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 2024-12-09.

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 7% 5% 4 4 4 6% 7% 37%	101452
	Night	14% 9% 3 7% 5% 4 4 3 5% 7% 38%	98314
Transparency	Day	44% 6% 11% 23% 15% 2	105135
	Night	44% 6% 11% 22% 16% 2	102770
Seeing	Day	17% 14% 27% 23% 13% 5%	15528
	Night	18% 11% 28% 27% 14% 2	25318

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	15% 17% 6% 8% 4 5% 4 3 5% 5% 27%	2527
	11th to 20th	Day	16% 13% 5% 14% 5% 7% 6% 3 5% 4 22%	2571
	21st to end	Day	27% 17% 9% 8% 5% 3 3 2 3 4 19%	3124
February	1st to 10th	Day	17% 14% 6% 8% 5% 3 4 3 4 6% 30%	1847
	11th to 20th	Day	14% 11% 5% 10% 5% 4 4 4 5% 6% 32%	1848
	21st to end	Day	19% 13% 5% 9% 4 3 4 2 4 5% 31%	1570
March	1st to 10th	Day	22% 17% 4 8% 4 4 3 3 4 5% 27%	3120
	11th to 20th	Day	14% 8% 3 5% 4 4 4 4 6% 6% 43%	3448
	21st to end	Day	17% 9% 3 7% 5% 4 5% 4 5% 7% 34%	3836
April	1st to 10th	Day	14% 11% 3 7% 5% 4 4 5% 6% 7% 34%	2280
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 3 5% 6% 39%	2277
	21st to end	Day	18% 8% 3 7% 5% 4 4 3 6% 7% 34%	2470
May	1st to 10th	Day	21% 9% 3 7% 4 5% 4 5% 5% 8% 29%	4082
	11th to 20th	Day	21% 8% 2 6% 3 3 4 3 4 7% 38%	4081
	21st to end	Day	11% 6% 3 8% 5% 6% 5% 5% 8% 9% 35%	4476
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	2554
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 3 6% 8% 45%	2660
	21st to end	Day	9% 6% 3 7% 5% 4 5% 5% 7% 10% 40%	2660
July	1st to 10th	Day	13% 6% 3 6% 5% 4 5% 4 7% 10% 37%	4440
	11th to 20th	Day	7% 4 1 5% 5% 4 5% 4 7% 11% 46%	4416
	21st to end	Day	10% 7% 3 8% 6% 7% 7% 5% 8% 9% 31%	4466
August	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 7% 11% 44%	4104
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 4 7% 11% 41%	3990
	21st to end	Day	6% 4 2 5% 4 4 4 4 5% 9% 53%	4180
September	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 4 6% 6% 46%	2193
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 6% 7% 48%	2090
	21st to end	Day	14% 7% 2 5% 4 3 4 3 4 6% 47%	2090
October	1st to 10th	Day	10% 5% 2 7% 4 5% 4 4 4 6% 49%	2090
	11th to 20th	Day	11% 7% 3 6% 4 4 4 3 6% 6% 47%	1960
	21st to end	Day	19% 10% 3 6% 4 4 3 3 5% 7% 38%	1900
November	1st to 10th	Day	9% 7% 2 6% 4 4 4 2 4 5% 53%	1762
	11th to 20th	Day	14% 11% 5% 8% 4 4 4 3 6% 5% 36%	1800
	21st to end	Day	18% 11% 4 7% 6% 3 3 2 4 6% 36%	1800
December	1st to 10th	Day	19% 13% 4 9% 5% 4 4 3 5% 5% 29%	1782
	11th to 20th	Day	15% 14% 6% 10% 6% 4 4 3 4 4 30%	2153
	21st to end	Day	21% 22% 6% 9% 4 4 4 3 4 5% 19%	2805
January	1st to 10th	Night	16% 16% 6% 10% 6% 4 4 4 5% 5% 23%	4428
	11th to 20th	Night	14% 17% 6% 15% 6% 5% 4 3 4 5% 21%	4380
	21st to end	Night	26% 17% 7% 8% 4 3 3 2 3 4 23%	4532
February	1st to 10th	Night	20% 12% 4 9% 5% 4 4 4 5% 5% 28%	2332
