Cheddar Ranch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cheddar Ranch Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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	8% 5% 2 5% 3 3 4 3 5% 6% 55%	131613
	Night	7% 5% 2 4 3 3 3 3 5% 7% 58%	130551
Transparency	Day	29% 4 9% 25% 25% 8%	135356
	Night	26% 4 9% 24% 27% 10%	134859
Seeing	Day	11% 9% 29% 31% 17% 3	17206
	Night	10% 10% 35% 34% 11% 1	27249

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	8% 5% 2 7% 5% 4 4 3 4 5% 52%	5289
	11th to 20th	Day	4 5% 2 4 2 3 2 2 3 4 68%	5310
	21st to end	Day	11% 8% 3 7% 3 3 3 2 3 5% 52%	5812
February	1st to 10th	Day	8% 11% 3 6% 3 3 3 2 4 4 53%	2053
	11th to 20th	Day	8% 7% 2 6% 4 4 4 2 3 5% 55%	2049
	21st to end	Day	8% 7% 3 5% 3 3 3 3 3 5% 58%	1730
March	1st to 10th	Day	10% 5% 2 5% 3 3 3 2 3 4 60%	4814
	11th to 20th	Day	6% 5% 2 5% 3 3 4 4 5% 6% 57%	5090
	21st to end	Day	8% 8% 3 7% 4 4 3 4 6% 6% 49%	5583
April	1st to 10th	Day	8% 5% 2 5% 4 4 3 3 5% 7% 54%	2211
	11th to 20th	Day	12% 6% 2 6% 4 3 3 3 5% 7% 49%	2418
	21st to end	Day	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2600
May	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 4 6% 42%	5862
	11th to 20th	Day	9% 4 2 4 3 3 3 3 4 7% 58%	5862
	21st to end	Day	12% 6% 2 6% 4 4 4 3 5% 6% 48%	6434
June	1st to 10th	Day	6% 4 2 5% 4 4 4 4 6% 9% 52%	2600
	11th to 20th	Day	5% 4 2 4 4 4 4 4 5% 8% 58%	2600
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 9% 59%	2600
July	1st to 10th	Day	8% 6% 2 7% 5% 4 5% 4 7% 9% 43%	5900
	11th to 20th	Day	5% 4 2 4 3 4 4 4 6% 9% 56%	5900
	21st to end	Day	3 2 1 4 3 3 4 4 6% 9% 62%	6422
August	1st to 10th	Day	6% 4 1 5% 4 4 4 4 7% 8% 54%	4234
	11th to 20th	Day	5% 4 2 4 3 4 3 4 6% 8% 56%	4151
	21st to end	Day	5% 4 1 4 3 3 3 3 5% 7% 63%	4312
September	1st to 10th	Day	10% 4 2 5% 3 3 3 2 6% 6% 57%	2205
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 5% 6% 51%	2200
	21st to end	Day	7% 3 1 5% 3 3 4 2 4 7% 59%	2200
October	1st to 10th	Day	8% 4 2 4 3 3 3 3 3 5% 62%	2200
	11th to 20th	Day	10% 2 1 3 2 2 2 2 3 4 68%	2019
	21st to end	Day	11% 4 2 4 3 3 2 2 3 5% 61%	2200
November	1st to 10th	Day	7% 3 2 3 2 3 2 3 4 6% 65%	1875
	11th to 20th	Day	6% 4 2 4 3 1 2 3 4 5% 66%	1890
	21st to end	Day	9% 5% 2 4 3 2 3 2 5% 6% 60%	1890
December	1st to 10th	Day	4 4 2 5% 3 3 3 3 5% 6% 63%	3030
	11th to 20th	Day	6% 4 2 4 2 3 3 2 5% 6% 63%	3481
	21st to end	Day	12% 11% 3 7% 4 4 4 3 4 5% 45%	4587
January	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 5% 6% 51%	7634
	11th to 20th	Night	3 3 1 3 2 3 3 3 4 6% 70%	7680
	21st to end	Night	13% 8% 2 6% 3 3 3 3 4 4 51%	8402
February	1st to 10th	Night	10% 8% 3 5% 4 3 3 3 3 4 53%	2583
	11th to 20th	Night	10% 7% 2 4 4 3 4 3 4 6% 53%	2458
	21st to end	Night	8% 5% 2 5% 3 3 3 3 4 6% 58%	2076
