Brookhaven
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brookhaven clear sky chart from 2005-11-05 to 2018-02-14.

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
32%
15633
18%
48782
9%
15602
43%
15602
20%
31714

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
20%
780
19%
2716
8%
776
35%
776
17%
1703
January16th to endNight
18%
832
19%
2860
6%
832
35%
832
17%
1823
February1st to 15thNight
18%
760
16%
2349
4%
757
32%
757
18%
1576
February16th to endNight
20%
631
24%
1893
8%
631
40%
631
21%
1194
March1st to 15thNight
21%
720
18%
2160
7%
720
34%
720
19%
1419
March16th to endNight
23%
768
24%
2004
7%
768
36%
768
24%
1303
April1st to 15thNight
28%
718
18%
1796
6%
718
34%
718
22%
1239
April16th to endNight
32%
666
18%
1746
5%
666
39%
666
23%
1192
May1st to 15thNight
31%
540
16%
1620
7%
540
38%
540
17%
1141
May16th to endNight
37%
393
14%
1516
12%
393
43%
393
14%
1000
June1st to 15thNight
41%
360
13%
1440
15%
360
47%
360
15%
935
June16th to endNight
52%
360
14%
1440
14%
360
59%
360
21%
848
July1st to 15thNight
50%
340
11%
1360
15%
340
51%
340
19%
825
July16th to endNight
52%
376
15%
1418
15%
354
61%
354
20%
870
August1st to 15thNight
57%
548
19%
1619
13%
546
60%
546
28%
986
August16th to endNight
61%
764
18%
1910
11%
764
63%
764
32%
1266
September1st to 15thNight
49%
720
21%
1824
13%
720
52%
720
30%
1222
September16th to endNight
48%
717
22%
1980
14%
717
55%
717
26%
1290
October1st to 15thNight
38%
720
20%
2052
14%
720
45%
720
24%
1368
October16th to endNight
29%
768
20%
2436
10%
768
41%
768
21%
1580
November1st to 15thNight
30%
760
19%
2472
10%
760
42%
760
20%
1591
November16th to endNight
27%
780
19%
2535
9%
780
40%
780
19%
1612
December1st to 15thNight
24%
780
14%
2724
5%
780
35%
780
14%
1818
December16th to endNight
17%
832
19%
2912
4%
832
35%
832
14%
1913

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
15%5%2442445%7%42%
49303
Night
16%6%24424446%41%
48952
Cloud CoverDay
15%5%2442445%7%42%
49313
Night
17%6%2442445%7%40%
48957
TransparencyDay
40%6%13%22%15%1
48991
Night
42%5%12%22%14%2
48782
SeeingDay
18%12%22%26%15%3
6312
Night
18%19%24%24%11%1
15634

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
16%6%245%34346%42%
1552
16th to endDay
16%6%45%325%336%42%
1716
February1st to 15thDay
17%5%25%434445%41%
1813
16th to endDay
16%5%15%424345%44%
1580
March1st to 15thDay
17%6%25%423348%39%
1790
16th to endDay
17%6%24323447%42%
2217
April1st to 15thDay
19%6%24334446%41%
2152
16th to endDay
18%5%2332345%6%44%
2214
May1st to 15thDay
20%7%33323446%38%
2332
16th to endDay
16%5%23425%45%9%40%
2607
June1st to 15thDay
19%5%2442445%8%37%
2520
16th to endDay
10%5%2443445%10%43%
2520
July1st to 15thDay
10%5%25%5%25%5%7%8%42%
2520
16th to endDay
13%414426%5%7%8%41%
2654
August1st to 15thDay
10%42342446%9%46%
2316
16th to endDay
11%4245%2445%7%46%
2292
September1st to 15thDay
12%4235%2435%7%47%
2136
16th to endDay
15%6%25%433346%45%
1980
October1st to 15thDay
18%5%24423346%43%
1905
16th to endDay
16%4145%2435%7%43%
1786
November1st to 15thDay
18%6%23433335%43%
1716
16th to endDay
14%5%244245%5%7%43%
1755
December1st to 15thDay
17%7%245%2345%7%39%
1566
16th to endDay
21%8%44323347%36%
1664
January1st to 15thNight
18%5%23423346%44%
2716
16th to endNight
17%9%24323335%44%
2860
February1st to 15thNight
15%8%36%424445%40%
2348
16th to endNight
17%414434346%44%
1893
March1st to 15thNight
17%6%24434345%43%
2147
16th to endNight
15%7%24323336%45%
2004
April1st to 15thNight
18%6%24433446%41%
1796
16th to endNight
19%6%24423447%39%
1746
May1st to 15thNight
19%8%245%35%445%34%
1620
16th to endNight
20%5%25%42446%8%33%
1514
June1st to 15thNight
21%6%2232435%6%37%
1440
16th to endNight
10%435%435%46%9%42%
1440
July1st to 15thNight
12%6%245%347%6%9%38%
1440
16th to endNight
10%6%245%36%6%5%10%37%
1514
August1st to 15thNight
8%4245%35%5%7%9%43%
1629
16th to endNight
11%434425%5%6%8%42%
1910
September1st to 15thNight
15%4245%2446%6%44%
1824
16th to endNight
15%5%24434245%46%
1980
October1st to 15thNight
17%6%26%423345%44%
2052
16th to endNight
18%5%15%413346%44%
2436
November1st to 15thNight
17%6%24423345%43%
2472
16th to endNight
17%5%24423447%43%
2535
December1st to 15thNight
20%7%24424446%37%
2724
16th to endNight
19%7%25%424446%38%
2912

