Astro Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Astro Ranch 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	9% 5% 2 5% 3 3 4 3 5% 7% 54%	149243
	Night	9% 5% 2 4 3 3 3 3 5% 6% 58%	142889
Transparency	Day	29% 5% 10% 25% 23% 7%	152947
	Night	26% 5% 9% 25% 26% 9%	147252
Seeing	Day	11% 8% 25% 29% 20% 6%	17952
	Night	11% 9% 31% 34% 14% 1	27710

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% 6% 2 5% 3 3 4 3 5% 6% 54%	5289
	11th to 20th	Day	5% 5% 2 4 2 2 3 2 3 5% 68%	5310
	21st to end	Day	15% 8% 3 6% 3 2 3 2 3 5% 50%	5812
February	1st to 10th	Day	13% 8% 2 5% 4 3 3 3 4 6% 49%	1979
	11th to 20th	Day	13% 7% 2 5% 3 3 5% 2 4 6% 50%	2049
	21st to end	Day	15% 8% 2 5% 3 2 2 2 3 5% 52%	1730
March	1st to 10th	Day	13% 6% 2 6% 3 3 3 2 4 5% 54%	5035
	11th to 20th	Day	7% 5% 2 5% 4 4 3 3 4 6% 56%	5090
	21st to end	Day	11% 9% 3 6% 4 4 4 3 5% 7% 44%	5583
April	1st to 10th	Day	11% 5% 2 5% 4 3 3 3 5% 6% 53%	2310
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 5% 6% 48%	2427
	21st to end	Day	13% 6% 3 6% 5% 4 4 3 5% 6% 44%	2730
May	1st to 10th	Day	15% 8% 3 6% 5% 4 4 3 4 6% 43%	7617
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 5% 6% 47%	7627
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	8392
June	1st to 10th	Day	8% 5% 2 6% 4 5% 4 5% 6% 9% 46%	2730
	11th to 20th	Day	7% 4 1 4 4 4 5% 4 6% 9% 51%	2730
	21st to end	Day	4 4 1 4 3 3 4 3 6% 7% 60%	2730
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 4 6% 10% 47%	7680
	11th to 20th	Day	4 3 1 5% 4 4 5% 4 7% 10% 53%	7680
	21st to end	Day	3 2 1 3 3 3 3 3 5% 8% 66%	8337
August	1st to 10th	Day	2 2 1 3 2 3 3 3 5% 7% 69%	6014
	11th to 20th	Day	3 2 1 3 2 2 3 3 5% 8% 67%	5948
	21st to end	Day	3 2 1 4 3 3 4 3 6% 9% 62%	6081
September	1st to 10th	Day	9% 4 2 5% 3 3 4 3 4 7% 56%	2310
	11th to 20th	Day	10% 5% 2 6% 3 4 5% 3 4 6% 51%	2310
	21st to end	Day	8% 2 1 4 3 4 4 4 5% 5% 59%	2310
October	1st to 10th	Day	7% 4 1 4 3 3 4 3 4 6% 61%	2253
	11th to 20th	Day	8% 4 2 4 2 2 2 2 3 5% 66%	2030
	21st to end	Day	13% 6% 2 4 2 2 3 2 3 4 58%	2200
November	1st to 10th	Day	10% 5% 1 3 3 3 3 3 4 6% 59%	2042
	11th to 20th	Day	7% 4 2 3 2 2 2 2 3 7% 66%	1890
	21st to end	Day	12% 5% 2 4 3 3 2 2 4 5% 58%	1890
December	1st to 10th	Day	11% 8% 2 5% 3 3 3 3 4 5% 54%	3030
	11th to 20th	Day	12% 6% 2 6% 3 3 3 2 4 4 54%	3481
	21st to end	Day	17% 9% 2 6% 3 3 3 2 3 4 47%	4587
January	1st to 10th	Night	11% 6% 2 6% 4 3 3 2 5% 6% 52%	7634
	11th to 20th	Night	4 3 1 2 2 2 2 2 3 5% 72%	7680
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 4 47%	8402
February	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 3 5% 52%	2656
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	2458
	21st to end	Night	10% 7% 2 5% 3 3 3 2 5% 4 56%	2076
March	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 5% 5% 52%	5039
