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 2024-07-16.

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%	89757
	Night	16% 10% 4 8% 5% 4 4 3 5% 6% 35%	89677
Transparency	Day	49% 6% 11% 20% 12% 1	89521
	Night	49% 6% 10% 21% 13% 2	89518
Seeing	Day	18% 9% 26% 23% 16% 8%	14996
	Night	19% 8% 26% 30% 15% 2	24775

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	11% 5% 2 6% 5% 5% 6% 5% 8% 9% 37%	2660
	11th to 20th	Day	9% 4 2 5% 5% 5% 5% 4 7% 10% 43%	2545
	21st to end	Day	9% 4 2 5% 6% 5% 6% 5% 7% 9% 41%	2547
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 6% 6% 10% 42%	2327
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 10% 39%	2303
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 8% 9% 43%	2437
September	1st to 10th	Day	11% 5% 2 7% 4 4 4 4 5% 6% 47%	2046
	11th to 20th	Day	10% 5% 1 6% 4 4 4 4 6% 7% 48%	1980
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 7% 46%	1980
October	1st to 10th	Day	13% 5% 2 7% 4 4 5% 3 4 7% 44%	1980
	11th to 20th	Day	16% 7% 3 6% 4 4 4 3 5% 7% 43%	1810
	21st to end	Day	20% 10% 3 7% 5% 4 3 3 5% 7% 31%	1841
November	1st to 10th	Day	11% 7% 2 5% 4 4 4 3 5% 7% 48%	1672
	11th to 20th	Day	15% 12% 4 8% 5% 5% 3 3 4 6% 35%	1710
	21st to end	Day	19% 12% 4 8% 5% 4 4 3 5% 6% 31%	1710
December	1st to 10th	Day	19% 16% 6% 10% 6% 4 4 3 4 5% 22%	1710
	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	9% 5% 2 8% 5% 4 5% 4 7% 8% 45%	1520
	11th to 20th	Night	8% 3 2 6% 4 4 4 4 6% 9% 51%	1532
	21st to end	Night	9% 5% 2 5% 4 4 4 5% 5% 7% 49%	1771
August	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 6% 10% 46%	1611
	11th to 20th	Night	6% 5% 2 7% 5% 5% 5% 4 6% 8% 48%	1598
	21st to end	Night	6% 4 2 5% 6% 4 5% 3 6% 9% 51%	1916
September	1st to 10th	Night	12% 5% 2 5% 3 3 4 3 5% 7% 50%	1895
	11th to 20th	Night	9% 5% 2 4 3 2 4 3 5% 7% 56%	1980
	21st to end	Night	13% 6% 2 5% 3 3 3 4 5% 6% 50%	1980
October	1st to 10th	Night	11% 4 2 6% 4 3 4 3 6% 7% 51%	1980
	11th to 20th	Night	17% 8% 2 7% 3 3 3 3 6% 6% 41%	2150
	21st to end	Night	20% 9% 3 6% 3 4 4 3 5% 5% 38%	2510
November	1st to 10th	Night	13% 8% 3 6% 3 3 4 4 5% 6% 46%	2406
	11th to 20th	Night	16% 10% 3 8% 4 3 3 3 5% 6% 38%	2470
	21st to end	Night	18% 13% 4 8% 5% 5% 5% 4 5% 6% 28%	2470
December	1st to 10th	Night	21% 14% 5% 9% 5% 4 4 3 4 7% 25%	2470
	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	40% 9% 12% 20% 15% 3	2591
	11th to 20th	Day	34% 8% 13% 27% 17% 1	2470
	21st to end	Day	36% 8% 15% 23% 15% 3	2405
August	1st to 10th	Day	34% 10% 13% 28% 14% 2	2314
	11th to 20th	Day	37% 8% 13% 28% 13% 1	2303
	21st to end	Day	34% 9% 12% 23% 16% 5%	2437
September	1st to 10th	Day	37% 6% 12% 22% 20% 4	2046
	11th to 20th	Day	34% 6% 12% 25% 21% 3	1980
	21st to end	Day	38% 4 10% 24% 22% 1	1980
