Brookhaven
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brookhaven clear sky chart from 2005-11-06 to 2024-03-18.

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
36%
23384
19%
80617
10%
23294
46%
23294
21%
51779

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 10thNight
21%
753
24%
4330
8%
737
39%
737
13%
2343
January11th to 20thNight
21%
760
22%
4338
8%
740
37%
740
10%
2613
January21st to endNight
20%
830
13%
4723
7%
814
38%
814
11%
3264
February1st to 10thNight
23%
760
18%
2383
7%
760
37%
760
20%
1581
February11th to 20thNight
17%
743
20%
2220
6%
740
34%
740
20%
1479
February21st to endNight
21%
628
23%
1884
8%
628
39%
628
21%
1230
March1st to 10thNight
25%
760
21%
2280
9%
760
39%
760
24%
1454
March11th to 20thNight
27%
752
22%
2202
10%
752
41%
752
25%
1412
March21st to endNight
28%
792
22%
1980
7%
792
39%
792
27%
1350
April1st to 10thNight
30%
720
20%
1800
7%
720
37%
720
25%
1268
April11th to 20thNight
36%
711
19%
1775
7%
711
42%
711
28%
1220
April21st to endNight
35%
639
19%
1719
8%
639
41%
639
27%
1174
May1st to 10thNight
35%
540
13%
1620
7%
540
40%
540
18%
1171
May11th to 20thNight
32%
475
17%
1537
11%
475
38%
475
18%
1057
May21st to endNight
53%
388
13%
1551
16%
388
55%
388
18%
1017
June1st to 10thNight
45%
360
15%
1440
15%
360
51%
360
18%
919
June11th to 20thNight
57%
360
15%
1440
19%
360
61%
360
21%
879
June21st to endNight
58%
360
15%
1440
16%
360
64%
360
21%
872
July1st to 10thNight
54%
350
12%
1400
16%
350
56%
350
20%
867
July11th to 20thNight
62%
348
15%
1392
20%
348
64%
348
22%
835
July21st to endNight
61%
394
19%
1486
22%
372
65%
372
22%
898
August1st to 10thNight
66%
460
18%
1524
15%
458
68%
458
29%
901
August11th to 20thNight
58%
710
15%
1774
9%
710
59%
710
28%
1225
August21st to endNight
62%
792
19%
1980
10%
792
63%
792
34%
1315
September1st to 10thNight
52%
720
20%
1800
11%
720
55%
720
30%
1244
September11th to 20thNight
56%
720
26%
1926
18%
720
60%
720
36%
1189
September21st to endNight
45%
720
21%
1980
15%
720
51%
720
26%
1337
October1st to 10thNight
37%
720
17%
1998
12%
720
43%
720
24%
1380
October11th to 20thNight
41%
720
22%
2160
14%
720
51%
720
29%
1345
October21st to endNight
28%
792
17%
2574
9%
792
39%
792
20%
1727
November1st to 10thNight
38%
740
22%
2407
13%
740
51%
740
25%
1469
November11th to 20thNight
29%
760
23%
2470
12%
760
44%
760
22%
1556
November21st to endNight
29%
760
22%
2470
9%
760
45%
760
24%
1490
December1st to 10thNight
24%
760
17%
2650
7%
760
38%
760
17%
1748
December11th to 20thNight
24%
760
18%
3176
6%
759
38%
759
14%
1968
December21st to endNight
22%
827
13%
4788
8%
817
38%
817
11%
2982

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%35%43445%7%42%
78551
Night
18%7%35%5%3445%6%42%
81229
TransparencyDay
40%7%13%23%15%2
78256
Night
43%6%12%23%14%2
81058
SeeingDay
18%9%26%29%15%3
14187
Night
20%9%29%28%12%2
23407

