Mt. Stony Brook Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. Stony Brook Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 41%	130758
	Night	17% 8% 3 6% 4 4 4 3 5% 6% 41%	130432
Transparency	Day	40% 7% 13% 22% 16% 2	134286
	Night	42% 6% 11% 22% 16% 3	134978
Seeing	Day	18% 7% 24% 29% 17% 4	16680
	Night	20% 8% 29% 30% 12% 2	25869

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% 6% 2 6% 4 4 4 3 5% 6% 47%	4661
	11th to 20th	Day	16% 8% 3 6% 4 3 4 3 4 5% 46%	4650
	21st to end	Day	16% 8% 3 6% 3 4 4 3 5% 6% 42%	5397
February	1st to 10th	Day	17% 6% 3 7% 4 4 4 3 5% 7% 41%	1964
	11th to 20th	Day	15% 7% 3 6% 4 4 4 3 4 6% 45%	2048
	21st to end	Day	18% 8% 3 7% 4 4 3 2 5% 6% 40%	1730
March	1st to 10th	Day	20% 9% 2 5% 3 3 3 3 4 6% 43%	4496
	11th to 20th	Day	11% 7% 3 6% 4 4 4 4 6% 8% 44%	4671
	21st to end	Day	18% 8% 2 6% 4 4 4 3 5% 6% 41%	5970
April	1st to 10th	Day	17% 9% 3 6% 4 3 4 4 5% 7% 38%	2400
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 4 6% 44%	2374
	21st to end	Day	19% 6% 2 4 3 4 4 3 6% 7% 43%	2481
May	1st to 10th	Day	27% 9% 2 5% 3 3 4 4 5% 8% 31%	5886
	11th to 20th	Day	22% 9% 2 6% 3 4 3 3 5% 6% 38%	6120
	21st to end	Day	20% 7% 2 5% 3 3 4 4 6% 8% 38%	6949
June	1st to 10th	Day	16% 6% 2 5% 4 4 4 4 6% 9% 41%	2800
	11th to 20th	Day	14% 7% 2 5% 4 4 5% 4 6% 10% 40%	2800
	21st to end	Day	10% 6% 3 6% 4 4 4 3 6% 10% 43%	2800
July	1st to 10th	Day	10% 8% 3 7% 5% 4 5% 5% 7% 11% 36%	6360
	11th to 20th	Day	10% 8% 2 7% 4 4 6% 5% 8% 10% 34%	6360
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	6926
August	1st to 10th	Day	17% 7% 2 6% 5% 4 5% 4 6% 9% 35%	4329
	11th to 20th	Day	11% 6% 3 6% 4 4 4 3 6% 9% 44%	3850
	21st to end	Day	8% 4 2 5% 4 4 4 4 7% 9% 49%	4312
September	1st to 10th	Day	14% 6% 2 5% 3 4 4 3 6% 6% 47%	2400
	11th to 20th	Day	12% 6% 2 4 3 3 4 3 5% 7% 51%	2270
	21st to end	Day	19% 6% 2 7% 4 4 5% 3 5% 7% 40%	2200
October	1st to 10th	Day	20% 6% 2 5% 2 3 4 3 4 7% 45%	2159
	11th to 20th	Day	15% 6% 2 6% 3 3 3 4 6% 6% 47%	2000
	21st to end	Day	18% 6% 2 6% 3 4 4 4 4 7% 43%	1986
November	1st to 10th	Day	14% 5% 2 6% 4 3 3 3 4 6% 50%	1851
	11th to 20th	Day	16% 6% 2 5% 4 3 3 3 4 8% 48%	1890
	21st to end	Day	16% 7% 2 5% 3 4 3 4 5% 6% 44%	1874
December	1st to 10th	Day	15% 7% 2 6% 4 3 3 3 6% 8% 43%	2680
	11th to 20th	Day	22% 9% 2 4 2 2 2 2 4 5% 47%	3066
	21st to end	Day	17% 10% 3 7% 4 4 3 3 4 6% 38%	4048
January	1st to 10th	Night	13% 8% 3 6% 3 3 3 3 4 6% 48%	8234
	11th to 20th	Night	16% 8% 3 7% 3 3 3 3 4 6% 44%	8235
	21st to end	Night	16% 8% 3 7% 4 4 4 3 5% 6% 41%	8781
February	1st to 10th	Night	15% 10% 3 6% 4 4 4 4 4 6% 40%	2654
	11th to 20th	Night	18% 8% 2 5% 5% 3 4 3 6% 6% 40%	2458
	21st to end	Night	19% 7% 2 6% 4 4 4 3 4 7% 40%	2076
March	1st to 10th	Night	20% 10% 2 5% 3 3 3 2 4 6% 42%	5389