October	1st to 10th	Day	39% 7% 11% 24% 18% 2	1980
	11th to 20th	Day	41% 4 10% 25% 18% 3	1810
	21st to end	Day	52% 5% 12% 19% 11% 1	1841
November	1st to 10th	Day	36% 5% 11% 27% 18% 3	1672
	11th to 20th	Day	52% 3 9% 21% 12% 2	1710
	21st to end	Day	55% 4 11% 20% 9% 1	1710
December	1st to 10th	Day	65% 4 11% 15% 5% 1	1710
	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	36% 8% 13% 20% 18% 4	1480
	11th to 20th	Night	29% 7% 13% 30% 19% 2	1484
	21st to end	Night	32% 7% 13% 28% 17% 3	1672
August	1st to 10th	Night	31% 9% 14% 28% 17% 1	1600
	11th to 20th	Night	32% 8% 13% 27% 19% 1	1598
	21st to end	Night	29% 9% 15% 21% 23% 4	1916
September	1st to 10th	Night	34% 5% 13% 25% 18% 5%	1895
	11th to 20th	Night	28% 6% 10% 27% 26% 3	1980
	21st to end	Night	35% 6% 9% 23% 24% 3	1980
October	1st to 10th	Night	33% 6% 11% 25% 23% 2	1980
	11th to 20th	Night	44% 4 8% 24% 18% 1	2150
	21st to end	Night	48% 5% 8% 22% 15% 3	2510
November	1st to 10th	Night	38% 5% 10% 23% 21% 3	2406
	11th to 20th	Night	47% 4 8% 22% 16% 3	2470
	21st to end	Night	56% 5% 10% 20% 8% 1	2470
December	1st to 10th	Night	60% 5% 10% 18% 7%	2470
	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% 3 18% 27% 27% 15%	597
	21st to end	Day	10% 3 16% 23% 28% 19%	488
August	1st to 10th	Day	12% 4 9% 24% 30% 22%	446
	11th to 20th	Day	13% 3 16% 26% 27% 15%	439
	21st to end	Day	10% 3 11% 28% 30% 18%	495
September	1st to 10th	Day	11% 3 15% 23% 32% 16%	450
	11th to 20th	Day	11% 4 11% 24% 31% 19%	450
	21st to end	Day	16% 7% 19% 26% 24% 9%	450
October	1st to 10th	Day	12% 8% 23% 27% 21% 10%	450
	11th to 20th	Day	15% 14% 30% 23% 16% 2	450
	21st to end	Day	25% 10% 34% 25% 5% 1	356
November	1st to 10th	Day	16% 11% 31% 27% 9% 6%	270
	11th to 20th	Day	23% 16% 26% 22% 11% 2	270
	21st to end	Day	28% 8% 38% 21% 5%	270
December	1st to 10th	Day	17% 19% 36% 20% 7% 1	270
	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 12% 39% 32% 6%	540
	21st to end	Night	10% 2 10% 39% 37% 2	591
August	1st to 10th	Night	8% 1 9% 39% 39% 4	537
	11th to 20th	Night	7% 1 11% 42% 34% 5%	534
	21st to end	Night	7% 2 11% 35% 38% 7%	594
September	1st to 10th	Night	15% 3 12% 32% 32% 5%	635
	11th to 20th	Night	12% 3 11% 34% 35% 5%	720
	21st to end	Night	14% 5% 19% 35% 25% 3	720
October	1st to 10th	Night	13% 5% 23% 32% 23% 5%	720
	11th to 20th	Night	21% 8% 26% 33% 11% 1	720
	21st to end	Night	23% 10% 29% 28% 9%	792
November	1st to 10th	Night	17% 9% 28% 29% 16% 1	740
	11th to 20th	Night	21% 14% 30% 27% 8% 1	760
	21st to end	Night	24% 10% 29% 29% 7% 1	760
December	1st to 10th	Night	27% 11% 32% 24% 5% 1	760
	11th to 20th	Night	26% 10% 34% 25% 5% 1	760
	21st to end	Night	30% 12% 30% 24% 4	983

Page generated at 2024-07-13 10:19:48. Version ac07

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