March	1st to 10th	Night	5% 5% 2 5% 3 3 3 3 4 5% 61%	5269
	11th to 20th	Night	6% 4 1 3 2 2 3 3 3 6% 66%	5090
	21st to end	Night	5% 6% 2 6% 4 4 4 4 6% 8% 50%	5588
April	1st to 10th	Night	8% 4 2 3 3 3 3 3 5% 7% 58%	2209
	11th to 20th	Night	12% 5% 2 3 3 3 3 3 5% 7% 53%	1928
	21st to end	Night	12% 5% 2 5% 3 3 3 3 5% 6% 53%	1800
May	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 4 5% 48%	4053
	11th to 20th	Night	8% 3 2 4 3 3 3 4 6% 8% 56%	4053
	21st to end	Night	11% 5% 2 4 3 3 3 3 5% 8% 54%	4443
June	1st to 10th	Night	5% 3 1 4 3 4 5% 4 6% 10% 56%	1800
	11th to 20th	Night	5% 3 1 4 3 3 3 4 6% 10% 58%	1800
	21st to end	Night	5% 2 1 2 3 3 4 3 5% 10% 63%	1800
July	1st to 10th	Night	5% 4 2 5% 4 5% 4 5% 8% 11% 48%	4078
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 11% 57%	4077
	21st to end	Night	3 2 1 4 3 3 3 3 6% 9% 61%	4449
August	1st to 10th	Night	2 2 1 3 3 3 4 4 7% 10% 61%	2926
	11th to 20th	Night	5% 4 1 4 3 4 4 5% 7% 11% 52%	2904
	21st to end	Night	3 2 1 3 3 3 3 3 5% 9% 64%	3597
September	1st to 10th	Night	7% 5% 1 4 3 3 3 4 5% 7% 59%	2195
	11th to 20th	Night	9% 6% 2 5% 3 4 4 3 5% 7% 53%	2200
	21st to end	Night	5% 4 2 5% 3 3 3 3 6% 6% 61%	2200
October	1st to 10th	Night	7% 4 1 3 2 3 3 3 5% 7% 61%	2200
	11th to 20th	Night	9% 3 1 4 2 2 2 2 3 4 68%	2360
	21st to end	Night	10% 4 1 3 3 3 2 2 3 4 65%	2640
November	1st to 10th	Night	8% 4 1 3 2 2 3 3 4 5% 66%	2640
	11th to 20th	Night	5% 4 2 4 3 2 3 2 4 5% 66%	2730
	21st to end	Night	9% 5% 1 3 3 2 2 3 4 6% 61%	2730
December	1st to 10th	Night	5% 3 1 4 3 2 3 3 4 7% 66%	4380
	11th to 20th	Night	5% 5% 2 5% 3 3 2 2 4 5% 64%	4946
	21st to end	Night	12% 8% 2 5% 3 3 3 2 4 5% 51%	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	34% 3 8% 25% 24% 6%	5290
	11th to 20th	Day	23% 2 8% 19% 40% 9%	5310
	21st to end	Day	38% 2 4 20% 29% 7%	5812
February	1st to 10th	Day	35% 3 8% 24% 22% 7%	2053
	11th to 20th	Day	33% 3 8% 25% 23% 7%	2059
	21st to end	Day	31% 3 5% 22% 28% 12%	1730
March	1st to 10th	Day	29% 3 6% 14% 23% 25%	4814
	11th to 20th	Day	27% 4 7% 18% 27% 17%	5090
	21st to end	Day	35% 5% 10% 30% 18% 3	5585
April	1st to 10th	Day	30% 4 10% 25% 21% 9%	2211
	11th to 20th	Day	36% 5% 8% 22% 20% 9%	2418
	21st to end	Day	33% 5% 11% 27% 21% 3	2600
May	1st to 10th	Day	43% 5% 11% 17% 18% 5%	5900
	11th to 20th	Day	28% 4 10% 25% 21% 12%	5866
	21st to end	Day	38% 5% 9% 21% 17% 9%	6447
June	1st to 10th	Day	27% 7% 18% 28% 15% 6%	2600
	11th to 20th	Day	25% 6% 13% 27% 24% 7%	2600
	21st to end	Day	22% 6% 15% 25% 25% 7%	2600
July	1st to 10th	Day	36% 7% 12% 20% 20% 5%	5836
	11th to 20th	Day	25% 5% 10% 31% 25% 4	5831
	21st to end	Day	18% 5% 15% 37% 23% 2	6240
August	1st to 10th	Day	25% 6% 9% 27% 26% 6%	4220
	11th to 20th	Day	25% 5% 13% 30% 23% 4	4190
	21st to end	Day	22% 5% 12% 34% 22% 5%	4311
