RCH Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the RCH Observatory clear sky chart from 2005-11-06 to 2025-01-22.

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 5% 4 4 4 3 5% 7% 48%	105340
	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	104137
Transparency	Day	35% 4 10% 23% 20% 8%	109083
	Night	32% 4 10% 22% 22% 11%	108508
Seeing	Day	13% 7% 21% 28% 24% 8%	15228
	Night	14% 7% 27% 35% 16% 2	25224

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	14% 7% 2 4 2 2 3 3 5% 7% 51%	2937
	11th to 20th	Day	16% 5% 2 3 3 3 2 3 4 6% 53%	2940
	21st to end	Day	22% 9% 3 6% 3 2 2 2 3 5% 44%	3144
February	1st to 10th	Day	20% 7% 2 6% 3 3 4 2 4 5% 43%	1871
	11th to 20th	Day	18% 7% 2 6% 4 4 3 3 3 5% 44%	1849
	21st to end	Day	19% 7% 3 5% 3 3 3 2 3 4 48%	1618
March	1st to 10th	Day	16% 7% 3 7% 4 4 3 3 5% 6% 40%	3436
	11th to 20th	Day	19% 8% 3 7% 5% 4 5% 4 5% 6% 34%	3480
	21st to end	Day	15% 8% 3 7% 4 4 4 3 5% 7% 40%	3814
April	1st to 10th	Day	14% 9% 2 6% 4 3 4 3 5% 7% 42%	2090
	11th to 20th	Day	15% 6% 3 6% 5% 4 4 4 5% 8% 40%	2141
	21st to end	Day	13% 6% 2 5% 4 3 4 3 5% 7% 47%	2355
May	1st to 10th	Day	20% 7% 3 8% 5% 4 5% 4 6% 7% 31%	4082
	11th to 20th	Day	15% 6% 3 6% 4 3 5% 3 5% 6% 45%	4082
	21st to end	Day	11% 7% 3 7% 5% 6% 6% 5% 8% 10% 34%	4477
June	1st to 10th	Day	5% 5% 1 5% 3 5% 5% 4 7% 10% 49%	2470
	11th to 20th	Day	5% 4 2 6% 3 5% 4 5% 7% 9% 49%	2470
	21st to end	Day	6% 5% 2 5% 5% 5% 5% 4 7% 10% 47%	2470
July	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 6% 8% 52%	4120
	11th to 20th	Day	4 3 1 4 3 4 4 4 7% 10% 57%	4120
	21st to end	Day	9% 7% 3 7% 5% 5% 5% 5% 6% 9% 38%	4465
August	1st to 10th	Day	2 1 1 2 2 2 3 3 5% 9% 70%	4104
	11th to 20th	Day	3 2 1 3 2 2 3 3 5% 10% 65%	4041
	21st to end	Day	4 2 1 5% 4 4 5% 4 7% 11% 52%	3938
September	1st to 10th	Day	8% 5% 2 6% 4 6% 6% 4 5% 7% 48%	2090
	11th to 20th	Day	11% 5% 1 7% 5% 5% 5% 4 6% 6% 46%	2090
	21st to end	Day	7% 3 2 4 4 4 5% 4 5% 7% 56%	2090
October	1st to 10th	Day	6% 4 1 4 3 3 3 3 5% 6% 61%	2049
	11th to 20th	Day	13% 3 2 4 3 3 3 3 3 4 59%	1900
	21st to end	Day	14% 4 1 4 2 3 3 2 3 4 60%	2090
November	1st to 10th	Day	13% 7% 2 5% 4 3 5% 3 4 6% 49%	1958
	11th to 20th	Day	11% 5% 2 5% 4 3 3 3 4 6% 55%	1940
	21st to end	Day	17% 7% 2 5% 4 3 4 3 4 6% 47%	1800
December	1st to 10th	Day	20% 6% 2 5% 4 2 3 3 4 5% 46%	2940
	11th to 20th	Day	19% 8% 2 6% 3 3 3 3 5% 6% 41%	3391
	21st to end	Day	15% 7% 2 7% 4 3 3 2 3 5% 49%	4488
January	1st to 10th	Night	17% 5% 2 4 2 2 2 2 3 5% 56%	4238
	11th to 20th	Night	15% 6% 1 3 2 1 2 2 3 4 61%	4250
	21st to end	Night	20% 7% 3 6% 3 3 3 2 4 5% 44%	4546
February	1st to 10th	Night	19% 7% 2 5% 3 3 4 2 4 5% 47%	2325
	11th to 20th	Night	19% 7% 2 5% 3 3 3 2 3 4 49%	2218
	21st to end	Night	19% 7% 1 4 3 3 3 3 3 5% 48%	1833
