Cabin in the Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cabin in the Sky Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	9% 9% 3 7% 4 4 4 4 5% 7% 43%	131347
	Night	11% 12% 3 7% 4 4 4 3 5% 6% 40%	129341
Transparency	Day	25% 39% 8% 16% 10% 3	131073
	Night	27% 38% 8% 15% 9% 2	129192
Seeing	Day	19% 12% 28% 26% 12% 3	16352
	Night	19% 7% 20% 33% 18% 3	25310

By Month

Percentage Cloud Cover NW (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% 21% 7% 13% 5% 4 3 3 5% 5% 19%	3719
	11th to 20th	Day	11% 10% 3 5% 4 4 3 3 6% 6% 45%	3696
	21st to end	Day	18% 20% 4 6% 3 3 3 3 4 5% 32%	4092
February	1st to 10th	Day	13% 15% 4 8% 4 5% 5% 3 5% 7% 30%	1650
	11th to 20th	Day	10% 15% 4 8% 6% 5% 5% 4 5% 5% 32%	1765
	21st to end	Day	10% 15% 3 9% 7% 6% 6% 4 6% 6% 26%	1650
March	1st to 10th	Day	8% 13% 4 8% 5% 5% 6% 5% 6% 7% 34%	4518
	11th to 20th	Day	8% 13% 4 7% 4 4 4 3 5% 6% 41%	4729
	21st to end	Day	14% 14% 4 9% 6% 5% 6% 5% 6% 7% 24%	5941
April	1st to 10th	Day	11% 12% 4 10% 7% 6% 5% 4 7% 7% 27%	2400
	11th to 20th	Day	9% 10% 3 8% 7% 5% 5% 5% 6% 8% 34%	2389
	21st to end	Day	9% 11% 4 9% 5% 5% 5% 4 6% 8% 34%	2695
May	1st to 10th	Day	8% 8% 3 7% 4 4 4 4 5% 7% 46%	6320
	11th to 20th	Day	9% 9% 3 9% 5% 5% 6% 5% 7% 9% 33%	6277
	21st to end	Day	10% 9% 3 7% 5% 5% 5% 4 7% 8% 36%	6903
June	1st to 10th	Day	14% 8% 3 7% 5% 5% 6% 4 6% 7% 36%	2945
	11th to 20th	Day	11% 8% 3 8% 6% 5% 7% 5% 7% 7% 32%	3004
	21st to end	Day	8% 6% 2 7% 4 4 5% 5% 6% 7% 46%	3167
July	1st to 10th	Day	2 2 1 3 2 2 3 3 4 6% 73%	6371
	11th to 20th	Day	3 2 1 3 2 2 3 2 3 5% 74%	6320
	21st to end	Day	4 3 1 4 3 3 3 3 5% 7% 64%	6874
August	1st to 10th	Day	2 3 1 3 3 3 4 4 6% 7% 64%	4541
	11th to 20th	Day	3 3 1 3 4 4 4 4 6% 7% 60%	4443
	21st to end	Day	6% 4 2 5% 4 4 5% 4 5% 7% 55%	4312
September	1st to 10th	Day	5% 4 1 5% 4 3 3 3 3 8% 62%	2400
	11th to 20th	Day	9% 5% 2 5% 5% 4 4 4 5% 7% 50%	2364
	21st to end	Day	10% 5% 2 6% 4 4 4 3 5% 7% 50%	2200
October	1st to 10th	Day	8% 6% 2 6% 4 4 4 3 5% 7% 50%	3285
	11th to 20th	Day	13% 8% 3 8% 5% 5% 4 4 5% 5% 40%	3163
	21st to end	Day	15% 13% 5% 7% 4 4 3 3 4 5% 37%	3200
November	1st to 10th	Day	21% 11% 3 6% 5% 5% 4 4 6% 7% 28%	1804
	11th to 20th	Day	19% 17% 3 8% 4 4 5% 2 3 5% 30%	1680
	21st to end	Day	16% 17% 6% 11% 4 4 4 4 5% 5% 25%	1601
December	1st to 10th	Day	12% 10% 3 8% 4 3 4 4 6% 8% 37%	2360
	11th to 20th	Day	16% 24% 8% 9% 4 4 5% 3 4 5% 17%	2712
	21st to end	Day	16% 20% 7% 13% 6% 5% 4 4 5% 5% 15%	3857
January	1st to 10th	Night	18% 24% 8% 12% 4 3 3 2 3 3 19%	6446
	11th to 20th	Night	12% 11% 3 6% 4 3 4 3 5% 7% 44%	6422
	21st to end	Night	17% 20% 7% 7% 3 3 3 2 4 4 31%	7106
February	1st to 10th	Night	14% 15% 5% 9% 5% 4 5% 3 5% 6% 29%	2747
	11th to 20th	Night	11% 13% 5% 8% 5% 4 4 4 5% 6% 36%	2522
	21st to end	Night	11% 15% 5% 8% 5% 5% 4 4 6% 7% 29%	1980
