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 2026-10-14.

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 2 4 3 3 3 3 5% 7% 59%	148889
	Night	7% 4 1 4 3 3 3 3 5% 7% 61%	143298
Transparency	Day	25% 3 9% 23% 25% 14%	152759
	Night	23% 3 9% 23% 26% 16%	147483
Seeing	Day	8% 7% 25% 33% 21% 6%	17535
	Night	8% 7% 35% 37% 12% 1	27388

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	7% 5% 2 4 2 2 3 2 4 6% 64%	5289
	11th to 20th	Day	6% 5% 2 3 3 2 2 2 4 6% 67%	5532
	21st to end	Day	14% 6% 2 6% 3 2 3 2 4 5% 52%	6468
February	1st to 10th	Day	9% 5% 3 5% 3 3 3 2 4 6% 57%	2100
	11th to 20th	Day	10% 6% 2 5% 2 4 3 3 5% 5% 55%	2049
	21st to end	Day	9% 5% 2 4 2 2 3 3 5% 6% 60%	1730
March	1st to 10th	Day	6% 5% 2 4 3 3 4 3 4 6% 61%	4600
	11th to 20th	Day	13% 4 1 3 2 2 3 3 4 5% 59%	5090
	21st to end	Day	4 5% 2 5% 4 3 4 3 5% 8% 58%	5583
April	1st to 10th	Day	8% 4 2 4 3 3 3 3 6% 7% 59%	2436
	11th to 20th	Day	10% 5% 2 5% 3 4 3 3 5% 6% 54%	2494
	21st to end	Day	9% 5% 2 4 3 2 3 2 3 6% 61%	2520
May	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 50%	7612
	11th to 20th	Day	10% 5% 2 6% 4 4 4 3 5% 6% 51%	7627
	21st to end	Day	8% 5% 2 5% 4 4 4 4 7% 9% 47%	8392
June	1st to 10th	Day	3 3 1 3 3 4 4 4 6% 9% 61%	2730
	11th to 20th	Day	3 2 1 3 3 3 4 4 6% 8% 62%	2730
	21st to end	Day	4 3 1 3 4 4 4 4 7% 8% 58%	2730
July	1st to 10th	Day	7% 4 2 5% 4 4 4 4 6% 9% 52%	7680
	11th to 20th	Day	6% 3 2 5% 4 4 5% 4 6% 9% 52%	7680
	21st to end	Day	2 2 1 2 2 2 3 3 4 8% 71%	8220
August	1st to 10th	Day	1 1 2 2 3 3 3 4 6% 76%	5545
	11th to 20th	Day	3 2 1 2 2 2 3 3 4 8% 72%	5483
	21st to end	Day	2 2 1 4 4 4 5% 4 5% 8% 61%	6079
September	1st to 10th	Day	8% 5% 2 6% 4 4 5% 3 5% 8% 51%	2378
	11th to 20th	Day	9% 3 2 4 5% 4 4 3 6% 8% 53%	2310
	21st to end	Day	9% 3 1 4 3 4 3 4 5% 7% 56%	2310
October	1st to 10th	Day	5% 4 1 3 3 3 3 3 4 6% 65%	2310
	11th to 20th	Day	7% 4 1 3 3 3 3 2 4 6% 65%	2233
	21st to end	Day	10% 3 1 4 2 2 2 2 3 4 67%	2218
November	1st to 10th	Day	8% 3 2 4 2 3 2 2 4 6% 63%	1853
	11th to 20th	Day	6% 4 1 3 2 2 3 3 4 6% 65%	1890
	21st to end	Day	9% 5% 2 4 3 3 3 3 4 6% 57%	1890
December	1st to 10th	Day	16% 7% 3 6% 3 3 2 2 3 5% 50%	3030
	11th to 20th	Day	7% 5% 2 5% 4 4 5% 3 5% 6% 54%	3481
	21st to end	Day	8% 4 1 3 2 1 1 2 2 4 70%	4587
January	1st to 10th	Night	6% 5% 2 4 3 3 3 3 4 5% 63%	7634
	11th to 20th	Night	4 3 1 3 2 2 2 2 3 5% 73%	7500
	21st to end	Night	16% 6% 2 5% 3 3 2 2 4 6% 50%	7746
February	1st to 10th	Night	8% 5% 1 3 3 3 3 2 4 6% 60%	2520
	11th to 20th	Night	11% 6% 2 4 4 3 3 2 4 6% 55%	2458
	21st to end	Night	8% 5% 1 4 3 2 3 3 4 6% 61%	2076
March	1st to 10th	Night	5% 4 1 4 3 3 3 3 5% 7% 62%	5477
	11th to 20th	Night	12% 4 1 3 2 2 3 3 4 6% 59%	5090
	21st to end	Night	4 4 2 4 3 3 4 3 5% 7% 61%	5588
