Troy Dark Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Troy Dark Sky Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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	13% 6% 2 6% 4 4 4 4 6% 7% 44%	113941
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	114357
Transparency	Day	38% 5% 10% 20% 20% 7%	117696
	Night	37% 5% 10% 18% 22% 8%	118741
Seeing	Day	17% 9% 23% 25% 19% 8%	15680
	Night	16% 9% 22% 30% 20% 3	25832

By Month

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
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 5% 2 5% 3 3 3 3 4 5% 52%	4056
	11th to 20th	Day	13% 6% 2 4 3 2 2 2 3 5% 58%	4060
	21st to end	Day	19% 7% 2 6% 3 3 3 2 3 5% 45%	4466
February	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 6% 7% 42%	1920
	11th to 20th	Day	22% 7% 2 4 3 3 3 3 4 6% 42%	1948
	21st to end	Day	17% 6% 3 6% 3 4 4 3 5% 6% 43%	1650
March	1st to 10th	Day	21% 7% 3 7% 4 3 4 3 4 4 39%	4903
	11th to 20th	Day	9% 4 1 5% 4 4 4 3 5% 7% 54%	4980
	21st to end	Day	17% 6% 3 7% 4 4 3 3 4 4 46%	5465
April	1st to 10th	Day	11% 4 2 4 4 4 4 4 5% 7% 50%	2200
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 5% 6% 49%	2193
	21st to end	Day	10% 5% 1 4 4 4 5% 4 6% 8% 51%	2470
May	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 7% 9% 42%	4082
	11th to 20th	Day	17% 8% 3 6% 4 4 4 4 5% 7% 38%	4081
	21st to end	Day	8% 4 2 5% 4 4 5% 4 7% 10% 48%	4476
June	1st to 10th	Day	9% 6% 2 7% 6% 5% 6% 6% 6% 10% 35%	2470
	11th to 20th	Day	6% 5% 2 5% 5% 6% 5% 5% 8% 10% 44%	2470
	21st to end	Day	6% 4 2 6% 6% 5% 7% 5% 7% 11% 40%	2470
July	1st to 10th	Day	10% 7% 3 8% 6% 6% 7% 5% 8% 10% 31%	4120
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 13% 36%	4120
	21st to end	Day	9% 8% 3 9% 6% 7% 8% 6% 10% 12% 22%	4466
August	1st to 10th	Day	17% 7% 3 8% 5% 6% 6% 5% 7% 10% 27%	4104
	11th to 20th	Day	10% 4 2 6% 5% 6% 6% 5% 8% 11% 38%	4044
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 50%	4094
September	1st to 10th	Day	8% 5% 2 7% 6% 6% 7% 5% 6% 9% 40%	2090
	11th to 20th	Day	14% 7% 2 7% 5% 5% 4 4 5% 8% 40%	2090
	21st to end	Day	15% 6% 2 6% 4 4 3 4 5% 7% 43%	2090
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 6% 52%	2057
	11th to 20th	Day	12% 4 2 4 2 3 3 2 4 6% 59%	1900
	21st to end	Day	11% 4 2 5% 3 3 4 3 4 4 58%	2090
November	1st to 10th	Day	14% 5% 2 6% 4 3 3 3 4 6% 51%	1947
	11th to 20th	Day	15% 5% 2 4 3 2 3 3 5% 6% 52%	1800
	21st to end	Day	13% 6% 2 4 4 4 4 3 5% 7% 49%	1800
December	1st to 10th	Day	13% 7% 2 5% 5% 3 3 3 5% 6% 49%	2930
	11th to 20th	Day	16% 6% 2 5% 3 3 4 3 5% 6% 45%	3373
	21st to end	Day	22% 7% 2 4 3 3 3 3 5% 6% 44%	4466
January	1st to 10th	Night	13% 4 2 5% 3 3 3 2 3 5% 56%	5889
	11th to 20th	Night	12% 6% 2 4 3 3 4 4 5% 6% 51%	5900
	21st to end	Night	18% 8% 2 5% 3 3 3 3 4 5% 47%	6490
February	1st to 10th	Night	16% 6% 2 4 4 3 3 4 5% 6% 46%	2475
	11th to 20th	Night	20% 6% 2 6% 4 3 3 3 4 4 44%	2338
