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 2026-08-15.

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 5% 4 6% 8% 42%	143443
	Night	13% 6% 2 6% 4 4 4 4 6% 8% 44%	138453
Transparency	Day	39% 5% 12% 20% 18% 6%	147154
	Night	39% 4 11% 19% 20% 7%	142814
Seeing	Day	17% 9% 23% 25% 19% 7%	17532
	Night	16% 9% 22% 30% 19% 3	27545

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	15% 6% 2 7% 4 3 4 3 5% 6% 46%	5258
	11th to 20th	Day	12% 5% 2 4 3 2 2 2 3 5% 61%	5280
	21st to end	Day	21% 8% 2 7% 3 4 3 2 4 5% 41%	5780
February	1st to 10th	Day	15% 6% 2 6% 4 4 4 4 6% 6% 43%	2016
	11th to 20th	Day	22% 7% 2 4 3 3 3 3 4 6% 43%	2048
	21st to end	Day	18% 6% 3 6% 3 4 4 3 5% 6% 43%	1730
March	1st to 10th	Day	21% 7% 3 7% 4 3 4 3 4 5% 39%	5012
	11th to 20th	Day	9% 4 1 5% 4 4 4 3 5% 7% 54%	5090
	21st to end	Day	16% 6% 2 7% 4 4 3 3 4 4 46%	5586
April	1st to 10th	Day	11% 4 2 4 4 5% 4 5% 6% 7% 49%	2310
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 5% 5% 51%	2428
	21st to end	Day	9% 5% 1 5% 4 4 5% 4 6% 8% 50%	2730
May	1st to 10th	Day	14% 6% 2 7% 5% 4 4 4 6% 9% 39%	7616
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 8% 38%	7626
	21st to end	Day	15% 7% 2 7% 5% 5% 6% 4 6% 8% 35%	8391
June	1st to 10th	Day	8% 6% 2 8% 6% 6% 6% 6% 6% 10% 35%	2730
	11th to 20th	Day	7% 4 2 5% 5% 6% 5% 5% 8% 11% 43%	2730
	21st to end	Day	6% 4 2 6% 6% 5% 7% 5% 7% 12% 40%	2730
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 9% 12% 37%	7680
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 14% 35%	7680
	21st to end	Day	7% 6% 2 7% 5% 6% 8% 6% 10% 14% 28%	8340
August	1st to 10th	Day	17% 7% 3 8% 5% 6% 6% 5% 7% 10% 26%	4364
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 11% 37%	4226
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 6% 9% 50%	4223
September	1st to 10th	Day	8% 5% 2 7% 6% 5% 7% 4 6% 9% 41%	2200
	11th to 20th	Day	14% 6% 2 7% 5% 4 4 4 5% 8% 41%	2200
	21st to end	Day	15% 6% 2 7% 4 4 3 4 5% 7% 42%	2200
October	1st to 10th	Day	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2165
	11th to 20th	Day	11% 5% 2 4 2 3 3 2 4 6% 59%	2000
	21st to end	Day	12% 4 2 5% 3 3 3 3 4 4 57%	2200
November	1st to 10th	Day	13% 5% 2 5% 4 2 3 3 4 6% 52%	2046
	11th to 20th	Day	14% 5% 2 4 3 2 3 3 5% 6% 54%	1890
	21st to end	Day	13% 5% 2 4 4 4 3 3 5% 7% 49%	1890
December	1st to 10th	Day	14% 7% 2 5% 4 3 3 3 5% 5% 48%	3020
	11th to 20th	Day	16% 6% 2 5% 3 3 4 3 5% 6% 46%	3463
	21st to end	Day	22% 7% 2 4 3 3 3 3 4 6% 45%	4565
January	1st to 10th	Night	13% 5% 2 6% 3 3 3 3 3 6% 52%	7635
	11th to 20th	Night	12% 6% 2 4 3 3 4 3 5% 6% 52%	7680
	21st to end	Night	20% 7% 2 5% 4 3 3 2 3 4 44%	8401
February	1st to 10th	Night	15% 6% 2 5% 4 3 3 4 5% 6% 47%	2599
	11th to 20th	Night	19% 6% 2 6% 4 3 4 3 4 4 44%	2458
	21st to end	Night	16% 7% 2 5% 3 4 4 4 5% 5% 46%	2076
March	1st to 10th	Night	20% 7% 3 6% 4 4 4 3 5% 7% 39%	5059
