Cascadia State Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cascadia State Park clear sky chart from 2005-11-06 to 2025-05-11.

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	20% 11% 2 5% 4 3 3 3 4 5% 40%	118001
	Night	23% 13% 3 5% 3 3 3 3 4 4 36%	119761
Transparency	Day	48% 4 9% 18% 14% 7%	117745
	Night	52% 4 9% 16% 13% 6%	119606
Seeing	Day	27% 12% 26% 22% 10% 3	15282
	Night	31% 7% 14% 28% 18% 2	24406

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	38% 22% 3 6% 4 3 3 2 3 3 13%	3639
	11th to 20th	Day	27% 14% 2 5% 3 3 3 2 3 3 35%	3640
	21st to end	Day	29% 11% 2 5% 3 3 3 3 4 4 35%	4446
February	1st to 10th	Day	30% 17% 5% 8% 5% 3 4 3 4 5% 19%	1800
	11th to 20th	Day	28% 20% 3 6% 4 3 4 4 4 4 21%	1755
	21st to end	Day	28% 23% 4 6% 3 4 3 2 3 4 20%	1594
March	1st to 10th	Day	20% 21% 4 7% 3 4 4 3 6% 6% 22%	4728
	11th to 20th	Day	25% 21% 3 7% 4 3 3 2 2 3 27%	5000
	21st to end	Day	26% 18% 4 8% 6% 6% 5% 4 5% 5% 13%	5613
April	1st to 10th	Day	29% 21% 4 8% 6% 3 3 3 4 4 16%	2400
	11th to 20th	Day	24% 18% 4 7% 5% 4 3 3 3 4 26%	2369
	21st to end	Day	28% 14% 3 6% 4 4 3 3 5% 6% 26%	2545
May	1st to 10th	Day	25% 14% 3 6% 3 3 3 2 3 4 33%	4459
	11th to 20th	Day	15% 6% 2 4 3 3 3 3 5% 6% 50%	4380
	21st to end	Day	23% 13% 2 6% 4 4 4 3 5% 6% 29%	4813
June	1st to 10th	Day	27% 9% 3 6% 5% 4 3 3 3 5% 33%	2660
	11th to 20th	Day	24% 9% 2 6% 4 4 4 3 6% 6% 32%	2660
	21st to end	Day	11% 5% 2 4 3 3 2 3 4 6% 57%	2660
July	1st to 10th	Day	4 2 1 2 2 1 1 1 2 3 81%	4420
	11th to 20th	Day	4 2 1 2 2 1 2 2 3 3 78%	4420
	21st to end	Day	3 3 1 2 1 2 2 2 3 4 78%	4774
August	1st to 10th	Day	3 2 1 3 2 2 2 2 3 5% 73%	4091
	11th to 20th	Day	4 3 2 4 3 3 3 3 4 6% 65%	4026
	21st to end	Day	6% 4 1 4 3 3 3 3 4 5% 64%	4251
September	1st to 10th	Day	7% 3 2 3 2 2 1 2 3 5% 69%	2280
	11th to 20th	Day	13% 8% 3 5% 3 3 3 3 5% 5% 49%	2280
	21st to end	Day	17% 5% 2 6% 4 4 3 2 4 5% 49%	2006
October	1st to 10th	Day	15% 7% 1 3 3 2 3 2 4 4 56%	3180
	11th to 20th	Day	20% 7% 2 5% 5% 4 4 3 5% 7% 38%	3161
	21st to end	Day	26% 10% 3 7% 5% 5% 5% 4 5% 6% 25%	3391
November	1st to 10th	Day	37% 11% 3 7% 4 4 3 2 2 5% 21%	1708
	11th to 20th	Day	35% 11% 3 8% 4 3 3 3 6% 4 20%	1600
	21st to end	Day	28% 13% 3 7% 4 4 4 2 4 5% 28%	1600
December	1st to 10th	Day	21% 10% 2 5% 3 3 3 3 3 5% 42%	2620
	11th to 20th	Day	30% 15% 3 9% 6% 4 5% 4 4 5% 14%	3028
	21st to end	Day	34% 20% 5% 9% 4 3 3 2 3 3 14%	4004
January	1st to 10th	Night	38% 19% 3 6% 4 3 4 3 3 4 13%	6306
	11th to 20th	Night	27% 16% 3 5% 3 2 2 2 3 4 33%	6320
	21st to end	Night	27% 13% 2 5% 3 3 3 1 3 3 36%	6513
February	1st to 10th	Night	27% 21% 4 7% 5% 4 4 3 4 4 19%	2600
	11th to 20th	Night	28% 20% 3 6% 4 3 3 2 4 4 23%	2532
	21st to end	Night	28% 24% 3 5% 3 3 2 3 3 5% 22%	2035
March	1st to 10th	Night	19% 22% 4 6% 4 4 4 4 6% 6% 21%	5162