March	1st to 10th	Night	17% 8% 2 6% 3 4 4 3 6% 6% 40%	3419
	11th to 20th	Night	17% 7% 3 7% 4 5% 4 4 6% 7% 38%	3480
	21st to end	Night	13% 6% 3 6% 4 4 4 3 5% 6% 45%	3817
April	1st to 10th	Night	11% 6% 3 4 3 3 4 3 5% 7% 50%	2090
	11th to 20th	Night	13% 6% 2 5% 4 4 4 4 6% 6% 48%	1985
	21st to end	Night	12% 6% 2 5% 4 3 3 3 5% 6% 50%	1820
May	1st to 10th	Night	19% 7% 3 6% 4 4 4 3 4 7% 39%	2823
	11th to 20th	Night	11% 5% 2 7% 4 3 4 4 5% 8% 46%	2823
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 43%	3090
June	1st to 10th	Night	4 3 2 4 3 3 4 3 6% 9% 59%	1710
	11th to 20th	Night	4 2 1 4 4 2 3 3 6% 10% 63%	1710
	21st to end	Night	4 4 2 4 4 3 4 4 7% 8% 57%	1710
July	1st to 10th	Night	4 3 1 4 3 4 4 4 7% 10% 57%	2850
	11th to 20th	Night	3 2 1 3 3 3 4 4 6% 10% 61%	2850
	21st to end	Night	6% 6% 2 6% 5% 4 5% 4 7% 9% 45%	3098
August	1st to 10th	Night	1 1 1 1 1 2 1 4 7% 80%	2836
	11th to 20th	Night	4 2 1 3 3 2 3 3 6% 11% 63%	2798
	21st to end	Night	3 2 1 3 3 3 4 4 8% 12% 58%	3700
September	1st to 10th	Night	7% 5% 1 5% 4 4 4 4 5% 8% 52%	2090
	11th to 20th	Night	9% 5% 2 5% 3 3 3 4 5% 8% 52%	2090
	21st to end	Night	7% 3 1 4 2 3 3 4 5% 8% 60%	2090
October	1st to 10th	Night	5% 2 1 3 2 3 3 3 4 6% 67%	2109
	11th to 20th	Night	12% 4 1 3 3 2 3 3 4 5% 61%	2280
	21st to end	Night	13% 4 2 4 2 2 2 2 4 4 61%	2508
November	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 3 5% 55%	2340
	11th to 20th	Night	9% 4 2 5% 3 3 4 3 4 5% 57%	2455
	21st to end	Night	17% 6% 2 4 3 3 3 3 4 6% 50%	2600
December	1st to 10th	Night	20% 6% 2 5% 3 2 2 3 5% 5% 47%	4250
	11th to 20th	Night	20% 7% 3 6% 3 2 3 2 3 5% 46%	4816
	21st to end	Night	17% 6% 2 5% 2 2 2 2 3 4 55%	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	34% 3 4 21% 28% 10%	2938
	11th to 20th	Day	33% 2 5% 22% 29% 8%	2940
	21st to end	Day	47% 2 3 16% 22% 10%	3144
February	1st to 10th	Day	45% 3 5% 15% 21% 11%	1871
	11th to 20th	Day	45% 2 6% 17% 20% 10%	1859
	21st to end	Day	43% 2 4 17% 18% 16%	1618
March	1st to 10th	Day	45% 3 6% 17% 21% 8%	3436
	11th to 20th	Day	51% 3 10% 14% 16% 6%	3480
	21st to end	Day	44% 3 9% 19% 20% 6%	3814
April	1st to 10th	Day	42% 4 9% 18% 16% 12%	2090
	11th to 20th	Day	43% 4 8% 17% 19% 9%	2141
	21st to end	Day	37% 4 9% 20% 23% 7%	2355
May	1st to 10th	Day	53% 3 10% 19% 11% 4	4120
	11th to 20th	Day	41% 4 8% 19% 26% 2	4086
	21st to end	Day	43% 5% 13% 25% 12% 2	4489
June	1st to 10th	Day	29% 6% 13% 30% 20% 3	2470
	11th to 20th	Day	29% 6% 12% 31% 16% 5%	2470
	21st to end	Day	31% 7% 13% 27% 20% 3	2470
July	1st to 10th	Day	28% 7% 21% 34% 9% 1	4056
	11th to 20th	Day	22% 6% 17% 36% 18% 2	4051
	21st to end	Day	39% 8% 15% 27% 10% 1	4325
August	1st to 10th	Day	12% 4 12% 39% 32% 1	4090
	11th to 20th	Day	16% 3 21% 41% 16% 2	4080
	21st to end	Day	24% 8% 21% 30% 15% 2	3937
