Cascade Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cascade Mountain Observatory clear sky chart from 2005-11-06 to 2026-06-16.

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	13% 6% 2 6% 4 4 4 3 5% 7% 45%	136699
	Night	13% 6% 2 5% 4 3 4 3 5% 6% 49%	135568
Transparency	Day	38% 4 10% 24% 19% 5%	140306
	Night	35% 4 9% 24% 21% 7%	140042
Seeing	Day	15% 10% 23% 26% 20% 7%	17518
	Night	15% 8% 26% 32% 17% 2	27337

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% 7% 2 6% 3 3 3 3 4 5% 46%	5289
	11th to 20th	Day	9% 8% 2 6% 4 3 3 2 3 4 56%	5310
	21st to end	Day	20% 7% 3 5% 3 2 2 2 3 4 49%	5812
February	1st to 10th	Day	22% 8% 2 6% 4 3 3 3 2 5% 42%	1890
	11th to 20th	Day	18% 7% 3 7% 4 4 3 3 4 5% 42%	1995
	21st to end	Day	21% 7% 3 7% 3 3 4 3 4 6% 40%	1755
March	1st to 10th	Day	21% 9% 3 7% 4 4 4 3 4 5% 37%	5046
	11th to 20th	Day	10% 5% 2 6% 4 4 3 3 5% 5% 52%	5090
	21st to end	Day	14% 8% 3 7% 4 4 3 3 4 5% 45%	5624
April	1st to 10th	Day	15% 7% 2 5% 4 4 4 3 6% 7% 44%	2520
	11th to 20th	Day	15% 6% 2 6% 5% 4 4 3 4 6% 45%	2494
	21st to end	Day	16% 6% 2 8% 5% 4 5% 4 5% 6% 39%	2520
May	1st to 10th	Day	16% 8% 3 7% 5% 4 4 3 5% 6% 38%	7021
	11th to 20th	Day	12% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	7505
	21st to end	Day	17% 8% 3 9% 6% 6% 5% 4 7% 8% 27%	8561
June	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2940
	11th to 20th	Day	5% 5% 2 6% 5% 6% 5% 4 7% 11% 45%	2871
	21st to end	Day	5% 3 1 4 4 4 6% 5% 7% 9% 52%	2735
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	5900
	11th to 20th	Day	4 3 1 4 3 4 4 4 6% 10% 55%	5900
	21st to end	Day	7% 7% 3 7% 5% 5% 6% 4 6% 8% 42%	6422
August	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 8% 62%	4234
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 7% 10% 43%	3854
	21st to end	Day	5% 4 1 5% 4 4 5% 4 7% 8% 52%	4312
September	1st to 10th	Day	11% 5% 2 4 4 4 5% 4 5% 7% 47%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 5% 4 6% 8% 45%	2400
	21st to end	Day	10% 5% 2 4 4 4 4 4 5% 6% 52%	2102
October	1st to 10th	Day	7% 4 2 5% 3 3 3 3 4 6% 60%	2000
	11th to 20th	Day	13% 5% 1 4 2 3 3 2 4 6% 58%	2000
	21st to end	Day	19% 5% 2 5% 2 2 3 2 4 5% 50%	2200
November	1st to 10th	Day	13% 5% 2 4 2 3 2 2 3 6% 57%	2058
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 5% 55%	1974
	21st to end	Day	19% 8% 2 6% 4 2 2 2 4 5% 46%	1853
December	1st to 10th	Day	17% 7% 3 6% 3 3 3 2 3 3 49%	2690
	11th to 20th	Day	17% 8% 3 7% 4 4 3 3 3 4 44%	3088
	21st to end	Day	20% 8% 2 7% 4 3 3 3 4 5% 39%	4334
January	1st to 10th	Night	17% 6% 2 5% 3 4 3 2 4 6% 47%	7634
	11th to 20th	Night	10% 4 1 4 2 2 2 2 4 4 64%	7680
	21st to end	Night	22% 8% 3 5% 3 3 3 2 3 4 44%	8402
February	1st to 10th	Night	21% 8% 3 5% 3 3 3 2 4 6% 42%	2730
	11th to 20th	Night	19% 7% 2 6% 3 3 2 3 3 6% 44%	2510
	21st to end	Night	16% 6% 2 5% 3 4 4 3 3 5% 48%	2069
