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 2026-08-30.

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 8% 3 6% 4 4 4 3 5% 6% 54%	152122
	Night	4 8% 3 7% 4 4 4 3 5% 6% 52%	147503
Transparency	Day	29% 5% 7% 17% 24% 18%	146984
	Night	31% 5% 8% 18% 23% 15%	143635
Seeing	Day	11% 17% 29% 25% 14% 3	17104
	Night	10% 9% 24% 32% 21% 4	26556

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% 8% 13% 4 2 3 3 4 6% 33%	4722
	11th to 20th	Day	4 11% 3 9% 4 4 3 2 4 6% 51%	4740
	21st to end	Day	3 11% 4 8% 4 4 5% 4 5% 7% 44%	5501
February	1st to 10th	Day	7% 12% 5% 10% 7% 5% 4 3 4 5% 38%	2021
	11th to 20th	Day	5% 14% 6% 10% 6% 4 4 3 5% 7% 36%	2050
	21st to end	Day	4 14% 4 10% 5% 4 4 3 5% 7% 41%	1730
March	1st to 10th	Day	3 20% 6% 13% 4 4 4 3 5% 7% 32%	4618
	11th to 20th	Day	3 11% 4 11% 4 3 4 3 5% 6% 45%	5009
	21st to end	Day	5% 20% 8% 11% 5% 4 4 3 5% 5% 31%	5713
April	1st to 10th	Day	6% 13% 3 9% 5% 5% 4 4 4 7% 40%	2520
	11th to 20th	Day	6% 11% 3 9% 4 4 4 3 5% 7% 44%	2506
	21st to end	Day	8% 11% 3 6% 3 3 3 3 5% 7% 47%	2730
May	1st to 10th	Day	3 7% 3 7% 4 4 5% 4 5% 5% 53%	7626
	11th to 20th	Day	4 9% 2 5% 3 3 3 3 4 6% 57%	7596
	21st to end	Day	4 5% 2 5% 4 5% 5% 4 5% 6% 55%	8517
June	1st to 10th	Day	5% 5% 2 5% 4 5% 4 4 5% 7% 54%	2940
	11th to 20th	Day	4 4 1 4 3 3 4 3 5% 6% 64%	2940
	21st to end	Day	2 3 1 3 3 3 3 2 4 5% 71%	2940
July	1st to 10th	Day	2 2 1 4 3 3 3 3 4 4 70%	8219
	11th to 20th	Day	2 3 1 5% 4 4 5% 4 6% 7% 59%	8220
	21st to end	Day	2 3 1 4 3 4 4 4 5% 6% 64%	8624
August	1st to 10th	Day	2 2 1 3 3 2 3 3 4 6% 72%	5909
	11th to 20th	Day	1 2 1 3 3 3 3 3 4 5% 73%	5838
	21st to end	Day	3 4 1 5% 3 3 3 3 5% 5% 64%	6093
September	1st to 10th	Day	2 2 1 4 3 3 3 3 4 5% 68%	2363
	11th to 20th	Day	5% 4 1 4 4 3 4 3 5% 5% 65%	2200
	21st to end	Day	5% 4 1 4 3 2 2 3 3 5% 69%	2200
October	1st to 10th	Day	4 5% 1 4 3 3 3 3 3 5% 66%	3600
	11th to 20th	Day	4 6% 1 3 2 2 3 2 3 5% 67%	3316
	21st to end	Day	3 5% 4 5% 4 3 3 3 5% 8% 58%	3200
November	1st to 10th	Day	6% 9% 3 6% 5% 4 3 3 6% 7% 48%	1854
	11th to 20th	Day	8% 9% 3 5% 4 4 4 4 5% 7% 46%	1890
	21st to end	Day	6% 12% 5% 7% 4 4 3 2 5% 6% 48%	1890
December	1st to 10th	Day	4 6% 2 4 3 2 3 2 4 6% 64%	3040
	11th to 20th	Day	6% 14% 4 8% 4 4 5% 4 8% 8% 34%	3155
	21st to end	Day	10% 15% 4 10% 5% 4 5% 3 5% 6% 34%	4092
January	1st to 10th	Night	5% 15% 7% 13% 5% 3 4 4 6% 7% 30%	8172
	11th to 20th	Night	6% 10% 3 8% 5% 5% 3 2 4 4 51%	8220
	21st to end	Night	4 9% 4 8% 4 3 3 3 5% 7% 49%	8693
February	1st to 10th	Night	6% 12% 5% 9% 5% 4 4 3 5% 6% 41%	2597
	11th to 20th	Night	5% 11% 6% 10% 5% 3 4 3 5% 6% 42%	2458
	21st to end	Night	4 12% 4 9% 4 4 5% 5% 7% 7% 40%	2076
March	1st to 10th	Night	3 16% 7% 11% 4 4 4 4 4 5% 39%	5496
	11th to 20th	Night	3 10% 5% 10% 4 4 5% 4 6% 7% 43%	5210
