River Moss Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the River Moss Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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% 7% 3 7% 4 4 4 4 6% 8% 42%	138272
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 42%	135015
Transparency	Day	39% 5% 12% 25% 17% 2	141852
	Night	40% 5% 11% 24% 18% 3	139515
Seeing	Day	14% 8% 24% 27% 20% 7%	18030
	Night	16% 7% 27% 34% 15% 1	27794

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	10% 11% 6% 13% 5% 4 4 3 5% 6% 32%	4691
	11th to 20th	Day	8% 8% 3 8% 6% 6% 5% 4 5% 7% 41%	5109
	21st to end	Day	17% 13% 5% 8% 4 3 3 2 4 4 35%	5812
February	1st to 10th	Day	15% 10% 4 9% 5% 4 5% 3 4 7% 34%	1890
	11th to 20th	Day	15% 9% 3 7% 6% 3 5% 4 5% 6% 37%	2012
	21st to end	Day	14% 10% 3 7% 5% 5% 4 3 4 6% 39%	1730
March	1st to 10th	Day	17% 9% 2 5% 3 3 2 2 3 4 49%	4673
	11th to 20th	Day	9% 6% 2 5% 3 3 4 4 6% 7% 51%	5090
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	5864
April	1st to 10th	Day	13% 6% 3 7% 6% 4 4 4 5% 8% 40%	2520
	11th to 20th	Day	16% 7% 2 8% 4 4 4 4 5% 5% 42%	2494
	21st to end	Day	15% 7% 3 8% 5% 5% 4 4 5% 7% 38%	2520
May	1st to 10th	Day	15% 8% 3 8% 5% 4 5% 4 6% 9% 33%	7175
	11th to 20th	Day	14% 8% 3 7% 4 4 4 4 6% 7% 40%	8198
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	9047
June	1st to 10th	Day	8% 6% 2 5% 4 4 5% 4 6% 9% 47%	2940
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 4 7% 9% 49%	2940
	21st to end	Day	5% 4 2 5% 4 5% 6% 5% 8% 11% 46%	2940
July	1st to 10th	Day	10% 5% 2 6% 4 4 4 4 6% 9% 47%	6500
	11th to 20th	Day	6% 5% 2 7% 6% 5% 6% 5% 7% 10% 41%	6500
	21st to end	Day	6% 4 2 6% 5% 5% 6% 5% 7% 12% 41%	6699
August	1st to 10th	Day	5% 4 2 5% 4 4 4 4 7% 10% 51%	4234
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 42%	3854
	21st to end	Day	6% 4 2 4 3 4 5% 4 6% 9% 52%	4312
September	1st to 10th	Day	12% 4 2 5% 4 3 3 3 5% 7% 51%	2400
	11th to 20th	Day	8% 5% 2 4 4 4 4 4 6% 8% 51%	2381
	21st to end	Day	8% 6% 2 6% 3 4 3 4 5% 7% 53%	2180
October	1st to 10th	Day	9% 4 2 5% 3 3 3 3 4 7% 56%	2000
	11th to 20th	Day	15% 6% 1 5% 3 3 3 2 4 6% 53%	2000
	21st to end	Day	18% 7% 3 4 3 3 3 2 3 6% 46%	2186
November	1st to 10th	Day	11% 6% 2 5% 4 4 2 2 5% 6% 54%	1852
	11th to 20th	Day	14% 8% 2 5% 4 3 3 4 4 6% 47%	1890
	21st to end	Day	16% 10% 4 7% 5% 3 4 3 4 6% 40%	1791
December	1st to 10th	Day	9% 10% 3 7% 3 4 3 2 4 6% 50%	2690
	11th to 20th	Day	12% 8% 5% 8% 5% 4 4 4 4 6% 39%	3088
	21st to end	Day	23% 15% 5% 10% 5% 4 4 3 4 4 23%	4070
January	1st to 10th	Night	13% 11% 5% 10% 5% 4 3 3 4 5% 37%	8232
	11th to 20th	Night	9% 9% 4 10% 5% 5% 5% 4 4 5% 40%	7878
	21st to end	Night	16% 13% 4 8% 4 4 4 3 4 6% 34%	8402
February	1st to 10th	Night	14% 12% 5% 7% 5% 4 4 3 5% 5% 36%	2730
	11th to 20th	Night	16% 9% 3 8% 4 3 3 3 4 6% 40%	2492
	21st to end	Night	14% 9% 4 8% 5% 3 4 3 4 5% 43%	2076
