Cabin-1800's Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabin-1800's Site clear sky chart from 2005-11-06 to 2026-01-17.

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% 7% 2 7% 5% 5% 5% 4 6% 7% 42%	126002
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 45%	125782
Transparency	Day	39% 7% 11% 19% 18% 7%	129640
	Night	36% 6% 11% 19% 19% 8%	130219
Seeing	Day	15% 8% 22% 25% 21% 9%	16535
	Night	15% 8% 24% 30% 19% 4	26303

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% 11% 3 7% 3 3 3 3 4 6% 43%	4167
	11th to 20th	Day	11% 6% 3 6% 4 4 4 3 4 6% 50%	4134
	21st to end	Day	21% 12% 3 6% 4 3 3 3 4 5% 38%	4589
February	1st to 10th	Day	17% 9% 3 6% 5% 4 3 3 4 6% 40%	2000
	11th to 20th	Day	19% 8% 3 6% 4 3 3 3 4 5% 42%	1948
	21st to end	Day	15% 10% 3 9% 5% 3 4 3 4 5% 39%	1650
March	1st to 10th	Day	18% 10% 3 8% 5% 5% 5% 3 5% 6% 33%	4771
	11th to 20th	Day	14% 6% 3 6% 4 4 4 3 5% 6% 45%	4980
	21st to end	Day	13% 9% 3 9% 5% 5% 5% 3 4 5% 40%	5684
April	1st to 10th	Day	11% 7% 3 6% 5% 4 5% 5% 6% 7% 42%	2400
	11th to 20th	Day	15% 7% 3 6% 4 4 4 4 5% 7% 42%	2374
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 8% 8% 44%	2400
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 7% 8% 35%	5405
	11th to 20th	Day	11% 7% 2 6% 5% 5% 5% 4 7% 9% 40%	5404
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 42%	5932
June	1st to 10th	Day	8% 5% 3 7% 7% 7% 7% 5% 7% 10% 35%	2688
	11th to 20th	Day	5% 5% 2 8% 7% 7% 7% 5% 8% 9% 38%	2753
	21st to end	Day	7% 5% 2 8% 7% 8% 8% 5% 8% 9% 33%	2600
July	1st to 10th	Day	6% 6% 2 8% 7% 7% 8% 6% 9% 11% 31%	5900
	11th to 20th	Day	6% 4 2 6% 7% 7% 8% 6% 9% 11% 33%	5900
	21st to end	Day	9% 7% 3 7% 6% 7% 7% 5% 7% 11% 31%	6204
August	1st to 10th	Day	3 3 2 4 4 4 4 4 5% 7% 60%	3905
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 6% 10% 47%	3849
	21st to end	Day	10% 5% 3 7% 6% 5% 5% 5% 5% 7% 41%	4312
September	1st to 10th	Day	10% 6% 2 7% 6% 6% 5% 5% 7% 8% 38%	2400
	11th to 20th	Day	12% 6% 2 7% 6% 5% 5% 4 4 5% 43%	2369
	21st to end	Day	10% 4 2 5% 4 4 4 4 5% 7% 52%	2200
October	1st to 10th	Day	6% 4 2 5% 4 4 4 4 5% 6% 56%	2042
	11th to 20th	Day	9% 5% 2 5% 3 3 3 2 5% 6% 58%	2000
	21st to end	Day	12% 5% 1 5% 3 4 3 2 4 6% 57%	2200
November	1st to 10th	Day	10% 5% 2 6% 4 3 4 3 5% 6% 52%	2058
	11th to 20th	Day	11% 7% 2 4 3 3 4 3 4 6% 53%	1979
	21st to end	Day	14% 6% 2 6% 4 3 4 2 5% 6% 47%	1890
December	1st to 10th	Day	16% 9% 3 6% 3 3 4 3 5% 6% 42%	3020
	11th to 20th	Day	12% 9% 3 7% 5% 4 5% 4 4 5% 43%	3373
	21st to end	Day	14% 7% 2 6% 3 3 3 3 4 5% 48%	4522
January	1st to 10th	Night	18% 8% 4 7% 3 3 3 2 3 4 46%	6018
	11th to 20th	Night	15% 7% 4 6% 3 3 3 2 4 5% 48%	5979
	21st to end	Night	24% 10% 2 5% 3 3 3 3 3 4 39%	6380
February	1st to 10th	Night	14% 9% 3 6% 4 4 3 3 5% 6% 42%	2400
	11th to 20th	Night	20% 9% 3 5% 4 3 3 2 4 5% 43%	2338
	21st to end	Night	19% 9% 3 7% 4 4 3 3 4 5% 39%	1980
