Butler Ranch
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Butler 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	11% 6% 2 6% 4 4 4 4 6% 8% 45%	149005
	Night	12% 6% 2 5% 3 3 4 3 5% 7% 50%	143160
Transparency	Day	35% 6% 11% 22% 16% 9%	152797
	Night	33% 5% 13% 22% 17% 11%	147435
Seeing	Day	12% 6% 20% 30% 24% 9%	17256
	Night	14% 6% 26% 38% 15% 1	27100

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	15% 9% 2 5% 3 4 3 2 4 5% 49%	5289
	11th to 20th	Day	14% 6% 2 5% 3 3 3 2 4 5% 53%	5310
	21st to end	Day	25% 8% 2 5% 3 2 3 2 4 6% 39%	6385
February	1st to 10th	Day	16% 10% 3 6% 3 2 2 3 4 5% 45%	2100
	11th to 20th	Day	17% 9% 3 6% 3 4 3 3 4 5% 43%	2049
	21st to end	Day	16% 5% 2 6% 3 3 3 2 4 5% 49%	1730
March	1st to 10th	Day	12% 6% 3 6% 4 4 4 3 5% 6% 47%	5035
	11th to 20th	Day	13% 7% 2 6% 4 4 3 3 3 4 50%	5090
	21st to end	Day	16% 7% 3 6% 4 3 3 3 4 6% 45%	5583
April	1st to 10th	Day	12% 6% 3 5% 5% 5% 4 4 6% 6% 43%	2310
	11th to 20th	Day	15% 7% 2 8% 5% 4 4 3 4 5% 42%	2286
	21st to end	Day	13% 7% 3 6% 5% 6% 4 3 4 7% 42%	2593
May	1st to 10th	Day	17% 8% 3 8% 5% 4 4 4 6% 8% 33%	7617
	11th to 20th	Day	7% 5% 2 7% 5% 5% 5% 3 6% 8% 46%	7627
	21st to end	Day	9% 6% 3 8% 5% 6% 5% 5% 7% 9% 37%	8392
June	1st to 10th	Day	5% 3 2 6% 5% 6% 6% 6% 8% 10% 44%	2730
	11th to 20th	Day	6% 5% 2 6% 6% 6% 6% 5% 6% 8% 44%	2730
	21st to end	Day	6% 5% 2 7% 7% 6% 7% 6% 7% 8% 38%	2730
July	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 10% 42%	7680
	11th to 20th	Day	8% 6% 2 7% 6% 6% 7% 5% 7% 10% 34%	7680
	21st to end	Day	6% 4 2 5% 4 5% 5% 5% 7% 10% 47%	8337
August	1st to 10th	Day	2 1 1 3 3 3 4 3 6% 10% 63%	5858
	11th to 20th	Day	3 2 1 3 3 4 5% 5% 8% 12% 55%	5483
	21st to end	Day	3 3 1 5% 4 5% 5% 4 7% 10% 52%	5757
September	1st to 10th	Day	9% 6% 3 7% 5% 6% 5% 5% 7% 7% 40%	2310
	11th to 20th	Day	8% 5% 3 6% 6% 6% 6% 5% 6% 8% 41%	2310
	21st to end	Day	6% 4 2 6% 5% 4 6% 5% 6% 8% 47%	2310
October	1st to 10th	Day	5% 4 1 5% 4 4 5% 4 5% 7% 55%	2310
	11th to 20th	Day	10% 5% 2 4 4 4 4 3 4 7% 53%	2113
	21st to end	Day	8% 5% 2 5% 3 3 3 3 4 6% 59%	2200
November	1st to 10th	Day	10% 7% 2 7% 4 4 4 3 5% 7% 46%	2058
	11th to 20th	Day	12% 6% 2 5% 3 4 3 3 5% 7% 50%	2025
	21st to end	Day	16% 8% 2 7% 5% 4 4 3 4 6% 42%	1890
December	1st to 10th	Day	31% 10% 2 6% 3 3 2 2 3 4 33%	3030
	11th to 20th	Day	15% 8% 2 7% 4 4 4 2 4 6% 43%	3481
	21st to end	Day	11% 6% 2 5% 4 3 3 3 5% 5% 53%	4587
January	1st to 10th	Night	17% 7% 2 5% 2 3 3 2 4 6% 49%	7634
	11th to 20th	Night	12% 5% 2 4 3 3 3 2 3 6% 58%	7680
	21st to end	Night	28% 6% 2 5% 3 2 3 3 4 6% 38%	7884
February	1st to 10th	Night	15% 7% 2 6% 3 4 3 3 4 7% 46%	2520
	11th to 20th	Night	19% 6% 3 5% 3 2 3 3 4 6% 48%	2458
	21st to end	Night	16% 6% 3 6% 3 3 3 3 4 5% 48%	2076
March	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 6% 43%	5039