	11th to 20th	Night	24% 21% 3 6% 3 3 3 2 3 3 28%	5000
	21st to end	Night	30% 22% 3 7% 4 4 3 3 3 5% 16%	5361
April	1st to 10th	Night	30% 21% 3 7% 4 3 3 3 4 4 20%	2000
	11th to 20th	Night	25% 19% 2 5% 3 3 3 3 4 4 28%	1979
	21st to end	Night	28% 15% 3 5% 4 4 3 3 4 5% 28%	1850
May	1st to 10th	Night	25% 18% 3 5% 3 2 2 2 3 4 33%	2724
	11th to 20th	Night	16% 9% 2 4 3 3 3 3 4 4 49%	2519
	21st to end	Night	23% 13% 2 4 4 3 3 3 4 6% 33%	2762
June	1st to 10th	Night	26% 9% 2 5% 4 3 4 4 4 6% 34%	1520
	11th to 20th	Night	21% 10% 2 5% 3 4 3 3 5% 6% 39%	1520
	21st to end	Night	11% 5% 2 5% 4 3 2 2 3 4 59%	1520
July	1st to 10th	Night	4 2 1 2 1 1 1 1 2 3 81%	2540
	11th to 20th	Night	3 2 1 3 1 2 2 2 2 3 79%	2540
	21st to end	Night	5% 1 1 1 1 1 2 1 2 3 81%	2774
August	1st to 10th	Night	2 2 1 2 2 2 3 2 4 5% 74%	2848
	11th to 20th	Night	4 3 1 4 4 3 4 3 4 6% 62%	2812
	21st to end	Night	8% 4 2 4 2 2 2 2 3 5% 67%	3424
September	1st to 10th	Night	7% 3 1 3 2 2 3 2 3 5% 69%	1900
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 4 5% 54%	1900
	21st to end	Night	16% 7% 1 3 3 3 3 3 4 6% 51%	2170
October	1st to 10th	Night	13% 7% 2 4 3 2 3 2 3 4 57%	3800
	11th to 20th	Night	18% 7% 2 5% 3 3 4 3 6% 8% 42%	3777
	21st to end	Night	26% 12% 3 8% 4 4 4 3 4 5% 27%	4175
November	1st to 10th	Night	34% 12% 3 7% 4 3 3 3 4 5% 23%	2590
	11th to 20th	Night	37% 12% 2 6% 3 3 3 3 4 4 21%	2800
	21st to end	Night	26% 13% 2 6% 4 4 3 3 4 4 31%	2800
December	1st to 10th	Night	18% 10% 2 4 4 3 3 2 4 5% 44%	4560
	11th to 20th	Night	31% 16% 3 8% 5% 5% 4 4 5% 4 15%	5176
	21st to end	Night	33% 17% 3 8% 5% 4 3 3 3 3 18%	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	78% 2 4 8% 7% 1	3639
	11th to 20th	Day	56% 2 5% 17% 15% 4	3640
	21st to end	Day	54% 3 6% 13% 17% 8%	4446
February	1st to 10th	Day	69% 5% 8% 10% 7% 1	1800
	11th to 20th	Day	69% 3 8% 10% 7% 3	1764
	21st to end	Day	71% 4 7% 10% 7% 3	1594
March	1st to 10th	Day	60% 5% 9% 17% 8%	4728
	11th to 20th	Day	65% 3 5% 17% 10%	5000
	21st to end	Day	72% 5% 11% 9% 3	5613
April	1st to 10th	Day	73% 4 9% 9% 5%	2400
	11th to 20th	Day	63% 4 9% 14% 8% 3	2372
	21st to end	Day	60% 5% 12% 15% 6% 2	2545
May	1st to 10th	Day	55% 4 10% 20% 10%	4459
	11th to 20th	Day	34% 5% 14% 32% 14%	4383
	21st to end	Day	56% 6% 21% 15% 2	4814
June	1st to 10th	Day	56% 4 14% 19% 6% 1	2660
	11th to 20th	Day	52% 5% 16% 20% 6% 1	2660
	21st to end	Day	29% 4 16% 32% 14% 5%	2660
July	1st to 10th	Day	11% 3 8% 16% 29% 34%	4351
	11th to 20th	Day	13% 2 5% 15% 28% 37%	4345
	21st to end	Day	12% 2 7% 25% 39% 14%	4623
August	1st to 10th	Day	15% 2 8% 26% 32% 16%	4076
	11th to 20th	Day	20% 5% 13% 38% 20% 5%	4065
	21st to end	Day	23% 5% 12% 24% 22% 15%	4250
September	1st to 10th	Day	20% 2 6% 31% 24% 16%	2280