	21st to end	Night	15% 7% 2 5% 3 4 4 4 5% 5% 46%	1980
March	1st to 10th	Night	20% 7% 3 6% 4 4 4 3 5% 6% 39%	4948
	11th to 20th	Night	10% 4 2 4 3 3 4 3 6% 7% 56%	4980
	21st to end	Night	16% 5% 2 5% 3 4 3 3 5% 6% 47%	5467
April	1st to 10th	Night	13% 4 2 5% 4 3 4 3 6% 7% 50%	2200
	11th to 20th	Night	12% 5% 2 6% 4 3 3 3 4 7% 52%	1955
	21st to end	Night	8% 5% 2 5% 4 3 4 4 5% 7% 54%	1710
May	1st to 10th	Night	10% 6% 2 7% 4 5% 5% 4 7% 9% 40%	2813
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 3 5% 8% 42%	2812
	21st to end	Night	6% 4 2 5% 3 5% 4 4 7% 11% 49%	3079
June	1st to 10th	Night	9% 6% 3 7% 5% 6% 7% 5% 8% 11% 33%	1710
	11th to 20th	Night	7% 4 2 6% 5% 5% 6% 5% 7% 11% 41%	1710
	21st to end	Night	8% 4 1 6% 5% 6% 7% 6% 9% 10% 38%	1710
July	1st to 10th	Night	13% 8% 3 7% 6% 5% 6% 5% 8% 9% 30%	2840
	11th to 20th	Night	8% 5% 2 8% 5% 6% 6% 6% 10% 13% 32%	2840
	21st to end	Night	11% 9% 3 9% 7% 7% 7% 5% 9% 12% 21%	3087
August	1st to 10th	Night	14% 8% 2 7% 6% 6% 8% 6% 8% 12% 23%	2825
	11th to 20th	Night	10% 6% 3 7% 5% 4 6% 5% 7% 10% 37%	2785
	21st to end	Night	7% 4 2 6% 4 4 6% 5% 7% 9% 47%	3525
September	1st to 10th	Night	10% 5% 2 6% 5% 4 6% 4 8% 9% 41%	2090
	11th to 20th	Night	14% 8% 2 7% 5% 4 4 3 6% 8% 39%	2090
	21st to end	Night	13% 6% 2 5% 4 4 4 4 5% 6% 47%	2090
October	1st to 10th	Night	11% 6% 2 6% 2 3 4 3 4 6% 53%	2121
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 5% 58%	2280
	21st to end	Night	10% 4 2 5% 3 3 3 3 4 5% 57%	2508
November	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 6% 54%	2348
	11th to 20th	Night	15% 6% 2 4 3 3 3 3 4 6% 52%	2600
	21st to end	Night	14% 4 1 5% 4 3 4 4 4 6% 50%	2600
December	1st to 10th	Night	14% 6% 2 5% 3 3 3 3 4 5% 53%	4250
	11th to 20th	Night	17% 6% 2 6% 3 3 3 3 4 6% 46%	4822
	21st to end	Night	21% 6% 2 6% 3 3 3 3 4 6% 42%	6490

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	35% 3 5% 21% 29% 7%	4057
	11th to 20th	Day	31% 2 4 19% 34% 9%	4060
	21st to end	Day	44% 1 4 18% 22% 10%	4466
February	1st to 10th	Day	41% 4 8% 22% 19% 6%	1920
	11th to 20th	Day	44% 3 4 19% 23% 7%	1958
	21st to end	Day	42% 4 8% 15% 22% 9%	1650
March	1st to 10th	Day	49% 3 5% 13% 20% 10%	4903
	11th to 20th	Day	30% 3 7% 15% 25% 19%	4980
	21st to end	Day	43% 3 6% 19% 23% 7%	5465
April	1st to 10th	Day	34% 4 11% 20% 25% 6%	2200
	11th to 20th	Day	36% 4 10% 18% 22% 11%	2193
	21st to end	Day	31% 5% 11% 27% 22% 4	2470
May	1st to 10th	Day	36% 9% 14% 23% 15% 3	4120
	11th to 20th	Day	45% 7% 11% 22% 15% 1	4085
	21st to end	Day	29% 8% 16% 23% 22% 2	4489
June	1st to 10th	Day	41% 8% 16% 22% 11% 2	2470
	11th to 20th	Day	33% 9% 17% 25% 16% 1	2470
	21st to end	Day	35% 10% 17% 21% 14% 4	2470
July	1st to 10th	Day	46% 7% 17% 21% 7% 2	4055
	11th to 20th	Day	37% 10% 25% 22% 4 2	4051
	21st to end	Day	50% 13% 19% 13% 5% 1	4324
August	1st to 10th	Day	52% 7% 16% 20% 5%	4091
