Rubicon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rubicon Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	9% 4 2 4 3 3 3 3 4 6% 61%	136259
	Night	11% 6% 2 5% 3 3 3 3 4 5% 54%	133212
Transparency	Day	25% 4 8% 20% 24% 19%	131119
	Night	31% 5% 8% 18% 21% 17%	129344
Seeing	Day	14% 10% 23% 27% 18% 8%	16849
	Night	14% 5% 13% 32% 28% 7%	26142

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	18% 8% 2 7% 4 4 5% 4 6% 7% 36%	3954
	11th to 20th	Day	20% 6% 3 5% 3 3 2 2 3 3 51%	4190
	21st to end	Day	20% 6% 3 4 3 3 3 2 4 6% 46%	4609
February	1st to 10th	Day	20% 7% 3 6% 4 3 3 3 4 5% 41%	1801
	11th to 20th	Day	16% 8% 3 6% 5% 5% 3 3 4 6% 41%	1755
	21st to end	Day	17% 8% 2 7% 4 3 3 3 5% 8% 41%	1655
March	1st to 10th	Day	23% 13% 3 6% 4 3 3 3 3 5% 34%	4952
	11th to 20th	Day	17% 8% 2 6% 4 4 4 3 4 6% 43%	5000
	21st to end	Day	18% 9% 3 7% 5% 5% 5% 4 6% 8% 29%	5486
April	1st to 10th	Day	14% 8% 3 7% 5% 3 4 4 6% 9% 38%	2390
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 5% 6% 49%	2369
	21st to end	Day	8% 5% 2 5% 3 4 4 4 6% 9% 51%	2428
May	1st to 10th	Day	4 3 1 3 2 2 2 2 4 7% 70%	5880
	11th to 20th	Day	3 2 1 2 2 2 3 3 3 5% 74%	5961
	21st to end	Day	3 2 1 3 2 2 2 2 4 6% 73%	6902
June	1st to 10th	Day	6% 4 1 5% 3 3 4 3 6% 7% 59%	2800
	11th to 20th	Day	2 2 1 3 2 2 2 3 4 6% 74%	2800
	21st to end	Day	3 1 1 2 2 2 2 2 3 4 78%	2800
July	1st to 10th	Day	1 2 1 1 1 1 2 2 88%	6319
	11th to 20th	Day	1 1 2 2 2 1 1 2 3 85%	5927
	21st to end	Day	1 1 2 1 2 2 1 3 4 82%	6402
August	1st to 10th	Day	1 1 1 2 1 1 2 2 4 5% 79%	5777
	11th to 20th	Day	1 1 1 2 2 2 2 2 2 3 83%	5736
	21st to end	Day	1 1 1 2 2 2 2 2 3 5% 80%	5984
September	1st to 10th	Day	1 2 2 1 2 2 2 3 5% 81%	2400
	11th to 20th	Day	3 3 2 4 3 3 3 2 4 6% 68%	2350
	21st to end	Day	3 2 1 3 2 2 2 2 3 5% 75%	2130
October	1st to 10th	Day	4 2 1 2 2 2 2 2 3 5% 73%	3280
	11th to 20th	Day	7% 3 1 3 2 2 3 2 5% 6% 64%	3261
	21st to end	Day	7% 3 2 5% 4 4 5% 4 5% 7% 53%	3558
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 5% 8% 50%	2043
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 3 5% 6% 42%	1780
	21st to end	Day	13% 4 2 5% 3 3 5% 3 4 5% 53%	1680
December	1st to 10th	Day	9% 5% 2 5% 2 4 4 4 6% 8% 52%	2700
	11th to 20th	Day	26% 9% 3 7% 5% 4 4 3 6% 7% 25%	3108
	21st to end	Day	24% 10% 3 8% 5% 4 5% 3 6% 7% 25%	4092
January	1st to 10th	Night	17% 9% 2 7% 6% 5% 5% 4 6% 8% 33%	6204
	11th to 20th	Night	17% 8% 2 6% 3 3 3 2 3 4 48%	6010
	21st to end	Night	20% 9% 3 6% 4 4 4 3 3 4 41%	6611
February	1st to 10th	Night	18% 8% 3 6% 4 3 4 3 5% 5% 42%	2600
	11th to 20th	Night	16% 9% 3 6% 5% 4 5% 3 5% 6% 38%	2532
	21st to end	Night	16% 8% 2 4 4 3 4 3 4 6% 47%	1970
March	1st to 10th	Night	19% 19% 4 6% 4 3 3 3 4 5% 30%	4942
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 4 4 41%	5000
