Keystone Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Keystone Ranch Observatory clear sky chart from 2005-11-06 to 2025-06-10.

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	7% 4 1 4 3 3 3 3 5% 7% 59%	119769
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	118727
Transparency	Day	26% 3 8% 21% 25% 16%	123681
	Night	23% 3 9% 22% 25% 18%	122933
Seeing	Day	9% 7% 25% 32% 21% 6%	15685
	Night	8% 7% 35% 37% 12% 1	25700

By Month

Percentage Cloud Cover SE (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% 2 4 2 2 3 2 3 5% 60%	4077
	11th to 20th	Day	6% 5% 2 2 2 2 2 2 4 6% 68%	4246
	21st to end	Day	14% 7% 2 4 2 2 3 2 3 5% 56%	4994
February	1st to 10th	Day	9% 5% 3 5% 3 3 3 3 4 6% 58%	2000
	11th to 20th	Day	10% 6% 2 5% 2 4 3 2 5% 5% 55%	1949
	21st to end	Day	10% 5% 2 4 2 2 3 3 5% 5% 58%	1650
March	1st to 10th	Day	6% 5% 2 4 3 3 4 3 4 6% 61%	4500
	11th to 20th	Day	13% 4 1 3 2 2 3 3 4 5% 59%	4980
	21st to end	Day	4 5% 2 5% 4 3 4 3 5% 8% 58%	5462
April	1st to 10th	Day	8% 4 2 4 2 3 3 3 5% 7% 60%	2320
	11th to 20th	Day	9% 5% 1 5% 3 4 3 3 5% 6% 55%	2374
	21st to end	Day	8% 4 2 4 3 2 3 2 4 6% 62%	2400
May	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 6% 53%	5857
	11th to 20th	Day	10% 5% 2 6% 4 3 3 3 5% 6% 54%	5862
	21st to end	Day	9% 4 2 4 3 4 4 4 7% 8% 51%	6434
June	1st to 10th	Day	3 2 1 3 3 3 4 3 6% 9% 62%	2588
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 8% 63%	2470
	21st to end	Day	4 4 1 3 4 4 4 4 7% 8% 56%	2470
July	1st to 10th	Day	4 2 1 5% 4 5% 5% 4 6% 8% 56%	4120
	11th to 20th	Day	2 1 3 3 3 4 3 6% 9% 66%	4120
	21st to end	Day	5% 3 1 4 4 3 3 3 5% 9% 60%	4404
August	1st to 10th	Day	1 1 2 2 3 4 3 5% 6% 73%	3785
	11th to 20th	Day	4 3 1 2 2 3 3 3 5% 8% 67%	3726
	21st to end	Day	3 3 1 5% 5% 5% 6% 4 6% 9% 52%	4180
September	1st to 10th	Day	9% 5% 1 5% 4 4 5% 3 5% 8% 50%	2152
	11th to 20th	Day	10% 4 2 5% 5% 4 4 3 6% 8% 51%	2090
	21st to end	Day	10% 3 1 3 3 4 3 3 4 7% 58%	2090
October	1st to 10th	Day	5% 4 1 3 3 3 3 3 4 6% 65%	2090
	11th to 20th	Day	7% 4 1 3 3 3 3 3 3 6% 65%	2090
	21st to end	Day	10% 3 1 4 2 2 2 2 3 4 66%	2107
November	1st to 10th	Day	9% 3 2 4 2 3 2 2 4 7% 61%	1763
	11th to 20th	Day	6% 4 1 3 2 3 3 3 4 6% 65%	1800
	21st to end	Day	9% 5% 1 4 3 3 3 3 4 7% 57%	1800
December	1st to 10th	Day	16% 8% 3 6% 3 3 3 2 3 5% 49%	2940
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 3 5% 6% 54%	3391
	21st to end	Day	8% 4 1 3 2 1 1 2 2 4 70%	4488
January	1st to 10th	Night	7% 5% 2 4 2 3 3 3 4 5% 62%	5888
	11th to 20th	Night	5% 4 1 2 1 1 2 2 4 5% 73%	5762
	21st to end	Night	17% 6% 2 4 3 3 2 2 4 5% 53%	5984
February	1st to 10th	Night	8% 6% 1 4 4 3 3 2 4 6% 60%	2400
	11th to 20th	Night	11% 6% 2 4 4 3 2 2 4 6% 55%	2338
	21st to end	Night	9% 5% 1 4 3 2 3 3 4 6% 59%	1980
