Moss Rock Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Moss Rock Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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	4 6% 2 6% 4 4 5% 4 6% 8% 52%	135570
	Night	3 5% 2 5% 3 3 3 3 5% 7% 59%	131932
Transparency	Day	27% 7% 12% 24% 23% 9%	130509
	Night	23% 5% 11% 25% 26% 10%	127997
Seeing	Day	10% 11% 35% 28% 13% 3	16964
	Night	9% 9% 36% 31% 13% 2	26946

By Month

Percentage Cloud Cover SW (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	6% 4 7% 4 3 3 2 4 8% 59%	3638
	11th to 20th	Day	4 3 8% 5% 4 4 3 6% 7% 57%	3778
	21st to end	Day	1 5% 3 7% 4 3 4 2 5% 7% 61%	4510
February	1st to 10th	Day	1 6% 4 8% 4 3 4 4 5% 8% 55%	1800
	11th to 20th	Day	4 3 6% 3 3 4 4 7% 8% 58%	1898
	21st to end	Day	1 7% 2 7% 3 4 3 3 5% 7% 58%	1650
March	1st to 10th	Day	1 9% 4 7% 3 3 3 3 5% 6% 54%	4500
	11th to 20th	Day	2 11% 5% 8% 4 4 4 3 5% 7% 48%	4992
	21st to end	Day	2 9% 2 6% 4 4 4 4 6% 10% 47%	5787
April	1st to 10th	Day	4 8% 2 5% 3 4 3 4 7% 8% 52%	2400
	11th to 20th	Day	8% 9% 5% 7% 4 4 4 4 5% 8% 43%	2371
	21st to end	Day	7% 10% 2 6% 4 5% 5% 4 7% 8% 43%	2400
May	1st to 10th	Day	10% 10% 3 6% 4 4 5% 4 6% 8% 40%	5782
	11th to 20th	Day	8% 6% 2 5% 4 4 5% 5% 7% 9% 44%	6298
	21st to end	Day	7% 6% 2 5% 4 5% 5% 5% 7% 9% 47%	6925
June	1st to 10th	Day	5% 5% 1 5% 4 5% 6% 6% 8% 10% 46%	2800
	11th to 20th	Day	6% 4 1 5% 4 4 6% 5% 7% 8% 51%	2800
	21st to end	Day	5% 4 1 5% 5% 4 6% 5% 8% 9% 47%	2800
July	1st to 10th	Day	4 3 1 4 3 4 5% 4 7% 9% 56%	6339
	11th to 20th	Day	3 3 1 5% 4 6% 7% 7% 11% 11% 41%	6340
	21st to end	Day	5% 4 2 7% 5% 5% 6% 5% 8% 9% 43%	6734
August	1st to 10th	Day	7% 4 2 5% 4 5% 7% 6% 9% 10% 41%	5779
	11th to 20th	Day	3 3 1 5% 4 4 5% 5% 7% 9% 53%	5416
	21st to end	Day	7% 5% 2 7% 6% 5% 7% 5% 7% 8% 42%	5984
September	1st to 10th	Day	4 4 1 4 4 3 5% 4 6% 7% 57%	2400
	11th to 20th	Day	5% 4 1 5% 3 4 4 3 6% 7% 57%	2393
	21st to end	Day	7% 4 1 4 3 3 4 3 3 5% 63%	2190
October	1st to 10th	Day	3 4 1 3 2 2 2 2 4 5% 71%	3270
	11th to 20th	Day	5% 4 2 6% 3 3 3 2 5% 7% 59%	3250
	21st to end	Day	4 7% 3 4 2 2 3 3 4 5% 64%	3452
November	1st to 10th	Day	2 5% 2 4 3 2 2 2 4 9% 66%	1781
	11th to 20th	Day	1 5% 2 3 3 3 3 3 6% 8% 63%	1800
	21st to end	Day	2 5% 3 7% 3 3 3 3 5% 8% 59%	1663
December	1st to 10th	Day	3 2 4 3 2 2 2 4 6% 71%	2620
	11th to 20th	Day	1 4 1 3 2 2 3 3 5% 8% 68%	3026
	21st to end	Day	2 6% 3 7% 4 4 4 3 6% 8% 54%	4004
January	1st to 10th	Night	5% 4 8% 4 3 3 3 4 8% 58%	6327
	11th to 20th	Night	2 1 5% 4 4 4 3 4 9% 63%	6230
	21st to end	Night	1 6% 2 6% 4 3 3 3 4 6% 62%	6468
February	1st to 10th	Night	1 5% 3 6% 3 3 4 3 5% 8% 58%	2600
	11th to 20th	Night	1 4 2 7% 4 3 4 4 5% 7% 59%	2388
	21st to end	Night	1 6% 3 6% 3 3 3 3 4 7% 61%	1980
March	1st to 10th	Night	1 8% 3 7% 3 4 3 3 4 7% 56%	5374