March	1st to 10th	Night	17% 10% 2 6% 4 4 4 4 5% 6% 38%	5110
	11th to 20th	Night	11% 6% 2 5% 4 4 4 3 5% 7% 48%	4980
	21st to end	Night	16% 7% 2 6% 4 4 4 4 5% 5% 42%	5243
April	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 7% 46%	2000
	11th to 20th	Night	12% 5% 2 6% 4 4 3 4 6% 7% 48%	1975
	21st to end	Night	8% 4 2 4 4 4 4 3 5% 8% 53%	2000
May	1st to 10th	Night	13% 6% 2 6% 5% 4 4 4 5% 8% 42%	4490
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 5% 9% 41%	4489
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 7% 11% 41%	4922
June	1st to 10th	Night	8% 6% 2 7% 5% 4 5% 5% 8% 8% 42%	1690
	11th to 20th	Night	4 4 2 5% 4 5% 5% 5% 7% 11% 49%	1628
	21st to end	Night	4 4 2 6% 5% 4 5% 6% 10% 12% 42%	1800
July	1st to 10th	Night	5% 5% 2 8% 5% 6% 6% 5% 7% 12% 38%	4078
	11th to 20th	Night	5% 4 2 6% 5% 5% 7% 7% 10% 13% 35%	4077
	21st to end	Night	7% 7% 2 7% 6% 5% 6% 5% 9% 11% 34%	4657
August	1st to 10th	Night	2 2 1 3 3 3 4 4 6% 11% 61%	3244
	11th to 20th	Night	4 3 1 4 4 5% 6% 5% 9% 13% 46%	3200
	21st to end	Night	7% 5% 3 8% 5% 5% 5% 5% 6% 7% 44%	3586
September	1st to 10th	Night	8% 5% 2 6% 5% 4 6% 5% 7% 9% 43%	2000
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 6% 9% 46%	2010
	21st to end	Night	9% 4 2 5% 3 3 4 3 5% 7% 55%	2200
October	1st to 10th	Night	6% 3 1 5% 3 3 3 4 5% 6% 60%	2353
	11th to 20th	Night	9% 5% 2 5% 3 3 3 3 4 5% 59%	2400
	21st to end	Night	14% 6% 2 4 2 3 3 3 3 4 56%	2640
November	1st to 10th	Night	12% 4 2 4 3 3 3 3 4 5% 58%	2460
	11th to 20th	Night	12% 5% 2 5% 3 3 3 2 4 6% 55%	2637
	21st to end	Night	12% 7% 2 5% 3 4 3 3 5% 7% 48%	2730
December	1st to 10th	Night	19% 8% 2 5% 4 3 3 3 4 5% 45%	4380
	11th to 20th	Night	14% 10% 3 6% 4 4 3 3 4 5% 45%	5028
	21st to end	Night	18% 10% 2 5% 2 3 3 2 3 5% 47%	6680

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	43% 4 5% 20% 24% 4	4168
	11th to 20th	Day	36% 4 5% 18% 29% 9%	4134
	21st to end	Day	50% 3 6% 11% 17% 13%	4589
February	1st to 10th	Day	47% 4 7% 14% 17% 11%	2000
	11th to 20th	Day	46% 4 5% 16% 20% 8%	1958
	21st to end	Day	47% 5% 6% 14% 17% 10%	1650
March	1st to 10th	Day	52% 5% 6% 12% 17% 8%	4771
	11th to 20th	Day	40% 5% 6% 14% 18% 18%	4980
	21st to end	Day	47% 5% 8% 16% 18% 7%	5686
April	1st to 10th	Day	39% 7% 9% 18% 18% 8%	2400
	11th to 20th	Day	41% 6% 8% 17% 21% 7%	2374
	21st to end	Day	34% 8% 11% 22% 21% 3	2400
May	1st to 10th	Day	43% 9% 13% 22% 11% 2	5440
	11th to 20th	Day	39% 8% 16% 19% 17% 1	5408
	21st to end	Day	33% 9% 15% 27% 14% 2	5945
June	1st to 10th	Day	41% 11% 14% 21% 12% 1	2688
	11th to 20th	Day	38% 10% 15% 21% 13% 2	2753
	21st to end	Day	42% 12% 17% 18% 9% 2	2600
July	1st to 10th	Day	42% 12% 19% 19% 8%	5836
	11th to 20th	Day	38% 13% 28% 18% 2 1	5831
	21st to end	Day	43% 12% 16% 24% 5%	6026
August	1st to 10th	Day	23% 7% 12% 28% 28% 2	3893
	11th to 20th	Day	30% 10% 19% 29% 12% 1	3885
