Blue Creek Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Creek Observing Site clear sky chart from 2005-11-06 to 2026-04-02.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	131139
	Night	14% 10% 4 8% 5% 4 4 4 5% 7% 37%	130806
Transparency	Day	46% 6% 11% 22% 14% 2	134780
	Night	46% 5% 10% 22% 15% 2	135237
Seeing	Day	17% 14% 28% 22% 13% 5%	17254
	Night	18% 11% 28% 27% 14% 2	27128

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% 14% 6% 10% 5% 5% 4 3 5% 6% 25%	4691
	11th to 20th	Day	11% 13% 5% 15% 7% 6% 5% 2 4 5% 28%	4803
	21st to end	Day	18% 13% 6% 10% 7% 5% 5% 4 4 5% 22%	5780
February	1st to 10th	Day	17% 14% 6% 8% 5% 3 3 3 4 7% 30%	2042
	11th to 20th	Day	15% 12% 5% 10% 5% 4 4 3 5% 6% 32%	2048
	21st to end	Day	18% 12% 5% 10% 4 3 4 2 4 6% 33%	1730
March	1st to 10th	Day	19% 14% 6% 9% 4 4 4 2 4 4 31%	4580
	11th to 20th	Day	12% 10% 3 6% 4 4 4 4 6% 7% 41%	5043
	21st to end	Day	17% 10% 3 7% 5% 4 5% 4 5% 7% 33%	5619
April	1st to 10th	Day	15% 11% 3 7% 5% 4 4 5% 6% 7% 34%	2412
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 3 5% 6% 39%	2398
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 6% 7% 34%	2600
May	1st to 10th	Day	21% 9% 3 7% 5% 5% 5% 5% 6% 8% 28%	5862
	11th to 20th	Day	20% 8% 2 7% 4 4 4 3 5% 7% 35%	5861
	21st to end	Day	14% 6% 3 8% 5% 6% 5% 5% 8% 9% 32%	6433
June	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 6% 7% 42%	2689
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 3 6% 9% 43%	2800
	21st to end	Day	9% 6% 3 7% 5% 4 5% 5% 7% 10% 40%	2800
July	1st to 10th	Day	10% 6% 3 6% 4 4 5% 4 6% 10% 41%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 10% 37%	6322
	21st to end	Day	10% 7% 3 8% 6% 6% 7% 5% 7% 9% 31%	6424
August	1st to 10th	Day	8% 5% 2 6% 4 4 4 4 7% 11% 45%	4234
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 4 7% 11% 41%	4118
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 9% 53%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 5% 5% 4 5% 6% 47%	2308
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 6% 7% 49%	2200
	21st to end	Day	14% 7% 2 5% 4 3 4 3 4 6% 47%	2200
October	1st to 10th	Day	10% 5% 2 7% 4 4 4 4 4 6% 50%	2200
	11th to 20th	Day	11% 6% 3 5% 4 4 4 3 6% 6% 47%	2063
	21st to end	Day	19% 10% 3 6% 4 4 3 3 5% 7% 38%	2000
November	1st to 10th	Day	9% 8% 2 6% 4 4 4 2 4 5% 52%	1852
	11th to 20th	Day	14% 11% 5% 8% 4 4 4 3 6% 5% 36%	1890
	21st to end	Day	18% 11% 4 8% 5% 3 3 2 4 6% 35%	1890
December	1st to 10th	Day	17% 11% 3 8% 4 3 3 2 4 5% 40%	3020
	11th to 20th	Day	17% 14% 6% 11% 5% 4 4 4 5% 4 27%	3463
	21st to end	Day	22% 20% 5% 9% 5% 4 4 3 4 5% 19%	4092
January	1st to 10th	Night	14% 14% 6% 12% 7% 5% 5% 4 5% 6% 22%	8232
	11th to 20th	Night	11% 15% 7% 15% 6% 5% 5% 3 4 5% 23%	8164
	21st to end	Night	17% 12% 5% 8% 4 4 4 3 4 5% 32%	8401
February	1st to 10th	Night	19% 12% 5% 9% 6% 4 3 4 5% 5% 28%	2576