April	1st to 10th	Night	5% 5% 1 4 3 3 3 3 6% 7% 60%	2179
	11th to 20th	Night	8% 4 1 4 3 3 3 3 4 7% 58%	2077
	21st to end	Night	6% 5% 1 3 3 2 3 3 4 6% 64%	2100
May	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 51%	5270
	11th to 20th	Night	6% 3 1 4 3 3 3 3 6% 8% 59%	5265
	21st to end	Night	6% 4 2 5% 3 3 5% 4 7% 11% 49%	5793
June	1st to 10th	Night	3 2 1 3 3 3 4 4 7% 10% 61%	1890
	11th to 20th	Night	2 2 1 2 2 3 4 4 5% 9% 65%	1890
	21st to end	Night	2 2 1 2 3 2 4 3 7% 9% 65%	1890
July	1st to 10th	Night	5% 3 1 5% 3 3 5% 3 6% 11% 54%	5308
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 9% 63%	5307
	21st to end	Night	2 2 1 2 2 3 2 3 4 6% 74%	5882
August	1st to 10th	Night	1 1 1 2 2 2 3 2 4 6% 76%	4625
	11th to 20th	Night	2 1 1 2 2 2 2 3 5% 9% 71%	4583
	21st to end	Night	2 2 1 3 3 3 4 3 6% 10% 63%	5069
September	1st to 10th	Night	7% 4 1 5% 3 4 4 4 5% 9% 54%	2239
	11th to 20th	Night	8% 3 1 4 3 4 4 3 6% 9% 55%	2310
	21st to end	Night	8% 3 1 4 3 3 4 3 5% 8% 59%	2310
October	1st to 10th	Night	5% 3 1 3 2 2 4 3 4 6% 66%	2310
	11th to 20th	Night	9% 3 1 3 2 2 3 3 3 4 67%	2233
	21st to end	Night	7% 4 1 4 2 2 2 3 4 4 67%	2600
November	1st to 10th	Night	9% 3 1 3 2 2 3 2 3 5% 67%	2660
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 6% 66%	2730
	21st to end	Night	10% 5% 1 5% 3 4 4 3 5% 6% 55%	2730
December	1st to 10th	Night	15% 6% 2 5% 3 3 3 3 5% 6% 51%	4380
	11th to 20th	Night	6% 5% 2 5% 3 3 3 3 4 7% 59%	4946
	21st to end	Night	6% 5% 1 4 2 2 3 3 4 6% 64%	6633

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	24% 2 4 25% 29% 16%	5290
	11th to 20th	Day	22% 2 5% 20% 33% 19%	5532
	21st to end	Day	36% 2 4 16% 26% 16%	6468
February	1st to 10th	Day	30% 2 5% 15% 24% 23%	2100
	11th to 20th	Day	32% 3 7% 21% 21% 17%	2059
	21st to end	Day	28% 2 4 15% 31% 21%	1730
March	1st to 10th	Day	27% 2 5% 13% 21% 32%	4600
	11th to 20th	Day	30% 2 6% 9% 20% 34%	5090
	21st to end	Day	25% 3 6% 17% 27% 21%	5585
April	1st to 10th	Day	25% 4 8% 20% 23% 20%	2436
	11th to 20th	Day	32% 3 8% 16% 22% 19%	2494
	21st to end	Day	27% 3 7% 20% 25% 17%	2520
May	1st to 10th	Day	35% 3 9% 15% 21% 17%	7650
	11th to 20th	Day	33% 4 6% 14% 28% 15%	7631
	21st to end	Day	33% 5% 13% 23% 15% 10%	8405
June	1st to 10th	Day	20% 4 14% 31% 24% 6%	2730
	11th to 20th	Day	20% 5% 12% 26% 28% 10%	2730
	21st to end	Day	23% 5% 14% 27% 24% 8%	2730
July	1st to 10th	Day	28% 5% 13% 33% 19% 1	7616
	11th to 20th	Day	28% 6% 17% 30% 16% 2	7611
	21st to end	Day	14% 4 12% 33% 34% 4	8039
August	1st to 10th	Day	11% 3 10% 36% 32% 8%	5532
	11th to 20th	Day	13% 3 14% 46% 23% 2	5519
	21st to end	Day	20% 7% 14% 26% 29% 5%	6078
September	1st to 10th	Day	32% 5% 9% 23% 21% 9%	2378
	11th to 20th	Day	31% 5% 13% 24% 21% 6%	2310
