Bridgewater Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bridgewater Hill Observatory clear sky chart from 2005-11-05 to 2017-09-21.

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
29%
14805
13%
46881
8%
14774
37%
14774
15%
33147

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%
720
10%
2506
5%
716
26%
716
8%
1869
January16th to endNight
17%
768
12%
2688
5%
768
27%
768
10%
1914
February1st to 15thNight
11%
704
10%
2243
1%
701
22%
701
8%
1747
February16th to endNight
19%
631
14%
1893
5%
631
30%
631
12%
1395
March1st to 15thNight
19%
720
13%
2160
7%
720
27%
720
12%
1602
March16th to endNight
25%
768
18%
2008
6%
768
35%
768
18%
1417
April1st to 15thNight
23%
718
15%
1796
4%
718
28%
718
17%
1318
April16th to endNight
24%
618
16%
1698
5%
618
32%
618
18%
1193
May1st to 15thNight
25%
486
15%
1566
9%
486
33%
486
15%
1109
May16th to endNight
37%
374
11%
1497
11%
374
43%
374
14%
1013
June1st to 15thNight
34%
360
9%
1440
7%
360
44%
360
12%
1030
June16th to endNight
45%
360
10%
1440
10%
360
52%
360
17%
929
July1st to 15thNight
53%
340
10%
1360
14%
340
57%
340
20%
849
July16th to endNight
51%
376
14%
1418
15%
354
58%
354
23%
843
August1st to 15thNight
49%
430
15%
1501
13%
428
53%
428
23%
947
August16th to endNight
54%
759
19%
1905
11%
759
55%
759
33%
1245
September1st to 15thNight
46%
720
17%
1821
9%
720
49%
720
29%
1211
September16th to endNight
47%
681
23%
1880
16%
681
53%
681
31%
1174
October1st to 15thNight
35%
660
17%
1895
11%
660
42%
660
18%
1362
October16th to endNight
26%
704
10%
2266
5%
704
34%
704
13%
1688
November1st to 15thNight
29%
700
15%
2277
10%
700
37%
700
15%
1589
November16th to endNight
26%
720
15%
2415
10%
720
33%
720
13%
1707
December1st to 15thNight
19%
720
8%
2520
3%
720
25%
720
8%
1936
December16th to endNight
13%
768
10%
2688
4%
768
21%
768
7%
2060

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
17%9%37%6%35%45%6%30%
47996
Night
15%8%36%5%35%5%5%6%32%
47051
Cloud CoverDay
16%7%36%6%35%5%5%7%32%
48006
Night
15%7%25%5%35%5%5%7%36%
47056
TransparencyDay
50%8%11%17%10%1
47684
Night
47%9%12%18%10%1
46881
SeeingDay
22%16%27%19%10%3
5748
Night
20%17%27%23%9%1
14806

