Rayville Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rayville Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	12% 6% 2 6% 4 4 4 4 5% 7% 44%	130936
	Night	13% 6% 2 5% 4 4 4 3 5% 7% 47%	131769
Transparency	Day	38% 4 11% 22% 19% 6%	134543
	Night	36% 4 10% 20% 21% 9%	136240
Seeing	Day	13% 7% 19% 24% 24% 13%	17032
	Night	14% 6% 23% 33% 21% 3	26881

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	16% 10% 3 7% 4 3 3 3 5% 7% 40%	5289
	11th to 20th	Day	13% 5% 2 4 3 3 3 2 4 7% 55%	5310
	21st to end	Day	24% 9% 3 7% 4 3 2 2 3 4 40%	5812
February	1st to 10th	Day	19% 9% 2 6% 4 4 3 2 4 5% 41%	1966
	11th to 20th	Day	21% 8% 2 6% 4 4 3 3 4 6% 38%	2048
	21st to end	Day	19% 8% 3 7% 4 4 4 2 4 5% 38%	1761
March	1st to 10th	Day	20% 8% 4 9% 5% 4 4 3 4 6% 34%	5046
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 6% 48%	5090
	21st to end	Day	13% 8% 3 9% 5% 4 4 3 5% 6% 41%	5647
April	1st to 10th	Day	13% 5% 2 6% 5% 4 5% 4 6% 7% 43%	2520
	11th to 20th	Day	15% 6% 3 6% 4 4 3 4 5% 7% 43%	2494
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 5% 7% 43%	2520
May	1st to 10th	Day	16% 8% 3 7% 4 4 5% 4 5% 8% 36%	5417
	11th to 20th	Day	11% 6% 2 8% 4 5% 5% 4 6% 8% 39%	5404
	21st to end	Day	9% 5% 2 6% 4 5% 6% 4 7% 10% 42%	6336
June	1st to 10th	Day	8% 4 2 4 5% 5% 6% 5% 8% 11% 42%	2600
	11th to 20th	Day	4 5% 2 5% 5% 5% 6% 4 8% 10% 46%	2600
	21st to end	Day	5% 4 2 5% 4 5% 5% 5% 8% 11% 46%	2600
July	1st to 10th	Day	5% 5% 3 8% 6% 6% 7% 6% 8% 10% 36%	5900
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 7% 10% 45%	5900
	21st to end	Day	11% 6% 3 6% 5% 5% 5% 5% 8% 10% 37%	6422
August	1st to 10th	Day	3 2 1 3 3 4 3 3 4 7% 67%	4013
	11th to 20th	Day	5% 3 2 4 3 4 5% 5% 8% 12% 49%	3849
	21st to end	Day	10% 5% 2 6% 5% 5% 4 4 5% 7% 46%	4312
September	1st to 10th	Day	8% 5% 1 6% 5% 6% 5% 6% 7% 8% 45%	2400
	11th to 20th	Day	12% 5% 2 6% 4 5% 5% 4 5% 8% 45%	2400
	21st to end	Day	10% 4 1 5% 4 4 4 3 5% 6% 55%	2131
October	1st to 10th	Day	6% 3 2 4 3 3 4 3 5% 6% 61%	2000
	11th to 20th	Day	10% 4 1 5% 3 3 3 2 4 5% 60%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 3 3 5% 55%	2200
November	1st to 10th	Day	11% 5% 2 5% 4 3 2 3 4 7% 54%	2058
	11th to 20th	Day	10% 5% 3 5% 3 3 4 2 5% 6% 55%	2100
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 6% 44%	1921
December	1st to 10th	Day	19% 7% 2 5% 4 3 4 3 4 5% 44%	2955
	11th to 20th	Day	16% 8% 3 8% 4 4 3 3 4 5% 42%	3328
	21st to end	Day	15% 6% 2 6% 3 4 3 3 4 6% 47%	4587
January	1st to 10th	Night	17% 7% 2 5% 3 3 3 2 4 5% 48%	7634
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 51%	7680
	21st to end	Night	26% 7% 2 6% 4 3 4 2 4 5% 37%	8402
February	1st to 10th	Night	15% 8% 3 6% 4 4 3 3 4 6% 45%	2651
	11th to 20th	Night	20% 7% 3 5% 5% 4 4 2 4 5% 41%	2458
	21st to end	Night	20% 8% 3 5% 3 3 4 3 3 5% 44%	2060
