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 2024-11-02.

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% 5% 5% 5% 4 6% 8% 44%	100952
	Night	11% 5% 2 5% 3 3 4 3 5% 7% 50%	98122
Transparency	Day	37% 7% 11% 21% 17% 8%	100763
	Night	33% 6% 13% 22% 17% 10%	97982
Seeing	Day	12% 6% 19% 30% 24% 9%	14667
	Night	14% 6% 26% 38% 16% 1	24582

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% 8% 2 6% 4 4 3 2 4 5% 47%	2847
	11th to 20th	Day	16% 5% 2 4 2 2 3 2 4 6% 55%	2850
	21st to end	Day	20% 8% 3 6% 3 3 3 3 6% 7% 39%	3432
February	1st to 10th	Day	17% 10% 3 7% 3 2 2 3 4 5% 43%	1900
	11th to 20th	Day	17% 10% 3 6% 3 3 3 3 4 5% 44%	1849
	21st to end	Day	17% 5% 3 6% 4 3 4 2 5% 5% 48%	1570
March	1st to 10th	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	3426
	11th to 20th	Day	19% 10% 4 9% 6% 5% 4 3 4 5% 31%	3480
	21st to end	Day	13% 6% 2 5% 4 3 3 3 5% 7% 47%	3814
April	1st to 10th	Day	12% 6% 3 5% 5% 5% 4 4 6% 6% 44%	2090
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 3 4 5% 43%	2066
	21st to end	Day	12% 7% 2 6% 5% 5% 4 3 4 7% 44%	2347
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 4 4 6% 8% 34%	4082
	11th to 20th	Day	10% 6% 3 7% 5% 5% 5% 3 6% 7% 43%	4082
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 7% 10% 43%	4477
June	1st to 10th	Day	4 3 1 5% 5% 6% 6% 6% 8% 10% 45%	2470
	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	8% 5% 2 5% 3 3 3 3 4 6% 58%	2090
November	1st to 10th	Day	11% 7% 3 7% 4 4 4 3 5% 7% 43%	1868
	11th to 20th	Day	13% 7% 2 5% 3 4 3 3 5% 7% 48%	1833
	21st to end	Day	16% 8% 2 7% 5% 4 4 3 4 6% 42%	1710
December	1st to 10th	Day	18% 8% 2 6% 4 3 3 3 4 6% 43%	1710
	11th to 20th	Day	18% 8% 2 6% 4 4 4 2 5% 6% 41%	2161
	21st to end	Day	13% 7% 3 6% 4 3 4 3 5% 5% 48%	3135
January	1st to 10th	Night	18% 6% 2 5% 2 2 3 3 4 7% 47%	4108
	11th to 20th	Night	13% 5% 1 3 2 2 2 2 3 5% 61%	4120
	21st to end	Night	23% 6% 2 6% 4 2 4 3 4 7% 39%	4262
February	1st to 10th	Night	16% 7% 2 6% 3 4 3 3 4 6% 44%	2280
	11th to 20th	Night	19% 6% 3 4 3 2 2 3 4 5% 48%	2218
	21st to end	Night	16% 6% 3 6% 4 3 4 3 4 6% 46%	1884
March	1st to 10th	Night	20% 8% 2 7% 4 4 4 3 5% 6% 38%	3429
	11th to 20th	Night	21% 9% 4 8% 5% 4 4 3 4 6% 32%	3480
	21st to end	Night	14% 6% 2 4 3 3 3 4 5% 6% 51%	3817
April	1st to 10th	Night	10% 7% 2 6% 4 4 4 5% 6% 8% 44%	2090
	11th to 20th	Night	13% 6% 3 7% 4 4 5% 4 4 7% 44%	2063
	21st to end	Night	14% 7% 3 6% 5% 4 4 4 4 6% 43%	1827
May	1st to 10th	Night	17% 9% 2 5% 4 4 4 4 5% 7% 38%	2823
	11th to 20th	Night	12% 6% 2 5% 4 4 4 3 5% 7% 47%	2823
	21st to end	Night	8% 5% 2 6% 4 4 4 4 7% 10% 45%	3090
June	1st to 10th	Night	4 4 1 3 3 3 3 4 6% 9% 60%	1710
	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% 6% 2 5% 4 4 3 3 4 6% 53%	2244
	11th to 20th	Night	11% 5% 2 5% 3 3 3 4 4 6% 53%	2343
	21st to end	Night	14% 6% 2 6% 4 3 4 3 5% 7% 47%	2470
December	1st to 10th	Night	16% 7% 2 5% 3 3 3 2 4 5% 50%	2470
	11th to 20th	Night	18% 8% 2 6% 4 3 4 3 4 6% 43%	3036
	21st to end	Night	14% 6% 2 5% 3 3 3 3 5% 7% 49%	4532

