Brown Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brown Observing Site clear sky chart from 2005-11-05 to 2017-09-24.

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
57%
14824
20%
46905
17%
14800
61%
14800
29%
28434

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
44%
716
8%
2506
7%
716
46%
716
12%
1920
January16th to endNight
48%
768
9%
2688
8%
768
50%
768
16%
1930
February1st to 15thNight
50%
696
10%
2244
8%
696
53%
696
19%
1562
February16th to endNight
46%
632
14%
1895
13%
632
51%
632
20%
1332
March1st to 15thNight
43%
720
11%
2156
8%
720
48%
720
20%
1523
March16th to endNight
42%
768
17%
2039
10%
768
46%
768
25%
1396
April1st to 15thNight
50%
719
20%
1799
10%
719
54%
719
32%
1179
April16th to endNight
55%
576
25%
1656
17%
576
59%
576
36%
960
May1st to 15thNight
55%
539
23%
1619
16%
539
61%
539
42%
863
May16th to endNight
61%
382
19%
1504
23%
382
65%
382
27%
808
June1st to 15thNight
58%
360
17%
1440
22%
360
64%
360
28%
766
June16th to endNight
67%
360
26%
1440
36%
360
75%
360
45%
598
July1st to 15thNight
77%
340
24%
1360
34%
340
83%
340
54%
520
July16th to endNight
77%
376
34%
1416
43%
354
84%
354
62%
495
August1st to 15thNight
75%
485
32%
1558
28%
483
80%
483
56%
679
August16th to endNight
80%
703
38%
1849
24%
703
82%
703
68%
867
September1st to 15thNight
80%
720
36%
1848
23%
720
84%
720
67%
896
September16th to endNight
70%
692
34%
1914
28%
692
73%
692
46%
1047
October1st to 15thNight
63%
660
28%
1848
23%
660
69%
660
40%
1058
October16th to endNight
61%
704
24%
2288
24%
704
65%
704
33%
1304
November1st to 15thNight
57%
700
22%
2275
22%
700
61%
700
27%
1398
November16th to endNight
50%
720
15%
2355
11%
720
56%
720
23%
1549
December1st to 15thNight
49%
720
10%
2520
8%
720
52%
720
16%
1780
December16th to endNight
37%
768
11%
2688
10%
768
41%
768
14%
2004

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
8%5%2442445%7%51%
48040
Night
10%6%25%42445%6%47%
47078
TransparencyDay
32%6%12%19%17%11%
47711
Night
35%6%12%19%15%9%
46906
SeeingDay
10%7%22%29%21%8%
5415
Night
12%418%36%24%3
14838