	21st to end	Night	4 13% 5% 9% 4 4 4 4 6% 7% 40%	5487
April	1st to 10th	Night	7% 11% 3 7% 4 4 5% 3 6% 6% 44%	2100
	11th to 20th	Night	7% 9% 4 6% 3 3 4 3 6% 6% 49%	2060
	21st to end	Night	9% 11% 3 5% 2 3 2 3 5% 6% 51%	1890
May	1st to 10th	Night	5% 8% 3 5% 3 3 3 2 4 5% 58%	5321
	11th to 20th	Night	4 6% 2 5% 3 3 3 2 4 6% 61%	5296
	21st to end	Night	4 5% 2 7% 4 4 5% 3 5% 6% 54%	5664
June	1st to 10th	Night	4 4 1 4 4 3 4 4 6% 9% 56%	1680
	11th to 20th	Night	5% 3 1 4 3 2 3 2 5% 5% 65%	1680
	21st to end	Night	2 3 1 4 3 3 3 2 3 6% 70%	1680
July	1st to 10th	Night	2 3 2 5% 4 3 4 3 4 4 67%	4740
	11th to 20th	Night	3 4 2 6% 5% 5% 5% 5% 6% 7% 52%	4740
	21st to end	Night	3 4 2 6% 5% 5% 5% 4 6% 7% 53%	5450
August	1st to 10th	Night	3 3 2 4 3 3 3 2 4 6% 68%	4123
	11th to 20th	Night	3 4 2 5% 3 3 3 3 3 5% 67%	4231
	21st to end	Night	4 4 2 5% 3 4 4 3 5% 5% 62%	5105
September	1st to 10th	Night	4 2 1 3 3 3 4 3 5% 6% 67%	2035
	11th to 20th	Night	4 4 1 4 3 4 3 3 5% 5% 64%	2200
	21st to end	Night	5% 3 2 3 2 2 2 2 3 4 73%	2200
October	1st to 10th	Night	4 3 1 3 2 3 3 2 4 4 72%	3600
	11th to 20th	Night	5% 6% 2 6% 3 3 3 2 3 5% 63%	3832
	21st to end	Night	3 5% 2 5% 3 3 4 3 5% 6% 62%	4600
November	1st to 10th	Night	6% 8% 2 6% 4 4 4 3 6% 6% 50%	2665
	11th to 20th	Night	6% 8% 2 6% 4 4 4 3 5% 6% 53%	2730
	21st to end	Night	4 9% 3 6% 5% 4 4 3 5% 8% 48%	2730
December	1st to 10th	Night	4 6% 2 5% 3 2 3 3 4 6% 61%	4370
	11th to 20th	Night	7% 12% 4 10% 5% 4 4 4 5% 8% 36%	5266
	21st to end	Night	8% 13% 5% 11% 6% 5% 5% 5% 6% 7% 28%	7106

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% 6% 16% 19% 12%	4723
	11th to 20th	Day	30% 8% 8% 17% 29% 8%	4740
	21st to end	Day	38% 9% 9% 17% 19% 8%	5501
February	1st to 10th	Day	45% 7% 12% 20% 12% 4	2021
	11th to 20th	Day	44% 7% 12% 20% 13% 5%	2060
	21st to end	Day	39% 7% 14% 23% 13% 4	1730
March	1st to 10th	Day	44% 10% 9% 16% 17% 5%	4618
	11th to 20th	Day	36% 6% 12% 28% 16% 2	5009
	21st to end	Day	47% 10% 14% 17% 10% 2	5713
April	1st to 10th	Day	40% 8% 11% 21% 16% 4	2520
	11th to 20th	Day	37% 5% 11% 22% 20% 4	2509
	21st to end	Day	35% 4 10% 28% 20% 2	2730
May	1st to 10th	Day	28% 5% 8% 27% 24% 7%	7626
	11th to 20th	Day	27% 3 5% 19% 35% 10%	7599
	21st to end	Day	27% 6% 5% 20% 37% 6%	8518
June	1st to 10th	Day	26% 7% 9% 17% 33% 9%	2940
	11th to 20th	Day	21% 4 5% 15% 32% 24%	2940
	21st to end	Day	15% 4 4 12% 32% 33%	2940
July	1st to 10th	Day	18% 3 1 5% 16% 56%	8151
	11th to 20th	Day	24% 4 3 8% 18% 42%	8145
	21st to end	Day	21% 3 4 9% 23% 41%	8432
August	1st to 10th	Day	18% 4 3 9% 27% 39%	4338
	11th to 20th	Day	15% 3 3 7% 24% 47%	4261
	21st to end	Day	17% 2 2 9% 30% 40%	4420
September	1st to 10th	Day	19% 4 4 10% 28% 35%	2363
	11th to 20th	Day	22% 4 5% 14% 30% 26%	2200
	21st to end	Day	19% 3 5% 15% 32% 26%	2200