September	1st to 10th	Day	28% 5% 10% 23% 24% 12%	2205
	11th to 20th	Day	32% 6% 9% 23% 24% 6%	2200
	21st to end	Day	25% 5% 9% 24% 29% 9%	2200
October	1st to 10th	Day	17% 3 6% 15% 44% 16%	3590
	11th to 20th	Day	18% 3 8% 23% 31% 18%	3259
	21st to end	Day	22% 3 6% 26% 32% 10%	3538
November	1st to 10th	Day	22% 3 7% 25% 31% 12%	1875
	11th to 20th	Day	22% 2 6% 28% 31% 11%	1890
	21st to end	Day	27% 3 6% 28% 26% 10%	1890
December	1st to 10th	Day	24% 2 6% 38% 26% 3	3030
	11th to 20th	Day	24% 2 7% 24% 34% 9%	3479
	21st to end	Day	42% 3 7% 23% 18% 6%	4587
January	1st to 10th	Night	34% 3 7% 27% 24% 4	7635
	11th to 20th	Night	17% 2 10% 21% 37% 13%	7680
	21st to end	Night	39% 2 5% 18% 28% 9%	8402
February	1st to 10th	Night	35% 3 7% 22% 24% 9%	2583
	11th to 20th	Night	33% 3 8% 23% 23% 9%	2458
	21st to end	Night	27% 4 6% 20% 28% 15%	2076
March	1st to 10th	Night	25% 3 5% 17% 25% 24%	5269
	11th to 20th	Night	21% 3 5% 20% 26% 24%	5090
	21st to end	Night	31% 4 12% 26% 21% 4	5588
April	1st to 10th	Night	25% 5% 10% 22% 25% 13%	2209
	11th to 20th	Night	30% 6% 9% 25% 20% 10%	1928
	21st to end	Night	33% 4 11% 27% 21% 5%	1800
May	1st to 10th	Night	40% 5% 12% 16% 21% 7%	4080
	11th to 20th	Night	26% 5% 11% 21% 25% 12%	4054
	21st to end	Night	30% 5% 14% 20% 18% 13%	4452
June	1st to 10th	Night	24% 6% 19% 30% 18% 5%	1800
	11th to 20th	Night	21% 5% 14% 30% 24% 5%	1800
	21st to end	Night	18% 4 15% 30% 23% 10%	1800
July	1st to 10th	Night	28% 7% 18% 27% 17% 3	4035
	11th to 20th	Night	20% 5% 11% 32% 26% 5%	4026
	21st to end	Night	19% 4 14% 34% 27% 2	4320
August	1st to 10th	Night	16% 5% 13% 31% 29% 7%	2916
	11th to 20th	Night	25% 4 12% 29% 25% 4	2931
	21st to end	Night	18% 5% 14% 36% 21% 5%	3597
September	1st to 10th	Night	25% 5% 9% 25% 25% 11%	2195
	11th to 20th	Night	31% 6% 10% 21% 25% 9%	2200
	21st to end	Night	23% 5% 7% 25% 30% 10%	2200
October	1st to 10th	Night	17% 4 7% 15% 36% 21%	3590
	11th to 20th	Night	16% 3 5% 22% 37% 18%	3833
	21st to end	Night	19% 2 5% 26% 37% 11%	4252
November	1st to 10th	Night	23% 3 8% 23% 30% 14%	2640
	11th to 20th	Night	22% 3 6% 26% 32% 12%	2730
	21st to end	Night	26% 3 7% 24% 28% 12%	2730
December	1st to 10th	Night	20% 3 7% 39% 26% 5%	4380
	11th to 20th	Night	25% 2 5% 24% 34% 10%	4947
	21st to end	Night	36% 3 7% 25% 22% 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	13% 14% 42% 24% 7% 1	330
	11th to 20th	Day	15% 21% 37% 22% 5% 1	330
	21st to end	Day	12% 14% 45% 23% 6%	363
February	1st to 10th	Day	13% 15% 43% 24% 5%	330
	11th to 20th	Day	16% 20% 43% 16% 4 1	330
	21st to end	Day	12% 22% 42% 19% 5%	273
March	1st to 10th	Day	15% 20% 39% 22% 5%	330
	11th to 20th	Day	19% 18% 39% 18% 5% 1	330
	21st to end	Day	15% 17% 45% 19% 4 1	363
April	1st to 10th	Day	17% 20% 35% 21% 7% 1	303
	11th to 20th	Day	15% 21% 43% 18% 3	536
	21st to end	Day	16% 18% 35% 24% 5% 2	700
