The Williams' Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Williams' Ranch clear sky chart from 2005-11-06 to 2025-11-03.

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	3 2 1 3 3 3 3 3 5% 7% 67%	134012
	Night	3 3 1 4 3 3 3 3 5% 7% 65%	133440
Transparency	Day	16% 4 7% 17% 27% 28%	129105
	Night	18% 4 7% 15% 26% 29%	129350
Seeing	Day	5% 7% 21% 28% 24% 14%	15768
	Night	5% 6% 26% 39% 23% 2	26068

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	4 4 2 5% 3 3 3 2 4 6% 64%	4098
	11th to 20th	Day	1 2 1 2 2 2 2 2 4 6% 78%	4100
	21st to end	Day	3 3 1 4 3 3 3 3 5% 7% 66%	4625
February	1st to 10th	Day	3 3 1 3 3 2 3 3 4 7% 69%	2000
	11th to 20th	Day	4 3 1 4 4 3 4 3 4 6% 65%	1949
	21st to end	Day	2 3 1 3 3 2 3 3 4 5% 71%	1650
March	1st to 10th	Day	2 3 2 4 2 2 3 3 5% 7% 68%	4720
	11th to 20th	Day	4 3 1 3 3 2 3 3 4 6% 67%	5000
	21st to end	Day	2 4 2 5% 3 3 3 3 4 6% 65%	5718
April	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 6% 70%	2400
	11th to 20th	Day	2 3 1 2 2 2 3 2 3 4 76%	2371
	21st to end	Day	2 2 1 2 1 1 2 3 4 4 79%	2400
May	1st to 10th	Day	1 1 1 1 2 2 2 3 4 83%	5440
	11th to 20th	Day	1 1 1 2 1 2 3 3 4 6% 75%	5405
	21st to end	Day	1 1 1 2 1 1 2 2 3 5% 80%	5944
June	1st to 10th	Day	2 2 1 3 2 2 3 3 5% 7% 70%	2725
	11th to 20th	Day	1 2 3 2 2 3 3 5% 6% 74%	2800
	21st to end	Day	4 3 1 3 4 3 5% 4 7% 9% 57%	2607
July	1st to 10th	Day	4 3 1 4 4 5% 5% 5% 7% 9% 53%	5879
	11th to 20th	Day	2 2 1 3 3 4 4 5% 7% 11% 59%	5880
	21st to end	Day	4 3 1 5% 4 5% 7% 6% 8% 12% 46%	6167
August	1st to 10th	Day	2 2 1 3 2 3 4 4 6% 10% 62%	5348
	11th to 20th	Day	2 2 1 4 4 5% 5% 5% 8% 12% 52%	5366
	21st to end	Day	2 2 1 4 4 4 5% 4 6% 11% 57%	5984
September	1st to 10th	Day	9% 5% 2 4 3 3 4 3 5% 7% 55%	2400
	11th to 20th	Day	7% 4 2 5% 4 4 4 3 5% 7% 54%	2380
	21st to end	Day	4 2 1 3 3 2 3 3 4 7% 69%	2200
October	1st to 10th	Day	3 1 1 2 2 2 2 2 3 5% 79%	3349
	11th to 20th	Day	3 2 1 2 2 2 2 2 4 4 76%	3250
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 85%	3548
November	1st to 10th	Day	4 2 1 3 2 2 2 2 4 6% 73%	1979
	11th to 20th	Day	2 2 1 3 2 3 3 3 5% 5% 71%	1865
	21st to end	Day	5% 3 1 3 3 3 3 4 5% 6% 65%	1800
December	1st to 10th	Day	6% 7% 2 5% 3 2 3 2 4 5% 61%	2940
	11th to 20th	Day	1 2 2 5% 4 3 3 3 4 6% 67%	3325
	21st to end	Day	2 2 1 2 2 2 2 3 5% 7% 72%	4400
January	1st to 10th	Night	4 5% 1 4 2 3 3 3 5% 6% 64%	5867
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 5% 76%	5880
	21st to end	Night	3 3 1 4 3 4 3 3 4 8% 64%	6336
February	1st to 10th	Night	3 3 1 3 2 2 2 3 4 7% 70%	2400
	11th to 20th	Night	4 3 1 4 3 3 4 3 5% 6% 64%	2338
	21st to end	Night	4 3 2 3 2 3 2 2 5% 6% 68%	1980
March	1st to 10th	Night	2 3 1 3 2 2 3 3 4 6% 70%	5159
	11th to 20th	Night	6% 5% 1 3 2 2 3 3 5% 6% 64%	5000
