Creston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Creston clear sky chart from 2005-11-06 to 2026-02-01.

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% 9% 3 8% 6% 5% 5% 4 6% 7% 39%	131409
	Night	7% 11% 4 9% 6% 5% 5% 4 6% 7% 36%	129321
Transparency	Day	44% 5% 14% 23% 11% 2	131128
	Night	49% 6% 15% 20% 9% 2	129179
Seeing	Day	13% 9% 24% 30% 18% 7%	16555
	Night	16% 3 16% 41% 21% 3	25791

By Month

Percentage Cloud Cover NW (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	5% 19% 8% 17% 10% 7% 6% 5% 5% 4 14%	3461
	11th to 20th	Day	5% 12% 4 10% 7% 7% 6% 4 5% 7% 33%	3696
	21st to end	Day	10% 20% 6% 10% 5% 4 5% 3 5% 6% 26%	4092
February	1st to 10th	Day	6% 13% 6% 14% 9% 8% 6% 5% 7% 7% 20%	1600
	11th to 20th	Day	4 17% 5% 12% 7% 6% 8% 5% 6% 6% 23%	1802
	21st to end	Day	5% 13% 5% 13% 9% 6% 7% 5% 5% 6% 24%	1650
March	1st to 10th	Day	4 12% 4 9% 5% 6% 6% 5% 9% 10% 30%	4518
	11th to 20th	Day	4 11% 4 9% 7% 6% 7% 6% 7% 7% 33%	4752
	21st to end	Day	8% 14% 5% 12% 9% 7% 7% 5% 6% 7% 20%	5827
April	1st to 10th	Day	8% 14% 4 11% 7% 7% 6% 6% 6% 7% 23%	2400
	11th to 20th	Day	7% 12% 5% 12% 6% 6% 6% 4 6% 7% 28%	2500
	21st to end	Day	10% 10% 4 10% 7% 6% 7% 5% 6% 7% 29%	2600
May	1st to 10th	Day	7% 9% 3 8% 5% 5% 5% 4 6% 6% 41%	6221
	11th to 20th	Day	8% 9% 4 10% 7% 7% 7% 6% 8% 9% 26%	6277
	21st to end	Day	8% 8% 4 8% 6% 6% 6% 5% 6% 8% 35%	7396
June	1st to 10th	Day	15% 8% 3 8% 6% 6% 5% 5% 6% 7% 32%	3000
	11th to 20th	Day	12% 9% 3 8% 8% 7% 6% 5% 7% 8% 27%	3000
	21st to end	Day	7% 4 2 7% 6% 5% 5% 5% 7% 8% 44%	3000
July	1st to 10th	Day	3 3 1 4 3 3 4 4 5% 7% 64%	6780
	11th to 20th	Day	3 2 1 3 2 2 3 3 5% 6% 71%	6420
	21st to end	Day	4 2 1 3 3 3 3 3 5% 7% 65%	6874
August	1st to 10th	Day	2 2 1 3 3 4 5% 4 6% 9% 60%	4541
	11th to 20th	Day	5% 2 1 3 3 3 4 4 6% 7% 63%	4283
	21st to end	Day	4 2 1 5% 4 4 5% 5% 7% 8% 55%	4536
September	1st to 10th	Day	5% 3 1 4 4 4 4 3 6% 8% 58%	2400
	11th to 20th	Day	7% 7% 3 6% 4 4 5% 4 6% 6% 48%	2262
	21st to end	Day	9% 6% 2 8% 4 5% 5% 5% 6% 8% 43%	2200
October	1st to 10th	Day	6% 4 1 6% 4 4 5% 4 7% 8% 52%	3514
	11th to 20th	Day	7% 5% 1 7% 5% 5% 6% 5% 7% 9% 43%	2975
	21st to end	Day	9% 10% 4 9% 6% 7% 7% 5% 7% 6% 31%	3200
November	1st to 10th	Day	14% 16% 5% 11% 6% 6% 6% 5% 6% 7% 19%	1854
	11th to 20th	Day	12% 15% 5% 10% 6% 7% 7% 4 7% 6% 20%	1661
	21st to end	Day	7% 16% 7% 15% 8% 8% 6% 5% 6% 4 19%	1470