March	1st to 10th	Night	18% 8% 3 6% 3 3 3 3 4 5% 44%	5029
	11th to 20th	Night	10% 6% 2 5% 4 3 3 3 4 6% 53%	5090
	21st to end	Night	12% 6% 2 7% 4 4 4 3 5% 7% 46%	5547
April	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 6% 7% 50%	2100
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 6% 7% 50%	2075
	21st to end	Night	14% 6% 2 6% 4 4 3 3 5% 7% 46%	2100
May	1st to 10th	Night	18% 9% 3 6% 4 4 4 4 5% 7% 37%	5832
	11th to 20th	Night	15% 8% 3 8% 5% 4 4 3 5% 6% 38%	5364
	21st to end	Night	20% 9% 3 8% 5% 5% 5% 4 5% 6% 32%	5614
June	1st to 10th	Night	6% 5% 1 4 3 4 4 4 7% 11% 51%	1680
	11th to 20th	Night	4 4 1 5% 4 4 4 4 5% 9% 57%	1640
	21st to end	Night	3 3 1 3 3 4 3 4 6% 9% 60%	1656
July	1st to 10th	Night	6% 4 2 7% 5% 5% 6% 5% 7% 9% 44%	4078
	11th to 20th	Night	3 2 1 4 3 4 4 4 6% 11% 57%	4077
	21st to end	Night	6% 6% 2 6% 4 4 4 4 6% 10% 48%	4449
August	1st to 10th	Night	2 2 1 4 3 3 3 3 6% 8% 67%	2926
	11th to 20th	Night	4 2 2 5% 4 3 4 4 7% 10% 54%	3195
	21st to end	Night	3 3 1 5% 4 4 5% 4 6% 9% 55%	3586
September	1st to 10th	Night	7% 4 2 6% 4 4 4 3 4 7% 55%	2000
	11th to 20th	Night	10% 4 2 6% 4 4 4 3 5% 7% 52%	2000
	21st to end	Night	8% 5% 2 4 3 3 3 3 4 7% 58%	2273
October	1st to 10th	Night	7% 3 2 5% 3 2 3 3 4 6% 62%	2400
	11th to 20th	Night	11% 3 1 4 3 2 3 3 3 5% 62%	2400
	21st to end	Night	17% 5% 2 5% 3 3 3 2 3 4 53%	2640
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 5% 57%	2460
	11th to 20th	Night	12% 4 2 3 3 2 3 3 4 6% 58%	2643
	21st to end	Night	15% 6% 2 5% 3 4 3 2 5% 5% 50%	2762
December	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 5% 51%	4720
	11th to 20th	Night	19% 8% 3 6% 4 3 3 3 3 4 46%	5339
	21st to end	Night	20% 7% 3 6% 4 4 4 3 4 5% 42%	6868

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	42% 2 9% 29% 16% 2	5290
	11th to 20th	Day	35% 2 6% 21% 30% 6%	5310
	21st to end	Day	42% 2 4 19% 27% 7%	5812
February	1st to 10th	Day	46% 4 9% 22% 15% 3	1890
	11th to 20th	Day	45% 4 8% 20% 18% 5%	2005
	21st to end	Day	46% 4 8% 16% 18% 8%	1755
March	1st to 10th	Day	51% 3 5% 16% 14% 12%	5046
	11th to 20th	Day	34% 4 7% 20% 24% 12%	5090
	21st to end	Day	43% 3 8% 26% 20% 1	5626
April	1st to 10th	Day	39% 5% 9% 23% 21% 3	2520
	11th to 20th	Day	41% 4 9% 22% 17% 6%	2494
	21st to end	Day	45% 5% 10% 22% 17% 2	2520
May	1st to 10th	Day	46% 4 9% 22% 17% 2	7056
	11th to 20th	Day	43% 5% 13% 23% 15% 2	7509
	21st to end	Day	53% 6% 12% 16% 11% 1	8575
June	1st to 10th	Day	36% 7% 12% 27% 17% 1	2940
	11th to 20th	Day	32% 7% 15% 27% 18% 2	2871
	21st to end	Day	26% 7% 14% 29% 20% 4	2735
July	1st to 10th	Day	34% 7% 16% 29% 13% 1	5836
	11th to 20th	Day	24% 6% 16% 37% 16% 2	5831
	21st to end	Day	38% 5% 15% 29% 12%	6240
August	1st to 10th	Day	20% 4 15% 32% 22% 5%	4220
	11th to 20th	Day	36% 6% 14% 31% 12% 1	3890