March	1st to 10th	Night	19% 11% 3 5% 3 3 3 2 3 4 43%	5432
	11th to 20th	Night	12% 7% 3 5% 3 3 3 3 4 7% 50%	5090
	21st to end	Night	16% 9% 3 7% 5% 5% 4 3 5% 6% 36%	5307
April	1st to 10th	Night	12% 6% 3 8% 5% 3 4 3 5% 7% 45%	2100
	11th to 20th	Night	13% 6% 3 6% 5% 4 4 3 6% 8% 44%	2075
	21st to end	Night	17% 7% 3 7% 4 4 3 3 5% 7% 41%	2100
May	1st to 10th	Night	16% 8% 3 7% 5% 4 4 4 6% 8% 35%	5663
	11th to 20th	Night	13% 6% 2 5% 4 4 5% 4 6% 8% 43%	4678
	21st to end	Night	14% 7% 3 8% 6% 5% 5% 4 6% 8% 33%	5139
June	1st to 10th	Night	7% 5% 2 6% 5% 4 4 4 7% 8% 49%	1680
	11th to 20th	Night	6% 4 2 5% 4 3 4 5% 6% 10% 52%	1680
	21st to end	Night	4 4 1 5% 4 4 5% 4 6% 10% 53%	1680
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 5% 9% 47%	3698
	11th to 20th	Night	4 4 1 6% 5% 5% 5% 5% 7% 10% 47%	3697
	21st to end	Night	6% 6% 2 7% 5% 4 6% 4 7% 11% 42%	4388
August	1st to 10th	Night	5% 3 2 4 3 4 4 4 6% 8% 57%	2926
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 6% 8% 47%	3192
	21st to end	Night	5% 3 2 3 3 4 4 3 6% 9% 58%	3586
September	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 52%	2000
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 4 6% 7% 52%	2000
	21st to end	Night	9% 5% 1 4 4 3 4 3 6% 7% 54%	2212
October	1st to 10th	Night	8% 3 2 4 3 3 4 3 4 6% 60%	2400
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 5% 6% 54%	2400
	21st to end	Night	19% 6% 2 5% 3 3 4 3 4 5% 47%	2652
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 6% 54%	2666
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 4 7% 49%	2730
	21st to end	Night	18% 9% 3 6% 3 3 3 3 5% 5% 43%	2825
December	1st to 10th	Night	11% 9% 3 7% 4 4 4 3 4 5% 46%	4720
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 6% 6% 41%	5339
	21st to end	Night	25% 17% 4 8% 5% 4 4 3 4 4 22%	7150

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	52% 5% 11% 22% 9% 1	4692
	11th to 20th	Day	40% 5% 13% 22% 16% 3	5109
	21st to end	Day	53% 3 10% 17% 14% 3	5812
February	1st to 10th	Day	51% 5% 11% 22% 10% 1	1890
	11th to 20th	Day	48% 4 11% 20% 15% 2	2022
	21st to end	Day	47% 3 8% 24% 14% 3	1730
March	1st to 10th	Day	41% 3 6% 23% 25% 3	4673
	11th to 20th	Day	31% 5% 10% 25% 24% 5%	5090
	21st to end	Day	44% 7% 13% 28% 8% 1	5866
April	1st to 10th	Day	42% 5% 12% 25% 16% 1	2520
	11th to 20th	Day	44% 4 10% 24% 14% 3	2494
	21st to end	Day	45% 5% 11% 25% 13% 1	2520
May	1st to 10th	Day	45% 7% 12% 23% 12%	7210
	11th to 20th	Day	42% 6% 9% 24% 17% 2	8202
	21st to end	Day	48% 6% 10% 19% 16% 1	9061
June	1st to 10th	Day	33% 7% 14% 27% 17% 2	2940
	11th to 20th	Day	30% 7% 12% 26% 22% 3	2940
	21st to end	Day	29% 8% 17% 25% 16% 4	2940
July	1st to 10th	Day	33% 5% 11% 30% 19% 2	6431
	11th to 20th	Day	35% 8% 16% 25% 14% 1	6425
	21st to end	Day	33% 8% 18% 30% 10% 1	6513
August	1st to 10th	Day	27% 6% 16% 36% 14%	4220
