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 2026-09-14.

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	17% 9% 2 5% 3 3 3 3 4 5% 45%	158473
	Night	20% 12% 2 5% 3 3 3 3 4 5% 40%	150014
Transparency	Day	43% 4 10% 21% 15% 7%	153280
	Night	48% 4 10% 18% 13% 7%	146204
Seeing	Day	26% 13% 26% 22% 10% 3	17054
	Night	30% 7% 15% 29% 17% 2	25973

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	33% 21% 3 8% 5% 4 4 3 3 3 11%	4722
	11th to 20th	Day	21% 10% 2 4 2 3 2 2 3 4 46%	4740
	21st to end	Day	25% 10% 2 6% 4 4 4 4 5% 5% 32%	5765
February	1st to 10th	Day	29% 16% 5% 8% 5% 3 3 3 4 5% 20%	1890
	11th to 20th	Day	26% 20% 3 7% 4 3 4 4 4 4 21%	1845
	21st to end	Day	29% 23% 4 6% 3 4 3 2 3 4 20%	1671
March	1st to 10th	Day	20% 21% 4 7% 3 4 4 3 6% 6% 22%	4832
	11th to 20th	Day	25% 20% 3 7% 4 3 3 2 3 3 27%	5110
	21st to end	Day	26% 18% 4 8% 6% 6% 6% 4 5% 5% 13%	5737
April	1st to 10th	Day	28% 21% 4 7% 5% 4 3 3 4 4 17%	2520
	11th to 20th	Day	23% 18% 4 7% 5% 4 3 3 3 5% 25%	2489
	21st to end	Day	27% 14% 3 6% 4 4 3 3 5% 6% 27%	2672
May	1st to 10th	Day	15% 9% 2 5% 3 3 4 3 4 5% 45%	8019
	11th to 20th	Day	18% 9% 2 6% 5% 5% 4 3 4 6% 38%	8160
	21st to end	Day	16% 9% 2 5% 4 4 4 3 5% 7% 40%	8992
June	1st to 10th	Day	26% 9% 3 5% 5% 3 3 3 4 5% 35%	2940
	11th to 20th	Day	22% 9% 2 5% 4 4 4 3 5% 6% 36%	2940
	21st to end	Day	12% 5% 2 4 3 3 2 3 4 6% 55%	2940
July	1st to 10th	Day	2 2 1 2 2 1 2 1 2 4 80%	8219
	11th to 20th	Day	3 2 1 3 2 1 2 2 3 3 80%	8220
	21st to end	Day	2 3 1 3 2 2 2 2 3 4 77%	8906
August	1st to 10th	Day	2 2 1 3 2 2 2 2 3 5% 77%	7547
	11th to 20th	Day	5% 3 1 3 2 3 3 3 4 6% 66%	7563
	21st to end	Day	5% 4 1 4 3 3 3 3 5% 7% 62%	7871
September	1st to 10th	Day	8% 4 2 3 2 2 2 2 4 5% 67%	2520
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 5% 6% 49%	2437
	21st to end	Day	17% 4 2 6% 4 4 3 2 3 5% 50%	2111
October	1st to 10th	Day	15% 7% 2 3 3 2 2 2 4 5% 55%	3280
	11th to 20th	Day	20% 7% 2 5% 5% 4 4 3 5% 7% 38%	3261
	21st to end	Day	26% 10% 3 7% 5% 5% 5% 4 5% 6% 25%	3499
November	1st to 10th	Day	36% 11% 3 7% 4 4 4 2 3 5% 21%	1795
	11th to 20th	Day	34% 11% 3 8% 4 3 4 3 6% 5% 20%	1680
	21st to end	Day	28% 13% 3 6% 4 4 4 2 4 5% 27%	1680
December	1st to 10th	Day	21% 10% 2 5% 3 3 3 3 3 5% 41%	2700
	11th to 20th	Day	30% 15% 4 9% 6% 4 5% 4 4 5% 14%	3108
	21st to end	Day	34% 20% 5% 9% 4 3 3 2 3 3 14%	4092
January	1st to 10th	Night	35% 19% 4 7% 4 4 4 3 3 4 12%	8172
	11th to 20th	Night	21% 12% 2 4 3 2 3 2 3 4 44%	8220
	21st to end	Night	23% 12% 3 8% 4 4 3 2 3 4 34%	8429
February	1st to 10th	Night	26% 20% 4 7% 5% 4 4 3 4 4 20%	2730
	11th to 20th	Night	27% 20% 3 6% 4 3 3 2 4 4 23%	2662
	21st to end	Night	28% 23% 3 5% 3 3 2 3 3 5% 22%	2134
March	1st to 10th	Night	19% 22% 4 7% 4 4 4 4 6% 6% 21%	5277
