Ruby Crest Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ruby Crest Observatory clear sky chart from 2005-11-06 to 2025-04-26.

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 9% 3 7% 4 3 4 3 5% 6% 53%	117427
	Night	5% 9% 3 7% 4 4 4 3 5% 6% 50%	120014
Transparency	Day	30% 6% 8% 17% 22% 17%	117177
	Night	32% 6% 9% 18% 22% 14%	119854
Seeing	Day	11% 17% 28% 26% 14% 3	15298
	Night	9% 9% 23% 32% 22% 4	24948

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	5% 18% 10% 14% 4 3 3 3 4 7% 30%	3639
	11th to 20th	Day	5% 15% 4 11% 5% 4 3 2 4 5% 42%	3640
	21st to end	Day	4 14% 4 8% 4 3 4 3 4 7% 46%	4249
February	1st to 10th	Day	7% 12% 5% 10% 7% 5% 4 3 4 5% 37%	1925
	11th to 20th	Day	5% 15% 5% 10% 5% 4 4 3 5% 7% 37%	1950
	21st to end	Day	4 14% 4 11% 5% 4 3 3 5% 6% 42%	1650
March	1st to 10th	Day	3 20% 6% 13% 4 4 4 3 5% 7% 31%	4518
	11th to 20th	Day	3 11% 4 12% 4 4 4 3 5% 6% 44%	4902
	21st to end	Day	5% 20% 8% 11% 5% 4 4 3 5% 5% 31%	5590
April	1st to 10th	Day	6% 13% 3 8% 5% 5% 4 4 4 7% 40%	2400
	11th to 20th	Day	6% 11% 3 9% 4 4 4 3 5% 7% 45%	2385
	21st to end	Day	8% 11% 3 6% 3 3 3 3 5% 7% 48%	2537
May	1st to 10th	Day	4 11% 5% 7% 4 4 5% 4 6% 6% 45%	4110
	11th to 20th	Day	4 5% 1 4 3 3 3 3 4 5% 63%	4071
	21st to end	Day	6% 7% 2 5% 4 4 4 3 5% 5% 56%	4559
June	1st to 10th	Day	5% 5% 2 5% 4 5% 5% 5% 5% 7% 52%	2660
	11th to 20th	Day	5% 4 1 4 3 3 4 3 5% 6% 62%	2660
	21st to end	Day	2 3 1 3 3 3 3 2 4 5% 71%	2660
July	1st to 10th	Day	2 2 1 3 2 2 3 2 3 4 76%	4420
	11th to 20th	Day	3 3 2 4 4 4 5% 4 6% 7% 60%	4420
	21st to end	Day	3 3 1 4 4 5% 4 4 6% 7% 59%	4622
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 6% 65%	4092
	11th to 20th	Day	2 2 1 3 3 3 2 2 4 5% 74%	3966
	21st to end	Day	3 4 1 3 2 2 2 1 3 4 77%	4180
September	1st to 10th	Day	2 2 1 3 2 3 3 3 4 4 71%	2245
	11th to 20th	Day	5% 4 1 4 4 3 4 2 5% 5% 65%	2090
	21st to end	Day	5% 4 1 4 2 2 2 3 3 4 69%	2090
October	1st to 10th	Day	4 5% 1 4 3 3 3 3 3 5% 66%	3490
	11th to 20th	Day	4 6% 1 3 2 2 3 2 3 5% 67%	3214
	21st to end	Day	3 5% 4 5% 4 3 3 3 5% 8% 57%	3101
November	1st to 10th	Day	6% 10% 3 5% 5% 4 3 4 6% 7% 48%	1764
	11th to 20th	Day	9% 9% 3 5% 3 3 3 4 5% 7% 47%	1800
	21st to end	Day	6% 12% 5% 7% 4 4 3 2 5% 6% 47%	1800
December	1st to 10th	Day	4 6% 2 4 3 2 3 2 4 6% 64%	2950
	11th to 20th	Day	6% 15% 4 8% 4 4 5% 4 8% 8% 34%	3074
	21st to end	Day	10% 15% 4 10% 5% 4 5% 3 5% 6% 34%	4004
January	1st to 10th	Night	5% 15% 7% 12% 5% 3 4 4 6% 8% 32%	6306
	11th to 20th	Night	7% 13% 4 11% 6% 6% 3 3 4 4 39%	6320
	21st to end	Night	5% 11% 5% 9% 4 3 3 3 4 6% 48%	6715
February	1st to 10th	Night	6% 12% 5% 9% 5% 4 4 3 5% 6% 41%	2474
	11th to 20th	Night	5% 11% 5% 9% 5% 3 4 3 5% 6% 43%	2338
	21st to end	Night	4 13% 4 10% 4 4 5% 4 7% 6% 40%	1980
March	1st to 10th	Night	3 17% 7% 11% 4 4 4 4 4 5% 39%	5376