	11th to 20th	Day	37% 6% 14% 25% 16% 1	3890
	21st to end	Day	28% 6% 16% 33% 15% 3	4311
September	1st to 10th	Day	32% 4 11% 26% 22% 5%	2400
	11th to 20th	Day	31% 4 10% 28% 25% 3	2381
	21st to end	Day	31% 5% 9% 27% 26% 3	2180
October	1st to 10th	Day	18% 3 6% 19% 47% 7%	3260
	11th to 20th	Day	23% 2 7% 22% 37% 8%	3240
	21st to end	Day	34% 3 7% 28% 26% 2	3512
November	1st to 10th	Day	32% 4 12% 27% 21% 4	1852
	11th to 20th	Day	38% 4 9% 26% 19% 5%	1890
	21st to end	Day	48% 4 10% 25% 14%	1791
December	1st to 10th	Day	38% 3 14% 30% 15%	2690
	11th to 20th	Day	47% 4 12% 23% 12% 2	3085
	21st to end	Day	65% 5% 12% 12% 6% 1	4070
January	1st to 10th	Night	50% 4 13% 24% 8%	8233
	11th to 20th	Night	46% 3 7% 23% 16% 3	7878
	21st to end	Night	52% 4 10% 18% 14% 1	8402
February	1st to 10th	Night	51% 4 10% 22% 12% 1	2730
	11th to 20th	Night	46% 3 9% 23% 17% 2	2493
	21st to end	Night	44% 3 11% 24% 15% 3	2076
March	1st to 10th	Night	46% 3 6% 22% 21% 2	5432
	11th to 20th	Night	36% 4 9% 24% 22% 6%	5090
	21st to end	Night	48% 6% 10% 24% 11% 1	5307
April	1st to 10th	Night	40% 4 10% 25% 18% 3	2100
	11th to 20th	Night	39% 4 11% 23% 19% 4	2075
	21st to end	Night	43% 5% 8% 25% 17% 2	2100
May	1st to 10th	Night	45% 7% 14% 22% 12% 1	5693
	11th to 20th	Night	37% 7% 12% 25% 16% 3	4679
	21st to end	Night	46% 6% 12% 19% 15% 1	5149
June	1st to 10th	Night	31% 6% 15% 26% 19% 3	1680
	11th to 20th	Night	26% 7% 12% 27% 25% 4	1680
	21st to end	Night	26% 6% 16% 27% 19% 7%	1680
July	1st to 10th	Night	34% 5% 10% 29% 19% 2	3660
	11th to 20th	Night	29% 8% 16% 30% 15% 2	3652
	21st to end	Night	34% 8% 20% 27% 11% 1	4263
August	1st to 10th	Night	23% 7% 17% 34% 18% 1	2916
	11th to 20th	Night	35% 5% 13% 26% 19% 2	3222
	21st to end	Night	22% 5% 16% 34% 19% 3	3586
September	1st to 10th	Night	32% 4 9% 24% 24% 6%	2000
	11th to 20th	Night	31% 5% 8% 26% 26% 5%	2000
	21st to end	Night	31% 5% 10% 23% 27% 5%	2212
October	1st to 10th	Night	17% 3 6% 18% 49% 7%	3920
	11th to 20th	Night	22% 3 7% 23% 36% 9%	3899
	21st to end	Night	31% 4 8% 26% 26% 5%	4276
November	1st to 10th	Night	32% 4 11% 26% 22% 6%	2666
	11th to 20th	Night	36% 3 8% 27% 22% 4	2730
	21st to end	Night	44% 4 11% 24% 16% 1	2825
December	1st to 10th	Night	42% 3 9% 32% 13% 1	4720
	11th to 20th	Night	43% 3 13% 23% 17%	5341
	21st to end	Night	66% 5% 11% 12% 5% 1	7150

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	19% 14% 35% 21% 9% 1	330
	11th to 20th	Day	22% 16% 37% 20% 5%	330
	21st to end	Day	16% 14% 43% 22% 4 1	363
February	1st to 10th	Day	22% 11% 41% 23% 3 1	330
	11th to 20th	Day	22% 12% 38% 19% 7% 1	330
	21st to end	Day	22% 13% 39% 23% 3	273
March	1st to 10th	Day	17% 13% 45% 22% 3	330
	11th to 20th	Day	23% 16% 34% 20% 7% 1	330
	21st to end	Day	26% 12% 35% 21% 6%	489
April	1st to 10th	Day	18% 11% 36% 23% 11% 1	540
	11th to 20th	Day	20% 14% 30% 23% 12%	532
