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 2025-01-07.

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%	104414
	Night	13% 6% 2 5% 4 4 4 3 5% 7% 48%	104681
Transparency	Day	38% 5% 10% 21% 20% 6%	108060
	Night	35% 5% 10% 20% 22% 9%	109176
Seeing	Day	13% 7% 19% 24% 24% 13%	15309
	Night	14% 6% 23% 32% 22% 4	25142

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% 11% 3 5% 3 2 3 3 5% 6% 41%	2901
	11th to 20th	Day	21% 8% 2 5% 3 2 3 2 3 5% 46%	2850
	21st to end	Day	22% 8% 3 6% 3 3 2 2 4 5% 42%	3135
February	1st to 10th	Day	19% 9% 3 6% 4 4 3 2 4 5% 40%	1778
	11th to 20th	Day	22% 7% 2 6% 4 4 3 3 4 5% 40%	1848
	21st to end	Day	20% 9% 3 7% 4 4 3 2 4 5% 37%	1598
March	1st to 10th	Day	20% 9% 4 10% 6% 4 4 3 4 6% 30%	3436
	11th to 20th	Day	17% 7% 2 6% 5% 4 5% 4 6% 8% 36%	3480
	21st to end	Day	14% 8% 3 9% 4 3 4 3 4 6% 42%	3863
April	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 4 5% 6% 43%	2280
	11th to 20th	Day	16% 6% 3 6% 4 4 4 4 4 7% 42%	2254
	21st to end	Day	11% 6% 2 6% 4 5% 4 4 5% 7% 46%	2280
May	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 5% 8% 39%	3765
	11th to 20th	Day	11% 5% 2 7% 3 5% 4 4 5% 8% 45%	3764
	21st to end	Day	9% 5% 2 6% 5% 6% 6% 5% 7% 10% 40%	4407
June	1st to 10th	Day	9% 4 2 4 5% 4 6% 5% 8% 10% 41%	2470
	11th to 20th	Day	4 5% 2 5% 6% 5% 6% 4 8% 9% 46%	2470
	21st to end	Day	6% 4 2 5% 4 5% 5% 5% 8% 11% 44%	2470
July	1st to 10th	Day	6% 5% 3 8% 7% 7% 7% 6% 8% 10% 35%	4120
	11th to 20th	Day	7% 5% 3 7% 6% 7% 7% 5% 8% 10% 36%	4120
	21st to end	Day	16% 8% 3 8% 5% 5% 5% 4 7% 9% 31%	4465
August	1st to 10th	Day	3 2 1 3 3 3 3 3 4 7% 67%	3890
	11th to 20th	Day	6% 3 2 4 3 4 5% 5% 8% 11% 49%	3729
	21st to end	Day	10% 5% 2 6% 5% 5% 4 4 6% 7% 46%	4180
September	1st to 10th	Day	8% 6% 1 6% 5% 6% 5% 6% 7% 8% 44%	2280
	11th to 20th	Day	13% 5% 3 7% 4 5% 4 4 5% 7% 44%	2280
	21st to end	Day	10% 4 1 5% 3 4 4 3 5% 6% 55%	2025
October	1st to 10th	Day	7% 3 2 4 3 3 4 3 4 6% 61%	1900
	11th to 20th	Day	11% 4 1 5% 3 3 3 2 4 5% 59%	1900
	21st to end	Day	14% 5% 2 5% 3 2 3 3 3 4 55%	2090
November	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 4 7% 53%	1958
	11th to 20th	Day	10% 5% 3 6% 3 3 4 2 4 6% 55%	2000
	21st to end	Day	15% 6% 3 6% 4 4 3 3 5% 6% 45%	1830
December	1st to 10th	Day	18% 7% 2 5% 4 3 4 3 4 5% 45%	2867
	11th to 20th	Day	16% 8% 3 8% 4 4 3 3 4 5% 41%	3243
	21st to end	Day	15% 6% 2 6% 3 4 3 3 4 6% 47%	4488
January	1st to 10th	Night	17% 7% 2 6% 3 3 3 2 4 4 47%	4187
	11th to 20th	Night	17% 7% 2 6% 4 3 3 2 3 4 48%	4120
	21st to end	Night	23% 6% 2 5% 3 3 3 2 4 4 45%	4532
February	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 6% 43%	2399
	11th to 20th	Night	20% 7% 3 5% 5% 4 4 2 4 5% 43%	2218
	21st to end	Night	21% 8% 3 5% 3 3 3 3 4 4 43%	1870
March	1st to 10th	Night	20% 9% 2 8% 5% 4 4 3 5% 7% 35%	3419
