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 2025-06-10.

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	17% 7% 2 6% 4 4 4 3 5% 7% 41%	119243
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 42%	118980
Transparency	Day	41% 7% 13% 22% 16% 2	122817
	Night	42% 5% 11% 22% 16% 3	123554
Seeing	Day	18% 9% 27% 29% 14% 3	15815
	Night	20% 9% 29% 28% 12% 2	24947

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	13% 7% 2 6% 3 3 3 3 4 5% 51%	3597
	11th to 20th	Day	14% 8% 2 6% 4 4 4 3 5% 5% 46%	3570
	21st to end	Day	19% 8% 5% 7% 4 4 3 3 3 4 40%	4144
February	1st to 10th	Day	17% 7% 3 7% 3 5% 3 4 5% 6% 40%	1890
	11th to 20th	Day	16% 5% 3 6% 4 5% 3 3 5% 5% 45%	1948
	21st to end	Day	19% 8% 2 6% 4 3 4 3 5% 5% 40%	1650
March	1st to 10th	Day	21% 9% 2 5% 3 3 3 3 4 6% 43%	4476
	11th to 20th	Day	12% 9% 2 5% 4 3 4 4 6% 8% 43%	4666
	21st to end	Day	19% 9% 3 6% 3 4 4 3 4 6% 40%	5970
April	1st to 10th	Day	18% 8% 2 6% 4 4 5% 3 5% 7% 39%	2400
	11th to 20th	Day	18% 6% 2 5% 3 4 4 4 4 6% 43%	2374
	21st to end	Day	19% 7% 2 4 3 4 3 4 6% 6% 43%	2490
May	1st to 10th	Day	28% 9% 2 5% 3 3 3 3 6% 8% 29%	5886
	11th to 20th	Day	22% 10% 3 6% 4 3 3 3 4 6% 37%	6057
	21st to end	Day	20% 7% 2 5% 4 3 4 4 6% 8% 36%	6949
June	1st to 10th	Day	17% 6% 2 5% 3 4 4 4 6% 9% 39%	2786
	11th to 20th	Day	14% 5% 3 5% 5% 4 4 5% 6% 9% 41%	2660
	21st to end	Day	12% 6% 2 5% 4 4 4 4 5% 11% 43%	2660
July	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 5% 6% 8% 36%	4440
	11th to 20th	Day	14% 6% 2 5% 4 4 5% 4 6% 9% 41%	4440
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 7% 7% 36%	4814
August	1st to 10th	Day	18% 6% 2 7% 4 5% 4 4 7% 9% 35%	4189
	11th to 20th	Day	11% 6% 3 6% 5% 4 4 4 5% 8% 46%	3730
	21st to end	Day	7% 4 1 5% 4 4 6% 4 6% 9% 50%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 3 5% 7% 47%	2280
	11th to 20th	Day	12% 5% 3 5% 3 3 3 3 5% 6% 52%	2147
	21st to end	Day	19% 7% 2 6% 4 4 4 3 4 5% 40%	2090
October	1st to 10th	Day	21% 5% 2 4 3 3 3 2 4 7% 44%	2067
	11th to 20th	Day	15% 5% 2 5% 4 3 4 3 6% 7% 48%	1897
	21st to end	Day	19% 5% 2 6% 4 3 4 3 5% 7% 42%	1881
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 3 5% 51%	1761
	11th to 20th	Day	17% 6% 2 4 3 3 3 3 4 6% 49%	1800
	21st to end	Day	15% 6% 2 5% 3 4 4 4 5% 6% 44%	1800
December	1st to 10th	Day	14% 7% 2 6% 4 4 3 3 6% 7% 44%	2608
	11th to 20th	Day	24% 7% 2 4 2 2 2 2 3 5% 47%	2986
	21st to end	Day	18% 9% 3 8% 4 4 3 3 4 5% 39%	3960
January	1st to 10th	Night	14% 5% 2 4 3 4 3 3 4 6% 52%	6349
	11th to 20th	Night	15% 8% 3 7% 3 4 3 2 4 4 47%	6315
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 6% 38%	6797
February	1st to 10th	Night	17% 9% 3 7% 4 3 3 3 4 6% 41%	2508
	11th to 20th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 41%	2338
	21st to end	Night	21% 6% 2 5% 4 4 4 3 5% 5% 42%	1980
