Cascade
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cascade clear sky chart from 2005-11-06 to 2025-06-10.

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	6% 10% 3 8% 4 4 4 3 4 6% 47%	123267
	Night	6% 10% 3 7% 4 4 4 3 5% 6% 47%	123758
Transparency	Day	36% 7% 10% 20% 18% 10%	123012
	Night	36% 7% 11% 21% 16% 7%	123600
Seeing	Day	13% 17% 31% 25% 11% 2	15089
	Night	14% 7% 21% 36% 20% 3	24447

By Month

Percentage Cloud Cover SW (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	4 15% 9% 16% 7% 6% 5% 3 4 5% 25%	3639
	11th to 20th	Day	6% 14% 5% 11% 4 4 3 3 4 7% 40%	3640
	21st to end	Day	9% 16% 4 6% 4 3 2 2 4 6% 43%	4004
February	1st to 10th	Day	8% 21% 6% 13% 6% 4 3 3 4 4 27%	1708
	11th to 20th	Day	8% 19% 7% 12% 5% 4 4 4 4 5% 27%	1950
	21st to end	Day	7% 21% 8% 16% 6% 3 3 3 4 5% 24%	1650
March	1st to 10th	Day	5% 14% 7% 14% 7% 5% 5% 4 6% 7% 28%	4518
	11th to 20th	Day	6% 15% 5% 10% 6% 5% 4 3 3 4 38%	4819
	21st to end	Day	6% 18% 5% 13% 8% 7% 6% 4 5% 7% 21%	5722
April	1st to 10th	Day	10% 18% 3 9% 6% 5% 5% 4 5% 6% 28%	2400
	11th to 20th	Day	8% 14% 3 9% 6% 5% 4 3 5% 6% 37%	2437
	21st to end	Day	9% 14% 3 7% 5% 5% 5% 4 5% 6% 36%	2600
May	1st to 10th	Day	5% 10% 4 7% 4 4 4 2 4 6% 51%	5880
	11th to 20th	Day	7% 8% 2 6% 4 5% 5% 4 6% 8% 46%	6096
	21st to end	Day	5% 8% 2 5% 4 4 4 3 5% 7% 53%	6902
June	1st to 10th	Day	12% 7% 3 7% 7% 6% 6% 5% 6% 7% 35%	2789
	11th to 20th	Day	12% 7% 3 6% 5% 4 4 4 5% 7% 45%	2660
	21st to end	Day	4 4 2 5% 3 3 3 3 4 5% 64%	2660
July	1st to 10th	Day	1 1 2 2 2 2 2 4 4 79%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 76%	4420
	21st to end	Day	2 2 1 3 2 3 3 3 4 5% 73%	4790
August	1st to 10th	Day	3 3 1 5% 3 5% 5% 4 6% 7% 59%	4260
	11th to 20th	Day	2 1 3 2 3 3 3 4 6% 74%	4028
	21st to end	Day	3 3 1 3 2 3 3 3 3 5% 71%	4253
September	1st to 10th	Day	4 3 1 3 2 3 3 3 5% 6% 70%	2280
	11th to 20th	Day	7% 4 2 4 4 4 4 3 5% 7% 56%	2104
	21st to end	Day	7% 4 1 4 2 4 4 3 4 6% 60%	2090
October	1st to 10th	Day	6% 5% 2 4 4 3 3 2 3 5% 63%	3490
	11th to 20th	Day	8% 6% 2 6% 4 4 4 4 4 7% 52%	3010
	21st to end	Day	8% 11% 4 9% 6% 5% 4 3 4 5% 40%	3101
November	1st to 10th	Day	14% 13% 4 9% 4 4 4 2 4 7% 34%	1764
	11th to 20th	Day	13% 15% 5% 9% 5% 4 4 3 6% 5% 33%	1800
	21st to end	Day	9% 15% 5% 8% 5% 4 5% 4 5% 6% 34%	1731