May	1st to 10th	Day	13% 11% 42% 25% 7% 1	703
	11th to 20th	Day	14% 11% 33% 26% 14% 2	725
	21st to end	Day	13% 8% 32% 29% 15% 3	789
June	1st to 10th	Day	8% 5% 25% 38% 21% 3	730
	11th to 20th	Day	7% 3 31% 40% 16% 3	730
	21st to end	Day	4 1 28% 42% 21% 5%	730
July	1st to 10th	Day	8% 2 13% 36% 33% 9%	721
	11th to 20th	Day	3 2 12% 46% 31% 6%	718
	21st to end	Day	6% 1 7% 39% 38% 8%	794
August	1st to 10th	Day	6% 2 13% 47% 28% 4	726
	11th to 20th	Day	7% 2 15% 37% 33% 6%	708
	21st to end	Day	7% 1 11% 36% 37% 9%	583
September	1st to 10th	Day	7% 10% 44% 30% 8%	335
	11th to 20th	Day	6% 2 18% 44% 27% 4	330
	21st to end	Day	9% 4 23% 34% 22% 7%	330
October	1st to 10th	Day	7% 7% 28% 33% 19% 5%	330
	11th to 20th	Day	9% 11% 38% 30% 12% 1	330
	21st to end	Day	21% 12% 31% 23% 12% 2	363
November	1st to 10th	Day	12% 9% 39% 32% 7% 2	330
	11th to 20th	Day	13% 13% 35% 26% 11% 2	330
	21st to end	Day	15% 15% 38% 23% 9% 1	330
December	1st to 10th	Day	8% 12% 45% 25% 9%	330
	11th to 20th	Day	11% 15% 41% 25% 9% 1	330
	21st to end	Day	15% 18% 39% 22% 5%	363
January	1st to 10th	Night	9% 13% 39% 33% 6%	1050
	11th to 20th	Night	11% 15% 37% 33% 4	1050
	21st to end	Night	11% 12% 41% 29% 6%	1155
February	1st to 10th	Night	14% 12% 41% 28% 4	903
	11th to 20th	Night	14% 17% 40% 25% 4	824
	21st to end	Night	12% 15% 45% 25% 3	692
March	1st to 10th	Night	6% 18% 41% 30% 5%	745
	11th to 20th	Night	17% 13% 39% 25% 6%	630
	21st to end	Night	11% 18% 43% 24% 5%	693
April	1st to 10th	Night	11% 15% 46% 22% 6%	603
	11th to 20th	Night	14% 16% 45% 22% 2	593
	21st to end	Night	16% 13% 41% 23% 7%	600
May	1st to 10th	Night	13% 11% 43% 29% 4	600
	11th to 20th	Night	10% 9% 34% 33% 13% 1	591
	21st to end	Night	8% 7% 38% 34% 11% 2	650
June	1st to 10th	Night	6% 5% 30% 42% 17% 1	600
	11th to 20th	Night	6% 4 36% 44% 10%	600
	21st to end	Night	4 2 34% 45% 14% 1	600
July	1st to 10th	Night	5% 2 17% 46% 28% 3	585
	11th to 20th	Night	4 1 18% 51% 26% 1	582
	21st to end	Night	4 1 11% 55% 27% 2	657
August	1st to 10th	Night	3 1 16% 51% 27% 1	597
	11th to 20th	Night	6% 1 16% 48% 27% 2	594
	21st to end	Night	5% 1 16% 41% 35% 2	660
September	1st to 10th	Night	8% 3 19% 43% 25% 2	600
	11th to 20th	Night	11% 2 26% 37% 22% 2	600
	21st to end	Night	6% 4 29% 41% 18% 2	600
October	1st to 10th	Night	9% 5% 29% 39% 18% 1	600
	11th to 20th	Night	11% 7% 40% 34% 8%	760
	21st to end	Night	12% 11% 36% 27% 13% 1	880
November	1st to 10th	Night	10% 7% 39% 34% 9% 1	1000
	11th to 20th	Night	7% 11% 40% 34% 7%	1050
	21st to end	Night	12% 13% 34% 31% 10%	1050
December	1st to 10th	Night	10% 12% 39% 31% 7% 1	1050
	11th to 20th	Night	13% 12% 42% 28% 6%	1050
	21st to end	Night	12% 13% 38% 32% 5%	1155

Page generated at 2026-03-30 01:18:44. Version ac07

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