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 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	12% 6% 2 6% 4 4 4 4 6% 7% 45%	119900
	Night	13% 6% 2 5% 3 3 4 3 5% 7% 49%	118582
Transparency	Day	36% 6% 10% 21% 17% 10%	123734
	Night	33% 5% 12% 20% 17% 12%	122878
Seeing	Day	12% 6% 20% 30% 23% 9%	15421
	Night	14% 6% 26% 38% 15% 1	25426

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	17% 8% 2 6% 3 4 3 2 4 5% 45%	4077
	11th to 20th	Day	16% 5% 2 4 3 2 3 2 4 6% 52%	4080
	21st to end	Day	28% 7% 2 5% 3 2 3 3 5% 6% 37%	4925
February	1st to 10th	Day	17% 10% 3 7% 3 2 2 3 4 5% 44%	2000
	11th to 20th	Day	18% 9% 3 6% 3 3 3 3 4 5% 43%	1949
	21st to end	Day	17% 5% 3 6% 3 3 3 2 5% 5% 47%	1650
March	1st to 10th	Day	12% 6% 3 6% 4 4 4 3 5% 6% 47%	4926
	11th to 20th	Day	13% 7% 2 7% 4 4 3 3 3 4 49%	4980
	21st to end	Day	16% 7% 3 6% 4 3 3 3 4 6% 45%	5462
April	1st to 10th	Day	12% 6% 3 5% 5% 5% 4 4 6% 6% 45%	2200
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 3 4 5% 43%	2176
	21st to end	Day	12% 7% 3 6% 5% 5% 4 3 4 7% 43%	2470
May	1st to 10th	Day	16% 7% 3 8% 5% 5% 5% 4 7% 8% 32%	5862
	11th to 20th	Day	7% 5% 2 6% 4 4 4 3 5% 8% 50%	5862
	21st to end	Day	7% 6% 3 7% 4 5% 5% 6% 7% 10% 40%	6434
June	1st to 10th	Day	4 3 1 5% 5% 6% 6% 6% 8% 10% 45%	2588
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 5% 6% 8% 46%	2470
	21st to end	Day	6% 5% 3 7% 8% 6% 7% 7% 7% 9% 36%	2470
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 9% 45%	4120
	11th to 20th	Day	6% 5% 2 8% 6% 6% 6% 5% 7% 10% 38%	4120
	21st to end	Day	8% 6% 3 7% 6% 7% 6% 5% 8% 10% 35%	4465
August	1st to 10th	Day	2 1 1 2 3 3 4 3 6% 10% 65%	3993
	11th to 20th	Day	3 2 1 4 4 4 6% 5% 9% 13% 50%	3726
	21st to end	Day	5% 4 1 6% 5% 6% 6% 5% 8% 11% 42%	3963
September	1st to 10th	Day	9% 7% 3 7% 6% 6% 5% 6% 7% 7% 38%	2090
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 4 6% 8% 41%	2090
	21st to end	Day	7% 4 2 6% 5% 5% 6% 5% 6% 8% 47%	2090
October	1st to 10th	Day	5% 4 1 5% 5% 5% 5% 4 6% 7% 54%	2090
	11th to 20th	Day	11% 5% 2 4 4 4 4 3 4 7% 52%	1976
	21st to end	Day	9% 5% 2 5% 3 3 3 3 4 6% 58%	2090
November	1st to 10th	Day	11% 7% 3 7% 4 4 4 3 6% 7% 44%	1958
	11th to 20th	Day	12% 7% 2 5% 3 4 3 3 5% 7% 49%	1929
	21st to end	Day	16% 7% 2 7% 5% 4 4 3 4 6% 42%	1800
December	1st to 10th	Day	31% 10% 2 6% 3 3 2 2 4 4 33%	2940
	11th to 20th	Day	15% 8% 2 7% 4 4 4 2 5% 6% 43%	3391
	21st to end	Day	11% 6% 2 5% 4 3 3 3 5% 5% 53%	4488
January	1st to 10th	Night	20% 7% 2 5% 2 2 2 2 4 6% 46%	5888
	11th to 20th	Night	12% 4 1 4 3 3 3 2 4 7% 56%	5900
	21st to end	Night	31% 5% 2 5% 3 2 3 3 4 6% 36%	6094
February	1st to 10th	Night	15% 7% 2 6% 3 4 3 3 4 6% 45%	2400
	11th to 20th	Night	20% 6% 3 5% 3 3 2 3 4 5% 47%	2338
	21st to end	Night	16% 6% 3 6% 4 3 3 3 4 5% 46%	1980
March	1st to 10th	Night	17% 7% 2 6% 4 4 4 3 4 6% 44%	4929
