The Troy Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Troy Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	15% 10% 4 8% 5% 5% 5% 4 5% 7% 33%	131388
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 34%	130507
Transparency	Day	48% 6% 11% 21% 13% 1	135013
	Night	48% 5% 10% 21% 13% 2	134952
Seeing	Day	18% 10% 26% 23% 16% 8%	17262
	Night	19% 8% 26% 30% 15% 2	27114

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	18% 16% 7% 15% 5% 4 4 3 4 5% 19%	4691
	11th to 20th	Day	12% 16% 5% 13% 9% 8% 7% 3 3 4 20%	4974
	21st to end	Day	19% 14% 7% 13% 6% 6% 5% 4 4 5% 18%	5780
February	1st to 10th	Day	18% 14% 6% 11% 6% 5% 5% 3 3 5% 23%	1955
	11th to 20th	Day	15% 12% 4 14% 7% 5% 4 3 5% 5% 24%	2048
	21st to end	Day	17% 13% 5% 10% 5% 5% 3 3 3 6% 30%	1730
March	1st to 10th	Day	19% 14% 7% 10% 5% 4 4 3 3 5% 27%	4641
	11th to 20th	Day	12% 10% 3 7% 5% 4 4 3 5% 7% 40%	5090
	21st to end	Day	18% 11% 3 9% 5% 5% 4 3 4 6% 30%	5586
April	1st to 10th	Day	16% 12% 4 9% 5% 4 4 3 5% 6% 31%	2361
	11th to 20th	Day	16% 10% 3 7% 4 4 4 3 5% 6% 37%	2457
	21st to end	Day	18% 8% 3 7% 5% 5% 5% 4 5% 7% 33%	2600
May	1st to 10th	Day	22% 9% 3 8% 5% 4 4 4 5% 6% 29%	5862
	11th to 20th	Day	19% 8% 3 7% 5% 4 4 3 5% 8% 34%	5861
	21st to end	Day	15% 7% 3 7% 5% 5% 6% 4 7% 10% 33%	6433
June	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 40%	2651
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	2800
	21st to end	Day	11% 5% 2 6% 5% 4 5% 5% 7% 10% 40%	2800
July	1st to 10th	Day	10% 6% 3 6% 5% 5% 5% 4 7% 9% 39%	6360
	11th to 20th	Day	9% 6% 2 6% 5% 5% 6% 4 7% 11% 38%	6228
	21st to end	Day	10% 6% 2 7% 6% 7% 7% 5% 7% 9% 35%	6424
August	1st to 10th	Day	7% 6% 2 6% 4 5% 5% 5% 7% 10% 42%	4234
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 5% 7% 11% 42%	4174
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 9% 46%	4418
September	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 5% 6% 50%	2272
	11th to 20th	Day	9% 5% 1 6% 4 4 3 4 6% 8% 50%	2200
	21st to end	Day	16% 6% 2 6% 4 3 4 3 5% 7% 44%	2200
October	1st to 10th	Day	13% 5% 2 7% 4 4 5% 3 4 7% 47%	2200
	11th to 20th	Day	15% 6% 2 5% 3 4 4 3 5% 7% 45%	2010
	21st to end	Day	19% 9% 3 7% 5% 4 3 3 6% 7% 33%	2045
November	1st to 10th	Day	11% 8% 2 5% 5% 4 4 3 5% 6% 47%	1852
	11th to 20th	Day	16% 12% 4 8% 5% 5% 3 3 4 6% 34%	1890
	21st to end	Day	19% 13% 5% 9% 4 4 4 3 4 6% 30%	1890
December	1st to 10th	Day	17% 12% 5% 8% 5% 4 3 3 4 5% 33%	3020
	11th to 20th	Day	19% 15% 6% 12% 5% 4 5% 3 4 5% 22%	3463
	21st to end	Day	24% 21% 6% 10% 5% 4 4 3 4 4 14%	4188
January	1st to 10th	Night	15% 16% 7% 14% 7% 5% 5% 4 4 5% 18%	8232
	11th to 20th	Night	11% 16% 8% 16% 6% 6% 5% 3 4 4 19%	7983
	21st to end	Night	19% 13% 5% 9% 5% 5% 4 3 5% 6% 27%	8401
February	1st to 10th	Night	19% 13% 4 10% 6% 5% 4 3 4 5% 26%	2662
	11th to 20th	Night	16% 12% 4 10% 6% 4 4 4 4 6% 30%	2458
