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 2025-03-27.

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 5% 8% 42%	109015
	Night	14% 8% 3 7% 4 4 4 3 5% 6% 43%	110511
Transparency	Day	39% 5% 12% 25% 17% 2	112634
	Night	40% 5% 11% 24% 18% 3	115033
Seeing	Day	14% 7% 24% 28% 20% 7%	16070
	Night	16% 7% 27% 34% 15% 1	26075

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	12% 12% 6% 13% 6% 4 4 4 5% 6% 29%	3617
	11th to 20th	Day	10% 8% 3 8% 5% 7% 5% 4 5% 7% 38%	3921
	21st to end	Day	18% 13% 5% 8% 4 3 3 2 4 4 34%	4488
February	1st to 10th	Day	16% 11% 4 9% 5% 4 4 4 4 8% 33%	1800
	11th to 20th	Day	15% 9% 3 7% 6% 3 5% 4 5% 6% 36%	1913
	21st to end	Day	14% 11% 3 6% 5% 4 4 3 4 6% 39%	1650
March	1st to 10th	Day	18% 10% 3 6% 3 3 3 3 4 5% 42%	3285
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 6% 7% 48%	3590
	21st to end	Day	15% 7% 3 7% 4 5% 5% 4 6% 7% 35%	4072
April	1st to 10th	Day	12% 6% 3 7% 6% 5% 4 4 5% 8% 41%	2280
	11th to 20th	Day	16% 7% 2 8% 4 4 4 4 5% 5% 42%	2254
	21st to end	Day	16% 7% 3 7% 5% 4 4 4 5% 6% 38%	2280
May	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 7% 10% 33%	3851
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 6% 8% 38%	4388
	21st to end	Day	9% 5% 2 7% 4 5% 5% 4 6% 7% 45%	4824
June	1st to 10th	Day	7% 6% 2 5% 4 4 5% 4 6% 9% 48%	2660
	11th to 20th	Day	6% 4 2 4 4 4 5% 4 7% 9% 51%	2660
	21st to end	Day	4 4 1 5% 5% 5% 5% 5% 8% 11% 46%	2660
July	1st to 10th	Day	13% 5% 2 7% 4 5% 4 4 6% 9% 42%	4440
	11th to 20th	Day	5% 3 2 5% 5% 4 5% 4 7% 10% 50%	4440
	21st to end	Day	6% 4 2 7% 5% 5% 6% 5% 7% 11% 42%	4566
August	1st to 10th	Day	5% 4 2 5% 4 4 5% 4 7% 10% 50%	4104
	11th to 20th	Day	12% 5% 2 6% 5% 4 5% 4 6% 9% 42%	3734
	21st to end	Day	6% 4 2 4 3 4 5% 4 6% 9% 52%	4180
September	1st to 10th	Day	13% 4 2 4 4 3 3 2 5% 8% 51%	2280
	11th to 20th	Day	9% 6% 2 4 4 4 4 4 6% 8% 50%	2262
	21st to end	Day	7% 5% 2 6% 4 4 3 4 5% 7% 53%	2071
October	1st to 10th	Day	10% 4 2 5% 3 3 2 3 4 7% 56%	1900
	11th to 20th	Day	15% 6% 1 5% 3 3 3 2 4 6% 53%	1900
	21st to end	Day	18% 7% 2 5% 4 3 3 2 4 6% 46%	2077
November	1st to 10th	Day	11% 6% 2 5% 3 4 2 2 5% 6% 54%	1762
	11th to 20th	Day	14% 8% 2 5% 4 3 3 4 4 6% 47%	1800
	21st to end	Day	15% 10% 4 6% 5% 3 4 3 4 6% 40%	1706
December	1st to 10th	Day	9% 10% 3 6% 3 4 2 2 4 6% 50%	2610
	11th to 20th	Day	12% 8% 5% 8% 6% 4 4 4 4 6% 38%	3008
	21st to end	Day	24% 15% 5% 10% 5% 4 4 3 4 4 23%	3982
January	1st to 10th	Night	15% 12% 6% 11% 5% 4 3 3 4 5% 32%	6348
	11th to 20th	Night	11% 10% 4 11% 6% 6% 5% 3 4 5% 35%	6056
	21st to end	Night	19% 13% 4 7% 4 3 3 3 4 6% 33%	6490
February	1st to 10th	Night	15% 12% 5% 7% 6% 4 4 3 5% 5% 34%	2600
	11th to 20th	Night	16% 9% 3 8% 5% 3 3 3 4 6% 40%	2371
	21st to end	Night	15% 9% 4 8% 5% 3 3 3 4 5% 42%	1980
March	1st to 10th	Night	21% 10% 3 5% 3 3 3 2 3 4 42%	3806