September	1st to 10th	Day	35% 5% 12% 20% 18% 10%	2090
	11th to 20th	Day	37% 6% 13% 19% 19% 7%	2090
	21st to end	Day	28% 5% 7% 24% 21% 15%	2090
October	1st to 10th	Day	15% 3 6% 13% 24% 40%	3400
	11th to 20th	Day	19% 2 4 19% 32% 23%	3140
	21st to end	Day	25% 2 5% 24% 32% 12%	3428
November	1st to 10th	Day	37% 5% 8% 19% 21% 11%	1958
	11th to 20th	Day	32% 4 8% 17% 24% 15%	1940
	21st to end	Day	40% 4 6% 18% 20% 12%	1800
December	1st to 10th	Day	41% 3 6% 19% 22% 9%	2940
	11th to 20th	Day	44% 4 6% 12% 25% 10%	3389
	21st to end	Day	40% 2 4 17% 26% 11%	4488
January	1st to 10th	Night	33% 2 4 19% 29% 13%	4238
	11th to 20th	Night	30% 2 4 17% 32% 15%	4250
	21st to end	Night	45% 1 3 16% 23% 12%	4546
February	1st to 10th	Night	42% 2 5% 13% 21% 16%	2325
	11th to 20th	Night	41% 3 5% 16% 22% 13%	2218
	21st to end	Night	40% 2 8% 14% 18% 17%	1833
March	1st to 10th	Night	45% 3 7% 15% 20% 9%	3419
	11th to 20th	Night	47% 4 9% 18% 16% 7%	3480
	21st to end	Night	40% 3 9% 17% 24% 7%	3817
April	1st to 10th	Night	33% 4 10% 20% 17% 14%	2090
	11th to 20th	Night	37% 3 11% 17% 17% 14%	1985
	21st to end	Night	35% 4 10% 18% 26% 8%	1820
May	1st to 10th	Night	46% 3 9% 22% 14% 5%	2850
	11th to 20th	Night	37% 3 11% 22% 22% 4	2824
	21st to end	Night	37% 4 14% 30% 11% 5%	3099
June	1st to 10th	Night	22% 5% 17% 31% 21% 3	1710
	11th to 20th	Night	19% 4 15% 35% 20% 7%	1710
	21st to end	Night	24% 6% 14% 28% 23% 5%	1710
July	1st to 10th	Night	21% 7% 24% 40% 7% 1	2805
	11th to 20th	Night	18% 6% 19% 39% 17% 2	2796
	21st to end	Night	33% 7% 20% 30% 9%	2999
August	1st to 10th	Night	6% 3 12% 38% 37% 4	2826
	11th to 20th	Night	17% 4 23% 39% 15% 3	2825
	21st to end	Night	17% 9% 23% 33% 15% 3	3700
September	1st to 10th	Night	30% 6% 13% 23% 18% 10%	2090
	11th to 20th	Night	31% 6% 14% 20% 20% 8%	2090
	21st to end	Night	22% 4 11% 24% 23% 15%	2090
October	1st to 10th	Night	13% 3 6% 15% 20% 43%	3512
	11th to 20th	Night	17% 2 4 16% 33% 28%	3779
	21st to end	Night	22% 3 4 21% 34% 17%	4120
November	1st to 10th	Night	33% 3 7% 19% 24% 14%	2340
	11th to 20th	Night	30% 3 7% 18% 24% 17%	2455
	21st to end	Night	38% 2 8% 17% 21% 14%	2600
December	1st to 10th	Night	40% 2 5% 20% 22% 9%	4250
	11th to 20th	Night	44% 3 5% 10% 25% 13%	4817
	21st to end	Night	36% 2 3 16% 28% 14%	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	20% 13% 37% 22% 7% 1	300
	11th to 20th	Day	21% 14% 33% 24% 7% 2	300
	21st to end	Day	21% 12% 35% 23% 8%	300
February	1st to 10th	Day	26% 15% 31% 20% 8%	270
	11th to 20th	Day	28% 16% 35% 18% 4	270
	21st to end	Day	27% 19% 38% 15% 2	225
March	1st to 10th	Day	23% 16% 39% 17% 5%	270
	11th to 20th	Day	20% 19% 36% 19% 6% 1	270
	21st to end	Day	18% 12% 46% 17% 6% 1	297
April	1st to 10th	Day	24% 14% 38% 19% 4	270
	11th to 20th	Day	22% 14% 30% 23% 9% 1	339
	21st to end	Day	17% 13% 36% 25% 8% 1	535