	11th to 20th	Night	16% 9% 3 6% 4 4 3 3 4 5% 42%	5283
	21st to end	Night	20% 9% 3 7% 4 4 4 3 5% 5% 38%	4962
April	1st to 10th	Night	19% 7% 3 6% 4 5% 4 3 5% 5% 38%	2000
	11th to 20th	Night	18% 8% 2 6% 4 4 3 3 4 6% 41%	1975
	21st to end	Night	17% 9% 3 6% 4 3 3 3 4 6% 42%	1917
May	1st to 10th	Night	25% 9% 4 7% 4 4 4 3 5% 7% 28%	4054
	11th to 20th	Night	23% 9% 3 6% 4 4 4 3 5% 6% 33%	3815
	21st to end	Night	21% 9% 3 6% 4 4 3 3 5% 7% 34%	3926
June	1st to 10th	Night	18% 6% 3 5% 4 4 4 3 5% 8% 40%	1600
	11th to 20th	Night	16% 8% 2 5% 3 4 4 4 6% 8% 40%	1600
	21st to end	Night	11% 6% 2 6% 6% 4 5% 5% 6% 9% 40%	1600
July	1st to 10th	Night	11% 10% 3 8% 5% 5% 5% 5% 6% 8% 34%	3600
	11th to 20th	Night	15% 11% 3 9% 5% 4 4 4 7% 9% 31%	3600
	21st to end	Night	13% 7% 3 7% 4 4 5% 3 6% 8% 41%	3926
August	1st to 10th	Night	22% 7% 2 5% 4 5% 4 5% 7% 8% 31%	2785
	11th to 20th	Night	12% 7% 2 6% 4 4 5% 4 6% 8% 40%	3197
	21st to end	Night	8% 4 2 5% 3 3 4 4 5% 8% 53%	3586
September	1st to 10th	Night	14% 5% 2 6% 4 5% 4 4 5% 7% 45%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 6% 52%	2130
	21st to end	Night	18% 6% 2 7% 4 4 3 3 5% 7% 41%	2200
October	1st to 10th	Night	19% 8% 2 6% 4 4 3 2 5% 6% 41%	2238
	11th to 20th	Night	16% 6% 2 5% 3 3 3 3 5% 6% 49%	2400
	21st to end	Night	19% 7% 3 6% 4 3 3 3 5% 7% 41%	2852
November	1st to 10th	Night	12% 6% 2 5% 5% 4 4 2 5% 6% 49%	2667
	11th to 20th	Night	17% 7% 2 4 4 3 3 3 5% 5% 47%	2730
	21st to end	Night	14% 7% 2 5% 4 3 4 3 6% 7% 45%	2744
December	1st to 10th	Night	16% 8% 2 5% 3 3 3 3 4 6% 47%	4720
	11th to 20th	Night	24% 8% 2 5% 3 3 3 2 4 4 42%	5348
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 5% 41%	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	36% 5% 12% 28% 17% 2	4662
	11th to 20th	Day	40% 4 10% 22% 17% 6%	4680
	21st to end	Day	41% 5% 12% 26% 16% 1	5397
February	1st to 10th	Day	43% 5% 14% 23% 14% 1	1964
	11th to 20th	Day	41% 4 10% 25% 17% 2	2057
	21st to end	Day	45% 5% 10% 22% 15% 3	1730
March	1st to 10th	Day	41% 6% 12% 18% 15% 9%	4499
	11th to 20th	Day	35% 7% 15% 28% 13% 2	4671
	21st to end	Day	42% 6% 9% 15% 23% 6%	5970
April	1st to 10th	Day	43% 5% 12% 20% 16% 4	2400
	11th to 20th	Day	41% 5% 12% 21% 18% 3	2374
	21st to end	Day	40% 6% 12% 22% 18% 4	2481
May	1st to 10th	Day	51% 7% 17% 14% 9% 1	5897
	11th to 20th	Day	46% 7% 12% 20% 13% 2	6124
	21st to end	Day	42% 8% 12% 23% 14% 1	6949
June	1st to 10th	Day	39% 9% 13% 23% 16% 1	2800
	11th to 20th	Day	39% 8% 13% 25% 13% 2	2800
	21st to end	Day	36% 8% 17% 23% 16% 1	2800
July	1st to 10th	Day	42% 10% 13% 20% 14% 1	6290
	11th to 20th	Day	40% 10% 19% 21% 10% 1	6285
	21st to end	Day	38% 8% 12% 22% 19% 1	6728
August	1st to 10th	Day	45% 8% 15% 22% 9% 1	4315
	11th to 20th	Day	35% 7% 14% 33% 11% 1	3886
	21st to end	Day	30% 7% 14% 27% 21% 1	4312
September	1st to 10th	Day	37% 7% 13% 22% 20% 2	2400