	11th to 20th	Night	15% 7% 3 6% 3 3 3 2 3 5% 50%	5090
	21st to end	Night	17% 7% 2 4 3 3 3 3 4 6% 48%	5588
April	1st to 10th	Night	10% 7% 2 6% 4 4 4 5% 6% 8% 43%	2310
	11th to 20th	Night	14% 6% 2 7% 4 4 5% 4 4 6% 44%	2283
	21st to end	Night	15% 8% 3 6% 5% 4 4 4 5% 6% 41%	2019
May	1st to 10th	Night	21% 10% 3 7% 4 4 4 4 5% 7% 32%	5266
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 7% 10% 46%	5265
	21st to end	Night	10% 7% 3 7% 4 4 5% 4 7% 11% 37%	5793
June	1st to 10th	Night	5% 4 1 4 3 3 3 4 6% 9% 57%	1890
	11th to 20th	Night	4 3 1 3 3 4 5% 4 6% 10% 58%	1890
	21st to end	Night	6% 4 2 4 4 4 4 4 6% 9% 52%	1890
July	1st to 10th	Night	6% 5% 2 6% 4 5% 4 4 6% 9% 48%	5308
	11th to 20th	Night	7% 5% 2 6% 4 4 5% 5% 7% 11% 43%	5307
	21st to end	Night	5% 4 2 5% 4 4 4 4 6% 8% 55%	5770
August	1st to 10th	Night	1 1 1 2 1 2 2 2 4 8% 76%	4293
	11th to 20th	Night	2 2 1 3 3 4 4 4 7% 10% 61%	4583
	21st to end	Night	3 2 1 3 3 3 4 3 6% 10% 63%	5374
September	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 7% 9% 46%	2310
	11th to 20th	Night	7% 5% 2 5% 4 4 5% 4 5% 7% 51%	2310
	21st to end	Night	6% 3 2 5% 3 4 5% 4 7% 9% 53%	2310
October	1st to 10th	Night	4 4 2 4 3 3 3 3 5% 7% 62%	2310
	11th to 20th	Night	9% 4 2 4 2 3 3 2 4 6% 61%	2330
	21st to end	Night	10% 4 1 4 3 2 3 3 3 4 64%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 4 6% 54%	2460
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 6% 55%	2591
	21st to end	Night	14% 6% 2 6% 4 3 4 3 5% 7% 47%	2730
December	1st to 10th	Night	31% 9% 3 6% 3 3 3 1 3 3 34%	4380
	11th to 20th	Night	15% 8% 3 6% 4 3 3 3 4 5% 48%	4946
	21st to end	Night	12% 6% 2 5% 3 2 3 2 4 6% 56%	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	39% 3 9% 18% 21% 11%	5290
	11th to 20th	Day	36% 2 3 18% 22% 18%	5310
	21st to end	Day	47% 3 5% 14% 22% 9%	6385
February	1st to 10th	Day	42% 3 7% 11% 21% 16%	2100
	11th to 20th	Day	44% 4 6% 17% 18% 11%	2059
	21st to end	Day	38% 4 8% 17% 21% 12%	1730
March	1st to 10th	Day	37% 5% 7% 19% 13% 19%	5035
	11th to 20th	Day	39% 4 5% 8% 18% 27%	5090
	21st to end	Day	39% 5% 7% 20% 18% 11%	5585
April	1st to 10th	Day	39% 6% 9% 16% 17% 13%	2310
	11th to 20th	Day	45% 5% 6% 17% 16% 12%	2286
	21st to end	Day	41% 6% 8% 18% 17% 10%	2593
May	1st to 10th	Day	46% 7% 11% 21% 11% 4	7655
	11th to 20th	Day	34% 7% 11% 22% 19% 7%	7631
	21st to end	Day	38% 9% 11% 30% 11% 1	8405
June	1st to 10th	Day	30% 9% 15% 25% 19% 2	2730
	11th to 20th	Day	34% 7% 12% 23% 20% 4	2730
	21st to end	Day	38% 9% 14% 25% 12% 2	2730
July	1st to 10th	Day	36% 9% 21% 30% 5%	7616
	11th to 20th	Day	41% 11% 19% 21% 8% 1	7611
	21st to end	Day	29% 9% 14% 33% 14% 1	8155
August	1st to 10th	Day	15% 7% 15% 42% 20% 2	5844
	11th to 20th	Day	19% 8% 26% 37% 8% 1	5519
	21st to end	Day	24% 10% 16% 31% 19% 1	5756