December	1st to 10th	Day	7% 12% 3 8% 4 3 3 2 4 5% 50%	2620
	11th to 20th	Day	8% 20% 8% 14% 7% 5% 5% 3 6% 6% 19%	3028
	21st to end	Day	9% 14% 6% 14% 5% 5% 5% 3 4 6% 29%	4004
January	1st to 10th	Night	7% 16% 7% 13% 6% 5% 5% 3 6% 5% 29%	6306
	11th to 20th	Night	7% 13% 5% 11% 5% 4 4 3 4 7% 38%	6320
	21st to end	Night	7% 18% 4 7% 3 3 3 3 4 5% 43%	6952
February	1st to 10th	Night	9% 19% 6% 10% 6% 4 4 3 5% 5% 28%	2687
	11th to 20th	Night	8% 16% 5% 11% 6% 4 3 3 6% 5% 32%	2338
	21st to end	Night	8% 18% 8% 13% 5% 4 4 3 5% 5% 28%	1980
March	1st to 10th	Night	4 16% 5% 9% 5% 4 4 2 4 8% 39%	5376
	11th to 20th	Night	8% 14% 5% 8% 5% 4 4 3 4 5% 40%	5180
	21st to end	Night	5% 11% 4 11% 7% 5% 6% 4 7% 8% 32%	5238
April	1st to 10th	Night	9% 12% 3 8% 4 4 4 4 6% 6% 40%	2000
	11th to 20th	Night	9% 10% 3 7% 4 4 4 3 5% 6% 44%	1909
	21st to end	Night	7% 8% 3 7% 4 4 4 4 6% 6% 47%	1800
May	1st to 10th	Night	7% 8% 2 4 2 3 3 3 4 6% 56%	4100
	11th to 20th	Night	5% 5% 2 6% 4 5% 4 4 6% 8% 52%	3800
	21st to end	Night	6% 6% 2 5% 3 4 3 2 4 6% 60%	3972
June	1st to 10th	Night	12% 5% 2 4 4 4 5% 5% 6% 9% 45%	1595
	11th to 20th	Night	10% 4 1 4 3 3 3 3 5% 7% 55%	1520
	21st to end	Night	4 3 2 3 4 3 4 3 5% 5% 64%	1520
July	1st to 10th	Night	1 1 1 1 1 2 2 3 4 5% 80%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 5% 79%	2540
	21st to end	Night	2 2 1 2 2 2 3 3 5% 6% 71%	2761
August	1st to 10th	Night	3 4 2 4 3 3 4 4 5% 7% 61%	2674
	11th to 20th	Night	1 1 1 2 2 2 3 2 4 5% 77%	2811
	21st to end	Night	4 2 1 3 2 2 2 2 4 4 73%	3420
September	1st to 10th	Night	3 3 1 3 2 3 3 3 4 5% 72%	1900
	11th to 20th	Night	5% 5% 1 5% 3 3 4 3 5% 6% 60%	2076
	21st to end	Night	8% 3 1 3 2 3 2 3 3 5% 65%	2090
October	1st to 10th	Night	5% 3 1 3 2 3 4 2 5% 6% 66%	3490
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 5% 7% 56%	3918
	21st to end	Night	8% 8% 2 6% 4 5% 5% 4 6% 7% 45%	4457
November	1st to 10th	Night	13% 11% 3 8% 5% 5% 4 3 4 7% 38%	2535
	11th to 20th	Night	14% 13% 3 6% 4 5% 5% 3 5% 6% 35%	2600
	21st to end	Night	9% 14% 3 8% 6% 5% 3 3 5% 6% 40%	2665
December	1st to 10th	Night	7% 9% 3 6% 4 4 4 3 4 6% 51%	4560
	11th to 20th	Night	7% 16% 6% 14% 7% 6% 6% 5% 6% 6% 23%	5176
	21st to end	Night	8% 16% 5% 11% 6% 6% 5% 4 5% 6% 28%	6952