	11th to 20th	Day	32% 5% 12% 24% 25% 2	2270
	21st to end	Day	44% 6% 11% 19% 18% 2	2200
October	1st to 10th	Day	33% 7% 11% 21% 23% 5%	3510
	11th to 20th	Day	26% 5% 11% 24% 30% 4	3240
	21st to end	Day	29% 5% 15% 26% 23% 2	3186
November	1st to 10th	Day	36% 5% 14% 22% 21% 2	1851
	11th to 20th	Day	38% 4 11% 22% 23% 3	1890
	21st to end	Day	40% 5% 14% 27% 12% 2	1874
December	1st to 10th	Day	39% 5% 16% 28% 11% 1	2680
	11th to 20th	Day	41% 3 14% 22% 17% 3	3066
	21st to end	Day	47% 6% 10% 20% 15% 3	4048
January	1st to 10th	Night	38% 4 12% 25% 19% 2	8234
	11th to 20th	Night	42% 4 11% 21% 16% 5%	8280
	21st to end	Night	44% 4 9% 21% 18% 4	8781
February	1st to 10th	Night	44% 6% 12% 23% 13% 2	2654
	11th to 20th	Night	44% 5% 10% 21% 16% 4	2460
	21st to end	Night	45% 4 8% 23% 16% 4	2076
March	1st to 10th	Night	44% 4 11% 19% 14% 8%	5399
	11th to 20th	Night	43% 6% 13% 24% 11% 4	5284
	21st to end	Night	47% 5% 7% 13% 23% 5%	4962
April	1st to 10th	Night	47% 5% 10% 18% 16% 4	2000
	11th to 20th	Night	44% 6% 10% 18% 20% 2	1975
	21st to end	Night	43% 6% 9% 21% 17% 4	1917
May	1st to 10th	Night	56% 7% 13% 13% 10% 1	4060
	11th to 20th	Night	51% 7% 13% 14% 13% 2	3816
	21st to end	Night	48% 7% 13% 20% 10% 1	3927
June	1st to 10th	Night	43% 6% 9% 20% 19% 2	1600
	11th to 20th	Night	40% 9% 12% 26% 12% 1	1600
	21st to end	Night	39% 8% 14% 24% 13% 2	1600
July	1st to 10th	Night	46% 9% 12% 22% 11% 1	3560
	11th to 20th	Night	47% 9% 14% 20% 9%	3552
	21st to end	Night	39% 8% 13% 24% 15% 1	3809
August	1st to 10th	Night	49% 9% 16% 17% 9% 1	2775
	11th to 20th	Night	39% 7% 15% 29% 9% 1	3227
	21st to end	Night	27% 8% 12% 33% 18% 1	3586
September	1st to 10th	Night	39% 7% 10% 25% 18% 2	2000
	11th to 20th	Night	33% 7% 10% 24% 23% 3	2130
	21st to end	Night	43% 7% 9% 20% 19% 3	2200
October	1st to 10th	Night	38% 7% 10% 24% 18% 3	3654
	11th to 20th	Night	25% 5% 10% 26% 30% 4	3899
	21st to end	Night	31% 5% 13% 27% 21% 2	4602
November	1st to 10th	Night	36% 5% 12% 23% 20% 3	2667
	11th to 20th	Night	40% 4 9% 24% 19% 4	2730
	21st to end	Night	37% 5% 13% 29% 14% 2	2744
December	1st to 10th	Night	40% 4 13% 30% 12% 2	4720
	11th to 20th	Night	47% 3 9% 23% 15% 3	5348
	21st to end	Night	44% 5% 11% 20% 16% 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	24% 15% 31% 22% 7%	220
	11th to 20th	Day	18% 22% 35% 19% 5%	220
	21st to end	Day	23% 11% 37% 21% 7% 1	292
February	1st to 10th	Day	23% 11% 37% 25% 4	404
	11th to 20th	Day	21% 13% 38% 26% 3	536
	21st to end	Day	23% 14% 32% 25% 6%	446
March	1st to 10th	Day	18% 12% 35% 29% 7%	508
	11th to 20th	Day	17% 7% 39% 30% 7% 1	500
	21st to end	Day	22% 9% 32% 27% 7% 2	550
April	1st to 10th	Day	23% 10% 30% 25% 11% 1	500
	11th to 20th	Day	20% 9% 25% 32% 13% 1	492
	21st to end	Day	24% 6% 25% 33% 11% 2	500
May	1st to 10th	Day	27% 5% 26% 31% 10% 2	503
