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 2024-12-09.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	101612
	Night	15% 10% 4 7% 5% 4 4 3 5% 6% 36%	98136
Transparency	Day	47% 6% 11% 22% 13% 1	105281
	Night	46% 6% 10% 22% 14% 2	102606
Seeing	Day	18% 9% 26% 23% 16% 8%	15534
	Night	19% 8% 26% 30% 15% 2	25308

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% 18% 8% 11% 4 4 4 3 4 5% 21%	2527
	11th to 20th	Day	16% 18% 6% 12% 5% 7% 9% 4 4 4 16%	2664
	21st to end	Day	28% 18% 10% 11% 5% 4 3 2 4 3 12%	3124
February	1st to 10th	Day	19% 14% 6% 10% 6% 5% 6% 3 3 5% 23%	1768
	11th to 20th	Day	15% 12% 4 14% 7% 5% 4 3 5% 6% 25%	1848
	21st to end	Day	18% 14% 5% 10% 4 5% 3 3 3 6% 29%	1570
March	1st to 10th	Day	23% 18% 5% 9% 5% 4 3 3 3 5% 23%	3166
	11th to 20th	Day	13% 9% 4 8% 4 4 4 3 5% 7% 38%	3480
	21st to end	Day	19% 11% 4 9% 5% 5% 4 3 5% 6% 30%	3815
April	1st to 10th	Day	14% 12% 4 9% 5% 4 4 3 5% 6% 32%	2232
	11th to 20th	Day	17% 9% 3 7% 4 4 4 3 5% 6% 38%	2333
	21st to end	Day	18% 8% 3 8% 5% 5% 5% 4 5% 7% 33%	2470
May	1st to 10th	Day	21% 10% 3 7% 5% 4 4 4 5% 7% 30%	4082
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 37%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 10% 34%	4476
June	1st to 10th	Day	13% 6% 3 6% 5% 5% 5% 4 6% 8% 39%	2518
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 4 7% 10% 41%	2660
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 9% 34%	4440
	11th to 20th	Day	8% 4 1 5% 4 5% 5% 4 7% 11% 46%	4350
	21st to end	Day	9% 6% 3 7% 6% 6% 7% 5% 7% 9% 36%	4466
August	1st to 10th	Day	7% 6% 2 6% 4 5% 5% 5% 7% 10% 42%	4104
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 5% 8% 10% 42%	4044
	21st to end	Day	7% 4 2 6% 5% 5% 5% 4 7% 9% 46%	4283
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 5% 6% 48%	2159
	11th to 20th	Day	10% 5% 1 6% 4 4 3 4 6% 8% 48%	2090
	21st to end	Day	16% 6% 2 6% 4 3 4 3 5% 7% 44%	2090
October	1st to 10th	Day	13% 5% 2 7% 4 4 5% 3 4 7% 46%	2090
	11th to 20th	Day	15% 7% 2 5% 4 4 4 3 5% 7% 44%	1910
	21st to end	Day	19% 10% 3 7% 5% 4 3 3 5% 7% 32%	1943
November	1st to 10th	Day	11% 7% 2 5% 5% 4 4 3 5% 7% 48%	1762
	11th to 20th	Day	16% 12% 4 8% 5% 5% 3 3 4 6% 33%	1800
	21st to end	Day	19% 13% 5% 8% 5% 4 4 3 4 6% 30%	1800
December	1st to 10th	Day	18% 16% 6% 10% 6% 4 4 3 4 5% 24%	1782
	11th to 20th	Day	16% 15% 5% 12% 5% 5% 5% 3 4 5% 25%	2153
	21st to end	Day	24% 22% 7% 10% 6% 5% 4 3 4 5% 11%	2872
January	1st to 10th	Night	17% 17% 8% 12% 5% 5% 4 4 4 4 20%	4428
	11th to 20th	Night	15% 17% 7% 15% 6% 5% 5% 4 4 4 17%	4283
	21st to end	Night	28% 19% 7% 9% 5% 4 3 2 3 4 17%	4532
February	1st to 10th	Night	21% 14% 4 10% 6% 5% 4 3 3 5% 27%	2409
	11th to 20th	Night	15% 12% 4 10% 6% 4 4 4 4 6% 31%	2218
	21st to end	Night	20% 13% 5% 11% 5% 4 3 3 4 5% 27%	1884