	11th to 20th	Night	10% 4 2 4 3 3 4 3 6% 7% 56%	5090
	21st to end	Night	16% 5% 2 5% 3 4 3 3 5% 6% 47%	5588
April	1st to 10th	Night	12% 4 2 5% 4 3 4 3 6% 7% 49%	2310
	11th to 20th	Night	11% 4 1 5% 4 3 3 3 4 7% 54%	2154
	21st to end	Night	8% 5% 2 6% 4 3 4 4 5% 7% 53%	1890
May	1st to 10th	Night	13% 7% 2 8% 5% 5% 5% 4 7% 8% 34%	5237
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 4 6% 9% 39%	5234
	21st to end	Night	15% 8% 3 6% 4 5% 4 4 6% 9% 36%	5761
June	1st to 10th	Night	10% 6% 3 7% 5% 5% 6% 5% 8% 10% 35%	1890
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 5% 7% 11% 40%	1890
	21st to end	Night	7% 5% 2 6% 6% 6% 7% 6% 9% 9% 38%	1890
July	1st to 10th	Night	9% 6% 3 8% 6% 5% 6% 5% 8% 12% 30%	5278
	11th to 20th	Night	7% 5% 2 7% 5% 6% 6% 6% 10% 14% 31%	5277
	21st to end	Night	10% 8% 3 8% 7% 7% 7% 6% 9% 13% 23%	5736
August	1st to 10th	Night	15% 9% 2 7% 6% 6% 8% 6% 8% 11% 22%	3005
	11th to 20th	Night	10% 6% 3 7% 5% 4 5% 5% 7% 11% 36%	2911
	21st to end	Night	7% 4 2 6% 4 4 6% 5% 7% 9% 47%	3637
September	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 8% 9% 42%	2200
	11th to 20th	Night	13% 8% 2 7% 5% 4 5% 3 6% 8% 40%	2200
	21st to end	Night	14% 6% 2 5% 4 4 4 4 5% 6% 47%	2200
October	1st to 10th	Night	11% 6% 2 6% 2 3 4 3 4 6% 53%	2233
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 4 5% 58%	2400
	21st to end	Night	11% 4 2 5% 3 3 3 3 4 5% 57%	2640
November	1st to 10th	Night	12% 5% 2 4 3 3 3 3 4 6% 55%	2469
	11th to 20th	Night	14% 5% 2 4 3 2 2 3 4 6% 53%	2730
	21st to end	Night	14% 5% 1 5% 4 3 4 4 4 6% 50%	2730
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 4 5% 53%	4380
	11th to 20th	Night	17% 6% 2 6% 3 3 3 3 4 6% 47%	4952
	21st to end	Night	20% 6% 2 6% 3 3 3 3 4 6% 43%	6633

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% 2 7% 22% 23% 5%	5259
	11th to 20th	Day	29% 2 3 16% 36% 13%	5280
	21st to end	Day	49% 1 4 16% 20% 10%	5780
February	1st to 10th	Day	41% 4 7% 23% 19% 6%	2016
	11th to 20th	Day	44% 3 4 20% 22% 7%	2058
	21st to end	Day	43% 4 8% 15% 22% 9%	1730
March	1st to 10th	Day	49% 3 6% 13% 20% 10%	5012
	11th to 20th	Day	30% 3 7% 16% 25% 19%	5090
	21st to end	Day	43% 3 6% 19% 22% 7%	5586
April	1st to 10th	Day	34% 4 11% 20% 25% 6%	2310
	11th to 20th	Day	34% 3 9% 20% 23% 10%	2428
	21st to end	Day	32% 5% 12% 26% 22% 4	2730
May	1st to 10th	Day	42% 5% 15% 24% 12% 2	7651
	11th to 20th	Day	43% 4 17% 21% 14% 1	7630
	21st to end	Day	46% 7% 13% 17% 16% 1	8405
June	1st to 10th	Day	41% 7% 16% 22% 12% 2	2730
	11th to 20th	Day	33% 8% 18% 25% 15% 1	2730
	21st to end	Day	34% 10% 17% 22% 13% 3	2730
July	1st to 10th	Day	39% 6% 23% 26% 5% 1	7615
	11th to 20th	Day	38% 8% 31% 20% 2 1	7611
	21st to end	Day	44% 8% 21% 19% 7% 1	8156
August	1st to 10th	Day	52% 6% 17% 19% 5%	4351
	11th to 20th	Day	40% 7% 16% 21% 15% 2	4265
