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 2026-10-14.

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	5% 8% 3 7% 4 4 4 3 5% 6% 51%	158125
	Night	6% 9% 3 7% 4 4 4 3 5% 6% 49%	150965
Transparency	Day	33% 6% 9% 20% 20% 11%	152873
	Night	34% 7% 11% 22% 18% 8%	147244
Seeing	Day	13% 17% 31% 25% 12% 2	16862
	Night	13% 7% 21% 36% 19% 2	26064

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 18% 9% 17% 7% 6% 5% 3 5% 5% 21%	4722
	11th to 20th	Day	5% 11% 4 8% 3 3 3 2 4 7% 51%	4740
	21st to end	Day	7% 13% 4 7% 5% 4 4 3 4 6% 44%	5187
February	1st to 10th	Day	8% 20% 6% 13% 6% 4 3 3 5% 5% 28%	1794
	11th to 20th	Day	8% 19% 7% 13% 5% 4 4 4 4 5% 26%	2050
	21st to end	Day	8% 21% 7% 15% 6% 3 3 3 4 5% 25%	1730
March	1st to 10th	Day	5% 14% 7% 13% 7% 5% 5% 4 6% 7% 28%	4618
	11th to 20th	Day	6% 15% 5% 10% 6% 5% 4 3 3 5% 38%	4925
	21st to end	Day	6% 18% 5% 13% 8% 7% 6% 4 5% 7% 21%	5848
April	1st to 10th	Day	9% 17% 3 9% 6% 5% 5% 4 5% 6% 29%	2520
	11th to 20th	Day	8% 14% 3 9% 6% 5% 4 3 5% 6% 36%	2561
	21st to end	Day	9% 14% 3 7% 5% 5% 5% 4 6% 6% 36%	2730
May	1st to 10th	Day	4 8% 3 5% 3 4 4 3 5% 6% 56%	7626
	11th to 20th	Day	5% 6% 1 5% 4 4 5% 4 6% 8% 50%	7929
	21st to end	Day	5% 7% 2 6% 4 4 5% 4 5% 7% 52%	8992
June	1st to 10th	Day	11% 6% 3 7% 7% 6% 6% 5% 6% 7% 36%	2940
	11th to 20th	Day	11% 7% 2 6% 5% 4 4 4 5% 6% 47%	2940
	21st to end	Day	4 4 2 5% 3 3 3 3 4 6% 63%	2940
July	1st to 10th	Day	2 1 1 2 2 2 2 2 4 6% 78%	8219
	11th to 20th	Day	1 2 1 4 3 3 3 3 4 5% 72%	8220
	21st to end	Day	2 2 1 3 2 3 3 3 5% 6% 70%	8922
August	1st to 10th	Day	2 2 1 4 2 3 4 3 6% 7% 67%	7858
	11th to 20th	Day	1 2 1 3 2 3 3 3 4 7% 71%	7565
	21st to end	Day	5% 4 1 4 3 4 4 4 5% 6% 61%	7885
September	1st to 10th	Day	4 3 1 3 2 3 3 3 5% 6% 68%	2520
	11th to 20th	Day	7% 4 1 5% 4 4 4 3 5% 7% 56%	2326
	21st to end	Day	7% 4 2 5% 3 4 4 3 4 6% 61%	2310
October	1st to 10th	Day	6% 5% 2 4 3 3 3 2 3 5% 64%	3710
	11th to 20th	Day	8% 6% 2 7% 4 4 4 3 4 7% 51%	3137
	21st to end	Day	8% 11% 4 9% 5% 5% 4 3 5% 6% 41%	3200
November	1st to 10th	Day	13% 13% 4 9% 5% 4 4 2 4 7% 34%	1854
	11th to 20th	Day	13% 15% 5% 9% 5% 4 4 3 6% 5% 32%	1890
	21st to end	Day	9% 15% 5% 8% 5% 4 5% 4 5% 6% 34%	1817
December	1st to 10th	Day	7% 12% 3 8% 4 3 4 2 4 5% 49%	2700
	11th to 20th	Day	8% 20% 8% 13% 7% 5% 5% 3 6% 6% 20%	3108
	21st to end	Day	9% 14% 6% 14% 5% 5% 5% 3 4 6% 29%	4092
January	1st to 10th	Night	5% 16% 7% 14% 7% 6% 5% 3 6% 5% 24%	8172
	11th to 20th	Night	5% 10% 4 8% 4 3 3 2 4 8% 49%	8220
	21st to end	Night	6% 14% 3 6% 3 4 5% 4 5% 6% 45%	8994
February	1st to 10th	Night	9% 19% 6% 9% 6% 4 4 3 5% 5% 29%	2821
	11th to 20th	Night	8% 16% 6% 11% 6% 5% 3 3 6% 6% 32%	2458
	21st to end	Night	8% 17% 7% 13% 5% 4 4 3 5% 5% 28%	2076
March	1st to 10th	Night	4 16% 5% 9% 5% 4 3 2 4 8% 39%	5496