	11th to 20th	Night	7% 4 1 3 3 2 3 3 4 6% 63%	5090
	21st to end	Night	11% 6% 2 5% 4 3 4 4 5% 7% 50%	5588
April	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 7% 53%	2310
	11th to 20th	Night	11% 5% 2 4 3 4 3 3 5% 6% 54%	2135
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 7% 50%	1890
May	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 7% 46%	5266
	11th to 20th	Night	10% 4 2 4 3 3 4 3 7% 10% 51%	5265
	21st to end	Night	12% 5% 2 6% 3 4 4 4 6% 8% 46%	5793
June	1st to 10th	Night	5% 3 1 4 2 3 4 5% 6% 11% 56%	1890
	11th to 20th	Night	5% 2 2 4 4 3 3 3 6% 10% 58%	1890
	21st to end	Night	3 2 1 3 2 2 4 3 5% 9% 65%	1890
July	1st to 10th	Night	4 4 2 5% 4 4 5% 4 7% 11% 52%	5308
	11th to 20th	Night	2 2 1 4 2 3 4 3 6% 10% 64%	5307
	21st to end	Night	2 1 1 2 2 2 3 3 4 7% 73%	5770
August	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 7% 72%	4156
	11th to 20th	Night	2 1 1 3 1 2 2 3 4 8% 73%	4118
	21st to end	Night	3 2 1 3 2 3 3 3 5% 8% 68%	5038
September	1st to 10th	Night	7% 5% 2 4 3 3 3 3 4 6% 61%	2310
	11th to 20th	Night	8% 4 2 6% 4 3 3 3 5% 7% 54%	2310
	21st to end	Night	6% 4 2 3 3 3 3 3 5% 8% 60%	2310
October	1st to 10th	Night	7% 3 1 4 3 3 2 2 4 6% 64%	2343
	11th to 20th	Night	9% 4 2 4 3 2 2 3 2 5% 65%	2437
	21st to end	Night	14% 4 2 4 3 2 2 1 3 4 61%	2640
November	1st to 10th	Night	9% 4 1 3 2 3 3 3 3 5% 64%	2471
	11th to 20th	Night	6% 4 2 5% 3 3 4 2 4 5% 63%	2730
	21st to end	Night	12% 6% 2 5% 3 3 3 2 4 5% 57%	2730
December	1st to 10th	Night	8% 6% 2 4 2 3 3 2 4 5% 61%	4380
	11th to 20th	Night	10% 7% 2 6% 4 3 3 2 3 3 58%	4946
	21st to end	Night	15% 7% 2 5% 2 3 3 2 3 4 52%	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	30% 4 9% 27% 27% 4	5290
	11th to 20th	Day	21% 3 8% 19% 43% 7%	5310
	21st to end	Day	39% 3 4 21% 25% 8%	5812
February	1st to 10th	Day	37% 4 8% 20% 23% 7%	1979
	11th to 20th	Day	36% 4 7% 25% 20% 7%	2059
	21st to end	Day	37% 2 5% 17% 28% 12%	1730
March	1st to 10th	Day	34% 4 6% 15% 21% 20%	5035
	11th to 20th	Day	29% 5% 7% 19% 21% 18%	5090
	21st to end	Day	39% 6% 9% 25% 17% 4	5585
April	1st to 10th	Day	33% 4 11% 23% 19% 9%	2310
	11th to 20th	Day	37% 5% 9% 21% 21% 7%	2427
	21st to end	Day	40% 6% 10% 22% 18% 4	2730
May	1st to 10th	Day	42% 6% 10% 20% 18% 4	7655
	11th to 20th	Day	36% 7% 10% 22% 22% 3	7631
	21st to end	Day	42% 8% 13% 18% 12% 5%	8405
June	1st to 10th	Day	33% 9% 15% 26% 15% 2	2730
	11th to 20th	Day	28% 8% 15% 26% 21% 3	2730
	21st to end	Day	23% 8% 15% 27% 23% 4	2730
July	1st to 10th	Day	31% 8% 13% 32% 15% 1	7616
	11th to 20th	Day	24% 9% 16% 31% 19% 1	7611
	21st to end	Day	16% 6% 13% 40% 24% 1	8155
August	1st to 10th	Day	15% 5% 9% 27% 33% 12%	6000
	11th to 20th	Day	15% 5% 12% 37% 27% 3	5987
	21st to end	Day	19% 6% 13% 40% 19% 3	6080