October	1st to 10th	Day	23% 3 5% 12% 33% 25%	3600
	11th to 20th	Day	21% 3 3 17% 37% 18%	3316
	21st to end	Day	24% 2 7% 29% 28% 9%	3200
November	1st to 10th	Day	31% 8% 9% 22% 21% 8%	1854
	11th to 20th	Day	34% 5% 11% 28% 19% 3	1890
	21st to end	Day	37% 4 10% 25% 18% 5%	1890
December	1st to 10th	Day	23% 4 10% 23% 31% 8%	3040
	11th to 20th	Day	43% 8% 13% 18% 16% 3	3155
	21st to end	Day	47% 8% 10% 20% 14% 2	4092
January	1st to 10th	Night	47% 7% 13% 19% 13% 1	8172
	11th to 20th	Night	30% 8% 8% 16% 25% 13%	8220
	21st to end	Night	33% 8% 9% 16% 26% 8%	8693
February	1st to 10th	Night	39% 7% 14% 24% 11% 5%	2597
	11th to 20th	Night	39% 7% 12% 20% 17% 5%	2459
	21st to end	Night	37% 8% 12% 22% 15% 7%	2076
March	1st to 10th	Night	41% 8% 8% 19% 16% 8%	5496
	11th to 20th	Night	37% 7% 12% 24% 18% 2	5210
	21st to end	Night	37% 8% 16% 24% 13% 2	5487
April	1st to 10th	Night	37% 7% 12% 22% 18% 5%	2100
	11th to 20th	Night	32% 5% 10% 26% 20% 7%	2060
	21st to end	Night	32% 5% 8% 25% 26% 2	1890
May	1st to 10th	Night	26% 5% 6% 26% 31% 5%	5319
	11th to 20th	Night	23% 3 4 17% 40% 13%	5297
	21st to end	Night	29% 5% 5% 19% 36% 6%	5665
June	1st to 10th	Night	21% 6% 11% 21% 31% 9%	1680
	11th to 20th	Night	20% 4 6% 13% 33% 24%	1680
	21st to end	Night	16% 3 4 13% 32% 32%	1680
July	1st to 10th	Night	22% 4 2 4 13% 55%	4700
	11th to 20th	Night	31% 4 4 8% 17% 35%	4692
	21st to end	Night	29% 4 4 8% 17% 38%	5330
August	1st to 10th	Night	24% 4 3 12% 18% 38%	3018
	11th to 20th	Night	20% 3 4 6% 17% 49%	3082
	21st to end	Night	18% 2 3 8% 24% 45%	3698
September	1st to 10th	Night	19% 3 5% 11% 27% 35%	2035
	11th to 20th	Night	22% 4 4 14% 27% 29%	2200
	21st to end	Night	17% 3 4 14% 31% 31%	2200
October	1st to 10th	Night	18% 3 4 11% 39% 24%	3600
	11th to 20th	Night	24% 3 4 15% 33% 21%	3832
	21st to end	Night	23% 3 4 28% 34% 8%	4600
November	1st to 10th	Night	29% 6% 11% 23% 22% 9%	2665
	11th to 20th	Night	30% 4 12% 25% 25% 5%	2730
	21st to end	Night	33% 5% 10% 26% 19% 6%	2730
December	1st to 10th	Night	25% 4 8% 21% 33% 8%	4370
	11th to 20th	Night	43% 6% 13% 18% 15% 5%	5266
	21st to end	Night	50% 7% 11% 18% 13% 2	7106

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% 14% 10% 2	220
	11th to 20th	Day	13% 23% 25% 19% 14% 5%	220
	21st to end	Day	12% 15% 33% 24% 13% 3	299
February	1st to 10th	Day	17% 25% 28% 18% 11% 2	461
	11th to 20th	Day	17% 23% 33% 19% 7% 1	535
	21st to end	Day	12% 26% 35% 21% 4 1	446
March	1st to 10th	Day	16% 21% 32% 24% 6% 1	540
	11th to 20th	Day	12% 26% 29% 22% 10% 1	540
	21st to end	Day	17% 24% 35% 17% 6% 1	594
April	1st to 10th	Day	16% 22% 29% 20% 10% 3	540
	11th to 20th	Day	15% 26% 35% 17% 6% 2	532
	21st to end	Day	15% 22% 31% 21% 8% 2	540
May	1st to 10th	Day	8% 27% 33% 19% 10% 3	543
	11th to 20th	Day	18% 17% 29% 24% 10% 2	568