	21st to end	Night	18% 14% 3 9% 4 4 4 3 4 6% 29%	5489
April	1st to 10th	Night	12% 9% 3 7% 5% 4 4 4 4 7% 39%	2009
	11th to 20th	Night	9% 8% 2 7% 4 4 3 3 5% 8% 47%	1979
	21st to end	Night	6% 6% 1 5% 3 4 4 3 5% 7% 56%	1969
May	1st to 10th	Night	5% 5% 1 3 2 2 2 2 3 5% 70%	4100
	11th to 20th	Night	3 3 2 5% 3 3 3 2 3 5% 68%	3946
	21st to end	Night	2 2 1 3 2 2 2 2 3 5% 75%	3972
June	1st to 10th	Night	4 5% 2 4 4 3 3 4 4 8% 59%	1600
	11th to 20th	Night	2 2 1 2 1 2 1 2 3 6% 78%	1600
	21st to end	Night	2 2 1 3 1 2 2 2 3 5% 77%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 3 89%	3640
	11th to 20th	Night	1 1 2 2 2 2 1 2 3 84%	4045
	21st to end	Night	2 1 3 2 2 2 2 4 5% 77%	4472
August	1st to 10th	Night	1 1 1 2 2 2 2 2 3 5% 79%	4035
	11th to 20th	Night	1 3 1 3 1 2 2 1 3 4 79%	4113
	21st to end	Night	1 2 1 2 1 2 2 1 2 4 81%	5015
September	1st to 10th	Night	1 1 1 2 2 2 2 1 3 4 80%	2000
	11th to 20th	Night	3 4 2 5% 3 3 3 3 4 6% 63%	2045
	21st to end	Night	3 3 1 3 3 2 3 2 3 5% 72%	2270
October	1st to 10th	Night	3 3 1 3 2 3 3 2 3 4 74%	3920
	11th to 20th	Night	5% 6% 2 7% 4 3 4 2 4 5% 57%	3897
	21st to end	Night	9% 4 2 5% 4 4 4 4 5% 7% 54%	4253
November	1st to 10th	Night	12% 6% 2 5% 4 4 5% 3 6% 7% 45%	2475
	11th to 20th	Night	13% 7% 3 6% 5% 5% 5% 4 6% 7% 40%	2837
	21st to end	Night	11% 6% 2 6% 4 3 4 3 4 7% 49%	2940
December	1st to 10th	Night	12% 4 1 5% 3 3 4 3 5% 7% 52%	4700
	11th to 20th	Night	28% 9% 3 7% 4 4 4 4 6% 7% 24%	5316
	21st to end	Night	29% 10% 4 8% 4 4 4 3 4 5% 27%	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	45% 8% 14% 17% 10% 7%	3954
	11th to 20th	Day	40% 4 6% 11% 22% 17%	4190
	21st to end	Day	41% 4 7% 13% 15% 21%	4609
February	1st to 10th	Day	45% 4 9% 16% 17% 9%	1801
	11th to 20th	Day	45% 5% 8% 18% 19% 5%	1764
	21st to end	Day	42% 5% 12% 20% 13% 7%	1655
March	1st to 10th	Day	51% 7% 6% 17% 17% 2	4952
	11th to 20th	Day	42% 7% 12% 20% 11% 8%	5000
	21st to end	Day	49% 10% 12% 20% 8% 1	5486
April	1st to 10th	Day	42% 5% 14% 23% 14% 2	2390
	11th to 20th	Day	35% 6% 15% 23% 16% 5%	2372
	21st to end	Day	28% 7% 15% 28% 18% 3	2428
May	1st to 10th	Day	14% 5% 12% 25% 28% 16%	5880
	11th to 20th	Day	13% 4 9% 30% 35% 9%	5964
	21st to end	Day	12% 4 9% 42% 31% 3	6904
June	1st to 10th	Day	24% 5% 12% 28% 26% 5%	2800
	11th to 20th	Day	11% 4 10% 24% 36% 15%	2800
	21st to end	Day	10% 2 4 19% 37% 27%	2800
July	1st to 10th	Day	4 2 2 10% 24% 57%	6251
	11th to 20th	Day	4 1 3 7% 20% 64%	5856
	21st to end	Day	6% 3 6% 17% 32% 36%	6217
August	1st to 10th	Day	8% 3 4 13% 30% 42%	4206
	11th to 20th	Day	5% 3 5% 17% 41% 29%	4147
	21st to end	Day	8% 2 5% 17% 35% 33%	4311
September	1st to 10th	Day	7% 2 4 13% 30% 44%	2400
	11th to 20th	Day	17% 4 7% 25% 34% 13%	2350
	21st to end	Day	13% 3 6% 19% 34% 24%	2130