	11th to 20th	Day	19% 6% 22% 34% 17% 2	527
	21st to end	Day	20% 2 13% 29% 29% 5%	573
June	1st to 10th	Day	18% 2 15% 36% 25% 4	530
	11th to 20th	Day	16% 5% 18% 36% 22% 3	530
	21st to end	Day	12% 4 18% 34% 26% 6%	530
July	1st to 10th	Day	12% 4 14% 31% 31% 9%	525
	11th to 20th	Day	9% 5% 13% 30% 34% 10%	524
	21st to end	Day	12% 5% 15% 33% 31% 4	576
August	1st to 10th	Day	12% 2 13% 36% 29% 9%	526
	11th to 20th	Day	13% 4 11% 34% 31% 8%	519
	21st to end	Day	10% 4 17% 34% 24% 11%	583
September	1st to 10th	Day	18% 5% 17% 26% 23% 12%	530
	11th to 20th	Day	14% 3 15% 33% 24% 12%	530
	21st to end	Day	21% 3 22% 31% 18% 5%	530
October	1st to 10th	Day	24% 5% 18% 25% 20% 9%	530
	11th to 20th	Day	16% 9% 28% 28% 16% 4	530
	21st to end	Day	22% 9% 30% 24% 13% 2	465
November	1st to 10th	Day	16% 11% 24% 33% 14% 3	425
	11th to 20th	Day	19% 15% 33% 26% 7% 1	430
	21st to end	Day	20% 13% 31% 25% 10% 1	414
December	1st to 10th	Day	21% 12% 35% 23% 9%	220
	11th to 20th	Day	28% 16% 27% 23% 5%	220
	21st to end	Day	24% 11% 34% 21% 9%	242
January	1st to 10th	Night	19% 13% 40% 22% 5%	840
	11th to 20th	Night	25% 12% 34% 26% 4	840
	21st to end	Night	20% 13% 40% 24% 3	924
February	1st to 10th	Night	21% 11% 36% 30% 3	840
	11th to 20th	Night	24% 9% 42% 23% 2	823
	21st to end	Night	23% 10% 37% 25% 4	692
March	1st to 10th	Night	21% 10% 36% 27% 6%	809
	11th to 20th	Night	19% 8% 38% 27% 7% 1	800
	21st to end	Night	21% 7% 38% 27% 6% 1	880
April	1st to 10th	Night	23% 6% 33% 31% 6% 1	800
	11th to 20th	Night	21% 6% 29% 34% 10%	791
	21st to end	Night	21% 4 29% 36% 9% 1	717
May	1st to 10th	Night	24% 6% 26% 31% 11% 2	600
	11th to 20th	Night	21% 5% 27% 35% 12% 1	504
	21st to end	Night	21% 3 13% 36% 24% 1	433
June	1st to 10th	Night	20% 2 21% 35% 20% 2	400
	11th to 20th	Night	19% 3 15% 42% 21% 1	400
	21st to end	Night	11% 4 16% 46% 22% 1	400
July	1st to 10th	Night	10% 5% 18% 39% 26% 3	390
	11th to 20th	Night	13% 2 13% 40% 29% 2	388
	21st to end	Night	13% 3 14% 46% 23% 1	438
August	1st to 10th	Night	9% 2 13% 44% 29% 3	506
	11th to 20th	Night	14% 3 13% 36% 30% 4	790
	21st to end	Night	13% 2 15% 36% 28% 5%	880
September	1st to 10th	Night	16% 3 19% 35% 22% 5%	800
	11th to 20th	Night	16% 2 16% 36% 22% 7%	800
	21st to end	Night	22% 3 22% 32% 18% 3	800
October	1st to 10th	Night	22% 6% 26% 24% 17% 4	800
	11th to 20th	Night	20% 8% 27% 31% 14% 1	800
	21st to end	Night	23% 9% 33% 27% 9% 1	880
November	1st to 10th	Night	16% 9% 31% 29% 13% 2	820
	11th to 20th	Night	21% 12% 33% 25% 9%	840
	21st to end	Night	18% 13% 32% 29% 8% 1	840
December	1st to 10th	Night	24% 13% 33% 23% 6% 1	840
	11th to 20th	Night	24% 10% 36% 25% 4	840
	21st to end	Night	23% 12% 37% 23% 5%	924

Page generated at 2026-02-28 01:40:26. Version ac07

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