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 7% 12% 25% 15%	2848
	11th to 20th	Day	33% 3 5% 21% 22% 17%	2850
	21st to end	Day	45% 3 4 14% 26% 8%	3432
February	1st to 10th	Day	44% 3 6% 11% 21% 15%	1900
	11th to 20th	Day	43% 4 6% 16% 19% 12%	1859
	21st to end	Day	39% 5% 8% 17% 19% 13%	1570
March	1st to 10th	Day	46% 6% 8% 17% 15% 7%	3426
	11th to 20th	Day	55% 5% 7% 8% 16% 8%	3480
	21st to end	Day	35% 5% 9% 20% 17% 14%	3814
April	1st to 10th	Day	39% 5% 9% 16% 18% 12%	2090
	11th to 20th	Day	44% 5% 6% 17% 16% 12%	2066
	21st to end	Day	40% 6% 7% 19% 18% 11%	2347
May	1st to 10th	Day	46% 6% 12% 18% 12% 6%	4120
	11th to 20th	Day	38% 7% 9% 19% 18% 8%	4086
	21st to end	Day	32% 8% 10% 37% 12% 1	4489
June	1st to 10th	Day	29% 8% 15% 25% 20% 3	2470
	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	28% 7% 9% 19% 24% 12%	2090
	11th to 20th	Day	32% 5% 10% 18% 19% 15%	1976
	21st to end	Day	28% 4 7% 15% 25% 21%	2090
November	1st to 10th	Day	40% 4 8% 18% 19% 11%	1868
	11th to 20th	Day	36% 4 10% 16% 21% 13%	1833
	21st to end	Day	44% 5% 6% 15% 18% 13%	1710
December	1st to 10th	Day	43% 4 7% 18% 20% 8%	1710
	11th to 20th	Day	44% 6% 7% 9% 19% 15%	2159
	21st to end	Day	38% 4 6% 18% 23% 11%	3135
January	1st to 10th	Night	37% 3 8% 12% 17% 22%	4108
	11th to 20th	Night	28% 2 5% 19% 25% 21%	4120
	21st to end	Night	47% 3 5% 12% 23% 10%	4262
February	1st to 10th	Night	41% 4 9% 11% 17% 18%	2280
	11th to 20th	Night	40% 3 6% 18% 19% 14%	2218
	21st to end	Night	40% 4 7% 19% 16% 14%	1884
March	1st to 10th	Night	48% 5% 10% 15% 15% 7%	3429
	11th to 20th	Night	53% 6% 9% 12% 11% 9%	3480
	21st to end	Night	34% 4 14% 15% 21% 12%	3817
April	1st to 10th	Night	37% 6% 12% 19% 15% 12%	2090
	11th to 20th	Night	39% 6% 10% 16% 13% 17%	2063
	21st to end	Night	42% 5% 10% 18% 15% 10%	1827
May	1st to 10th	Night	44% 5% 12% 19% 13% 7%	2850
	11th to 20th	Night	35% 8% 11% 20% 18% 8%	2824
	21st to end	Night	31% 9% 17% 33% 10%	3099
June	1st to 10th	Night	20% 7% 18% 32% 21% 2	1710
	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	21% 6% 12% 27% 21% 14%	2090
	11th to 20th	Night	27% 5% 9% 22% 21% 16%	2182
	21st to end	Night	25% 3 7% 19% 25% 21%	2508
November	1st to 10th	Night	31% 5% 11% 19% 22% 12%	2244
	11th to 20th	Night	32% 4 13% 18% 18% 15%	2343
	21st to end	Night	38% 4 10% 18% 18% 12%	2470
December	1st to 10th	Night	38% 4 8% 20% 19% 11%	2470
	11th to 20th	Night	43% 4 7% 13% 16% 17%	3037
	21st to end	Night	35% 5% 7% 15% 25% 12%	4532