March	1st to 10th	Night	21% 13% 4 6% 4 4 3 2 4 5% 34%	3687
	11th to 20th	Night	13% 8% 2 6% 5% 4 4 3 5% 6% 43%	3480
	21st to end	Night	20% 8% 3 7% 4 3 3 3 4 6% 39%	3817
April	1st to 10th	Night	16% 9% 3 6% 4 5% 4 4 6% 7% 36%	1962
	11th to 20th	Night	17% 8% 3 5% 5% 4 4 4 5% 7% 38%	1796
	21st to end	Night	18% 7% 3 6% 4 5% 4 4 6% 6% 37%	1710
May	1st to 10th	Night	22% 10% 3 8% 4 4 5% 4 5% 6% 28%	2813
	11th to 20th	Night	19% 5% 2 6% 4 4 4 3 4 8% 40%	2812
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 7% 10% 45%	3079
June	1st to 10th	Night	8% 6% 2 5% 4 5% 5% 4 5% 9% 47%	1656
	11th to 20th	Night	8% 5% 2 5% 5% 5% 4 4 6% 10% 48%	1520
	21st to end	Night	7% 4 2 4 4 5% 6% 5% 6% 10% 47%	1520
July	1st to 10th	Night	11% 6% 2 7% 4 4 4 3 6% 8% 44%	2530
	11th to 20th	Night	7% 3 2 5% 4 4 4 4 6% 9% 53%	2613
	21st to end	Night	10% 6% 3 7% 4 5% 4 5% 7% 9% 41%	3087
August	1st to 10th	Night	10% 7% 2 7% 5% 5% 5% 4 5% 9% 41%	2825
	11th to 20th	Night	6% 7% 3 7% 5% 4 5% 4 6% 8% 47%	2785
	21st to end	Night	4 3 1 5% 5% 5% 5% 4 7% 9% 51%	3370
September	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 52%	2001
	11th to 20th	Night	9% 5% 2 4 3 2 4 3 5% 7% 57%	2090
	21st to end	Night	14% 7% 2 5% 3 3 3 4 4 6% 48%	2090
October	1st to 10th	Night	10% 4 2 6% 4 3 4 3 6% 7% 52%	2090
	11th to 20th	Night	16% 8% 2 7% 3 3 3 3 5% 6% 42%	2270
	21st to end	Night	19% 8% 3 6% 4 4 4 3 5% 6% 39%	2650
November	1st to 10th	Night	13% 7% 3 6% 3 3 4 3 5% 6% 46%	2536
	11th to 20th	Night	16% 10% 3 8% 4 4 4 3 5% 6% 38%	2600
	21st to end	Night	18% 14% 4 8% 5% 5% 4 4 5% 6% 27%	2600
December	1st to 10th	Night	21% 13% 5% 9% 5% 4 3 3 5% 6% 26%	2581
	11th to 20th	Night	19% 14% 6% 9% 5% 3 5% 4 4 5% 27%	3042
	21st to end	Night	24% 19% 7% 10% 6% 4 4 2 4 4 16%	4770

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	65% 5% 7% 14% 9%	2528
	11th to 20th	Day	73% 4 9% 9% 5%	2664
	21st to end	Day	77% 4 7% 9% 3	3124
February	1st to 10th	Day	65% 4 9% 13% 9% 1	1768
	11th to 20th	Day	60% 5% 10% 16% 8% 1	1858
	21st to end	Day	58% 5% 10% 17% 8% 1	1570
March	1st to 10th	Day	65% 5% 7% 17% 6%	3166
	11th to 20th	Day	44% 6% 9% 28% 12% 1	3480
	21st to end	Day	54% 6% 11% 21% 7%	3815
April	1st to 10th	Day	52% 5% 11% 19% 12% 1	2232
	11th to 20th	Day	47% 5% 12% 20% 13% 3	2333
	21st to end	Day	50% 5% 11% 21% 11% 2	2470
May	1st to 10th	Day	52% 7% 10% 22% 8% 1	4120
	11th to 20th	Day	46% 6% 13% 24% 9% 1	4085
	21st to end	Day	41% 10% 12% 21% 13% 2	4489
June	1st to 10th	Day	42% 6% 11% 24% 14% 3	2518
	11th to 20th	Day	36% 8% 13% 24% 16% 1	2660
	21st to end	Day	37% 9% 15% 21% 15% 3	2660
July	1st to 10th	Day	43% 7% 10% 21% 16% 2	4371
	11th to 20th	Day	31% 7% 13% 28% 20% 1	4275
	21st to end	Day	41% 9% 12% 18% 16% 4	4324
August	1st to 10th	Day	33% 9% 15% 28% 14% 1	4091
	11th to 20th	Day	33% 7% 11% 31% 16%	4083
	21st to end	Day	32% 8% 12% 30% 15% 3	4282