	21st to end	Day	28% 6% 11% 32% 20% 3	4311
September	1st to 10th	Day	36% 6% 11% 22% 18% 7%	2400
	11th to 20th	Day	36% 6% 11% 20% 23% 5%	2400
	21st to end	Day	32% 4 9% 20% 28% 7%	2102
October	1st to 10th	Day	16% 3 5% 14% 37% 25%	3260
	11th to 20th	Day	18% 2 4 20% 39% 17%	3240
	21st to end	Day	32% 3 5% 28% 26% 6%	3538
November	1st to 10th	Day	31% 3 8% 26% 25% 7%	2058
	11th to 20th	Day	31% 3 8% 23% 26% 9%	1974
	21st to end	Day	42% 3 6% 21% 23% 6%	1853
December	1st to 10th	Day	42% 2 6% 27% 20% 3	2690
	11th to 20th	Day	45% 3 5% 23% 20% 4	3085
	21st to end	Day	46% 4 7% 25% 14% 4	4334
January	1st to 10th	Night	40% 2 7% 30% 18% 2	7635
	11th to 20th	Night	26% 3 6% 20% 36% 9%	7680
	21st to end	Night	45% 3 7% 18% 22% 6%	8402
February	1st to 10th	Night	44% 3 8% 22% 18% 5%	2730
	11th to 20th	Night	44% 3 6% 22% 19% 6%	2511
	21st to end	Night	41% 3 7% 18% 22% 9%	2069
March	1st to 10th	Night	44% 3 7% 15% 19% 13%	5029
	11th to 20th	Night	33% 4 7% 24% 18% 14%	5090
	21st to end	Night	39% 3 9% 26% 20% 3	5547
April	1st to 10th	Night	34% 4 10% 25% 19% 7%	2100
	11th to 20th	Night	34% 4 8% 24% 20% 10%	2075
	21st to end	Night	38% 4 11% 23% 21% 3	2100
May	1st to 10th	Night	47% 4 12% 21% 14% 2	5862
	11th to 20th	Night	46% 4 12% 20% 13% 4	5365
	21st to end	Night	53% 5% 13% 16% 10% 3	5625
June	1st to 10th	Night	27% 6% 14% 29% 22% 2	1680
	11th to 20th	Night	25% 4 13% 30% 25% 3	1640
	21st to end	Night	21% 5% 13% 30% 24% 7%	1656
July	1st to 10th	Night	35% 6% 14% 27% 18% 1	4035
	11th to 20th	Night	20% 6% 16% 38% 17% 3	4026
	21st to end	Night	33% 5% 16% 29% 17%	4320
August	1st to 10th	Night	15% 4 15% 37% 23% 5%	2916
	11th to 20th	Night	23% 5% 19% 35% 16% 2	3225
	21st to end	Night	25% 5% 12% 28% 27% 3	3586
September	1st to 10th	Night	29% 6% 10% 26% 21% 9%	2000
	11th to 20th	Night	33% 5% 9% 22% 25% 6%	2000
	21st to end	Night	27% 5% 9% 21% 29% 9%	2273
October	1st to 10th	Night	15% 3 6% 15% 31% 31%	3920
	11th to 20th	Night	17% 2 3 16% 39% 23%	3899
	21st to end	Night	30% 2 5% 21% 29% 12%	4252
November	1st to 10th	Night	30% 4 7% 23% 26% 10%	2460
	11th to 20th	Night	28% 3 6% 27% 26% 10%	2643
	21st to end	Night	37% 3 7% 21% 26% 5%	2762
December	1st to 10th	Night	37% 3 7% 31% 18% 3	4720
	11th to 20th	Night	44% 3 5% 21% 21% 6%	5341
	21st to end	Night	46% 3 8% 24% 16% 3	6868

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	27% 17% 32% 18% 5% 1	330
	11th to 20th	Day	22% 20% 44% 12% 1 1	330
	21st to end	Day	21% 23% 37% 16% 3	363
February	1st to 10th	Day	28% 19% 33% 15% 4 1	330
	11th to 20th	Day	25% 28% 33% 12% 2	330
	21st to end	Day	30% 22% 34% 10% 3	273
March	1st to 10th	Day	26% 20% 35% 16% 4	330
	11th to 20th	Day	26% 25% 32% 16% 1	330
	21st to end	Day	23% 22% 39% 14% 3	379
April	1st to 10th	Day	22% 16% 32% 23% 7% 1	540
	11th to 20th	Day	22% 17% 34% 19% 7% 1	532
	21st to end	Day	19% 16% 32% 22% 9% 2	540
