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 2024-02-20.

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% 5% 2 5% 5% 3 5% 5% 6% 8% 44%	78762
	Night	13% 5% 2 5% 5% 3 4 5% 5% 7% 45%	80434
Transparency	Day	39% 6% 11% 20% 19% 5%	78485
	Night	38% 6% 10% 19% 21% 7%	80230
Seeing	Day	16% 9% 23% 25% 19% 8%	14138
	Night	16% 9% 22% 30% 20% 3	24298

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	17% 6% 2 4 5% 2 4 4 5% 7% 44%	2836
	11th to 20th	Day	15% 5% 2 4 4 2 3 3 4 6% 53%	2840
	21st to end	Day	23% 7% 2 4 3 2 3 3 3 5% 44%	3124
February	1st to 10th	Day	16% 6% 2 5% 5% 3 4 5% 5% 7% 41%	1824
	11th to 20th	Day	21% 6% 3 3 3 3 3 4 4 6% 43%	1838
	21st to end	Day	19% 6% 3 5% 4 3 5% 4 5% 6% 42%	1480
March	1st to 10th	Day	15% 5% 2 4 4 2 4 3 3 5% 52%	1962
	11th to 20th	Day	18% 5% 2 5% 4 3 4 5% 5% 6% 43%	1980
	21st to end	Day	16% 5% 2 7% 5% 3 4 5% 5% 7% 42%	2178
April	1st to 10th	Day	11% 3 3 4 4 3 4 5% 5% 7% 50%	1980
	11th to 20th	Day	12% 5% 3 5% 5% 3 4 4 5% 6% 48%	2075
	21st to end	Day	9% 5% 1 3 5% 3 5% 5% 5% 8% 52%	2340
May	1st to 10th	Day	9% 4 2 4 4 3 5% 5% 6% 8% 50%	2340
	11th to 20th	Day	12% 5% 2 6% 5% 3 6% 6% 6% 8% 41%	2301
	21st to end	Day	12% 5% 2 5% 5% 3 6% 5% 7% 9% 42%	2531
June	1st to 10th	Day	9% 6% 2 6% 7% 4 6% 7% 6% 10% 36%	2340
	11th to 20th	Day	7% 4 2 4 5% 4 6% 6% 7% 10% 44%	2340
	21st to end	Day	6% 4 2 5% 7% 4 7% 7% 6% 11% 40%	2340
July	1st to 10th	Day	8% 6% 3 6% 9% 5% 7% 7% 8% 9% 33%	2340
	11th to 20th	Day	8% 5% 2 6% 6% 4 8% 7% 8% 12% 33%	2340
	21st to end	Day	7% 4 2 5% 7% 4 9% 8% 10% 12% 33%	2547
August	1st to 10th	Day	10% 6% 2 7% 7% 5% 6% 7% 9% 10% 32%	2327
	11th to 20th	Day	12% 4 3 6% 7% 5% 7% 6% 8% 8% 33%	2303
	21st to end	Day	11% 7% 3 7% 7% 4 6% 6% 7% 10% 31%	2332
September	1st to 10th	Day	9% 5% 2 6% 7% 4 6% 6% 6% 9% 40%	1980
	11th to 20th	Day	13% 6% 2 6% 6% 3 4 5% 5% 8% 42%	1980
	21st to end	Day	15% 6% 2 6% 4 3 4 4 5% 7% 43%	1980
October	1st to 10th	Day	13% 6% 2 4 4 2 3 3 4 6% 53%	1949
	11th to 20th	Day	12% 5% 2 4 3 2 3 3 4 6% 57%	1800
	21st to end	Day	11% 4 2 4 4 2 4 4 4 4 57%	1980
November	1st to 10th	Day	13% 5% 2 5% 4 2 3 4 4 5% 53%	1848
	11th to 20th	Day	14% 5% 2 4 3 2 3 4 5% 6% 53%	1710
	21st to end	Day	14% 5% 3 4 4 3 4 4 5% 7% 48%	1710
December	1st to 10th	Day	17% 8% 3 5% 6% 3 4 4 5% 6% 39%	1710
	11th to 20th	Day	17% 5% 2 4 4 2 3 4 5% 6% 49%	2153
	21st to end	Day	21% 7% 2 4 4 2 4 4 5% 6% 41%	3124
January	1st to 10th	Night	17% 5% 2 5% 4 2 4 4 4 6% 46%	4109
	11th to 20th	Night	11% 6% 2 4 5% 3 4 6% 5% 7% 46%	4120
	21st to end	Night	22% 7% 3 5% 4 2 3 3 3 3 44%	4532
February	1st to 10th	Night	16% 6% 2 4 4 2 4 5% 5% 6% 45%	2352
	11th to 20th	Night	20% 5% 2 5% 4 2 4 4 4 4 45%	2213
	21st to end	Night	16% 7% 3 4 4 3 4 5% 5% 5% 45%	1776
March	1st to 10th	Night	14% 6% 3 4 4 3 4 4 5% 6% 49%	1996