	11th to 20th	Night	2 12% 3 5% 3 3 3 3 5% 7% 55%	5004
	21st to end	Night	4 7% 3 6% 4 4 3 3 5% 8% 54%	5173
April	1st to 10th	Night	2 7% 2 5% 3 3 3 4 5% 8% 57%	2000
	11th to 20th	Night	7% 9% 3 5% 3 3 3 3 4 7% 52%	1977
	21st to end	Night	9% 10% 2 6% 3 3 4 3 5% 7% 49%	2000
May	1st to 10th	Night	13% 9% 3 4 3 3 4 3 5% 7% 46%	4193
	11th to 20th	Night	8% 6% 2 6% 4 4 4 4 6% 8% 48%	3617
	21st to end	Night	7% 6% 1 4 3 3 3 3 4 6% 60%	3972
June	1st to 10th	Night	5% 3 2 4 2 3 4 4 6% 9% 59%	1600
	11th to 20th	Night	6% 3 1 2 2 2 4 4 6% 7% 64%	1600
	21st to end	Night	4 2 1 3 3 3 3 4 7% 8% 62%	1600
July	1st to 10th	Night	3 3 1 4 3 3 4 4 6% 8% 63%	3640
	11th to 20th	Night	4 3 1 3 3 3 4 4 8% 12% 55%	3640
	21st to end	Night	4 2 1 4 4 4 5% 4 8% 10% 55%	4122
August	1st to 10th	Night	5% 3 1 3 3 5% 5% 6% 8% 9% 50%	4033
	11th to 20th	Night	3 2 1 3 2 4 4 4 7% 9% 61%	4432
	21st to end	Night	4 4 2 5% 5% 5% 5% 4 7% 8% 51%	4993
September	1st to 10th	Night	6% 3 2 4 2 3 3 3 5% 8% 60%	2000
	11th to 20th	Night	6% 3 1 2 3 3 3 3 4 7% 66%	2006
	21st to end	Night	7% 3 1 3 3 3 3 3 4 5% 64%	2207
October	1st to 10th	Night	3 4 1 2 2 1 2 2 3 4 75%	3920
	11th to 20th	Night	3 4 2 3 3 3 3 2 5% 7% 65%	3898
	21st to end	Night	4 6% 3 4 2 2 2 2 3 5% 66%	4316
November	1st to 10th	Night	2 5% 2 4 2 2 3 2 5% 7% 67%	2562
	11th to 20th	Night	1 5% 2 4 3 2 3 2 4 7% 67%	2600
	21st to end	Night	2 5% 2 6% 3 3 3 3 5% 6% 62%	2735
December	1st to 10th	Night	1 3 1 3 2 2 2 2 3 6% 76%	4570
	11th to 20th	Night	1 4 2 4 2 3 3 3 5% 7% 66%	5181
	21st to end	Night	2 7% 3 7% 4 4 5% 3 6% 7% 53%	6974

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	23% 5% 11% 31% 23% 7%	3639
	11th to 20th	Day	26% 6% 13% 23% 23% 9%	3778
	21st to end	Day	22% 5% 10% 27% 23% 12%	4510
February	1st to 10th	Day	27% 6% 15% 26% 19% 7%	1800
	11th to 20th	Day	22% 7% 13% 28% 25% 5%	1908
	21st to end	Day	26% 4 11% 25% 25% 9%	1650
March	1st to 10th	Day	27% 6% 8% 18% 30% 11%	4500
	11th to 20th	Day	30% 8% 9% 25% 23% 4	4992
	21st to end	Day	26% 11% 12% 35% 14% 2	5787
April	1st to 10th	Day	26% 7% 13% 27% 20% 8%	2400
	11th to 20th	Day	36% 6% 13% 21% 19% 5%	2374
	21st to end	Day	35% 8% 14% 25% 16% 1	2400
May	1st to 10th	Day	36% 8% 13% 24% 16% 2	5782
	11th to 20th	Day	31% 9% 13% 21% 19% 7%	6302
	21st to end	Day	30% 7% 15% 23% 21% 4	6926
June	1st to 10th	Day	27% 10% 18% 21% 17% 7%	2800
	11th to 20th	Day	27% 9% 12% 17% 22% 14%	2800
	21st to end	Day	26% 8% 13% 20% 22% 11%	2800
July	1st to 10th	Day	21% 7% 7% 17% 28% 20%	6271
	11th to 20th	Day	29% 6% 15% 24% 16% 9%	6265
	21st to end	Day	32% 7% 12% 18% 17% 15%	6539
August	1st to 10th	Day	38% 8% 15% 18% 15% 7%	4208
	11th to 20th	Day	26% 7% 15% 18% 21% 12%	3920
	21st to end	Day	27% 6% 12% 19% 26% 10%	4311
September	1st to 10th	Day	23% 5% 10% 22% 24% 15%	2400
	11th to 20th	Day	23% 6% 12% 22% 26% 12%	2393