	11th to 20th	Night	3 10% 6% 10% 4 4 5% 4 6% 7% 42%	5098
	21st to end	Night	5% 13% 5% 9% 4 4 4 4 6% 7% 39%	5368
April	1st to 10th	Night	7% 11% 3 6% 4 4 5% 3 6% 6% 44%	2000
	11th to 20th	Night	7% 10% 4 6% 3 3 4 3 6% 5% 48%	1961
	21st to end	Night	8% 11% 3 5% 3 3 3 3 5% 6% 51%	1759
May	1st to 10th	Night	8% 12% 4 7% 3 3 3 2 4 5% 48%	2860
	11th to 20th	Night	4 5% 1 4 3 3 3 2 4 5% 65%	2834
	21st to end	Night	7% 6% 3 6% 4 4 4 3 5% 7% 52%	3021
June	1st to 10th	Night	4 4 1 4 4 3 4 4 6% 9% 56%	1520
	11th to 20th	Night	6% 4 2 4 3 2 3 3 4 5% 64%	1520
	21st to end	Night	2 3 1 4 3 3 3 3 3 6% 69%	1520
July	1st to 10th	Night	2 3 2 3 3 3 3 3 4 3 72%	2540
	11th to 20th	Night	4 5% 2 6% 5% 5% 6% 5% 6% 7% 49%	2540
	21st to end	Night	4 4 2 6% 5% 5% 6% 4 6% 6% 52%	2918
August	1st to 10th	Night	5% 4 2 5% 4 3 4 3 5% 6% 59%	2847
	11th to 20th	Night	2 3 1 4 3 3 3 3 3 5% 69%	2871
	21st to end	Night	3 3 1 3 2 2 3 2 3 4 75%	3498
September	1st to 10th	Night	4 2 1 3 3 3 4 2 4 6% 68%	1933
	11th to 20th	Night	4 3 1 4 3 4 3 3 5% 5% 65%	2090
	21st to end	Night	5% 3 1 3 2 1 2 1 2 4 74%	2090
October	1st to 10th	Night	4 3 1 3 2 3 3 2 4 4 72%	3490
	11th to 20th	Night	5% 5% 2 6% 4 3 3 2 4 5% 63%	3715
	21st to end	Night	3 5% 2 5% 3 3 4 3 5% 6% 62%	4457
November	1st to 10th	Night	7% 8% 2 6% 4 4 4 3 6% 6% 50%	2535
	11th to 20th	Night	6% 8% 2 6% 4 3 4 3 5% 6% 54%	2600
	21st to end	Night	5% 10% 3 7% 5% 4 4 3 5% 7% 47%	2600
December	1st to 10th	Night	4 6% 2 5% 3 2 3 3 4 6% 61%	4240
	11th to 20th	Night	7% 13% 4 10% 5% 4 4 4 5% 8% 36%	5128
	21st to end	Night	8% 13% 5% 11% 6% 5% 5% 5% 6% 7% 28%	6952

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	47% 8% 16% 14% 15%	3639
	11th to 20th	Day	37% 11% 10% 19% 18% 5%	3640
	21st to end	Day	37% 11% 9% 16% 20% 6%	4249
February	1st to 10th	Day	45% 7% 12% 19% 11% 4	1925
	11th to 20th	Day	43% 7% 12% 20% 13% 5%	1960
	21st to end	Day	39% 7% 14% 22% 13% 5%	1650
March	1st to 10th	Day	44% 10% 9% 16% 16% 5%	4518
	11th to 20th	Day	36% 6% 12% 28% 16% 2	4902
	21st to end	Day	48% 10% 14% 17% 10% 2	5590
April	1st to 10th	Day	41% 8% 11% 21% 15% 4	2400
	11th to 20th	Day	36% 5% 11% 22% 20% 4	2388
	21st to end	Day	34% 4 11% 29% 20% 3	2537
May	1st to 10th	Day	29% 9% 11% 26% 18% 7%	4110
	11th to 20th	Day	21% 4 9% 18% 39% 9%	4074
	21st to end	Day	26% 6% 8% 21% 33% 5%	4559
June	1st to 10th	Day	26% 7% 10% 17% 31% 8%	2660
	11th to 20th	Day	22% 4 6% 16% 32% 20%	2660
	21st to end	Day	14% 4 5% 12% 30% 35%	2660
July	1st to 10th	Day	14% 3 3 6% 12% 64%	4351
	11th to 20th	Day	22% 5% 3 7% 18% 44%	4345
	21st to end	Day	24% 4 4 11% 22% 35%	4476
August	1st to 10th	Day	19% 4 3 9% 26% 39%	4078
	11th to 20th	Day	15% 3 3 6% 24% 49%	4005
	21st to end	Day	15% 2 2 7% 31% 43%	4179
September	1st to 10th	Day	17% 4 4 10% 29% 37%	2245
	11th to 20th	Day	21% 4 5% 15% 29% 26%	2090