	11th to 20th	Night	14% 5% 2 5% 4 3 4 5% 5% 7% 45%	1980
	21st to end	Night	13% 5% 2 6% 5% 2 5% 5% 5% 7% 44%	2178
April	1st to 10th	Night	13% 3 2 4 5% 2 4 3 6% 7% 51%	1980
	11th to 20th	Night	12% 4 2 5% 4 2 3 3 4 7% 53%	1845
	21st to end	Night	9% 4 2 4 4 2 4 4 5% 7% 54%	1620
May	1st to 10th	Night	8% 4 2 4 5% 3 4 5% 6% 8% 50%	1620
	11th to 20th	Night	9% 5% 2 5% 8% 4 6% 6% 6% 10% 40%	1592
	21st to end	Night	9% 6% 3 5% 5% 3 5% 6% 6% 10% 42%	1744
June	1st to 10th	Night	9% 6% 3 6% 6% 4 7% 7% 9% 11% 33%	1620
	11th to 20th	Night	7% 4 2 5% 6% 4 6% 7% 7% 11% 41%	1620
	21st to end	Night	8% 4 2 5% 6% 4 8% 8% 9% 10% 37%	1620
July	1st to 10th	Night	9% 6% 3 6% 7% 4 7% 7% 10% 11% 30%	1620
	11th to 20th	Night	8% 4 3 6% 7% 5% 6% 7% 9% 12% 34%	1620
	21st to end	Night	6% 5% 3 6% 7% 5% 7% 7% 8% 14% 32%	1771
August	1st to 10th	Night	9% 5% 2 5% 7% 4 8% 8% 10% 13% 29%	1611
	11th to 20th	Night	10% 4 4 6% 6% 3 6% 7% 8% 11% 36%	1598
	21st to end	Night	11% 5% 3 6% 7% 4 7% 7% 8% 10% 32%	2001
September	1st to 10th	Night	10% 4 2 4 7% 3 5% 6% 8% 9% 42%	1980
	11th to 20th	Night	13% 7% 2 6% 6% 3 4 5% 6% 8% 41%	1980
	21st to end	Night	13% 6% 2 4 4 3 4 5% 5% 6% 48%	1980
October	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 53%	2009
	11th to 20th	Night	11% 5% 2 4 4 3 3 4 4 5% 56%	2160
	21st to end	Night	11% 4 2 4 4 3 3 4 4 5% 56%	2376
November	1st to 10th	Night	12% 5% 2 3 4 2 3 4 4 6% 57%	2227
	11th to 20th	Night	14% 5% 2 4 3 2 3 4 4 6% 53%	2470
	21st to end	Night	15% 4 2 4 4 2 4 5% 4 6% 49%	2470
December	1st to 10th	Night	17% 7% 2 5% 4 3 3 4 5% 6% 43%	2470
	11th to 20th	Night	17% 5% 2 5% 3 2 2 3 4 5% 53%	3042
	21st to end	Night	20% 7% 2 6% 5% 3 4 4 5% 7% 38%	4532

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	39% 4 6% 22% 22% 7%	2837
	11th to 20th	Day	34% 3 5% 20% 28% 9%	2840
	21st to end	Day	44% 2 4 16% 26% 7%	3124
February	1st to 10th	Day	42% 4 8% 20% 20% 6%	1824
	11th to 20th	Day	43% 3 5% 19% 23% 7%	1848
	21st to end	Day	44% 5% 8% 15% 20% 9%	1480
March	1st to 10th	Day	37% 3 7% 20% 23% 11%	1962
	11th to 20th	Day	41% 5% 10% 19% 17% 8%	1980
	21st to end	Day	43% 5% 10% 20% 18% 5%	2178
April	1st to 10th	Day	33% 4 11% 19% 25% 6%	1980
	11th to 20th	Day	37% 4 9% 19% 21% 10%	2075
	21st to end	Day	31% 5% 11% 27% 22% 4	2340
May	1st to 10th	Day	31% 6% 13% 24% 21% 5%	2340
	11th to 20th	Day	40% 7% 15% 20% 16% 1	2305
	21st to end	Day	38% 8% 12% 25% 17% 2	2531
June	1st to 10th	Day	41% 8% 16% 22% 11% 2	2340
	11th to 20th	Day	33% 9% 17% 25% 14% 1	2340
	21st to end	Day	35% 11% 17% 21% 13% 4	2340
July	1st to 10th	Day	43% 9% 18% 17% 10% 3	2275
	11th to 20th	Day	40% 12% 20% 19% 7% 3	2271
	21st to end	Day	38% 13% 20% 20% 9% 1	2405
August	1st to 10th	Day	44% 11% 16% 20% 7% 1	2314
	11th to 20th	Day	45% 11% 15% 17% 10% 2	2303
	21st to end	Day	46% 10% 16% 17% 9% 2	2332