	21st to end	Day	23% 5% 6% 22% 34% 10%	2190
October	1st to 10th	Day	16% 4 7% 15% 36% 22%	3270
	11th to 20th	Day	24% 4 8% 30% 27% 7%	3250
	21st to end	Day	22% 3 6% 27% 34% 8%	3452
November	1st to 10th	Day	18% 5% 13% 24% 30% 11%	1781
	11th to 20th	Day	18% 4 10% 30% 29% 9%	1800
	21st to end	Day	24% 3 10% 26% 26% 11%	1663
December	1st to 10th	Day	15% 4 10% 33% 29% 9%	2620
	11th to 20th	Day	15% 4 9% 36% 29% 7%	3024
	21st to end	Day	26% 5% 10% 26% 26% 8%	4004
January	1st to 10th	Night	26% 4 8% 28% 27% 7%	6327
	11th to 20th	Night	20% 5% 13% 25% 23% 13%	6230
	21st to end	Night	25% 4 11% 24% 23% 13%	6468
February	1st to 10th	Night	25% 6% 14% 27% 22% 7%	2600
	11th to 20th	Night	25% 6% 11% 29% 23% 5%	2388
	21st to end	Night	24% 4 8% 27% 24% 12%	1980
March	1st to 10th	Night	27% 5% 7% 19% 33% 10%	5374
	11th to 20th	Night	27% 6% 9% 18% 30% 10%	5004
	21st to end	Night	28% 6% 12% 35% 17% 2	5173
April	1st to 10th	Night	25% 7% 12% 25% 23% 8%	2000
	11th to 20th	Night	31% 6% 11% 25% 20% 6%	1977
	21st to end	Night	34% 6% 10% 26% 20% 3	2000
May	1st to 10th	Night	34% 8% 13% 26% 15% 5%	4190
	11th to 20th	Night	31% 8% 14% 22% 20% 5%	3618
	21st to end	Night	24% 6% 18% 26% 21% 5%	3972
June	1st to 10th	Night	22% 7% 18% 29% 15% 9%	1600
	11th to 20th	Night	18% 5% 13% 20% 24% 20%	1600
	21st to end	Night	17% 8% 12% 22% 28% 14%	1600
July	1st to 10th	Night	19% 5% 9% 16% 33% 18%	3600
	11th to 20th	Night	20% 5% 18% 29% 19% 9%	3592
	21st to end	Night	22% 5% 17% 23% 17% 15%	4005
August	1st to 10th	Night	29% 7% 18% 22% 19% 6%	2928
	11th to 20th	Night	19% 5% 22% 21% 22% 11%	3204
	21st to end	Night	21% 5% 15% 22% 30% 7%	3597
September	1st to 10th	Night	21% 5% 11% 22% 24% 17%	2000
	11th to 20th	Night	18% 4 11% 25% 26% 16%	2006
	21st to end	Night	22% 4 9% 22% 31% 12%	2207
October	1st to 10th	Night	14% 4 6% 20% 32% 24%	3920
	11th to 20th	Night	20% 3 9% 28% 31% 8%	3898
	21st to end	Night	22% 2 5% 25% 33% 12%	4316
November	1st to 10th	Night	18% 5% 9% 25% 29% 13%	2562
	11th to 20th	Night	18% 4 9% 29% 31% 9%	2600
	21st to end	Night	22% 4 11% 24% 30% 10%	2735
December	1st to 10th	Night	13% 4 9% 32% 35% 8%	4570
	11th to 20th	Night	16% 5% 8% 32% 35% 4	5182
	21st to end	Night	27% 6% 10% 24% 27% 7%	6974

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	11% 10% 40% 27% 9% 3	300
	11th to 20th	Day	8% 17% 48% 22% 5% 1	300
	21st to end	Day	6% 14% 46% 27% 6% 1	330
February	1st to 10th	Day	13% 24% 43% 17% 3 1	300
	11th to 20th	Day	12% 13% 53% 19% 3	300
	21st to end	Day	13% 16% 51% 18% 2	249
March	1st to 10th	Day	8% 15% 47% 22% 8%	300
	11th to 20th	Day	15% 16% 38% 25% 5% 1	492
	21st to end	Day	15% 21% 41% 18% 5% 1	550
April	1st to 10th	Day	13% 23% 37% 22% 4 1	500
	11th to 20th	Day	13% 25% 36% 21% 4 1	492
	21st to end	Day	16% 14% 40% 25% 5% 1	500