	11th to 20th	Night	24% 21% 3 6% 3 3 3 2 3 3 28%	5110
	21st to end	Night	30% 22% 3 7% 4 4 3 3 4 5% 16%	5479
April	1st to 10th	Night	29% 20% 3 7% 3 3 3 3 4 4 21%	2100
	11th to 20th	Night	25% 19% 2 5% 4 3 3 3 4 4 28%	2079
	21st to end	Night	27% 15% 2 5% 4 4 3 3 3 5% 29%	1943
May	1st to 10th	Night	16% 12% 3 4 3 3 3 3 4 6% 43%	4908
	11th to 20th	Night	19% 16% 3 5% 4 3 3 3 4 5% 36%	4700
	21st to end	Night	19% 10% 2 4 3 3 3 3 5% 7% 41%	5181
June	1st to 10th	Night	25% 9% 2 5% 4 3 3 4 4 6% 36%	1680
	11th to 20th	Night	19% 9% 2 5% 3 4 3 3 5% 6% 41%	1680
	21st to end	Night	13% 5% 1 5% 4 3 2 2 3 4 58%	1680
July	1st to 10th	Night	3 3 1 2 1 1 2 2 4 4 75%	4740
	11th to 20th	Night	2 2 1 3 2 2 2 2 2 3 79%	4740
	21st to end	Night	4 2 1 2 2 2 2 2 3 3 78%	5166
August	1st to 10th	Night	3 2 1 2 2 2 3 2 4 5% 73%	5275
	11th to 20th	Night	6% 4 1 4 4 4 4 3 4 6% 61%	5292
	21st to end	Night	6% 3 2 3 2 2 3 3 3 5% 67%	6371
September	1st to 10th	Night	8% 4 2 3 2 2 3 3 3 5% 65%	2100
	11th to 20th	Night	13% 5% 2 4 3 4 4 3 4 5% 53%	2030
	21st to end	Night	15% 7% 1 4 2 3 4 3 4 6% 52%	2285
October	1st to 10th	Night	13% 7% 2 4 3 2 3 2 3 4 57%	3920
	11th to 20th	Night	18% 7% 2 5% 3 3 4 3 5% 8% 42%	3897
	21st to end	Night	26% 12% 3 8% 4 4 4 3 4 5% 27%	4309
November	1st to 10th	Night	33% 12% 3 7% 3 3 3 3 4 5% 22%	2723
	11th to 20th	Night	36% 12% 3 6% 3 4 4 3 4 4 21%	2940
	21st to end	Night	26% 13% 2 6% 4 4 3 3 4 4 30%	2940
December	1st to 10th	Night	19% 10% 2 4 3 3 3 2 4 6% 43%	4700
	11th to 20th	Night	31% 16% 3 8% 5% 5% 4 4 5% 5% 15%	5316
	21st to end	Night	33% 16% 3 8% 5% 4 3 3 3 3 19%	7106

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% 3 5% 8% 6% 1	4723
	11th to 20th	Day	44% 2 4 16% 18% 17%	4740
	21st to end	Day	54% 4 5% 13% 17% 7%	5765
February	1st to 10th	Day	68% 5% 8% 10% 7% 2	1890
	11th to 20th	Day	69% 3 8% 10% 7% 3	1854
	21st to end	Day	70% 3 6% 10% 8% 3	1671
March	1st to 10th	Day	60% 5% 9% 17% 8%	4832
	11th to 20th	Day	65% 3 5% 17% 10% 1	5110
	21st to end	Day	72% 5% 11% 9% 4	5737
April	1st to 10th	Day	72% 4 10% 10% 5%	2520
	11th to 20th	Day	63% 4 9% 13% 7% 3	2492
	21st to end	Day	59% 5% 12% 16% 6% 2	2672
May	1st to 10th	Day	41% 5% 11% 30% 14%	8019
	11th to 20th	Day	46% 7% 15% 25% 8%	8163
	21st to end	Day	43% 6% 22% 23% 6%	8994
June	1st to 10th	Day	52% 5% 13% 21% 8% 1	2940
	11th to 20th	Day	49% 5% 14% 21% 9% 1	2940
	21st to end	Day	31% 4 16% 30% 14% 5%	2940
July	1st to 10th	Day	11% 3 9% 31% 28% 19%	8151
	11th to 20th	Day	12% 2 5% 15% 33% 34%	8145
	21st to end	Day	13% 3 9% 37% 30% 9%	8707
August	1st to 10th	Day	11% 2 6% 21% 36% 24%	5976
	11th to 20th	Day	17% 5% 13% 41% 20% 4	5962
	21st to end	Day	21% 4 10% 32% 22% 11%	6174
September	1st to 10th	Day	22% 3 7% 30% 23% 15%	2520
	11th to 20th	Day	35% 4 11% 24% 17% 9%	2437
