Ross Creek Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ross Creek Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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 4 2 4 3 3 3 3 5% 6% 62%	139528
	Night	5% 6% 2 6% 4 4 4 3 5% 7% 54%	138809
Transparency	Day	21% 6% 9% 20% 26% 19%	134472
	Night	26% 7% 11% 19% 22% 16%	134851
Seeing	Day	7% 11% 23% 31% 21% 7%	17044
	Night	8% 6% 15% 34% 31% 7%	26531

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	9% 7% 3 6% 5% 5% 5% 4 5% 8% 42%	5063
	11th to 20th	Day	6% 5% 2 6% 3 3 4 3 4 5% 58%	5340
	21st to end	Day	8% 8% 3 8% 5% 5% 4 4 6% 8% 42%	5843
February	1st to 10th	Day	10% 8% 3 6% 4 4 4 3 5% 6% 46%	1890
	11th to 20th	Day	10% 7% 2 6% 4 6% 4 4 6% 7% 43%	1915
	21st to end	Day	10% 9% 2 7% 5% 4 4 3 5% 6% 45%	1650
March	1st to 10th	Day	8% 8% 3 9% 7% 7% 7% 5% 7% 7% 32%	4855
	11th to 20th	Day	6% 5% 2 6% 4 6% 5% 5% 6% 8% 46%	5000
	21st to end	Day	7% 7% 3 8% 6% 7% 7% 5% 7% 10% 34%	5589
April	1st to 10th	Day	10% 7% 2 6% 4 4 4 4 5% 7% 47%	2400
	11th to 20th	Day	8% 6% 2 4 3 3 3 3 6% 7% 55%	2369
	21st to end	Day	5% 5% 2 4 3 4 3 3 6% 8% 58%	2400
May	1st to 10th	Day	4 2 1 2 2 1 2 2 3 6% 74%	5524
	11th to 20th	Day	2 2 1 2 1 2 2 2 4 6% 77%	5965
	21st to end	Day	2 2 1 3 2 3 2 2 4 6% 74%	6902
June	1st to 10th	Day	4 4 1 4 3 2 3 3 5% 7% 65%	2800
	11th to 20th	Day	2 2 1 1 1 1 3 2 4 7% 77%	2800
	21st to end	Day	1 2 1 1 1 2 2 2 3 4 80%	2800
July	1st to 10th	Day	1 1 1 1 1 2 4 87%	6319
	11th to 20th	Day	1 1 2 1 2 2 3 4 83%	5887
	21st to end	Day	1 1 2 2 3 2 2 4 6% 76%	6402
August	1st to 10th	Day	1 1 2 2 2 2 3 5% 6% 76%	5777
	11th to 20th	Day	1 1 2 2 2 2 2 4 4 81%	5456
	21st to end	Day	1 2 2 3 3 2 4 6% 76%	5984
September	1st to 10th	Day	1 1 1 2 2 2 2 1 2 5% 81%	2400
	11th to 20th	Day	2 3 2 4 3 3 3 2 5% 7% 66%	2400
	21st to end	Day	2 1 1 2 2 2 3 2 3 5% 76%	2130
October	1st to 10th	Day	2 2 1 3 2 2 2 2 4 7% 74%	3280
	11th to 20th	Day	2 3 1 3 2 3 3 3 4 6% 69%	3261
	21st to end	Day	2 3 1 3 3 3 4 3 6% 8% 63%	3558
November	1st to 10th	Day	5% 6% 1 5% 3 4 4 4 5% 7% 56%	2018
	11th to 20th	Day	5% 6% 2 5% 5% 5% 4 4 6% 7% 49%	1890
	21st to end	Day	8% 5% 2 5% 4 3 4 3 6% 8% 53%	1761
December	1st to 10th	Day	4 3 2 5% 4 4 5% 4 6% 9% 54%	2700
	11th to 20th	Day	10% 7% 4 9% 6% 6% 6% 4 6% 8% 34%	3108
	21st to end	Day	8% 11% 5% 11% 6% 7% 7% 5% 7% 7% 27%	4092
January	1st to 10th	Night	9% 9% 3 8% 5% 5% 5% 3 6% 7% 40%	7832
	11th to 20th	Night	6% 5% 3 8% 4 3 4 3 4 6% 53%	7650
	21st to end	Night	8% 8% 4 9% 5% 5% 5% 4 5% 8% 39%	8371
February	1st to 10th	Night	9% 7% 3 7% 5% 4 4 4 4 5% 48%	2730
	11th to 20th	Night	9% 9% 2 6% 4 5% 4 4 6% 7% 43%	2482
	21st to end	Night	8% 8% 4 5% 4 4 4 4 5% 7% 49%	1980
March	1st to 10th	Night	10% 13% 4 10% 6% 5% 5% 4 6% 6% 32%	5036
	11th to 20th	Night	9% 7% 3 7% 5% 5% 4 3 5% 6% 45%	5000
