Brown's Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brown's Observatory clear sky chart from 2005-11-05 to 2017-09-22.

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%
14830
16%
46884
7%
14798
37%
14798
17%
31922

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
17%
720
17%
2506
6%
716
30%
716
11%
1728
January16th to endNight
13%
768
15%
2688
4%
768
28%
768
11%
1834
February1st to 15thNight
10%
704
13%
2232
2%
701
24%
701
10%
1635
February16th to endNight
16%
631
17%
1893
5%
631
30%
631
14%
1344
March1st to 15thNight
17%
720
15%
2160
6%
720
28%
720
13%
1520
March16th to endNight
23%
768
21%
2014
7%
768
34%
768
20%
1380
April1st to 15thNight
20%
718
17%
1796
3%
718
28%
718
18%
1299
April16th to endNight
23%
604
17%
1684
5%
604
32%
604
19%
1167
May1st to 15thNight
22%
516
16%
1596
8%
516
31%
516
15%
1140
May16th to endNight
32%
375
11%
1497
9%
374
40%
374
14%
1018
June1st to 15thNight
31%
360
11%
1440
7%
360
43%
360
13%
981
June16th to endNight
41%
360
10%
1440
10%
360
49%
360
18%
928
July1st to 15thNight
49%
340
12%
1360
16%
340
54%
340
23%
809
July16th to endNight
50%
376
16%
1418
17%
354
57%
354
25%
822
August1st to 15thNight
47%
455
15%
1526
10%
453
51%
453
25%
934
August16th to endNight
51%
738
20%
1884
11%
738
53%
738
34%
1197
September1st to 15thNight
41%
720
19%
1836
8%
720
47%
720
32%
1189
September16th to endNight
46%
685
26%
1891
16%
685
53%
685
35%
1138
October1st to 15thNight
34%
660
19%
1873
10%
660
42%
660
19%
1303
October16th to endNight
23%
704
13%
2280
5%
704
34%
704
14%
1617
November1st to 15thNight
28%
700
18%
2277
10%
700
40%
700
17%
1516
November16th to endNight
24%
720
17%
2385
9%
720
36%
720
15%
1627
December1st to 15thNight
16%
720
13%
2520
4%
720
28%
720
10%
1802
December16th to endNight
12%
768
13%
2688
3%
768
24%
768
9%
1994

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%6%35%45%7%34%
48000
Night
15%7%36%5%3445%6%36%
47054
Cloud CoverDay
16%7%36%6%3445%7%34%
48010
Night
15%7%35%5%3445%6%36%
47059
TransparencyDay
48%6%11%20%11%1
47688
Night
46%6%12%21%12%1
46884
SeeingDay
20%17%29%19%10%3
5798
Night
21%18%28%23%8%
14830