March	1st to 10th	Night	7% 11% 3 10% 5% 5% 5% 4 6% 8% 36%	5376
	11th to 20th	Night	10% 14% 4 6% 4 3 3 3 5% 6% 43%	5268
	21st to end	Night	14% 16% 5% 10% 6% 5% 5% 4 5% 6% 23%	5033
April	1st to 10th	Night	12% 10% 3 7% 4 4 5% 4 7% 8% 35%	2000
	11th to 20th	Night	8% 8% 3 7% 5% 5% 6% 4 6% 6% 41%	1961
	21st to end	Night	9% 9% 3 6% 5% 4 6% 4 6% 8% 41%	1703
May	1st to 10th	Night	6% 9% 2 7% 4 4 4 3 5% 7% 50%	3640
	11th to 20th	Night	7% 8% 3 7% 5% 5% 4 4 6% 8% 43%	3616
	21st to end	Night	9% 6% 3 6% 4 4 4 4 7% 9% 44%	3972
June	1st to 10th	Night	12% 6% 2 5% 4 5% 6% 5% 7% 8% 40%	1450
	11th to 20th	Night	9% 7% 2 6% 6% 6% 5% 4 6% 7% 40%	1385
	21st to end	Night	7% 5% 2 5% 4 4 5% 4 6% 7% 52%	1228
July	1st to 10th	Night	2 3 1 2 1 2 2 2 4 6% 73%	3566
	11th to 20th	Night	2 2 1 2 2 2 3 2 3 6% 75%	3640
	21st to end	Night	2 2 1 4 3 3 3 3 4 6% 67%	3969
August	1st to 10th	Night	3 3 1 5% 3 3 4 3 5% 5% 65%	2609
	11th to 20th	Night	3 2 1 4 3 3 4 3 7% 10% 61%	2601
	21st to end	Night	8% 4 2 4 3 3 4 4 5% 7% 56%	3608
September	1st to 10th	Night	3 4 2 3 2 3 3 4 5% 7% 63%	2000
	11th to 20th	Night	8% 5% 1 5% 3 3 3 3 5% 8% 55%	2035
	21st to end	Night	8% 6% 3 6% 4 4 3 3 5% 7% 51%	2200
October	1st to 10th	Night	8% 5% 2 5% 4 5% 5% 4 5% 6% 51%	3911
	11th to 20th	Night	16% 9% 3 7% 4 3 4 3 5% 6% 42%	3992
	21st to end	Night	18% 9% 3 7% 4 4 3 3 5% 4 42%	4600
November	1st to 10th	Night	20% 14% 3 6% 4 4 3 3 6% 7% 29%	2712
	11th to 20th	Night	22% 14% 3 7% 4 4 4 4 5% 6% 28%	2940
	21st to end	Night	18% 17% 4 10% 5% 3 4 4 4 5% 24%	2993
December	1st to 10th	Night	10% 11% 3 8% 4 4 4 4 5% 7% 40%	5040
	11th to 20th	Night	20% 21% 6% 10% 5% 4 5% 3 4 5% 18%	5700
	21st to end	Night	18% 23% 5% 10% 4 4 3 3 4 5% 20%	7370

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	46% 28% 9% 12% 4	3719
	11th to 20th	Day	26% 28% 15% 21% 7% 3	3720
	21st to end	Day	40% 30% 9% 12% 9%	4092
February	1st to 10th	Day	22% 64% 3 5% 6% 2	1650
	11th to 20th	Day	22% 62% 4 7% 3	1774
	21st to end	Day	19% 64% 5% 8% 5%	1650
March	1st to 10th	Day	31% 33% 8% 12% 15% 1	4518
	11th to 20th	Day	29% 30% 10% 23% 7%	4729
	21st to end	Day	40% 33% 9% 11% 7%	5941
April	1st to 10th	Day	19% 64% 7% 8% 3	2400
	11th to 20th	Day	16% 64% 5% 11% 3 1	2389
	21st to end	Day	17% 63% 4 9% 6%	2695
May	1st to 10th	Day	25% 31% 5% 21% 18% 1	6320
	11th to 20th	Day	35% 32% 8% 18% 7%	6280
	21st to end	Day	29% 31% 14% 20% 6%	6904
June	1st to 10th	Day	16% 63% 4 9% 6% 1	2945
	11th to 20th	Day	17% 64% 4 9% 6%	3004
	21st to end	Day	15% 62% 5% 11% 7% 1	3167
July	1st to 10th	Day	9% 27% 8% 17% 23% 16%	6302
	11th to 20th	Day	9% 26% 6% 22% 20% 18%	6245
	21st to end	Day	15% 28% 6% 25% 18% 7%	6681
August	1st to 10th	Day	10% 37% 3 18% 25% 7%	4524
	11th to 20th	Day	15% 40% 10% 28% 7% 1	4485
	21st to end	Day	19% 38% 9% 18% 13% 3	4311
