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 2026-10-29.

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 Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(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

By Month

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 Forecast	Data Points
Cloud Cover	Day	16% 7% 2 6% 4 4 4 3 5% 7% 42%	149244
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	142840
Transparency	Day	40% 6% 13% 23% 15% 2	152775
	Night	43% 5% 11% 23% 16% 3	147385
Seeing	Day	17% 9% 27% 29% 14% 3	17665
	Night	19% 9% 29% 28% 12% 2	26666

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	12% 7% 2 7% 4 4 4 3 4 5% 47%	4661
	11th to 20th	Day	15% 8% 2 6% 3 3 3 3 4 5% 47%	4650
	21st to end	Day	16% 7% 4 6% 3 3 3 3 4 6% 43%	5364
February	1st to 10th	Day	16% 7% 3 7% 3 5% 4 4 5% 5% 41%	1985
	11th to 20th	Day	16% 5% 3 6% 4 5% 3 3 5% 5% 45%	2048
	21st to end	Day	19% 8% 2 5% 4 4 4 3 5% 5% 40%	1730
March	1st to 10th	Day	21% 9% 2 5% 3 3 3 3 4 6% 43%	4576
	11th to 20th	Day	12% 9% 2 5% 4 3 4 4 6% 8% 43%	4769
	21st to end	Day	18% 9% 3 6% 3 4 4 3 4 6% 40%	6102
April	1st to 10th	Day	18% 8% 2 6% 4 4 4 3 5% 7% 39%	2520
	11th to 20th	Day	17% 6% 2 5% 3 4 4 4 4 6% 44%	2494
	21st to end	Day	19% 7% 2 4 3 4 3 3 6% 6% 42%	2614
May	1st to 10th	Day	26% 9% 2 5% 3 3 3 3 5% 7% 33%	7639
	11th to 20th	Day	20% 9% 2 5% 3 3 3 3 4 6% 41%	7877
	21st to end	Day	23% 8% 2 5% 4 3 4 4 6% 7% 34%	9060
June	1st to 10th	Day	16% 6% 2 5% 3 4 4 4 6% 9% 41%	2940
	11th to 20th	Day	14% 6% 3 5% 5% 4 4 5% 6% 8% 41%	2940
	21st to end	Day	12% 6% 2 5% 4 5% 4 4 6% 11% 43%	2940
July	1st to 10th	Day	16% 7% 2 6% 4 4 5% 4 6% 9% 37%	8280
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 7% 11% 40%	8280
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 43%	8993
August	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 5% 8% 9% 34%	6144
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 8% 46%	5488
	21st to end	Day	10% 4 2 5% 4 4 5% 4 6% 9% 45%	6080
September	1st to 10th	Day	14% 5% 2 6% 4 4 4 3 5% 7% 47%	2520
	11th to 20th	Day	12% 5% 2 5% 4 3 3 3 5% 6% 52%	2373
	21st to end	Day	20% 7% 2 7% 4 4 4 3 5% 6% 38%	2310
October	1st to 10th	Day	20% 5% 2 4 3 3 3 2 5% 6% 46%	2285
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 6% 7% 47%	2097
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 7% 44%	2052
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 3 5% 51%	1851