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 4 6% 8% 39%	3480
	21st to end	Night	14% 6% 2 6% 5% 5% 4 3 5% 6% 42%	3768
April	1st to 10th	Night	11% 5% 2 5% 4 3 4 4 6% 7% 49%	1900
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 6% 48%	1875
	21st to end	Night	9% 6% 2 5% 4 4 4 4 5% 6% 51%	1900
May	1st to 10th	Night	12% 4 2 6% 4 4 5% 4 6% 9% 43%	3130
	11th to 20th	Night	14% 5% 2 6% 4 4 3 4 6% 9% 43%	3129
	21st to end	Night	9% 5% 3 6% 5% 5% 5% 5% 6% 9% 43%	3153
June	1st to 10th	Night	7% 4 2 7% 4 4 4 5% 7% 11% 45%	1710
	11th to 20th	Night	4 4 2 4 3 3 4 4 7% 10% 56%	1710
	21st to end	Night	5% 4 2 5% 4 4 4 4 7% 9% 51%	1710
July	1st to 10th	Night	5% 5% 2 6% 5% 5% 7% 4 8% 11% 41%	2850
	11th to 20th	Night	7% 4 1 6% 5% 5% 6% 6% 9% 11% 39%	2850
	21st to end	Night	12% 8% 3 7% 4 3 4 4 7% 10% 39%	3098
August	1st to 10th	Night	2 2 1 2 2 3 3 3 5% 8% 69%	3040
	11th to 20th	Night	4 2 1 4 3 4 5% 5% 8% 13% 50%	3100
	21st to end	Night	7% 5% 2 5% 4 4 5% 3 7% 7% 51%	3476
September	1st to 10th	Night	7% 4 2 6% 4 4 5% 5% 7% 9% 48%	1900
	11th to 20th	Night	9% 6% 3 6% 4 4 4 3 5% 7% 50%	1900
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 7% 58%	2132
October	1st to 10th	Night	7% 3 1 5% 3 3 3 3 5% 6% 62%	2280
	11th to 20th	Night	9% 5% 2 5% 3 3 3 2 4 6% 58%	2280
	21st to end	Night	12% 5% 1 4 3 3 3 2 3 5% 59%	2508
November	1st to 10th	Night	13% 5% 2 4 3 3 3 3 3 5% 56%	2340
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 4 6% 55%	2400
	21st to end	Night	14% 6% 2 5% 4 4 3 3 5% 6% 48%	2563
December	1st to 10th	Night	21% 6% 2 4 3 3 4 3 4 5% 45%	4310
	11th to 20th	Night	16% 6% 2 7% 4 4 4 3 4 5% 45%	4954
	21st to end	Night	16% 8% 2 5% 3 3 3 3 3 5% 49%	6490

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	44% 3 4 20% 24% 5%	2902
	11th to 20th	Day	43% 4 7% 21% 22% 4	2850
	21st to end	Day	47% 3 4 17% 22% 7%	3135
February	1st to 10th	Day	47% 4 7% 16% 17% 10%	1778
	11th to 20th	Day	48% 4 5% 17% 19% 8%	1858
	21st to end	Day	50% 4 6% 12% 19% 8%	1598
March	1st to 10th	Day	56% 5% 7% 13% 15% 5%	3436
	11th to 20th	Day	45% 6% 7% 22% 15% 5%	3480
	21st to end	Day	44% 4 8% 20% 21% 4	3863
April	1st to 10th	Day	40% 5% 9% 21% 16% 8%	2280
	11th to 20th	Day	42% 5% 9% 17% 19% 8%	2254
	21st to end	Day	37% 5% 10% 23% 23% 3	2280
May	1st to 10th	Day	42% 5% 11% 27% 12% 3	3800
	11th to 20th	Day	37% 4 13% 22% 22% 2	3768
	21st to end	Day	36% 6% 15% 29% 12% 2	4419
June	1st to 10th	Day	34% 7% 17% 26% 15% 1	2470
	11th to 20th	Day	32% 8% 16% 24% 17% 2	2470
	21st to end	Day	31% 9% 17% 25% 16% 3	2470
July	1st to 10th	Day	41% 7% 23% 21% 7% 1	4056
	11th to 20th	Day	40% 6% 21% 24% 8% 1	4051
	21st to end	Day	50% 6% 15% 22% 7% 1	4325
August	1st to 10th	Day	18% 5% 12% 33% 29% 3	3876
	11th to 20th	Day	27% 6% 20% 33% 13% 1	3765
	21st to end	Day	37% 4 14% 21% 21% 2	4179