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%8%47%6%46%6%7%8%27%
1432
16th to endDay
11%8%35%7%35%5%7%8%32%
1536
February1st to 15thDay
13%11%46%7%46%5%5%7%27%
1626
16th to endDay
13%11%5%7%7%35%35%6%30%
1580
March1st to 15thDay
17%10%47%5%35%5%6%5%28%
1790
16th to endDay
15%8%36%5%345%36%36%
2206
April1st to 15thDay
17%8%46%5%2435%6%34%
2152
16th to endDay
17%7%36%5%335%45%36%
2274
May1st to 15thDay
23%9%46%5%34335%30%
2364
16th to endDay
18%6%36%5%35%437%34%
2618
June1st to 15thDay
23%7%36%5%34447%29%
2520
16th to endDay
18%6%36%6%35%5%5%7%30%
2520
July1st to 15thDay
13%5%36%7%35%5%6%8%33%
2520
16th to endDay
13%5%25%5%46%5%6%8%34%
2654
August1st to 15thDay
13%5%36%7%46%5%6%8%33%
2412
16th to endDay
11%425%6%35%5%5%8%40%
2315
September1st to 15thDay
14%6%15%5%3445%7%41%
2123
16th to endDay
13%5%24433346%49%
1881
October1st to 15thDay
20%8%36%5%24345%36%
1756
16th to endDay
18%9%26%6%44447%30%
1592
November1st to 15thDay
15%7%26%5%2445%6%38%
1581
16th to endDay
17%8%37%5%2334438%
1572
December1st to 15thDay
17%10%46%6%35%5%5%6%29%
1440
16th to endDay
19%13%5%7%5%35%45%5%25%
1536
January1st to 15thNight
12%9%47%6%35%5%6%7%31%
2506
16th to endNight
11%9%47%6%45%5%5%6%32%
2688
February1st to 15thNight
12%9%47%7%46%6%6%7%27%
2231
16th to endNight
13%10%46%5%35%47%7%31%
1893
March1st to 15thNight
17%9%36%5%35%45%5%32%
2147
16th to endNight
15%7%35%5%34445%40%
2014
April1st to 15thNight
17%7%35%5%3445%6%34%
1796
16th to endNight
19%6%35%43435%6%37%
1684
May1st to 15thNight
19%9%35%5%34335%33%
1596
16th to endNight
18%6%36%7%2445%6%34%
1495
June1st to 15thNight
24%5%35%5%35%35%6%31%
1440
16th to endNight
16%4246%25%5%6%7%38%
1440
July1st to 15thNight
13%4245%35%5%5%8%42%
1440
16th to endNight
12%434335%5%5%9%42%
1514
August1st to 15thNight
11%5%15%5%35%5%6%7%43%
1536
16th to endNight
12%5%1332446%8%44%
1884
September1st to 15thNight
14%5%24323346%48%
1836
16th to endNight
12%424323447%51%
1891
October1st to 15thNight
20%7%25%433345%38%
1873
16th to endNight
18%10%35%5%2445%6%32%
2280
November1st to 15thNight
13%8%36%6%43445%39%
2277
16th to endNight
15%8%36%5%25%5%46%36%
2385
December1st to 15thNight
17%8%47%6%35%5%46%30%
2520
16th to endNight
17%11%47%6%35%5%5%6%26%
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%37%7%36%6%5%8%28%
1432
16th to endDay
12%7%36%6%45%5%7%8%33%
1536
February1st to 15thDay
13%11%37%6%36%5%5%7%28%
1626
16th to endDay
13%11%48%7%44346%31%
1580
March1st to 15thDay
17%10%47%5%35%45%7%28%
1800
16th to endDay
16%8%46%5%33446%37%
2206
April1st to 15thDay
16%9%36%5%2435%6%34%
2152
16th to endDay
18%7%35%5%25%445%36%
2274
May1st to 15thDay
22%10%37%5%34335%30%
2364
16th to endDay
17%8%25%5%44447%35%
2618
June1st to 15thDay
23%6%26%5%3435%7%29%
2520
16th to endDay
17%6%36%7%35%5%5%7%30%
2520
July1st to 15thDay
13%5%36%6%45%5%6%8%33%
2520
16th to endDay
13%5%35%6%35%5%6%8%34%
2654
August1st to 15thDay
14%5%26%7%46%5%6%8%33%
2412
16th to endDay
11%425%5%35%5%6%8%40%
2315
September1st to 15thDay
13%6%25%5%3445%7%41%
2123
16th to endDay
13%6%24422346%49%
1881
October1st to 15thDay
19%10%26%5%2344436%
1756
16th to endDay
19%7%37%5%3445%7%31%
1592
November1st to 15thDay
14%8%25%5%245%5%6%39%
1581
16th to endDay
17%8%47%5%2334439%
1572
December1st to 15thDay
17%9%47%5%35%5%5%5%30%
1440
16th to endDay
19%13%47%5%35%5%45%25%
1536
January1st to 15thNight
11%9%37%7%35%5%5%8%32%
2506
16th to endNight
12%9%46%6%445%6%6%33%
2688
February1st to 15thNight
12%9%47%6%46%6%6%6%28%
2224
16th to endNight
13%10%46%435%46%7%31%
1893
March1st to 15thNight
17%9%36%5%3446%5%32%
2160
16th to endNight
14%7%35%5%25%445%40%
2013
April1st to 15thNight
17%8%36%5%3445%5%35%
1796
16th to endNight
19%7%26%434446%38%
1684
May1st to 15thNight
19%9%36%5%35%335%33%
1596
16th to endNight
18%6%37%6%34447%33%
1495
June1st to 15thNight
23%6%35%424446%32%
1440
16th to endNight
15%5%35%5%35%46%7%38%
1440
July1st to 15thNight
13%3246%35%45%9%42%
1440
16th to endNight
11%5%234245%6%8%43%
1514
August1st to 15thNight
10%6%25%4336%6%8%42%
1536
16th to endNight
11%6%1342446%8%46%
1884
September1st to 15thNight
14%5%2442335%5%48%
1836
16th to endNight
12%5%2432335%6%51%
1891
October1st to 15thNight