December	1st to 10th	Day	6% 10% 4 9% 8% 7% 5% 4 8% 9% 30%	2360
	11th to 20th	Day	8% 17% 7% 16% 8% 7% 7% 4 6% 5% 14%	2712
	21st to end	Day	9% 18% 7% 19% 11% 8% 6% 4 4 4 10%	3575
January	1st to 10th	Night	6% 26% 9% 17% 8% 6% 5% 3 5% 4 12%	6749
	11th to 20th	Night	5% 12% 5% 13% 8% 8% 6% 4 7% 6% 26%	6422
	21st to end	Night	11% 21% 6% 10% 4 5% 5% 4 4 5% 25%	7106
February	1st to 10th	Night	7% 17% 5% 13% 8% 6% 6% 4 5% 8% 22%	2800
	11th to 20th	Night	6% 16% 5% 10% 7% 6% 6% 4 6% 7% 27%	2483
	21st to end	Night	5% 14% 4 12% 8% 6% 6% 5% 7% 8% 25%	1980
March	1st to 10th	Night	4 11% 4 10% 6% 6% 6% 4 7% 8% 34%	5376
	11th to 20th	Night	5% 13% 4 9% 6% 6% 7% 6% 6% 6% 32%	5234
	21st to end	Night	8% 13% 4 11% 7% 7% 7% 6% 7% 9% 21%	5144
April	1st to 10th	Night	6% 10% 3 7% 5% 6% 6% 5% 8% 9% 36%	2000
	11th to 20th	Night	9% 7% 3 8% 5% 6% 4 6% 6% 7% 38%	1851
	21st to end	Night	5% 9% 3 8% 6% 6% 6% 4 7% 7% 39%	1800
May	1st to 10th	Night	6% 6% 3 7% 5% 4 5% 4 7% 8% 46%	3741
	11th to 20th	Night	6% 6% 3 8% 5% 7% 7% 6% 8% 9% 36%	3616
	21st to end	Night	9% 8% 3 7% 4 4 5% 4 6% 9% 41%	3479
June	1st to 10th	Night	12% 5% 2 8% 5% 5% 5% 5% 6% 8% 39%	1400
	11th to 20th	Night	10% 5% 2 7% 5% 6% 6% 6% 8% 9% 36%	1400
	21st to end	Night	3 4 2 5% 4 5% 4 5% 7% 8% 54%	1400
July	1st to 10th	Night	3 1 1 1 1 2 2 3 5% 7% 72%	3180
	11th to 20th	Night	4 1 1 2 2 2 2 2 4 5% 74%	3537
	21st to end	Night	4 2 1 2 2 3 3 3 4 6% 70%	3969
August	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 9% 67%	2609
	11th to 20th	Night	2 2 1 3 3 3 3 4 6% 8% 64%	2770
	21st to end	Night	3 2 1 3 2 4 5% 4 7% 9% 60%	3373
September	1st to 10th	Night	4 2 1 4 3 3 3 4 5% 8% 64%	2000
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 9% 51%	2137
	21st to end	Night	7% 5% 2 4 3 5% 5% 4 6% 7% 53%	2200
October	1st to 10th	Night	5% 4 2 4 3 3 4 3 5% 8% 60%	3683
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 5% 6% 7% 48%	4174
	21st to end	Night	8% 8% 3 8% 6% 5% 7% 6% 7% 8% 35%	4600
November	1st to 10th	Night	13% 15% 4 11% 7% 6% 5% 4 6% 9% 21%	2665
	11th to 20th	Night	11% 18% 4 12% 7% 6% 6% 4 6% 7% 19%	2930
	21st to end	Night	9% 21% 6% 13% 7% 6% 6% 5% 5% 5% 19%	3150
December	1st to 10th	Night	7% 11% 4 11% 9% 6% 6% 5% 7% 6% 30%	5040
	11th to 20th	Night	8% 20% 7% 18% 8% 6% 6% 5% 5% 5% 12%	5700
	21st to end	Night	9% 23% 7% 17% 10% 7% 6% 4 4 4 9%	7623