	11th to 20th	Night	15% 7% 3 6% 3 3 3 2 3 5% 50%	4980
	21st to end	Night	17% 7% 2 4 3 3 3 3 4 6% 47%	5467
April	1st to 10th	Night	10% 7% 2 6% 4 4 4 5% 6% 8% 45%	2200
	11th to 20th	Night	13% 6% 3 7% 4 4 5% 4 4 6% 45%	2173
	21st to end	Night	14% 7% 3 6% 5% 4 4 4 5% 6% 42%	1922
May	1st to 10th	Night	20% 9% 2 6% 4 4 4 4 5% 7% 35%	4053
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 7% 10% 50%	4053
	21st to end	Night	9% 7% 3 7% 5% 4 5% 4 7% 11% 38%	4443
June	1st to 10th	Night	5% 4 1 3 3 3 3 4 6% 10% 59%	1796
	11th to 20th	Night	3 2 1 3 3 4 4 4 6% 10% 59%	1710
	21st to end	Night	6% 5% 2 5% 4 4 4 4 6% 9% 51%	1710
July	1st to 10th	Night	4 4 2 4 4 5% 4 4 6% 9% 53%	2850
	11th to 20th	Night	5% 5% 2 5% 3 3 5% 5% 7% 11% 49%	2850
	21st to end	Night	5% 4 2 6% 4 5% 5% 5% 7% 10% 45%	3098
August	1st to 10th	Night	1 1 1 1 1 2 2 3 4 7% 77%	2928
	11th to 20th	Night	2 1 1 3 2 4 4 4 7% 12% 60%	3113
	21st to end	Night	4 3 1 4 3 3 4 3 6% 11% 58%	3694
September	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 6% 9% 46%	2090
	11th to 20th	Night	8% 6% 2 6% 5% 4 5% 4 5% 7% 50%	2090
	21st to end	Night	6% 3 2 5% 3 4 5% 3 7% 8% 54%	2090
October	1st to 10th	Night	4 4 2 4 3 3 3 3 5% 8% 62%	2090
	11th to 20th	Night	9% 5% 2 4 2 3 3 3 4 6% 59%	2182
	21st to end	Night	10% 5% 1 4 2 2 3 2 3 4 63%	2508
November	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 4 6% 52%	2340
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 6% 54%	2467
	21st to end	Night	14% 6% 2 6% 4 3 4 3 4 7% 48%	2600
December	1st to 10th	Night	31% 9% 3 6% 3 3 3 1 3 3 34%	4250
	11th to 20th	Night	15% 8% 3 6% 4 3 3 3 4 5% 48%	4816
	21st to end	Night	12% 6% 2 5% 2 2 3 2 4 6% 56%	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	41% 3 7% 16% 22% 11%	4078
	11th to 20th	Day	35% 3 4 21% 21% 16%	4080
	21st to end	Day	49% 2 4 13% 21% 11%	4925
February	1st to 10th	Day	43% 3 7% 11% 20% 16%	2000
	11th to 20th	Day	44% 4 6% 17% 18% 12%	1959
	21st to end	Day	39% 5% 8% 17% 18% 13%	1650
March	1st to 10th	Day	37% 5% 7% 19% 13% 19%	4926
	11th to 20th	Day	39% 4 5% 8% 17% 27%	4980
	21st to end	Day	40% 5% 7% 20% 17% 11%	5464
April	1st to 10th	Day	38% 6% 9% 16% 18% 13%	2200
	11th to 20th	Day	43% 5% 6% 17% 17% 13%	2176
	21st to end	Day	40% 6% 7% 18% 17% 11%	2470
May	1st to 10th	Day	45% 7% 11% 19% 12% 4	5900
	11th to 20th	Day	31% 7% 11% 23% 19% 9%	5866
	21st to end	Day	35% 8% 10% 33% 13% 1	6447
June	1st to 10th	Day	29% 9% 15% 26% 20% 2	2588
	11th to 20th	Day	33% 7% 11% 24% 20% 5%	2470
	21st to end	Day	39% 10% 14% 25% 10% 2	2470
July	1st to 10th	Day	33% 9% 18% 33% 7%	4056
	11th to 20th	Day	38% 9% 16% 23% 12% 1	4051
	21st to end	Day	40% 11% 17% 25% 7%	4325
August	1st to 10th	Day	15% 6% 13% 45% 20% 1	3979
	11th to 20th	Day	23% 9% 26% 33% 9% 1	3762
	21st to end	Day	32% 12% 17% 23% 15% 1	3962