	11th to 20th	Day	36% 4 11% 23% 17% 9%	2280
	21st to end	Day	39% 4 9% 22% 17% 9%	2006
October	1st to 10th	Day	33% 3 5% 21% 22% 15%	3180
	11th to 20th	Day	45% 6% 13% 19% 13% 3	3161
	21st to end	Day	59% 5% 8% 13% 13% 2	3391
November	1st to 10th	Day	69% 3 6% 11% 9% 2	1708
	11th to 20th	Day	67% 4 8% 9% 9% 3	1600
	21st to end	Day	61% 4 6% 11% 13% 5%	1600
December	1st to 10th	Day	45% 4 7% 12% 22% 10%	2620
	11th to 20th	Day	73% 5% 9% 8% 6%	3028
	21st to end	Day	78% 2 5% 12% 2 1	4004
January	1st to 10th	Night	76% 3 6% 8% 6% 2	6306
	11th to 20th	Night	57% 3 6% 16% 14% 5%	6320
	21st to end	Night	57% 2 4 10% 18% 10%	6513
February	1st to 10th	Night	70% 3 8% 9% 7% 2	2600
	11th to 20th	Night	67% 4 8% 10% 7% 4	2534
	21st to end	Night	67% 3 7% 13% 8% 2	2035
March	1st to 10th	Night	62% 6% 12% 14% 4 3	5162
	11th to 20th	Night	63% 3 8% 16% 10%	5000
	21st to end	Night	73% 5% 11% 10% 1	5361
April	1st to 10th	Night	70% 4 10% 10% 6%	2000
	11th to 20th	Night	60% 5% 12% 13% 8% 3	1979
	21st to end	Night	61% 5% 11% 14% 6% 2	1850
May	1st to 10th	Night	56% 5% 10% 16% 12% 1	2724
	11th to 20th	Night	37% 7% 9% 35% 10% 1	2519
	21st to end	Night	52% 7% 23% 15% 3	2762
June	1st to 10th	Night	52% 7% 14% 22% 6%	1520
	11th to 20th	Night	48% 6% 16% 23% 6% 1	1520
	21st to end	Night	31% 3 16% 31% 14% 5%	1520
July	1st to 10th	Night	12% 2 7% 18% 26% 35%	2500
	11th to 20th	Night	13% 2 5% 16% 29% 35%	2492
	21st to end	Night	10% 2 8% 26% 41% 12%	2687
August	1st to 10th	Night	13% 3 7% 31% 30% 14%	2839
	11th to 20th	Night	23% 5% 15% 31% 20% 5%	2839
	21st to end	Night	22% 3 13% 25% 24% 13%	3424
September	1st to 10th	Night	19% 3 9% 28% 26% 15%	1900
	11th to 20th	Night	34% 4 10% 26% 14% 12%	1900
	21st to end	Night	35% 4 11% 22% 17% 11%	2170
October	1st to 10th	Night	32% 4 6% 19% 28% 12%	3800
	11th to 20th	Night	41% 5% 18% 21% 11% 4	3777
	21st to end	Night	61% 5% 6% 13% 11% 4	4175
November	1st to 10th	Night	65% 6% 6% 9% 12% 2	2590
	11th to 20th	Night	67% 4 8% 9% 8% 4	2800
	21st to end	Night	58% 3 7% 11% 13% 7%	2800
December	1st to 10th	Night	43% 4 7% 14% 24% 8%	4560
	11th to 20th	Night	72% 4 10% 7% 6% 1	5176
	21st to end	Night	71% 5% 8% 12% 3 1	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	54% 12% 15% 12% 7% 1	200
	11th to 20th	Day	41% 9% 23% 18% 8% 4	200
	21st to end	Day	37% 11% 18% 20% 12% 2	415
February	1st to 10th	Day	40% 11% 21% 18% 10% 1	400
	11th to 20th	Day	41% 16% 24% 13% 5% 1	395
	21st to end	Day	44% 14% 19% 18% 5%	387
March	1st to 10th	Day	38% 18% 20% 19% 4	500
	11th to 20th	Day	39% 11% 25% 18% 5% 2	500
	21st to end	Day	43% 17% 21% 15% 3 1	550
April	1st to 10th	Day	39% 15% 23% 16% 7% 1	500
	11th to 20th	Day	36% 16% 25% 15% 6% 1	492
	21st to end	Day	38% 15% 22% 16% 7% 2	500
May	1st to 10th	Day	26% 17% 27% 21% 7% 2	503
	11th to 20th	Day	33% 11% 24% 21% 8% 4	489