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 7% 51%	3590
	21st to end	Night	18% 10% 3 7% 5% 5% 4 3 5% 5% 34%	3697
April	1st to 10th	Night	11% 7% 3 7% 5% 3 4 3 4 7% 46%	1900
	11th to 20th	Night	13% 6% 3 6% 5% 3 4 3 6% 8% 44%	1875
	21st to end	Night	17% 7% 3 7% 4 3 3 3 5% 6% 41%	1900
May	1st to 10th	Night	16% 8% 2 6% 3 4 4 4 6% 8% 39%	3043
	11th to 20th	Night	15% 7% 2 5% 4 4 5% 4 5% 7% 43%	2514
	21st to end	Night	8% 5% 2 6% 5% 4 4 5% 7% 10% 44%	2743
June	1st to 10th	Night	7% 5% 2 5% 4 4 3 5% 7% 8% 50%	1520
	11th to 20th	Night	5% 4 1 4 4 3 4 5% 5% 10% 54%	1520
	21st to end	Night	4 4 1 4 4 4 5% 4 6% 11% 53%	1520
July	1st to 10th	Night	12% 6% 2 6% 4 3 4 4 5% 9% 44%	2530
	11th to 20th	Night	4 3 1 4 4 4 4 4 6% 10% 56%	2530
	21st to end	Night	7% 6% 2 8% 6% 4 6% 4 6% 10% 41%	2993
August	1st to 10th	Night	5% 3 2 4 3 4 4 4 6% 8% 58%	2836
	11th to 20th	Night	11% 5% 2 6% 4 4 4 4 6% 7% 47%	3092
	21st to end	Night	5% 3 2 3 3 4 4 4 6% 9% 57%	3476
September	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 7% 53%	1900
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 3 6% 7% 52%	1900
	21st to end	Night	9% 5% 1 4 4 3 4 3 6% 7% 54%	2101
October	1st to 10th	Night	9% 4 2 4 3 3 4 3 4 5% 60%	2280
	11th to 20th	Night	13% 5% 2 4 3 2 3 3 5% 6% 54%	2280
	21st to end	Night	18% 6% 2 4 3 3 4 3 4 5% 47%	2519
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 6% 55%	2536
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 4 7% 49%	2600
	21st to end	Night	18% 9% 3 6% 3 3 3 3 5% 5% 44%	2690
December	1st to 10th	Night	11% 9% 3 6% 4 4 3 3 4 5% 47%	4580
	11th to 20th	Night	15% 8% 3 6% 4 4 4 3 6% 6% 41%	5199
	21st to end	Night	25% 17% 4 8% 5% 4 4 3 4 4 21%	6996

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	55% 5% 11% 19% 9% 1	3618
	11th to 20th	Day	43% 6% 13% 19% 15% 3	3921
	21st to end	Day	53% 3 9% 15% 16% 3	4488
February	1st to 10th	Day	52% 5% 11% 21% 10% 1	1800
	11th to 20th	Day	48% 4 11% 20% 15% 2	1923
	21st to end	Day	47% 3 8% 24% 15% 3	1650
March	1st to 10th	Day	46% 4 7% 21% 19% 3	3285
	11th to 20th	Day	35% 6% 11% 25% 18% 5%	3590
	21st to end	Day	45% 7% 15% 24% 7% 1	4072
April	1st to 10th	Day	40% 5% 12% 26% 17% 1	2280
	11th to 20th	Day	44% 4 10% 24% 15% 3	2254
	21st to end	Day	46% 5% 11% 23% 13% 1	2280
May	1st to 10th	Day	44% 9% 14% 23% 10% 1	3886
	11th to 20th	Day	44% 7% 10% 26% 12% 1	4392
	21st to end	Day	35% 7% 11% 24% 21% 1	4837
June	1st to 10th	Day	32% 7% 14% 28% 17% 2	2660
	11th to 20th	Day	29% 7% 12% 27% 23% 3	2660
	21st to end	Day	28% 8% 17% 26% 16% 5%	2660
July	1st to 10th	Day	38% 5% 12% 29% 14% 2	4371
	11th to 20th	Day	27% 7% 15% 31% 19% 2	4365
	21st to end	Day	33% 9% 19% 26% 11% 1	4422
August	1st to 10th	Day	27% 7% 16% 36% 14%	4090
	11th to 20th	Day	38% 6% 13% 25% 17% 1	3770
	21st to end	Day	28% 6% 16% 33% 15% 3	4179