September	1st to 10th	Day	39% 10% 13% 18% 15% 6%	2310
	11th to 20th	Day	37% 8% 14% 23% 14% 4	2310
	21st to end	Day	31% 8% 13% 20% 17% 11%	2310
October	1st to 10th	Day	20% 5% 7% 13% 22% 34%	3700
	11th to 20th	Day	22% 5% 11% 20% 22% 21%	3405
	21st to end	Day	23% 5% 9% 18% 27% 18%	3538
November	1st to 10th	Day	37% 5% 8% 17% 20% 13%	2058
	11th to 20th	Day	34% 4 9% 17% 21% 15%	2025
	21st to end	Day	43% 5% 7% 15% 18% 12%	1890
December	1st to 10th	Day	56% 3 5% 13% 16% 7%	3030
	11th to 20th	Day	41% 6% 8% 15% 17% 13%	3479
	21st to end	Day	33% 4 6% 20% 23% 14%	4587
January	1st to 10th	Night	38% 3 9% 16% 19% 15%	7635
	11th to 20th	Night	31% 2 4 18% 24% 21%	7680
	21st to end	Night	48% 3 4 14% 20% 11%	7884
February	1st to 10th	Night	39% 4 9% 11% 18% 19%	2520
	11th to 20th	Night	40% 3 6% 19% 18% 14%	2458
	21st to end	Night	39% 4 7% 18% 18% 14%	2076
March	1st to 10th	Night	44% 4 7% 15% 15% 14%	5039
	11th to 20th	Night	38% 4 7% 11% 16% 24%	5090
	21st to end	Night	38% 4 11% 17% 19% 10%	5588
April	1st to 10th	Night	37% 6% 12% 19% 14% 12%	2310
	11th to 20th	Night	40% 6% 10% 16% 13% 16%	2283
	21st to end	Night	44% 5% 10% 17% 14% 9%	2019
May	1st to 10th	Night	51% 5% 10% 17% 12% 4	5293
	11th to 20th	Night	31% 7% 13% 22% 18% 7%	5266
	21st to end	Night	39% 9% 17% 27% 7%	5805
June	1st to 10th	Night	23% 7% 18% 31% 19% 2	1890
	11th to 20th	Night	22% 6% 13% 33% 22% 4	1890
	21st to end	Night	26% 9% 20% 30% 14% 1	1890
July	1st to 10th	Night	31% 7% 25% 33% 3	5265
	11th to 20th	Night	33% 10% 27% 23% 7%	5256
	21st to end	Night	25% 8% 18% 34% 13% 1	5641
August	1st to 10th	Night	8% 4 21% 47% 19% 1	4283
	11th to 20th	Night	17% 7% 34% 35% 7%	4613
	21st to end	Night	17% 9% 23% 35% 16% 1	5374
September	1st to 10th	Night	32% 8% 20% 20% 14% 6%	2310
	11th to 20th	Night	31% 7% 17% 26% 15% 4	2310
	21st to end	Night	25% 7% 16% 25% 17% 10%	2310
October	1st to 10th	Night	15% 5% 8% 17% 20% 35%	3700
	11th to 20th	Night	18% 4 10% 21% 27% 20%	3764
	21st to end	Night	19% 3 8% 24% 30% 17%	4252
November	1st to 10th	Night	31% 5% 10% 18% 22% 14%	2460
	11th to 20th	Night	31% 4 12% 18% 19% 16%	2591
	21st to end	Night	38% 5% 10% 18% 18% 12%	2730
December	1st to 10th	Night	57% 3 5% 12% 13% 10%	4380
	11th to 20th	Night	39% 5% 9% 13% 18% 16%	4947
	21st to end	Night	30% 5% 6% 17% 26% 16%	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	21% 10% 32% 26% 9% 2	330
	11th to 20th	Day	21% 9% 33% 27% 10% 1	330
	21st to end	Day	22% 9% 33% 25% 9% 1	363
February	1st to 10th	Day	21% 12% 25% 30% 11% 1	330
	11th to 20th	Day	22% 12% 36% 21% 9%	330
	21st to end	Day	19% 10% 41% 24% 6%	273
March	1st to 10th	Day	17% 13% 38% 22% 9%	330
	11th to 20th	Day	14% 9% 42% 27% 8%	330
	21st to end	Day	15% 11% 36% 29% 8% 1	363
April	1st to 10th	Day	16% 11% 37% 28% 8% 1	330
	11th to 20th	Day	17% 15% 30% 27% 11% 1	324