	21st to end	Day	30% 10% 24% 22% 11% 4	529
June	1st to 10th	Day	29% 13% 21% 24% 10% 3	490
	11th to 20th	Day	26% 15% 29% 18% 10% 1	490
	21st to end	Day	10% 10% 36% 29% 12% 4	490
July	1st to 10th	Day	9% 9% 39% 25% 13% 5%	486
	11th to 20th	Day	4 12% 37% 28% 14% 6%	484
	21st to end	Day	3 9% 37% 28% 17% 7%	531
August	1st to 10th	Day	2 11% 33% 33% 13% 8%	487
	11th to 20th	Day	3 10% 28% 33% 18% 7%	477
	21st to end	Day	8% 11% 33% 31% 12% 5%	539
September	1st to 10th	Day	9% 10% 33% 28% 16% 4	490
	11th to 20th	Day	17% 9% 30% 30% 14% 1	490
	21st to end	Day	17% 13% 27% 28% 12% 2	490
October	1st to 10th	Day	22% 10% 27% 25% 13% 2	490
	11th to 20th	Day	27% 8% 20% 26% 16% 4	490
	21st to end	Day	33% 14% 22% 18% 11% 3	506
November	1st to 10th	Day	38% 11% 17% 22% 10% 1	272
	11th to 20th	Day	37% 14% 21% 15% 12% 3	200
	21st to end	Day	33% 12% 19% 25% 10% 3	200
December	1st to 10th	Day	37% 11% 19% 15% 14% 5%	200
	11th to 20th	Day	47% 10% 23% 17% 3 1	200
	21st to end	Day	45% 14% 15% 17% 6% 1	220
January	1st to 10th	Night	48% 8% 17% 17% 8% 2	800
	11th to 20th	Night	39% 8% 19% 20% 10% 4	800
	21st to end	Night	36% 8% 18% 26% 10% 2	880
February	1st to 10th	Night	37% 8% 19% 26% 9% 1	800
	11th to 20th	Night	40% 9% 18% 25% 8%	784
	21st to end	Night	43% 9% 18% 22% 7%	660
March	1st to 10th	Night	39% 10% 14% 28% 9% 1	800
	11th to 20th	Night	36% 8% 16% 28% 12% 1	800
	21st to end	Night	43% 10% 14% 22% 10% 1	880
April	1st to 10th	Night	42% 9% 14% 22% 11% 2	800
	11th to 20th	Night	37% 8% 14% 26% 13% 2	791
	21st to end	Night	37% 9% 13% 27% 13% 2	650
May	1st to 10th	Night	27% 9% 16% 26% 19% 2	465
	11th to 20th	Night	28% 5% 8% 29% 27% 2	375
	21st to end	Night	31% 5% 13% 23% 27% 1	411
June	1st to 10th	Night	29% 6% 10% 26% 27% 3	380
	11th to 20th	Night	25% 5% 12% 35% 22% 2	380
	21st to end	Night	14% 2 11% 37% 36% 1	380
July	1st to 10th	Night	8% 3 7% 40% 40% 2	370
	11th to 20th	Night	5% 1 11% 50% 32%	368
	21st to end	Night	3 3 6% 38% 48% 2	417
August	1st to 10th	Night	6% 1 4 33% 47% 9%	567
	11th to 20th	Night	6% 2 7% 31% 45% 9%	564
	21st to end	Night	6% 4 12% 42% 32% 3	788
September	1st to 10th	Night	9% 2 15% 39% 33% 4	760
	11th to 20th	Night	15% 4 8% 35% 32% 5%	760
	21st to end	Night	18% 5% 11% 38% 26% 3	760
October	1st to 10th	Night	22% 5% 14% 32% 23% 4	760
	11th to 20th	Night	25% 6% 11% 33% 21% 4	760
	21st to end	Night	31% 9% 17% 26% 11% 5%	836
November	1st to 10th	Night	41% 6% 16% 28% 8% 1	780
	11th to 20th	Night	46% 8% 17% 21% 8% 1	800
	21st to end	Night	34% 8% 18% 31% 9% 1	800
December	1st to 10th	Night	35% 7% 20% 25% 10% 2	800
	11th to 20th	Night	43% 8% 19% 21% 9% 1	800
	21st to end	Night	48% 9% 19% 18% 6% 1	880

Page generated at 2025-05-08 06:33:58. Version ac07

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