	21st to end	Day	32% 6% 14% 27% 17% 5%	4223
September	1st to 10th	Day	38% 8% 15% 18% 18% 4	2200
	11th to 20th	Day	42% 5% 11% 18% 21% 3	2200
	21st to end	Day	41% 5% 11% 20% 17% 6%	2200
October	1st to 10th	Day	32% 2 5% 17% 33% 10%	3529
	11th to 20th	Day	19% 2 4 17% 28% 29%	3240
	21st to end	Day	26% 2 4 23% 32% 13%	3538
November	1st to 10th	Day	34% 3 8% 17% 26% 11%	2046
	11th to 20th	Day	31% 4 8% 22% 26% 10%	1890
	21st to end	Day	35% 4 8% 20% 25% 7%	1890
December	1st to 10th	Day	39% 3 6% 17% 28% 8%	3020
	11th to 20th	Day	40% 3 9% 20% 20% 8%	3460
	21st to end	Day	42% 3 7% 22% 24% 2	4565
January	1st to 10th	Night	36% 2 8% 22% 26% 6%	7636
	11th to 20th	Night	34% 2 5% 16% 32% 11%	7680
	21st to end	Night	45% 2 3 15% 24% 9%	8401
February	1st to 10th	Night	39% 4 8% 20% 22% 8%	2599
	11th to 20th	Night	44% 3 6% 18% 22% 7%	2459
	21st to end	Night	40% 5% 7% 16% 21% 12%	2076
March	1st to 10th	Night	46% 3 9% 12% 23% 7%	5059
	11th to 20th	Night	28% 3 7% 13% 29% 20%	5090
	21st to end	Night	38% 3 6% 20% 20% 12%	5588
April	1st to 10th	Night	34% 4 9% 21% 24% 8%	2310
	11th to 20th	Night	31% 4 6% 24% 25% 10%	2154
	21st to end	Night	31% 5% 10% 26% 21% 6%	1890
May	1st to 10th	Night	45% 6% 14% 21% 13% 2	5264
	11th to 20th	Night	43% 4 17% 21% 13% 2	5235
	21st to end	Night	45% 6% 13% 18% 16% 1	5771
June	1st to 10th	Night	41% 9% 17% 18% 12% 3	1890
	11th to 20th	Night	37% 7% 18% 25% 12% 2	1890
	21st to end	Night	37% 10% 18% 19% 12% 3	1890
July	1st to 10th	Night	46% 5% 24% 20% 5% 1	5235
	11th to 20th	Night	41% 9% 31% 15% 2 1	5226
	21st to end	Night	51% 8% 20% 13% 6% 2	5606
August	1st to 10th	Night	55% 8% 18% 17% 3	2994
	11th to 20th	Night	42% 6% 21% 20% 10% 1	2938
	21st to end	Night	33% 6% 14% 28% 16% 3	3637
September	1st to 10th	Night	37% 8% 16% 18% 16% 5%	2200
	11th to 20th	Night	43% 6% 12% 16% 20% 4	2200
	21st to end	Night	38% 5% 11% 20% 19% 6%	2200
October	1st to 10th	Night	31% 3 8% 17% 33% 7%	3649
	11th to 20th	Night	19% 2 5% 16% 27% 30%	3899
	21st to end	Night	25% 2 4 20% 31% 18%	4252
November	1st to 10th	Night	32% 4 7% 18% 27% 12%	2469
	11th to 20th	Night	33% 3 6% 18% 27% 13%	2730
	21st to end	Night	35% 4 7% 19% 26% 9%	2730
December	1st to 10th	Night	36% 3 6% 17% 31% 8%	4380
	11th to 20th	Night	40% 2 12% 17% 21% 8%	4954
	21st to end	Night	44% 2 5% 25% 20% 3	6633

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% 18% 39% 16% 3	330
	11th to 20th	Day	26% 18% 35% 17% 3 1	330
	21st to end	Day	25% 25% 36% 10% 3 1	363
February	1st to 10th	Day	22% 18% 40% 17% 3	330
	11th to 20th	Day	37% 15% 34% 12% 2	330
	21st to end	Day	26% 18% 37% 15% 3	273
March	1st to 10th	Day	26% 15% 29% 24% 6%	520
	11th to 20th	Day	20% 16% 37% 20% 7% 1	540
	21st to end	Day	21% 16% 36% 20% 8%	594
April	1st to 10th	Day	20% 13% 31% 25% 10% 1	540
	11th to 20th	Day	13% 13% 32% 27% 13% 2	532
	21st to end	Day	14% 11% 27% 28% 15% 5%	540
