Blue Creek
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Creek clear sky chart from 2005-11-06 to 2025-07-21.

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	15% 10% 3 7% 5% 4 5% 4 5% 7% 35%	120305
	Night	15% 10% 3 7% 5% 4 4 3 5% 7% 36%	118847
Transparency	Day	47% 3 10% 23% 15% 2	124021
	Night	48% 3 9% 23% 15% 2	123267
Seeing	Day	22% 13% 27% 21% 12% 5%	15741
	Night	20% 11% 24% 27% 15% 2	25805

By Month

Percentage Cloud Cover SE (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	16% 20% 6% 11% 5% 4 3 3 4 5% 23%	4056
	11th to 20th	Day	14% 16% 8% 13% 5% 4 3 2 3 3 28%	4060
	21st to end	Day	21% 14% 6% 8% 5% 5% 4 3 4 4 25%	4466
February	1st to 10th	Day	20% 13% 4 10% 5% 4 4 4 4 5% 29%	1843
	11th to 20th	Day	18% 13% 5% 11% 4 4 5% 3 4 5% 28%	1948
	21st to end	Day	20% 14% 3 7% 4 4 3 2 5% 5% 32%	1650
March	1st to 10th	Day	17% 15% 4 8% 4 4 4 3 4 5% 31%	4659
	11th to 20th	Day	13% 7% 4 6% 4 4 5% 4 5% 7% 40%	4980
	21st to end	Day	18% 9% 2 7% 5% 5% 4 4 5% 7% 34%	5465
April	1st to 10th	Day	16% 10% 3 7% 4 4 4 3 5% 8% 36%	2260
	11th to 20th	Day	17% 9% 3 6% 5% 3 4 3 5% 6% 39%	2482
	21st to end	Day	17% 8% 3 7% 6% 5% 5% 3 5% 7% 34%	2600
May	1st to 10th	Day	19% 10% 3 8% 5% 4 6% 4 6% 7% 27%	5862
	11th to 20th	Day	21% 9% 3 8% 5% 4 5% 4 5% 7% 30%	5861
	21st to end	Day	15% 8% 2 7% 5% 5% 6% 5% 7% 8% 32%	6433
June	1st to 10th	Day	12% 5% 2 6% 5% 5% 6% 5% 6% 9% 39%	2600
	11th to 20th	Day	10% 6% 2 7% 6% 5% 5% 5% 7% 9% 39%	2600
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 7% 10% 40%	2600
July	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 9% 40%	4250
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 7% 10% 41%	4250
	21st to end	Day	13% 9% 4 9% 6% 6% 6% 4 7% 8% 28%	4466
August	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 8% 10% 40%	4104
	11th to 20th	Day	9% 6% 2 6% 4 5% 6% 5% 7% 10% 38%	4044
	21st to end	Day	6% 3 2 4 4 4 4 4 6% 9% 55%	4346
September	1st to 10th	Day	14% 5% 2 5% 3 4 4 3 5% 6% 48%	2104
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 6% 7% 49%	2090
	21st to end	Day	19% 6% 2 6% 3 3 4 3 4 5% 44%	2090
October	1st to 10th	Day	12% 4 2 5% 4 4 4 3 6% 7% 49%	2067
	11th to 20th	Day	15% 7% 3 5% 3 4 4 3 4 5% 48%	1900
	21st to end	Day	21% 8% 3 6% 4 3 3 3 4 6% 40%	2038
November	1st to 10th	Day	13% 7% 2 5% 4 3 4 3 4 6% 50%	1762
	11th to 20th	Day	15% 13% 3 6% 4 3 4 3 6% 6% 37%	1800