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
16%7%24434448%39%
1552
16th to endDay
16%7%34424347%40%
1716
February1st to 15thDay
17%5%35%425%45%7%38%
1813
16th to endDay
15%5%246%245%5%6%42%
1580
March1st to 15thDay
18%6%25%423348%39%
1800
16th to endDay
16%8%25%224445%43%
2217
April1st to 15thDay
18%6%24314447%42%
2152
16th to endDay
16%7%13324447%44%
2214
May1st to 15thDay
21%7%25%423446%37%
2332
16th to endDay
16%5%25%3245%5%9%38%
2607
June1st to 15thDay
19%6%2442435%8%39%
2520
16th to endDay
11%4145%25%45%9%44%
2520
July1st to 15thDay
10%5%24425%5%7%9%41%
2520
16th to endDay
12%425%4246%6%9%42%
2654
August1st to 15thDay
10%41443446%8%48%
2316
16th to endDay
11%3244335%5%7%48%
2292
September1st to 15thDay
12%41432445%7%47%
2136
16th to endDay
15%5%25%433346%44%
1980
October1st to 15thDay
19%5%25%423446%44%
1905
16th to endDay
16%41442446%7%42%
1786
November1st to 15thDay
18%7%24423336%42%
1716
16th to endDay
14%5%24425%5%5%7%42%
1755
December1st to 15thDay
17%7%345%2446%7%37%
1566
16th to endDay
21%8%25%42345%7%34%
1664
January1st to 15thNight
18%5%2442336%8%40%
2716
16th to endNight
18%8%24233346%41%
2860
February1st to 15thNight
15%8%35%42445%6%38%
2341
16th to endNight
17%5%24425%45%6%43%
1893
March1st to 15thNight
17%6%34424346%42%
2160
16th to endNight
16%7%23322345%46%
2003
April1st to 15thNight
19%6%2442435%6%40%
1796
16th to endNight
20%6%2442435%7%39%
1746
May1st to 15thNight
18%8%35%5%35%446%34%
1620
16th to endNight
20%6%25%435%45%8%33%
1514
June1st to 15thNight
21%6%34433337%38%
1440
16th to endNight
10%415%5%35%5%5%10%41%
1440
July1st to 15thNight
12%6%24425%5%6%9%39%
1440
16th to endNight
11%5%35%5%35%48%10%36%
1514
August1st to 15thNight
7%5%245%346%6%8%44%
1629
16th to endNight
12%5%245%335%6%8%44%
1910
September1st to 15thNight
15%5%1442446%6%43%
1824
16th to endNight
17%6%25%42435%5%44%
1980
October1st to 15thNight
18%6%25%323345%43%
2052
16th to endNight
18%5%25%422446%42%
2436
November1st to 15thNight
18%5%25%423435%43%
2472
16th to endNight
17%6%25%42435%7%40%
2535
December1st to 15thNight
20%6%25%5%24448%35%
2724
16th to endNight
19%7%2442335%7%36%
2912