	21st to end	Day	15% 16% 23% 29% 15% 3	617
June	1st to 10th	Day	7% 17% 30% 28% 17% 2	570
	11th to 20th	Day	11% 20% 34% 22% 12% 1	570
	21st to end	Day	6% 14% 32% 28% 17% 3	570
July	1st to 10th	Day	7% 11% 35% 31% 13% 4	566
	11th to 20th	Day	6% 8% 27% 41% 16% 3	564
	21st to end	Day	5% 8% 26% 35% 20% 6%	619
August	1st to 10th	Day	6% 11% 29% 30% 20% 5%	567
	11th to 20th	Day	6% 10% 26% 30% 20% 7%	557
	21st to end	Day	7% 9% 25% 33% 19% 6%	620
September	1st to 10th	Day	5% 9% 26% 32% 21% 8%	530
	11th to 20th	Day	9% 12% 28% 25% 20% 6%	530
	21st to end	Day	7% 12% 25% 31% 19% 5%	530
October	1st to 10th	Day	11% 14% 24% 25% 20% 5%	530
	11th to 20th	Day	9% 17% 25% 24% 21% 4	425
	21st to end	Day	11% 16% 25% 29% 18%	363
November	1st to 10th	Day	14% 19% 22% 26% 16% 4	330
	11th to 20th	Day	13% 18% 25% 28% 12% 4	330
	21st to end	Day	14% 15% 26% 27% 16% 2	330
December	1st to 10th	Day	12% 19% 27% 21% 15% 6%	330
	11th to 20th	Day	14% 19% 30% 25% 9% 3	236
	21st to end	Day	24% 19% 27% 21% 8%	242
January	1st to 10th	Night	15% 14% 20% 26% 20% 5%	840
	11th to 20th	Night	11% 12% 25% 29% 19% 4	840
	21st to end	Night	11% 10% 25% 32% 15% 6%	924
February	1st to 10th	Night	13% 11% 30% 28% 15% 3	840
	11th to 20th	Night	12% 13% 27% 32% 14% 2	824
	21st to end	Night	10% 14% 32% 31% 11% 2	692
March	1st to 10th	Night	11% 11% 29% 34% 11% 4	840
	11th to 20th	Night	10% 11% 26% 32% 18% 3	840
	21st to end	Night	13% 14% 31% 30% 12% 1	924
April	1st to 10th	Night	11% 12% 30% 29% 17% 1	840
	11th to 20th	Night	11% 13% 30% 27% 15% 3	812
	21st to end	Night	13% 13% 27% 27% 19% 1	630
May	1st to 10th	Night	8% 12% 30% 31% 17% 2	630
	11th to 20th	Night	13% 10% 23% 30% 21% 3	621
	21st to end	Night	11% 10% 25% 30% 23% 2	627
June	1st to 10th	Night	6% 7% 24% 37% 22% 4	420
	11th to 20th	Night	7% 10% 26% 32% 25% 1	420
	21st to end	Night	3 8% 19% 41% 25% 3	420
July	1st to 10th	Night	4 2 22% 37% 29% 4	410
	11th to 20th	Night	5% 2 20% 41% 31% 1	408
	21st to end	Night	5% 3 17% 41% 31% 3	565
August	1st to 10th	Night	7% 4 22% 39% 27% 2	627
	11th to 20th	Night	5% 3 18% 40% 29% 5%	662
	21st to end	Night	5% 4 17% 39% 30% 5%	916
September	1st to 10th	Night	6% 5% 14% 37% 33% 7%	800
	11th to 20th	Night	6% 6% 15% 35% 30% 9%	800
	21st to end	Night	7% 5% 19% 29% 30% 9%	800
October	1st to 10th	Night	8% 7% 21% 33% 25% 7%	800
	11th to 20th	Night	9% 9% 19% 31% 27% 6%	800
	21st to end	Night	8% 9% 23% 29% 24% 7%	880
November	1st to 10th	Night	10% 9% 22% 30% 23% 5%	820
	11th to 20th	Night	10% 10% 25% 34% 18% 4	840
	21st to end	Night	8% 10% 26% 28% 20% 7%	840
December	1st to 10th	Night	9% 9% 27% 33% 16% 5%	840
	11th to 20th	Night	15% 8% 26% 32% 16% 3	840
	21st to end	Night	18% 14% 29% 27% 11% 2	924

Page generated at 2026-08-27 01:07:11. Version ac07

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