March	1st to 10th	Night	17% 8% 2 7% 5% 4 4 3 5% 7% 39%	5029
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 48%	5090
	21st to end	Night	15% 7% 2 6% 4 4 4 3 5% 6% 44%	5524
April	1st to 10th	Night	12% 4 2 5% 4 3 4 4 6% 7% 48%	2100
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 50%	2075
	21st to end	Night	11% 6% 2 5% 4 4 4 4 5% 6% 50%	2100
May	1st to 10th	Night	14% 5% 2 7% 4 4 5% 4 6% 9% 39%	4500
	11th to 20th	Night	15% 5% 3 7% 5% 5% 4 4 7% 9% 36%	4489
	21st to end	Night	10% 5% 2 6% 4 4 5% 5% 7% 10% 43%	4532
June	1st to 10th	Night	6% 3 2 6% 4 4 4 5% 7% 12% 46%	1800
	11th to 20th	Night	4 4 2 4 3 4 4 4 7% 10% 55%	1800
	21st to end	Night	5% 4 1 5% 4 4 4 5% 8% 10% 51%	1800
July	1st to 10th	Night	4 4 2 6% 5% 5% 7% 5% 9% 12% 40%	4078
	11th to 20th	Night	5% 4 1 5% 4 4 6% 5% 8% 12% 45%	4077
	21st to end	Night	9% 6% 2 6% 4 3 4 4 8% 12% 42%	4449
August	1st to 10th	Night	2 2 1 3 2 2 3 3 5% 8% 69%	3137
	11th to 20th	Night	4 2 1 4 3 4 5% 5% 8% 13% 50%	3200
	21st to end	Night	7% 5% 2 5% 4 4 5% 3 7% 7% 51%	3586
September	1st to 10th	Night	7% 4 2 6% 4 4 5% 4 7% 9% 48%	2000
	11th to 20th	Night	8% 6% 2 6% 4 4 4 3 5% 7% 50%	2000
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 7% 59%	2244
October	1st to 10th	Night	7% 3 1 5% 3 3 3 3 5% 6% 62%	2400
	11th to 20th	Night	9% 5% 2 5% 3 3 3 2 3 6% 60%	2400
	21st to end	Night	12% 5% 1 4 3 3 3 2 3 5% 59%	2640
November	1st to 10th	Night	12% 5% 2 4 3 3 3 3 3 5% 57%	2460
	11th to 20th	Night	11% 4 2 4 4 3 3 3 4 6% 56%	2520
	21st to end	Night	14% 6% 2 5% 4 4 3 3 5% 6% 48%	2691
December	1st to 10th	Night	21% 7% 2 5% 3 3 4 3 4 5% 45%	4442
	11th to 20th	Night	16% 6% 2 7% 4 4 4 3 4 4 46%	5088
	21st to end	Night	16% 8% 2 5% 3 3 3 3 3 5% 49%	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	46% 2 6% 25% 18% 3	5290
	11th to 20th	Day	31% 3 5% 26% 29% 6%	5310
	21st to end	Day	51% 2 3 12% 21% 11%	5812
February	1st to 10th	Day	46% 4 6% 17% 18% 9%	1966
	11th to 20th	Day	50% 3 6% 17% 17% 7%	2058
	21st to end	Day	49% 4 7% 11% 19% 9%	1761
March	1st to 10th	Day	53% 4 5% 15% 15% 8%	5046
	11th to 20th	Day	37% 4 5% 18% 21% 14%	5090
	21st to end	Day	46% 3 9% 20% 17% 5%	5649
April	1st to 10th	Day	40% 5% 9% 22% 16% 8%	2520
	11th to 20th	Day	41% 4 10% 17% 20% 7%	2494
	21st to end	Day	39% 4 11% 22% 21% 3	2520
May	1st to 10th	Day	45% 4 11% 25% 13% 2	5452
	11th to 20th	Day	42% 4 16% 20% 18% 2	5408
	21st to end	Day	36% 5% 14% 28% 15% 2	6349
June	1st to 10th	Day	34% 7% 17% 26% 15% 1	2600
	11th to 20th	Day	32% 8% 17% 25% 17% 2	2600
	21st to end	Day	30% 8% 18% 26% 16% 3	2600
July	1st to 10th	Day	40% 6% 22% 22% 10% 1	5836
	11th to 20th	Day	33% 5% 25% 30% 6%	5831
	21st to end	Day	40% 5% 18% 27% 8%	6240
August	1st to 10th	Day	18% 5% 12% 33% 29% 3	3999
	11th to 20th	Day	27% 6% 20% 33% 12% 1	3885
	21st to end	Day	37% 4 14% 21% 21% 2	4311