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	57% 11% 10% 16% 5% 1	3639
	11th to 20th	Day	44% 7% 15% 24% 9% 2	3640
	21st to end	Day	44% 6% 11% 19% 15% 4	4004
February	1st to 10th	Day	61% 7% 11% 16% 4 1	1708
	11th to 20th	Day	59% 8% 11% 15% 7% 1	1960
	21st to end	Day	61% 7% 10% 13% 7% 1	1650
March	1st to 10th	Day	51% 8% 10% 17% 12% 3	4518
	11th to 20th	Day	45% 8% 10% 30% 8%	4819
	21st to end	Day	56% 13% 12% 15% 3	5722
April	1st to 10th	Day	54% 9% 14% 17% 6% 1	2400
	11th to 20th	Day	46% 7% 12% 20% 13% 2	2440
	21st to end	Day	47% 8% 13% 22% 10%	2600
May	1st to 10th	Day	33% 6% 8% 27% 24% 2	5880
	11th to 20th	Day	30% 8% 10% 29% 18% 4	6099
	21st to end	Day	29% 7% 10% 30% 22% 3	6904
June	1st to 10th	Day	44% 10% 12% 21% 12% 1	2789
	11th to 20th	Day	37% 6% 10% 22% 19% 6%	2660
	21st to end	Day	22% 4 8% 21% 34% 11%	2660
July	1st to 10th	Day	10% 3 4 10% 22% 51%	4351
	11th to 20th	Day	11% 3 4 8% 22% 52%	4345
	21st to end	Day	11% 3 5% 10% 31% 39%	4639
August	1st to 10th	Day	22% 5% 7% 13% 27% 27%	4244
	11th to 20th	Day	11% 4 5% 17% 42% 22%	4067
	21st to end	Day	16% 3 6% 22% 35% 18%	4252
September	1st to 10th	Day	16% 4 7% 19% 32% 22%	2280
	11th to 20th	Day	27% 5% 10% 21% 26% 11%	2104
	21st to end	Day	25% 4 9% 28% 26% 7%	2090
October	1st to 10th	Day	26% 4 6% 21% 37% 7%	3490
	11th to 20th	Day	33% 5% 14% 25% 22% 2	3010
	21st to end	Day	43% 8% 8% 24% 16% 1	3101
November	1st to 10th	Day	51% 6% 12% 20% 9% 1	1764
	11th to 20th	Day	53% 6% 14% 18% 8% 1	1800
	21st to end	Day	50% 7% 14% 16% 11% 2	1731
December	1st to 10th	Day	37% 6% 17% 33% 7% 1	2620
	11th to 20th	Day	62% 11% 14% 10% 4	3028
	21st to end	Day	53% 9% 14% 16% 7% 1	4004
January	1st to 10th	Night	54% 9% 13% 18% 5% 1	6306
	11th to 20th	Night	44% 8% 14% 21% 12% 1	6320
	21st to end	Night	44% 7% 9% 20% 13% 7%	6952
February	1st to 10th	Night	57% 8% 13% 15% 5% 1	2687
	11th to 20th	Night	53% 8% 13% 16% 10% 1	2339
	21st to end	Night	58% 6% 10% 16% 8% 2	1980
March	1st to 10th	Night	43% 7% 10% 22% 14% 5%	5376
	11th to 20th	Night	43% 8% 7% 29% 12% 1	5180
	21st to end	Night	43% 14% 18% 20% 5%	5238
April	1st to 10th	Night	43% 8% 14% 24% 11% 1	2000
	11th to 20th	Night	39% 6% 13% 24% 14% 3	1909
	21st to end	Night	36% 8% 16% 27% 13%	1800
May	1st to 10th	Night	26% 7% 8% 31% 25% 2	4097
	11th to 20th	Night	27% 7% 13% 32% 21% 1	3800
	21st to end	Night	26% 6% 11% 36% 19% 2	3972
June	1st to 10th	Night	34% 9% 17% 29% 11%	1595
	11th to 20th	Night	29% 5% 12% 29% 20% 6%	1520
	21st to end	Night	21% 4 8% 24% 31% 12%	1520
July	1st to 10th	Night	8% 3 4 12% 25% 48%	2500
	11th to 20th	Night	7% 3 5% 12% 20% 54%	2492
	21st to end	Night	12% 4 4 12% 31% 37%	2674
August	1st to 10th	Night	20% 5% 6% 13% 36% 21%	2666
	11th to 20th	Night	10% 3 5% 16% 40% 26%	2838
	21st to end	Night	15% 3 5% 20% 33% 23%	3420
September	1st to 10th	Night	16% 3 6% 21% 31% 22%	1900
	11th to 20th	Night	24% 5% 9% 21% 26% 15%	2076
	21st to end	Night	22% 4 8% 29% 26% 11%	2090
October	1st to 10th	Night	20% 5% 7% 20% 37% 12%	3490
	11th to 20th	Night	27% 5% 14% 24% 26% 3	3918
	21st to end	Night	36% 8% 12% 24% 20% 1	4457
November	1st to 10th	Night	46% 7% 13% 22% 11% 1	2535
	11th to 20th	Night	49% 8% 13% 21% 9%	2600
	21st to end	Night	46% 6% 12% 20% 12% 3	2665
December	1st to 10th	Night	34% 8% 22% 26% 10% 1	4560
	11th to 20th	Night	59% 10% 14% 13% 4 1	5176
	21st to end	Night	52% 9% 15% 15% 7% 2	6952