	21st to end	Day	41% 8% 14% 19% 16% 3	4311
September	1st to 10th	Day	40% 10% 15% 16% 15% 5%	2400
	11th to 20th	Day	41% 7% 10% 17% 18% 7%	2369
	21st to end	Day	31% 6% 12% 23% 18% 10%	2200
October	1st to 10th	Day	21% 4 7% 12% 24% 32%	3328
	11th to 20th	Day	18% 2 6% 14% 29% 31%	3240
	21st to end	Day	22% 3 4 15% 36% 19%	3538
November	1st to 10th	Day	33% 5% 7% 17% 26% 11%	2058
	11th to 20th	Day	33% 3 7% 16% 27% 13%	1979
	21st to end	Day	39% 4 8% 17% 22% 10%	1890
December	1st to 10th	Day	44% 4 6% 16% 24% 6%	3020
	11th to 20th	Day	44% 5% 8% 14% 20% 10%	3370
	21st to end	Day	38% 5% 6% 22% 27% 2	4522
January	1st to 10th	Night	43% 3 5% 19% 24% 6%	6018
	11th to 20th	Night	40% 4 5% 13% 27% 10%	5979
	21st to end	Night	50% 3 4 13% 20% 9%	6380
February	1st to 10th	Night	43% 4 8% 15% 20% 10%	2400
	11th to 20th	Night	45% 4 5% 15% 21% 10%	2339
	21st to end	Night	47% 5% 7% 14% 15% 12%	1980
March	1st to 10th	Night	45% 6% 8% 12% 21% 8%	5110
	11th to 20th	Night	35% 6% 8% 16% 19% 17%	4980
	21st to end	Night	43% 6% 9% 17% 18% 7%	5243
April	1st to 10th	Night	36% 8% 11% 18% 18% 10%	2000
	11th to 20th	Night	34% 6% 9% 19% 20% 10%	1975
	21st to end	Night	28% 6% 11% 23% 26% 6%	2000
May	1st to 10th	Night	39% 8% 14% 22% 15% 3	4520
	11th to 20th	Night	38% 7% 16% 24% 14% 1	4490
	21st to end	Night	34% 9% 18% 26% 12% 2	4931
June	1st to 10th	Night	35% 10% 17% 22% 15% 2	1690
	11th to 20th	Night	28% 9% 19% 27% 14% 3	1628
	21st to end	Night	29% 12% 23% 22% 12% 2	1800
July	1st to 10th	Night	37% 13% 24% 20% 6% 1	4035
	11th to 20th	Night	35% 16% 30% 16% 3 1	4026
	21st to end	Night	41% 14% 17% 23% 5%	4522
August	1st to 10th	Night	17% 8% 19% 32% 23% 1	3232
	11th to 20th	Night	26% 11% 22% 30% 9% 1	3230
	21st to end	Night	37% 9% 13% 19% 18% 4	3586
September	1st to 10th	Night	33% 11% 16% 19% 14% 6%	2000
	11th to 20th	Night	33% 8% 15% 17% 19% 7%	2010
	21st to end	Night	30% 5% 10% 23% 22% 10%	2200
October	1st to 10th	Night	20% 4 8% 12% 24% 31%	3847
	11th to 20th	Night	18% 3 4 20% 22% 33%	3899
	21st to end	Night	21% 3 5% 12% 38% 21%	4252
November	1st to 10th	Night	30% 4 8% 18% 26% 14%	2460
	11th to 20th	Night	31% 3 7% 16% 28% 15%	2637
	21st to end	Night	36% 4 8% 19% 21% 12%	2730
December	1st to 10th	Night	43% 3 6% 17% 25% 5%	4380
	11th to 20th	Night	43% 4 7% 16% 23% 7%	5030
	21st to end	Night	42% 3 5% 20% 25% 5%	6680

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	26% 22% 33% 15% 2 1	330
	11th to 20th	Day	25% 20% 35% 16% 3 1	318
	21st to end	Day	29% 20% 37% 12% 2	330
February	1st to 10th	Day	27% 18% 35% 14% 5% 1	300
	11th to 20th	Day	37% 17% 32% 11% 3	300
	21st to end	Day	27% 25% 37% 10% 1	249
March	1st to 10th	Day	27% 16% 39% 15% 3	300
	11th to 20th	Day	29% 19% 36% 14% 2 1	300
	21st to end	Day	21% 18% 35% 22% 4	423
April	1st to 10th	Day	19% 17% 35% 22% 7% 1	500
	11th to 20th	Day	22% 14% 31% 21% 11% 1	492