	21st to end	Night	2 3 1 4 2 2 3 3 5% 7% 67%	5244
April	1st to 10th	Night	3 3 1 3 2 2 2 3 4 6% 69%	2000
	11th to 20th	Night	3 2 1 2 2 3 3 3 4 6% 72%	1977
	21st to end	Night	1 2 1 2 2 2 2 2 4 5% 77%	2000
May	1st to 10th	Night	3 3 1 2 2 2 2 1 3 4 77%	4540
	11th to 20th	Night	3 3 1 3 3 3 3 3 6% 7% 65%	4510
	21st to end	Night	1 2 1 2 1 2 2 2 4 5% 78%	4953
June	1st to 10th	Night	2 2 1 4 4 3 4 4 7% 8% 60%	1654
	11th to 20th	Night	2 2 1 2 3 4 2 3 6% 6% 68%	1600
	21st to end	Night	4 4 2 6% 5% 4 6% 5% 7% 9% 50%	1779
July	1st to 10th	Night	5% 4 1 5% 5% 5% 5% 5% 7% 11% 47%	4100
	11th to 20th	Night	2 2 1 4 3 4 6% 5% 9% 12% 52%	4100
	21st to end	Night	4 3 2 7% 5% 7% 8% 6% 9% 11% 38%	4689
August	1st to 10th	Night	2 2 1 3 3 4 6% 5% 7% 11% 55%	4464
	11th to 20th	Night	3 4 2 8% 5% 5% 6% 6% 8% 11% 43%	4483
	21st to end	Night	3 3 1 5% 5% 5% 6% 6% 8% 12% 47%	4993
September	1st to 10th	Night	5% 5% 1 5% 3 4 5% 4 6% 9% 52%	2000
	11th to 20th	Night	7% 3 2 5% 4 4 5% 4 5% 8% 52%	2018
	21st to end	Night	6% 3 1 5% 3 3 3 3 6% 6% 62%	2200
October	1st to 10th	Night	3 2 1 3 2 3 2 2 3 5% 75%	3821
	11th to 20th	Night	5% 3 1 4 2 2 2 2 3 5% 68%	3898
	21st to end	Night	2 1 1 2 1 1 2 2 3 4 82%	4252
November	1st to 10th	Night	4 3 1 2 1 2 2 2 4 6% 73%	2364
	11th to 20th	Night	2 3 1 3 2 3 3 3 4 5% 72%	2510
	21st to end	Night	6% 2 1 3 2 2 3 3 5% 7% 65%	2600
December	1st to 10th	Night	4 6% 2 4 3 2 3 2 4 6% 63%	4250
	11th to 20th	Night	2 2 1 4 3 3 3 2 5% 6% 70%	4875
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 77%	6606

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	20% 3 4 11% 35% 26%	4099
	11th to 20th	Day	9% 2 3 17% 46% 23%	4100
	21st to end	Day	18% 4 6% 20% 26% 26%	4625
February	1st to 10th	Day	16% 3 7% 13% 27% 33%	2000
	11th to 20th	Day	21% 4 6% 16% 27% 26%	1959
	21st to end	Day	16% 3 4 11% 29% 37%	1650
March	1st to 10th	Day	15% 3 4 7% 26% 44%	4720
	11th to 20th	Day	18% 4 4 13% 28% 34%	5000
	21st to end	Day	18% 4 5% 8% 25% 39%	5718
April	1st to 10th	Day	15% 3 4 12% 29% 37%	2400
	11th to 20th	Day	13% 3 4 9% 27% 45%	2374
	21st to end	Day	9% 2 5% 10% 27% 46%	2400
May	1st to 10th	Day	7% 3 3 13% 25% 49%	5440
	11th to 20th	Day	10% 3 4 12% 23% 49%	5408
	21st to end	Day	8% 3 5% 13% 13% 59%	5945
June	1st to 10th	Day	12% 4 8% 17% 28% 31%	2725
	11th to 20th	Day	11% 3 5% 12% 24% 45%	2800
	21st to end	Day	21% 6% 11% 17% 21% 23%	2607
July	1st to 10th	Day	25% 8% 13% 22% 23% 10%	5816
	11th to 20th	Day	16% 7% 16% 28% 24% 9%	5811
	21st to end	Day	25% 10% 18% 26% 18% 3	5989
August	1st to 10th	Day	17% 8% 11% 30% 28% 7%	3892
	11th to 20th	Day	20% 7% 11% 22% 28% 11%	3882
	21st to end	Day	23% 8% 15% 29% 21% 4	4311
September	1st to 10th	Day	28% 7% 11% 18% 20% 16%	2400
	11th to 20th	Day	28% 6% 10% 17% 24% 15%	2380