September	1st to 10th	Day	35% 7% 14% 20% 18% 6%	2400
	11th to 20th	Day	37% 7% 10% 21% 20% 6%	2400
	21st to end	Day	29% 5% 10% 23% 22% 10%	2131
October	1st to 10th	Day	16% 3 5% 14% 26% 36%	3260
	11th to 20th	Day	17% 2 3 15% 34% 29%	3240
	21st to end	Day	26% 2 4 17% 38% 13%	3538
November	1st to 10th	Day	32% 4 7% 18% 29% 10%	2058
	11th to 20th	Day	32% 3 6% 19% 27% 13%	2100
	21st to end	Day	41% 3 6% 19% 24% 7%	1921
December	1st to 10th	Day	44% 3 6% 20% 22% 5%	2955
	11th to 20th	Day	47% 2 5% 18% 19% 8%	3326
	21st to end	Day	40% 2 6% 21% 29% 2	4587
January	1st to 10th	Night	41% 2 6% 24% 22% 4	7635
	11th to 20th	Night	36% 3 3 18% 30% 10%	7680
	21st to end	Night	52% 2 4 16% 18% 8%	8402
February	1st to 10th	Night	42% 3 5% 18% 23% 9%	2651
	11th to 20th	Night	46% 3 5% 17% 19% 8%	2459
	21st to end	Night	45% 3 7% 14% 17% 14%	2060
March	1st to 10th	Night	46% 4 7% 13% 21% 9%	5029
	11th to 20th	Night	36% 4 8% 15% 19% 18%	5090
	21st to end	Night	41% 4 9% 18% 21% 6%	5524
April	1st to 10th	Night	34% 5% 11% 21% 19% 10%	2100
	11th to 20th	Night	35% 4 10% 19% 22% 11%	2075
	21st to end	Night	34% 4 9% 23% 25% 5%	2100
May	1st to 10th	Night	41% 5% 13% 22% 16% 3	4530
	11th to 20th	Night	43% 4 15% 23% 14% 1	4490
	21st to end	Night	36% 5% 17% 24% 15% 3	4541
June	1st to 10th	Night	29% 8% 17% 26% 18% 1	1800
	11th to 20th	Night	25% 5% 19% 30% 17% 4	1800
	21st to end	Night	26% 8% 18% 27% 18% 4	1800
July	1st to 10th	Night	36% 7% 25% 22% 10% 1	4035
	11th to 20th	Night	30% 5% 31% 24% 9% 1	4026
	21st to end	Night	34% 7% 19% 29% 10% 1	4320
August	1st to 10th	Night	14% 5% 16% 31% 30% 4	3127
	11th to 20th	Night	23% 6% 22% 35% 13% 2	3230
	21st to end	Night	32% 6% 13% 21% 23% 5%	3586
September	1st to 10th	Night	31% 7% 14% 24% 15% 9%	2000
	11th to 20th	Night	33% 6% 13% 18% 21% 9%	2000
	21st to end	Night	25% 4 10% 24% 23% 13%	2244
October	1st to 10th	Night	17% 4 5% 15% 23% 37%	3920
	11th to 20th	Night	18% 2 4 14% 30% 32%	3899
	21st to end	Night	20% 2 6% 14% 39% 18%	4252
November	1st to 10th	Night	31% 3 7% 19% 26% 14%	2460
	11th to 20th	Night	31% 4 8% 17% 28% 13%	2520
	21st to end	Night	37% 4 6% 19% 23% 11%	2691
December	1st to 10th	Night	44% 3 5% 18% 24% 5%	4442
	11th to 20th	Night	43% 2 8% 16% 23% 8%	5089
	21st to end	Night	40% 2 6% 21% 26% 4	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	22% 15% 39% 18% 5% 1	330
	11th to 20th	Day	20% 17% 33% 21% 8% 2	330
	21st to end	Day	22% 16% 34% 24% 4	363
February	1st to 10th	Day	24% 10% 36% 24% 6% 1	330
	11th to 20th	Day	31% 17% 31% 16% 5%	330
	21st to end	Day	26% 19% 32% 16% 5% 1	273
March	1st to 10th	Day	21% 15% 31% 25% 8%	330
	11th to 20th	Day	17% 20% 38% 20% 4 1	330
	21st to end	Day	16% 14% 37% 21% 12%	389
April	1st to 10th	Day	17% 11% 33% 27% 11% 1	540
	11th to 20th	Day	20% 13% 29% 24% 13% 2	532
	21st to end	Day	15% 11% 26% 29% 17% 3	540