October	1st to 10th	Day	15% 2 4 16% 22% 41%	3280
	11th to 20th	Day	20% 4 7% 24% 30% 15%	3261
	21st to end	Day	28% 7% 9% 20% 26% 11%	3558
November	1st to 10th	Day	32% 5% 10% 24% 18% 11%	2043
	11th to 20th	Day	41% 6% 11% 18% 16% 8%	1780
	21st to end	Day	33% 4 11% 17% 21% 14%	1680
December	1st to 10th	Day	29% 5% 7% 21% 27% 10%	2700
	11th to 20th	Day	58% 5% 7% 14% 12% 4	3108
	21st to end	Day	56% 8% 9% 16% 8% 3	4092
January	1st to 10th	Night	46% 10% 16% 12% 11% 4	6204
	11th to 20th	Night	41% 5% 6% 14% 19% 14%	6010
	21st to end	Night	45% 4 7% 8% 17% 19%	6611
February	1st to 10th	Night	43% 5% 9% 16% 17% 9%	2600
	11th to 20th	Night	45% 7% 7% 21% 15% 5%	2534
	21st to end	Night	38% 5% 14% 23% 16% 5%	1970
March	1st to 10th	Night	54% 7% 7% 18% 12% 2	4942
	11th to 20th	Night	42% 6% 10% 20% 15% 7%	5000
	21st to end	Night	53% 9% 13% 16% 8% 1	5489
April	1st to 10th	Night	41% 8% 14% 20% 13% 3	2009
	11th to 20th	Night	34% 6% 15% 23% 17% 6%	1979
	21st to end	Night	26% 6% 15% 31% 19% 3	1969
May	1st to 10th	Night	16% 4 8% 25% 32% 15%	4097
	11th to 20th	Night	19% 4 8% 25% 35% 11%	3947
	21st to end	Night	11% 4 9% 44% 29% 2	3972
June	1st to 10th	Night	23% 5% 11% 29% 26% 5%	1600
	11th to 20th	Night	10% 4 9% 24% 39% 14%	1600
	21st to end	Night	12% 2 5% 18% 35% 29%	1600
July	1st to 10th	Night	3 2 2 13% 24% 56%	3600
	11th to 20th	Night	5% 2 3 6% 22% 62%	3993
	21st to end	Night	8% 4 6% 17% 29% 36%	4345
August	1st to 10th	Night	9% 2 3 12% 29% 44%	2929
	11th to 20th	Night	7% 2 5% 17% 40% 29%	2977
	21st to end	Night	10% 2 4 20% 31% 33%	3608
September	1st to 10th	Night	8% 2 3 14% 28% 45%	2000
	11th to 20th	Night	19% 3 9% 19% 35% 15%	2045
	21st to end	Night	15% 3 6% 21% 28% 28%	2270
October	1st to 10th	Night	15% 3 5% 14% 22% 42%	3920
	11th to 20th	Night	24% 7% 8% 19% 27% 14%	3897
	21st to end	Night	28% 5% 9% 22% 21% 15%	4253
November	1st to 10th	Night	35% 7% 11% 20% 17% 10%	2475
	11th to 20th	Night	39% 8% 14% 16% 15% 7%	2837
	21st to end	Night	35% 5% 10% 18% 20% 13%	2940
December	1st to 10th	Night	30% 5% 8% 24% 21% 13%	4700
	11th to 20th	Night	58% 6% 7% 11% 14% 4	5316
	21st to end	Night	60% 6% 9% 13% 9% 3	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	32% 13% 21% 18% 13% 3	325
	11th to 20th	Day	23% 10% 19% 19% 20% 8%	430
	21st to end	Day	22% 9% 19% 26% 18% 7%	473
February	1st to 10th	Day	28% 12% 19% 20% 15% 5%	401
	11th to 20th	Day	28% 16% 21% 19% 14% 2	395
	21st to end	Day	21% 14% 22% 27% 14% 2	411
March	1st to 10th	Day	34% 17% 23% 16% 8% 2	500
	11th to 20th	Day	30% 12% 24% 19% 11% 3	500
	21st to end	Day	25% 20% 26% 19% 7% 3	550
April	1st to 10th	Day	22% 16% 29% 18% 9% 5%	500
	11th to 20th	Day	20% 17% 27% 23% 9% 5%	492
	21st to end	Day	15% 18% 33% 18% 11% 5%	500
May	1st to 10th	Day	12% 18% 28% 24% 13% 5%	503
	11th to 20th	Day	13% 16% 29% 24% 11% 8%	528