	21st to end	Day	16% 5% 7% 22% 31% 19%	2200
October	1st to 10th	Day	11% 3 5% 10% 24% 47%	3349
	11th to 20th	Day	13% 3 7% 17% 25% 35%	3250
	21st to end	Day	7% 2 3 13% 37% 38%	3548
November	1st to 10th	Day	14% 3 5% 20% 31% 27%	1979
	11th to 20th	Day	15% 3 6% 19% 31% 27%	1865
	21st to end	Day	20% 3 7% 18% 32% 21%	1800
December	1st to 10th	Day	27% 3 6% 17% 30% 17%	2940
	11th to 20th	Day	19% 3 6% 15% 36% 21%	3323
	21st to end	Day	12% 3 5% 15% 43% 23%	4400
January	1st to 10th	Night	21% 3 4 12% 35% 25%	5867
	11th to 20th	Night	11% 3 4 18% 40% 24%	5880
	21st to end	Night	19% 4 5% 20% 30% 22%	6336
February	1st to 10th	Night	15% 2 6% 15% 25% 36%	2400
	11th to 20th	Night	21% 3 7% 13% 28% 29%	2338
	21st to end	Night	18% 2 5% 9% 27% 39%	1980
March	1st to 10th	Night	15% 4 3 8% 23% 47%	5159
	11th to 20th	Night	19% 5% 5% 10% 27% 34%	5000
	21st to end	Night	15% 3 5% 12% 27% 37%	5244
April	1st to 10th	Night	16% 2 6% 9% 31% 36%	2000
	11th to 20th	Night	15% 3 6% 11% 23% 43%	1977
	21st to end	Night	11% 3 4 11% 26% 45%	2000
May	1st to 10th	Night	12% 3 4 15% 16% 51%	4537
	11th to 20th	Night	16% 4 5% 12% 20% 43%	4511
	21st to end	Night	10% 3 7% 10% 11% 59%	4954
June	1st to 10th	Night	20% 6% 8% 15% 21% 29%	1654
	11th to 20th	Night	14% 4 7% 11% 21% 43%	1600
	21st to end	Night	28% 6% 13% 14% 17% 21%	1779
July	1st to 10th	Night	29% 8% 15% 24% 17% 7%	4055
	11th to 20th	Night	20% 7% 16% 25% 23% 8%	4046
	21st to end	Night	34% 9% 15% 23% 16% 2	4554
August	1st to 10th	Night	23% 8% 12% 25% 23% 9%	3244
	11th to 20th	Night	23% 7% 11% 21% 30% 9%	3243
	21st to end	Night	26% 10% 15% 23% 20% 6%	3597
September	1st to 10th	Night	28% 8% 12% 17% 18% 17%	2000
	11th to 20th	Night	28% 7% 11% 18% 17% 18%	2018
	21st to end	Night	22% 5% 9% 17% 28% 20%	2200
October	1st to 10th	Night	13% 4 6% 8% 22% 47%	3821
	11th to 20th	Night	19% 3 7% 12% 19% 40%	3898
	21st to end	Night	9% 2 3 9% 34% 42%	4252
November	1st to 10th	Night	14% 2 5% 15% 33% 31%	2364
	11th to 20th	Night	15% 3 4 17% 32% 29%	2510
	21st to end	Night	19% 3 6% 17% 32% 23%	2600
December	1st to 10th	Night	23% 3 6% 20% 32% 17%	4250
	11th to 20th	Night	16% 2 5% 14% 36% 28%	4876
	21st to end	Night	12% 2 3 15% 41% 26%	6606

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	10% 12% 32% 25% 17% 4	300
	11th to 20th	Day	6% 10% 34% 32% 17% 2	300
	21st to end	Day	5% 8% 32% 32% 17% 7%	330
February	1st to 10th	Day	6% 6% 33% 32% 16% 7%	300
	11th to 20th	Day	9% 12% 36% 29% 12% 2	300
	21st to end	Day	10% 17% 32% 29% 10% 3	249
March	1st to 10th	Day	9% 16% 33% 30% 10% 1	390
	11th to 20th	Day	12% 16% 31% 26% 12% 4	500
	21st to end	Day	7% 20% 37% 24% 9% 3	550
April	1st to 10th	Day	8% 14% 38% 25% 13% 3	500
	11th to 20th	Day	7% 17% 37% 27% 10% 2	492