May	1st to 10th	Day	16% 8% 28% 28% 18% 2	507
	11th to 20th	Day	12% 5% 21% 31% 22% 9%	527
	21st to end	Day	8% 3 18% 32% 26% 12%	743
June	1st to 10th	Day	10% 2 13% 26% 33% 15%	730
	11th to 20th	Day	5% 2 11% 31% 39% 12%	730
	21st to end	Day	6% 2 9% 23% 35% 25%	730
July	1st to 10th	Day	5% 1 6% 23% 39% 26%	721
	11th to 20th	Day	5% 1 5% 21% 38% 30%	718
	21st to end	Day	8% 2 4 19% 41% 26%	794
August	1st to 10th	Day	6% 1 4 18% 43% 29%	596
	11th to 20th	Day	9% 1 2 16% 39% 32%	519
	21st to end	Day	13% 2 4 16% 32% 33%	583
September	1st to 10th	Day	7% 1 8% 25% 39% 20%	530
	11th to 20th	Day	10% 2 7% 19% 33% 29%	530
	21st to end	Day	12% 2 9% 26% 34% 17%	421
October	1st to 10th	Day	7% 2 17% 35% 26% 13%	330
	11th to 20th	Day	7% 5% 22% 30% 25% 11%	330
	21st to end	Day	15% 9% 25% 25% 17% 9%	363
November	1st to 10th	Day	16% 7% 25% 30% 17% 5%	330
	11th to 20th	Day	10% 10% 30% 32% 14% 4	330
	21st to end	Day	17% 12% 32% 26% 9% 4	330
December	1st to 10th	Day	21% 8% 39% 23% 7% 2	330
	11th to 20th	Day	22% 11% 33% 22% 11% 2	330
	21st to end	Day	18% 15% 33% 24% 7% 2	363
January	1st to 10th	Night	20% 10% 32% 32% 5%	1050
	11th to 20th	Night	22% 10% 33% 27% 8%	1050
	21st to end	Night	24% 7% 36% 29% 4	1155
February	1st to 10th	Night	20% 10% 36% 30% 5%	971
	11th to 20th	Night	23% 11% 34% 27% 4	823
	21st to end	Night	22% 14% 35% 26% 2	676
March	1st to 10th	Night	18% 10% 33% 33% 7%	630
	11th to 20th	Night	15% 9% 37% 33% 7%	630
	21st to end	Night	16% 8% 34% 31% 11%	693
April	1st to 10th	Night	13% 8% 37% 33% 9%	630
	11th to 20th	Night	14% 9% 34% 33% 9% 1	623
	21st to end	Night	11% 7% 33% 34% 14% 1	630
May	1st to 10th	Night	11% 5% 25% 44% 14% 1	603
	11th to 20th	Night	10% 3 26% 38% 22% 1	591
	21st to end	Night	7% 3 16% 39% 31% 5%	650
June	1st to 10th	Night	7% 2 10% 39% 38% 4	600
	11th to 20th	Night	4 2 9% 36% 46% 5%	600
	21st to end	Night	6% 1 7% 30% 45% 12%	600
July	1st to 10th	Night	6% 2 3 30% 50% 10%	585
	11th to 20th	Night	5% 1 4 25% 52% 13%	582
	21st to end	Night	5% 1 4 26% 52% 12%	657
August	1st to 10th	Night	3 1 4 26% 50% 16%	597
	11th to 20th	Night	7% 1 2 27% 51% 12%	594
	21st to end	Night	10% 2 5% 24% 46% 13%	660
September	1st to 10th	Night	7% 2 7% 34% 44% 6%	600
	11th to 20th	Night	9% 2 9% 31% 42% 8%	600
	21st to end	Night	10% 2 8% 38% 37% 5%	740
October	1st to 10th	Night	8% 2 14% 37% 33% 6%	800
	11th to 20th	Night	11% 5% 21% 39% 22% 3	800
	21st to end	Night	14% 5% 22% 36% 21% 3	880
November	1st to 10th	Night	13% 4 21% 38% 21% 3	820
	11th to 20th	Night	13% 6% 26% 39% 15% 1	840
	21st to end	Night	17% 7% 28% 33% 13% 2	840
December	1st to 10th	Night	21% 9% 28% 34% 9%	876
	11th to 20th	Night	21% 8% 30% 33% 8%	1050
	21st to end	Night	23% 9% 29% 31% 8%	1155

Page generated at 2026-04-29 01:21:48. Version ac07

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