March	1st to 10th	Night	20% 10% 3 5% 3 3 3 2 4 5% 43%	5364
	11th to 20th	Night	16% 10% 3 6% 3 4 4 3 5% 5% 42%	5288
	21st to end	Night	20% 8% 3 6% 4 3 4 4 4 5% 38%	4962
April	1st to 10th	Night	19% 8% 2 6% 4 5% 4 4 4 6% 38%	2000
	11th to 20th	Night	19% 7% 2 6% 4 3 3 3 4 6% 42%	1975
	21st to end	Night	19% 7% 2 5% 3 4 4 4 4 6% 41%	1909
May	1st to 10th	Night	25% 11% 4 7% 4 3 4 3 5% 7% 27%	4054
	11th to 20th	Night	24% 10% 3 6% 4 3 3 3 5% 6% 33%	3878
	21st to end	Night	23% 9% 3 7% 4 4 4 3 6% 7% 32%	3926
June	1st to 10th	Night	19% 8% 3 4 4 4 5% 3 5% 7% 39%	1598
	11th to 20th	Night	15% 6% 2 4 3 4 4 4 7% 9% 41%	1520
	21st to end	Night	11% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	1520
July	1st to 10th	Night	14% 11% 3 7% 5% 5% 4 5% 7% 7% 33%	2520
	11th to 20th	Night	15% 7% 3 6% 5% 4 4 5% 6% 9% 35%	2520
	21st to end	Night	20% 8% 3 6% 4 4 4 3 6% 8% 32%	2739
August	1st to 10th	Night	23% 7% 3 6% 3 5% 5% 4 6% 8% 30%	2706
	11th to 20th	Night	11% 6% 3 6% 5% 5% 5% 4 8% 8% 39%	3097
	21st to end	Night	8% 4 2 4 4 3 4 4 6% 8% 53%	3476
September	1st to 10th	Night	15% 6% 2 6% 5% 4 5% 4 5% 7% 43%	1900
	11th to 20th	Night	13% 6% 1 5% 3 3 4 3 5% 6% 52%	2033
	21st to end	Night	19% 6% 3 6% 4 4 4 2 5% 6% 41%	2090
October	1st to 10th	Night	20% 7% 2 7% 4 4 3 2 4 6% 41%	2109
	11th to 20th	Night	15% 6% 2 6% 3 3 3 3 4 6% 50%	2281
	21st to end	Night	20% 6% 2 6% 4 3 3 3 4 6% 43%	2717
November	1st to 10th	Night	12% 6% 2 6% 5% 4 3 3 5% 6% 49%	2537
	11th to 20th	Night	19% 7% 2 5% 4 3 3 3 4 5% 47%	2600
	21st to end	Night	15% 5% 1 5% 4 4 4 3 5% 7% 46%	2600
December	1st to 10th	Night	15% 7% 2 5% 4 3 3 3 4 5% 49%	4570
	11th to 20th	Night	26% 7% 2 4 3 3 2 2 3 4 43%	5208
	21st to end	Night	18% 8% 3 6% 3 4 4 3 4 5% 42%	6996

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	34% 4 9% 31% 20% 1	3598
	11th to 20th	Day	39% 5% 11% 24% 16% 5%	3600
	21st to end	Day	46% 6% 10% 22% 14% 1	4144
February	1st to 10th	Day	44% 6% 15% 22% 12% 1	1890
	11th to 20th	Day	42% 5% 10% 25% 16% 2	1957
	21st to end	Day	45% 5% 11% 21% 14% 3	1650
March	1st to 10th	Day	41% 5% 12% 20% 14% 8%	4479
	11th to 20th	Day	36% 7% 15% 27% 14% 1	4666
	21st to end	Day	45% 4 9% 15% 22% 5%	5970
April	1st to 10th	Day	42% 6% 12% 20% 17% 4	2400
	11th to 20th	Day	40% 6% 13% 22% 17% 3	2374
	21st to end	Day	41% 6% 10% 22% 18% 4	2490
May	1st to 10th	Day	52% 9% 16% 14% 9% 1	5897
	11th to 20th	Day	49% 6% 12% 19% 13% 2	6061
	21st to end	Day	45% 9% 11% 22% 12% 1	6949
June	1st to 10th	Day	41% 8% 13% 21% 16% 1	2786
	11th to 20th	Day	38% 8% 15% 24% 13% 2	2660
	21st to end	Day	35% 8% 17% 24% 15% 1	2660
July	1st to 10th	Day	46% 10% 10% 17% 16% 1	4370
	11th to 20th	Day	38% 7% 19% 23% 12% 1	4365
	21st to end	Day	44% 8% 13% 18% 14% 2	4661
August	1st to 10th	Day	43% 8% 17% 22% 9%	4175
	11th to 20th	Day	36% 7% 14% 32% 10% 1	3766
	21st to end	Day	28% 10% 14% 29% 18% 1	4180