	21st to end	Day	38% 4 9% 22% 18% 9%	2111
October	1st to 10th	Day	34% 4 6% 21% 22% 15%	3280
	11th to 20th	Day	45% 6% 12% 19% 13% 3	3261
	21st to end	Day	59% 5% 8% 13% 12% 2	3499
November	1st to 10th	Day	68% 3 7% 12% 8% 2	1795
	11th to 20th	Day	67% 5% 8% 9% 8% 3	1680
	21st to end	Day	61% 4 7% 11% 12% 5%	1680
December	1st to 10th	Day	46% 4 7% 12% 21% 10%	2700
	11th to 20th	Day	72% 5% 9% 8% 6% 1	3108
	21st to end	Day	78% 2 5% 12% 2 1	4092
January	1st to 10th	Night	76% 4 6% 8% 5% 1	8172
	11th to 20th	Night	45% 3 5% 14% 14% 19%	8220
	21st to end	Night	56% 2 4 9% 18% 10%	8429
February	1st to 10th	Night	69% 3 8% 10% 8% 2	2730
	11th to 20th	Night	66% 4 9% 11% 6% 4	2664
	21st to end	Night	66% 3 7% 13% 9% 2	2134
March	1st to 10th	Night	62% 6% 12% 14% 4 3	5277
	11th to 20th	Night	63% 3 8% 16% 10% 1	5110
	21st to end	Night	73% 5% 11% 10% 2	5479
April	1st to 10th	Night	69% 4 11% 11% 5%	2100
	11th to 20th	Night	61% 5% 12% 13% 7% 2	2079
	21st to end	Night	60% 5% 11% 15% 7% 2	1943
May	1st to 10th	Night	42% 6% 12% 27% 13%	4906
	11th to 20th	Night	51% 7% 11% 26% 5% 1	4700
	21st to end	Night	42% 6% 24% 22% 5%	5182
June	1st to 10th	Night	50% 6% 14% 23% 7%	1680
	11th to 20th	Night	45% 5% 15% 25% 8% 2	1680
	21st to end	Night	33% 3 16% 30% 14% 5%	1680
July	1st to 10th	Night	13% 3 10% 34% 22% 19%	4700
	11th to 20th	Night	14% 2 6% 15% 34% 29%	4692
	21st to end	Night	13% 3 12% 35% 29% 7%	5053
August	1st to 10th	Night	10% 2 5% 26% 34% 22%	4169
	11th to 20th	Night	23% 5% 14% 33% 22% 3	4169
	21st to end	Night	19% 4 12% 35% 21% 9%	4990
September	1st to 10th	Night	22% 4 10% 27% 24% 14%	2100
	11th to 20th	Night	33% 4 11% 26% 13% 12%	2030
	21st to end	Night	34% 4 10% 22% 17% 12%	2285
October	1st to 10th	Night	33% 4 6% 19% 27% 11%	3920
	11th to 20th	Night	41% 5% 17% 21% 12% 4	3897
	21st to end	Night	61% 5% 6% 14% 11% 4	4309
November	1st to 10th	Night	65% 6% 6% 9% 11% 2	2723
	11th to 20th	Night	67% 4 8% 9% 8% 4	2940
	21st to end	Night	58% 4 7% 11% 13% 7%	2940
December	1st to 10th	Night	44% 4 7% 14% 23% 8%	4700
	11th to 20th	Night	72% 4 10% 7% 6% 1	5316
	21st to end	Night	71% 5% 9% 12% 3 1	7106

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	52% 13% 15% 14% 6%	220
	11th to 20th	Day	37% 8% 22% 21% 9% 4	220
	21st to end	Day	36% 12% 19% 19% 12% 2	447
February	1st to 10th	Day	39% 11% 21% 18% 10% 1	430
	11th to 20th	Day	40% 17% 24% 14% 5% 1	425
	21st to end	Day	44% 14% 20% 17% 5%	416
March	1st to 10th	Day	38% 19% 20% 18% 4	540
	11th to 20th	Day	39% 12% 25% 17% 5% 2	540
	21st to end	Day	42% 17% 22% 14% 3 1	594
April	1st to 10th	Day	37% 15% 23% 16% 8% 1	540
	11th to 20th	Day	36% 17% 25% 16% 6% 1	532
	21st to end	Day	37% 15% 23% 17% 7% 2	540
May	1st to 10th	Day	24% 16% 28% 22% 7% 3	543
	11th to 20th	Day	33% 13% 23% 20% 7% 3	568