	21st to end	Night	8% 12% 4 9% 6% 7% 5% 4 5% 6% 34%	5385
April	1st to 10th	Night	9% 7% 3 5% 4 4 5% 5% 6% 7% 47%	2000
	11th to 20th	Night	7% 6% 3 6% 4 4 4 3 5% 7% 53%	1979
	21st to end	Night	4 5% 2 4 3 4 5% 4 5% 9% 56%	2000
May	1st to 10th	Night	2 4 2 4 2 2 4 2 4 5% 70%	4454
	11th to 20th	Night	1 2 1 3 4 3 4 4 7% 10% 60%	3941
	21st to end	Night	1 3 1 3 2 3 3 2 6% 7% 69%	3972
June	1st to 10th	Night	2 3 1 3 3 2 3 3 5% 9% 64%	1600
	11th to 20th	Night	1 2 1 3 2 1 3 4 5% 7% 73%	1600
	21st to end	Night	1 3 1 3 2 2 3 2 3 6% 76%	1600
July	1st to 10th	Night	1 1 1 1 1 1 3 5% 85%	3640
	11th to 20th	Night	1 1 3 2 3 3 2 4 6% 75%	4088
	21st to end	Night	3 2 5% 4 7% 5% 3 6% 6% 59%	4472
August	1st to 10th	Night	1 2 1 3 3 5% 3 2 6% 7% 67%	4035
	11th to 20th	Night	2 1 3 2 3 3 3 4 6% 73%	4408
	21st to end	Night	1 1 1 3 4 5% 4 3 6% 6% 67%	5015
September	1st to 10th	Night	1 2 1 2 3 2 3 2 5% 7% 73%	2000
	11th to 20th	Night	2 4 2 5% 3 4 4 3 5% 5% 64%	2000
	21st to end	Night	2 3 1 3 2 2 3 2 4 6% 73%	2264
October	1st to 10th	Night	2 2 1 2 2 2 3 3 4 6% 74%	3920
	11th to 20th	Night	3 5% 2 5% 3 4 4 3 4 6% 60%	3897
	21st to end	Night	3 4 2 4 3 4 5% 3 5% 6% 60%	4253
November	1st to 10th	Night	5% 5% 2 5% 4 3 3 3 5% 8% 57%	2497
	11th to 20th	Night	7% 5% 2 6% 3 4 4 4 6% 8% 50%	2730
	21st to end	Night	6% 5% 1 5% 5% 4 5% 4 4 7% 54%	2856
December	1st to 10th	Night	5% 4 2 4 3 3 5% 3 5% 7% 58%	4700
	11th to 20th	Night	11% 9% 4 8% 5% 5% 5% 4 8% 11% 29%	5316
	21st to end	Night	11% 10% 4 11% 8% 8% 8% 5% 6% 7% 25%	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	34% 10% 16% 21% 15% 4	5064
	11th to 20th	Day	27% 6% 6% 11% 25% 25%	5340
	21st to end	Day	36% 7% 7% 16% 21% 13%	5843
February	1st to 10th	Day	38% 6% 8% 19% 18% 12%	1890
	11th to 20th	Day	38% 7% 11% 18% 20% 7%	1924
	21st to end	Day	39% 7% 8% 21% 19% 7%	1650
March	1st to 10th	Day	43% 12% 9% 15% 18% 2	4855
	11th to 20th	Day	30% 9% 13% 28% 11% 8%	5000
	21st to end	Day	37% 13% 17% 21% 11% 2	5589
April	1st to 10th	Day	35% 6% 12% 22% 20% 5%	2400
	11th to 20th	Day	27% 7% 15% 25% 20% 7%	2372
	21st to end	Day	24% 6% 14% 28% 23% 5%	2400
May	1st to 10th	Day	12% 4 7% 31% 32% 14%	5524
	11th to 20th	Day	10% 3 13% 35% 31% 9%	5968
	21st to end	Day	12% 4 11% 30% 39% 4	6904
June	1st to 10th	Day	19% 6% 12% 28% 27% 9%	2800
	11th to 20th	Day	9% 4 9% 23% 37% 18%	2800
	21st to end	Day	9% 3 6% 15% 32% 36%	2800
July	1st to 10th	Day	4 1 3 8% 23% 61%	6251
	11th to 20th	Day	5% 2 5% 7% 22% 60%	5817
	21st to end	Day	7% 4 5% 20% 33% 31%	6217
August	1st to 10th	Day	10% 3 6% 12% 35% 34%	4206
	11th to 20th	Day	5% 2 4 10% 42% 36%	3949
	21st to end	Day	7% 3 6% 14% 34% 35%	4311
September	1st to 10th	Day	8% 3 4 12% 30% 43%	2400
	11th to 20th	Day	16% 4 8% 24% 30% 17%	2400
	21st to end	Day	11% 3 7% 20% 35% 23%	2130