September	1st to 10th	Day	38% 9% 14% 18% 17% 4	1980
	11th to 20th	Day	41% 6% 10% 18% 22% 3	1980
	21st to end	Day	41% 5% 9% 20% 18% 7%	1980
October	1st to 10th	Day	34% 4 7% 21% 26% 7%	1949
	11th to 20th	Day	31% 3 7% 25% 26% 8%	1800
	21st to end	Day	32% 3 5% 17% 31% 11%	1980
November	1st to 10th	Day	34% 4 7% 16% 27% 11%	1848
	11th to 20th	Day	32% 4 7% 21% 25% 10%	1710
	21st to end	Day	36% 4 8% 20% 24% 8%	1710
December	1st to 10th	Day	46% 4 8% 18% 17% 7%	1710
	11th to 20th	Day	37% 3 7% 19% 22% 11%	2150
	21st to end	Day	43% 4 7% 18% 24% 3	3124
January	1st to 10th	Night	39% 3 6% 18% 26% 8%	4109
	11th to 20th	Night	37% 4 7% 19% 25% 8%	4120
	21st to end	Night	46% 3 4 13% 26% 8%	4532
February	1st to 10th	Night	40% 5% 8% 18% 22% 8%	2352
	11th to 20th	Night	43% 3 6% 18% 22% 8%	2214
	21st to end	Night	40% 5% 8% 16% 20% 10%	1776
March	1st to 10th	Night	37% 3 9% 15% 23% 12%	1996
	11th to 20th	Night	39% 5% 11% 15% 22% 8%	1980
	21st to end	Night	40% 6% 9% 20% 20% 7%	2178
April	1st to 10th	Night	32% 5% 9% 21% 25% 8%	1980
	11th to 20th	Night	32% 4 7% 22% 24% 10%	1845
	21st to end	Night	30% 5% 10% 26% 22% 7%	1620
May	1st to 10th	Night	32% 7% 13% 23% 20% 6%	1620
	11th to 20th	Night	39% 8% 16% 19% 16% 2	1593
	21st to end	Night	37% 9% 12% 23% 16% 3	1745
June	1st to 10th	Night	41% 9% 17% 19% 12% 2	1620
	11th to 20th	Night	36% 7% 18% 26% 12% 2	1620
	21st to end	Night	37% 12% 19% 17% 11% 3	1620
July	1st to 10th	Night	42% 11% 22% 12% 10% 3	1575
	11th to 20th	Night	39% 10% 24% 18% 6% 2	1566
	21st to end	Night	40% 12% 22% 19% 7% 1	1672
August	1st to 10th	Night	42% 13% 18% 21% 5%	1600
	11th to 20th	Night	40% 11% 18% 19% 11% 1	1598
	21st to end	Night	43% 11% 16% 18% 9% 2	2001
September	1st to 10th	Night	36% 9% 16% 17% 16% 5%	1980
	11th to 20th	Night	41% 6% 13% 15% 20% 4	1980
	21st to end	Night	37% 6% 11% 20% 21% 7%	1980
October	1st to 10th	Night	33% 5% 10% 18% 29% 6%	2009
	11th to 20th	Night	32% 4 8% 21% 26% 10%	2160
	21st to end	Night	30% 3 6% 18% 30% 13%	2376
November	1st to 10th	Night	31% 4 7% 18% 27% 13%	2227
	11th to 20th	Night	34% 3 6% 18% 26% 13%	2470
	21st to end	Night	36% 4 7% 19% 26% 8%	2470
December	1st to 10th	Night	41% 5% 9% 20% 20% 6%	2470
	11th to 20th	Night	37% 2 8% 17% 27% 9%	3044
	21st to end	Night	47% 3 7% 20% 20% 3	4532

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% 17% 39% 16% 3	270
	11th to 20th	Day	25% 17% 34% 20% 3 1	270
	21st to end	Day	24% 24% 37% 10% 3 1	297
February	1st to 10th	Day	24% 19% 37% 17% 3	270
	11th to 20th	Day	35% 16% 35% 13% 1	267
	21st to end	Day	23% 22% 39% 15% 2	198
March	1st to 10th	Day	23% 15% 32% 25% 5%	402
	11th to 20th	Day	22% 15% 36% 20% 7% 1	420
	21st to end	Day	22% 16% 34% 18% 9%	462
April	1st to 10th	Day	20% 13% 29% 26% 11% 1	420
	11th to 20th	Day	15% 14% 30% 26% 13% 2	412
	21st to end	Day	15% 11% 28% 26% 15% 5%	420