	21st to end	Day	19% 3 5% 14% 33% 26%	2090
October	1st to 10th	Day	22% 3 5% 12% 33% 25%	3490
	11th to 20th	Day	21% 3 4 17% 38% 18%	3214
	21st to end	Day	24% 3 7% 29% 28% 9%	3101
November	1st to 10th	Day	32% 9% 9% 21% 21% 8%	1764
	11th to 20th	Day	33% 5% 11% 28% 20% 3	1800
	21st to end	Day	38% 4 10% 24% 18% 6%	1800
December	1st to 10th	Day	23% 5% 9% 23% 32% 8%	2950
	11th to 20th	Day	43% 8% 13% 18% 16% 2	3074
	21st to end	Day	47% 8% 10% 20% 14% 2	4004
January	1st to 10th	Night	42% 9% 14% 16% 17% 1	6306
	11th to 20th	Night	38% 11% 10% 18% 17% 6%	6320
	21st to end	Night	36% 10% 8% 16% 24% 6%	6715
February	1st to 10th	Night	39% 7% 15% 24% 11% 5%	2474
	11th to 20th	Night	38% 7% 12% 20% 17% 5%	2339
	21st to end	Night	37% 8% 11% 21% 15% 7%	1980
March	1st to 10th	Night	41% 8% 8% 18% 16% 8%	5376
	11th to 20th	Night	38% 7% 12% 24% 18% 1	5098
	21st to end	Night	38% 8% 16% 24% 12% 2	5368
April	1st to 10th	Night	38% 7% 13% 21% 16% 5%	2000
	11th to 20th	Night	33% 6% 10% 26% 19% 6%	1961
	21st to end	Night	32% 6% 9% 26% 24% 3	1759
May	1st to 10th	Night	30% 8% 8% 25% 23% 5%	2860
	11th to 20th	Night	19% 5% 7% 20% 40% 10%	2835
	21st to end	Night	26% 8% 9% 23% 27% 7%	3021
June	1st to 10th	Night	21% 7% 12% 23% 29% 9%	1520
	11th to 20th	Night	21% 4 7% 14% 33% 21%	1520
	21st to end	Night	16% 3 5% 13% 29% 34%	1520
July	1st to 10th	Night	17% 4 4 5% 11% 59%	2500
	11th to 20th	Night	31% 5% 3 8% 18% 34%	2492
	21st to end	Night	32% 4 4 10% 17% 34%	2826
August	1st to 10th	Night	25% 4 3 13% 18% 37%	2838
	11th to 20th	Night	19% 4 4 6% 16% 52%	2898
	21st to end	Night	16% 2 3 7% 25% 48%	3498
September	1st to 10th	Night	17% 3 5% 11% 26% 37%	1933
	11th to 20th	Night	21% 4 5% 14% 26% 29%	2090
	21st to end	Night	16% 3 4 14% 32% 31%	2090
October	1st to 10th	Night	18% 3 4 11% 39% 25%	3490
	11th to 20th	Night	24% 3 4 15% 33% 21%	3715
	21st to end	Night	23% 3 4 28% 33% 8%	4457
November	1st to 10th	Night	29% 6% 11% 23% 22% 9%	2535
	11th to 20th	Night	29% 4 11% 25% 26% 5%	2600
	21st to end	Night	34% 5% 10% 25% 19% 7%	2600
December	1st to 10th	Night	25% 5% 8% 20% 34% 9%	4240
	11th to 20th	Night	44% 6% 13% 18% 15% 5%	5128
	21st to end	Night	50% 7% 11% 18% 13% 2	6952

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% 23% 30% 15% 11% 2	200
	11th to 20th	Day	15% 25% 23% 19% 15% 5%	200
	21st to end	Day	11% 15% 33% 23% 14% 4	272
February	1st to 10th	Day	17% 26% 26% 18% 11% 1	425
	11th to 20th	Day	16% 23% 34% 19% 7% 1	495
	21st to end	Day	11% 26% 35% 22% 4 1	414
March	1st to 10th	Day	15% 21% 33% 24% 6% 1	500
	11th to 20th	Day	11% 26% 29% 23% 10% 1	500
	21st to end	Day	17% 24% 35% 18% 5% 1	550
April	1st to 10th	Day	16% 23% 29% 22% 9% 2	500
	11th to 20th	Day	14% 26% 34% 18% 7% 2	492
	21st to end	Day	13% 22% 31% 22% 9% 2	481
May	1st to 10th	Day	7% 29% 32% 20% 10% 3	463
	11th to 20th	Day	14% 17% 28% 26% 12% 2	488