	21st to end	Day	28% 10% 25% 22% 11% 4	617
June	1st to 10th	Day	28% 13% 23% 24% 10% 4	570
	11th to 20th	Day	24% 15% 31% 20% 9% 2	570
	21st to end	Day	11% 10% 35% 28% 12% 4	570
July	1st to 10th	Day	8% 8% 40% 27% 13% 5%	566
	11th to 20th	Day	4 13% 38% 27% 13% 6%	564
	21st to end	Day	3 9% 37% 27% 16% 7%	619
August	1st to 10th	Day	3 11% 33% 32% 12% 8%	567
	11th to 20th	Day	4 10% 30% 32% 17% 7%	557
	21st to end	Day	8% 11% 33% 30% 12% 5%	627
September	1st to 10th	Day	10% 12% 31% 28% 15% 4	570
	11th to 20th	Day	16% 9% 30% 30% 14% 2	543
	21st to end	Day	16% 13% 27% 28% 13% 2	530
October	1st to 10th	Day	22% 11% 28% 25% 12% 2	530
	11th to 20th	Day	27% 10% 20% 25% 15% 4	530
	21st to end	Day	32% 14% 22% 18% 10% 3	548
November	1st to 10th	Day	38% 12% 18% 22% 10% 1	299
	11th to 20th	Day	35% 12% 23% 15% 12% 3	220
	21st to end	Day	33% 12% 20% 25% 9% 2	220
December	1st to 10th	Day	38% 11% 18% 15% 13% 5%	220
	11th to 20th	Day	46% 10% 22% 17% 4 1	220
	21st to end	Day	45% 15% 15% 16% 8% 1	242
January	1st to 10th	Night	47% 9% 17% 17% 8% 2	840
	11th to 20th	Night	37% 8% 21% 21% 10% 4	840
	21st to end	Night	34% 8% 19% 26% 10% 2	924
February	1st to 10th	Night	36% 8% 20% 27% 8% 1	840
	11th to 20th	Night	39% 9% 19% 25% 8%	824
	21st to end	Night	43% 10% 19% 21% 7%	692
March	1st to 10th	Night	40% 10% 14% 27% 8% 1	840
	11th to 20th	Night	36% 8% 17% 27% 11% 1	840
	21st to end	Night	43% 10% 14% 22% 9% 1	924
April	1st to 10th	Night	41% 9% 14% 23% 11% 2	840
	11th to 20th	Night	37% 8% 14% 26% 13% 2	831
	21st to end	Night	36% 9% 14% 28% 12% 2	683
May	1st to 10th	Night	26% 9% 16% 28% 19% 2	488
	11th to 20th	Night	29% 7% 9% 28% 25% 2	414
	21st to end	Night	29% 5% 13% 25% 26% 1	455
June	1st to 10th	Night	27% 5% 13% 26% 26% 3	420
	11th to 20th	Night	23% 5% 13% 37% 21% 1	420
	21st to end	Night	15% 2 11% 37% 35%	420
July	1st to 10th	Night	7% 3 7% 40% 40% 2	410
	11th to 20th	Night	5% 2 13% 49% 31%	408
	21st to end	Night	3 3 6% 39% 46% 2	461
August	1st to 10th	Night	6% 1 5% 34% 46% 8%	627
	11th to 20th	Night	7% 2 7% 32% 44% 8%	624
	21st to end	Night	6% 4 13% 43% 31% 3	872
September	1st to 10th	Night	10% 2 15% 38% 31% 3	840
	11th to 20th	Night	15% 4 9% 36% 32% 5%	812
	21st to end	Night	17% 5% 11% 39% 25% 3	800
October	1st to 10th	Night	22% 5% 15% 33% 22% 3	800
	11th to 20th	Night	26% 7% 12% 33% 20% 4	800
	21st to end	Night	31% 9% 18% 27% 11% 5%	880
November	1st to 10th	Night	41% 7% 17% 27% 8% 1	820
	11th to 20th	Night	45% 8% 17% 22% 8% 1	840
	21st to end	Night	33% 8% 18% 31% 9% 1	840
December	1st to 10th	Night	36% 7% 21% 25% 10% 2	840
	11th to 20th	Night	43% 8% 20% 20% 8% 1	840
	21st to end	Night	47% 10% 19% 18% 5% 1	924

Page generated at 2026-09-11 01:11:58. Version ac07

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