	11th to 20th	Night	16% 11% 3 8% 5% 5% 4 4 5% 6% 33%	2458
	21st to end	Night	19% 13% 4 8% 4 3 3 3 5% 5% 32%	2076
March	1st to 10th	Night	19% 12% 3 6% 4 4 3 3 4 5% 37%	5493
	11th to 20th	Night	11% 7% 3 7% 4 4 4 3 5% 7% 45%	5121
	21st to end	Night	17% 9% 3 8% 4 4 5% 4 5% 6% 34%	5588
April	1st to 10th	Night	15% 9% 3 6% 5% 4 4 4 6% 7% 37%	2016
	11th to 20th	Night	15% 8% 3 6% 4 4 5% 4 6% 7% 39%	1950
	21st to end	Night	16% 7% 2 7% 5% 4 5% 4 6% 8% 37%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 4 5% 4 5% 7% 29%	4033
	11th to 20th	Night	16% 8% 3 7% 5% 4 4 5% 6% 8% 35%	4032
	21st to end	Night	13% 7% 2 6% 5% 5% 4 4 6% 8% 39%	4421
June	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 4 4 8% 48%	1706
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 4 7% 10% 45%	1600
	21st to end	Night	6% 3 2 4 5% 4 4 5% 8% 9% 49%	1600
July	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 48%	3618
	11th to 20th	Night	8% 5% 2 5% 5% 5% 5% 4 6% 10% 45%	3651
	21st to end	Night	9% 6% 2 7% 4 4 5% 4 8% 10% 39%	4427
August	1st to 10th	Night	9% 6% 2 6% 5% 5% 4 4 6% 9% 45%	2915
	11th to 20th	Night	7% 7% 2 6% 5% 4 5% 3 6% 8% 48%	2927
	21st to end	Night	4 3 2 4 4 4 4 3 6% 10% 57%	3586
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 52%	2070
	11th to 20th	Night	8% 5% 1 5% 4 3 5% 3 4 7% 55%	2200
	21st to end	Night	14% 6% 2 5% 4 3 3 3 4 6% 49%	2200
October	1st to 10th	Night	10% 4 2 5% 3 4 3 3 5% 7% 56%	2200
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 4 6% 47%	2335
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 4 6% 41%	2835
November	1st to 10th	Night	12% 7% 2 5% 4 3 4 3 5% 6% 48%	2666
	11th to 20th	Night	15% 8% 3 7% 4 4 3 3 6% 7% 40%	2730
	21st to end	Night	18% 12% 3 8% 6% 4 3 3 5% 6% 32%	2730
December	1st to 10th	Night	17% 11% 4 7% 4 4 3 3 4 5% 38%	4380
	11th to 20th	Night	18% 14% 6% 10% 5% 4 5% 3 4 5% 26%	4952
	21st to end	Night	24% 17% 6% 10% 5% 4 4 3 4 5% 18%	7117

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	60% 4 11% 19% 6%	4692
	11th to 20th	Day	58% 4 9% 15% 13% 1	4803
	21st to end	Day	64% 3 8% 16% 6% 3	5780
February	1st to 10th	Day	55% 5% 11% 18% 10% 1	2042
	11th to 20th	Day	54% 5% 10% 21% 10% 1	2058
	21st to end	Day	54% 3 10% 22% 9% 2	1730
March	1st to 10th	Day	56% 4 6% 18% 13% 3	4580
	11th to 20th	Day	42% 5% 9% 28% 15% 1	5043
	21st to end	Day	50% 5% 13% 19% 11% 2	5621
April	1st to 10th	Day	47% 7% 11% 21% 13% 1	2412
	11th to 20th	Day	47% 5% 10% 22% 13% 3	2398
	21st to end	Day	47% 6% 10% 23% 12% 2	2600
May	1st to 10th	Day	53% 7% 12% 17% 11% 1	5897
	11th to 20th	Day	49% 6% 11% 24% 10%	5865
	21st to end	Day	44% 8% 14% 21% 12% 1	6447
June	1st to 10th	Day	39% 8% 10% 24% 16% 3	2689
	11th to 20th	Day	37% 8% 14% 21% 19% 2	2800
	21st to end	Day	38% 7% 15% 20% 16% 4	2800
July	1st to 10th	Day	38% 6% 10% 26% 19% 1	6291
	11th to 20th	Day	39% 7% 13% 25% 15% 1	6247
	21st to end	Day	47% 7% 13% 18% 13% 2	6240