	11th to 20th	Night	7% 14% 4 8% 5% 4 4 3 4 5% 40%	5294
	21st to end	Night	5% 11% 4 11% 7% 5% 7% 4 7% 8% 32%	5354
April	1st to 10th	Night	9% 12% 3 8% 4 4 4 4 6% 7% 41%	2100
	11th to 20th	Night	9% 10% 3 7% 4 4 4 3 6% 6% 44%	2005
	21st to end	Night	7% 8% 3 7% 4 4 4 4 6% 6% 47%	1890
May	1st to 10th	Night	6% 6% 2 4 2 3 3 3 5% 8% 60%	5321
	11th to 20th	Night	4 4 2 5% 4 4 4 4 6% 8% 56%	4938
	21st to end	Night	5% 5% 2 5% 4 4 4 3 5% 7% 56%	5181
June	1st to 10th	Night	11% 5% 2 4 4 3 5% 5% 6% 8% 46%	1680
	11th to 20th	Night	9% 4 2 4 4 3 3 3 5% 6% 56%	1680
	21st to end	Night	4 4 2 3 4 3 4 3 5% 5% 63%	1680
July	1st to 10th	Night	1 1 1 2 2 2 3 3 4 6% 75%	4740
	11th to 20th	Night	1 2 1 2 1 2 3 2 4 5% 77%	4740
	21st to end	Night	1 2 1 3 2 2 3 3 6% 7% 71%	5153
August	1st to 10th	Night	2 2 1 3 2 3 3 3 4 6% 71%	4935
	11th to 20th	Night	1 1 1 2 2 3 3 3 5% 7% 74%	5291
	21st to end	Night	5% 3 2 4 3 4 4 3 6% 6% 60%	6340
September	1st to 10th	Night	3 3 1 3 2 3 3 3 4 5% 70%	2100
	11th to 20th	Night	5% 5% 1 5% 3 3 4 3 5% 6% 59%	2294
	21st to end	Night	7% 3 1 3 2 3 3 3 4 6% 66%	2310
October	1st to 10th	Night	5% 3 1 3 2 3 4 2 5% 5% 67%	3710
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 5% 7% 56%	4077
	21st to end	Night	8% 8% 2 6% 4 4 5% 4 7% 7% 45%	4600
November	1st to 10th	Night	13% 11% 3 8% 5% 5% 4 3 4 7% 38%	2665
	11th to 20th	Night	13% 13% 3 6% 5% 5% 5% 4 6% 6% 34%	2730
	21st to end	Night	9% 13% 3 8% 6% 5% 4 3 5% 6% 39%	2798
December	1st to 10th	Night	7% 9% 3 7% 4 4 4 3 4 6% 50%	4700
	11th to 20th	Night	7% 16% 6% 14% 6% 6% 6% 4 6% 6% 23%	5316
	21st to end	Night	9% 15% 5% 11% 6% 6% 5% 4 5% 6% 28%	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	59% 13% 11% 13% 4	4723
	11th to 20th	Day	34% 6% 14% 29% 14% 3	4740
	21st to end	Day	42% 6% 11% 20% 17% 4	5187
February	1st to 10th	Day	59% 7% 12% 16% 5% 1	1794
	11th to 20th	Day	59% 9% 11% 15% 7% 1	2060
	21st to end	Day	60% 7% 10% 14% 7% 1	1730
March	1st to 10th	Day	51% 8% 10% 17% 12% 3	4618
	11th to 20th	Day	45% 8% 10% 30% 8%	4925
	21st to end	Day	56% 13% 12% 15% 4	5848
April	1st to 10th	Day	53% 9% 13% 19% 6% 1	2520
	11th to 20th	Day	46% 8% 12% 20% 12% 2	2564
	21st to end	Day	46% 8% 13% 22% 11%	2730
May	1st to 10th	Day	27% 5% 8% 30% 27% 2	7626
	11th to 20th	Day	26% 8% 10% 29% 23% 4	7932
	21st to end	Day	28% 8% 10% 29% 23% 3	8994
June	1st to 10th	Day	43% 10% 12% 21% 14% 1	2940
	11th to 20th	Day	36% 6% 9% 21% 21% 6%	2940
	21st to end	Day	23% 4 7% 21% 34% 11%	2940
July	1st to 10th	Day	10% 3 4 14% 31% 38%	8151
	11th to 20th	Day	15% 3 3 9% 26% 44%	8145
	21st to end	Day	13% 4 5% 15% 30% 33%	8723
August	1st to 10th	Day	15% 3 5% 10% 24% 42%	6227
	11th to 20th	Day	13% 5% 7% 19% 39% 16%	5964
	21st to end	Day	18% 4 7% 25% 32% 14%	6188
September	1st to 10th	Day	17% 4 7% 20% 32% 20%	2520
	11th to 20th	Day	27% 5% 10% 21% 26% 10%	2326