March	1st to 10th	Night	5% 4 1 4 3 3 3 3 5% 7% 62%	5357
	11th to 20th	Night	12% 4 1 3 2 2 3 3 4 6% 59%	4980
	21st to end	Night	4 4 2 4 3 3 4 4 5% 7% 60%	5467
April	1st to 10th	Night	5% 5% 1 4 3 3 3 3 5% 7% 61%	2075
	11th to 20th	Night	8% 4 1 4 3 3 3 3 4 7% 59%	1977
	21st to end	Night	6% 4 1 3 3 2 3 3 4 6% 65%	2000
May	1st to 10th	Night	10% 5% 2 5% 3 4 4 3 5% 6% 54%	4057
	11th to 20th	Night	6% 3 1 4 3 3 4 3 6% 8% 59%	4053
	21st to end	Night	6% 4 2 5% 3 3 5% 4 7% 10% 52%	4443
June	1st to 10th	Night	3 2 1 3 2 4 4 4 7% 10% 61%	1796
	11th to 20th	Night	2 2 1 2 2 2 4 4 6% 9% 67%	1710
	21st to end	Night	3 2 1 2 3 2 4 3 7% 8% 65%	1710
July	1st to 10th	Night	3 2 1 4 3 3 4 3 6% 9% 62%	2850
	11th to 20th	Night	1 1 1 2 2 2 3 4 8% 75%	2850
	21st to end	Night	3 3 1 4 3 3 3 3 5% 8% 66%	3154
August	1st to 10th	Night	1 2 1 3 2 3 3 3 5% 6% 72%	3155
	11th to 20th	Night	2 2 1 3 3 3 3 4 5% 9% 65%	3113
	21st to end	Night	2 3 1 4 3 4 5% 4 7% 12% 55%	3487
September	1st to 10th	Night	8% 4 1 4 3 4 4 4 5% 9% 54%	2025
	11th to 20th	Night	9% 3 1 4 3 4 4 3 6% 9% 54%	2090
	21st to end	Night	8% 3 1 3 3 2 3 3 5% 8% 61%	2090
October	1st to 10th	Night	4 3 3 2 2 4 3 5% 7% 67%	2090
	11th to 20th	Night	9% 3 1 3 2 2 4 3 3 4 65%	2090
	21st to end	Night	8% 4 1 4 2 2 3 3 4 4 66%	2470
November	1st to 10th	Night	9% 3 1 3 2 2 3 2 4 5% 65%	2530
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 6% 65%	2600
	21st to end	Night	10% 4 1 4 3 4 4 3 5% 6% 56%	2600
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 50%	4250
	11th to 20th	Night	6% 4 2 5% 3 3 3 3 4 7% 59%	4816
	21st to end	Night	6% 5% 1 4 2 2 3 3 4 6% 64%	6490

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	28% 2 4 22% 27% 17%	4078
	11th to 20th	Day	21% 2 5% 20% 32% 20%	4246
	21st to end	Day	34% 2 5% 14% 27% 18%	4994
February	1st to 10th	Day	29% 3 5% 15% 24% 24%	2000
	11th to 20th	Day	32% 3 6% 21% 20% 18%	1959
	21st to end	Day	29% 2 4 13% 30% 22%	1650
March	1st to 10th	Day	26% 2 5% 13% 21% 33%	4500
	11th to 20th	Day	30% 2 6% 9% 20% 33%	4980
	21st to end	Day	25% 3 6% 16% 27% 21%	5464
April	1st to 10th	Day	24% 4 8% 19% 24% 21%	2320
	11th to 20th	Day	31% 3 7% 16% 23% 20%	2374
	21st to end	Day	26% 3 7% 20% 25% 18%	2400
May	1st to 10th	Day	33% 3 8% 13% 22% 21%	5895
	11th to 20th	Day	31% 3 7% 14% 27% 18%	5866
	21st to end	Day	30% 5% 11% 24% 17% 12%	6447
June	1st to 10th	Day	20% 4 14% 32% 25% 7%	2588
	11th to 20th	Day	20% 4 11% 28% 27% 10%	2470
	21st to end	Day	24% 5% 13% 25% 23% 9%	2470
July	1st to 10th	Day	24% 6% 14% 37% 17% 1	4056
	11th to 20th	Day	15% 4 15% 37% 24% 4	4051
	21st to end	Day	21% 4 12% 31% 28% 4	4265
August	1st to 10th	Day	13% 3 11% 34% 36% 4	3772
	11th to 20th	Day	17% 3 13% 41% 23% 3	3762
	21st to end	Day	26% 8% 17% 22% 23% 4	4179