September	1st to 10th	Day	41% 10% 12% 17% 15% 6%	2090
	11th to 20th	Day	38% 8% 12% 22% 15% 4	2090
	21st to end	Day	32% 8% 12% 18% 18% 11%	2090
October	1st to 10th	Day	19% 5% 6% 12% 22% 35%	3480
	11th to 20th	Day	23% 5% 11% 19% 22% 21%	3268
	21st to end	Day	23% 5% 9% 17% 28% 18%	3428
November	1st to 10th	Day	38% 5% 8% 17% 19% 12%	1958
	11th to 20th	Day	35% 4 9% 16% 21% 15%	1929
	21st to end	Day	43% 5% 7% 15% 18% 13%	1800
December	1st to 10th	Day	57% 3 5% 14% 16% 6%	2940
	11th to 20th	Day	41% 6% 8% 15% 16% 14%	3389
	21st to end	Day	33% 4 6% 20% 23% 15%	4488
January	1st to 10th	Night	40% 3 7% 15% 18% 17%	5888
	11th to 20th	Night	30% 2 5% 20% 23% 19%	5900
	21st to end	Night	50% 3 4 11% 19% 13%	6094
February	1st to 10th	Night	40% 4 8% 11% 17% 19%	2400
	11th to 20th	Night	41% 3 6% 18% 18% 14%	2338
	21st to end	Night	41% 4 7% 18% 16% 14%	1980
March	1st to 10th	Night	43% 4 8% 16% 15% 15%	4929
	11th to 20th	Night	39% 4 7% 11% 15% 24%	4980
	21st to end	Night	38% 4 11% 17% 19% 10%	5467
April	1st to 10th	Night	36% 6% 12% 19% 15% 12%	2200
	11th to 20th	Night	38% 5% 9% 16% 14% 17%	2173
	21st to end	Night	43% 5% 10% 17% 15% 9%	1922
May	1st to 10th	Night	48% 6% 10% 18% 13% 5%	4080
	11th to 20th	Night	28% 7% 14% 22% 20% 9%	4054
	21st to end	Night	38% 9% 16% 29% 8%	4452
June	1st to 10th	Night	21% 6% 18% 33% 20% 2	1796
	11th to 20th	Night	20% 6% 13% 34% 22% 5%	1710
	21st to end	Night	27% 9% 18% 31% 14% 1	1710
July	1st to 10th	Night	25% 8% 25% 36% 5%	2805
	11th to 20th	Night	27% 10% 25% 25% 13% 1	2796
	21st to end	Night	31% 10% 24% 28% 6%	2999
August	1st to 10th	Night	8% 5% 16% 52% 19%	2918
	11th to 20th	Night	16% 8% 35% 34% 6% 1	3143
	21st to end	Night	20% 11% 27% 29% 12% 1	3694
September	1st to 10th	Night	33% 8% 20% 19% 14% 6%	2090
	11th to 20th	Night	33% 7% 16% 24% 16% 4	2090
	21st to end	Night	25% 7% 16% 23% 18% 11%	2090
October	1st to 10th	Night	14% 5% 8% 17% 19% 37%	3480
	11th to 20th	Night	18% 4 10% 19% 27% 21%	3616
	21st to end	Night	19% 3 8% 23% 30% 16%	4120
November	1st to 10th	Night	32% 5% 11% 19% 21% 13%	2340
	11th to 20th	Night	31% 4 12% 18% 19% 16%	2467
	21st to end	Night	37% 4 10% 18% 18% 13%	2600
December	1st to 10th	Night	58% 3 5% 12% 13% 9%	4250
	11th to 20th	Night	39% 5% 10% 13% 17% 16%	4817
	21st to end	Night	30% 4 6% 17% 26% 16%	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	22% 10% 33% 26% 8% 2	300
	11th to 20th	Day	22% 8% 33% 28% 8% 1	300
	21st to end	Day	22% 10% 33% 25% 9% 2	330
February	1st to 10th	Day	22% 12% 25% 29% 11% 1	300
	11th to 20th	Day	24% 12% 35% 21% 9%	300
	21st to end	Day	21% 10% 42% 22% 5%	249
March	1st to 10th	Day	17% 13% 40% 20% 9%	300
	11th to 20th	Day	15% 9% 43% 26% 8%	300
	21st to end	Day	17% 12% 38% 27% 5% 1	330
April	1st to 10th	Day	16% 11% 37% 27% 8% 1	300
	11th to 20th	Day	16% 14% 30% 27% 12% 1	294