	21st to end	Day	19% 10% 34% 28% 9% 1	540
May	1st to 10th	Day	16% 10% 28% 32% 11% 3	543
	11th to 20th	Day	16% 8% 26% 29% 17% 5%	763
	21st to end	Day	13% 6% 24% 33% 17% 8%	844
June	1st to 10th	Day	10% 5% 18% 29% 27% 11%	780
	11th to 20th	Day	8% 4 18% 33% 29% 8%	780
	21st to end	Day	5% 4 22% 34% 28% 7%	780
July	1st to 10th	Day	10% 3 9% 27% 34% 17%	771
	11th to 20th	Day	6% 3 12% 35% 33% 12%	768
	21st to end	Day	9% 2 11% 30% 37% 11%	845
August	1st to 10th	Day	7% 2 10% 35% 33% 12%	726
	11th to 20th	Day	11% 2 11% 24% 37% 14%	524
	21st to end	Day	8% 2 8% 28% 38% 16%	583
September	1st to 10th	Day	12% 2 12% 30% 29% 15%	530
	11th to 20th	Day	8% 2 10% 25% 36% 19%	530
	21st to end	Day	10% 5% 19% 27% 26% 14%	510
October	1st to 10th	Day	9% 5% 21% 32% 23% 10%	330
	11th to 20th	Day	13% 14% 29% 26% 15% 4	330
	21st to end	Day	27% 14% 26% 24% 8%	363
November	1st to 10th	Day	14% 9% 29% 29% 15% 4	330
	11th to 20th	Day	16% 16% 33% 20% 13% 2	330
	21st to end	Day	24% 10% 28% 26% 12% 1	330
December	1st to 10th	Day	13% 12% 38% 28% 7% 3	330
	11th to 20th	Day	18% 12% 35% 25% 8% 1	330
	21st to end	Day	25% 16% 36% 17% 5% 1	363
January	1st to 10th	Night	18% 12% 36% 28% 5%	1050
	11th to 20th	Night	20% 14% 37% 25% 4	1050
	21st to end	Night	19% 11% 38% 29% 2	1155
February	1st to 10th	Night	21% 9% 36% 30% 3	1050
	11th to 20th	Night	21% 13% 37% 27% 2	857
	21st to end	Night	19% 12% 40% 27% 2	692
March	1st to 10th	Night	21% 11% 37% 27% 4	818
	11th to 20th	Night	21% 10% 32% 30% 7%	630
	21st to end	Night	23% 10% 33% 29% 5%	693
April	1st to 10th	Night	15% 12% 37% 30% 6%	630
	11th to 20th	Night	16% 10% 33% 32% 9%	623
	21st to end	Night	21% 6% 35% 32% 6%	630
May	1st to 10th	Night	17% 4 30% 39% 9%	630
	11th to 20th	Night	18% 2 26% 35% 18% 1	621
	21st to end	Night	11% 3 20% 43% 20% 3	683
June	1st to 10th	Night	9% 2 19% 41% 28% 1	630
	11th to 20th	Night	8% 2 15% 50% 23% 1	630
	21st to end	Night	5% 1 19% 50% 24% 1	630
July	1st to 10th	Night	10% 2 7% 35% 38% 7%	615
	11th to 20th	Night	5% 1 9% 43% 38% 4	612
	21st to end	Night	6% 2 9% 42% 38% 3	687
August	1st to 10th	Night	8% 2 7% 41% 40% 3	597
	11th to 20th	Night	8% 3 8% 40% 37% 5%	594
	21st to end	Night	7% 8% 39% 41% 5%	660
September	1st to 10th	Night	12% 2 12% 37% 33% 6%	600
	11th to 20th	Night	9% 2 11% 34% 39% 6%	600
	21st to end	Night	10% 4 19% 35% 27% 5%	797
October	1st to 10th	Night	10% 3 21% 36% 27% 4	800
	11th to 20th	Night	16% 6% 29% 34% 15% 1	800
	21st to end	Night	22% 8% 31% 32% 6%	880
November	1st to 10th	Night	14% 6% 33% 36% 11% 1	820
	11th to 20th	Night	15% 11% 34% 33% 7%	840
	21st to end	Night	22% 10% 31% 30% 6%	935
December	1st to 10th	Night	20% 10% 34% 31% 5% 1	1050
	11th to 20th	Night	23% 7% 30% 34% 6%	1050
	21st to end	Night	27% 10% 32% 28% 3	1155

Page generated at 2026-07-28 02:25:24. Version ac07

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