September	1st to 10th	Day	36% 8% 13% 19% 17% 6%	2280
	11th to 20th	Day	38% 7% 10% 19% 20% 6%	2280
	21st to end	Day	30% 5% 10% 23% 22% 11%	2025
October	1st to 10th	Day	16% 3 5% 14% 26% 36%	3160
	11th to 20th	Day	18% 2 3 14% 34% 29%	3140
	21st to end	Day	26% 2 4 17% 38% 14%	3428
November	1st to 10th	Day	33% 4 7% 17% 29% 9%	1958
	11th to 20th	Day	32% 3 6% 18% 28% 14%	2000
	21st to end	Day	41% 4 6% 18% 24% 7%	1830
December	1st to 10th	Day	43% 3 6% 20% 23% 5%	2867
	11th to 20th	Day	47% 2 5% 18% 19% 8%	3241
	21st to end	Day	40% 2 6% 21% 29% 2	4488
January	1st to 10th	Night	41% 3 5% 16% 28% 7%	4187
	11th to 20th	Night	42% 3 4 22% 20% 8%	4120
	21st to end	Night	44% 3 6% 19% 21% 7%	4532
February	1st to 10th	Night	43% 4 6% 18% 22% 8%	2399
	11th to 20th	Night	45% 3 5% 17% 21% 9%	2219
	21st to end	Night	46% 4 6% 13% 17% 13%	1870
March	1st to 10th	Night	49% 5% 8% 15% 16% 7%	3419
	11th to 20th	Night	42% 6% 11% 16% 19% 6%	3480
	21st to end	Night	42% 5% 9% 17% 24% 3	3768
April	1st to 10th	Night	33% 6% 11% 21% 19% 10%	1900
	11th to 20th	Night	36% 4 10% 18% 20% 11%	1875
	21st to end	Night	33% 4 8% 23% 27% 5%	1900
May	1st to 10th	Night	36% 7% 11% 24% 18% 5%	3160
	11th to 20th	Night	38% 6% 10% 27% 19% 1	3130
	21st to end	Night	37% 6% 15% 25% 16% 1	3162
June	1st to 10th	Night	30% 8% 17% 26% 17% 1	1710
	11th to 20th	Night	23% 5% 18% 31% 18% 4	1710
	21st to end	Night	26% 8% 16% 27% 19% 4	1710
July	1st to 10th	Night	36% 8% 25% 22% 8% 1	2805
	11th to 20th	Night	35% 6% 24% 25% 9% 1	2796
	21st to end	Night	40% 8% 18% 26% 7% 1	2999
August	1st to 10th	Night	14% 5% 16% 31% 30% 4	3030
	11th to 20th	Night	23% 6% 21% 35% 13% 2	3130
	21st to end	Night	31% 6% 13% 21% 23% 5%	3476
September	1st to 10th	Night	31% 8% 14% 24% 14% 9%	1900
	11th to 20th	Night	33% 6% 14% 17% 21% 10%	1900
	21st to end	Night	26% 4 9% 24% 23% 13%	2132
October	1st to 10th	Night	17% 4 5% 15% 23% 37%	3800
	11th to 20th	Night	18% 3 4 14% 29% 32%	3779
	21st to end	Night	20% 2 6% 14% 39% 18%	4120
November	1st to 10th	Night	32% 4 7% 18% 25% 14%	2340
	11th to 20th	Night	31% 4 7% 16% 28% 14%	2400
	21st to end	Night	37% 4 6% 18% 24% 12%	2563
December	1st to 10th	Night	44% 3 5% 18% 25% 5%	4310
	11th to 20th	Night	43% 2 8% 16% 23% 8%	4955
	21st to end	Night	40% 2 6% 21% 26% 4	6490

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	23% 15% 38% 19% 5% 1	288
	11th to 20th	Day	23% 16% 33% 19% 8% 1	270
	21st to end	Day	21% 16% 34% 24% 4	297
February	1st to 10th	Day	24% 11% 37% 21% 6% 1	270
	11th to 20th	Day	33% 16% 31% 16% 4	270
	21st to end	Day	30% 23% 29% 15% 3	225
March	1st to 10th	Day	22% 16% 31% 23% 9%	270
	11th to 20th	Day	19% 20% 37% 19% 4 1	270
	21st to end	Day	17% 16% 39% 17% 11%	321
April	1st to 10th	Day	17% 11% 34% 26% 11% 1	460
	11th to 20th	Day	22% 13% 25% 25% 13% 2	452
	21st to end	Day	12% 12% 27% 30% 18% 2	460