May	1st to 10th	Day	13% 13% 36% 28% 8% 1	653
	11th to 20th	Day	15% 5% 26% 34% 17% 3	675
	21st to end	Day	14% 5% 25% 34% 18% 4	734
June	1st to 10th	Day	5% 2 17% 41% 26% 9%	680
	11th to 20th	Day	6% 1 17% 42% 28% 6%	680
	21st to end	Day	5% 2 12% 32% 35% 13%	680
July	1st to 10th	Day	7% 2 9% 34% 36% 14%	671
	11th to 20th	Day	2 1 5% 33% 45% 14%	668
	21st to end	Day	4 1 4 28% 48% 14%	739
August	1st to 10th	Day	3 1 3 29% 46% 18%	676
	11th to 20th	Day	7% 1 4 28% 46% 15%	665
	21st to end	Day	7% 1 5% 19% 39% 28%	401
September	1st to 10th	Day	9% 5% 24% 34% 27%	300
	11th to 20th	Day	5% 1 7% 24% 42% 21%	300
	21st to end	Day	8% 3 6% 29% 35% 19%	300
October	1st to 10th	Day	4 2 12% 35% 35% 12%	300
	11th to 20th	Day	10% 6% 20% 33% 24% 7%	300
	21st to end	Day	21% 10% 23% 23% 18% 5%	330
November	1st to 10th	Day	20% 6% 28% 26% 15% 5%	300
	11th to 20th	Day	15% 9% 29% 27% 18% 2	300
	21st to end	Day	22% 14% 28% 23% 11% 2	300
December	1st to 10th	Day	18% 9% 29% 25% 13% 6%	300
	11th to 20th	Day	25% 12% 30% 25% 7% 1	300
	21st to end	Day	21% 8% 40% 21% 9% 1	330
January	1st to 10th	Night	19% 10% 35% 29% 7% 1	1000
	11th to 20th	Night	21% 12% 31% 30% 7%	1000
	21st to end	Night	20% 11% 31% 32% 7%	1051
February	1st to 10th	Night	24% 10% 34% 27% 5%	805
	11th to 20th	Night	24% 11% 37% 24% 4	744
	21st to end	Night	24% 14% 41% 19% 2	577
March	1st to 10th	Night	22% 9% 40% 24% 5%	570
	11th to 20th	Night	18% 10% 38% 28% 6%	570
	21st to end	Night	14% 12% 44% 27% 4	627
April	1st to 10th	Night	12% 12% 43% 28% 5%	570
	11th to 20th	Night	15% 12% 39% 26% 8%	563
	21st to end	Night	15% 8% 42% 26% 9% 1	570
May	1st to 10th	Night	10% 10% 43% 29% 7% 1	570
	11th to 20th	Night	11% 4 36% 34% 13% 1	561
	21st to end	Night	9% 4 31% 41% 15% 1	617
June	1st to 10th	Night	4 3 19% 45% 27% 2	570
	11th to 20th	Night	4 1 22% 56% 15% 2	570
	21st to end	Night	5% 1 14% 44% 32% 4	570
July	1st to 10th	Night	4 1 10% 43% 37% 4	555
	11th to 20th	Night	3 8% 51% 36% 3	552
	21st to end	Night	2 2 8% 47% 37% 4	624
August	1st to 10th	Night	2 5% 48% 40% 5%	567
	11th to 20th	Night	9% 4 46% 36% 5%	564
	21st to end	Night	4 1 6% 39% 42% 8%	627
September	1st to 10th	Night	8% 2 7% 40% 38% 5%	570
	11th to 20th	Night	11% 1 8% 36% 39% 4	570
	21st to end	Night	8% 1 11% 38% 37% 6%	570
October	1st to 10th	Night	6% 1 14% 48% 26% 5%	589
	11th to 20th	Night	13% 4 22% 39% 20% 3	760
	21st to end	Night	14% 8% 28% 32% 16% 1	836
November	1st to 10th	Night	13% 5% 27% 36% 18% 1	780
	11th to 20th	Night	11% 7% 30% 39% 13% 1	855
	21st to end	Night	20% 11% 27% 29% 13% 1	1000
December	1st to 10th	Night	18% 8% 32% 33% 8% 1	1000
	11th to 20th	Night	22% 8% 31% 32% 6%	1000
	21st to end	Night	21% 8% 32% 31% 8%	1100

Page generated at 2025-01-19 02:06:46. Version ac07

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