September	1st to 10th	Day	13% 57% 6% 14% 9% 2	2400
	11th to 20th	Day	16% 59% 5% 12% 7% 1	2364
	21st to end	Day	16% 58% 8% 12% 5% 1	2200
October	1st to 10th	Day	18% 38% 14% 24% 6%	3285
	11th to 20th	Day	29% 40% 5% 18% 8%	3163
	21st to end	Day	31% 39% 7% 16% 7%	3200
November	1st to 10th	Day	25% 56% 8% 9% 2	1804
	11th to 20th	Day	28% 55% 6% 6% 3 1	1680
	21st to end	Day	27% 55% 6% 6% 5% 1	1601
December	1st to 10th	Day	28% 38% 19% 13% 2	2360
	11th to 20th	Day	49% 33% 7% 6% 4	2714
	21st to end	Day	57% 28% 7% 6% 2	3857
January	1st to 10th	Night	54% 27% 6% 11% 3	6446
	11th to 20th	Night	25% 28% 11% 20% 11% 3	6460
	21st to end	Night	43% 29% 9% 10% 9%	7106
February	1st to 10th	Night	23% 63% 3 4 5% 1	2747
	11th to 20th	Night	22% 61% 5% 8% 3 2	2524
	21st to end	Night	18% 63% 5% 9% 6%	1980
March	1st to 10th	Night	29% 32% 7% 16% 16% 1	5376
	11th to 20th	Night	28% 29% 9% 23% 11%	5269
	21st to end	Night	46% 32% 7% 10% 5%	5033
April	1st to 10th	Night	17% 64% 7% 10% 3 1	2000
	11th to 20th	Night	16% 64% 4 10% 6%	1961
	21st to end	Night	14% 63% 3 12% 7%	1703
May	1st to 10th	Night	23% 31% 4 22% 20% 1	3637
	11th to 20th	Night	28% 30% 9% 26% 5% 2	3616
	21st to end	Night	21% 30% 17% 26% 5% 1	3972
June	1st to 10th	Night	12% 63% 6% 12% 7% 1	1450
	11th to 20th	Night	15% 63% 4 9% 8% 1	1385
	21st to end	Night	13% 60% 5% 15% 6% 1	1228
July	1st to 10th	Night	8% 25% 9% 16% 28% 13%	3526
	11th to 20th	Night	10% 26% 9% 19% 19% 18%	3592
	21st to end	Night	13% 26% 5% 28% 20% 8%	3857
August	1st to 10th	Night	13% 36% 3 16% 27% 5%	2601
	11th to 20th	Night	12% 39% 11% 26% 10% 1	2625
	21st to end	Night	18% 37% 9% 22% 12% 2	3608
September	1st to 10th	Night	10% 58% 7% 13% 11% 1	2000
	11th to 20th	Night	14% 58% 5% 14% 8%	2035
	21st to end	Night	16% 58% 7% 13% 5%	2200
October	1st to 10th	Night	16% 37% 15% 23% 8%	3911
	11th to 20th	Night	28% 37% 7% 18% 10%	3992
	21st to end	Night	26% 38% 10% 21% 5%	4600
November	1st to 10th	Night	26% 56% 8% 8% 3	2712
	11th to 20th	Night	26% 56% 6% 6% 4 2	2940
	21st to end	Night	27% 56% 6% 6% 4 1	2993
December	1st to 10th	Night	25% 37% 15% 18% 5%	5040
	11th to 20th	Night	48% 34% 8% 6% 4	5697
	21st to end	Night	49% 28% 8% 10% 5%	7370

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	30% 10% 23% 27% 6% 3	220
	11th to 20th	Day	30% 6% 25% 25% 10% 3	220
	21st to end	Day	27% 7% 28% 25% 10% 3	242
February	1st to 10th	Day	20% 13% 35% 26% 5% 2	250
	11th to 20th	Day	22% 13% 35% 21% 9% 1	400
	21st to end	Day	18% 12% 34% 28% 8%	414
March	1st to 10th	Day	25% 18% 29% 18% 9% 1	500
	11th to 20th	Day	24% 16% 24% 21% 13% 2	500
	21st to end	Day	28% 16% 25% 19% 11% 3	550
April	1st to 10th	Day	26% 14% 30% 20% 9% 2	500
	11th to 20th	Day	24% 18% 29% 18% 8% 4	492
	21st to end	Day	20% 17% 26% 22% 10% 4	500
May	1st to 10th	Day	19% 16% 27% 24% 10% 2	503
	11th to 20th	Day	24% 8% 28% 26% 9% 4	528
