Coal Creek Crest
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Coal Creek Crest clear sky chart from 2005-11-06 to 2026-07-31.

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% 10% 3 8% 5% 4 4 3 5% 6% 31%	143476
	Night	22% 11% 3 7% 4 4 4 3 5% 6% 31%	138285
Transparency	Day	51% 7% 12% 17% 10% 2	143196
	Night	52% 7% 13% 17% 8% 2	138147
Seeing	Day	22% 13% 25% 26% 12% 2	17289
	Night	25% 7% 17% 32% 17% 1	25590

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	40% 18% 5% 9% 5% 4 4 2 4 3 6%	4723
	11th to 20th	Day	22% 10% 3 7% 4 4 4 3 6% 9% 29%	4716
	21st to end	Day	32% 9% 3 7% 4 3 3 3 4 5% 26%	5637
February	1st to 10th	Day	33% 15% 4 8% 5% 4 4 3 5% 6% 13%	1890
	11th to 20th	Day	30% 17% 5% 10% 6% 4 3 3 3 3 16%	1845
	21st to end	Day	31% 18% 3 9% 6% 3 4 3 4 4 15%	1615
March	1st to 10th	Day	21% 17% 5% 11% 6% 5% 6% 4 5% 4 15%	4857
	11th to 20th	Day	30% 18% 3 8% 4 3 3 2 3 4 23%	5110
	21st to end	Day	29% 16% 4 12% 6% 6% 6% 4 4 4 10%	5711
April	1st to 10th	Day	29% 14% 4 9% 6% 5% 4 3 5% 6% 15%	2520
	11th to 20th	Day	26% 13% 4 9% 5% 5% 4 3 5% 5% 23%	2524
	21st to end	Day	22% 13% 4 9% 6% 5% 5% 4 5% 5% 23%	2730
May	1st to 10th	Day	13% 8% 2 6% 4 4 5% 4 6% 7% 40%	8098
	11th to 20th	Day	14% 10% 4 9% 6% 6% 7% 5% 8% 8% 24%	8160
	21st to end	Day	16% 8% 3 7% 5% 5% 5% 4 7% 9% 32%	8993
June	1st to 10th	Day	21% 11% 3 8% 5% 4 4 4 5% 7% 28%	2940
	11th to 20th	Day	20% 10% 3 10% 6% 5% 5% 4 6% 7% 23%	3097
	21st to end	Day	13% 7% 3 7% 5% 5% 6% 4 6% 7% 37%	3150
July	1st to 10th	Day	5% 4 1 5% 3 3 3 3 4 6% 64%	6895
	11th to 20th	Day	4 3 1 3 2 3 3 2 4 7% 68%	6460
	21st to end	Day	7% 4 2 4 3 3 4 3 6% 8% 57%	7015
August	1st to 10th	Day	5% 4 2 4 3 3 4 3 5% 8% 60%	4290
	11th to 20th	Day	8% 6% 2 7% 5% 6% 7% 5% 7% 9% 38%	4156
	21st to end	Day	14% 8% 3 7% 3 4 4 3 6% 7% 43%	4480
September	1st to 10th	Day	8% 7% 2 5% 3 4 4 4 6% 8% 48%	2400
	11th to 20th	Day	13% 9% 3 6% 5% 5% 4 3 6% 8% 37%	2388
	21st to end	Day	19% 7% 3 6% 4 4 4 4 5% 7% 38%	2110
October	1st to 10th	Day	18% 7% 2 5% 4 3 4 3 6% 8% 40%	3280
	11th to 20th	Day	25% 9% 3 8% 4 4 5% 4 6% 7% 24%	3261
	21st to end	Day	30% 11% 3 9% 5% 4 5% 4 5% 5% 18%	3470
November	1st to 10th	Day	34% 14% 4 9% 4 5% 5% 4 5% 4 12%	1698
	11th to 20th	Day	38% 14% 4 7% 4 3 4 3 5% 5% 14%	1680
	21st to end	Day	32% 12% 3 10% 4 5% 4 3 5% 4 18%	1680
December	1st to 10th	Day	25% 10% 3 7% 5% 5% 5% 4 6% 6% 25%	2700
	11th to 20th	Day	39% 15% 4 10% 6% 4 4 3 3 3 9%	3105
	21st to end	Day	38% 18% 4 11% 5% 4 4 3 3 3 7%	4092
January	1st to 10th	Night	37% 17% 4 10% 6% 4 4 3 3 4 9%	8172
	11th to 20th	Night	21% 10% 3 7% 3 3 3 3 5% 9% 34%	8182
	21st to end	Night	32% 11% 3 6% 3 3 2 2 4 6% 28%	8564
February	1st to 10th	Night	29% 14% 5% 9% 6% 5% 4 3 4 5% 17%	2730
	11th to 20th	Night	27% 14% 4 9% 5% 4 4 3 4 5% 22%	2662