September	1st to 10th	Day	32% 4 11% 26% 22% 5%	2280
	11th to 20th	Day	32% 4 9% 27% 25% 3	2262
	21st to end	Day	31% 5% 9% 28% 24% 3	2071
October	1st to 10th	Day	18% 2 6% 19% 48% 7%	3160
	11th to 20th	Day	23% 2 7% 21% 38% 8%	3140
	21st to end	Day	34% 3 7% 28% 26% 2	3403
November	1st to 10th	Day	32% 4 12% 26% 21% 5%	1762
	11th to 20th	Day	38% 4 9% 25% 20% 5%	1800
	21st to end	Day	47% 4 10% 26% 14%	1706
December	1st to 10th	Day	37% 3 14% 30% 15%	2610
	11th to 20th	Day	47% 4 12% 23% 13% 2	3005
	21st to end	Day	65% 5% 12% 12% 6% 1	3982
January	1st to 10th	Night	55% 4 10% 22% 8% 1	6348
	11th to 20th	Night	52% 4 8% 21% 13% 3	6056
	21st to end	Night	53% 4 8% 18% 16% 1	6490
February	1st to 10th	Night	52% 4 10% 21% 12% 1	2600
	11th to 20th	Night	47% 3 9% 21% 17% 2	2372
	21st to end	Night	45% 4 10% 23% 15% 3	1980
March	1st to 10th	Night	47% 3 8% 24% 16% 2	3806
	11th to 20th	Night	35% 4 11% 24% 21% 5%	3590
	21st to end	Night	51% 6% 10% 25% 7%	3697
April	1st to 10th	Night	38% 4 10% 26% 18% 3	1900
	11th to 20th	Night	38% 4 10% 24% 20% 4	1875
	21st to end	Night	44% 5% 8% 24% 17% 2	1900
May	1st to 10th	Night	41% 8% 17% 19% 14% 1	3073
	11th to 20th	Night	40% 8% 12% 28% 12% 1	2515
	21st to end	Night	34% 8% 15% 23% 19% 2	2751
June	1st to 10th	Night	29% 7% 15% 26% 19% 4	1520
	11th to 20th	Night	25% 7% 11% 27% 26% 4	1520
	21st to end	Night	26% 6% 16% 27% 19% 7%	1520
July	1st to 10th	Night	36% 6% 11% 30% 14% 3	2490
	11th to 20th	Night	22% 6% 12% 37% 20% 3	2482
	21st to end	Night	36% 7% 18% 24% 13% 2	2898
August	1st to 10th	Night	22% 7% 16% 35% 18% 1	2826
	11th to 20th	Night	35% 5% 12% 26% 20% 2	3122
	21st to end	Night	22% 5% 16% 34% 18% 3	3476
September	1st to 10th	Night	32% 4 9% 24% 24% 7%	1900
	11th to 20th	Night	32% 5% 8% 25% 25% 5%	1900
	21st to end	Night	30% 5% 9% 24% 27% 4	2101
October	1st to 10th	Night	17% 3 6% 18% 49% 7%	3800
	11th to 20th	Night	22% 3 7% 23% 37% 9%	3779
	21st to end	Night	30% 4 9% 26% 26% 5%	4143
November	1st to 10th	Night	32% 4 11% 26% 22% 6%	2536
	11th to 20th	Night	36% 3 8% 26% 22% 4	2600
	21st to end	Night	43% 4 11% 24% 17% 1	2690
December	1st to 10th	Night	41% 3 9% 33% 14% 1	4580
	11th to 20th	Night	43% 3 13% 23% 17%	5201
	21st to end	Night	67% 5% 11% 12% 5% 1	6996

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	20% 14% 35% 20% 10% 1	300
	11th to 20th	Day	24% 14% 36% 21% 5%	300
	21st to end	Day	16% 14% 42% 23% 5% 1	330
February	1st to 10th	Day	23% 11% 40% 23% 3	300
	11th to 20th	Day	24% 12% 36% 20% 7% 1	300
	21st to end	Day	22% 12% 39% 23% 3	249
March	1st to 10th	Day	17% 11% 45% 23% 4	300
	11th to 20th	Day	23% 16% 32% 20% 8% 1	300
	21st to end	Day	27% 11% 33% 21% 7%	430
April	1st to 10th	Day	15% 10% 37% 24% 12% 1	460
	11th to 20th	Day	20% 12% 30% 25% 13%	452
	21st to end	Day	20% 10% 34% 27% 8% 1	460