	21st to end	Night	19% 13% 5% 11% 5% 4 3 3 4 6% 28%	2076
March	1st to 10th	Night	20% 12% 4 7% 4 4 3 2 3 5% 36%	5432
	11th to 20th	Night	12% 7% 2 6% 5% 5% 5% 4 6% 7% 41%	5090
	21st to end	Night	19% 9% 4 8% 5% 4 4 3 4 6% 35%	5588
April	1st to 10th	Night	17% 9% 3 6% 4 4 4 4 6% 7% 35%	2077
	11th to 20th	Night	16% 7% 3 5% 5% 4 4 4 5% 8% 38%	1892
	21st to end	Night	17% 7% 3 6% 4 5% 4 4 6% 7% 38%	1800
May	1st to 10th	Night	22% 10% 3 8% 5% 4 4 4 5% 6% 29%	4033
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 5% 9% 38%	4032
	21st to end	Night	13% 6% 2 6% 5% 4 5% 3 6% 9% 39%	4421
June	1st to 10th	Night	9% 6% 2 5% 4 5% 4 4 5% 8% 47%	1743
	11th to 20th	Night	9% 5% 2 5% 5% 5% 4 4 6% 10% 47%	1600
	21st to end	Night	6% 4 3 4 3 5% 6% 5% 6% 10% 47%	1600
July	1st to 10th	Night	9% 5% 2 7% 4 4 5% 4 6% 9% 44%	3618
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 5% 6% 10% 44%	3739
	21st to end	Night	9% 6% 3 7% 4 5% 4 4 7% 9% 41%	4427
August	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 5% 9% 42%	2915
	11th to 20th	Night	6% 7% 3 7% 5% 4 5% 4 6% 8% 47%	2875
	21st to end	Night	4 3 1 5% 5% 5% 5% 4 7% 9% 51%	3477
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 5% 7% 52%	2107
	11th to 20th	Night	9% 5% 2 4 3 2 4 4 5% 7% 57%	2200
	21st to end	Night	14% 7% 2 5% 4 4 3 4 5% 6% 48%	2200
October	1st to 10th	Night	10% 4 2 6% 4 3 4 3 6% 6% 53%	2200
	11th to 20th	Night	16% 8% 2 7% 3 3 3 3 5% 6% 44%	2390
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	2790
November	1st to 10th	Night	13% 7% 3 6% 3 3 4 3 5% 6% 46%	2666
	11th to 20th	Night	16% 10% 3 8% 4 4 4 3 5% 6% 39%	2730
	21st to end	Night	19% 14% 4 8% 4 5% 4 3 5% 6% 27%	2730
December	1st to 10th	Night	18% 11% 4 8% 5% 5% 4 3 5% 7% 31%	4380
	11th to 20th	Night	19% 16% 7% 11% 5% 3 4 3 4 5% 22%	4952
	21st to end	Night	26% 19% 7% 10% 5% 4 4 2 4 5% 15%	6991

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	68% 4 10% 13% 5%	4692
	11th to 20th	Day	70% 3 7% 9% 10% 1	4974
	21st to end	Day	69% 4 9% 11% 6% 1	5780
February	1st to 10th	Day	65% 4 9% 14% 8% 1	1955
	11th to 20th	Day	61% 5% 9% 16% 8% 1	2058
	21st to end	Day	56% 5% 11% 18% 8% 1	1730
March	1st to 10th	Day	61% 4 6% 19% 9% 1	4641
	11th to 20th	Day	44% 5% 9% 29% 13% 1	5090
	21st to end	Day	55% 5% 11% 19% 9% 2	5586
April	1st to 10th	Day	53% 5% 11% 19% 12% 1	2361
	11th to 20th	Day	48% 5% 12% 21% 12% 3	2457
	21st to end	Day	49% 5% 11% 22% 11% 2	2600
May	1st to 10th	Day	54% 7% 9% 18% 11% 1	5897
	11th to 20th	Day	48% 6% 12% 24% 10%	5865
	21st to end	Day	44% 9% 11% 23% 11% 2	6447
June	1st to 10th	Day	42% 6% 11% 24% 16% 2	2651
	11th to 20th	Day	38% 8% 13% 24% 16% 1	2800
	21st to end	Day	37% 9% 15% 21% 15% 3	2800
July	1st to 10th	Day	40% 7% 9% 27% 17% 2	6291
	11th to 20th	Day	38% 8% 14% 24% 15% 1	6153
	21st to end	Day	42% 9% 12% 19% 16% 3	6240
August	1st to 10th	Day	33% 9% 15% 28% 14% 1	4221