	21st to end	Day	21% 7% 27% 26% 15% 4	573
June	1st to 10th	Day	19% 12% 29% 27% 10% 3	675
	11th to 20th	Day	20% 11% 27% 27% 12% 3	734
	21st to end	Day	13% 10% 26% 29% 17% 4	897
July	1st to 10th	Day	7% 13% 30% 34% 11% 4	551
	11th to 20th	Day	7% 15% 31% 31% 11% 4	524
	21st to end	Day	6% 9% 30% 32% 18% 5%	575
August	1st to 10th	Day	8% 7% 29% 33% 17% 7%	527
	11th to 20th	Day	9% 8% 30% 29% 17% 9%	517
	21st to end	Day	14% 11% 25% 29% 14% 6%	583
September	1st to 10th	Day	11% 11% 27% 30% 16% 6%	530
	11th to 20th	Day	20% 8% 25% 28% 15% 4	530
	21st to end	Day	21% 13% 29% 22% 12% 3	530
October	1st to 10th	Day	16% 15% 30% 25% 13% 2	530
	11th to 20th	Day	22% 12% 28% 24% 12% 2	470
	21st to end	Day	28% 8% 33% 20% 9% 1	363
November	1st to 10th	Day	30% 10% 27% 25% 8%	302
	11th to 20th	Day	34% 14% 22% 19% 12%	220
	21st to end	Day	26% 5% 34% 19% 11% 5%	220
December	1st to 10th	Day	25% 6% 23% 24% 17% 5%	220
	11th to 20th	Day	27% 12% 27% 25% 6% 3	220
	21st to end	Day	29% 8% 21% 29% 10% 2	242
January	1st to 10th	Night	32% 9% 21% 26% 10% 2	840
	11th to 20th	Night	27% 7% 20% 31% 13% 2	840
	21st to end	Night	22% 5% 21% 36% 14% 1	924
February	1st to 10th	Night	23% 8% 25% 31% 11% 3	800
	11th to 20th	Night	18% 7% 25% 34% 14% 2	784
	21st to end	Night	20% 7% 20% 33% 17% 3	660
March	1st to 10th	Night	20% 9% 20% 31% 17% 3	800
	11th to 20th	Night	19% 9% 21% 34% 16% 2	800
	21st to end	Night	23% 8% 20% 31% 15% 3	880
April	1st to 10th	Night	18% 6% 19% 35% 19% 3	800
	11th to 20th	Night	13% 10% 23% 34% 16% 3	773
	21st to end	Night	14% 8% 25% 33% 17% 2	503
May	1st to 10th	Night	13% 9% 20% 36% 19% 3	400
	11th to 20th	Night	14% 6% 19% 31% 25% 5%	394
	21st to end	Night	16% 5% 18% 31% 26% 5%	433
June	1st to 10th	Night	16% 7% 20% 35% 19% 2	400
	11th to 20th	Night	15% 8% 23% 33% 19% 2	400
	21st to end	Night	9% 6% 21% 34% 27% 4	400
July	1st to 10th	Night	7% 6% 18% 37% 28% 3	390
	11th to 20th	Night	5% 8% 19% 40% 24% 3	388
	21st to end	Night	4 5% 15% 39% 31% 5%	438
August	1st to 10th	Night	4 4 16% 32% 38% 6%	398
	11th to 20th	Night	5% 3 17% 35% 33% 6%	407
	21st to end	Night	9% 5% 15% 39% 27% 4	880
September	1st to 10th	Night	5% 5% 16% 38% 29% 7%	800
	11th to 20th	Night	12% 4 14% 33% 29% 8%	800
	21st to end	Night	14% 7% 16% 37% 23% 4	800
October	1st to 10th	Night	19% 7% 15% 33% 24% 2	800
	11th to 20th	Night	22% 7% 18% 33% 17% 3	800
	21st to end	Night	26% 6% 20% 32% 13% 3	880
November	1st to 10th	Night	30% 8% 23% 31% 7% 1	820
	11th to 20th	Night	31% 10% 19% 25% 13% 2	840
	21st to end	Night	28% 7% 20% 30% 13% 3	893
December	1st to 10th	Night	21% 9% 20% 37% 13% 2	1050
	11th to 20th	Night	31% 8% 20% 32% 8% 1	1050
	21st to end	Night	29% 9% 24% 29% 9% 1	1045

Page generated at 2026-01-28 22:21:44. Version ac07

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