Sky Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sky Ranch Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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	2 3 1 3 3 3 4 4 6% 8% 64%	151764
	Night	2 3 1 4 3 4 4 4 6% 8% 60%	147998
Transparency	Day	18% 5% 10% 22% 28% 18%	146779
	Night	20% 5% 10% 21% 28% 16%	143991
Seeing	Day	5% 11% 27% 29% 21% 6%	17563
	Night	4 7% 29% 41% 18% 1	27618

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	1 5% 2 5% 3 3 4 3 5% 8% 61%	5241
	11th to 20th	Day	1 3 1 2 1 1 2 2 3 5% 77%	5340
	21st to end	Day	2 4 2 4 3 3 4 4 5% 7% 62%	5844
February	1st to 10th	Day	1 4 1 5% 3 3 3 3 5% 7% 63%	1918
	11th to 20th	Day	2 3 1 4 3 3 4 3 5% 8% 62%	2049
	21st to end	Day	1 3 2 3 3 3 3 3 4 7% 67%	1730
March	1st to 10th	Day	1 4 1 2 2 2 3 3 5% 7% 69%	4853
	11th to 20th	Day	2 3 2 4 4 4 4 4 6% 9% 58%	5110
	21st to end	Day	1 3 1 4 3 3 4 4 6% 8% 64%	5784
April	1st to 10th	Day	2 2 1 3 3 3 3 3 5% 8% 67%	2520
	11th to 20th	Day	2 3 1 4 3 3 3 3 5% 7% 65%	2491
	21st to end	Day	2 3 1 2 2 3 3 3 5% 7% 70%	2520
May	1st to 10th	Day	3 2 1 3 4 4 4 4 6% 7% 62%	7057
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 7% 72%	7294
	21st to end	Day	2 2 1 2 2 3 3 3 4 7% 72%	9026
June	1st to 10th	Day	1 2 1 3 2 3 4 4 7% 9% 64%	2940
	11th to 20th	Day	1 1 1 3 2 2 3 3 5% 7% 72%	2940
	21st to end	Day	4 3 1 3 3 3 4 4 6% 8% 60%	2940
July	1st to 10th	Day	2 2 1 3 4 4 4 4 7% 7% 63%	8095
	11th to 20th	Day	1 2 1 4 4 5% 6% 6% 9% 12% 50%	7650
	21st to end	Day	2 3 1 4 4 5% 7% 6% 9% 12% 46%	8304
August	1st to 10th	Day	2 2 1 3 3 4 4 5% 8% 12% 56%	5745
	11th to 20th	Day	2 2 1 3 3 3 5% 4 6% 10% 59%	5486
	21st to end	Day	2 3 1 5% 4 6% 6% 6% 8% 11% 48%	6093
September	1st to 10th	Day	5% 4 1 3 3 4 5% 4 7% 9% 55%	2400
	11th to 20th	Day	4 3 1 3 4 3 5% 4 6% 7% 61%	2400
	21st to end	Day	2 2 1 3 2 2 4 2 4 7% 70%	2175
October	1st to 10th	Day	3 2 1 2 2 2 3 2 4 5% 74%	3270
	11th to 20th	Day	3 3 1 3 3 3 3 3 6% 6% 67%	3250
	21st to end	Day	2 2 1 2 2 2 2 2 3 5% 79%	3548
November	1st to 10th	Day	2 2 1 1 2 2 3 3 5% 7% 73%	2003
	11th to 20th	Day	1 2 1 2 2 2 3 3 4 7% 72%	1890
	21st to end	Day	2 3 1 3 3 3 3 2 5% 7% 66%	1890
December	1st to 10th	Day	1 2 2 2 2 2 2 3 6% 78%	2770
	11th to 20th	Day	1 2 1 4 3 3 4 2 5% 7% 68%	3106
	21st to end	Day	1 3 1 3 3 3 3 3 4 6% 70%	4092
January	1st to 10th	Night	1 6% 3 6% 4 4 5% 4 6% 7% 54%	7723
	11th to 20th	Night	1 3 1 2 2 2 3 2 3 5% 75%	7650
	21st to end	Night	3 5% 2 4 2 3 4 4 6% 8% 61%	8370
February	1st to 10th	Night	1 4 2 4 2 3 3 3 5% 7% 66%	2699
	11th to 20th	Night	3 5% 2 4 4 4 3 4 5% 8% 59%	2458
	21st to end	Night	2 4 2 4 3 3 4 2 4 5% 68%	2076
March	1st to 10th	Night	2 4 1 3 3 3 4 3 5% 5% 67%	5237
	11th to 20th	Night	3 4 2 5% 4 4 5% 4 5% 6% 59%	5110
	21st to end	Night	2 2 1 4 3 3 3 3 5% 7% 67%	5423