	11th to 20th	Day	40% 7% 15% 21% 16% 2	4083
	21st to end	Day	31% 6% 14% 28% 17% 5%	4094
September	1st to 10th	Day	38% 8% 15% 17% 17% 4	2090
	11th to 20th	Day	43% 6% 10% 17% 21% 3	2090
	21st to end	Day	41% 5% 10% 20% 18% 7%	2090
October	1st to 10th	Day	31% 3 5% 17% 34% 11%	3421
	11th to 20th	Day	19% 2 4 17% 28% 30%	3140
	21st to end	Day	26% 2 4 23% 32% 13%	3428
November	1st to 10th	Day	35% 4 8% 16% 26% 11%	1947
	11th to 20th	Day	32% 4 7% 21% 26% 10%	1800
	21st to end	Day	35% 4 8% 20% 25% 7%	1800
December	1st to 10th	Day	38% 3 6% 17% 28% 8%	2930
	11th to 20th	Day	40% 3 9% 21% 19% 8%	3370
	21st to end	Day	42% 3 7% 21% 24% 2	4466
January	1st to 10th	Night	32% 2 5% 19% 34% 8%	5889
	11th to 20th	Night	35% 3 5% 17% 32% 8%	5900
	21st to end	Night	42% 2 4 15% 29% 8%	6490
February	1st to 10th	Night	39% 5% 8% 20% 21% 8%	2475
	11th to 20th	Night	44% 3 6% 18% 22% 8%	2339
	21st to end	Night	39% 5% 8% 16% 21% 12%	1980
March	1st to 10th	Night	46% 3 8% 12% 24% 7%	4948
	11th to 20th	Night	28% 3 7% 13% 29% 20%	4980
	21st to end	Night	39% 3 6% 19% 21% 13%	5467
April	1st to 10th	Night	33% 5% 9% 21% 24% 8%	2200
	11th to 20th	Night	32% 4 7% 22% 24% 10%	1955
	21st to end	Night	30% 5% 10% 26% 22% 7%	1710
May	1st to 10th	Night	37% 9% 14% 22% 15% 3	2840
	11th to 20th	Night	40% 7% 14% 21% 16% 1	2813
	21st to end	Night	27% 9% 15% 25% 22% 2	3087
June	1st to 10th	Night	41% 9% 17% 19% 12% 2	1710
	11th to 20th	Night	35% 7% 17% 25% 13% 2	1710
	21st to end	Night	37% 11% 19% 18% 12% 4	1710
July	1st to 10th	Night	48% 8% 20% 15% 7% 2	2795
	11th to 20th	Night	41% 10% 25% 20% 4 1	2786
	21st to end	Night	55% 12% 17% 11% 4 1	2988
August	1st to 10th	Night	53% 8% 18% 17% 3	2814
	11th to 20th	Night	41% 7% 21% 19% 11% 1	2812
	21st to end	Night	32% 6% 15% 28% 16% 3	3525
September	1st to 10th	Night	38% 9% 15% 17% 16% 5%	2090
	11th to 20th	Night	43% 6% 12% 16% 19% 4	2090
	21st to end	Night	38% 5% 11% 20% 20% 7%	2090
October	1st to 10th	Night	31% 3 8% 16% 34% 7%	3537
	11th to 20th	Night	19% 2 5% 16% 27% 31%	3779
	21st to end	Night	24% 2 4 21% 31% 18%	4120
November	1st to 10th	Night	33% 4 7% 18% 25% 13%	2348
	11th to 20th	Night	35% 3 6% 17% 27% 13%	2600
	21st to end	Night	35% 4 7% 19% 26% 8%	2600
December	1st to 10th	Night	35% 3 6% 17% 32% 8%	4250
	11th to 20th	Night	40% 2 12% 17% 21% 8%	4824
	21st to end	Night	45% 2 5% 25% 20% 3	6490

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	25% 17% 40% 15% 3	300
	11th to 20th	Day	25% 17% 35% 19% 3 1	300
	21st to end	Day	24% 25% 37% 10% 3 1	330
February	1st to 10th	Day	22% 18% 38% 18% 3	300
	11th to 20th	Day	37% 16% 33% 13% 1	300
	21st to end	Day	23% 20% 39% 16% 2	249
March	1st to 10th	Day	26% 16% 28% 24% 5%	481
	11th to 20th	Day	20% 16% 37% 20% 6% 1	500
	21st to end	Day	22% 16% 35% 19% 8%	550
April	1st to 10th	Day	21% 14% 32% 24% 10% 1	500
	11th to 20th	Day	15% 14% 32% 25% 12% 2	452
	21st to end	Day	15% 12% 28% 26% 15% 5%	460