	21st to end	Day	15% 10% 36% 30% 8% 1	613
May	1st to 10th	Day	15% 11% 36% 29% 8%	753
	11th to 20th	Day	11% 10% 31% 33% 14% 2	775
	21st to end	Day	10% 6% 27% 37% 18% 2	844
June	1st to 10th	Day	7% 2 13% 39% 31% 8%	780
	11th to 20th	Day	8% 3 12% 47% 25% 5%	780
	21st to end	Day	8% 2 11% 34% 33% 12%	780
July	1st to 10th	Day	10% 3 9% 30% 35% 14%	771
	11th to 20th	Day	5% 2 6% 30% 43% 13%	768
	21st to end	Day	6% 1 6% 31% 41% 15%	849
August	1st to 10th	Day	3 1 4 27% 43% 23%	711
	11th to 20th	Day	3 1 6% 32% 36% 23%	557
	21st to end	Day	5% 2 5% 21% 36% 31%	487
September	1st to 10th	Day	12% 1 5% 15% 43% 23%	360
	11th to 20th	Day	8% 2 4 16% 41% 30%	360
	21st to end	Day	8% 2 4 25% 41% 21%	360
October	1st to 10th	Day	5% 4 8% 29% 34% 21%	360
	11th to 20th	Day	9% 1 9% 37% 29% 14%	339
	21st to end	Day	12% 8% 18% 29% 23% 10%	363
November	1st to 10th	Day	13% 6% 19% 31% 24% 7%	330
	11th to 20th	Day	16% 6% 18% 33% 21% 5%	330
	21st to end	Day	18% 12% 32% 23% 11% 4	330
December	1st to 10th	Day	23% 5% 24% 29% 14% 5%	330
	11th to 20th	Day	18% 12% 25% 29% 15% 1	330
	21st to end	Day	16% 8% 28% 31% 15% 2	363
January	1st to 10th	Night	20% 8% 27% 34% 11%	1050
	11th to 20th	Night	19% 8% 31% 35% 7%	1050
	21st to end	Night	23% 9% 28% 34% 7%	971
February	1st to 10th	Night	20% 11% 32% 32% 5%	840
	11th to 20th	Night	23% 10% 34% 29% 4	824
	21st to end	Night	21% 12% 41% 24% 2	692
March	1st to 10th	Night	22% 11% 41% 24% 3	640
	11th to 20th	Night	18% 13% 35% 30% 4	630
	21st to end	Night	16% 9% 44% 28% 2	693
April	1st to 10th	Night	14% 10% 48% 25% 3	630
	11th to 20th	Night	16% 13% 40% 28% 4	623
	21st to end	Night	17% 11% 42% 26% 4	630
May	1st to 10th	Night	14% 9% 47% 26% 4	630
	11th to 20th	Night	10% 7% 40% 34% 7% 1	621
	21st to end	Night	9% 6% 37% 39% 8%	683
June	1st to 10th	Night	6% 3 22% 51% 18%	630
	11th to 20th	Night	5% 2 25% 53% 14%	630
	21st to end	Night	5% 3 16% 50% 24% 2	630
July	1st to 10th	Night	6% 3 12% 51% 27% 2	615
	11th to 20th	Night	3 1 13% 52% 28% 2	612
	21st to end	Night	4 1 9% 58% 26% 2	690
August	1st to 10th	Night	2 1 7% 57% 31% 2	627
	11th to 20th	Night	4 1 9% 53% 31% 2	624
	21st to end	Night	5% 1 6% 43% 41% 3	693
September	1st to 10th	Night	7% 1 9% 43% 36% 4	630
	11th to 20th	Night	10% 1 8% 39% 39% 3	630
	21st to end	Night	7% 1 13% 38% 37% 4	630
October	1st to 10th	Night	5% 2 12% 46% 32% 3	630
	11th to 20th	Night	11% 3 18% 47% 19% 2	706
	21st to end	Night	11% 6% 26% 36% 20% 2	880
November	1st to 10th	Night	11% 5% 23% 39% 20% 1	820
	11th to 20th	Night	13% 5% 26% 40% 15% 1	911
	21st to end	Night	17% 7% 28% 33% 14% 1	1050
December	1st to 10th	Night	21% 6% 24% 35% 13% 1	1050
	11th to 20th	Night	23% 8% 26% 33% 10%	1050
	21st to end	Night	20% 7% 26% 38% 9%	1155

Page generated at 2026-10-11 04:06:29. Version ac07

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