	21st to end	Day	14% 16% 22% 28% 16% 3	529
June	1st to 10th	Day	6% 16% 28% 29% 19% 2	490
	11th to 20th	Day	11% 19% 33% 22% 13% 2	490
	21st to end	Day	3 14% 31% 31% 18% 3	490
July	1st to 10th	Day	6% 9% 34% 33% 14% 4	486
	11th to 20th	Day	5% 7% 27% 42% 16% 3	484
	21st to end	Day	5% 7% 24% 36% 22% 6%	531
August	1st to 10th	Day	7% 9% 28% 32% 21% 5%	487
	11th to 20th	Day	6% 10% 24% 31% 22% 7%	477
	21st to end	Day	6% 10% 24% 33% 20% 7%	539
September	1st to 10th	Day	5% 9% 25% 31% 21% 9%	490
	11th to 20th	Day	10% 12% 27% 24% 21% 6%	490
	21st to end	Day	7% 12% 25% 31% 20% 6%	490
October	1st to 10th	Day	9% 14% 24% 26% 21% 6%	490
	11th to 20th	Day	8% 16% 24% 25% 23% 4	390
	21st to end	Day	10% 17% 26% 28% 19%	330
November	1st to 10th	Day	13% 20% 21% 26% 17% 4	300
	11th to 20th	Day	12% 18% 24% 29% 12% 5%	300
	21st to end	Day	13% 16% 24% 28% 17% 2	300
December	1st to 10th	Day	11% 20% 25% 22% 16% 6%	300
	11th to 20th	Day	12% 20% 31% 25% 9% 3	215
	21st to end	Day	22% 20% 29% 22% 8%	220
January	1st to 10th	Night	14% 14% 20% 27% 21% 5%	800
	11th to 20th	Night	12% 12% 25% 27% 19% 5%	800
	21st to end	Night	11% 11% 24% 33% 16% 6%	880
February	1st to 10th	Night	13% 12% 28% 28% 16% 4	800
	11th to 20th	Night	11% 13% 27% 32% 14% 2	784
	21st to end	Night	10% 14% 30% 32% 12% 3	660
March	1st to 10th	Night	11% 11% 27% 35% 12% 4	800
	11th to 20th	Night	10% 12% 24% 33% 19% 3	800
	21st to end	Night	13% 14% 29% 31% 12% 1	880
April	1st to 10th	Night	11% 12% 29% 29% 18% 1	800
	11th to 20th	Night	11% 13% 30% 27% 16% 3	773
	21st to end	Night	12% 14% 24% 27% 20% 2	587
May	1st to 10th	Night	8% 12% 28% 31% 19% 2	570
	11th to 20th	Night	11% 11% 22% 30% 23% 3	561
	21st to end	Night	12% 9% 21% 30% 26% 2	567
June	1st to 10th	Night	5% 7% 22% 37% 24% 5%	380
	11th to 20th	Night	7% 10% 23% 32% 27% 1	380
	21st to end	Night	3 7% 16% 42% 28% 3	380
July	1st to 10th	Night	5% 2 18% 38% 32% 5%	370
	11th to 20th	Night	5% 1 16% 41% 34% 2	368
	21st to end	Night	4 3 15% 41% 34% 3	511
August	1st to 10th	Night	7% 4 19% 39% 29% 2	567
	11th to 20th	Night	4 3 16% 41% 31% 5%	598
	21st to end	Night	4 4 15% 39% 33% 5%	836
September	1st to 10th	Night	5% 4 13% 36% 34% 7%	760
	11th to 20th	Night	5% 6% 13% 35% 31% 9%	760
	21st to end	Night	7% 5% 18% 29% 32% 10%	760
October	1st to 10th	Night	7% 6% 21% 34% 26% 7%	760
	11th to 20th	Night	7% 8% 18% 32% 28% 7%	760
	21st to end	Night	8% 9% 22% 28% 26% 7%	836
November	1st to 10th	Night	10% 9% 21% 30% 24% 5%	780
	11th to 20th	Night	9% 9% 25% 34% 19% 5%	800
	21st to end	Night	8% 10% 25% 29% 21% 8%	800
December	1st to 10th	Night	9% 10% 26% 34% 17% 5%	800
	11th to 20th	Night	15% 8% 26% 32% 16% 3	800
	21st to end	Night	16% 14% 29% 28% 11% 2	880

Page generated at 2025-04-23 06:25:05. Version ac07

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