	11th to 20th	Day	33% 7% 11% 32% 16%	4213
	21st to end	Day	32% 8% 12% 30% 15% 3	4417
September	1st to 10th	Day	34% 6% 11% 23% 21% 4	2272
	11th to 20th	Day	32% 5% 11% 25% 23% 3	2200
	21st to end	Day	41% 4 9% 22% 23% 2	2200
October	1st to 10th	Day	25% 4 10% 31% 28% 2	3590
	11th to 20th	Day	32% 4 10% 28% 25% 2	3250
	21st to end	Day	40% 4 10% 24% 20% 2	3282
November	1st to 10th	Day	37% 5% 11% 27% 17% 3	1852
	11th to 20th	Day	52% 3 9% 21% 12% 2	1890
	21st to end	Day	57% 4 10% 20% 8% 1	1890
December	1st to 10th	Day	54% 4 13% 22% 7%	3020
	11th to 20th	Day	65% 5% 10% 15% 5%	3460
	21st to end	Day	74% 4 7% 11% 4	4188
January	1st to 10th	Night	68% 5% 9% 13% 5%	8233
	11th to 20th	Night	69% 4 7% 11% 8% 1	7983
	21st to end	Night	59% 4 12% 17% 7% 1	8401
February	1st to 10th	Night	61% 4 10% 16% 8% 1	2662
	11th to 20th	Night	55% 5% 10% 18% 11% 1	2459
	21st to end	Night	58% 4 11% 16% 9% 2	2076
March	1st to 10th	Night	52% 4 6% 26% 11% 2	5432
	11th to 20th	Night	41% 6% 11% 28% 13% 1	5090
	21st to end	Night	51% 4 7% 26% 11% 1	5588
April	1st to 10th	Night	47% 5% 13% 21% 14% 2	2077
	11th to 20th	Night	44% 5% 12% 19% 17% 4	1892
	21st to end	Night	45% 6% 11% 22% 13% 2	1800
May	1st to 10th	Night	54% 8% 9% 18% 9% 3	4060
	11th to 20th	Night	44% 7% 12% 23% 13% 1	4033
	21st to end	Night	40% 7% 13% 27% 11% 3	4429
June	1st to 10th	Night	34% 7% 10% 23% 23% 2	1743
	11th to 20th	Night	33% 7% 14% 26% 17% 3	1600
	21st to end	Night	30% 8% 17% 24% 18% 4	1600
July	1st to 10th	Night	35% 7% 11% 24% 18% 4	3580
	11th to 20th	Night	34% 8% 14% 27% 16% 1	3694
	21st to end	Night	37% 7% 13% 21% 19% 3	4297
August	1st to 10th	Night	37% 8% 13% 28% 13% 1	2904
	11th to 20th	Night	34% 7% 10% 29% 19% 1	2902
	21st to end	Night	27% 8% 15% 27% 21% 2	3477
September	1st to 10th	Night	32% 5% 11% 26% 20% 6%	2107
	11th to 20th	Night	27% 6% 9% 27% 29% 3	2200
	21st to end	Night	38% 6% 9% 21% 24% 2	2200
October	1st to 10th	Night	22% 4 7% 30% 34% 2	3590
	11th to 20th	Night	31% 3 10% 26% 28% 1	3889
	21st to end	Night	35% 4 7% 26% 24% 4	4503
November	1st to 10th	Night	38% 4 10% 24% 21% 2	2666
	11th to 20th	Night	47% 4 8% 22% 15% 3	2730
	21st to end	Night	58% 5% 10% 19% 8% 1	2730
December	1st to 10th	Night	53% 4 12% 23% 8% 1	4380
	11th to 20th	Night	64% 4 11% 15% 6%	4954
	21st to end	Night	73% 3 8% 12% 3	6991

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	27% 15% 33% 19% 6%	330
	11th to 20th	Day	23% 21% 38% 14% 4	330
	21st to end	Day	30% 16% 31% 17% 5%	363
February	1st to 10th	Day	25% 14% 40% 15% 6%	330
	11th to 20th	Day	24% 21% 36% 14% 5%	330
	21st to end	Day	22% 18% 46% 12% 3	273
March	1st to 10th	Day	28% 17% 36% 15% 4	352
	11th to 20th	Day	24% 15% 33% 19% 8% 1	540
	21st to end	Day	26% 17% 34% 17% 6%	594
April	1st to 10th	Day	23% 14% 31% 21% 9% 2	504
	11th to 20th	Day	19% 18% 29% 21% 10% 3	492
	21st to end	Day	21% 11% 34% 22% 10% 2	500