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	73% 6% 8% 12% 1	3461
	11th to 20th	Day	52% 6% 21% 13% 5% 3	3720
	21st to end	Day	61% 5% 14% 16% 4	4092
February	1st to 10th	Day	63% 5% 13% 9% 8% 2	1600
	11th to 20th	Day	63% 5% 14% 13% 6%	1811
	21st to end	Day	62% 3 13% 14% 8%	1650
March	1st to 10th	Day	48% 6% 15% 19% 11%	4518
	11th to 20th	Day	48% 8% 20% 21% 4	4753
	21st to end	Day	62% 7% 11% 16% 4	5827
April	1st to 10th	Day	58% 4 15% 17% 5%	2400
	11th to 20th	Day	56% 3 14% 19% 7% 1	2500
	21st to end	Day	55% 3 13% 22% 5% 1	2600
May	1st to 10th	Day	43% 5% 11% 27% 13%	6221
	11th to 20th	Day	52% 5% 15% 22% 6%	6280
	21st to end	Day	48% 4 18% 25% 6%	7397
June	1st to 10th	Day	52% 2 14% 20% 11% 1	3000
	11th to 20th	Day	54% 3 12% 20% 9% 1	3000
	21st to end	Day	36% 3 16% 28% 15% 2	3000
July	1st to 10th	Day	20% 4 11% 29% 29% 7%	6706
	11th to 20th	Day	15% 3 12% 28% 28% 14%	6342
	21st to end	Day	19% 4 12% 31% 23% 11%	6681
August	1st to 10th	Day	20% 4 13% 34% 23% 6%	4524
	11th to 20th	Day	21% 3 18% 39% 17% 2	4325
	21st to end	Day	25% 6% 18% 34% 16% 2	4535
September	1st to 10th	Day	27% 2 13% 34% 19% 5%	2400
	11th to 20th	Day	36% 2 15% 29% 15% 2	2262
	21st to end	Day	40% 4 22% 23% 10% 1	2200
October	1st to 10th	Day	30% 7% 21% 36% 7%	3514
	11th to 20th	Day	39% 9% 18% 26% 9%	2975
	21st to end	Day	49% 12% 17% 17% 6%	3200
November	1st to 10th	Day	66% 6% 13% 12% 4	1854
	11th to 20th	Day	64% 7% 14% 10% 6%	1661
	21st to end	Day	68% 6% 14% 9% 2	1470
December	1st to 10th	Day	53% 12% 21% 13% 1	2360
	11th to 20th	Day	72% 7% 12% 8% 2	2714
	21st to end	Day	77% 7% 7% 6% 3	3575
January	1st to 10th	Night	77% 5% 9% 7% 1	6749
	11th to 20th	Night	57% 9% 11% 15% 6% 2	6460
	21st to end	Night	63% 5% 14% 14% 3	7106
February	1st to 10th	Night	63% 5% 12% 10% 9% 1	2800
	11th to 20th	Night	59% 5% 15% 15% 6%	2485
	21st to end	Night	58% 4 16% 13% 8% 1	1980
March	1st to 10th	Night	49% 5% 17% 19% 11%	5376
	11th to 20th	Night	53% 6% 18% 18% 5%	5234
	21st to end	Night	60% 8% 14% 15% 3	5144
April	1st to 10th	Night	44% 5% 22% 21% 7% 1	2000
	11th to 20th	Night	46% 3 17% 23% 11%	1851
	21st to end	Night	46% 3 16% 27% 8% 1	1800
May	1st to 10th	Night	38% 4 13% 33% 12% 1	3738
	11th to 20th	Night	43% 6% 17% 26% 8% 1	3616
	21st to end	Night	41% 2 22% 28% 6%	3479
June	1st to 10th	Night	44% 2 16% 25% 11% 2	1400
	11th to 20th	Night	43% 4 15% 27% 10% 2	1400
	21st to end	Night	27% 3 18% 30% 20% 2	1400
July	1st to 10th	Night	13% 3 15% 31% 31% 8%	3145
	11th to 20th	Night	14% 3 13% 26% 25% 19%	3492
	21st to end	Night	16% 4 9% 34% 22% 15%	3857
August	1st to 10th	Night	16% 3 14% 33% 26% 8%	2601
	11th to 20th	Night	19% 3 22% 34% 20% 2	2794
	21st to end	Night	20% 5% 19% 34% 19% 3	3373
September	1st to 10th	Night	20% 3 13% 37% 22% 5%	2000
	11th to 20th	Night	32% 3 16% 32% 15% 3	2137
	21st to end	Night	31% 4 24% 27% 12% 2	2200
October	1st to 10th	Night	24% 6% 24% 39% 7%	3683
	11th to 20th	Night	36% 7% 16% 32% 9% 1	4174
	21st to end	Night	43% 11% 22% 20% 4	4600
November	1st to 10th	Night	63% 7% 14% 12% 3	2665
	11th to 20th	Night	66% 7% 12% 10% 5%	2930
	21st to end	Night	69% 6% 12% 11% 2	3150
December	1st to 10th	Night	54% 11% 19% 14% 1	5040
	11th to 20th	Night	75% 8% 10% 5% 1	5697
	21st to end	Night	80% 5% 7% 6% 2	7623