	21st to end	Day	12% 10% 34% 30% 14% 1	500
May	1st to 10th	Day	17% 9% 31% 26% 13% 4	503
	11th to 20th	Day	13% 8% 25% 29% 19% 7%	527
	21st to end	Day	8% 5% 21% 34% 24% 9%	573
June	1st to 10th	Day	12% 4 15% 35% 26% 10%	693
	11th to 20th	Day	6% 3 16% 37% 30% 8%	730
	21st to end	Day	8% 2 12% 29% 30% 19%	730
July	1st to 10th	Day	5% 2 7% 28% 41% 16%	721
	11th to 20th	Day	6% 2 4 25% 42% 20%	718
	21st to end	Day	8% 3 5% 28% 39% 17%	702
August	1st to 10th	Day	5% 2 6% 25% 35% 26%	526
	11th to 20th	Day	8% 2 3 24% 41% 23%	519
	21st to end	Day	13% 3 7% 23% 33% 21%	583
September	1st to 10th	Day	11% 3 7% 30% 32% 17%	530
	11th to 20th	Day	12% 2 12% 24% 35% 16%	530
	21st to end	Day	12% 4 13% 30% 29% 11%	530
October	1st to 10th	Day	8% 4 23% 37% 19% 8%	372
	11th to 20th	Day	8% 6% 24% 36% 18% 7%	330
	21st to end	Day	20% 10% 25% 26% 16% 4	363
November	1st to 10th	Day	14% 10% 29% 32% 12% 3	330
	11th to 20th	Day	17% 8% 38% 28% 9% 1	330
	21st to end	Day	16% 18% 37% 17% 10% 2	330
December	1st to 10th	Day	26% 13% 35% 21% 5% 1	330
	11th to 20th	Day	18% 16% 41% 19% 6% 1	330
	21st to end	Day	25% 15% 35% 15% 9%	363
January	1st to 10th	Night	24% 14% 35% 22% 5%	1050
	11th to 20th	Night	23% 15% 30% 25% 7%	1031
	21st to end	Night	24% 15% 33% 24% 3	1055
February	1st to 10th	Night	21% 15% 37% 23% 4	800
	11th to 20th	Night	27% 16% 30% 22% 4	783
	21st to end	Night	25% 19% 35% 18% 2	660
March	1st to 10th	Night	19% 14% 36% 25% 6%	693
	11th to 20th	Night	19% 12% 37% 27% 5%	600
	21st to end	Night	19% 9% 38% 27% 7%	660
April	1st to 10th	Night	16% 12% 36% 26% 9% 1	600
	11th to 20th	Night	15% 11% 36% 28% 9% 1	593
	21st to end	Night	9% 8% 35% 33% 13% 1	600
May	1st to 10th	Night	11% 5% 30% 38% 13% 2	600
	11th to 20th	Night	12% 3 27% 36% 19% 3	591
	21st to end	Night	6% 3 22% 39% 26% 5%	650
June	1st to 10th	Night	10% 1 12% 41% 30% 6%	600
	11th to 20th	Night	5% 2 10% 40% 38% 7%	600
	21st to end	Night	6% 2 10% 31% 37% 15%	600
July	1st to 10th	Night	5% 2 6% 29% 46% 12%	585
	11th to 20th	Night	5% 2 5% 27% 44% 17%	582
	21st to end	Night	5% 2 5% 33% 43% 12%	657
August	1st to 10th	Night	4 1 4 25% 48% 19%	597
	11th to 20th	Night	6% 2 4 25% 47% 15%	594
	21st to end	Night	12% 2 7% 25% 39% 16%	660
September	1st to 10th	Night	9% 3 8% 32% 40% 9%	600
	11th to 20th	Night	9% 2 9% 28% 42% 10%	610
	21st to end	Night	11% 2 12% 35% 35% 6%	800
October	1st to 10th	Night	7% 3 17% 36% 32% 5%	800
	11th to 20th	Night	12% 6% 23% 35% 23% 2	800
	21st to end	Night	18% 4 23% 32% 19% 3	880
November	1st to 10th	Night	14% 6% 22% 38% 17% 3	820
	11th to 20th	Night	15% 8% 28% 36% 13% 1	840
	21st to end	Night	16% 11% 30% 28% 13% 2	840
December	1st to 10th	Night	23% 10% 28% 30% 8%	840
	11th to 20th	Night	21% 9% 35% 28% 7%	877
	21st to end	Night	25% 12% 31% 26% 6%	1155

Page generated at 2026-01-14 00:14:25. Version ac07

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