October	1st to 10th	Day	12% 3 6% 12% 29% 38%	3280
	11th to 20th	Day	15% 5% 8% 26% 30% 16%	3261
	21st to end	Day	17% 5% 8% 22% 31% 17%	3558
November	1st to 10th	Day	27% 6% 10% 20% 25% 12%	2018
	11th to 20th	Day	31% 7% 11% 20% 22% 8%	1890
	21st to end	Day	29% 5% 9% 18% 29% 9%	1761
December	1st to 10th	Day	25% 6% 8% 21% 32% 7%	2700
	11th to 20th	Day	44% 8% 11% 17% 15% 6%	3108
	21st to end	Day	48% 12% 12% 16% 10% 2	4092
January	1st to 10th	Night	35% 12% 16% 19% 15% 3	7832
	11th to 20th	Night	28% 7% 9% 12% 27% 17%	7650
	21st to end	Night	38% 9% 7% 16% 18% 13%	8371
February	1st to 10th	Night	37% 5% 9% 18% 19% 13%	2730
	11th to 20th	Night	37% 7% 12% 18% 20% 7%	2484
	21st to end	Night	32% 6% 12% 27% 17% 6%	1980
March	1st to 10th	Night	44% 11% 11% 17% 14% 3	5036
	11th to 20th	Night	34% 8% 14% 21% 16% 7%	5000
	21st to end	Night	40% 13% 14% 20% 11% 3	5385
April	1st to 10th	Night	32% 8% 14% 22% 18% 5%	2000
	11th to 20th	Night	29% 6% 14% 27% 17% 8%	1979
	21st to end	Night	21% 7% 17% 27% 21% 7%	2000
May	1st to 10th	Night	13% 6% 12% 27% 28% 14%	4451
	11th to 20th	Night	12% 5% 28% 29% 19% 7%	3942
	21st to end	Night	11% 5% 16% 30% 35% 4	3972
June	1st to 10th	Night	16% 5% 16% 28% 25% 10%	1600
	11th to 20th	Night	10% 4 11% 20% 31% 24%	1600
	21st to end	Night	10% 4 8% 17% 26% 35%	1600
July	1st to 10th	Night	4 2 3 11% 22% 59%	3600
	11th to 20th	Night	7% 3 6% 10% 20% 55%	4035
	21st to end	Night	14% 5% 13% 17% 26% 25%	4345
August	1st to 10th	Night	14% 4 9% 13% 33% 27%	2929
	11th to 20th	Night	9% 2 6% 10% 38% 35%	3183
	21st to end	Night	10% 4 10% 14% 34% 28%	3608
September	1st to 10th	Night	10% 3 5% 13% 27% 42%	2000
	11th to 20th	Night	16% 6% 9% 23% 27% 18%	2000
	21st to end	Night	12% 4 7% 18% 33% 26%	2264
October	1st to 10th	Night	11% 4 7% 11% 24% 42%	3920
	11th to 20th	Night	19% 5% 10% 24% 25% 17%	3897
	21st to end	Night	20% 6% 8% 21% 26% 19%	4253
November	1st to 10th	Night	24% 6% 10% 24% 22% 15%	2497
	11th to 20th	Night	31% 6% 11% 21% 23% 9%	2730
	21st to end	Night	29% 5% 10% 19% 22% 15%	2856
December	1st to 10th	Night	24% 6% 8% 25% 26% 11%	4700
	11th to 20th	Night	44% 8% 10% 16% 17% 5%	5316
	21st to end	Night	50% 12% 13% 12% 8% 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	23% 18% 24% 20% 14% 2	340
	11th to 20th	Day	12% 13% 25% 30% 15% 5%	430
	21st to end	Day	13% 13% 26% 28% 18% 3	473
February	1st to 10th	Day	15% 20% 24% 23% 14% 4	430
	11th to 20th	Day	16% 21% 29% 23% 9% 2	469
	21st to end	Day	11% 19% 30% 27% 12% 2	414
March	1st to 10th	Day	17% 20% 30% 24% 8% 1	500
	11th to 20th	Day	16% 20% 25% 22% 13% 4	500
	21st to end	Day	13% 20% 33% 23% 9% 2	550
April	1st to 10th	Day	12% 17% 31% 24% 13% 3	500
	11th to 20th	Day	12% 18% 29% 24% 14% 3	492
	21st to end	Day	6% 19% 27% 29% 13% 7%	500
May	1st to 10th	Day	7% 16% 31% 28% 14% 4	503