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 10thDay
16%7%26%5%34445%46%
2517
11th to 20thDay
17%8%35%5%35%45%5%40%
2520
21st to endDay
22%10%6%7%424335%33%
2900
February1st to 10thDay
17%7%36%4445%5%6%40%
1795
11th to 20thDay
16%5%35%5%34345%46%
1848
21st to endDay
20%8%35%5%3445%5%38%
1570
March1st to 10thDay
16%7%35%433447%44%
1897
11th to 20thDay
15%5%26%5%3446%8%42%
1919
21st to endDay
19%8%35%433446%41%
2376
April1st to 10thDay
18%7%36%435%347%41%
2160
11th to 20thDay
18%6%25%4345%46%44%
2134
21st to endDay
20%6%2433346%6%42%
2240
May1st to 10thDay
23%8%24423446%38%
2340
11th to 20thDay
18%6%3442445%7%42%
2371
21st to endDay
15%6%245%245%5%9%43%
2725
June1st to 10thDay
18%5%25%4245%6%10%39%
2520
11th to 20thDay
14%5%35%5%35%6%6%9%40%
2520
21st to endDay
12%6%244445%5%11%44%
2520
July1st to 10thDay
11%5%25%5%35%6%7%8%44%
2520
11th to 20thDay
11%5%25%5%35%5%7%9%44%
2520
21st to endDay
13%5%25%5%35%5%7%9%42%
2744
August1st to 10thDay
10%5%24435%5%8%9%44%
2383
11th to 20thDay
12%5%35%6%345%6%8%44%
2126
21st to endDay
12%5%25%5%35%5%6%8%45%
2376
September1st to 10thDay
14%5%245%3445%7%46%
2160
11th to 20thDay
12%5%3443335%6%52%
2034
21st to endDay
19%6%25%5%33446%42%
1980
October1st to 10thDay
22%5%24423346%44%
1959
11th to 20thDay
16%4245%3446%7%46%
1798
21st to endDay
20%5%25%5%25%45%7%41%
1782
November1st to 10thDay
14%6%245%34435%50%
1671
11th to 20thDay
17%6%24423347%48%
1710
21st to endDay
15%6%25%4245%5%7%44%
1710
December1st to 10thDay
19%6%25%5%3446%8%38%
1528
11th to 20thDay
22%6%24322346%46%
1906
21st to endDay
18%10%47%5%3445%7%33%
2772
January1st to 10thNight
18%6%23433346%47%
4429
11th to 20thNight
18%9%36%43333444%
4440
21st to endNight
23%11%47%5%33345%33%
4755
February1st to 10thNight
16%8%36%5%34447%41%
2383
11th to 20thNight
18%8%25%5%24446%41%
2218
21st to endNight
22%5%15%5%34345%43%
1884
March1st to 10thNight
17%6%25%433446%45%
2270
11th to 20thNight
16%7%25%43446%6%43%
2201
21st to endNight
16%8%36%5%24447%41%
1980
April1st to 10thNight
19%7%35%5%34446%40%
1800
11th to 20thNight
18%7%245%24446%42%
1775
21st to endNight
20%7%25%434446%41%
1719
May1st to 10thNight
21%9%36%6%25%45%6%33%
1620
11th to 20thNight
20%7%35%6%345%6%7%34%
1536
21st to endNight
19%7%25%4345%7%8%36%
1550
June1st to 10thNight
19%7%345%35%45%7%38%
1440
11th to 20thNight
16%6%2444447%9%40%
1440
21st to endNight
11%6%36%5%36%5%6%10%40%
1440
July1st to 10thNight
13%6%346%45%6%7%8%39%
1440
11th to 20thNight
12%6%35%5%25%7%5%9%40%
1440
21st to endNight
12%7%25%5%45%5%6%9%40%
1574
August1st to 10thNight
7%425%5%45%7%7%10%43%
1534
11th to 20thNight
11%6%36%7%45%5%8%8%37%
1774
21st to endNight
12%5%34435%5%6%9%44%
1980
September1st to 10thNight
15%5%26%6%35%5%5%7%42%
1800
11th to 20thNight
14%5%2442345%6%52%
1926
21st to endNight
19%5%35%5%35%35%6%42%
1980
October1st to 10thNight
21%6%26%5%24346%40%
1998
11th to 20thNight
16%6%25%423446%48%
2160
21st to endNight
21%6%35%5%23445%41%
2574
November1st to 10thNight
13%6%35%5%3445%6%47%
2407
11th to 20thNight
20%6%34423345%47%
2470
21st to endNight
15%425%43435%7%47%
2470
December1st to 10thNight
21%6%35%5%2445%6%40%
2650
11th to 20thNight
20%7%24433345%43%
3288
21st to endNight
20%9%36%434445%39%
4884