August	1st to 10th	Day	32% 7% 15% 32% 13% 1	4221
	11th to 20th	Day	35% 6% 12% 32% 15% 1	4157
	21st to end	Day	28% 7% 13% 30% 19% 4	4311
September	1st to 10th	Day	36% 6% 10% 24% 19% 5%	2308
	11th to 20th	Day	32% 6% 12% 26% 22% 3	2200
	21st to end	Day	37% 5% 10% 23% 22% 2	2200
October	1st to 10th	Day	24% 4 8% 31% 32% 2	3590
	11th to 20th	Day	29% 4 9% 30% 26% 2	3342
	21st to end	Day	38% 3 9% 24% 24% 2	3212
November	1st to 10th	Day	35% 4 9% 30% 19% 3	1852
	11th to 20th	Day	48% 4 8% 21% 15% 3	1890
	21st to end	Day	52% 4 9% 22% 13% 1	1890
December	1st to 10th	Day	48% 4 9% 26% 12%	3020
	11th to 20th	Day	60% 5% 8% 18% 8%	3460
	21st to end	Day	68% 4 10% 12% 6%	4092
January	1st to 10th	Night	61% 4 10% 16% 8%	8233
	11th to 20th	Night	63% 4 8% 12% 11% 2	8164
	21st to end	Night	55% 3 9% 22% 10% 1	8401
February	1st to 10th	Night	58% 5% 10% 20% 7% 1	2576
	11th to 20th	Night	52% 5% 11% 20% 11% 1	2459
	21st to end	Night	54% 4 10% 19% 10% 2	2076
March	1st to 10th	Night	50% 4 6% 26% 11% 3	5493
	11th to 20th	Night	38% 5% 11% 27% 18% 1	5121
	21st to end	Night	49% 5% 8% 26% 11% 1	5588
April	1st to 10th	Night	45% 6% 10% 24% 12% 2	2016
	11th to 20th	Night	41% 7% 11% 20% 17% 5%	1950
	21st to end	Night	44% 7% 11% 21% 14% 3	1800
May	1st to 10th	Night	53% 7% 10% 19% 8% 2	4060
	11th to 20th	Night	46% 6% 13% 25% 9% 1	4033
	21st to end	Night	40% 8% 17% 23% 11% 3	4429
June	1st to 10th	Night	33% 8% 10% 25% 21% 2	1706
	11th to 20th	Night	32% 7% 14% 22% 21% 4	1600
	21st to end	Night	29% 9% 17% 22% 19% 5%	1600
July	1st to 10th	Night	33% 7% 11% 24% 20% 4	3580
	11th to 20th	Night	33% 7% 15% 28% 15% 1	3606
	21st to end	Night	37% 7% 14% 26% 13% 3	4297
August	1st to 10th	Night	35% 7% 13% 31% 12% 1	2904
	11th to 20th	Night	33% 6% 12% 31% 17% 1	2954
	21st to end	Night	23% 6% 14% 32% 22% 3	3586
September	1st to 10th	Night	32% 6% 12% 24% 19% 7%	2070
	11th to 20th	Night	28% 6% 10% 26% 27% 3	2200
	21st to end	Night	37% 4 9% 22% 25% 3	2200
October	1st to 10th	Night	21% 3 7% 26% 41% 3	3590
	11th to 20th	Night	28% 3 9% 24% 34% 2	3795
	21st to end	Night	35% 4 7% 23% 26% 5%	4573
November	1st to 10th	Night	37% 4 10% 23% 23% 3	2666
	11th to 20th	Night	43% 4 10% 23% 17% 4	2730
	21st to end	Night	54% 4 10% 21% 11% 1	2730
December	1st to 10th	Night	49% 3 11% 25% 11%	4380
	11th to 20th	Night	60% 4 10% 17% 9% 1	4954
	21st to end	Night	70% 4 9% 13% 4	7117

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	25% 23% 34% 15% 3 1	330
	11th to 20th	Day	30% 28% 30% 10% 2	330
	21st to end	Day	27% 28% 30% 13% 2	363
February	1st to 10th	Day	23% 28% 33% 14% 2	330
	11th to 20th	Day	25% 30% 34% 10% 1	330
	21st to end	Day	25% 25% 41% 8% 1	273
March	1st to 10th	Day	25% 29% 32% 12% 2	330
	11th to 20th	Day	25% 24% 34% 13% 3 1	519
	21st to end	Day	29% 20% 31% 14% 5% 1	594
April	1st to 10th	Day	22% 22% 31% 17% 7% 2	504
	11th to 20th	Day	21% 23% 26% 19% 9% 2	492