	11th to 20th	Night	14% 11% 3 8% 5% 6% 4 4 5% 7% 33%	2218
	21st to end	Night	20% 14% 4 8% 4 4 3 3 5% 5% 30%	1884
March	1st to 10th	Night	21% 13% 3 7% 4 4 4 3 4 5% 32%	3733
	11th to 20th	Night	11% 8% 3 6% 4 4 4 3 5% 7% 46%	3496
	21st to end	Night	17% 8% 3 6% 5% 4 4 3 4 6% 40%	3817
April	1st to 10th	Night	13% 9% 3 6% 5% 4 4 4 6% 7% 38%	1906
	11th to 20th	Night	15% 8% 3 5% 4 4 5% 4 6% 7% 39%	1851
	21st to end	Night	17% 7% 2 7% 5% 4 5% 4 6% 7% 37%	1710
May	1st to 10th	Night	21% 11% 3 9% 5% 4 5% 3 5% 7% 27%	2813
	11th to 20th	Night	18% 7% 3 6% 4 3 4 4 6% 7% 39%	2812
	21st to end	Night	9% 6% 2 5% 4 4 4 4 6% 9% 46%	3079
June	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 4 4 8% 47%	1621
	11th to 20th	Night	7% 5% 2 5% 4 5% 4 4 8% 10% 46%	1520
	21st to end	Night	6% 3 2 5% 5% 4 4 5% 8% 9% 49%	1520
July	1st to 10th	Night	9% 6% 2 5% 4 4 4 4 6% 8% 48%	2530
	11th to 20th	Night	6% 4 1 4 4 4 4 4 6% 9% 54%	2551
	21st to end	Night	11% 7% 2 6% 5% 4 5% 4 8% 10% 37%	3087
August	1st to 10th	Night	9% 6% 2 6% 5% 5% 4 4 6% 9% 45%	2825
	11th to 20th	Night	7% 7% 2 6% 5% 4 5% 3 6% 8% 48%	2835
	21st to end	Night	3 3 2 4 4 4 4 3 6% 10% 57%	3476
September	1st to 10th	Night	11% 6% 2 5% 3 3 4 3 5% 6% 51%	1966
	11th to 20th	Night	8% 5% 1 5% 4 3 5% 3 5% 8% 54%	2090
	21st to end	Night	14% 6% 2 6% 4 3 3 3 4 6% 49%	2090
October	1st to 10th	Night	10% 4 2 5% 3 4 3 3 5% 7% 56%	2090
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 4 6% 46%	2218
	21st to end	Night	16% 7% 3 7% 5% 4 4 3 4 6% 41%	2693
November	1st to 10th	Night	12% 7% 2 5% 5% 3 4 3 4 6% 48%	2536
	11th to 20th	Night	15% 8% 2 7% 4 4 3 3 6% 7% 40%	2600
	21st to end	Night	17% 12% 4 7% 6% 4 3 3 5% 6% 32%	2600
December	1st to 10th	Night	19% 12% 4 7% 5% 4 4 3 5% 6% 31%	2581
	11th to 20th	Night	18% 13% 5% 8% 5% 4 4 3 4 5% 30%	3042
	21st to end	Night	21% 17% 6% 10% 5% 4 5% 3 4 5% 18%	4852

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	58% 5% 9% 18% 10%	2528
	11th to 20th	Day	62% 6% 11% 14% 6%	2571
	21st to end	Day	70% 4 8% 14% 3 1	3124
February	1st to 10th	Day	56% 5% 11% 17% 11% 1	1847
	11th to 20th	Day	53% 5% 10% 21% 10% 1	1858
	21st to end	Day	56% 3 9% 21% 8% 2	1570
March	1st to 10th	Day	60% 5% 8% 18% 9% 1	3120
	11th to 20th	Day	40% 6% 10% 26% 16% 1	3448
	21st to end	Day	48% 6% 13% 21% 10% 1	3836
April	1st to 10th	Day	46% 7% 11% 21% 13% 1	2280
	11th to 20th	Day	46% 5% 10% 21% 14% 3	2277
	21st to end	Day	47% 6% 10% 22% 12% 2	2470
May	1st to 10th	Day	52% 8% 13% 19% 8% 1	4120
	11th to 20th	Day	46% 7% 13% 23% 12% 1	4085
	21st to end	Day	40% 9% 16% 18% 15% 2	4489
June	1st to 10th	Day	40% 8% 11% 24% 16% 3	2554
	11th to 20th	Day	36% 8% 14% 21% 19% 2	2660
	21st to end	Day	38% 7% 15% 20% 15% 4	2660
July	1st to 10th	Day	41% 6% 11% 21% 18% 1	4371
	11th to 20th	Day	31% 7% 11% 30% 20% 1	4341