September	1st to 10th	Day	33% 5% 10% 23% 20% 10%	2152
	11th to 20th	Day	33% 4 12% 21% 23% 7%	2090
	21st to end	Day	27% 4 9% 22% 23% 15%	2090
October	1st to 10th	Day	13% 3 6% 14% 26% 38%	3480
	11th to 20th	Day	21% 4 6% 18% 23% 28%	3458
	21st to end	Day	17% 2 4 24% 40% 13%	3445
November	1st to 10th	Day	25% 3 7% 25% 23% 17%	1763
	11th to 20th	Day	22% 3 9% 20% 25% 21%	1800
	21st to end	Day	29% 3 6% 19% 28% 15%	1800
December	1st to 10th	Day	40% 2 5% 17% 24% 12%	2940
	11th to 20th	Day	32% 2 8% 16% 28% 15%	3389
	21st to end	Day	21% 2 3 20% 32% 22%	4488
January	1st to 10th	Night	26% 2 5% 20% 30% 16%	5888
	11th to 20th	Night	17% 1 4 21% 31% 25%	5762
	21st to end	Night	36% 2 5% 14% 24% 19%	5984
February	1st to 10th	Night	28% 2 6% 14% 26% 24%	2400
	11th to 20th	Night	32% 2 6% 17% 21% 21%	2338
	21st to end	Night	27% 3 6% 12% 26% 25%	1980
March	1st to 10th	Night	22% 3 5% 16% 28% 26%	5357
	11th to 20th	Night	29% 2 7% 12% 16% 34%	4980
	21st to end	Night	24% 4 8% 17% 26% 22%	5467
April	1st to 10th	Night	23% 4 9% 22% 18% 24%	2075
	11th to 20th	Night	26% 4 9% 19% 21% 20%	1977
	21st to end	Night	21% 4 11% 24% 26% 14%	2000
May	1st to 10th	Night	31% 4 9% 19% 19% 18%	4084
	11th to 20th	Night	24% 4 11% 18% 24% 19%	4054
	21st to end	Night	27% 5% 14% 24% 17% 13%	4452
June	1st to 10th	Night	17% 5% 17% 33% 24% 4	1796
	11th to 20th	Night	15% 5% 13% 30% 27% 10%	1710
	21st to end	Night	17% 5% 14% 27% 27% 9%	1710
July	1st to 10th	Night	20% 5% 17% 38% 19% 2	2805
	11th to 20th	Night	9% 3 12% 40% 30% 6%	2796
	21st to end	Night	17% 3 12% 38% 28% 2	3054
August	1st to 10th	Night	13% 3 10% 28% 41% 5%	3144
	11th to 20th	Night	16% 4 15% 40% 22% 2	3143
	21st to end	Night	21% 7% 22% 24% 22% 4	3487
September	1st to 10th	Night	26% 7% 12% 23% 22% 11%	2025
	11th to 20th	Night	27% 6% 15% 22% 23% 7%	2090
	21st to end	Night	23% 3 11% 25% 20% 17%	2090
October	1st to 10th	Night	11% 3 7% 15% 23% 41%	3480
	11th to 20th	Night	17% 3 10% 17% 24% 29%	3460
	21st to end	Night	15% 2 4 26% 36% 17%	4058
November	1st to 10th	Night	23% 3 8% 22% 24% 20%	2530
	11th to 20th	Night	21% 4 7% 22% 26% 22%	2600
	21st to end	Night	30% 3 6% 17% 27% 17%	2600
December	1st to 10th	Night	37% 2 5% 19% 21% 15%	4250
	11th to 20th	Night	26% 3 7% 18% 28% 17%	4817
	21st to end	Night	23% 2 4 23% 28% 20%	6490

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% 14% 43% 24% 8% 1	300
	11th to 20th	Day	12% 15% 34% 30% 7% 3	300
	21st to end	Day	13% 13% 38% 27% 8% 1	330
February	1st to 10th	Day	15% 14% 39% 23% 9% 1	300
	11th to 20th	Day	18% 13% 43% 23% 3	300
	21st to end	Day	14% 12% 47% 23% 3	249
March	1st to 10th	Day	15% 12% 44% 24% 5%	300
	11th to 20th	Day	18% 14% 37% 22% 9% 1	300
	21st to end	Day	10% 17% 47% 20% 6% 1	330
April	1st to 10th	Day	11% 15% 42% 25% 6%	420