	21st to end	Day	4 16% 36% 29% 13% 2	500
May	1st to 10th	Day	3 11% 38% 31% 14% 3	503
	11th to 20th	Day	2 10% 32% 34% 18% 5%	528
	21st to end	Day	2 7% 28% 40% 20% 3	573
June	1st to 10th	Day	2 2 19% 40% 28% 9%	705
	11th to 20th	Day	2 2 15% 38% 38% 6%	730
	21st to end	Day	4 2 11% 30% 32% 21%	537
July	1st to 10th	Day	4 1 6% 22% 38% 29%	526
	11th to 20th	Day	4 1 3 22% 34% 36%	524
	21st to end	Day	2 1 5% 21% 36% 35%	575
August	1st to 10th	Day	4 2 16% 33% 45%	527
	11th to 20th	Day	6% 5% 21% 37% 31%	517
	21st to end	Day	5% 1 5% 19% 34% 36%	583
September	1st to 10th	Day	9% 1 7% 24% 32% 27%	530
	11th to 20th	Day	6% 1 7% 26% 37% 24%	510
	21st to end	Day	4 1 7% 25% 36% 28%	330
October	1st to 10th	Day	8% 1 12% 26% 33% 20%	330
	11th to 20th	Day	5% 3 17% 32% 30% 15%	330
	21st to end	Day	3 8% 19% 29% 24% 17%	363
November	1st to 10th	Day	6% 7% 22% 32% 24% 9%	306
	11th to 20th	Day	3 7% 19% 31% 30% 10%	300
	21st to end	Day	9% 13% 28% 27% 17% 6%	300
December	1st to 10th	Day	10% 7% 27% 28% 24% 5%	300
	11th to 20th	Day	4 9% 25% 36% 21% 5%	300
	21st to end	Day	8% 11% 29% 36% 12% 4	330
January	1st to 10th	Night	7% 6% 34% 38% 13% 2	800
	11th to 20th	Night	5% 8% 32% 39% 14% 2	800
	21st to end	Night	5% 8% 35% 37% 14% 1	880
February	1st to 10th	Night	4 9% 33% 38% 15% 1	800
	11th to 20th	Night	6% 8% 40% 36% 9% 1	784
	21st to end	Night	5% 11% 47% 31% 5%	660
March	1st to 10th	Night	6% 13% 37% 35% 9% 1	800
	11th to 20th	Night	7% 12% 32% 38% 11% 1	800
	21st to end	Night	3 10% 44% 36% 8%	777
April	1st to 10th	Night	5% 11% 44% 32% 8%	600
	11th to 20th	Night	5% 10% 47% 29% 10%	593
	21st to end	Night	2 12% 42% 36% 9%	600
May	1st to 10th	Night	5% 7% 43% 38% 7%	600
	11th to 20th	Night	3 7% 34% 43% 12% 1	591
	21st to end	Night	3 4 30% 51% 12%	650
June	1st to 10th	Night	3 2 23% 49% 22% 1	600
	11th to 20th	Night	3 2 19% 54% 23% 1	600
	21st to end	Night	4 2 12% 46% 35% 2	600
July	1st to 10th	Night	4 1 7% 40% 46% 2	585
	11th to 20th	Night	3 1 5% 34% 54% 4	582
	21st to end	Night	4 1 4 31% 53% 6%	657
August	1st to 10th	Night	5% 5% 26% 58% 7%	597
	11th to 20th	Night	3 1 6% 28% 54% 9%	594
	21st to end	Night	5% 1 4 28% 57% 5%	660
September	1st to 10th	Night	6% 1 6% 38% 47% 3	600
	11th to 20th	Night	9% 1 5% 43% 40% 3	600
	21st to end	Night	7% 2 11% 45% 34% 3	600
October	1st to 10th	Night	6% 2 16% 45% 30% 2	740
	11th to 20th	Night	6% 3 22% 42% 25% 2	800
	21st to end	Night	4 5% 19% 43% 25% 4	880
November	1st to 10th	Night	5% 2 24% 39% 27% 3	788
	11th to 20th	Night	4 3 26% 43% 23% 1	910
	21st to end	Night	7% 7% 26% 39% 19% 2	1000
December	1st to 10th	Night	7% 7% 29% 40% 17% 1	1000
	11th to 20th	Night	4 7% 34% 42% 13% 1	1000
	21st to end	Night	5% 9% 31% 41% 12% 1	940

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