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 10thDay
40%413%29%12%2
2518
11th to 20thDay
44%7%12%25%10%1
2520
21st to endDay
52%7%9%21%9%1
2900
February1st to 10thDay
45%6%15%22%12%1
1795
11th to 20thDay
41%5%10%25%16%2
1857
21st to endDay
46%5%11%21%13%3
1570
March1st to 10thDay
39%6%15%23%14%3
1900
11th to 20thDay
39%6%13%26%15%1
1919
21st to endDay
45%5%9%19%17%4
2376
April1st to 10thDay
41%6%11%21%17%4
2160
11th to 20thDay
40%6%12%22%17%3
2134
21st to endDay
42%6%10%21%17%4
2240
May1st to 10thDay
47%7%11%20%13%3
2340
11th to 20thDay
42%6%12%21%16%3
2375
21st to endDay
39%9%16%22%12%2
2725
June1st to 10thDay
42%8%13%22%14%1
2520
11th to 20thDay
40%8%15%23%13%1
2520
21st to endDay
35%8%17%25%15%1
2520
July1st to 10thDay
36%11%14%21%16%1
2450
11th to 20thDay
34%8%16%27%13%2
2445
21st to endDay
35%9%14%24%15%3
2591
August1st to 10thDay
32%10%18%25%14%1
2369
11th to 20thDay
35%11%14%25%14%1
2126
21st to endDay
34%11%12%22%20%2
2376
September1st to 10thDay
37%7%15%21%18%2
2160
11th to 20thDay
32%6%13%24%23%2
2034
21st to endDay
44%6%9%19%19%2
1980
October1st to 10thDay
41%7%11%22%16%2
1959
11th to 20thDay
36%6%13%26%18%1
1798
21st to endDay
43%7%12%21%16%2
1782
November1st to 10thDay
38%5%13%23%19%2
1671
11th to 20thDay
39%5%13%22%18%3
1710
21st to endDay
40%5%15%28%10%2
1710
December1st to 10thDay
45%6%15%19%13%1
1528
11th to 20thDay
41%5%15%20%17%2
1906
21st to endDay
49%8%12%23%8%
2772
January1st to 10thNight
39%5%12%30%12%3
4429
11th to 20thNight
46%5%11%25%11%2
4440
21st to endNight
53%6%10%20%10%1
4755
February1st to 10thNight
44%7%14%23%12%2
2383
11th to 20thNight
45%5%10%21%15%4
2220
21st to endNight
45%410%22%16%4
1884
March1st to 10thNight
41%5%13%23%15%4
2280
11th to 20thNight
41%5%10%24%16%4
2202
21st to endNight
42%6%10%19%18%6%
1980
April1st to 10thNight
45%6%10%20%17%3
1800
11th to 20thNight
42%6%10%20%21%2
1775
21st to endNight
45%410%20%15%5%
1719
May1st to 10thNight
52%7%10%16%14%2
1620
11th to 20thNight
49%7%10%17%16%3
1537
21st to endNight
47%7%14%20%11%1
1551
June1st to 10thNight
45%7%10%19%17%2
1440
11th to 20thNight
40%7%13%25%13%1
1440
21st to endNight
39%8%13%26%12%1
1440
July1st to 10thNight
40%9%16%19%14%1
1400
11th to 20thNight
37%10%13%27%12%1
1392
21st to endNight
38%9%13%25%13%2
1486
August1st to 10thNight
33%9%19%24%14%1
1524
11th to 20thNight
39%10%14%25%11%1
1774
21st to endNight
35%9%13%24%17%2
1980
September1st to 10thNight
42%7%10%24%16%2
1800
11th to 20thNight
34%6%11%25%23%3
1926
21st to endNight
44%6%9%20%19%3
1980
October1st to 10thNight
46%5%10%21%15%3
1998
11th to 20thNight
38%6%13%28%15%1
2160
21st to endNight
46%6%11%21%16%1
2574
November1st to 10thNight
38%6%12%23%18%3
2407
11th to 20thNight
41%6%10%23%17%4
2470
21st to endNight
37%5%15%29%13%1
2470
December1st to 10thNight
46%6%15%19%12%1
2650
11th to 20thNight
43%6%12%20%17%1
3288
21st to endNight
46%6%12%24%11%1
4884