	21st to end	Day	21% 20% 33% 16% 8% 2	500
May	1st to 10th	Day	24% 14% 35% 16% 7% 3	503
	11th to 20th	Day	22% 14% 26% 23% 11% 5%	527
	21st to end	Day	14% 10% 29% 23% 15% 9%	573
June	1st to 10th	Day	13% 9% 30% 27% 17% 5%	619
	11th to 20th	Day	11% 7% 26% 33% 17% 6%	730
	21st to end	Day	12% 8% 26% 30% 17% 7%	730
July	1st to 10th	Day	12% 4 18% 29% 22% 14%	721
	11th to 20th	Day	7% 5% 25% 32% 22% 9%	706
	21st to end	Day	12% 4 18% 32% 23% 11%	576
August	1st to 10th	Day	8% 4 20% 28% 24% 16%	526
	11th to 20th	Day	10% 5% 22% 30% 20% 13%	519
	21st to end	Day	7% 5% 18% 27% 29% 13%	583
September	1st to 10th	Day	11% 5% 18% 29% 30% 7%	530
	11th to 20th	Day	10% 2 17% 26% 30% 15%	530
	21st to end	Day	18% 10% 19% 23% 22% 7%	530
October	1st to 10th	Day	10% 7% 25% 30% 21% 8%	530
	11th to 20th	Day	13% 15% 33% 22% 13% 3	530
	21st to end	Day	25% 15% 39% 17% 3 1	383
November	1st to 10th	Day	14% 20% 35% 21% 8% 2	330
	11th to 20th	Day	22% 24% 27% 20% 5% 2	330
	21st to end	Day	28% 16% 37% 16% 3	330
December	1st to 10th	Day	23% 23% 36% 13% 4	330
	11th to 20th	Day	24% 23% 39% 10% 4	330
	21st to end	Day	31% 23% 30% 13% 2	363
January	1st to 10th	Night	22% 20% 33% 20% 4	1050
	11th to 20th	Night	25% 21% 36% 16% 2	1012
	21st to end	Night	24% 18% 35% 20% 2	924
February	1st to 10th	Night	25% 15% 36% 22% 2	840
	11th to 20th	Night	23% 20% 35% 19% 2	823
	21st to end	Night	27% 17% 36% 20% 1	692
March	1st to 10th	Night	23% 18% 33% 23% 3	840
	11th to 20th	Night	20% 17% 38% 20% 5% 1	840
	21st to end	Night	24% 14% 32% 25% 6%	924
April	1st to 10th	Night	19% 13% 36% 25% 6%	609
	11th to 20th	Night	21% 13% 29% 27% 9% 1	593
	21st to end	Night	20% 10% 39% 22% 9% 1	600
May	1st to 10th	Night	20% 11% 34% 24% 11% 1	600
	11th to 20th	Night	17% 6% 28% 29% 17% 3	591
	21st to end	Night	11% 4 27% 30% 26% 3	650
June	1st to 10th	Night	11% 4 24% 41% 20% 1	600
	11th to 20th	Night	10% 4 22% 40% 22% 2	600
	21st to end	Night	7% 3 26% 39% 24% 3	600
July	1st to 10th	Night	11% 2 16% 32% 31% 8%	585
	11th to 20th	Night	8% 4 17% 35% 31% 4	582
	21st to end	Night	11% 4 13% 38% 31% 4	657
August	1st to 10th	Night	7% 3 14% 37% 35% 4	597
	11th to 20th	Night	8% 1 15% 38% 33% 5%	594
	21st to end	Night	6% 2 17% 34% 36% 5%	660
September	1st to 10th	Night	13% 4 15% 34% 30% 4	670
	11th to 20th	Night	9% 3 16% 33% 33% 6%	800
	21st to end	Night	17% 5% 23% 30% 23% 3	800
October	1st to 10th	Night	12% 7% 22% 32% 23% 4	800
	11th to 20th	Night	16% 11% 28% 30% 14% 2	800
	21st to end	Night	20% 14% 33% 23% 10%	880
November	1st to 10th	Night	15% 10% 33% 27% 14% 1	820
	11th to 20th	Night	20% 17% 29% 24% 8% 1	840
	21st to end	Night	24% 16% 25% 29% 6%	840
December	1st to 10th	Night	25% 17% 33% 21% 5%	840
	11th to 20th	Night	23% 15% 35% 21% 5% 1	840
	21st to end	Night	29% 19% 31% 17% 3	1135

Page generated at 2026-03-30 02:18:41. Version ac07

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