May	1st to 10th	Day	25% 12% 29% 20% 11% 3	503
	11th to 20th	Day	22% 7% 26% 23% 13% 9%	527
	21st to end	Day	16% 7% 23% 26% 16% 12%	573
June	1st to 10th	Day	15% 5% 22% 30% 21% 8%	581
	11th to 20th	Day	12% 5% 21% 34% 21% 7%	730
	21st to end	Day	12% 5% 24% 29% 20% 9%	730
July	1st to 10th	Day	13% 3 17% 24% 25% 18%	721
	11th to 20th	Day	10% 4 18% 28% 26% 15%	664
	21st to end	Day	10% 3 16% 24% 28% 19%	576
August	1st to 10th	Day	11% 4 12% 23% 30% 20%	526
	11th to 20th	Day	12% 3 16% 28% 26% 15%	519
	21st to end	Day	9% 3 12% 28% 31% 16%	583
September	1st to 10th	Day	10% 5% 15% 22% 32% 16%	530
	11th to 20th	Day	10% 3 10% 24% 33% 21%	530
	21st to end	Day	19% 7% 17% 25% 23% 10%	530
October	1st to 10th	Day	11% 7% 21% 28% 22% 11%	530
	11th to 20th	Day	15% 13% 28% 25% 16% 3	530
	21st to end	Day	23% 11% 34% 24% 6% 1	428
November	1st to 10th	Day	16% 12% 33% 25% 8% 5%	330
	11th to 20th	Day	24% 16% 30% 20% 9% 2	330
	21st to end	Day	29% 10% 38% 19% 4	330
December	1st to 10th	Day	17% 18% 38% 19% 6% 1	330
	11th to 20th	Day	24% 14% 42% 15% 4 1	330
	21st to end	Day	34% 14% 33% 16% 4	363
January	1st to 10th	Night	22% 16% 33% 23% 5% 1	1050
	11th to 20th	Night	27% 13% 36% 20% 4	948
	21st to end	Night	24% 12% 35% 25% 4	924
February	1st to 10th	Night	25% 11% 33% 28% 3	840
	11th to 20th	Night	23% 13% 34% 27% 3	823
	21st to end	Night	24% 13% 38% 22% 2	692
March	1st to 10th	Night	23% 12% 32% 27% 7%	840
	11th to 20th	Night	20% 14% 34% 26% 6%	840
	21st to end	Night	24% 11% 32% 27% 6%	924
April	1st to 10th	Night	21% 10% 33% 29% 6% 1	670
	11th to 20th	Night	18% 11% 33% 26% 11% 1	593
	21st to end	Night	19% 7% 38% 25% 10% 1	600
May	1st to 10th	Night	23% 7% 30% 28% 11% 1	600
	11th to 20th	Night	21% 4 25% 31% 17% 2	591
	21st to end	Night	11% 3 22% 36% 25% 3	650
June	1st to 10th	Night	11% 2 23% 41% 23% 1	600
	11th to 20th	Night	12% 3 18% 42% 23% 1	600
	21st to end	Night	7% 2 21% 40% 29% 2	600
July	1st to 10th	Night	10% 2 13% 36% 32% 6%	585
	11th to 20th	Night	8% 3 13% 38% 31% 6%	582
	21st to end	Night	11% 2 9% 38% 37% 3	657
August	1st to 10th	Night	8% 1 10% 39% 38% 4	597
	11th to 20th	Night	7% 2 10% 42% 34% 5%	594
	21st to end	Night	6% 2 11% 36% 38% 7%	660
September	1st to 10th	Night	13% 4 12% 34% 32% 5%	707
	11th to 20th	Night	11% 3 10% 36% 36% 5%	800
	21st to end	Night	15% 5% 18% 35% 24% 3	800
October	1st to 10th	Night	12% 5% 23% 33% 24% 5%	800
	11th to 20th	Night	20% 8% 25% 34% 12% 1	800
	21st to end	Night	21% 10% 30% 29% 9%	880
November	1st to 10th	Night	16% 9% 29% 30% 14% 1	820
	11th to 20th	Night	21% 14% 30% 27% 7% 1	840
	21st to end	Night	25% 11% 29% 28% 7% 1	840
December	1st to 10th	Night	27% 11% 33% 24% 5%	840
	11th to 20th	Night	26% 10% 34% 24% 5% 1	840
	21st to end	Night	31% 13% 29% 23% 3	1087

Page generated at 2026-03-30 02:18:45. Version ac07

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