September	1st to 10th	Day	37% 7% 14% 21% 19% 2	2280
	11th to 20th	Day	32% 6% 13% 25% 23% 2	2147
	21st to end	Day	46% 6% 9% 18% 18% 2	2090
October	1st to 10th	Day	33% 6% 13% 23% 21% 5%	3431
	11th to 20th	Day	26% 5% 11% 23% 32% 3	3125
	21st to end	Day	31% 5% 15% 27% 20% 2	3081
November	1st to 10th	Day	37% 5% 14% 23% 20% 2	1761
	11th to 20th	Day	38% 5% 12% 22% 20% 3	1800
	21st to end	Day	40% 5% 15% 28% 9% 2	1800
December	1st to 10th	Day	40% 5% 15% 28% 12% 1	2608
	11th to 20th	Day	42% 4 13% 21% 17% 3	2986
	21st to end	Day	46% 6% 9% 21% 14% 3	3960
January	1st to 10th	Night	34% 4 9% 26% 24% 3	6349
	11th to 20th	Night	41% 4 11% 23% 16% 5%	6360
	21st to end	Night	47% 5% 10% 20% 15% 3	6797
February	1st to 10th	Night	44% 7% 13% 23% 11% 2	2508
	11th to 20th	Night	44% 5% 9% 22% 15% 4	2340
	21st to end	Night	45% 4 9% 22% 16% 4	1980
March	1st to 10th	Night	45% 4 11% 20% 13% 8%	5374
	11th to 20th	Night	43% 6% 12% 24% 11% 4	5289
	21st to end	Night	47% 5% 7% 14% 22% 5%	4962
April	1st to 10th	Night	46% 6% 10% 19% 16% 3	2000
	11th to 20th	Night	43% 6% 10% 19% 20% 2	1975
	21st to end	Night	44% 4 10% 21% 17% 5%	1909
May	1st to 10th	Night	55% 7% 13% 13% 9% 2	4060
	11th to 20th	Night	52% 6% 12% 15% 12% 3	3879
	21st to end	Night	50% 7% 12% 20% 10% 1	3927
June	1st to 10th	Night	44% 7% 10% 19% 18% 2	1598
	11th to 20th	Night	39% 7% 14% 26% 13% 1	1520
	21st to end	Night	40% 8% 13% 26% 12% 1	1520
July	1st to 10th	Night	48% 8% 13% 18% 13% 1	2480
	11th to 20th	Night	40% 9% 16% 25% 10% 1	2472
	21st to end	Night	50% 6% 13% 16% 13% 1	2651
August	1st to 10th	Night	50% 8% 17% 16% 9% 1	2696
	11th to 20th	Night	39% 8% 15% 29% 9%	3127
	21st to end	Night	28% 7% 12% 34% 18% 1	3476
September	1st to 10th	Night	41% 7% 10% 24% 17% 2	1900
	11th to 20th	Night	33% 6% 10% 25% 23% 3	2033
	21st to end	Night	45% 6% 9% 20% 18% 2	2090
October	1st to 10th	Night	40% 4 11% 24% 18% 2	3512
	11th to 20th	Night	26% 4 10% 26% 30% 5%	3792
	21st to end	Night	31% 5% 13% 27% 21% 2	4467
November	1st to 10th	Night	37% 6% 12% 23% 18% 4	2537
	11th to 20th	Night	40% 5% 10% 22% 18% 4	2600
	21st to end	Night	38% 5% 14% 29% 13% 1	2600
December	1st to 10th	Night	39% 4 13% 28% 13% 3	4570
	11th to 20th	Night	46% 4 9% 24% 15% 2	5208
	21st to end	Night	44% 5% 11% 21% 17% 3	6996

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	19% 20% 35% 21% 6% 1	200
	11th to 20th	Day	16% 29% 34% 18% 4	200
	21st to end	Day	19% 14% 40% 20% 6% 1	260
February	1st to 10th	Day	23% 12% 36% 24% 4	390
	11th to 20th	Day	21% 20% 36% 21% 2	496
	21st to end	Day	22% 15% 36% 22% 4 1	414
March	1st to 10th	Day	17% 16% 39% 24% 5%	500
	11th to 20th	Day	16% 13% 39% 25% 6% 1	500
	21st to end	Day	21% 13% 37% 21% 6% 1	550
April	1st to 10th	Day	23% 13% 31% 25% 8%	500
	11th to 20th	Day	18% 14% 31% 27% 9% 1	492
	21st to end	Day	22% 8% 30% 31% 7% 2	500