May	1st to 10th	Day	13% 12% 30% 27% 14% 4	463
	11th to 20th	Day	15% 7% 18% 31% 18% 9%	487
	21st to end	Day	15% 7% 17% 25% 25% 11%	529
June	1st to 10th	Day	12% 3 16% 29% 30% 10%	490
	11th to 20th	Day	8% 4 17% 28% 28% 14%	490
	21st to end	Day	9% 3 16% 31% 28% 13%	490
July	1st to 10th	Day	9% 3 9% 23% 34% 22%	485
	11th to 20th	Day	8% 2 7% 27% 38% 18%	484
	21st to end	Day	8% 2 7% 30% 37% 15%	532
August	1st to 10th	Day	15% 4 11% 24% 31% 15%	486
	11th to 20th	Day	12% 3 9% 28% 32% 16%	479
	21st to end	Day	12% 2 11% 24% 33% 17%	539
September	1st to 10th	Day	10% 3 13% 27% 34% 14%	490
	11th to 20th	Day	20% 2 12% 28% 28% 11%	490
	21st to end	Day	18% 4 19% 25% 25% 9%	490
October	1st to 10th	Day	17% 4 22% 32% 16% 11%	490
	11th to 20th	Day	12% 8% 23% 30% 21% 6%	490
	21st to end	Day	14% 7% 25% 29% 20% 5%	539
November	1st to 10th	Day	20% 8% 24% 27% 16% 5%	485
	11th to 20th	Day	17% 12% 40% 22% 7% 2	300
	21st to end	Day	15% 11% 36% 26% 11% 2	300
December	1st to 10th	Day	25% 17% 32% 21% 5%	300
	11th to 20th	Day	24% 11% 36% 22% 6% 1	300
	21st to end	Day	30% 15% 32% 16% 4 3	330
January	1st to 10th	Night	21% 17% 28% 28% 5%	800
	11th to 20th	Night	20% 17% 29% 27% 7%	800
	21st to end	Night	22% 17% 34% 24% 3	880
February	1st to 10th	Night	20% 14% 31% 30% 6%	800
	11th to 20th	Night	25% 15% 36% 22% 3	783
	21st to end	Night	20% 17% 33% 26% 4	660
March	1st to 10th	Night	19% 14% 32% 31% 5%	800
	11th to 20th	Night	17% 11% 37% 28% 8% 1	800
	21st to end	Night	18% 12% 34% 28% 8%	880
April	1st to 10th	Night	15% 15% 31% 28% 12%	800
	11th to 20th	Night	15% 13% 28% 29% 15%	726
	21st to end	Night	11% 9% 25% 38% 16% 1	570
May	1st to 10th	Night	11% 9% 24% 38% 18% 1	570
	11th to 20th	Night	13% 3 24% 34% 22% 4	561
	21st to end	Night	12% 3 11% 36% 33% 6%	617
June	1st to 10th	Night	11% 3 13% 39% 30% 5%	570
	11th to 20th	Night	6% 3 14% 31% 35% 11%	570
	21st to end	Night	10% 2 13% 32% 34% 8%	570
July	1st to 10th	Night	10% 3 7% 30% 38% 12%	555
	11th to 20th	Night	10% 1 6% 30% 43% 10%	552
	21st to end	Night	8% 1 8% 30% 43% 9%	624
August	1st to 10th	Night	11% 2 8% 31% 39% 9%	567
	11th to 20th	Night	12% 2 9% 31% 40% 7%	564
	21st to end	Night	12% 3 11% 28% 38% 8%	717
September	1st to 10th	Night	12% 3 10% 32% 36% 7%	760
	11th to 20th	Night	16% 3 13% 25% 38% 5%	760
	21st to end	Night	17% 3 15% 31% 29% 5%	760
October	1st to 10th	Night	14% 5% 18% 31% 26% 6%	760
	11th to 20th	Night	14% 6% 21% 34% 22% 4	760
	21st to end	Night	12% 8% 22% 32% 23% 2	836
November	1st to 10th	Night	15% 7% 22% 32% 21% 3	780
	11th to 20th	Night	17% 11% 29% 28% 14% 1	800
	21st to end	Night	18% 11% 25% 28% 16% 1	800
December	1st to 10th	Night	21% 14% 25% 30% 10% 1	800
	11th to 20th	Night	20% 11% 26% 31% 12% 1	800
	21st to end	Night	27% 14% 23% 29% 7%	880

Page generated at 2025-04-08 09:17:26. Version ac07

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