April	1st to 10th	Night	3 3 1 3 3 3 3 3 5% 7% 64%	2100
	11th to 20th	Night	3 4 2 4 3 3 4 3 5% 6% 63%	2077
	21st to end	Night	3 2 1 3 3 3 3 3 5% 6% 68%	2100
May	1st to 10th	Night	4 4 1 4 3 4 5% 4 7% 8% 55%	5890
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 7% 64%	5565
	21st to end	Night	2 2 1 4 3 3 4 3 5% 7% 65%	5181
June	1st to 10th	Night	2 3 1 4 4 5% 4 5% 8% 10% 55%	1680
	11th to 20th	Night	2 2 1 3 2 3 4 4 5% 7% 67%	1680
	21st to end	Night	3 4 2 5% 5% 5% 4 5% 8% 8% 52%	1680
July	1st to 10th	Night	2 3 1 6% 5% 4 5% 5% 7% 8% 54%	4874
	11th to 20th	Night	3 3 2 5% 5% 6% 7% 7% 11% 13% 39%	5340
	21st to end	Night	5% 4 2 7% 6% 7% 7% 7% 10% 13% 33%	5805
August	1st to 10th	Night	2 3 2 8% 6% 7% 8% 6% 8% 10% 40%	4285
	11th to 20th	Night	2 3 1 5% 5% 6% 7% 6% 9% 13% 42%	4583
	21st to end	Night	4 3 3 7% 6% 7% 7% 6% 10% 13% 33%	5083
September	1st to 10th	Night	4 3 1 5% 5% 5% 5% 5% 8% 11% 48%	2000
	11th to 20th	Night	4 3 2 5% 3 4 4 4 7% 7% 58%	2000
	21st to end	Night	3 3 1 3 3 3 3 3 4 7% 67%	2221
October	1st to 10th	Night	3 2 1 3 2 2 2 2 4 5% 74%	3920
	11th to 20th	Night	4 3 1 4 3 2 4 3 5% 7% 64%	3898
	21st to end	Night	2 1 1 1 2 1 2 2 3 6% 79%	4252
November	1st to 10th	Night	1 2 1 2 2 2 2 3 4 5% 77%	2492
	11th to 20th	Night	1 2 1 2 2 2 2 2 5% 6% 73%	2730
	21st to end	Night	3 4 2 4 4 3 3 3 4 6% 65%	2730
December	1st to 10th	Night	1 2 1 2 1 2 2 2 4 7% 75%	4637
	11th to 20th	Night	2 2 1 2 2 3 3 2 5% 9% 69%	5321
	21st to end	Night	2 3 1 4 3 3 3 3 4 7% 68%	7128

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% 4 9% 36% 27% 4	5244
	11th to 20th	Day	11% 3 8% 27% 37% 14%	5340
	21st to end	Day	20% 4 6% 17% 40% 12%	5844
February	1st to 10th	Day	20% 4 11% 24% 29% 12%	1918
	11th to 20th	Day	20% 5% 7% 26% 30% 12%	2059
	21st to end	Day	16% 3 5% 20% 33% 21%	1730
March	1st to 10th	Day	14% 4 4 13% 38% 27%	4853
	11th to 20th	Day	18% 7% 7% 21% 32% 15%	5110
	21st to end	Day	16% 3 6% 29% 36% 10%	5784
April	1st to 10th	Day	16% 4 7% 20% 34% 20%	2520
	11th to 20th	Day	19% 4 6% 18% 30% 23%	2494
	21st to end	Day	14% 4 7% 20% 37% 18%	2520
May	1st to 10th	Day	19% 5% 10% 19% 29% 18%	7057
	11th to 20th	Day	12% 4 7% 14% 30% 34%	7297
	21st to end	Day	13% 4 8% 16% 19% 40%	9027
June	1st to 10th	Day	15% 5% 9% 22% 25% 24%	2940
	11th to 20th	Day	13% 3 6% 14% 23% 42%	2940
	21st to end	Day	21% 5% 9% 19% 21% 25%	2940
July	1st to 10th	Day	19% 7% 9% 14% 20% 32%	8029
	11th to 20th	Day	21% 9% 20% 28% 14% 7%	7581
	21st to end	Day	26% 9% 20% 30% 12% 3	8120
August	1st to 10th	Day	22% 10% 19% 24% 16% 8%	4218
	11th to 20th	Day	19% 6% 12% 26% 22% 14%	4002
	21st to end	Day	28% 10% 18% 23% 17% 3	4420
September	1st to 10th	Day	24% 7% 12% 22% 24% 11%	2400
	11th to 20th	Day	23% 6% 9% 18% 26% 18%	2400