	21st to end	Day	18% 11% 3 8% 4 4 3 3 4 6% 37%	1800
December	1st to 10th	Day	20% 12% 4 8% 5% 3 3 3 3 4 34%	2930
	11th to 20th	Day	22% 15% 5% 8% 4 4 5% 3 4 5% 25%	3373
	21st to end	Day	20% 18% 5% 11% 5% 5% 4 3 4 5% 21%	4466
January	1st to 10th	Night	16% 18% 6% 16% 6% 4 4 3 4 4 18%	5889
	11th to 20th	Night	17% 15% 6% 13% 7% 4 4 2 4 4 23%	5900
	21st to end	Night	20% 12% 4 8% 5% 5% 4 2 3 5% 31%	6490
February	1st to 10th	Night	21% 14% 5% 8% 5% 4 3 3 4 5% 28%	2555
	11th to 20th	Night	21% 13% 4 9% 4 5% 4 3 4 5% 29%	2338
	21st to end	Night	22% 15% 4 8% 4 3 3 2 4 6% 29%	1980
March	1st to 10th	Night	17% 14% 2 6% 3 4 4 3 4 6% 37%	5194
	11th to 20th	Night	13% 9% 3 5% 3 4 4 4 5% 8% 43%	4980
	21st to end	Night	16% 10% 4 8% 5% 5% 5% 4 5% 6% 32%	5467
April	1st to 10th	Night	16% 9% 3 6% 6% 3 4 3 5% 7% 37%	2156
	11th to 20th	Night	14% 7% 3 6% 4 4 5% 5% 5% 7% 40%	1866
	21st to end	Night	17% 8% 3 6% 5% 4 5% 5% 5% 7% 37%	1800
May	1st to 10th	Night	22% 10% 3 7% 4 4 4 3 5% 7% 29%	4033
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 5% 6% 9% 33%	4032
	21st to end	Night	14% 7% 2 6% 4 5% 4 5% 7% 9% 36%	4421
June	1st to 10th	Night	8% 5% 2 6% 5% 5% 5% 4 7% 9% 46%	1800
	11th to 20th	Night	7% 4 2 6% 5% 4 5% 4 6% 12% 45%	1800
	21st to end	Night	7% 5% 1 5% 5% 4 5% 5% 8% 10% 45%	1800
July	1st to 10th	Night	8% 6% 2 6% 4 5% 4 4 7% 9% 45%	2930
	11th to 20th	Night	7% 5% 2 6% 5% 5% 5% 3 7% 10% 46%	2930
	21st to end	Night	10% 9% 3 8% 5% 4 5% 5% 6% 8% 36%	3087
August	1st to 10th	Night	7% 6% 2 6% 4 5% 6% 4 7% 9% 44%	2825
	11th to 20th	Night	10% 7% 2 6% 4 6% 5% 3 6% 7% 42%	2785
	21st to end	Night	4 3 1 4 3 3 4 4 6% 9% 59%	3306
September	1st to 10th	Night	11% 5% 2 4 4 4 4 3 5% 7% 51%	2057
	11th to 20th	Night	10% 4 2 5% 4 4 4 2 4 7% 54%	2090
	21st to end	Night	13% 6% 3 6% 4 3 4 4 5% 6% 46%	2090
October	1st to 10th	Night	10% 5% 1 5% 4 4 4 3 5% 7% 53%	2111
	11th to 20th	Night	14% 6% 2 5% 2 4 4 3 7% 7% 45%	2280
	21st to end	Night	19% 8% 2 5% 4 4 4 3 4 5% 42%	2557
November	1st to 10th	Night	13% 7% 2 5% 4 3 3 3 4 5% 49%	2536
	11th to 20th	Night	16% 10% 3 7% 5% 4 4 3 4 6% 39%	2600
	21st to end	Night	19% 12% 3 6% 5% 4 4 3 4 6% 34%	2600
December	1st to 10th	Night	18% 13% 4 7% 5% 3 3 2 4 5% 35%	4250
	11th to 20th	Night	20% 14% 7% 9% 4 3 3 3 4 5% 28%	4822
	21st to end	Night	20% 18% 6% 10% 6% 4 4 3 4 5% 19%	6490