September	1st to 10th	Day	27% 5% 11% 22% 24% 11%	2310
	11th to 20th	Day	34% 6% 9% 24% 20% 7%	2310
	21st to end	Day	25% 6% 9% 24% 27% 9%	2310
October	1st to 10th	Day	16% 4 6% 13% 38% 23%	3604
	11th to 20th	Day	17% 3 6% 21% 35% 18%	3270
	21st to end	Day	25% 4 8% 27% 28% 8%	3538
November	1st to 10th	Day	28% 4 8% 22% 29% 9%	2042
	11th to 20th	Day	22% 3 8% 26% 30% 11%	1890
	21st to end	Day	31% 3 6% 22% 28% 9%	1890
December	1st to 10th	Day	33% 4 7% 31% 21% 3	3030
	11th to 20th	Day	35% 3 5% 18% 30% 9%	3479
	21st to end	Day	41% 3 6% 24% 20% 5%	4587
January	1st to 10th	Night	32% 4 7% 27% 25% 5%	7635
	11th to 20th	Night	16% 3 8% 20% 38% 14%	7680
	21st to end	Night	40% 4 6% 15% 25% 9%	8402
February	1st to 10th	Night	35% 4 8% 21% 24% 9%	2656
	11th to 20th	Night	36% 4 8% 20% 24% 8%	2458
	21st to end	Night	32% 4 6% 17% 26% 15%	2076
March	1st to 10th	Night	34% 4 6% 16% 21% 19%	5039
	11th to 20th	Night	23% 4 7% 23% 18% 25%	5090
	21st to end	Night	33% 6% 10% 21% 27% 3	5588
April	1st to 10th	Night	31% 4 11% 21% 20% 13%	2310
	11th to 20th	Night	32% 6% 9% 21% 21% 11%	2135
	21st to end	Night	32% 8% 10% 25% 21% 5%	1890
May	1st to 10th	Night	38% 6% 13% 19% 18% 6%	5293
	11th to 20th	Night	28% 6% 14% 27% 19% 7%	5266
	21st to end	Night	34% 8% 17% 21% 15% 6%	5805
June	1st to 10th	Night	21% 9% 14% 32% 20% 3	1890
	11th to 20th	Night	22% 6% 15% 29% 24% 4	1890
	21st to end	Night	16% 4 16% 28% 29% 7%	1890
July	1st to 10th	Night	25% 8% 14% 33% 18% 2	5265
	11th to 20th	Night	15% 8% 18% 34% 24% 2	5256
	21st to end	Night	12% 4 13% 41% 28% 3	5641
August	1st to 10th	Night	11% 4 8% 28% 35% 13%	4146
	11th to 20th	Night	11% 4 8% 38% 35% 4	4145
	21st to end	Night	15% 5% 12% 40% 24% 5%	5038
September	1st to 10th	Night	24% 6% 10% 23% 26% 11%	2310
	11th to 20th	Night	30% 6% 9% 24% 22% 9%	2310
	21st to end	Night	22% 5% 8% 25% 29% 11%	2310
October	1st to 10th	Night	15% 3 6% 14% 37% 26%	3759
	11th to 20th	Night	17% 3 4 18% 36% 22%	3936
	21st to end	Night	23% 3 5% 31% 27% 11%	4252
November	1st to 10th	Night	25% 3 7% 22% 29% 13%	2471
	11th to 20th	Night	24% 4 7% 24% 29% 13%	2730
	21st to end	Night	31% 4 6% 20% 29% 10%	2730
December	1st to 10th	Night	27% 3 6% 30% 28% 6%	4380
	11th to 20th	Night	33% 3 5% 17% 28% 13%	4947
	21st to end	Night	36% 3 7% 24% 23% 7%	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	16% 15% 39% 23% 6% 1	330
	11th to 20th	Day	19% 19% 32% 23% 5% 1	330
	21st to end	Day	13% 12% 45% 20% 8% 1	363
February	1st to 10th	Day	19% 16% 39% 20% 7%	330
	11th to 20th	Day	19% 19% 44% 16% 2	330
	21st to end	Day	23% 16% 42% 14% 5%	273
March	1st to 10th	Day	19% 18% 40% 16% 7%	330
	11th to 20th	Day	20% 19% 41% 13% 6% 1	330
	21st to end	Day	16% 17% 47% 16% 4	363
April	1st to 10th	Day	16% 23% 32% 23% 5% 1	330
	11th to 20th	Day	18% 19% 41% 19% 3	465
	21st to end	Day	16% 18% 37% 20% 6% 1	750