May	1st to 10th	Day	14% 8% 28% 29% 19% 2	463
	11th to 20th	Day	12% 3 20% 32% 23% 10%	487
	21st to end	Day	7% 4 19% 32% 26% 12%	690
June	1st to 10th	Day	10% 3 12% 26% 33% 16%	680
	11th to 20th	Day	5% 2 11% 30% 39% 12%	680
	21st to end	Day	7% 2 9% 24% 35% 23%	680
July	1st to 10th	Day	5% 1 7% 24% 38% 25%	671
	11th to 20th	Day	5% 1 5% 22% 37% 30%	668
	21st to end	Day	9% 2 4 20% 41% 24%	739
August	1st to 10th	Day	6% 1 4 18% 42% 29%	553
	11th to 20th	Day	9% 1 2 18% 40% 30%	479
	21st to end	Day	13% 1 4 16% 31% 34%	539
September	1st to 10th	Day	7% 1 8% 24% 39% 21%	490
	11th to 20th	Day	11% 2 8% 20% 33% 26%	490
	21st to end	Day	12% 2 8% 25% 34% 18%	387
October	1st to 10th	Day	7% 2 17% 35% 26% 13%	300
	11th to 20th	Day	8% 5% 23% 30% 24% 9%	300
	21st to end	Day	15% 9% 27% 24% 16% 9%	330
November	1st to 10th	Day	18% 6% 24% 31% 17% 4	300
	11th to 20th	Day	11% 11% 30% 30% 14% 4	300
	21st to end	Day	17% 12% 32% 24% 10% 4	300
December	1st to 10th	Day	20% 8% 39% 24% 8% 2	300
	11th to 20th	Day	22% 12% 33% 21% 10% 1	300
	21st to end	Day	19% 15% 35% 21% 7% 2	330
January	1st to 10th	Night	20% 11% 32% 32% 5%	981
	11th to 20th	Night	24% 9% 32% 27% 8%	950
	21st to end	Night	23% 8% 35% 30% 4	1045
February	1st to 10th	Night	21% 10% 35% 29% 5%	879
	11th to 20th	Night	23% 11% 35% 27% 4	743
	21st to end	Night	24% 14% 36% 25% 2	614
March	1st to 10th	Night	18% 10% 32% 33% 8%	570
	11th to 20th	Night	16% 8% 36% 33% 6%	570
	21st to end	Night	16% 9% 34% 30% 11%	627
April	1st to 10th	Night	13% 9% 36% 32% 9%	570
	11th to 20th	Night	14% 10% 33% 33% 10% 1	563
	21st to end	Night	10% 8% 33% 34% 14% 1	570
May	1st to 10th	Night	10% 5% 26% 44% 15% 1	570
	11th to 20th	Night	9% 3 24% 40% 23% 1	561
	21st to end	Night	6% 3 16% 38% 31% 6%	617
June	1st to 10th	Night	7% 2 10% 38% 39% 4	570
	11th to 20th	Night	3 2 9% 35% 46% 5%	570
	21st to end	Night	6% 1 7% 31% 43% 12%	570
July	1st to 10th	Night	6% 2 3 30% 48% 11%	555
	11th to 20th	Night	5% 1 4 26% 52% 12%	552
	21st to end	Night	6% 1 4 27% 51% 12%	624
August	1st to 10th	Night	3 1 4 26% 50% 16%	567
	11th to 20th	Night	8% 1 2 28% 51% 11%	564
	21st to end	Night	10% 2 5% 24% 46% 13%	627
September	1st to 10th	Night	7% 2 7% 33% 45% 6%	570
	11th to 20th	Night	10% 2 9% 31% 42% 7%	570
	21st to end	Night	10% 2 9% 37% 37% 5%	703
October	1st to 10th	Night	8% 2 13% 37% 33% 6%	760
	11th to 20th	Night	11% 5% 22% 38% 21% 3	760
	21st to end	Night	14% 5% 22% 35% 22% 3	836
November	1st to 10th	Night	14% 3 22% 38% 20% 3	780
	11th to 20th	Night	14% 6% 26% 38% 15% 1	800
	21st to end	Night	17% 7% 28% 32% 14% 2	800
December	1st to 10th	Night	20% 9% 28% 34% 9%	834
	11th to 20th	Night	21% 8% 29% 33% 8%	1000
	21st to end	Night	23% 10% 29% 31% 7%	1100

Page generated at 2025-01-04 02:24:02. Version ac07

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