	21st to end	Day	15% 10% 37% 29% 8% 1	570
May	1st to 10th	Day	14% 10% 37% 29% 9%	703
	11th to 20th	Day	11% 9% 30% 33% 14% 2	725
	21st to end	Day	10% 6% 27% 38% 18% 1	789
June	1st to 10th	Day	6% 2 13% 38% 32% 9%	726
	11th to 20th	Day	7% 1 12% 48% 26% 5%	680
	21st to end	Day	8% 2 10% 32% 36% 12%	680
July	1st to 10th	Day	9% 3 8% 30% 36% 14%	671
	11th to 20th	Day	4 1 5% 29% 46% 15%	668
	21st to end	Day	6% 1 6% 31% 41% 15%	739
August	1st to 10th	Day	3 1 4 28% 43% 22%	617
	11th to 20th	Day	4 1 5% 31% 37% 23%	477
	21st to end	Day	6% 2 6% 22% 33% 31%	413
September	1st to 10th	Day	13% 1 6% 17% 44% 20%	300
	11th to 20th	Day	8% 1 4 14% 39% 33%	300
	21st to end	Day	9% 2 4 23% 40% 22%	300
October	1st to 10th	Day	5% 3 7% 30% 35% 20%	300
	11th to 20th	Day	10% 1 10% 38% 26% 15%	300
	21st to end	Day	13% 8% 18% 29% 21% 10%	330
November	1st to 10th	Day	14% 6% 19% 32% 23% 7%	300
	11th to 20th	Day	18% 7% 17% 31% 21% 6%	300
	21st to end	Day	19% 12% 30% 24% 11% 4	300
December	1st to 10th	Day	22% 5% 25% 28% 14% 6%	300
	11th to 20th	Day	19% 12% 25% 30% 13% 1	300
	21st to end	Day	17% 8% 28% 32% 13% 2	330
January	1st to 10th	Night	21% 8% 27% 34% 11%	1000
	11th to 20th	Night	20% 8% 31% 35% 7%	1000
	21st to end	Night	22% 9% 28% 34% 7%	925
February	1st to 10th	Night	21% 11% 32% 32% 5%	800
	11th to 20th	Night	24% 10% 34% 29% 3	784
	21st to end	Night	22% 12% 41% 23% 2	660
March	1st to 10th	Night	21% 11% 41% 24% 3	610
	11th to 20th	Night	18% 12% 35% 30% 4	600
	21st to end	Night	17% 9% 45% 28% 2	660
April	1st to 10th	Night	14% 10% 48% 26% 3	600
	11th to 20th	Night	15% 12% 40% 29% 4	593
	21st to end	Night	16% 10% 42% 26% 5% 1	600
May	1st to 10th	Night	13% 9% 48% 26% 4	600
	11th to 20th	Night	10% 7% 41% 35% 7% 1	591
	21st to end	Night	9% 5% 38% 39% 8%	650
June	1st to 10th	Night	5% 3 22% 51% 19% 1	599
	11th to 20th	Night	5% 1 26% 54% 14%	570
	21st to end	Night	5% 2 16% 51% 24% 2	570
July	1st to 10th	Night	5% 2 12% 51% 27% 2	555
	11th to 20th	Night	3 1 12% 52% 30% 2	552
	21st to end	Night	4 1 8% 58% 27% 2	624
August	1st to 10th	Night	2 1 7% 57% 31% 2	567
	11th to 20th	Night	4 1 8% 52% 32% 3	564
	21st to end	Night	6% 1 7% 42% 42% 3	627
September	1st to 10th	Night	7% 1 8% 42% 36% 5%	570
	11th to 20th	Night	11% 1 8% 38% 40% 4	570
	21st to end	Night	8% 1 12% 37% 39% 4	570
October	1st to 10th	Night	5% 2 12% 46% 32% 3	570
	11th to 20th	Night	11% 3 18% 48% 18% 2	662
	21st to end	Night	11% 6% 26% 35% 20% 2	836
November	1st to 10th	Night	12% 5% 23% 39% 20% 2	780
	11th to 20th	Night	13% 5% 25% 40% 16% 1	867
	21st to end	Night	17% 7% 28% 33% 15% 1	1000
December	1st to 10th	Night	20% 6% 23% 35% 14% 1	1000
	11th to 20th	Night	23% 8% 27% 33% 9%	1000
	21st to end	Night	20% 7% 26% 38% 9%	1100

Page generated at 2025-06-07 09:28:14. Version ac07

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