	21st to end	Day	25% 4 9% 28% 27% 7%	2310
October	1st to 10th	Day	25% 4 6% 21% 38% 6%	3710
	11th to 20th	Day	33% 5% 13% 24% 22% 2	3137
	21st to end	Day	43% 8% 8% 24% 16% 1	3200
November	1st to 10th	Day	50% 7% 13% 21% 9% 1	1854
	11th to 20th	Day	54% 7% 14% 18% 7%	1890
	21st to end	Day	49% 8% 15% 16% 11% 2	1817
December	1st to 10th	Day	38% 6% 16% 32% 7% 1	2700
	11th to 20th	Day	61% 11% 14% 10% 4	3108
	21st to end	Day	52% 9% 14% 16% 7% 1	4092
January	1st to 10th	Night	53% 14% 14% 15% 4	8172
	11th to 20th	Night	34% 7% 15% 27% 15% 3	8220
	21st to end	Night	39% 8% 10% 22% 13% 7%	8994
February	1st to 10th	Night	56% 8% 14% 16% 5% 1	2821
	11th to 20th	Night	52% 8% 13% 16% 10% 1	2459
	21st to end	Night	57% 6% 10% 17% 8% 2	2076
March	1st to 10th	Night	43% 7% 10% 22% 14% 5%	5496
	11th to 20th	Night	43% 8% 7% 28% 13% 1	5294
	21st to end	Night	43% 14% 17% 20% 5%	5354
April	1st to 10th	Night	42% 8% 14% 26% 10% 1	2100
	11th to 20th	Night	40% 6% 13% 24% 14% 3	2005
	21st to end	Night	35% 8% 15% 26% 14%	1890
May	1st to 10th	Night	21% 6% 9% 33% 28% 2	5319
	11th to 20th	Night	24% 6% 11% 32% 27% 1	4938
	21st to end	Night	27% 6% 11% 38% 17% 2	5182
June	1st to 10th	Night	33% 9% 16% 28% 14%	1680
	11th to 20th	Night	28% 5% 11% 28% 22% 6%	1680
	21st to end	Night	22% 4 8% 24% 30% 11%	1680
July	1st to 10th	Night	10% 3 5% 17% 29% 37%	4700
	11th to 20th	Night	10% 3 4 11% 29% 42%	4692
	21st to end	Night	12% 3 5% 18% 29% 33%	5040
August	1st to 10th	Night	14% 3 4 10% 32% 36%	3906
	11th to 20th	Night	11% 3 8% 19% 37% 21%	4168
	21st to end	Night	19% 4 6% 24% 30% 17%	4972
September	1st to 10th	Night	17% 3 7% 22% 31% 20%	2100
	11th to 20th	Night	24% 5% 9% 23% 25% 13%	2294
	21st to end	Night	21% 4 8% 29% 28% 10%	2310
October	1st to 10th	Night	20% 5% 7% 21% 36% 12%	3710
	11th to 20th	Night	27% 5% 14% 24% 27% 3	4077
	21st to end	Night	35% 7% 12% 25% 19% 1	4600
November	1st to 10th	Night	46% 6% 14% 22% 11% 1	2665
	11th to 20th	Night	49% 8% 13% 21% 9%	2730
	21st to end	Night	46% 6% 12% 20% 12% 3	2798
December	1st to 10th	Night	34% 8% 22% 26% 10% 1	4700
	11th to 20th	Night	58% 10% 14% 13% 4 1	5316
	21st to end	Night	52% 9% 15% 15% 7% 2	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	16% 17% 31% 24% 10% 3	220
	11th to 20th	Day	17% 15% 25% 24% 12% 8%	220
	21st to end	Day	14% 14% 32% 22% 13% 5%	242
February	1st to 10th	Day	19% 22% 30% 16% 9% 3	292
	11th to 20th	Day	19% 20% 35% 19% 7% 1	535
	21st to end	Day	21% 23% 31% 18% 7%	446
March	1st to 10th	Day	20% 24% 34% 18% 3 1	540
	11th to 20th	Day	20% 20% 31% 19% 8% 1	540
	21st to end	Day	20% 25% 30% 18% 6% 1	594
April	1st to 10th	Day	18% 23% 28% 19% 11% 2	540
	11th to 20th	Day	19% 28% 26% 18% 7% 2	532
	21st to end	Day	15% 23% 31% 20% 9% 2	540
May	1st to 10th	Day	10% 20% 34% 24% 10% 1	543