	11th to 20th	Day	16% 6% 2 4 3 3 3 3 4 7% 49%	1890
	21st to end	Day	16% 6% 2 5% 4 4 4 4 5% 6% 44%	1890
December	1st to 10th	Day	14% 7% 2 6% 4 4 4 3 6% 7% 44%	2688
	11th to 20th	Day	24% 7% 2 4 2 2 2 2 3 5% 48%	3066
	21st to end	Day	18% 9% 3 8% 4 4 4 3 4 5% 39%	4048
January	1st to 10th	Night	14% 7% 3 5% 3 3 3 3 4 5% 49%	8234
	11th to 20th	Night	16% 8% 3 7% 3 3 3 2 4 5% 46%	8235
	21st to end	Night	15% 9% 3 7% 4 4 4 3 5% 5% 41%	8802
February	1st to 10th	Night	16% 9% 3 7% 4 4 3 3 4 6% 41%	2633
	11th to 20th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 41%	2458
	21st to end	Night	20% 6% 1 6% 4 4 4 3 5% 5% 41%	2076
March	1st to 10th	Night	20% 10% 3 5% 3 3 3 2 4 5% 43%	5484
	11th to 20th	Night	16% 9% 3 6% 3 4 4 3 5% 5% 42%	5405
	21st to end	Night	20% 8% 3 6% 4 3 4 4 4 5% 38%	5072
April	1st to 10th	Night	20% 7% 2 6% 4 5% 4 3 4 6% 38%	2100
	11th to 20th	Night	18% 8% 2 6% 4 3 3 3 4 7% 42%	2075
	21st to end	Night	20% 7% 2 5% 3 4 4 3 4 6% 41%	2004
May	1st to 10th	Night	22% 10% 4 6% 4 3 4 3 5% 7% 32%	5259
	11th to 20th	Night	21% 9% 3 6% 4 3 4 3 5% 7% 35%	5038
	21st to end	Night	25% 10% 3 7% 4 4 4 3 5% 6% 29%	5114
June	1st to 10th	Night	18% 8% 3 5% 4 4 5% 2 5% 7% 40%	1680
	11th to 20th	Night	17% 6% 2 4 3 4 4 4 7% 9% 40%	1680
	21st to end	Night	12% 6% 3 7% 6% 5% 5% 4 6% 9% 38%	1680
July	1st to 10th	Night	17% 10% 3 7% 5% 4 4 4 6% 8% 33%	4680
	11th to 20th	Night	15% 9% 3 7% 4 4 4 4 6% 9% 35%	4680
	21st to end	Night	15% 7% 3 6% 4 4 4 4 6% 8% 40%	5084
August	1st to 10th	Night	19% 7% 3 7% 4 5% 6% 4 7% 8% 32%	3956
	11th to 20th	Night	11% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	4557
	21st to end	Night	10% 5% 2 5% 4 4 4 4 5% 8% 49%	5045
September	1st to 10th	Night	15% 6% 2 6% 5% 4 5% 4 5% 7% 42%	2100
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 6% 52%	2247
	21st to end	Night	20% 6% 3 6% 4 4 4 2 5% 6% 39%	2310
October	1st to 10th	Night	19% 6% 2 6% 4 3 3 2 4 6% 43%	2331
	11th to 20th	Night	16% 6% 2 5% 3 2 3 3 4 6% 49%	2521
	21st to end	Night	19% 6% 2 6% 4 3 3 3 4 6% 44%	2965
November	1st to 10th	Night	12% 6% 2 6% 4 4 3 3 5% 7% 49%	2667
	11th to 20th	Night	18% 7% 2 4 4 3 3 3 4 5% 47%	2730
	21st to end	Night	15% 5% 1 6% 4 4 4 3 5% 7% 47%	2730
December	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 4 5% 49%	4710
	11th to 20th	Night	26% 7% 2 4 3 3 3 2 3 4 44%	5348
	21st to end	Night	18% 8% 3 6% 3 4 4 3 4 5% 43%	7150