19%7%25%324445%38%
1873
16th to endNight
19%9%25%5%3445%5%33%
2280
November1st to 15thNight
14%7%46%6%34445%39%
2277
16th to endNight
15%7%36%5%3445%5%37%
2385
December1st to 15thNight
16%8%47%6%36%45%6%31%
2520
16th to endNight
17%11%46%6%36%46%7%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
50%8%10%19%9%1
1433
16th to endDay
44%7%15%21%9%1
1536
February1st to 15thDay
54%7%11%17%8%
1629
16th to endDay
54%5%8%16%12%1
1580
March1st to 15thDay
54%5%11%15%11%2
1800
16th to endDay
47%5%9%17%15%4
2206
April1st to 15thDay
49%5%8%19%13%3
2152
16th to endDay
48%5%7%19%15%2
2274
May1st to 15thDay
56%47%18%11%2
2364
16th to endDay
49%6%11%19%10%1
2622
June1st to 15thDay
53%5%12%17%9%
2520
16th to endDay
51%7%11%16%11%1
2520
July1st to 15thDay
45%7%12%20%12%
2371
16th to endDay
42%7%15%23%10%
2487
August1st to 15thDay
46%7%13%22%9%1
2398
16th to endDay
38%6%14%26%13%1
2315
September1st to 15thDay
41%6%10%23%16%1
2123
16th to endDay
35%49%27%20%1
1881
October1st to 15thDay
49%5%8%23%10%2
1756
16th to endDay
50%6%12%20%8%1
1592
November1st to 15thDay
43%7%13%22%11%1
1581
16th to endDay
48%412%18%13%1
1572
December1st to 15thDay
52%7%14%16%9%
1440
16th to endDay
58%6%11%14%8%
1536
January1st to 15thNight
47%8%11%19%11%1
2506
16th to endNight
47%7%11%18%12%1
2688
February1st to 15thNight
51%8%11%16%10%
2232
16th to endNight
48%7%11%20%9%2
1893
March1st to 15thNight
50%5%12%19%10%1
2160
16th to endNight
43%5%11%15%18%4
2014
April1st to 15thNight
47%5%9%19%15%1
1796
16th to endNight
46%5%10%18%16%2
1684
May1st to 15thNight
52%5%6%18%15%1
1596
16th to endNight
50%6%13%18%9%1
1497
June1st to 15thNight
52%5%14%19%8%
1440
16th to endNight
43%8%14%21%12%
1440
July1st to 15thNight
37%7%16%27%11%
1360
16th to endNight
35%7%16%27%11%1
1418
August1st to 15thNight
37%7%16%24%13%1
1526
16th to endNight
33%6%14%30%14%
1884
September1st to 15thNight
36%5%13%26%17%
1836
16th to endNight
32%49%30%20%1
1891
October1st to 15thNight
48%411%20%12%2
1873
16th to endNight
50%7%11%21%8%
2280
November1st to 15thNight
45%5%12%20%13%1
2277
16th to endNight
46%7%12%21%9%2
2385
December1st to 15thNight
52%7%13%18%7%
2520
16th to endNight
54%9%10%17%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
20%30%30%13%6%
60
16th to endDay
25%26%31%15%1
64
February1st to 15thDay
27%25%27%18%2
172
16th to endDay
18%30%27%19%4
239
March1st to 15thDay
29%22%30%12%4
270
16th to endDay
22%27%25%13%8%2
288
April1st to 15thDay
26%20%27%15%6%2
270
16th to endDay
21%20%32%13%9%3
270
May1st to 15thDay
30%22%22%13%7%3
288
16th to endDay
21%15%35%19%6%1
331
June1st to 15thDay
26%14%30%18%8%1
315
16th to endDay
20%15%32%20%10%1
315
July1st to 15thDay
17%8%35%22%13%2
305
16th to endDay
10%9%36%30%10%3
326
August1st to 15thDay
12%13%30%24%13%5%
313
16th to endDay
12%9%28%23%17%9%
332
September1st to 15thDay
14%13%26%22%16%6%
315
16th to endDay
16%11%25%17%16%13%
278
October1st to 15thDay
21%15%22%21%13%5%
255
16th to endDay
23%20%26%21%8%
241
November1st to 15thDay
20%16%27%21%12%
235
16th to endDay
28%19%23%17%9%1
192
December1st to 15thDay
21%28%26%21%1
60
16th to endDay
35%28%29%41
64
January1st to 15thNight
19%28%30%18%2
720
16th to endNight
23%24%32%16%2
768
February1st to 15thNight
20%26%34%16%1
704
16th to endNight
22%25%30%16%3
631
March1st to 15thNight
27%22%29%18%2
720
16th to endNight
22%22%26%20%7%
768
April1st to 15thNight
21%25%28%20%4
718
16th to endNight
25%18%28%20%6%
604
May1st to 15thNight
24%15%31%22%5%
516
16th to endNight
20%8%31%30%9%
375
June1st to 15thNight
25%10%27%26%10%
360
16th to endNight
17%8%26%38%9%
360
July1st to 15thNight
14%5%25%39%13%1
340
16th to endNight
13%327%38%16%1
376
August1st to 15thNight
13%5%26%37%14%1
455
16th to endNight
13%5%23%33%20%2
738
September1st to 15thNight
16%9%29%25%18%
720
16th to endNight
14%8%26%28%16%5%
685
October1st to 15thNight
23%15%20%25%12%1
660
16th to endNight
23%23%24%22%6%
704
November1st to 15thNight
18%19%27%25%8%1
700
16th to endNight
22%20%27%19%8%
720
December1st to 15thNight
22%26%30%16%3
720
16th to endNight
28%24%30%14%2
768
Page generated at 2017-09-23 06:43:41. Version ac04

Copyright 2017 A. Danko


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