September	1st to 10th	Day	35% 6% 11% 22% 21% 4	2159
	11th to 20th	Day	33% 6% 11% 25% 22% 3	2090
	21st to end	Day	41% 4 9% 23% 21% 1	2090
October	1st to 10th	Day	25% 5% 10% 31% 27% 1	3480
	11th to 20th	Day	32% 4 10% 28% 25% 2	3150
	21st to end	Day	40% 4 10% 24% 20% 2	3180
November	1st to 10th	Day	37% 5% 11% 27% 18% 3	1762
	11th to 20th	Day	52% 4 9% 21% 12% 2	1800
	21st to end	Day	56% 4 10% 20% 8% 1	1800
December	1st to 10th	Day	63% 4 11% 16% 6% 1	1782
	11th to 20th	Day	61% 5% 9% 18% 7%	2150
	21st to end	Day	77% 4 6% 8% 5%	2872
January	1st to 10th	Night	67% 6% 8% 13% 6%	4428
	11th to 20th	Night	71% 4 8% 12% 4	4283
	21st to end	Night	72% 4 8% 11% 4	4532
February	1st to 10th	Night	61% 4 10% 16% 8% 1	2409
	11th to 20th	Night	54% 5% 10% 18% 12% 1	2219
	21st to end	Night	60% 5% 9% 15% 9% 2	1884
March	1st to 10th	Night	53% 5% 7% 23% 11% 1	3687
	11th to 20th	Night	40% 7% 11% 27% 14% 2	3480
	21st to end	Night	48% 5% 9% 29% 10% 1	3817
April	1st to 10th	Night	46% 5% 13% 21% 14% 2	1962
	11th to 20th	Night	44% 6% 11% 18% 17% 4	1796
	21st to end	Night	46% 6% 12% 21% 13% 3	1710
May	1st to 10th	Night	53% 8% 9% 21% 8% 2	2840
	11th to 20th	Night	43% 7% 13% 26% 10% 1	2813
	21st to end	Night	32% 9% 14% 28% 14% 4	3087
June	1st to 10th	Night	34% 7% 11% 23% 22% 2	1656
	11th to 20th	Night	32% 7% 14% 26% 17% 4	1520
	21st to end	Night	30% 8% 17% 24% 17% 4	1520
July	1st to 10th	Night	37% 7% 10% 24% 16% 5%	2490
	11th to 20th	Night	27% 6% 13% 30% 22% 1	2565
	21st to end	Night	37% 7% 13% 21% 18% 4	2988
August	1st to 10th	Night	38% 8% 13% 28% 13% 1	2814
	11th to 20th	Night	34% 7% 10% 29% 19% 1	2812
	21st to end	Night	28% 8% 15% 27% 20% 2	3370
September	1st to 10th	Night	33% 5% 12% 25% 19% 6%	2001
	11th to 20th	Night	27% 6% 10% 27% 27% 3	2090
	21st to end	Night	38% 6% 9% 22% 23% 2	2090
October	1st to 10th	Night	22% 4 7% 31% 34% 2	3480
	11th to 20th	Night	31% 3 10% 26% 28% 1	3769
	21st to end	Night	35% 4 7% 26% 25% 4	4363
November	1st to 10th	Night	38% 5% 10% 24% 21% 3	2536
	11th to 20th	Night	47% 4 8% 22% 16% 3	2600
	21st to end	Night	57% 5% 10% 19% 8% 1	2600
December	1st to 10th	Night	59% 5% 10% 18% 7%	2581
	11th to 20th	Night	59% 4 10% 19% 7% 1	3044
	21st to end	Night	73% 4 8% 11% 4	4770

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	26% 14% 34% 19% 7%	270
	11th to 20th	Day	25% 16% 39% 15% 5%	270
	21st to end	Day	34% 14% 30% 17% 5%	297
February	1st to 10th	Day	27% 14% 38% 15% 6%	270
	11th to 20th	Day	24% 21% 37% 13% 5%	270
	21st to end	Day	26% 16% 44% 11% 3	225
March	1st to 10th	Day	28% 17% 34% 16% 4	290
	11th to 20th	Day	23% 13% 34% 21% 8% 1	460
	21st to end	Day	27% 15% 34% 18% 7%	506
April	1st to 10th	Day	22% 15% 31% 22% 9% 2	460
	11th to 20th	Day	19% 18% 27% 21% 11% 4	452
	21st to end	Day	22% 11% 34% 23% 8% 2	460
May	1st to 10th	Day	25% 12% 29% 20% 11% 3	463