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% 9% 32% 28% 9% 2	270
	11th to 20th	Day	22% 8% 31% 29% 9% 1	270
	21st to end	Day	20% 10% 33% 26% 9% 2	297
February	1st to 10th	Day	23% 13% 27% 26% 9% 1	270
	11th to 20th	Day	23% 11% 35% 21% 10%	270
	21st to end	Day	20% 11% 45% 20% 4	225
March	1st to 10th	Day	19% 12% 39% 21% 9%	270
	11th to 20th	Day	16% 9% 40% 28% 7%	270
	21st to end	Day	15% 12% 39% 27% 6% 1	297
April	1st to 10th	Day	17% 10% 37% 27% 8% 1	270
	11th to 20th	Day	17% 14% 30% 27% 12%	264
	21st to end	Day	16% 9% 36% 30% 9% 1	527
May	1st to 10th	Day	14% 10% 36% 30% 9%	653
	11th to 20th	Day	12% 9% 29% 33% 15% 2	675
	21st to end	Day	10% 6% 26% 38% 18% 1	734
June	1st to 10th	Day	6% 2 12% 37% 33% 9%	680
	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% 19% 29% 21% 10%	330
November	1st to 10th	Day	15% 5% 17% 30% 25% 7%	273
	11th to 20th	Day	19% 7% 16% 31% 21% 5%	270
	21st to end	Day	21% 13% 30% 23% 10% 3	270
December	1st to 10th	Day	20% 3 25% 29% 16% 6%	270
	11th to 20th	Day	21% 11% 26% 30% 11% 1	270
	21st to end	Day	18% 9% 28% 32% 12% 2	297
January	1st to 10th	Night	20% 7% 26% 35% 11%	950
	11th to 20th	Night	20% 7% 30% 36% 7%	950
	21st to end	Night	21% 9% 28% 35% 8%	879
February	1st to 10th	Night	21% 11% 32% 31% 5%	760
	11th to 20th	Night	24% 9% 34% 30% 3	744
	21st to end	Night	21% 13% 41% 22% 2	628
March	1st to 10th	Night	21% 10% 42% 24% 3	580
	11th to 20th	Night	19% 11% 34% 31% 4	570
	21st to end	Night	16% 9% 45% 28% 2	627
April	1st to 10th	Night	14% 9% 48% 26% 4	570
	11th to 20th	Night	15% 12% 40% 29% 4	563
	21st to end	Night	16% 10% 42% 27% 5% 1	570
May	1st to 10th	Night	12% 9% 48% 26% 4	570
	11th to 20th	Night	10% 6% 39% 35% 7% 1	561
	21st to end	Night	9% 5% 37% 40% 9%	617
June	1st to 10th	Night	5% 2 22% 52% 19% 1	570
	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	11% 4 23% 39% 21% 2	748
	11th to 20th	Night	13% 5% 25% 41% 15% 1	823
	21st to end	Night	17% 6% 28% 33% 15% 1	950
December	1st to 10th	Night	19% 6% 23% 36% 14% 1	950
	11th to 20th	Night	23% 7% 27% 33% 9%	950
	21st to end	Night	20% 7% 26% 38% 9%	1045

Page generated at 2024-10-30 12:19:24. Version ac07

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