	21st to end	Day	15% 5% 6% 22% 32% 20%	2175
October	1st to 10th	Day	14% 4 4 13% 33% 32%	3270
	11th to 20th	Day	18% 4 7% 16% 31% 24%	3250
	21st to end	Day	10% 2 4 20% 40% 24%	3548
November	1st to 10th	Day	12% 2 6% 22% 41% 17%	2003
	11th to 20th	Day	13% 3 8% 24% 40% 12%	1890
	21st to end	Day	17% 4 6% 24% 32% 16%	1890
December	1st to 10th	Day	11% 3 7% 31% 39% 11%	2770
	11th to 20th	Day	17% 2 8% 31% 35% 7%	3104
	21st to end	Day	16% 3 10% 27% 33% 12%	4092
January	1st to 10th	Night	25% 4 10% 29% 25% 7%	7724
	11th to 20th	Night	12% 3 7% 26% 38% 13%	7650
	21st to end	Night	19% 4 7% 18% 41% 11%	8370
February	1st to 10th	Night	17% 4 10% 25% 29% 14%	2699
	11th to 20th	Night	21% 5% 9% 25% 26% 13%	2458
	21st to end	Night	17% 3 6% 16% 36% 22%	2076
March	1st to 10th	Night	17% 4 4 12% 39% 25%	5237
	11th to 20th	Night	22% 7% 6% 17% 32% 16%	5110
	21st to end	Night	14% 4 7% 26% 38% 12%	5423
April	1st to 10th	Night	18% 4 6% 20% 31% 22%	2100
	11th to 20th	Night	20% 3 6% 19% 26% 25%	2077
	21st to end	Night	16% 4 8% 20% 34% 17%	2100
May	1st to 10th	Night	23% 7% 10% 20% 22% 18%	5888
	11th to 20th	Night	18% 5% 7% 15% 24% 30%	5566
	21st to end	Night	17% 6% 11% 11% 14% 41%	5182
June	1st to 10th	Night	22% 7% 13% 17% 18% 23%	1680
	11th to 20th	Night	16% 4 6% 14% 21% 39%	1680
	21st to end	Night	25% 8% 12% 15% 16% 24%	1680
July	1st to 10th	Night	25% 7% 10% 18% 15% 25%	4832
	11th to 20th	Night	28% 10% 26% 22% 9% 6%	5286
	21st to end	Night	36% 11% 21% 21% 9% 1	5677
August	1st to 10th	Night	36% 12% 16% 17% 13% 6%	3136
	11th to 20th	Night	28% 11% 16% 18% 17% 11%	3343
	21st to end	Night	37% 10% 18% 22% 9% 4	3687
September	1st to 10th	Night	26% 10% 15% 20% 20% 10%	2000
	11th to 20th	Night	22% 8% 11% 17% 22% 19%	2000
	21st to end	Night	18% 5% 9% 18% 29% 21%	2221
October	1st to 10th	Night	14% 3 6% 15% 30% 32%	3920
	11th to 20th	Night	19% 5% 7% 14% 28% 27%	3898
	21st to end	Night	10% 2 4 22% 39% 23%	4252
November	1st to 10th	Night	11% 3 5% 21% 42% 18%	2492
	11th to 20th	Night	12% 3 5% 23% 41% 16%	2730
	21st to end	Night	21% 4 6% 20% 32% 17%	2730
December	1st to 10th	Night	11% 3 6% 34% 32% 14%	4637
	11th to 20th	Night	13% 3 7% 30% 37% 10%	5322
	21st to end	Night	17% 3 9% 27% 36% 8%	7128

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	5% 15% 38% 31% 10%	330
	11th to 20th	Day	8% 14% 38% 27% 12% 2	330
	21st to end	Day	3 9% 39% 34% 13% 2	363
February	1st to 10th	Day	4 17% 38% 28% 12% 1	330
	11th to 20th	Day	8% 19% 41% 26% 6%	330
	21st to end	Day	10% 21% 40% 22% 6% 1	273
March	1st to 10th	Day	6% 22% 33% 28% 10%	440
	11th to 20th	Day	10% 25% 31% 23% 9% 1	540
	21st to end	Day	9% 29% 32% 20% 9% 1	594
April	1st to 10th	Day	9% 22% 35% 23% 9% 2	540
	11th to 20th	Day	9% 26% 39% 17% 8% 1	532
	21st to end	Day	7% 24% 38% 23% 6% 1	540