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	65% 2 9% 17% 8%	4057
	11th to 20th	Day	63% 1 6% 16% 12% 2	4060
	21st to end	Day	63% 2 7% 20% 7% 2	4466
February	1st to 10th	Day	59% 3 9% 18% 10% 1	1843
	11th to 20th	Day	59% 2 8% 19% 11% 1	1958
	21st to end	Day	55% 2 8% 23% 11% 2	1650
March	1st to 10th	Day	56% 1 5% 21% 12% 4	4659
	11th to 20th	Day	44% 1 8% 27% 17% 2	4980
	21st to end	Day	50% 2 10% 23% 13% 2	5465
April	1st to 10th	Day	49% 2 9% 24% 13% 3	2260
	11th to 20th	Day	47% 2 8% 23% 16% 3	2482
	21st to end	Day	50% 3 11% 21% 12% 3	2600
May	1st to 10th	Day	55% 3 10% 19% 11% 1	5897
	11th to 20th	Day	54% 3 9% 23% 10% 1	5865
	21st to end	Day	47% 4 12% 21% 14% 2	6447
June	1st to 10th	Day	41% 4 11% 25% 17% 2	2600
	11th to 20th	Day	41% 5% 14% 23% 16% 2	2600
	21st to end	Day	39% 6% 13% 24% 15% 4	2600
July	1st to 10th	Day	42% 5% 11% 23% 17% 2	4185
	11th to 20th	Day	36% 5% 12% 32% 15% 1	4181
	21st to end	Day	51% 5% 11% 17% 14% 2	4324
August	1st to 10th	Day	36% 5% 13% 33% 11% 1	4091
	11th to 20th	Day	38% 5% 9% 30% 16% 1	4083
	21st to end	Day	26% 4 15% 31% 21% 3	4345
September	1st to 10th	Day	37% 3 10% 23% 20% 6%	2104
	11th to 20th	Day	35% 2 10% 25% 25% 2	2090
	21st to end	Day	43% 3 6% 25% 21% 2	2090
October	1st to 10th	Day	24% 2 7% 31% 34% 2	3431
	11th to 20th	Day	30% 2 8% 31% 27% 2	3140
	21st to end	Day	34% 2 10% 28% 23% 3	3340
November	1st to 10th	Day	37% 2 9% 28% 20% 3	1762
	11th to 20th	Day	48% 2 7% 22% 18% 4	1800
	21st to end	Day	50% 2 9% 22% 15% 2	1800
December	1st to 10th	Day	54% 3 8% 24% 10%	2930
	11th to 20th	Day	63% 3 7% 18% 8% 1	3370
	21st to end	Day	67% 2 9% 15% 6%	4466
January	1st to 10th	Night	71% 2 7% 14% 6%	5889
	11th to 20th	Night	66% 2 7% 14% 8% 3	5900
	21st to end	Night	58% 2 7% 22% 10% 1	6490
February	1st to 10th	Night	59% 3 8% 19% 9% 1	2555
	11th to 20th	Night	60% 2 6% 19% 12% 1	2339
	21st to end	Night	58% 2 9% 17% 11% 3	1980
March	1st to 10th	Night	51% 1 4 28% 13% 2	5194
	11th to 20th	Night	41% 2 8% 30% 19% 1	4980
	21st to end	Night	54% 2 7% 23% 13% 1	5467
April	1st to 10th	Night	47% 3 8% 23% 16% 2	2156
	11th to 20th	Night	43% 3 10% 21% 17% 6%	1866
	21st to end	Night	46% 4 10% 23% 15% 2	1800
May	1st to 10th	Night	55% 4 10% 19% 9% 3	4060
	11th to 20th	Night	48% 6% 13% 22% 10% 1	4033
	21st to end	Night	44% 4 14% 24% 11% 3	4429
June	1st to 10th	Night	35% 4 14% 24% 20% 3	1800
	11th to 20th	Night	33% 5% 16% 21% 22% 3	1800
	21st to end	Night	32% 6% 17% 25% 18% 3	1800
July	1st to 10th	Night	36% 5% 12% 25% 17% 4	2885
	11th to 20th	Night	33% 4 15% 28% 19% 1	2876
	21st to end	Night	45% 5% 11% 22% 13% 4	2988
August	1st to 10th	Night	35% 6% 13% 33% 12% 1	2814
	11th to 20th	Night	40% 5% 10% 25% 19% 1	2812
	21st to end	Night	22% 4 16% 32% 22% 4	3306
September	1st to 10th	Night	33% 3 13% 24% 19% 7%	2057
	11th to 20th	Night	33% 3 9% 25% 27% 3	2090
	21st to end	Night	39% 3 7% 22% 26% 2	2090
October	1st to 10th	Night	26% 2 7% 25% 37% 3	3527
	11th to 20th	Night	27% 3 7% 27% 30% 6%	3779
	21st to end	Night	33% 2 8% 23% 26% 8%	4205
November	1st to 10th	Night	38% 3 8% 24% 23% 3	2536
	11th to 20th	Night	48% 2 6% 21% 20% 4	2600
	21st to end	Night	53% 2 8% 22% 13% 2	2600
December	1st to 10th	Night	52% 2 10% 24% 10% 1	4250
	11th to 20th	Night	59% 2 11% 19% 7% 1	4824
	21st to end	Night	68% 3 8% 16% 6%	6490