	21st to end	Day	11% 12% 26% 28% 15% 8%	573
June	1st to 10th	Day	10% 11% 30% 28% 15% 6%	530
	11th to 20th	Day	7% 14% 34% 26% 14% 5%	530
	21st to end	Day	2 8% 26% 35% 18% 11%	530
July	1st to 10th	Day	2 4 26% 40% 20% 9%	526
	11th to 20th	Day	1 2 20% 41% 24% 12%	524
	21st to end	Day	1 2 14% 44% 25% 15%	575
August	1st to 10th	Day	2 3 26% 37% 22% 11%	527
	11th to 20th	Day	2 3 24% 35% 22% 13%	517
	21st to end	Day	2 3 21% 37% 25% 12%	583
September	1st to 10th	Day	3 4 24% 36% 23% 10%	530
	11th to 20th	Day	6% 7% 22% 34% 20% 11%	530
	21st to end	Day	6% 8% 20% 32% 20% 13%	530
October	1st to 10th	Day	8% 9% 22% 25% 24% 12%	530
	11th to 20th	Day	10% 7% 20% 24% 24% 15%	530
	21st to end	Day	12% 8% 21% 27% 23% 9%	583
November	1st to 10th	Day	15% 9% 19% 24% 22% 10%	519
	11th to 20th	Day	18% 8% 13% 23% 24% 14%	272
	21st to end	Day	16% 7% 18% 20% 24% 16%	220
December	1st to 10th	Day	21% 5% 15% 24% 20% 15%	220
	11th to 20th	Day	29% 10% 15% 16% 17% 13%	220
	21st to end	Day	29% 10% 17% 20% 17% 7%	242
January	1st to 10th	Night	26% 5% 15% 30% 19% 5%	840
	11th to 20th	Night	23% 4 13% 32% 23% 6%	840
	21st to end	Night	22% 4 13% 33% 23% 5%	924
February	1st to 10th	Night	24% 5% 13% 33% 20% 4	800
	11th to 20th	Night	22% 7% 16% 31% 19% 5%	784
	21st to end	Night	20% 8% 19% 33% 17% 2	660
March	1st to 10th	Night	27% 8% 16% 30% 15% 4	800
	11th to 20th	Night	25% 6% 13% 32% 20% 4	800
	21st to end	Night	23% 10% 17% 33% 16% 3	880
April	1st to 10th	Night	19% 7% 23% 28% 19% 5%	800
	11th to 20th	Night	14% 9% 18% 33% 20% 5%	791
	21st to end	Night	10% 9% 22% 30% 24% 5%	769
May	1st to 10th	Night	8% 7% 21% 32% 27% 6%	600
	11th to 20th	Night	7% 3 18% 34% 32% 5%	542
	21st to end	Night	6% 6% 20% 31% 32% 6%	433
June	1st to 10th	Night	5% 6% 19% 38% 29% 4	400
	11th to 20th	Night	2 6% 20% 40% 28% 4	400
	21st to end	Night	2 2 11% 45% 35% 6%	400
July	1st to 10th	Night	1 1 12% 35% 44% 7%	390
	11th to 20th	Night	2 1 5% 28% 46% 18%	555
	21st to end	Night	1 4 26% 52% 17%	657
August	1st to 10th	Night	3 4 31% 47% 15%	597
	11th to 20th	Night	1 1 6% 30% 49% 14%	616
	21st to end	Night	2 2 5% 30% 50% 12%	880
September	1st to 10th	Night	2 2 9% 32% 45% 11%	800
	11th to 20th	Night	6% 3 10% 35% 35% 11%	800
	21st to end	Night	5% 4 10% 32% 36% 12%	800
October	1st to 10th	Night	7% 3 11% 36% 35% 9%	800
	11th to 20th	Night	10% 3 9% 33% 35% 10%	800
	21st to end	Night	11% 4 11% 31% 31% 12%	880
November	1st to 10th	Night	15% 3 15% 29% 30% 8%	820
	11th to 20th	Night	18% 5% 12% 34% 26% 5%	840
	21st to end	Night	15% 6% 14% 35% 25% 6%	840
December	1st to 10th	Night	24% 3 11% 32% 22% 8%	840
	11th to 20th	Night	30% 5% 11% 32% 18% 5%	840
	21st to end	Night	30% 7% 16% 27% 16% 4	924

Page generated at 2026-01-28 22:21:42. Version ac07

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