	21st to end	Day	28% 4 10% 22% 22% 14%	2310
October	1st to 10th	Day	13% 3 6% 14% 26% 37%	3700
	11th to 20th	Day	21% 4 6% 20% 23% 27%	3601
	21st to end	Day	17% 2 4 23% 40% 14%	3556
November	1st to 10th	Day	24% 3 7% 23% 25% 18%	1853
	11th to 20th	Day	22% 3 9% 20% 25% 21%	1890
	21st to end	Day	30% 3 5% 19% 28% 15%	1890
December	1st to 10th	Day	39% 2 5% 17% 24% 13%	3030
	11th to 20th	Day	31% 2 7% 15% 28% 16%	3479
	21st to end	Day	20% 2 3 21% 32% 21%	4587
January	1st to 10th	Night	25% 2 4 23% 31% 16%	7635
	11th to 20th	Night	17% 1 4 20% 32% 26%	7500
	21st to end	Night	38% 2 5% 13% 25% 17%	7746
February	1st to 10th	Night	27% 2 6% 14% 27% 23%	2520
	11th to 20th	Night	32% 3 6% 17% 22% 21%	2458
	21st to end	Night	26% 3 6% 12% 28% 25%	2076
March	1st to 10th	Night	22% 3 5% 17% 28% 26%	5477
	11th to 20th	Night	28% 2 7% 12% 17% 34%	5090
	21st to end	Night	23% 4 8% 17% 26% 22%	5588
April	1st to 10th	Night	24% 4 10% 22% 18% 23%	2179
	11th to 20th	Night	27% 5% 10% 19% 20% 19%	2077
	21st to end	Night	22% 4 11% 24% 25% 14%	2100
May	1st to 10th	Night	32% 4 11% 20% 18% 14%	5297
	11th to 20th	Night	24% 4 9% 20% 28% 16%	5266
	21st to end	Night	29% 5% 17% 24% 14% 11%	5805
June	1st to 10th	Night	17% 5% 17% 33% 24% 4	1890
	11th to 20th	Night	16% 5% 14% 30% 26% 9%	1890
	21st to end	Night	16% 5% 14% 29% 27% 8%	1890
July	1st to 10th	Night	25% 5% 16% 35% 19% 1	5265
	11th to 20th	Night	20% 5% 18% 33% 21% 3	5256
	21st to end	Night	12% 3 12% 37% 33% 3	5752
August	1st to 10th	Night	11% 2 10% 32% 37% 9%	4614
	11th to 20th	Night	12% 3 16% 46% 22% 2	4613
	21st to end	Night	17% 6% 16% 26% 30% 5%	5069
September	1st to 10th	Night	26% 7% 12% 24% 22% 10%	2239
	11th to 20th	Night	26% 6% 15% 23% 23% 7%	2310
	21st to end	Night	24% 4 11% 25% 21% 16%	2310
October	1st to 10th	Night	12% 3 7% 15% 24% 40%	3700
	11th to 20th	Night	17% 3 10% 18% 24% 28%	3603
	21st to end	Night	15% 2 4 25% 36% 18%	4188
November	1st to 10th	Night	22% 2 8% 21% 26% 21%	2660
	11th to 20th	Night	20% 3 7% 21% 27% 22%	2730
	21st to end	Night	31% 3 6% 17% 27% 17%	2730
December	1st to 10th	Night	36% 2 5% 19% 22% 15%	4380
	11th to 20th	Night	26% 3 7% 18% 28% 18%	4947
	21st to end	Night	23% 2 4 23% 28% 20%	6633

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	9% 15% 43% 24% 8% 1	330
	11th to 20th	Day	12% 15% 33% 31% 8% 3	330
	21st to end	Day	13% 13% 39% 26% 8% 1	363
February	1st to 10th	Day	14% 13% 38% 24% 10% 1	330
	11th to 20th	Day	16% 13% 44% 24% 4	330
	21st to end	Day	13% 12% 47% 24% 4	273
March	1st to 10th	Day	14% 12% 44% 25% 5%	330
	11th to 20th	Day	17% 13% 37% 23% 9% 1	330
	21st to end	Day	9% 15% 44% 23% 7% 1	363
April	1st to 10th	Day	11% 14% 42% 26% 7%	456
	11th to 20th	Day	12% 18% 44% 21% 4	532
	21st to end	Day	13% 15% 39% 27% 6% 1	540