	11th to 20th	Day	19% 20% 29% 23% 8% 2	568
	21st to end	Day	12% 16% 27% 28% 14% 3	617
June	1st to 10th	Day	13% 19% 27% 24% 13% 4	570
	11th to 20th	Day	15% 23% 32% 19% 10% 2	570
	21st to end	Day	5% 14% 34% 30% 14% 3	570
July	1st to 10th	Day	3 10% 40% 31% 13% 3	566
	11th to 20th	Day	3 14% 38% 31% 12% 2	564
	21st to end	Day	3 9% 38% 31% 17% 2	619
August	1st to 10th	Day	5% 11% 36% 32% 13% 3	567
	11th to 20th	Day	2 9% 32% 36% 18% 3	557
	21st to end	Day	5% 11% 32% 31% 18% 3	627
September	1st to 10th	Day	6% 10% 30% 34% 17% 4	570
	11th to 20th	Day	8% 13% 29% 31% 17% 3	570
	21st to end	Day	7% 14% 31% 30% 16% 2	570
October	1st to 10th	Day	13% 13% 27% 28% 17% 2	570
	11th to 20th	Day	15% 15% 26% 24% 16% 4	407
	21st to end	Day	18% 17% 26% 26% 10% 3	363
November	1st to 10th	Day	23% 14% 24% 24% 13% 2	330
	11th to 20th	Day	15% 22% 25% 26% 10% 2	330
	21st to end	Day	15% 13% 28% 28% 13% 3	291
December	1st to 10th	Day	22% 14% 20% 21% 13% 9%	220
	11th to 20th	Day	25% 20% 25% 18% 10% 1	220
	21st to end	Day	21% 19% 26% 22% 9% 3	242
January	1st to 10th	Night	20% 10% 23% 29% 15% 2	840
	11th to 20th	Night	21% 7% 23% 28% 18% 3	840
	21st to end	Night	15% 8% 25% 30% 19% 3	924
February	1st to 10th	Night	20% 11% 26% 30% 11% 2	840
	11th to 20th	Night	17% 10% 27% 31% 14% 1	824
	21st to end	Night	18% 10% 26% 32% 10% 2	692
March	1st to 10th	Night	18% 12% 27% 30% 12% 2	840
	11th to 20th	Night	17% 10% 26% 31% 15% 1	840
	21st to end	Night	16% 11% 28% 34% 10% 1	924
April	1st to 10th	Night	14% 11% 21% 34% 17% 2	840
	11th to 20th	Night	13% 14% 28% 30% 13% 2	757
	21st to end	Night	11% 10% 24% 40% 15% 1	630
May	1st to 10th	Night	9% 8% 24% 39% 19% 1	630
	11th to 20th	Night	9% 7% 24% 35% 22% 3	498
	21st to end	Night	11% 4 20% 39% 24% 2	455
June	1st to 10th	Night	13% 5% 16% 41% 24% 1	420
	11th to 20th	Night	13% 2 20% 44% 20% 2	420
	21st to end	Night	4 3 13% 49% 30% 1	420
July	1st to 10th	Night	1 1 10% 54% 31% 2	410
	11th to 20th	Night	2 1 14% 59% 23% 1	408
	21st to end	Night	2 11% 55% 30% 1	460
August	1st to 10th	Night	5% 2 11% 50% 30% 3	506
	11th to 20th	Night	1 1 12% 50% 33% 3	624
	21st to end	Night	4 3 13% 44% 31% 4	866
September	1st to 10th	Night	4 2 13% 46% 32% 3	840
	11th to 20th	Night	7% 3 13% 40% 32% 5%	840
	21st to end	Night	9% 4 15% 38% 28% 6%	840
October	1st to 10th	Night	10% 4 18% 42% 23% 2	840
	11th to 20th	Night	11% 7% 19% 37% 22% 4	812
	21st to end	Night	14% 7% 22% 37% 16% 3	880
November	1st to 10th	Night	19% 8% 20% 35% 16% 2	820
	11th to 20th	Night	22% 8% 24% 31% 13% 2	840
	21st to end	Night	16% 8% 25% 30% 17% 3	840
December	1st to 10th	Night	17% 9% 24% 31% 16% 2	840
	11th to 20th	Night	22% 11% 24% 29% 12% 2	840
	21st to end	Night	23% 12% 25% 26% 12% 1	924

Page generated at 2026-10-11 03:06:26. Version ac07

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