	21st to end	Night	28% 16% 4 8% 5% 4 4 4 3 5% 20%	2191
March	1st to 10th	Night	19% 16% 4 10% 6% 5% 5% 4 6% 6% 19%	5254
	11th to 20th	Night	26% 16% 3 8% 4 4 3 3 3 3 26%	5109
	21st to end	Night	34% 16% 5% 9% 5% 4 4 3 5% 5% 12%	5505
April	1st to 10th	Night	23% 16% 4 8% 7% 5% 4 3 4 5% 21%	2100
	11th to 20th	Night	20% 13% 3 8% 5% 4 4 4 5% 6% 27%	2043
	21st to end	Night	20% 12% 3 8% 5% 4 4 4 5% 7% 28%	1890
May	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 5% 7% 47%	4826
	11th to 20th	Night	12% 11% 4 9% 6% 5% 5% 4 6% 8% 30%	4700
	21st to end	Night	8% 10% 3 7% 5% 5% 5% 4 7% 8% 38%	5178
June	1st to 10th	Night	16% 7% 3 8% 5% 5% 4 4 4 7% 37%	1680
	11th to 20th	Night	15% 9% 2 6% 4 4 3 4 6% 8% 38%	1516
	21st to end	Night	9% 7% 2 7% 4 4 4 3 5% 6% 50%	1470
July	1st to 10th	Night	2 2 1 3 3 3 3 2 5% 7% 68%	3268
	11th to 20th	Night	2 2 1 3 2 2 2 3 4 5% 73%	3720
	21st to end	Night	3 3 1 3 2 2 3 3 5% 8% 66%	4049
August	1st to 10th	Night	2 2 1 3 2 3 3 3 5% 7% 69%	2865
	11th to 20th	Night	8% 5% 1 5% 5% 4 5% 4 6% 8% 50%	2902
	21st to end	Night	11% 7% 3 6% 4 3 4 3 4 6% 50%	3434
September	1st to 10th	Night	8% 4 1 4 3 3 4 3 5% 8% 57%	2000
	11th to 20th	Night	12% 7% 3 7% 5% 3 4 3 5% 6% 46%	2010
	21st to end	Night	14% 7% 3 5% 4 4 4 3 5% 8% 44%	2290
October	1st to 10th	Night	13% 7% 2 6% 4 3 4 3 6% 9% 44%	3920
	11th to 20th	Night	21% 9% 3 8% 4 4 4 3 5% 7% 31%	3897
	21st to end	Night	28% 10% 4 7% 5% 4 4 3 5% 6% 24%	4336
November	1st to 10th	Night	35% 11% 3 7% 5% 4 4 3 4 5% 19%	2816
	11th to 20th	Night	37% 12% 4 7% 5% 3 3 3 5% 4 18%	2940
	21st to end	Night	29% 13% 4 8% 5% 5% 4 3 3 4 22%	2940
December	1st to 10th	Night	23% 11% 3 6% 4 4 4 4 7% 7% 27%	4700
	11th to 20th	Night	40% 17% 4 9% 4 4 3 2 3 3 11%	5320
	21st to end	Night	39% 18% 5% 10% 5% 4 3 2 3 3 9%	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	82% 6% 5% 5% 2	4723
	11th to 20th	Day	53% 8% 10% 14% 13% 3	4740
	21st to end	Day	61% 6% 6% 13% 12% 2	5638
February	1st to 10th	Day	70% 7% 11% 9% 3	1890
	11th to 20th	Day	72% 6% 7% 9% 5% 1	1854
	21st to end	Day	72% 4 9% 8% 6% 1	1615
March	1st to 10th	Day	66% 8% 13% 9% 4	4857
	11th to 20th	Day	65% 5% 8% 19% 3	5110
	21st to end	Day	74% 8% 10% 7% 1	5711
April	1st to 10th	Day	68% 9% 11% 11% 1	2520
	11th to 20th	Day	62% 7% 10% 12% 8% 1	2524
	21st to end	Day	62% 7% 11% 13% 6%	2730
May	1st to 10th	Day	39% 7% 14% 28% 12%	8098
	11th to 20th	Day	50% 11% 19% 16% 3	8163
	21st to end	Day	46% 8% 23% 19% 4	8994
June	1st to 10th	Day	56% 7% 16% 15% 7%	2940
	11th to 20th	Day	57% 8% 15% 15% 5%	3097
	21st to end	Day	43% 9% 15% 22% 9% 2	3150
July	1st to 10th	Day	23% 5% 12% 26% 18% 16%	6821
	11th to 20th	Day	16% 4 10% 24% 40% 5%	6385
	21st to end	Day	23% 5% 19% 33% 19% 2	6822
August	1st to 10th	Day	23% 6% 11% 28% 26% 6%	4273
	11th to 20th	Day	35% 11% 19% 22% 10% 3	4195
	21st to end	Day	40% 6% 11% 23% 17% 3	4479