	11th to 20th	Day	11% 18% 43% 23% 5%	492
	21st to end	Day	11% 15% 39% 27% 7% 1	500
May	1st to 10th	Day	9% 10% 43% 31% 6%	698
	11th to 20th	Day	12% 8% 34% 33% 11% 2	725
	21st to end	Day	8% 5% 34% 39% 12% 1	789
June	1st to 10th	Day	3 2 21% 50% 20% 3	726
	11th to 20th	Day	3 2 26% 46% 19% 3	680
	21st to end	Day	3 2 12% 43% 32% 8%	680
July	1st to 10th	Day	5% 1 8% 40% 35% 11%	671
	11th to 20th	Day	2 4 42% 43% 9%	668
	21st to end	Day	2 1 4 36% 42% 15%	702
August	1st to 10th	Day	1 3 29% 48% 17%	487
	11th to 20th	Day	9% 1 3 31% 42% 15%	477
	21st to end	Day	4 1 5% 28% 44% 18%	539
September	1st to 10th	Day	10% 1 4 28% 38% 19%	362
	11th to 20th	Day	5% 2 6% 25% 43% 20%	300
	21st to end	Day	13% 2 7% 28% 34% 15%	300
October	1st to 10th	Day	2 4 13% 36% 33% 11%	300
	11th to 20th	Day	11% 2 24% 42% 19% 3	300
	21st to end	Day	14% 8% 27% 30% 18% 4	330
November	1st to 10th	Day	16% 6% 32% 31% 13% 1	300
	11th to 20th	Day	13% 13% 29% 26% 14% 6%	300
	21st to end	Day	13% 16% 39% 20% 9% 3	300
December	1st to 10th	Day	16% 9% 30% 34% 9% 2	300
	11th to 20th	Day	7% 14% 38% 29% 12% 1	300
	21st to end	Day	14% 12% 39% 27% 7%	330
January	1st to 10th	Night	9% 9% 43% 31% 7%	1000
	11th to 20th	Night	11% 10% 38% 35% 6%	940
	21st to end	Night	12% 10% 43% 29% 6%	880
February	1st to 10th	Night	11% 9% 50% 25% 5%	800
	11th to 20th	Night	13% 15% 45% 24% 3	784
	21st to end	Night	12% 16% 48% 21% 2	660
March	1st to 10th	Night	11% 14% 51% 20% 4	796
	11th to 20th	Night	13% 12% 46% 26% 3	600
	21st to end	Night	7% 13% 55% 22% 3	660
April	1st to 10th	Night	6% 12% 56% 23% 3	600
	11th to 20th	Night	8% 12% 55% 22% 3	593
	21st to end	Night	8% 12% 55% 22% 4	600
May	1st to 10th	Night	6% 10% 53% 28% 3	600
	11th to 20th	Night	9% 6% 45% 32% 8%	591
	21st to end	Night	4 5% 47% 35% 8%	650
June	1st to 10th	Night	3 3 33% 51% 10% 1	599
	11th to 20th	Night	2 2 41% 49% 6%	570
	21st to end	Night	3 1 21% 56% 18% 1	570
July	1st to 10th	Night	6% 2 10% 56% 25% 2	555
	11th to 20th	Night	3 13% 59% 25% 1	552
	21st to end	Night	1 1 8% 63% 26% 1	624
August	1st to 10th	Night	1 1 8% 58% 31% 2	567
	11th to 20th	Night	5% 1 7% 54% 29% 3	564
	21st to end	Night	3 2 8% 46% 36% 4	627
September	1st to 10th	Night	8% 2 9% 47% 32% 2	570
	11th to 20th	Night	9% 8% 48% 32% 2	570
	21st to end	Night	11% 1 16% 48% 23% 2	570
October	1st to 10th	Night	4 2 25% 48% 20%	570
	11th to 20th	Night	9% 5% 30% 42% 13% 1	570
	21st to end	Night	10% 7% 32% 36% 13% 2	798
November	1st to 10th	Night	10% 6% 33% 37% 13% 1	970
	11th to 20th	Night	6% 7% 37% 36% 12% 2	1000
	21st to end	Night	12% 11% 37% 31% 9% 1	1000
December	1st to 10th	Night	11% 7% 38% 36% 7%	1000
	11th to 20th	Night	11% 11% 39% 33% 7%	1000
	21st to end	Night	11% 9% 41% 33% 6%	1100

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