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 10thDay
19%18%33%22%7%1
180
11th to 20thDay
17%28%33%18%4
180
21st to endDay
20%12%38%22%7%1
234
February1st to 10thDay
23%12%36%24%5%
355
11th to 20thDay
20%19%37%21%2
456
21st to endDay
24%16%34%21%5%1
382
March1st to 10thDay
17%15%38%25%5%
460
11th to 20thDay
16%12%41%25%5%1
449
21st to endDay
21%12%37%22%6%2
462
April1st to 10thDay
21%12%32%24%9%
420
11th to 20thDay
18%12%29%29%11%1
412
21st to endDay
25%8%30%29%5%2
420
May1st to 10thDay
29%8%28%26%7%2
423
11th to 20thDay
22%8%26%34%10%1
447
21st to endDay
20%220%29%25%4
485
June1st to 10thDay
18%5%22%35%18%3
450
11th to 20thDay
14%425%37%18%3
450
21st to endDay
12%6%19%38%21%5%
450
July1st to 10thDay
13%315%38%26%5%
445
11th to 20thDay
9%314%39%29%7%
444
21st to endDay
13%316%40%26%3
488
August1st to 10thDay
10%314%38%30%6%
446
11th to 20thDay
13%212%39%26%7%
439
21st to endDay
12%316%37%24%8%
495
September1st to 10thDay
19%321%26%23%8%
450
11th to 20thDay
13%422%30%22%9%
450
21st to endDay
21%427%28%18%3
450
October1st to 10thDay
26%6%19%22%20%6%
450
11th to 20thDay
15%8%29%29%16%3
448
21st to endDay
23%10%33%22%11%2
396
November1st to 10thDay
16%8%27%32%14%3
365
11th to 20thDay
20%16%29%25%9%
370
21st to endDay
15%15%31%27%11%1
370
December1st to 10thDay
19%13%37%24%6%1
188
11th to 20thDay
23%23%32%18%4
180
21st to endDay
26%13%34%18%9%1
198
January1st to 10thNight
19%15%40%22%5%
760
11th to 20thNight
25%16%33%23%3
760
21st to endNight
20%14%41%22%3
836
February1st to 10thNight
21%11%39%27%3
760
11th to 20thNight
23%15%42%18%2
743
21st to endNight
23%15%36%20%5%
628
March1st to 10thNight
21%12%37%23%6%
760
11th to 20thNight
19%11%38%24%6%1
752
21st to endNight
20%9%37%27%7%1
792
April1st to 10thNight
23%8%34%27%7%
720
11th to 20thNight
22%7%31%31%10%
711
21st to endNight
24%431%31%8%1
639
May1st to 10thNight
26%6%25%30%12%2
540
11th to 20thNight
22%432%32%9%
475
21st to endNight
22%214%38%23%2
389
June1st to 10thNight
20%321%34%20%1
360
11th to 20thNight
16%217%44%20%1
360
21st to endNight
12%119%44%23%1
360
July1st to 10thNight
13%317%42%23%3
350
11th to 20thNight
13%5%10%39%29%4
348
21st to endNight
13%414%45%23%2
394
August1st to 10thNight
7%115%46%29%2
460
11th to 20thNight
13%315%35%31%3
710
21st to endNight
14%313%35%29%7%
792
September1st to 10thNight
17%318%35%22%5%
720
11th to 20thNight
18%219%31%22%8%
720
21st to endNight
22%421%31%19%3
720
October1st to 10thNight
23%6%26%21%19%5%
720
11th to 20thNight
19%8%26%30%17%1
720
21st to endNight
24%9%32%24%9%1
792
November1st to 10thNight
16%9%30%28%14%2
740
11th to 20thNight
20%13%31%24%10%1
760
21st to endNight
16%15%34%26%8%1
760
December1st to 10thNight
24%14%34%21%6%1
760
11th to 20thNight
23%15%33%24%5%
760
21st to endNight
23%11%38%24%4
836
Page generated at 2024-03-15 14:01:39. Version ac07

Copyright © 2024 Attilla Danko.


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