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	16% 16% 32% 25% 10% 3	200
	11th to 20th	Day	19% 16% 26% 22% 11% 7%	200
	21st to end	Day	15% 15% 32% 20% 11% 6%	220
February	1st to 10th	Day	20% 23% 32% 16% 8% 2	266
	11th to 20th	Day	19% 20% 35% 18% 7%	495
	21st to end	Day	21% 23% 30% 18% 7%	414
March	1st to 10th	Day	20% 23% 35% 18% 3 1	500
	11th to 20th	Day	20% 20% 30% 20% 8% 1	500
	21st to end	Day	20% 24% 31% 18% 6% 1	550
April	1st to 10th	Day	19% 23% 28% 19% 9% 2	500
	11th to 20th	Day	19% 28% 26% 18% 6% 2	492
	21st to end	Day	15% 23% 31% 19% 9% 2	500
May	1st to 10th	Day	11% 21% 33% 24% 10% 1	503
	11th to 20th	Day	19% 19% 29% 23% 8% 2	528
	21st to end	Day	13% 16% 26% 28% 14% 4	573
June	1st to 10th	Day	13% 18% 27% 24% 13% 4	527
	11th to 20th	Day	16% 22% 32% 18% 10% 1	490
	21st to end	Day	4 14% 34% 32% 13% 3	490
July	1st to 10th	Day	3 11% 40% 33% 11% 2	486
	11th to 20th	Day	3 14% 38% 31% 12% 2	484
	21st to end	Day	3 8% 39% 33% 15% 1	531
August	1st to 10th	Day	5% 10% 37% 32% 13% 3	487
	11th to 20th	Day	2 9% 31% 37% 17% 3	477
	21st to end	Day	6% 12% 32% 32% 17% 3	539
September	1st to 10th	Day	6% 10% 30% 34% 17% 3	490
	11th to 20th	Day	9% 12% 31% 31% 15% 1	490
	21st to end	Day	8% 14% 31% 31% 15% 1	490
October	1st to 10th	Day	14% 14% 27% 29% 15% 1	490
	11th to 20th	Day	15% 14% 25% 23% 17% 5%	362
	21st to end	Day	19% 17% 27% 25% 9% 3	330
November	1st to 10th	Day	24% 13% 24% 24% 13% 1	300
	11th to 20th	Day	15% 21% 26% 25% 11% 3	300
	21st to end	Day	16% 12% 28% 28% 12% 3	265
December	1st to 10th	Day	22% 13% 21% 22% 14% 10%	200
	11th to 20th	Day	26% 19% 27% 18% 10% 2	200
	21st to end	Day	20% 18% 28% 23% 8% 3	220
January	1st to 10th	Night	21% 10% 23% 29% 16% 2	800
	11th to 20th	Night	22% 8% 23% 27% 18% 3	800
	21st to end	Night	16% 8% 25% 28% 20% 3	880
February	1st to 10th	Night	21% 11% 27% 30% 10% 2	800
	11th to 20th	Night	18% 10% 26% 31% 13% 2	784
	21st to end	Night	18% 11% 25% 33% 11% 2	660
March	1st to 10th	Night	19% 12% 26% 30% 12% 2	800
	11th to 20th	Night	17% 10% 24% 33% 16% 1	800
	21st to end	Night	16% 11% 27% 34% 10% 1	880
April	1st to 10th	Night	15% 11% 22% 33% 17% 2	800
	11th to 20th	Night	13% 13% 28% 30% 13% 2	721
	21st to end	Night	11% 9% 24% 40% 14% 2	600
May	1st to 10th	Night	10% 8% 24% 37% 20% 1	600
	11th to 20th	Night	9% 7% 24% 33% 23% 4	474
	21st to end	Night	12% 4 19% 38% 25% 2	433
June	1st to 10th	Night	13% 5% 15% 40% 25% 2	399
	11th to 20th	Night	14% 1 20% 42% 21% 2	380
	21st to end	Night	4 2 13% 49% 31% 1	380
July	1st to 10th	Night	1 2 10% 54% 31% 2	370
	11th to 20th	Night	2 1 14% 59% 23% 1	368
	21st to end	Night	3 11% 55% 30% 1	416
August	1st to 10th	Night	5% 2 10% 48% 32% 3	458
	11th to 20th	Night	1 1 12% 47% 35% 3	564
	21st to end	Night	4 2 13% 43% 33% 5%	784
September	1st to 10th	Night	4 2 13% 47% 32% 3	760
	11th to 20th	Night	7% 3 13% 39% 32% 6%	760
	21st to end	Night	10% 4 16% 37% 27% 7%	760
October	1st to 10th	Night	10% 4 17% 42% 24% 2	760
	11th to 20th	Night	12% 7% 16% 38% 23% 5%	760
	21st to end	Night	14% 7% 22% 37% 17% 3	836
November	1st to 10th	Night	19% 8% 20% 35% 17% 2	780
	11th to 20th	Night	22% 8% 24% 30% 13% 2	800
	21st to end	Night	17% 8% 25% 30% 17% 3	800
December	1st to 10th	Night	18% 8% 23% 32% 17% 2	800
	11th to 20th	Night	22% 11% 24% 29% 13% 2	800
	21st to end	Night	23% 12% 26% 27% 12% 1	880

Page generated at 2025-06-07 08:28:13. Version ac07

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