	11th to 20th	Day	8% 13% 30% 28% 16% 6%	528
	21st to end	Day	5% 11% 27% 32% 18% 7%	573
June	1st to 10th	Day	3 11% 26% 31% 21% 8%	530
	11th to 20th	Day	2 9% 30% 36% 18% 5%	530
	21st to end	Day	2 4 21% 35% 27% 10%	530
July	1st to 10th	Day	1 2 17% 39% 32% 9%	526
	11th to 20th	Day	1 10% 44% 32% 13%	524
	21st to end	Day	1 12% 40% 32% 15%	575
August	1st to 10th	Day	1 2 20% 39% 27% 11%	527
	11th to 20th	Day	3 19% 39% 25% 15%	517
	21st to end	Day	2 12% 41% 32% 13%	583
September	1st to 10th	Day	1 5% 15% 40% 30% 10%	530
	11th to 20th	Day	2 5% 24% 38% 21% 9%	530
	21st to end	Day	3 6% 19% 36% 28% 7%	530
October	1st to 10th	Day	2 9% 23% 30% 25% 11%	530
	11th to 20th	Day	5% 11% 19% 29% 27% 10%	530
	21st to end	Day	6% 8% 22% 34% 23% 6%	583
November	1st to 10th	Day	7% 12% 20% 26% 28% 7%	494
	11th to 20th	Day	5% 11% 24% 28% 24% 8%	330
	21st to end	Day	8% 10% 22% 31% 25% 5%	261
December	1st to 10th	Day	8% 13% 21% 32% 19% 8%	220
	11th to 20th	Day	8% 21% 19% 25% 18% 10%	220
	21st to end	Day	12% 25% 26% 21% 13% 4	242
January	1st to 10th	Night	16% 9% 18% 30% 22% 6%	840
	11th to 20th	Night	10% 7% 16% 37% 23% 8%	840
	21st to end	Night	14% 6% 14% 36% 24% 6%	924
February	1st to 10th	Night	13% 10% 16% 34% 22% 4	840
	11th to 20th	Night	13% 11% 20% 33% 18% 5%	804
	21st to end	Night	13% 9% 21% 37% 18% 3	660
March	1st to 10th	Night	15% 12% 21% 30% 20% 3	800
	11th to 20th	Night	14% 10% 20% 28% 24% 4	800
	21st to end	Night	10% 11% 23% 34% 19% 3	880
April	1st to 10th	Night	11% 6% 25% 33% 22% 3	800
	11th to 20th	Night	9% 11% 20% 34% 19% 5%	791
	21st to end	Night	6% 7% 26% 32% 23% 6%	800
May	1st to 10th	Night	5% 4 26% 35% 26% 5%	759
	11th to 20th	Night	5% 3 12% 38% 36% 6%	537
	21st to end	Night	3 5% 17% 41% 31% 3	433
June	1st to 10th	Night	3 4 15% 44% 30% 5%	400
	11th to 20th	Night	1 3 12% 49% 33% 2	400
	21st to end	Night	1 10% 38% 48% 3	400
July	1st to 10th	Night	1 4 36% 55% 4	390
	11th to 20th	Night	1 1 30% 55% 13%	571
	21st to end	Night	1 1 2 29% 55% 12%	657
August	1st to 10th	Night	3 3 34% 52% 8%	597
	11th to 20th	Night	1 1 8% 40% 43% 8%	744
	21st to end	Night	1 1 7% 32% 50% 10%	880
September	1st to 10th	Night	1 1 9% 31% 47% 11%	800
	11th to 20th	Night	4 2 10% 38% 39% 8%	800
	21st to end	Night	3 2 12% 34% 39% 10%	800
October	1st to 10th	Night	4 3 12% 37% 35% 10%	800
	11th to 20th	Night	6% 5% 12% 31% 37% 10%	800
	21st to end	Night	5% 5% 11% 33% 30% 15%	880
November	1st to 10th	Night	6% 5% 16% 30% 34% 10%	820
	11th to 20th	Night	10% 4 18% 33% 29% 7%	840
	21st to end	Night	9% 5% 16% 39% 26% 7%	840
December	1st to 10th	Night	12% 5% 18% 34% 25% 6%	840
	11th to 20th	Night	16% 6% 18% 34% 22% 5%	840
	21st to end	Night	15% 13% 22% 30% 17% 3	924

Page generated at 2026-02-12 22:42:14. Version ac07

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