May	1st to 10th	Day	10% 10% 42% 31% 6%	748
	11th to 20th	Day	12% 10% 34% 32% 11% 2	775
	21st to end	Day	9% 6% 34% 38% 13% 1	844
June	1st to 10th	Day	3 2 22% 50% 20% 3	780
	11th to 20th	Day	3 2 27% 46% 19% 3	780
	21st to end	Day	3 1 13% 44% 31% 7%	780
July	1st to 10th	Day	6% 2 9% 40% 34% 9%	771
	11th to 20th	Day	3 1 6% 42% 40% 8%	768
	21st to end	Day	2 1 4 37% 41% 15%	808
August	1st to 10th	Day	1 1 3 30% 48% 17%	567
	11th to 20th	Day	7% 1 4 31% 42% 15%	557
	21st to end	Day	3 1 5% 28% 44% 18%	627
September	1st to 10th	Day	8% 2 4 27% 40% 19%	428
	11th to 20th	Day	4 2 5% 27% 44% 17%	360
	21st to end	Day	12% 2 8% 31% 33% 14%	360
October	1st to 10th	Day	3 4 12% 34% 33% 13%	360
	11th to 20th	Day	10% 2 23% 43% 19% 3	339
	21st to end	Day	13% 9% 27% 31% 17% 4	363
November	1st to 10th	Day	14% 6% 31% 31% 16% 2	330
	11th to 20th	Day	12% 12% 28% 29% 13% 6%	330
	21st to end	Day	13% 16% 38% 20% 10% 3	330
December	1st to 10th	Day	15% 8% 33% 32% 9% 2	330
	11th to 20th	Day	8% 13% 36% 31% 12% 1	330
	21st to end	Day	13% 11% 38% 29% 8% 1	363
January	1st to 10th	Night	9% 9% 44% 31% 7%	1050
	11th to 20th	Night	11% 10% 38% 35% 6%	987
	21st to end	Night	12% 10% 43% 29% 6%	924
February	1st to 10th	Night	11% 9% 49% 25% 5%	840
	11th to 20th	Night	13% 16% 45% 24% 3	824
	21st to end	Night	11% 16% 49% 22% 2	692
March	1st to 10th	Night	11% 14% 51% 19% 5%	836
	11th to 20th	Night	13% 12% 47% 25% 3	630
	21st to end	Night	7% 12% 54% 24% 3	693
April	1st to 10th	Night	6% 12% 55% 24% 3	630
	11th to 20th	Night	9% 13% 55% 21% 3	623
	21st to end	Night	8% 12% 54% 21% 4	630
May	1st to 10th	Night	6% 10% 52% 28% 3	630
	11th to 20th	Night	9% 7% 44% 31% 8%	621
	21st to end	Night	5% 5% 46% 35% 8%	683
June	1st to 10th	Night	3 3 33% 50% 10% 1	630
	11th to 20th	Night	2 2 40% 49% 6%	630
	21st to end	Night	3 1 23% 55% 17%	630
July	1st to 10th	Night	6% 2 11% 57% 23% 1	615
	11th to 20th	Night	4 1 12% 59% 24%	612
	21st to end	Night	1 1 8% 64% 25% 2	690
August	1st to 10th	Night	1 1 8% 58% 30% 1	627
	11th to 20th	Night	4 1 7% 56% 28% 3	624
	21st to end	Night	3 1 9% 47% 36% 4	693
September	1st to 10th	Night	8% 2 9% 49% 31% 2	630
	11th to 20th	Night	9% 9% 51% 29% 2	630
	21st to end	Night	10% 1 17% 50% 21% 2	630
October	1st to 10th	Night	5% 2 23% 48% 21%	630
	11th to 20th	Night	9% 5% 30% 43% 13%	609
	21st to end	Night	10% 7% 32% 37% 12% 2	840
November	1st to 10th	Night	10% 6% 33% 38% 13% 1	1020
	11th to 20th	Night	6% 7% 37% 36% 12% 1	1050
	21st to end	Night	11% 11% 37% 31% 9% 1	1050
December	1st to 10th	Night	11% 7% 39% 36% 7%	1050
	11th to 20th	Night	11% 10% 39% 33% 7%	1050
	21st to end	Night	10% 8% 41% 33% 7%	1155

Page generated at 2026-10-11 04:06:32. Version ac07

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