May	1st to 10th	Day	16% 10% 26% 33% 12% 3	463
	11th to 20th	Day	17% 6% 23% 30% 18% 5%	664
	21st to end	Day	12% 4 25% 33% 17% 9%	734
June	1st to 10th	Day	9% 5% 16% 30% 28% 11%	680
	11th to 20th	Day	7% 2 18% 33% 31% 9%	680
	21st to end	Day	4 4 23% 35% 27% 7%	680
July	1st to 10th	Day	11% 3 9% 26% 34% 18%	671
	11th to 20th	Day	6% 2 12% 36% 32% 12%	668
	21st to end	Day	9% 2 11% 30% 37% 11%	739
August	1st to 10th	Day	8% 2 10% 35% 33% 12%	676
	11th to 20th	Day	12% 2 12% 25% 36% 13%	484
	21st to end	Day	8% 2 8% 27% 39% 16%	539
September	1st to 10th	Day	12% 2 12% 29% 29% 16%	490
	11th to 20th	Day	9% 2 10% 25% 35% 18%	490
	21st to end	Day	10% 5% 20% 27% 25% 12%	471
October	1st to 10th	Day	10% 6% 22% 32% 21% 10%	300
	11th to 20th	Day	13% 13% 30% 26% 15% 3	300
	21st to end	Day	26% 13% 27% 25% 9%	330
November	1st to 10th	Day	15% 7% 28% 29% 16% 4	300
	11th to 20th	Day	16% 16% 33% 19% 13% 3	300
	21st to end	Day	23% 9% 29% 27% 11% 1	300
December	1st to 10th	Day	12% 12% 39% 26% 7% 3	300
	11th to 20th	Day	18% 13% 34% 26% 8% 1	300
	21st to end	Day	26% 16% 35% 17% 5% 1	330
January	1st to 10th	Night	19% 13% 36% 27% 5%	1000
	11th to 20th	Night	21% 13% 37% 25% 4	1000
	21st to end	Night	19% 11% 38% 30% 2	1100
February	1st to 10th	Night	22% 10% 36% 29% 3	1000
	11th to 20th	Night	21% 13% 37% 27% 2	816
	21st to end	Night	19% 12% 40% 27% 2	660
March	1st to 10th	Night	20% 11% 37% 27% 4	779
	11th to 20th	Night	22% 9% 31% 31% 7%	600
	21st to end	Night	24% 9% 32% 29% 5%	647
April	1st to 10th	Night	15% 12% 36% 29% 7%	570
	11th to 20th	Night	15% 9% 32% 34% 9%	563
	21st to end	Night	21% 5% 36% 31% 6%	570
May	1st to 10th	Night	17% 4 30% 39% 10%	570
	11th to 20th	Night	19% 2 24% 36% 18% 1	561
	21st to end	Night	11% 3 20% 42% 21% 3	617
June	1st to 10th	Night	9% 2 17% 41% 30% 1	570
	11th to 20th	Night	8% 1 14% 51% 25% 1	570
	21st to end	Night	5% 1 19% 49% 24% 2	570
July	1st to 10th	Night	11% 2 7% 35% 37% 8%	555
	11th to 20th	Night	5% 1 10% 44% 36% 4	552
	21st to end	Night	6% 2 9% 42% 38% 3	624
August	1st to 10th	Night	8% 2 7% 39% 41% 3	567
	11th to 20th	Night	8% 3 8% 40% 36% 5%	564
	21st to end	Night	7% 8% 39% 41% 5%	627
September	1st to 10th	Night	12% 1 11% 36% 33% 6%	570
	11th to 20th	Night	9% 2 11% 34% 38% 5%	570
	21st to end	Night	11% 4 19% 36% 26% 4	757
October	1st to 10th	Night	11% 3 22% 34% 27% 4	760
	11th to 20th	Night	16% 6% 29% 34% 14% 1	760
	21st to end	Night	22% 8% 31% 32% 6%	836
November	1st to 10th	Night	14% 6% 32% 36% 11% 1	780
	11th to 20th	Night	16% 11% 33% 32% 7%	800
	21st to end	Night	22% 10% 31% 30% 7%	890
December	1st to 10th	Night	20% 9% 34% 31% 5% 1	1000
	11th to 20th	Night	23% 7% 29% 34% 6%	1000
	21st to end	Night	28% 10% 32% 27% 3	1100

Page generated at 2025-03-24 08:04:31. Version ac07

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