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 2024-11-02.

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%	101201
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	97791
Transparency	Day	41% 7% 13% 22% 14% 2	100956
	Night	44% 6% 12% 22% 14% 2	97656
Seeing	Day	18% 9% 26% 30% 15% 3	15090
	Night	20% 9% 29% 29% 12% 2	24137

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	16% 7% 2 7% 4 4 3 3 4 5% 45%	2517
	11th to 20th	Day	17% 9% 2 6% 4 4 5% 3 5% 5% 39%	2520
	21st to end	Day	22% 10% 5% 8% 3 4 3 3 4 5% 33%	2900
February	1st to 10th	Day	17% 7% 3 7% 3 5% 3 4 5% 6% 40%	1795
	11th to 20th	Day	16% 5% 3 6% 4 5% 3 2 5% 5% 46%	1848
	21st to end	Day	20% 9% 2 6% 4 4 4 3 5% 5% 38%	1570
March	1st to 10th	Day	26% 10% 2 6% 3 3 3 2 3 5% 36%	3116
	11th to 20th	Day	9% 4 2 5% 4 3 4 4 6% 8% 49%	3264
	21st to end	Day	20% 7% 3 5% 3 3 3 3 5% 6% 41%	4166
April	1st to 10th	Day	18% 8% 3 6% 4 4 5% 3 5% 7% 39%	2280
	11th to 20th	Day	18% 7% 2 5% 3 4 4 4 4 6% 44%	2254
	21st to end	Day	20% 7% 2 4 3 4 3 4 6% 6% 42%	2365
May	1st to 10th	Day	27% 11% 2 5% 3 3 3 3 5% 7% 31%	4106
	11th to 20th	Day	25% 9% 3 6% 4 3 3 3 4 6% 34%	4221
	21st to end	Day	14% 6% 2 5% 4 3 4 4 6% 9% 44%	4837
June	1st to 10th	Day	17% 6% 2 5% 3 4 4 4 6% 9% 39%	2660
	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	14% 6% 2 5% 4 4 4 3 3 5% 50%	1681
	11th to 20th	Day	17% 7% 1 4 3 3 3 3 4 6% 48%	1710
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 6% 44%	1710
December	1st to 10th	Day	19% 6% 2 6% 4 4 3 4 7% 8% 37%	1528
	11th to 20th	Day	22% 7% 2 4 3 3 2 3 4 5% 46%	1906
	21st to end	Day	18% 10% 3 8% 5% 5% 4 3 5% 7% 33%	2772
January	1st to 10th	Night	18% 6% 2 4 4 4 3 3 4 6% 47%	4429
	11th to 20th	Night	18% 9% 3 7% 3 3 3 3 3 4 44%	4440
	21st to end	Night	23% 11% 3 8% 4 4 3 3 4 4 33%	4755
February	1st to 10th	Night	16% 9% 3 7% 4 3 3 3 4 6% 41%	2383
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 6% 41%	2218
	21st to end	Night	22% 6% 1 5% 4 4 3 3 4 5% 42%	1884
March	1st to 10th	Night	24% 12% 3 6% 3 3 3 2 3 4 36%	3724
	11th to 20th	Night	13% 6% 2 6% 4 4 4 3 6% 6% 46%	3690
	21st to end	Night	22% 8% 3 6% 4 2 3 4 4 5% 39%	3466
April	1st to 10th	Night	19% 7% 2 6% 4 4 4 4 4 6% 39%	1900
	11th to 20th	Night	19% 8% 2 6% 4 3 3 3 4 6% 42%	1875
	21st to end	Night	19% 7% 2 5% 3 4 4 4 4 6% 41%	1814
May	1st to 10th	Night	23% 12% 4 7% 4 4 4 3 5% 6% 27%	2834
	11th to 20th	Night	26% 9% 3 7% 5% 4 4 4 5% 6% 29%	2700
	21st to end	Night	18% 9% 3 6% 4 3 3 4 6% 8% 37%	2738
June	1st to 10th	Night	19% 7% 3 4 4 4 5% 3 5% 8% 39%	1520
	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	13% 6% 2 6% 5% 4 3 3 5% 6% 47%	2431
	11th to 20th	Night	19% 7% 2 5% 4 3 3 3 4 5% 47%	2470
	21st to end	Night	15% 5% 1 5% 4 4 4 3 5% 7% 47%	2470
December	1st to 10th	Night	20% 7% 3 5% 4 4 4 3 5% 6% 39%	2650
	11th to 20th	Night	20% 7% 2 5% 4 4 3 3 5% 5% 43%	3288
	21st to end	Night	20% 9% 3 7% 3 4 4 3 4 5% 39%	4884