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%9%5%8%10%47%6%6%6%21%
1432
16th to endDay
11%9%48%9%35%6%6%7%26%
1536
February1st to 15thDay
12%10%49%9%46%5%5%6%24%
1616
16th to endDay
12%12%6%9%9%34445%26%
1580
March1st to 15thDay
15%12%5%9%6%36%5%5%425%
1790
16th to endDay
15%9%5%7%6%34345%33%
2214
April1st to 15thDay
17%10%5%6%5%245%45%31%
2152
16th to endDay
19%9%36%5%33446%33%
2262
May1st to 15thDay
22%10%36%6%23335%30%
2396
16th to endDay
19%7%36%5%35%45%7%32%
2618
June1st to 15thDay
25%7%36%5%345%5%5%27%
2520
16th to endDay
19%7%37%6%45%5%6%6%27%
2520
July1st to 15thDay
15%6%35%6%47%5%5%7%31%
2520
16th to endDay
15%6%35%6%36%6%6%8%32%
2654
August1st to 15thDay
15%6%36%7%46%5%7%7%29%
2436
16th to endDay
13%6%26%5%346%6%9%35%
2297
September1st to 15thDay
15%7%26%435%45%6%38%
2139
16th to endDay
14%5%24423347%46%
1873
October1st to 15thDay
22%10%36%5%2324433%
1734
16th to endDay
19%13%37%6%34446%26%
1605
November1st to 15thDay
15%10%26%435%5%5%7%34%
1581
16th to endDay
18%8%48%5%34445%31%
1545
December1st to 15thDay
19%10%5%10%7%44445%23%
1440
16th to endDay
18%15%7%7%6%25%5%5%6%19%
1536
January1st to 15thNight
13%9%37%8%47%6%7%6%24%
2506
16th to endNight
11%9%5%8%6%36%6%6%8%26%
2688
February1st to 15thNight
12%11%5%7%9%47%6%6%7%22%
2242
16th to endNight
13%10%46%8%46%5%6%5%27%
1893
March1st to 15thNight
15%8%5%7%6%45%45%430%
2147
16th to endNight
15%8%37%5%43346%36%
2008
April1st to 15thNight
17%8%27%6%45%5%5%7%30%
1796
16th to endNight
18%7%35%43445%6%35%
1698
May1st to 15thNight
21%8%37%423346%33%
1566
16th to endNight
18%7%35%4335%6%7%33%
1495
June1st to 15thNight
25%5%25%425%5%5%6%30%
1440
16th to endNight
15%5%35%5%35%46%6%38%
1440
July1st to 15thNight
13%5%34435%5%7%8%37%
1440
16th to endNight
13%4243245%6%8%43%
1514
August1st to 15thNight
12%5%16%5%3446%8%41%
1511
16th to endNight
12%425%42446%8%43%
1905
September1st to 15thNight
14%5%25%434445%45%
1821
16th to endNight
12%6%14324345%50%
1880
October1st to 15thNight
17%8%46%434446%35%
1895
16th to endNight
20%10%36%6%35%5%5%6%27%
2266
November1st to 15thNight
15%9%36%6%35%5%46%33%
2277
16th to endNight
15%9%37%5%3446%5%33%
2415
December1st to 15thNight
18%9%6%10%6%45%5%5%6%21%
2520
16th to endNight
18%11%5%8%7%35%5%5%6%21%
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
12%8%36%8%36%6%7%7%25%
1432
16th to endDay
10%9%45%7%45%5%6%9%30%
1536
February1st to 15thDay
11%10%47%8%46%5%5%8%26%
1616
16th to endDay
12%10%6%7%7%5%45%5%6%29%
1580
March1st to 15thDay
15%10%5%7%5%46%5%6%6%26%
1800
16th to endDay
14%8%46%7%34446%36%
2214
April1st to 15thDay
17%8%35%5%35%446%34%
2152
16th to endDay
17%8%36%5%3445%5%35%
2262
May1st to 15thDay
22%8%37%6%34446%30%
2396
16th to endDay
18%6%35%6%3445%7%34%
2618
June1st to 15thDay
24%5%35%6%35%45%7%28%
2520
16th to endDay
16%6%37%6%5%5%5%6%7%28%
2520
July1st to 15thDay
13%5%36%7%35%6%5%8%32%
2520
16th to endDay
13%5%25%5%35%6%7%9%33%
2654
August1st to 15thDay
13%6%26%7%46%5%7%7%31%
2436
16th to endDay
11%5%25%5%45%5%6%8%38%
2297
September1st to 15thDay
14%425%5%35%5%5%7%39%
2139
16th to endDay
13%5%1442445%6%48%
1873
October1st to 15thDay
21%8%25%5%23345%34%
1734
16th to endDay
18%8%27%7%35%45%6%29%
1605
November1st to 15thDay
14%8%25%5%246%6%7%36%
1581
16th to endDay
16%8%445%345%47%34%
1545
December1st to 15thDay
17%9%5%7%7%35%45%7%26%
1440
16th to endDay
18%13%5%7%5%35%5%46%24%
1536
January1st to 15thNight
12%8%26%6%36%6%6%8%30%
2506
16th to endNight
11%7%47%6%35%6%7%7%33%
2688
February1st to 15thNight
10%9%47%7%47%5%6%7%27%
2235
16th to endNight
13%8%45%5%45%6%6%8%31%
1893
March1st to 15thNight
16%8%26%6%36%5%5%6%32%
2160
16th to endNight
13%7%35%43445%6%39%
2007
April1st to 15thNight
15%7%25%6%35%46%8%34%
1796
16th to endNight
19%6%25%5%2445%7%38%
1698
May1st to 15thNight
21%7%25%5%33446%34%
1566
16th to endNight
18%5%35%435%5%5%8%34%
1495
June1st to 15thNight
24%4144345%6%8%32%
1440
16th to endNight
14%425%5%25%46%7%40%
1440
July1st to 15thNight
13%324326%6%6%9%41%
1440
16th to endNight
13%42343446%9%44%
1514
August1st to 15thNight
11%415%5%235%5%9%44%
1511
16th to endNight
12%41343445%9%46%
1905
September1st to 15thNight
13%423434447%47%
1821
16th to endNight
12%5%13323346%51%
1880
October1st to 15thNight