May	1st to 10th	Day	18% 15% 29% 27% 10% 1	543
	11th to 20th	Day	16% 8% 27% 27% 17% 4	729
	21st to end	Day	11% 6% 24% 34% 19% 5%	844
June	1st to 10th	Day	10% 5% 19% 31% 26% 9%	780
	11th to 20th	Day	6% 3 17% 37% 28% 9%	756
	21st to end	Day	5% 3 16% 35% 30% 12%	730
July	1st to 10th	Day	6% 2 10% 31% 37% 15%	721
	11th to 20th	Day	6% 3 9% 32% 34% 17%	718
	21st to end	Day	7% 1 10% 30% 37% 15%	794
August	1st to 10th	Day	6% 2 10% 36% 36% 11%	726
	11th to 20th	Day	14% 3 7% 27% 35% 13%	524
	21st to end	Day	11% 2 7% 26% 36% 19%	583
September	1st to 10th	Day	10% 2 12% 26% 36% 14%	530
	11th to 20th	Day	11% 2 12% 22% 40% 13%	530
	21st to end	Day	13% 6% 16% 33% 23% 8%	367
October	1st to 10th	Day	8% 3 28% 34% 22% 4	330
	11th to 20th	Day	11% 14% 32% 25% 15% 3	330
	21st to end	Day	27% 13% 30% 20% 9% 1	363
November	1st to 10th	Day	18% 12% 33% 22% 13% 1	330
	11th to 20th	Day	12% 15% 39% 26% 8% 1	330
	21st to end	Day	26% 17% 28% 19% 9% 1	330
December	1st to 10th	Day	22% 21% 30% 20% 6% 2	330
	11th to 20th	Day	25% 17% 34% 16% 7%	330
	21st to end	Day	25% 19% 35% 17% 2 1	363
January	1st to 10th	Night	24% 10% 35% 27% 5%	1050
	11th to 20th	Night	20% 13% 38% 26% 3 1	1050
	21st to end	Night	21% 12% 38% 27% 3	1155
February	1st to 10th	Night	24% 13% 32% 27% 4	1050
	11th to 20th	Night	23% 15% 36% 24% 2	875
	21st to end	Night	22% 16% 38% 22% 3	685
March	1st to 10th	Night	20% 11% 37% 29% 4	630
	11th to 20th	Night	19% 13% 37% 27% 4	630
	21st to end	Night	16% 12% 37% 27% 8%	693
April	1st to 10th	Night	15% 12% 36% 29% 7%	630
	11th to 20th	Night	15% 12% 36% 27% 10% 1	623
	21st to end	Night	16% 9% 34% 30% 10%	630
May	1st to 10th	Night	16% 5% 25% 42% 11%	630
	11th to 20th	Night	15% 7% 29% 29% 17% 2	621
	21st to end	Night	11% 5% 21% 37% 21% 5%	683
June	1st to 10th	Night	6% 3 15% 37% 34% 4	630
	11th to 20th	Night	4 2 10% 46% 34% 3	615
	21st to end	Night	4 1 10% 40% 39% 6%	600
July	1st to 10th	Night	5% 1 7% 35% 42% 9%	585
	11th to 20th	Night	4 2 7% 32% 46% 9%	582
	21st to end	Night	6% 1 7% 36% 44% 7%	657
August	1st to 10th	Night	2 1 7% 41% 40% 8%	597
	11th to 20th	Night	7% 1 8% 38% 38% 7%	594
	21st to end	Night	7% 1 10% 33% 43% 7%	660
September	1st to 10th	Night	9% 2 7% 40% 37% 5%	600
	11th to 20th	Night	9% 3 12% 34% 38% 4	600
	21st to end	Night	9% 4 14% 36% 33% 4	715
October	1st to 10th	Night	9% 2 17% 36% 31% 4	800
	11th to 20th	Night	13% 5% 27% 36% 19% 1	800
	21st to end	Night	18% 7% 27% 32% 14% 1	880
November	1st to 10th	Night	15% 7% 26% 37% 15% 1	820
	11th to 20th	Night	14% 8% 31% 35% 11%	840
	21st to end	Night	18% 10% 32% 28% 12%	872
December	1st to 10th	Night	21% 8% 34% 31% 6%	1050
	11th to 20th	Night	23% 10% 29% 32% 6%	1050
	21st to end	Night	23% 13% 31% 27% 5%	1155

Page generated at 2026-06-13 01:31:45. Version ac07

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