September	1st to 10th	Day	33% 7% 14% 23% 17% 6%	2400
	11th to 20th	Day	44% 8% 13% 15% 16% 4	2388
	21st to end	Day	44% 6% 15% 17% 14% 3	2110
October	1st to 10th	Day	42% 6% 9% 29% 13% 1	3280
	11th to 20th	Day	56% 8% 13% 13% 10%	3261
	21st to end	Day	64% 7% 9% 13% 8%	3470
November	1st to 10th	Day	72% 8% 8% 10% 2	1698
	11th to 20th	Day	71% 7% 9% 7% 5% 2	1680
	21st to end	Day	69% 6% 7% 7% 11%	1680
December	1st to 10th	Day	59% 6% 8% 13% 13% 1	2700
	11th to 20th	Day	77% 8% 7% 7% 1	3108
	21st to end	Day	79% 8% 7% 4 2	4092
January	1st to 10th	Night	79% 7% 7% 5% 2 1	8172
	11th to 20th	Night	49% 11% 14% 16% 9% 1	8220
	21st to end	Night	59% 5% 10% 16% 9% 1	8564
February	1st to 10th	Night	70% 8% 9% 11% 2 1	2730
	11th to 20th	Night	65% 7% 10% 9% 6% 2	2664
	21st to end	Night	68% 7% 11% 9% 6%	2191
March	1st to 10th	Night	62% 9% 14% 11% 4	5254
	11th to 20th	Night	61% 8% 11% 16% 4	5110
	21st to end	Night	73% 8% 11% 7% 1	5505
April	1st to 10th	Night	65% 6% 13% 12% 3	2100
	11th to 20th	Night	56% 9% 12% 14% 8% 2	2043
	21st to end	Night	54% 8% 15% 17% 5% 1	1890
May	1st to 10th	Night	34% 7% 16% 31% 11%	4823
	11th to 20th	Night	45% 10% 21% 18% 5% 1	4700
	21st to end	Night	39% 11% 27% 21% 2	5182
June	1st to 10th	Night	47% 7% 18% 21% 7%	1680
	11th to 20th	Night	43% 9% 20% 23% 5%	1516
	21st to end	Night	32% 7% 21% 29% 8% 3	1470
July	1st to 10th	Night	12% 4 19% 32% 16% 17%	3233
	11th to 20th	Night	13% 3 9% 30% 40% 5%	3672
	21st to end	Night	15% 5% 23% 36% 17% 3	3937
August	1st to 10th	Night	14% 5% 13% 36% 28% 4	2857
	11th to 20th	Night	28% 9% 26% 24% 9% 4	2929
	21st to end	Night	34% 6% 15% 26% 17% 2	3434
September	1st to 10th	Night	27% 6% 15% 28% 17% 8%	2000
	11th to 20th	Night	40% 7% 12% 20% 14% 6%	2010
	21st to end	Night	37% 7% 17% 21% 15% 3	2290
October	1st to 10th	Night	37% 8% 11% 33% 11% 1	3920
	11th to 20th	Night	52% 7% 14% 18% 8% 1	3897
	21st to end	Night	60% 6% 10% 16% 8%	4336
November	1st to 10th	Night	67% 7% 10% 11% 5%	2816
	11th to 20th	Night	70% 7% 8% 8% 5% 2	2940
	21st to end	Night	65% 7% 8% 9% 10% 1	2940
December	1st to 10th	Night	55% 9% 13% 15% 8%	4700
	11th to 20th	Night	77% 7% 7% 7% 3	5316
	21st to end	Night	78% 8% 7% 6% 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	40% 15% 22% 15% 5% 3	220
	11th to 20th	Day	32% 13% 24% 18% 10% 4	220
	21st to end	Day	34% 11% 19% 23% 10% 3	398
February	1st to 10th	Day	40% 12% 21% 15% 11% 1	430
	11th to 20th	Day	39% 16% 24% 16% 5%	425
	21st to end	Day	38% 16% 22% 18% 6%	386
March	1st to 10th	Day	31% 18% 27% 17% 7% 1	540
	11th to 20th	Day	33% 15% 24% 18% 8% 2	540
	21st to end	Day	34% 19% 23% 16% 8% 1	594
April	1st to 10th	Day	28% 17% 22% 22% 8% 3	540
	11th to 20th	Day	25% 22% 24% 19% 8% 2	532
	21st to end	Day	25% 15% 29% 22% 8% 1	540
May	1st to 10th	Day	17% 15% 28% 25% 12% 2	543