May	1st to 10th	Day	15% 12% 39% 25% 7% 1	613
	11th to 20th	Day	18% 9% 32% 27% 12% 2	725
	21st to end	Day	13% 6% 34% 28% 17% 2	789
June	1st to 10th	Day	6% 6% 27% 35% 20% 6%	730
	11th to 20th	Day	7% 8% 34% 32% 15% 4	730
	21st to end	Day	7% 7% 32% 32% 15% 6%	730
July	1st to 10th	Day	7% 4 22% 35% 26% 6%	721
	11th to 20th	Day	4 3 24% 36% 27% 6%	718
	21st to end	Day	6% 4 20% 35% 27% 8%	726
August	1st to 10th	Day	6% 6% 30% 34% 19% 5%	527
	11th to 20th	Day	4 4 31% 34% 22% 5%	517
	21st to end	Day	7% 3 29% 39% 18% 5%	583
September	1st to 10th	Day	6% 4 30% 36% 18% 5%	530
	11th to 20th	Day	6% 7% 32% 35% 16% 4	523
	21st to end	Day	8% 7% 31% 30% 19% 5%	330
October	1st to 10th	Day	11% 10% 31% 23% 18% 7%	330
	11th to 20th	Day	8% 12% 39% 29% 8% 3	330
	21st to end	Day	10% 16% 40% 25% 8% 1	363
November	1st to 10th	Day	8% 14% 42% 23% 12% 1	306
	11th to 20th	Day	12% 14% 38% 26% 7% 2	300
	21st to end	Day	11% 15% 41% 27% 5% 1	300
December	1st to 10th	Day	12% 14% 41% 23% 8% 1	300
	11th to 20th	Day	10% 17% 49% 20% 4	300
	21st to end	Day	12% 17% 43% 21% 6%	330
January	1st to 10th	Night	10% 14% 42% 25% 8% 2	1000
	11th to 20th	Night	11% 18% 43% 19% 7% 1	955
	21st to end	Night	9% 14% 48% 23% 5% 1	880
February	1st to 10th	Night	10% 16% 53% 17% 4	800
	11th to 20th	Night	8% 15% 49% 22% 6%	784
	21st to end	Night	8% 14% 53% 21% 3	660
March	1st to 10th	Night	6% 13% 48% 26% 6% 1	800
	11th to 20th	Night	11% 11% 37% 33% 8% 1	800
	21st to end	Night	11% 11% 45% 28% 3 1	745
April	1st to 10th	Night	10% 10% 49% 28% 4	600
	11th to 20th	Night	12% 11% 41% 26% 8% 1	593
	21st to end	Night	14% 8% 39% 28% 11% 1	600
May	1st to 10th	Night	13% 7% 35% 33% 11% 1	600
	11th to 20th	Night	17% 5% 23% 40% 13% 3	591
	21st to end	Night	9% 5% 29% 35% 20% 2	650
June	1st to 10th	Night	6% 3 21% 43% 23% 4	600
	11th to 20th	Night	6% 3 28% 45% 16% 2	600
	21st to end	Night	5% 2 22% 42% 27% 3	600
July	1st to 10th	Night	6% 3 14% 36% 36% 5%	585
	11th to 20th	Night	3 1 13% 42% 36% 5%	582
	21st to end	Night	7% 2 7% 39% 38% 6%	657
August	1st to 10th	Night	7% 2 12% 42% 34% 4	597
	11th to 20th	Night	4 2 14% 45% 30% 5%	594
	21st to end	Night	5% 2 14% 41% 31% 7%	660
September	1st to 10th	Night	6% 2 17% 43% 27% 6%	600
	11th to 20th	Night	7% 4 24% 40% 18% 6%	600
	21st to end	Night	7% 4 24% 43% 17% 4	607
October	1st to 10th	Night	9% 7% 34% 33% 14% 4	800
	11th to 20th	Night	8% 8% 38% 34% 11% 3	800
	21st to end	Night	9% 12% 42% 27% 8% 2	920
November	1st to 10th	Night	6% 10% 43% 30% 9% 2	986
	11th to 20th	Night	9% 15% 45% 26% 4 1	1000
	21st to end	Night	7% 11% 47% 29% 6%	1000
December	1st to 10th	Night	10% 14% 44% 28% 5%	1000
	11th to 20th	Night	8% 14% 48% 24% 5% 1	1000
	21st to end	Night	9% 17% 45% 23% 6% 1	1100

Page generated at 2025-10-31 00:02:39. Version ac07

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