	11th to 20th	Day	22% 7% 26% 22% 14% 9%	487
	21st to end	Day	14% 6% 23% 27% 17% 13%	529
June	1st to 10th	Day	15% 4 21% 30% 21% 8%	538
	11th to 20th	Day	11% 5% 21% 34% 22% 7%	680
	21st to end	Day	12% 5% 25% 30% 19% 9%	680
July	1st to 10th	Day	13% 3 17% 23% 24% 19%	671
	11th to 20th	Day	9% 4 19% 27% 26% 15%	617
	21st to end	Day	10% 3 17% 24% 27% 19%	532
August	1st to 10th	Day	12% 4 11% 24% 29% 20%	486
	11th to 20th	Day	12% 3 18% 27% 26% 14%	479
	21st to end	Day	10% 3 11% 28% 32% 18%	539
September	1st to 10th	Day	11% 4 14% 23% 32% 15%	490
	11th to 20th	Day	10% 3 10% 25% 33% 19%	490
	21st to end	Day	18% 8% 18% 26% 22% 8%	490
October	1st to 10th	Day	11% 7% 22% 29% 21% 10%	490
	11th to 20th	Day	14% 14% 29% 23% 16% 4	490
	21st to end	Day	23% 10% 35% 24% 7% 1	392
November	1st to 10th	Day	16% 11% 34% 25% 8% 6%	300
	11th to 20th	Day	23% 15% 28% 21% 10% 2	300
	21st to end	Day	28% 9% 38% 20% 4	300
December	1st to 10th	Day	16% 18% 38% 19% 7% 1	294
	11th to 20th	Day	23% 13% 42% 16% 5% 1	270
	21st to end	Day	34% 14% 32% 16% 5%	297
January	1st to 10th	Night	23% 16% 33% 23% 5% 1	950
	11th to 20th	Night	29% 12% 34% 21% 4	858
	21st to end	Night	25% 11% 33% 26% 4	836
February	1st to 10th	Night	27% 10% 32% 28% 3	760
	11th to 20th	Night	23% 13% 34% 27% 3	743
	21st to end	Night	26% 12% 37% 22% 3	628
March	1st to 10th	Night	22% 11% 32% 28% 7%	760
	11th to 20th	Night	21% 13% 34% 26% 6% 1	760
	21st to end	Night	25% 9% 31% 27% 7%	836
April	1st to 10th	Night	20% 10% 33% 30% 6% 1	632
	11th to 20th	Night	19% 10% 33% 26% 11% 1	563
	21st to end	Night	19% 7% 38% 25% 9% 1	570
May	1st to 10th	Night	23% 7% 31% 28% 11% 1	570
	11th to 20th	Night	21% 3 25% 32% 18% 1	561
	21st to end	Night	10% 2 22% 36% 26% 4	617
June	1st to 10th	Night	11% 2 22% 41% 23% 1	570
	11th to 20th	Night	11% 3 19% 41% 24% 1	570
	21st to end	Night	7% 2 21% 40% 28% 2	570
July	1st to 10th	Night	10% 3 13% 35% 32% 6%	555
	11th to 20th	Night	8% 3 13% 38% 32% 6%	552
	21st to end	Night	11% 2 9% 39% 36% 2	624
August	1st to 10th	Night	8% 1 10% 38% 38% 4	567
	11th to 20th	Night	7% 2 11% 43% 32% 5%	564
	21st to end	Night	7% 2 11% 35% 39% 7%	627
September	1st to 10th	Night	14% 4 12% 34% 32% 5%	671
	11th to 20th	Night	11% 3 11% 36% 35% 5%	760
	21st to end	Night	15% 5% 19% 34% 24% 3	760
October	1st to 10th	Night	12% 5% 23% 33% 23% 5%	760
	11th to 20th	Night	20% 8% 26% 32% 12% 1	760
	21st to end	Night	22% 10% 30% 29% 10%	836
November	1st to 10th	Night	17% 9% 28% 30% 15% 1	780
	11th to 20th	Night	21% 14% 30% 27% 8% 1	800
	21st to end	Night	25% 11% 29% 28% 7% 1	800
December	1st to 10th	Night	27% 11% 33% 24% 5% 1	795
	11th to 20th	Night	26% 10% 34% 25% 5% 1	760
	21st to end	Night	30% 12% 30% 24% 4	983

Page generated at 2024-12-06 03:54:49. Version ac07

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