19%7%25%42345%6%37%
1895
16th to endNight
19%7%25%5%36%5%5%6%32%
2266
November1st to 15thNight
14%8%26%5%35%5%47%36%
2277
16th to endNight
14%8%35%5%35%5%5%5%37%
2415
December1st to 15thNight
17%9%37%6%35%45%7%27%
2520
16th to endNight
17%10%47%5%25%45%8%26%
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
54%11%13%14%6%
1433
16th to endDay
50%10%14%16%7%
1536
February1st to 15thDay
56%10%12%13%7%
1619
16th to endDay
54%8%11%12%11%1
1580
March1st to 15thDay
57%8%11%11%9%1
1800
16th to endDay
50%5%10%14%14%3
2214
April1st to 15thDay
51%8%8%18%11%2
2152
16th to endDay
49%7%7%19%12%2
2262
May1st to 15thDay
55%6%6%18%11%1
2396
16th to endDay
50%7%11%18%10%1
2622
June1st to 15thDay
56%7%13%14%8%
2520
16th to endDay
54%8%10%15%9%
2520
July1st to 15thDay
47%9%10%20%10%
2371
16th to endDay
44%10%15%21%8%
2487
August1st to 15thDay
47%10%14%18%8%
2422
16th to endDay
40%8%14%23%11%
2297
September1st to 15thDay
42%7%11%23%13%
2139
16th to endDay
36%5%12%28%15%1
1873
October1st to 15thDay
52%5%8%21%9%2
1734
16th to endDay
55%7%13%16%6%
1605
November1st to 15thDay
45%9%14%17%11%1
1581
16th to endDay
51%6%13%15%11%1
1545
December1st to 15thDay
58%8%13%12%6%
1440
16th to endDay
63%8%10%10%7%
1536
January1st to 15thNight
50%14%13%12%7%1
2506
16th to endNight
48%11%12%15%11%1
2688
February1st to 15thNight
54%13%11%13%8%
2243
16th to endNight
49%10%12%16%8%1
1893
March1st to 15thNight
52%8%12%16%8%1
2160
16th to endNight
45%6%13%15%15%4
2008
April1st to 15thNight
48%7%11%18%12%1
1796
16th to endNight
45%7%9%19%14%2
1698
May1st to 15thNight
51%6%6%16%16%1
1566
16th to endNight
47%9%12%19%10%1
1497
June1st to 15thNight
50%10%13%15%9%
1440
16th to endNight
42%10%12%22%11%
1440
July1st to 15thNight
36%12%17%23%9%
1360
16th to endNight
35%9%15%27%10%1
1418
August1st to 15thNight
37%10%13%26%12%
1501
16th to endNight
33%9%15%30%10%
1905
September1st to 15thNight
37%8%13%27%13%
1821
16th to endNight
34%6%10%29%17%1
1880
October1st to 15thNight
48%8%10%19%10%2
1895
16th to endNight
52%9%12%17%7%
2266
November1st to 15thNight
48%8%14%15%11%2
2277
16th to endNight
48%9%13%17%8%1
2415
December1st to 15thNight
56%10%14%14%4
2520
16th to endNight
57%10%12%12%6%
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%28%20%20%5%1
60
16th to endDay
21%26%32%14%4
64
February1st to 15thDay
24%25%31%16%3
150
16th to endDay
22%25%27%21%4
239
March1st to 15thDay
27%19%29%18%4
270
16th to endDay
23%24%27%13%7%3
288
April1st to 15thDay
32%17%23%17%8%1
270
16th to endDay
25%18%28%15%7%4
270
May1st to 15thDay
29%17%27%13%8%4
288
16th to endDay
21%17%32%21%5%1
331
June1st to 15thDay
29%13%31%17%7%
315
16th to endDay
21%15%31%20%9%
315
July1st to 15thDay
17%11%34%20%12%3
305
16th to endDay
14%9%35%26%12%2
326
August1st to 15thDay
16%12%28%21%15%5%
313
16th to endDay
14%10%21%23%16%12%
332
September1st to 15thDay
14%13%25%20%17%8%
315
16th to endDay
14%10%25%17%16%16%
275
October1st to 15thDay
25%12%21%21%17%3
255
16th to endDay
26%21%20%24%6%
243
November1st to 15thDay
19%17%25%24%10%1
235
16th to endDay
27%18%24%18%9%1
165
December1st to 15thDay
20%33%28%10%6%1
60
16th to endDay
35%17%42%4
64
January1st to 15thNight
21%23%31%19%3
720
16th to endNight
20%21%35%19%3
768
February1st to 15thNight
18%25%36%17%1
704
16th to endNight
21%23%30%20%3
631
March1st to 15thNight
23%21%30%20%3
720
16th to endNight
21%23%25%20%7%
768
April1st to 15thNight
21%24%27%21%5%
718
16th to endNight
25%19%27%18%8%
618
May1st to 15thNight
24%17%26%23%8%
486
16th to endNight
19%11%26%29%12%1
375
June1st to 15thNight
23%11%25%29%10%
360
16th to endNight
15%7%25%39%10%1
360
July1st to 15thNight
15%6%20%40%14%2
340
16th to endNight
13%325%35%20%1
376
August1st to 15thNight
15%423%34%19%1
430
16th to endNight
14%5%20%31%24%3
759
September1st to 15thNight
15%9%22%29%20%1
720
16th to endNight
15%9%22%26%19%6%
681
October1st to 15thNight
24%15%19%25%13%2
660
16th to endNight
25%18%25%24%5%
704
November1st to 15thNight
19%19%27%24%9%1
700
16th to endNight
21%21%27%19%8%2
720
December1st to 15thNight
25%24%27%18%4
720
16th to endNight
27%24%30%15%2
768
Page generated at 2017-09-22 07:35:36. Version ac04

Copyright 2017 A. Danko


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