May	1st to 10th	Day	6% 20% 39% 24% 8% 3	543
	11th to 20th	Day	6% 15% 32% 30% 14% 3	593
	21st to end	Day	4 12% 30% 36% 16% 2	844
June	1st to 10th	Day	2 7% 23% 39% 25% 5%	780
	11th to 20th	Day	2 6% 25% 35% 27% 4	780
	21st to end	Day	6% 4 22% 33% 28% 6%	780
July	1st to 10th	Day	3 2 15% 35% 33% 12%	723
	11th to 20th	Day	2 7% 28% 40% 23%	564
	21st to end	Day	3 1 7% 23% 42% 24%	619
August	1st to 10th	Day	3 2 10% 28% 38% 19%	567
	11th to 20th	Day	3 1 14% 31% 36% 15%	557
	21st to end	Day	4 1 11% 30% 37% 16%	620
September	1st to 10th	Day	4 3 16% 32% 31% 14%	530
	11th to 20th	Day	3 2 26% 36% 26% 8%	530
	21st to end	Day	4 6% 22% 33% 27% 8%	355
October	1st to 10th	Day	5% 10% 25% 24% 28% 9%	330
	11th to 20th	Day	6% 12% 29% 26% 21% 6%	330
	21st to end	Day	8% 11% 26% 29% 21% 5%	363
November	1st to 10th	Day	5% 14% 33% 32% 15% 2	330
	11th to 20th	Day	4 8% 28% 38% 21% 2	330
	21st to end	Day	8% 13% 35% 29% 14% 1	330
December	1st to 10th	Day	4 14% 34% 32% 14% 2	330
	11th to 20th	Day	6% 10% 35% 35% 12% 2	330
	21st to end	Day	7% 18% 36% 33% 6% 1	363
January	1st to 10th	Night	4 6% 34% 41% 14% 1	883
	11th to 20th	Night	4 5% 35% 41% 14% 1	840
	21st to end	Night	5% 8% 33% 40% 12% 2	924
February	1st to 10th	Night	4 8% 38% 40% 10% 1	840
	11th to 20th	Night	5% 11% 40% 36% 7%	824
	21st to end	Night	5% 10% 47% 34% 3	692
March	1st to 10th	Night	5% 10% 40% 36% 8%	840
	11th to 20th	Night	6% 13% 33% 38% 11%	840
	21st to end	Night	4 12% 38% 40% 8%	840
April	1st to 10th	Night	5% 13% 40% 37% 5%	630
	11th to 20th	Night	4 17% 37% 36% 6%	623
	21st to end	Night	5% 13% 42% 34% 6%	630
May	1st to 10th	Night	5% 9% 36% 40% 10%	630
	11th to 20th	Night	6% 9% 32% 39% 14% 1	621
	21st to end	Night	4 8% 35% 42% 12%	683
June	1st to 10th	Night	2 6% 26% 46% 19% 1	630
	11th to 20th	Night	2 3 29% 49% 15% 1	630
	21st to end	Night	4 4 19% 47% 24% 1	630
July	1st to 10th	Night	3 2 16% 46% 31% 2	615
	11th to 20th	Night	3 1 9% 38% 44% 4	612
	21st to end	Night	6% 1 8% 31% 47% 6%	690
August	1st to 10th	Night	3 1 10% 34% 46% 7%	627
	11th to 20th	Night	4 2 10% 44% 37% 3	624
	21st to end	Night	3 1 8% 43% 41% 5%	687
September	1st to 10th	Night	4 3 12% 49% 30% 2	600
	11th to 20th	Night	5% 3 14% 49% 29% 1	600
	21st to end	Night	4 4 20% 48% 22% 2	646
October	1st to 10th	Night	5% 4 22% 48% 21% 1	800
	11th to 20th	Night	4 5% 27% 46% 17% 2	800
	21st to end	Night	5% 5% 26% 41% 20% 2	880
November	1st to 10th	Night	2 5% 27% 45% 19% 1	852
	11th to 20th	Night	2 6% 28% 44% 18% 2	1050
	21st to end	Night	5% 9% 31% 38% 16% 2	1050
December	1st to 10th	Night	4 9% 31% 43% 11% 1	1050
	11th to 20th	Night	5% 9% 33% 42% 11%	1050
	21st to end	Night	5% 9% 38% 41% 7%	1155

Page generated at 2026-08-27 02:07:15. Version ac07

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