May	1st to 10th	Day	14% 12% 31% 26% 13% 3	543
	11th to 20th	Day	15% 8% 20% 30% 18% 8%	567
	21st to end	Day	18% 8% 17% 24% 22% 10%	617
June	1st to 10th	Day	12% 4 16% 29% 29% 10%	570
	11th to 20th	Day	9% 4 17% 29% 28% 13%	570
	21st to end	Day	8% 3 14% 31% 31% 14%	570
July	1st to 10th	Day	8% 3 9% 24% 35% 21%	565
	11th to 20th	Day	8% 2 7% 27% 38% 19%	564
	21st to end	Day	8% 2 8% 32% 36% 14%	620
August	1st to 10th	Day	18% 4 11% 25% 29% 14%	566
	11th to 20th	Day	12% 3 9% 28% 32% 17%	535
	21st to end	Day	13% 2 12% 25% 31% 17%	583
September	1st to 10th	Day	9% 3 13% 29% 33% 13%	530
	11th to 20th	Day	18% 2 12% 28% 29% 11%	530
	21st to end	Day	18% 4 18% 27% 24% 9%	530
October	1st to 10th	Day	16% 4 22% 33% 15% 10%	530
	11th to 20th	Day	12% 8% 23% 30% 22% 6%	530
	21st to end	Day	16% 8% 26% 28% 18% 4	583
November	1st to 10th	Day	20% 8% 25% 27% 15% 4	524
	11th to 20th	Day	16% 12% 41% 22% 8% 2	330
	21st to end	Day	14% 12% 37% 25% 10% 2	330
December	1st to 10th	Day	27% 17% 31% 20% 5%	330
	11th to 20th	Day	24% 11% 35% 23% 6% 1	330
	21st to end	Day	28% 15% 33% 16% 5% 3	363
January	1st to 10th	Night	21% 16% 28% 29% 5%	840
	11th to 20th	Night	20% 18% 29% 26% 7%	840
	21st to end	Night	23% 17% 34% 24% 2	924
February	1st to 10th	Night	20% 14% 31% 29% 6%	840
	11th to 20th	Night	25% 15% 36% 22% 3	823
	21st to end	Night	21% 17% 33% 26% 4	692
March	1st to 10th	Night	18% 13% 32% 32% 5%	840
	11th to 20th	Night	17% 11% 36% 28% 7%	840
	21st to end	Night	17% 12% 34% 28% 8%	924
April	1st to 10th	Night	15% 14% 31% 29% 11%	840
	11th to 20th	Night	13% 12% 30% 30% 15%	799
	21st to end	Night	11% 8% 25% 40% 15% 1	630
May	1st to 10th	Night	12% 9% 23% 39% 16% 1	630
	11th to 20th	Night	14% 4 25% 33% 21% 3	621
	21st to end	Night	14% 4 11% 35% 31% 5%	683
June	1st to 10th	Night	12% 3 13% 38% 29% 5%	630
	11th to 20th	Night	8% 3 15% 30% 34% 10%	630
	21st to end	Night	10% 2 12% 32% 36% 8%	630
July	1st to 10th	Night	10% 3 7% 30% 39% 12%	615
	11th to 20th	Night	10% 1 5% 29% 43% 11%	612
	21st to end	Night	9% 1 9% 30% 42% 9%	690
August	1st to 10th	Night	14% 2 7% 32% 36% 9%	627
	11th to 20th	Night	12% 2 8% 30% 41% 7%	606
	21st to end	Night	12% 2 11% 30% 37% 8%	755
September	1st to 10th	Night	12% 3 10% 34% 35% 7%	800
	11th to 20th	Night	15% 2 12% 27% 39% 5%	800
	21st to end	Night	18% 3 15% 31% 29% 5%	800
October	1st to 10th	Night	14% 5% 19% 32% 26% 6%	800
	11th to 20th	Night	14% 6% 20% 34% 22% 4	800
	21st to end	Night	13% 9% 22% 31% 23% 2	880
November	1st to 10th	Night	15% 6% 23% 33% 20% 3	820
	11th to 20th	Night	17% 11% 29% 28% 14% 1	840
	21st to end	Night	18% 11% 26% 28% 15% 1	840
December	1st to 10th	Night	22% 14% 25% 30% 9% 1	840
	11th to 20th	Night	20% 11% 26% 31% 12%	840
	21st to end	Night	26% 14% 24% 29% 7%	924

Page generated at 2026-08-12 03:37:13. Version ac07

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