By Month

Percentage Cloud Cover SW (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%9%48%9%36%5%47%25%
1432
16th to endDay
14%9%411%9%46%5%47%22%
1536
February1st to 15thDay
11%8%47%6%35%5%47%34%
1616
16th to endDay
9%11%36%6%35%5%5%7%35%
1579
March1st to 15thDay
11%8%46%437%6%6%7%33%
1803
16th to endDay
11%7%36%5%35%6%6%8%34%
2184
April1st to 15thDay
8%6%25%5%35%6%7%8%39%
2149
16th to endDay
8%5%1342445%7%51%
2302
May1st to 15thDay
5%31343345%7%57%
2331
16th to endDay
8%4243345%5%8%49%
2604
June1st to 15thDay
8%4244345%5%8%47%
2520
16th to endDay
2212212345%71%
2520
July1st to 15thDay
221113345%71%
2520
16th to endDay
211121223376%
2657
August1st to 15thDay
3112313335%70%
2394
16th to endDay
212112235%76%
2351
September1st to 15thDay
211211223475%
2112
16th to endDay
7%314212345%62%
1904
October1st to 15thDay
8%41432435%8%54%
1780
16th to endDay
8%5%15%4345%5%7%46%
1584
November1st to 15thDay
11%7%25%5%25%448%42%
1584
16th to endDay
12%8%26%7%36%5%6%8%30%
1602
December1st to 15thDay
16%8%39%5%35%5%5%6%30%
1440
16th to endDay
19%10%48%8%445%5%5%23%
1536
January1st to 15thNight
14%9%48%7%46%5%5%5%26%
2506
16th to endNight
14%9%49%5%345%46%32%
2688
February1st to 15thNight
13%8%36%435%5%6%7%35%
2233
16th to endNight
10%8%47%5%46%7%6%7%32%
1895
March1st to 15thNight
13%9%36%5%35%45%7%34%
2156
16th to endNight
13%8%35%5%25%5%5%8%37%
2039
April1st to 15thNight
9%5%25%436%5%7%7%43%
1799
16th to endNight
9%42342435%7%52%
1656
May1st to 15thNight
441332445%8%57%
1619
16th to endNight
5%4246%36%5%6%8%46%
1504
June1st to 15thNight
9%322425%45%7%52%
1440
16th to endNight
322313346%67%
1440
July1st to 15thNight
111122345%7%69%
1440
16th to endNight
211212336%75%
1512
August1st to 15thNight
31232235%6%68%
1566
16th to endNight
211212336%75%
1849
September1st to 15thNight
212212235%74%
1848
16th to endNight
7%314324335%61%
1914
October1st to 15thNight
7%314435%445%56%
1848
16th to endNight
10%42442445%7%49%
2288
November1st to 15thNight
14%5%2442445%6%44%
2275
16th to endNight
15%7%36%5%2445%6%39%
2355
December1st to 15thNight
15%9%27%5%35%45%7%34%
2520
16th to endNight
20%11%36%6%34345%29%
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
57%12%17%12%
1432
16th to endDay
60%12%15%11%
1536
February1st to 15thDay
48%9%19%17%4
1616
16th to endDay
47%7%18%19%5%
1579
March1st to 15thDay
45%9%16%21%6%
1803
16th to endDay
44%8%17%20%8%
2184
April1st to 15thDay
38%7%15%23%13%1
2149
16th to endDay
31%5%13%27%17%3
2302
May1st to 15thDay
24%5%10%28%25%5%
2331
16th to endDay
30%7%14%25%17%4
2608
June1st to 15thDay
32%6%11%21%21%6%
2520
16th to endDay
13%36%20%35%19%
2520
July1st to 15thDay
14%35%15%29%32%
2371
16th to endDay
11%247%25%48%
2490
August1st to 15thDay
16%36%15%25%32%
2377
16th to endDay
11%26%15%35%28%
2351
September1st to 15thDay
12%26%17%32%28%
2112
16th to endDay
24%38%26%27%10%
1904
October1st to 15thDay
26%5%16%27%20%4
1780
16th to endDay
34%6%15%28%14%
1584
November1st to 15thDay
40%6%21%23%8%
1584
16th to endDay
48%10%21%16%3
1602
December1st to 15thDay
52%10%21%13%2
1440
16th to endDay
59%7%17%12%2
1536
January1st to 15thNight
56%10%16%13%2
2506
16th to endNight
50%9%18%17%3
2688
February1st to 15thNight
44%10%19%19%5%
2244
16th to endNight
46%8%17%20%5%
1895
March1st to 15thNight
47%7%13%21%8%1
2156
16th to endNight
43%6%14%24%10%
2039
April1st to 15thNight
36%6%13%26%15%1
1799
16th to endNight
31%410%27%23%2
1656
May1st to 15thNight
23%5%11%20%33%4
1620
16th to endNight
30%7%12%26%15%6%
1504
June1st to 15thNight
29%7%11%22%23%5%
1440
16th to endNight
17%46%18%30%22%
1440
July1st to 15thNight
14%26%14%27%34%
1360
16th to endNight
11%247%25%48%
1416
August1st to 15thNight
15%27%14%23%36%
1558
16th to endNight
11%3415%27%37%
1849
September1st to 15thNight
12%25%14%31%33%
1848
16th to endNight
25%36%19%27%18%
1914
October1st to 15thNight
28%5%9%26%22%7%
1848
16th to endNight
32%413%25%21%2
2288
November1st to 15thNight
39%5%14%24%15%1
2275
16th to endNight
44%6%17%24%7%
2355
December1st to 15thNight
49%9%20%16%4
2520
16th to endNight
56%7%15%17%3
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
26%313%20%23%13%
60
16th to endDay
26%20%23%21%7%
64
February1st to 15thDay
17%5%22%19%27%7%
179
16th to endDay
16%14%26%27%10%4
238
March1st to 15thDay
18%11%28%28%10%2
270
16th to endDay
15%14%32%27%7%1
288
April1st to 15thDay
9%16%30%20%14%9%
269
16th to endDay
12%15%30%24%14%2
270
May1st to 15thDay
9%11%26%31%16%4
288
16th to endDay
12%5%15%33%20%12%
330
June1st to 15thDay
8%9%25%33%16%6%
315
16th to endDay
37%20%34%24%9%
315
July1st to 15thDay
4121%33%28%10%
305
16th to endDay
1319%41%25%8%
327
August1st to 15thDay
4224%28%31%7%
315
16th to endDay
3216%30%32%15%
332
September1st to 15thDay
5%116%26%29%20%
315
16th to endDay
7%5%19%28%24%13%
288
October1st to 15thDay
10%423%24%24%11%
250
16th to endDay
23%321%28%16%6%
96
November1st to 15thDay
15%116%25%41%
90
16th to endDay
26%11%25%26%10%
87
December1st to 15thDay
35%325%28%35%
60
16th to endDay
35%9%9%21%15%7%
64
January1st to 15thNight
16%6%18%31%22%3
720
16th to endNight
18%416%29%28%3
768
February1st to 15thNight
16%418%37%20%2
704
16th to endNight
13%6%26%34%18%1
632
March1st to 15thNight
16%6%25%31%17%2
720
16th to endNight
15%8%27%34%11%1
768
April1st to 15thNight
10%7%24%35%19%2
719
16th to endNight
11%7%22%38%17%2
576
May1st to 15thNight
7%6%26%33%23%2
540
16th to endNight
7%417%35%29%5%
383
June1st to 15thNight
11%6%20%36%21%4
360
16th to endNight
5%418%37%31%2
360
July1st to 15thNight
1114%47%33%2
340
16th to endNight
2114%47%31%2
376
August1st to 15thNight
4113%44%35%1
485
16th to endNight
2211%43%35%4
703
September1st to 15thNight
2111%37%38%8%
720
16th to endNight
9%313%32%31%10%
692
October1st to 15thNight
8%317%36%26%6%
660
16th to endNight
12%315%37%24%6%
704
November1st to 15thNight
14%417%36%24%3
700
16th to endNight
18%419%34%19%4
720
December1st to 15thNight
18%5%17%37%19%2
720
16th to endNight
25%7%19%31%14%1
768
Page generated at 2017-09-25 07:02:20. Version ac04

Copyright 2017 A. Danko


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