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	40% 4 13% 29% 12% 2	2518
	11th to 20th	Day	44% 7% 12% 25% 10% 1	2520
	21st to end	Day	52% 7% 9% 21% 9% 1	2900
February	1st to 10th	Day	45% 6% 15% 22% 12% 1	1795
	11th to 20th	Day	41% 5% 10% 25% 16% 2	1857
	21st to end	Day	46% 5% 11% 21% 13% 3	1570
March	1st to 10th	Day	47% 6% 13% 23% 10% 2	3119
	11th to 20th	Day	29% 8% 14% 32% 15% 1	3264
	21st to end	Day	44% 5% 8% 19% 20% 5%	4166
April	1st to 10th	Day	41% 6% 12% 20% 17% 3	2280
	11th to 20th	Day	40% 6% 13% 22% 17% 3	2254
	21st to end	Day	41% 6% 10% 21% 18% 4	2365
May	1st to 10th	Day	52% 10% 13% 14% 10% 1	4120
	11th to 20th	Day	50% 7% 15% 18% 9% 1	4225
	21st to end	Day	36% 10% 13% 23% 16% 2	4837
June	1st to 10th	Day	40% 8% 13% 21% 15% 1	2660
	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	40% 7% 11% 22% 16% 3	2067
	11th to 20th	Day	35% 6% 13% 25% 20% 1	1897
	21st to end	Day	41% 7% 12% 22% 16% 2	1881
November	1st to 10th	Day	38% 5% 13% 23% 19% 2	1681
	11th to 20th	Day	39% 5% 13% 22% 18% 3	1710
	21st to end	Day	40% 5% 15% 28% 10% 2	1710
December	1st to 10th	Day	45% 6% 15% 19% 13% 1	1528
	11th to 20th	Day	41% 5% 15% 20% 17% 2	1906
	21st to end	Day	49% 8% 12% 23% 8%	2772
January	1st to 10th	Night	39% 5% 12% 30% 12% 3	4429
	11th to 20th	Night	46% 5% 11% 25% 11% 2	4440
	21st to end	Night	53% 6% 10% 20% 10% 1	4755
February	1st to 10th	Night	44% 7% 14% 23% 12% 2	2383
	11th to 20th	Night	45% 5% 10% 21% 15% 4	2220
	21st to end	Night	45% 4 10% 22% 16% 4	1884
March	1st to 10th	Night	52% 4 10% 21% 10% 2	3734
	11th to 20th	Night	36% 7% 12% 27% 14% 3	3691
	21st to end	Night	46% 5% 7% 15% 21% 6%	3466
April	1st to 10th	Night	45% 6% 10% 19% 16% 3	1900
	11th to 20th	Night	43% 6% 9% 20% 20% 2	1875
	21st to end	Night	44% 4 10% 21% 15% 5%	1814
May	1st to 10th	Night	56% 8% 10% 15% 9% 1	2840
	11th to 20th	Night	55% 8% 13% 14% 9% 2	2701
	21st to end	Night	43% 8% 14% 21% 13% 2	2739
June	1st to 10th	Night	44% 7% 10% 20% 18% 2	1520
	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	46% 5% 10% 21% 15% 3	2109
	11th to 20th	Night	36% 5% 12% 27% 17% 2	2281
	21st to end	Night	44% 6% 11% 21% 16% 1	2717
November	1st to 10th	Night	38% 6% 12% 23% 18% 3	2431
	11th to 20th	Night	41% 6% 10% 23% 17% 4	2470
	21st to end	Night	37% 5% 15% 29% 13% 1	2470
December	1st to 10th	Night	46% 6% 15% 19% 12% 1	2650
	11th to 20th	Night	43% 6% 12% 20% 17% 1	3288
	21st to end	Night	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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	19% 18% 33% 22% 7% 1	180
	11th to 20th	Day	17% 28% 33% 18% 4	180
	21st to end	Day	20% 12% 38% 22% 7% 1	234
February	1st to 10th	Day	23% 12% 36% 24% 5%	355
	11th to 20th	Day	20% 19% 37% 21% 2	456
	21st to end	Day	24% 16% 34% 21% 5% 1	382
March	1st to 10th	Day	17% 15% 38% 25% 5%	460
	11th to 20th	Day	15% 13% 41% 25% 5% 1	460
	21st to end	Day	21% 13% 36% 22% 6% 1	506
April	1st to 10th	Day	22% 13% 31% 25% 9%	460
	11th to 20th	Day	18% 13% 30% 29% 10% 1	452
	21st to end	Day	24% 8% 30% 30% 7% 2	460
May	1st to 10th	Day	29% 8% 27% 27% 7% 2	463
	11th to 20th	Day	22% 8% 25% 35% 9% 1	487
	21st to end	Day	18% 3 20% 32% 24% 3	529
June	1st to 10th	Day	16% 5% 22% 36% 18% 2	490
	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	16% 9% 27% 32% 14% 3	369
	11th to 20th	Day	20% 16% 29% 25% 9%	370
	21st to end	Day	15% 15% 31% 27% 11% 1	370
December	1st to 10th	Day	19% 13% 37% 24% 6% 1	188
	11th to 20th	Day	23% 23% 32% 18% 4	180
	21st to end	Day	26% 13% 34% 18% 9% 1	198
January	1st to 10th	Night	19% 15% 40% 22% 5%	760
	11th to 20th	Night	25% 16% 33% 23% 3	760
	21st to end	Night	20% 14% 41% 22% 3	836
February	1st to 10th	Night	21% 11% 39% 27% 3	760
	11th to 20th	Night	23% 15% 42% 18% 2	743
	21st to end	Night	23% 15% 36% 20% 5%	628
March	1st to 10th	Night	21% 12% 37% 23% 6%	760
	11th to 20th	Night	19% 12% 38% 24% 6% 1	760
	21st to end	Night	20% 11% 36% 25% 7% 1	836
April	1st to 10th	Night	23% 8% 34% 29% 6%	760
	11th to 20th	Night	21% 7% 32% 30% 9%	751
	21st to end	Night	23% 4 32% 32% 8% 1	674
May	1st to 10th	Night	26% 6% 25% 30% 12% 2	570
	11th to 20th	Night	23% 4 31% 32% 10%	501
	21st to end	Night	21% 2 14% 38% 22% 2	411
June	1st to 10th	Night	19% 3 21% 36% 20% 1	380
	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% 27% 14% 2	748
	11th to 20th	Night	20% 13% 31% 24% 10% 1	760
	21st to end	Night	16% 15% 34% 26% 8% 1	760
December	1st to 10th	Night	24% 14% 34% 21% 6% 1	760
	11th to 20th	Night	23% 15% 33% 24% 5%	760
	21st to end	Night	23% 11% 38% 24% 4	836

Page generated at 2024-10-30 13:19:25. Version ac07

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