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	15% 9% 21% 32% 16% 7%	220
	11th to 20th	Day	15% 4 17% 42% 16% 6%	220
	21st to end	Day	16% 4 23% 36% 15% 7%	242
February	1st to 10th	Day	12% 4 31% 40% 11% 2	200
	11th to 20th	Day	14% 7% 22% 35% 19% 2	382
	21st to end	Day	13% 9% 31% 30% 15% 3	414
March	1st to 10th	Day	18% 10% 30% 25% 14% 3	500
	11th to 20th	Day	15% 13% 25% 22% 17% 8%	500
	21st to end	Day	19% 13% 29% 25% 11% 3	550
April	1st to 10th	Day	20% 13% 26% 24% 12% 4	500
	11th to 20th	Day	19% 17% 25% 23% 10% 5%	492
	21st to end	Day	17% 13% 27% 22% 16% 5%	500
May	1st to 10th	Day	14% 11% 28% 27% 14% 6%	503
	11th to 20th	Day	18% 10% 23% 27% 16% 7%	528
	21st to end	Day	18% 7% 17% 27% 24% 8%	780
June	1st to 10th	Day	18% 8% 21% 30% 19% 4	730
	11th to 20th	Day	15% 10% 28% 28% 16% 4	730
	21st to end	Day	10% 8% 20% 31% 21% 10%	730
July	1st to 10th	Day	6% 6% 23% 32% 26% 7%	721
	11th to 20th	Day	5% 9% 28% 31% 20% 7%	569
	21st to end	Day	4 8% 30% 25% 22% 10%	575
August	1st to 10th	Day	6% 7% 22% 33% 23% 10%	527
	11th to 20th	Day	6% 6% 22% 33% 19% 14%	517
	21st to end	Day	8% 9% 23% 27% 23% 10%	583
September	1st to 10th	Day	10% 6% 23% 26% 24% 11%	530
	11th to 20th	Day	11% 8% 21% 32% 18% 9%	530
	21st to end	Day	16% 8% 26% 29% 15% 6%	530
October	1st to 10th	Day	13% 7% 25% 33% 14% 7%	530
	11th to 20th	Day	14% 8% 21% 33% 17% 8%	360
	21st to end	Day	21% 6% 28% 29% 13% 3	363
November	1st to 10th	Day	19% 8% 21% 33% 17% 1	330
	11th to 20th	Day	23% 6% 21% 31% 15% 3	267
	21st to end	Day	13% 5% 21% 38% 15% 9%	220
December	1st to 10th	Day	15% 5% 15% 35% 21% 8%	220
	11th to 20th	Day	12% 12% 18% 41% 14% 4	220
	21st to end	Day	11% 5% 16% 40% 20% 8%	242
January	1st to 10th	Night	25% 5% 17% 38% 13% 2	974
	11th to 20th	Night	20% 5% 18% 40% 15% 2	840
	21st to end	Night	19% 3 19% 43% 14% 2	924
February	1st to 10th	Night	17% 4 20% 43% 14% 3	800
	11th to 20th	Night	16% 4 17% 45% 15% 2	784
	21st to end	Night	15% 4 17% 41% 19% 4	660
March	1st to 10th	Night	17% 4 18% 40% 19% 2	800
	11th to 20th	Night	18% 3 20% 36% 22% 2	800
	21st to end	Night	19% 3 18% 38% 19% 3	880
April	1st to 10th	Night	13% 3 17% 44% 21% 2	800
	11th to 20th	Night	15% 2 21% 43% 18% 1	663
	21st to end	Night	13% 5% 19% 42% 22% 1	600
May	1st to 10th	Night	9% 3 16% 47% 23% 2	449
	11th to 20th	Night	11% 2 12% 37% 34% 3	394
	21st to end	Night	18% 3 9% 37% 29% 4	433
June	1st to 10th	Night	16% 2 15% 41% 25% 2	400
	11th to 20th	Night	15% 2 15% 45% 22% 2	400
	21st to end	Night	6% 2 14% 43% 33% 4	400
July	1st to 10th	Night	4 1 6% 45% 42% 2	390
	11th to 20th	Night	4 1 10% 52% 31% 2	388
	21st to end	Night	3 8% 46% 41% 2	438
August	1st to 10th	Night	3 2 7% 41% 43% 4	398
	11th to 20th	Night	5% 1 9% 39% 42% 4	511
	21st to end	Night	5% 2 11% 43% 36% 3	736
September	1st to 10th	Night	6% 1 11% 45% 31% 7%	800
	11th to 20th	Night	8% 1 11% 38% 36% 6%	800
	21st to end	Night	13% 1 15% 42% 24% 5%	800
October	1st to 10th	Night	14% 3 14% 41% 25% 3	800
	11th to 20th	Night	17% 2 17% 39% 22% 4	800
	21st to end	Night	19% 5% 19% 40% 14% 3	880
November	1st to 10th	Night	25% 4 17% 42% 11% 1	820
	11th to 20th	Night	26% 6% 17% 36% 14% 1	924
	21st to end	Night	22% 4 17% 38% 17% 2	1050
December	1st to 10th	Night	19% 3 16% 42% 17% 2	1050
	11th to 20th	Night	21% 5% 19% 41% 13% 1	1050
	21st to end	Night	21% 5% 18% 40% 15% 1	1155

Page generated at 2026-01-28 22:21:40. Version ac07

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