May	1st to 10th	Day	15% 12% 35% 32% 6%	753
	11th to 20th	Day	15% 9% 33% 26% 14% 2	775
	21st to end	Day	13% 8% 31% 31% 14% 5%	844
June	1st to 10th	Day	8% 5% 19% 37% 25% 6%	780
	11th to 20th	Day	8% 4 23% 39% 22% 4	780
	21st to end	Day	5% 2 16% 43% 26% 8%	780
July	1st to 10th	Day	6% 1 11% 34% 35% 12%	771
	11th to 20th	Day	3 2 10% 34% 41% 12%	768
	21st to end	Day	4 1 5% 32% 42% 16%	849
August	1st to 10th	Day	3 1 8% 39% 39% 9%	776
	11th to 20th	Day	6% 2 7% 39% 37% 8%	765
	21st to end	Day	4 1 8% 33% 38% 15%	632
September	1st to 10th	Day	6% 1 8% 33% 36% 17%	360
	11th to 20th	Day	4 2 9% 34% 38% 13%	360
	21st to end	Day	12% 3 13% 34% 28% 11%	360
October	1st to 10th	Day	6% 3 23% 35% 23% 10%	360
	11th to 20th	Day	9% 8% 31% 34% 17% 2	339
	21st to end	Day	23% 10% 25% 27% 13% 2	363
November	1st to 10th	Day	15% 7% 37% 29% 8% 2	330
	11th to 20th	Day	13% 9% 38% 26% 12% 2	330
	21st to end	Day	19% 11% 38% 22% 8% 2	330
December	1st to 10th	Day	13% 13% 38% 26% 9% 1	330
	11th to 20th	Day	20% 11% 38% 25% 6%	330
	21st to end	Day	20% 15% 38% 23% 4	363
January	1st to 10th	Night	13% 12% 38% 29% 7%	1050
	11th to 20th	Night	13% 15% 36% 31% 5%	1050
	21st to end	Night	14% 12% 37% 32% 6%	1155
February	1st to 10th	Night	16% 13% 40% 27% 4	976
	11th to 20th	Night	17% 17% 38% 25% 2	824
	21st to end	Night	15% 16% 43% 23% 3	692
March	1st to 10th	Night	15% 16% 36% 29% 4	640
	11th to 20th	Night	16% 14% 42% 25% 4	630
	21st to end	Night	14% 13% 44% 24% 4	693
April	1st to 10th	Night	13% 13% 43% 27% 4	630
	11th to 20th	Night	13% 13% 45% 25% 4	623
	21st to end	Night	13% 12% 41% 26% 6% 1	630
May	1st to 10th	Night	10% 11% 40% 33% 6% 1	630
	11th to 20th	Night	13% 7% 33% 34% 13% 1	621
	21st to end	Night	7% 5% 34% 37% 15% 2	683
June	1st to 10th	Night	5% 4 22% 44% 24% 2	630
	11th to 20th	Night	6% 2 28% 48% 17%	630
	21st to end	Night	3 2 21% 47% 24% 3	630
July	1st to 10th	Night	4 2 11% 46% 33% 4	615
	11th to 20th	Night	2 1 11% 46% 37% 2	612
	21st to end	Night	3 1 8% 45% 41% 3	690
August	1st to 10th	Night	1 1 9% 50% 37% 2	627
	11th to 20th	Night	5% 8% 49% 35% 3	624
	21st to end	Night	3 2 6% 46% 38% 6%	693
September	1st to 10th	Night	8% 2 10% 45% 33% 2	630
	11th to 20th	Night	9% 1 14% 43% 30% 3	630
	21st to end	Night	8% 3 19% 42% 25% 4	630
October	1st to 10th	Night	8% 2 24% 40% 23% 3	663
	11th to 20th	Night	11% 7% 33% 35% 13%	813
	21st to end	Night	18% 9% 30% 31% 12% 1	880
November	1st to 10th	Night	11% 7% 37% 33% 12% 1	831
	11th to 20th	Night	9% 12% 34% 34% 10%	1050
	21st to end	Night	15% 11% 37% 28% 9%	1050
December	1st to 10th	Night	13% 11% 37% 32% 6%	1050
	11th to 20th	Night	17% 11% 37% 29% 6%	1050
	21st to end	Night	17% 13% 36% 29% 6%	1155

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

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