	11th to 20th	Day	25% 15% 25% 22% 10% 2	568
	21st to end	Day	19% 12% 27% 27% 12% 3	617
June	1st to 10th	Day	24% 11% 25% 28% 10% 2	570
	11th to 20th	Day	16% 18% 34% 24% 8% 1	727
	21st to end	Day	10% 13% 31% 31% 13% 2	780
July	1st to 10th	Day	9% 6% 31% 37% 15% 2	762
	11th to 20th	Day	8% 9% 26% 38% 17% 3	564
	21st to end	Day	6% 5% 24% 40% 23% 3	616
August	1st to 10th	Day	9% 5% 22% 43% 17% 4	527
	11th to 20th	Day	5% 4 22% 41% 23% 6%	517
	21st to end	Day	10% 8% 29% 34% 14% 4	583
September	1st to 10th	Day	9% 7% 28% 37% 16% 3	530
	11th to 20th	Day	14% 9% 24% 35% 15% 3	530
	21st to end	Day	18% 13% 24% 29% 15% 1	530
October	1st to 10th	Day	22% 12% 26% 26% 13% 2	530
	11th to 20th	Day	25% 12% 22% 23% 15% 3	530
	21st to end	Day	31% 13% 26% 18% 11% 2	531
November	1st to 10th	Day	31% 16% 26% 18% 7% 2	247
	11th to 20th	Day	40% 12% 16% 21% 9% 2	220
	21st to end	Day	30% 19% 21% 15% 14% 1	220
December	1st to 10th	Day	35% 11% 20% 16% 15% 3	220
	11th to 20th	Day	40% 20% 23% 11% 6%	220
	21st to end	Day	40% 14% 20% 16% 9%	242
January	1st to 10th	Night	41% 10% 17% 19% 11% 2	840
	11th to 20th	Night	35% 9% 15% 24% 16% 1	840
	21st to end	Night	32% 7% 14% 28% 17% 2	924
February	1st to 10th	Night	33% 9% 15% 29% 12% 1	840
	11th to 20th	Night	31% 9% 17% 29% 13% 1	824
	21st to end	Night	33% 11% 16% 29% 11%	692
March	1st to 10th	Night	30% 10% 21% 25% 13% 1	840
	11th to 20th	Night	33% 10% 17% 28% 12% 1	840
	21st to end	Night	34% 10% 18% 27% 10% 1	924
April	1st to 10th	Night	29% 9% 20% 26% 15% 1	840
	11th to 20th	Night	23% 9% 23% 32% 12% 2	795
	21st to end	Night	25% 6% 17% 36% 15% 1	630
May	1st to 10th	Night	19% 7% 22% 34% 18%	457
	11th to 20th	Night	18% 5% 18% 37% 20% 2	414
	21st to end	Night	14% 6% 17% 40% 22% 1	455
June	1st to 10th	Night	18% 4 20% 41% 17%	420
	11th to 20th	Night	16% 6% 22% 44% 12%	420
	21st to end	Night	10% 2 22% 49% 18%	420
July	1st to 10th	Night	5% 2 18% 51% 23%	410
	11th to 20th	Night	4 2 21% 53% 19% 1	408
	21st to end	Night	3 2 12% 58% 25% 1	458
August	1st to 10th	Night	6% 2 10% 48% 32% 3	550
	11th to 20th	Night	7% 2 9% 43% 36% 3	594
	21st to end	Night	11% 3 16% 44% 24% 1	771
September	1st to 10th	Night	10% 3 18% 42% 25% 2	800
	11th to 20th	Night	13% 4 16% 34% 29% 3	800
	21st to end	Night	15% 6% 18% 39% 21% 2	800
October	1st to 10th	Night	20% 5% 16% 37% 21% 3	800
	11th to 20th	Night	25% 5% 15% 30% 23% 3	800
	21st to end	Night	29% 7% 19% 27% 17% 1	880
November	1st to 10th	Night	34% 8% 19% 29% 10%	820
	11th to 20th	Night	40% 9% 16% 22% 13% 1	840
	21st to end	Night	32% 8% 17% 25% 16% 2	840
December	1st to 10th	Night	31% 7% 16% 27% 16% 2	840
	11th to 20th	Night	39% 9% 18% 24% 10% 1	840
	21st to end	Night	41% 11% 20% 19% 7% 1	924

Page generated at 2026-07-28 00:25:21. Version ac07

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