay
43%5%13%22%12%1
2487
August1st to 15thDay
44%5%13%24%11%1
2377
16th to endDay
38%5%11%27%15%2
2309
September1st to 15thDay
40%410%22%19%1
2115
16th to endDay
36%37%25%24%2
1815
October1st to 15thDay
47%38%23%13%3
1768
16th to endDay
44%413%21%13%2
1592
November1st to 15thDay
43%313%23%13%2
1581
16th to endDay
43%313%22%15%2
1602
December1st to 15thDay
48%214%19%14%
1440
16th to endDay
50%6%12%18%11%
1536
January1st to 15thNight
45%411%22%13%2
2506
16th to endNight
45%311%21%15%2
2688
February1st to 15thNight
46%414%20%11%1
2204
16th to endNight
44%311%23%13%3
1893
March1st to 15thNight
47%311%21%12%2
2160
16th to endNight
42%310%18%18%5%
2018
April1st to 15thNight
46%310%20%17%1
1796
16th to endNight
46%29%20%17%3
1686
May1st to 15thNight
50%39%19%13%2
1620
16th to endNight
49%5%13%18%11%1
1502
June1st to 15thNight
49%5%11%22%10%
1440
16th to endNight
40%7%13%23%14%
1440
July1st to 15thNight
40%6%13%26%13%
1360
16th to endNight
37%416%27%11%1
1418
August1st to 15thNight
37%6%15%22%16%1
1547
16th to endNight
36%5%11%28%16%
1890
September1st to 15thNight
36%413%25%19%1
1845
16th to endNight
33%39%27%23%2
1818
October1st to 15thNight
43%212%22%14%3
1859
16th to endNight
45%314%24%11%
2279
November1st to 15thNight
42%311%20%18%2
2277
16th to endNight
45%214%21%13%2
2355
December1st to 15thNight
46%414%22%11%
2520
16th to endNight
46%5%11%23%11%
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
31%21%33%10%3
60
16th to endDay
25%28%34%10%1
64
February1st to 15thDay
27%25%35%8%2
194
16th to endDay
25%23%35%14%1
239
March1st to 15thDay
32%24%29%10%2
270
16th to endDay
23%27%31%9%5%1
288
April1st to 15thDay
24%20%38%10%5%
270
16th to endDay
24%15%34%17%5%3
270
May1st to 15thDay
35%17%24%14%5%2
288
16th to endDay
20%13%34%22%6%
331
June1st to 15thDay
26%13%29%23%6%
315
16th to endDay
16%12%40%21%6%1
315
July1st to 15thDay
18%7%33%29%9%
305
16th to endDay
16%7%29%32%11%3
326
August1st to 15thDay
14%9%30%25%15%3
313
16th to endDay
12%8%28%28%16%5%
332
September1st to 15thDay
17%11%30%20%15%5%
315
16th to endDay
17%9%28%22%11%11%
254
October1st to 15thDay
24%11%26%22%10%3
255
16th to endDay
23%20%32%19%4
241
November1st to 15thDay
24%19%29%14%9%1
235
16th to endDay
29%18%24%18%6%1
222
December1st to 15thDay
26%31%26%13%1
60
16th to endDay
39%28%26%6%
64
January1st to 15thNight
25%26%34%11%2
720
16th to endNight
26%24%33%13%2
768
February1st to 15thNight
21%26%38%11%1
704
16th to endNight
25%28%30%13%1
631
March1st to 15thNight
26%23%33%14%1
720
16th to endNight
23%23%31%16%4
768
April1st to 15thNight
24%21%34%17%2
718
16th to endNight
25%19%31%17%5%
606
May1st to 15thNight
25%15%34%19%4
540
16th to endNight
21%10%37%25%5%
380
June1st to 15thNight
25%11%33%21%8%
360
16th to endNight
17%7%37%32%5%
360
July1st to 15thNight
15%5%34%35%9%
340
16th to endNight
11%5%34%34%12%
376
August1st to 15thNight
12%436%34%10%
476
16th to endNight
14%427%35%16%1
744
September1st to 15thNight
16%8%36%26%9%1
720
16th to endNight
16%8%29%27%15%2
658
October1st to 15thNight
23%14%30%20%8%1
660
16th to endNight
26%18%29%18%5%
704
November1st to 15thNight
20%22%33%15%7%1
700
16th to endNight
26%20%30%15%6%
720
December1st to 15thNight
27%23%32%12%3
720
16th to endNight
29%25%31%11%2
768
Page generated at 2017-09-16 17:06:58. Version ac04

Copyright 2017 A. Danko


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