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
42%5%14%23%11%2
1553
16th to endDay
42%5%14%24%11%2
1716
February1st to 15thDay
44%5%14%21%12%1
1816
16th to endDay
42%411%26%13%1
1580
March1st to 15thDay
44%415%23%10%1
1800
16th to endDay
42%411%21%16%2
2217
April1st to 15thDay
41%5%11%21%16%3
2152
16th to endDay
39%5%10%22%19%3
2214
May1st to 15thDay
46%5%11%20%14%2
2332
16th to endDay
42%7%15%20%12%2
2611
June1st to 15thDay
43%6%13%22%12%1
2520
16th to endDay
34%8%16%24%14%1
2520
July1st to 15thDay
36%8%14%22%17%1
2371
16th to endDay
35%7%15%24%12%3
2487
August1st to 15thDay
32%8%17%23%17%1
2302
16th to endDay
32%9%9%25%20%1
2292
September1st to 15thDay
34%7%14%21%21%1
2136
16th to endDay
41%39%20%21%2
1980
October1st to 15thDay
41%6%11%21%17%1
1905
16th to endDay
37%6%13%24%16%1
1786
November1st to 15thDay
43%414%19%16%1
1716
16th to endDay
40%5%14%24%12%2
1755
December1st to 15thDay
43%6%17%17%13%
1566
16th to endDay
49%5%15%18%10%
1664
January1st to 15thNight
41%5%14%23%13%1
2716
16th to endNight
42%5%12%24%13%1
2860
February1st to 15thNight
45%6%13%22%11%1
2349
16th to endNight
40%411%24%16%2
1893
March1st to 15thNight
43%413%20%13%3
2160
16th to endNight
39%410%22%18%4
2004
April1st to 15thNight
43%5%11%21%15%2
1796
16th to endNight
44%410%17%19%3
1746
May1st to 15thNight
49%5%9%17%14%2
1620
16th to endNight
47%6%14%17%12%1
1516
June1st to 15thNight
47%6%11%18%15%1
1440
16th to endNight
37%6%13%26%14%1
1440
July1st to 15thNight
38%8%15%21%14%1
1360
16th to endNight
38%9%13%25%10%2
1418
August1st to 15thNight
34%7%15%24%17%1
1619
16th to endNight
36%6%13%25%17%1
1910
September1st to 15thNight
38%6%10%25%17%1
1824
16th to endNight
41%48%20%23%1
1980
October1st to 15thNight
42%5%10%21%16%2
2052
16th to endNight
42%5%12%23%15%1
2436
November1st to 15thNight
42%5%11%21%16%2
2472
16th to endNight
42%5%13%24%11%2
2535
December1st to 15thNight
47%5%17%18%10%
2724
16th to endNight
46%5%14%21%10%1
2912

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
14%16%42%18%6%1
90
16th to endDay
24%5%39%25%4
108
February1st to 15thDay
23%17%36%20%2
249
16th to endDay
15%20%35%21%6%
239
March1st to 15thDay
21%22%29%19%6%
270
16th to endDay
18%19%34%19%6%2
288
April1st to 15thDay
20%17%23%26%11%
270
16th to endDay
21%13%22%30%8%3
270
May1st to 15thDay
30%15%21%24%6%2
288
16th to endDay
20%12%18%26%19%1
331
June1st to 15thDay
18%12%20%26%19%3
315
16th to endDay
12%11%17%38%15%3
315
July1st to 15thDay
14%7%14%35%24%3
305
16th to endDay
11%9%11%34%26%5%
326
August1st to 15thDay
14%5%11%34%28%6%
313
16th to endDay
10%5%7%37%27%10%
332
September1st to 15thDay
16%6%21%24%22%8%
315
16th to endDay
20%6%15%23%23%10%
314
October1st to 15thDay
23%7%15%23%21%7%
315
16th to endDay
15%12%29%25%13%1
302
November1st to 15thDay
20%16%23%25%12%1
280
16th to endDay
12%22%29%22%11%1
285
December1st to 15thDay
20%13%34%20%9%1
96
16th to endDay
32%15%37%13%1
96
January1st to 15thNight
16%27%32%20%3
780
16th to endNight
21%24%32%20%1
832
February1st to 15thNight
18%22%36%21%1
760
16th to endNight
17%26%32%20%3
631
March1st to 15thNight
20%23%29%19%6%
720
16th to endNight
17%23%30%20%7%1
768
April1st to 15thNight
20%20%27%25%6%
718
16th to endNight
19%18%25%27%8%1
666
May1st to 15thNight
19%19%22%25%10%1
540
16th to endNight
21%18%13%30%14%1
394
June1st to 15thNight
17%18%15%29%18%
360
16th to endNight
10%17%12%38%20%
360
July1st to 15thNight
14%13%13%36%21%1
340
16th to endNight
10%17%10%35%22%2
376
August1st to 15thNight
10%10%13%40%24%1
548
16th to endNight
13%9%7%31%31%6%
764
September1st to 15thNight
15%9%18%31%20%4
720
16th to endNight
19%10%13%26%22%7%
717
October1st to 15thNight
19%12%23%23%18%2
720
16th to endNight
20%15%29%21%11%
768
November1st to 15thNight
18%20%25%19%13%1
760
16th to endNight
15%24%27%21%9%
780
December1st to 15thNight
22%22%26%19%7%1
780
16th to endNight
22%25%30%18%1
832
Page generated at 2018-02-15 10:58:48. Version ac04

Copyright 2018 A. Danko


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