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	33% 21% 30% 13% 3	300
	11th to 20th	Day	38% 25% 30% 7% 1	300
	21st to end	Day	33% 28% 27% 11% 2	330
February	1st to 10th	Day	33% 25% 29% 10% 2	300
	11th to 20th	Day	41% 26% 27% 4 2	300
	21st to end	Day	37% 23% 34% 6%	249
March	1st to 10th	Day	34% 21% 30% 12% 4	375
	11th to 20th	Day	28% 21% 32% 14% 5%	500
	21st to end	Day	36% 18% 27% 15% 3	550
April	1st to 10th	Day	27% 20% 27% 17% 8% 1	500
	11th to 20th	Day	27% 20% 25% 16% 8% 2	492
	21st to end	Day	26% 18% 34% 16% 5% 2	500
May	1st to 10th	Day	26% 17% 31% 17% 8% 2	503
	11th to 20th	Day	24% 12% 30% 21% 9% 5%	527
	21st to end	Day	16% 12% 26% 26% 14% 6%	573
June	1st to 10th	Day	15% 9% 25% 31% 14% 6%	530
	11th to 20th	Day	13% 7% 28% 29% 16% 7%	530
	21st to end	Day	13% 8% 30% 23% 18% 8%	530
July	1st to 10th	Day	15% 4 18% 25% 19% 19%	525
	11th to 20th	Day	11% 7% 23% 29% 20% 10%	524
	21st to end	Day	14% 4 21% 28% 22% 11%	532
August	1st to 10th	Day	12% 6% 19% 27% 23% 13%	486
	11th to 20th	Day	11% 5% 21% 33% 22% 8%	479
	21st to end	Day	8% 5% 19% 31% 25% 12%	539
September	1st to 10th	Day	14% 5% 19% 27% 26% 10%	490
	11th to 20th	Day	9% 3 19% 27% 31% 11%	490
	21st to end	Day	22% 10% 19% 25% 20% 4	490
October	1st to 10th	Day	12% 7% 26% 31% 18% 6%	490
	11th to 20th	Day	17% 16% 29% 21% 12% 5%	490
	21st to end	Day	26% 12% 32% 22% 8%	487
November	1st to 10th	Day	18% 18% 32% 24% 6% 1	300
	11th to 20th	Day	30% 16% 31% 16% 6% 2	300
	21st to end	Day	27% 21% 34% 14% 4 1	300
December	1st to 10th	Day	30% 23% 31% 12% 4	300
	11th to 20th	Day	30% 19% 33% 13% 4 1	300
	21st to end	Day	32% 22% 32% 11% 3	330
January	1st to 10th	Night	31% 19% 28% 18% 5%	800
	11th to 20th	Night	32% 19% 30% 17% 2	800
	21st to end	Night	25% 21% 30% 21% 2	880
February	1st to 10th	Night	31% 17% 28% 22% 2	800
	11th to 20th	Night	33% 20% 27% 19% 1	783
	21st to end	Night	30% 19% 31% 18% 2	660
March	1st to 10th	Night	25% 18% 29% 24% 5%	800
	11th to 20th	Night	25% 18% 31% 19% 7%	800
	21st to end	Night	25% 14% 30% 24% 7%	880
April	1st to 10th	Night	23% 15% 30% 23% 8%	756
	11th to 20th	Night	19% 12% 31% 25% 11% 1	593
	21st to end	Night	22% 9% 33% 29% 7%	600
May	1st to 10th	Night	20% 11% 28% 29% 11% 1	600
	11th to 20th	Night	19% 7% 25% 31% 17% 1	591
	21st to end	Night	10% 7% 21% 32% 26% 4	650
June	1st to 10th	Night	9% 4 21% 42% 23% 2	600
	11th to 20th	Night	9% 3 18% 38% 27% 4	600
	21st to end	Night	8% 4 22% 34% 29% 4	600
July	1st to 10th	Night	12% 3 14% 31% 30% 9%	585
	11th to 20th	Night	8% 2 14% 36% 37% 4	582
	21st to end	Night	10% 3 12% 38% 33% 4	624
August	1st to 10th	Night	8% 3 10% 37% 37% 5%	567
	11th to 20th	Night	8% 2 12% 38% 32% 7%	564
	21st to end	Night	8% 2 13% 33% 37% 7%	627
September	1st to 10th	Night	13% 2 12% 33% 33% 6%	727
	11th to 20th	Night	10% 4 15% 30% 33% 8%	760
	21st to end	Night	15% 6% 20% 33% 23% 3	760
October	1st to 10th	Night	12% 6% 22% 32% 25% 4	760
	11th to 20th	Night	18% 11% 26% 30% 15% 1	760
	21st to end	Night	23% 12% 26% 27% 12% 1	836
November	1st to 10th	Night	18% 10% 24% 28% 18% 1	780
	11th to 20th	Night	24% 16% 27% 23% 10% 1	800
	21st to end	Night	27% 12% 24% 29% 8% 1	800
December	1st to 10th	Night	31% 17% 26% 21% 6%	800
	11th to 20th	Night	28% 14% 31% 22% 5% 1	800
	21st to end	Night	30% 16% 27% 23% 4	880

Page generated at 2025-07-18 01:10:05. Version ac07

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