May	1st to 10th	Day	13% 11% 31% 28% 14% 4	423
	11th to 20th	Day	14% 6% 19% 31% 19% 10%	447
	21st to end	Day	16% 7% 15% 25% 25% 12%	485
June	1st to 10th	Day	12% 3 16% 28% 31% 10%	450
	11th to 20th	Day	9% 4 17% 26% 29% 14%	450
	21st to end	Day	9% 3 16% 31% 27% 14%	450
July	1st to 10th	Day	9% 3 9% 23% 34% 22%	445
	11th to 20th	Day	9% 2 8% 26% 38% 17%	444
	21st to end	Day	8% 2 7% 29% 39% 16%	488
August	1st to 10th	Day	14% 3 11% 25% 31% 16%	446
	11th to 20th	Day	13% 3 7% 27% 33% 17%	439
	21st to end	Day	14% 3 12% 23% 33% 16%	495
September	1st to 10th	Day	10% 3 10% 27% 35% 15%	450
	11th to 20th	Day	16% 2 12% 29% 29% 12%	450
	21st to end	Day	18% 4 17% 27% 24% 10%	450
October	1st to 10th	Day	17% 4 22% 31% 16% 11%	450
	11th to 20th	Day	13% 7% 23% 28% 22% 7%	450
	21st to end	Day	15% 8% 26% 28% 17% 5%	495
November	1st to 10th	Day	19% 9% 25% 26% 16% 5%	446
	11th to 20th	Day	16% 13% 41% 22% 7% 1	270
	21st to end	Day	14% 11% 36% 25% 11% 3	270
December	1st to 10th	Day	26% 16% 30% 22% 6%	270
	11th to 20th	Day	24% 12% 36% 21% 6% 1	270
	21st to end	Day	30% 16% 32% 14% 4 3	297
January	1st to 10th	Night	22% 16% 28% 28% 6%	760
	11th to 20th	Night	19% 17% 29% 27% 8%	760
	21st to end	Night	22% 17% 34% 25% 3	836
February	1st to 10th	Night	20% 14% 30% 29% 6%	760
	11th to 20th	Night	24% 15% 37% 22% 3	742
	21st to end	Night	21% 17% 32% 27% 4	592
March	1st to 10th	Night	17% 13% 33% 32% 5%	720
	11th to 20th	Night	18% 10% 36% 28% 8% 1	720
	21st to end	Night	18% 12% 32% 29% 9%	792
April	1st to 10th	Night	15% 14% 31% 28% 13%	720
	11th to 20th	Night	14% 12% 28% 30% 15%	685
	21st to end	Night	11% 8% 25% 38% 16% 1	540
May	1st to 10th	Night	11% 9% 24% 38% 17% 1	540
	11th to 20th	Night	12% 3 24% 34% 22% 4	531
	21st to end	Night	13% 3 10% 35% 34% 6%	584
June	1st to 10th	Night	11% 3 12% 39% 30% 5%	540
	11th to 20th	Night	6% 4 13% 31% 34% 11%	540
	21st to end	Night	10% 2 13% 32% 34% 9%	540
July	1st to 10th	Night	10% 3 7% 30% 38% 13%	525
	11th to 20th	Night	10% 1 6% 30% 43% 10%	522
	21st to end	Night	7% 1 8% 30% 44% 10%	591
August	1st to 10th	Night	11% 1 7% 31% 40% 10%	537
	11th to 20th	Night	12% 2 8% 31% 41% 7%	534
	21st to end	Night	13% 3 11% 28% 38% 8%	679
September	1st to 10th	Night	11% 3 9% 32% 38% 8%	720
	11th to 20th	Night	15% 2 13% 25% 39% 6%	720
	21st to end	Night	16% 3 16% 31% 29% 5%	720
October	1st to 10th	Night	15% 5% 18% 31% 25% 6%	720
	11th to 20th	Night	15% 6% 20% 33% 22% 4	720
	21st to end	Night	13% 9% 22% 32% 22% 2	792
November	1st to 10th	Night	14% 7% 23% 32% 21% 3	740
	11th to 20th	Night	17% 11% 29% 29% 13% 1	760
	21st to end	Night	19% 10% 25% 29% 16% 1	760
December	1st to 10th	Night	21% 13% 24% 30% 10% 1	760
	11th to 20th	Night	19% 11% 26% 31% 12% 1	760
	21st to end	Night	27% 14% 23% 28% 7%	836

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