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	37% 4 11% 29% 18% 1	4662
	11th to 20th	Day	39% 4 10% 23% 18% 6%	4680
	21st to end	Day	41% 5% 12% 25% 16% 1	5364
February	1st to 10th	Day	44% 5% 15% 22% 13% 1	1985
	11th to 20th	Day	42% 5% 10% 25% 16% 2	2057
	21st to end	Day	45% 5% 11% 22% 14% 3	1730
March	1st to 10th	Day	41% 5% 12% 19% 14% 8%	4579
	11th to 20th	Day	36% 7% 15% 27% 14% 1	4769
	21st to end	Day	45% 4 9% 15% 22% 5%	6102
April	1st to 10th	Day	42% 6% 12% 20% 17% 3	2520
	11th to 20th	Day	39% 6% 14% 22% 17% 2	2494
	21st to end	Day	41% 6% 10% 22% 18% 4	2614
May	1st to 10th	Day	50% 8% 13% 17% 11% 1	7651
	11th to 20th	Day	45% 6% 11% 23% 13% 1	7881
	21st to end	Day	48% 7% 12% 20% 12% 1	9061
June	1st to 10th	Day	40% 8% 13% 22% 17% 1	2940
	11th to 20th	Day	39% 8% 15% 24% 13% 2	2940
	21st to end	Day	36% 8% 17% 23% 16% 1	2940
July	1st to 10th	Day	44% 9% 13% 19% 14% 1	8210
	11th to 20th	Day	39% 7% 17% 25% 11% 1	8205
	21st to end	Day	38% 7% 12% 23% 19% 1	8795
August	1st to 10th	Day	43% 9% 20% 20% 8%	6130
	11th to 20th	Day	34% 7% 18% 33% 7% 1	5524
	21st to end	Day	33% 8% 14% 29% 15% 1	6080
September	1st to 10th	Day	37% 7% 14% 22% 18% 2	2520
	11th to 20th	Day	32% 5% 13% 25% 22% 2	2373
	21st to end	Day	48% 5% 10% 18% 17% 2	2310
October	1st to 10th	Day	33% 5% 12% 24% 21% 5%	3649
	11th to 20th	Day	26% 5% 12% 23% 31% 3	3325
	21st to end	Day	30% 5% 15% 27% 20% 2	3252
November	1st to 10th	Day	37% 4 14% 23% 20% 2	1851
	11th to 20th	Day	37% 5% 12% 22% 20% 3	1890
	21st to end	Day	40% 5% 15% 28% 10% 2	1890
December	1st to 10th	Day	40% 5% 15% 28% 12% 1	2688
	11th to 20th	Day	42% 4 13% 21% 17% 3	3066
	21st to end	Day	46% 6% 9% 22% 15% 3	4048
January	1st to 10th	Night	38% 4 11% 25% 21% 2	8234
	11th to 20th	Night	42% 4 11% 22% 16% 5%	8280
	21st to end	Night	45% 4 9% 21% 18% 4	8802
February	1st to 10th	Night	44% 6% 13% 23% 12% 2	2633
	11th to 20th	Night	45% 5% 9% 22% 15% 4	2460
	21st to end	Night	45% 3 9% 23% 16% 4	2076
March	1st to 10th	Night	45% 4 11% 20% 13% 8%	5494
	11th to 20th	Night	43% 5% 12% 24% 11% 4	5406
	21st to end	Night	48% 5% 7% 14% 22% 5%	5072
April	1st to 10th	Night	46% 6% 10% 19% 16% 4	2100
	11th to 20th	Night	42% 6% 10% 20% 20% 2	2075
	21st to end	Night	44% 4 10% 20% 17% 5%	2004
May	1st to 10th	Night	51% 7% 12% 17% 12% 1	5265
	11th to 20th	Night	49% 6% 13% 20% 11% 2	5039
	21st to end	Night	54% 6% 10% 19% 10% 1	5115
June	1st to 10th	Night	43% 7% 10% 20% 19% 1	1680
	11th to 20th	Night	40% 7% 13% 25% 13% 1	1680
	21st to end	Night	41% 8% 13% 25% 11% 2	1680
July	1st to 10th	Night	49% 6% 11% 21% 12% 1	4640
	11th to 20th	Night	44% 7% 13% 24% 11% 1	4632
	21st to end	Night	42% 6% 13% 22% 16% 1	4967
August	1st to 10th	Night	49% 8% 17% 17% 10% 1	3946
	11th to 20th	Night	39% 7% 17% 29% 7%	4587
	21st to end	Night	34% 6% 12% 31% 16% 1	5045
September	1st to 10th	Night	41% 6% 10% 24% 16% 2	2100
	11th to 20th	Night	34% 6% 10% 25% 22% 3	2247
	21st to end	Night	47% 6% 9% 19% 17% 2	2310
October	1st to 10th	Night	39% 4 11% 24% 18% 3	3734
	11th to 20th	Night	26% 4 10% 26% 30% 5%	4032
	21st to end	Night	31% 5% 13% 27% 22% 2	4715
November	1st to 10th	Night	37% 6% 12% 24% 18% 4	2667
	11th to 20th	Night	40% 5% 9% 22% 19% 5%	2730
	21st to end	Night	38% 5% 14% 29% 13% 1	2730
December	1st to 10th	Night	39% 4 13% 28% 13% 2	4710
	11th to 20th	Night	46% 4 9% 24% 15% 2	5348
	21st to end	Night	43% 5% 11% 21% 17% 3	7150