May	1st to 10th	Day	29% 8% 28% 27% 7% 1	503
	11th to 20th	Day	21% 8% 26% 35% 9% 1	527
	21st to end	Day	20% 4 20% 32% 22% 3	573
June	1st to 10th	Day	17% 5% 21% 36% 18% 2	526
	11th to 20th	Day	13% 4 25% 38% 18% 3	490
	21st to end	Day	12% 6% 19% 38% 20% 4	490
July	1st to 10th	Day	13% 3 15% 39% 26% 5%	485
	11th to 20th	Day	10% 3 15% 39% 28% 6%	484
	21st to end	Day	13% 2 17% 41% 25% 2	532
August	1st to 10th	Day	11% 4 14% 37% 29% 5%	486
	11th to 20th	Day	13% 3 12% 39% 25% 8%	479
	21st to end	Day	12% 3 16% 37% 25% 7%	539
September	1st to 10th	Day	18% 3 22% 26% 24% 8%	490
	11th to 20th	Day	13% 4 21% 30% 24% 9%	490
	21st to end	Day	22% 4 26% 29% 16% 3	490
October	1st to 10th	Day	25% 6% 20% 23% 20% 6%	490
	11th to 20th	Day	14% 7% 30% 30% 15% 3	487
	21st to end	Day	22% 9% 32% 23% 11% 2	429
November	1st to 10th	Day	15% 8% 31% 30% 13% 3	395
	11th to 20th	Day	19% 16% 31% 26% 9%	400
	21st to end	Day	16% 16% 32% 26% 10% 1	400
December	1st to 10th	Day	18% 13% 37% 25% 7%	208
	11th to 20th	Day	24% 23% 31% 18% 4	200
	21st to end	Day	25% 13% 33% 19% 10%	220
January	1st to 10th	Night	18% 16% 40% 21% 4	800
	11th to 20th	Night	25% 16% 33% 23% 3	800
	21st to end	Night	19% 15% 41% 22% 3	880
February	1st to 10th	Night	21% 11% 39% 26% 3	800
	11th to 20th	Night	23% 16% 41% 18% 2	783
	21st to end	Night	23% 14% 37% 20% 5%	660
March	1st to 10th	Night	21% 13% 37% 23% 6%	800
	11th to 20th	Night	20% 12% 37% 25% 6% 1	800
	21st to end	Night	20% 11% 37% 25% 7% 1	880
April	1st to 10th	Night	23% 9% 34% 28% 6%	800
	11th to 20th	Night	22% 8% 32% 29% 9%	791
	21st to end	Night	23% 4 32% 32% 8% 1	709
May	1st to 10th	Night	26% 7% 24% 31% 12% 2	600
	11th to 20th	Night	23% 5% 30% 32% 9% 1	528
	21st to end	Night	23% 3 14% 38% 21% 2	433
June	1st to 10th	Night	20% 3 21% 36% 20% 1	400
	11th to 20th	Night	16% 2 16% 44% 21% 1	380
	21st to end	Night	12% 1 20% 44% 22% 1	380
July	1st to 10th	Night	13% 3 16% 41% 23% 3	370
	11th to 20th	Night	13% 5% 11% 39% 29% 4	368
	21st to end	Night	14% 4 15% 44% 22% 1	416
August	1st to 10th	Night	9% 2 15% 46% 28% 2	487
	11th to 20th	Night	13% 3 15% 35% 31% 3	750
	21st to end	Night	14% 3 13% 36% 29% 7%	836
September	1st to 10th	Night	16% 3 19% 35% 22% 5%	760
	11th to 20th	Night	17% 2 18% 31% 24% 8%	760
	21st to end	Night	22% 4 22% 31% 19% 3	760
October	1st to 10th	Night	23% 6% 26% 22% 19% 5%	760
	11th to 20th	Night	18% 8% 27% 29% 16% 1	760
	21st to end	Night	23% 9% 32% 25% 10% 1	836
November	1st to 10th	Night	16% 9% 31% 28% 14% 2	780
	11th to 20th	Night	20% 13% 33% 24% 10% 1	800
	21st to end	Night	16% 16% 33% 26% 8% 1	800
December	1st to 10th	Night	23% 15% 34% 21% 7% 1	800
	11th to 20th	Night	24% 15% 33% 24% 5%	800
	21st to end	Night	23% 11% 38% 24% 5%	880

Page generated at 2025-06-07 10:28:15. Version ac07

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