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	18% 20% 35% 21% 6%	220
	11th to 20th	Day	16% 30% 34% 17% 4	220
	21st to end	Day	19% 14% 41% 19% 6% 1	286
February	1st to 10th	Day	21% 12% 37% 25% 4	425
	11th to 20th	Day	21% 19% 37% 21% 3	536
	21st to end	Day	22% 15% 35% 23% 4 1	446
March	1st to 10th	Day	18% 15% 37% 24% 5%	540
	11th to 20th	Day	16% 13% 40% 25% 6% 1	540
	21st to end	Day	21% 14% 38% 20% 6% 1	594
April	1st to 10th	Day	22% 13% 32% 24% 8% 1	540
	11th to 20th	Day	17% 14% 30% 30% 9% 1	532
	21st to end	Day	22% 8% 31% 31% 6% 2	540
May	1st to 10th	Day	28% 8% 29% 27% 7% 1	543
	11th to 20th	Day	21% 8% 25% 35% 10% 1	567
	21st to end	Day	20% 6% 20% 30% 21% 3	617
June	1st to 10th	Day	16% 5% 21% 37% 19% 2	570
	11th to 20th	Day	14% 6% 24% 37% 16% 2	570
	21st to end	Day	12% 6% 19% 38% 20% 5%	570
July	1st to 10th	Day	13% 3 14% 38% 27% 4	565
	11th to 20th	Day	10% 3 16% 37% 28% 5%	564
	21st to end	Day	13% 2 16% 41% 25% 2	620
August	1st to 10th	Day	11% 4 14% 37% 29% 5%	566
	11th to 20th	Day	12% 3 13% 40% 25% 7%	559
	21st to end	Day	11% 3 17% 38% 24% 7%	627
September	1st to 10th	Day	17% 4 24% 26% 23% 7%	570
	11th to 20th	Day	13% 3 20% 32% 24% 8%	570
	21st to end	Day	22% 5% 25% 28% 16% 3	570
October	1st to 10th	Day	22% 6% 21% 25% 20% 6%	570
	11th to 20th	Day	16% 8% 31% 29% 14% 3	567
	21st to end	Day	21% 10% 31% 22% 12% 3	486
November	1st to 10th	Day	15% 9% 32% 29% 13% 2	425
	11th to 20th	Day	18% 17% 31% 25% 9%	430
	21st to end	Day	17% 17% 31% 25% 9% 1	430
December	1st to 10th	Day	18% 14% 37% 25% 7%	228
	11th to 20th	Day	24% 22% 33% 18% 4	220
	21st to end	Day	24% 15% 33% 18% 10%	242
January	1st to 10th	Night	18% 16% 41% 21% 5%	840
	11th to 20th	Night	24% 18% 33% 22% 3	840
	21st to end	Night	19% 17% 40% 21% 3	924
February	1st to 10th	Night	21% 12% 38% 27% 3	840
	11th to 20th	Night	23% 15% 40% 19% 2	823
	21st to end	Night	23% 14% 38% 21% 5%	692
March	1st to 10th	Night	21% 13% 36% 23% 6%	840
	11th to 20th	Night	19% 12% 36% 25% 6% 1	840
	21st to end	Night	20% 11% 37% 25% 6% 1	924
April	1st to 10th	Night	23% 9% 34% 27% 7% 1	840
	11th to 20th	Night	21% 7% 32% 30% 9%	831
	21st to end	Night	23% 4 32% 33% 8% 1	744
May	1st to 10th	Night	25% 7% 25% 30% 11% 2	630
	11th to 20th	Night	22% 5% 30% 32% 9% 1	555
	21st to end	Night	23% 3 15% 37% 20% 2	455
June	1st to 10th	Night	19% 3 20% 37% 20% 1	420
	11th to 20th	Night	17% 3 18% 43% 19% 1	420
	21st to end	Night	13% 1 20% 42% 23% 1	420
July	1st to 10th	Night	13% 4 16% 41% 23% 3	410
	11th to 20th	Night	13% 5% 12% 38% 28% 3	408
	21st to end	Night	13% 4 15% 45% 22% 2	460
August	1st to 10th	Night	10% 1 14% 44% 29% 2	539
	11th to 20th	Night	13% 3 15% 35% 30% 3	830
	21st to end	Night	14% 3 13% 37% 27% 6%	924
September	1st to 10th	Night	17% 3 19% 35% 22% 4	840
	11th to 20th	Night	16% 2 18% 33% 24% 7%	840
	21st to end	Night	23% 4 22% 31% 18% 3	840
October	1st to 10th	Night	21% 6% 26% 23% 18% 6%	840
	11th to 20th	Night	18% 9% 27% 28% 16% 1	840
	21st to end	Night	22% 9% 32% 25% 11% 1	913
November	1st to 10th	Night	16% 10% 31% 28% 14% 2	820
	11th to 20th	Night	19% 15% 32% 23% 10%	840
	21st to end	Night	16% 17% 32% 26% 8% 1	840
December	1st to 10th	Night	23% 16% 34% 21% 6% 1	840
	11th to 20th	Night	24% 16% 33% 24% 4	840
	21st to end	Night	23% 13% 37% 23% 4	924

Page generated at 2026-10-26 04:37:47. Version ac07

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