	21st to end	Day	46% 8% 13% 16% 13% 3	4324
August	1st to 10th	Day	32% 8% 15% 31% 13% 1	4091
	11th to 20th	Day	35% 6% 12% 32% 15% 1	4029
	21st to end	Day	28% 6% 14% 30% 19% 4	4179
September	1st to 10th	Day	37% 6% 11% 23% 19% 6%	2193
	11th to 20th	Day	33% 6% 12% 25% 21% 3	2090
	21st to end	Day	37% 5% 10% 24% 21% 2	2090
October	1st to 10th	Day	24% 4 8% 31% 32% 2	3480
	11th to 20th	Day	29% 4 9% 30% 26% 2	3239
	21st to end	Day	37% 3 9% 24% 24% 2	3112
November	1st to 10th	Day	35% 4 9% 30% 19% 3	1762
	11th to 20th	Day	48% 4 9% 21% 15% 3	1800
	21st to end	Day	51% 4 9% 22% 13% 1	1800
December	1st to 10th	Day	58% 4 11% 19% 8% 1	1782
	11th to 20th	Day	57% 5% 8% 18% 11%	2150
	21st to end	Day	69% 5% 9% 11% 7%	2805
January	1st to 10th	Night	61% 6% 10% 14% 9%	4428
	11th to 20th	Night	66% 4 10% 13% 7%	4380
	21st to end	Night	66% 4 7% 16% 7% 1	4532
February	1st to 10th	Night	58% 4 10% 19% 7% 1	2332
	11th to 20th	Night	51% 5% 12% 20% 12% 1	2219
	21st to end	Night	56% 5% 10% 17% 10% 2	1884
March	1st to 10th	Night	52% 5% 8% 23% 11% 1	3733
	11th to 20th	Night	37% 6% 13% 25% 18% 2	3496
	21st to end	Night	45% 6% 9% 28% 11% 1	3817
April	1st to 10th	Night	44% 6% 10% 25% 13% 2	1906
	11th to 20th	Night	41% 7% 11% 19% 18% 5%	1851
	21st to end	Night	45% 7% 11% 21% 14% 3	1710
May	1st to 10th	Night	55% 8% 10% 19% 7% 1	2840
	11th to 20th	Night	43% 7% 14% 26% 9% 1	2813
	21st to end	Night	31% 9% 16% 26% 14% 4	3087
June	1st to 10th	Night	33% 8% 11% 26% 20% 2	1621
	11th to 20th	Night	30% 7% 14% 23% 22% 4	1520
	21st to end	Night	29% 9% 16% 23% 18% 5%	1520
July	1st to 10th	Night	34% 7% 11% 24% 18% 6%	2490
	11th to 20th	Night	25% 7% 12% 33% 20% 2	2503
	21st to end	Night	39% 7% 14% 23% 13% 4	2988
August	1st to 10th	Night	35% 7% 13% 31% 12% 1	2814
	11th to 20th	Night	33% 7% 12% 30% 18% 1	2862
	21st to end	Night	23% 6% 15% 32% 22% 3	3476
September	1st to 10th	Night	32% 6% 13% 23% 19% 7%	1966
	11th to 20th	Night	28% 6% 10% 26% 26% 4	2090
	21st to end	Night	37% 4 9% 23% 24% 3	2090
October	1st to 10th	Night	21% 3 7% 26% 40% 3	3480
	11th to 20th	Night	28% 3 9% 25% 34% 2	3678
	21st to end	Night	35% 4 8% 23% 26% 5%	4431
November	1st to 10th	Night	37% 4 10% 22% 22% 3	2536
	11th to 20th	Night	43% 4 10% 22% 17% 4	2600
	21st to end	Night	54% 4 10% 20% 11% 1	2600
December	1st to 10th	Night	54% 4 11% 20% 10% 1	2581
	11th to 20th	Night	55% 5% 10% 19% 11% 1	3044
	21st to end	Night	68% 5% 10% 12% 5%	4852

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	24% 24% 34% 14% 3 1	270
	11th to 20th	Day	30% 27% 30% 11% 2	270
	21st to end	Day	29% 25% 30% 13% 3	297
February	1st to 10th	Day	25% 30% 30% 13% 1	270
	11th to 20th	Day	25% 30% 33% 10% 1	270
	21st to end	Day	29% 23% 39% 8% 1	225
March	1st to 10th	Day	24% 31% 31% 11% 2	270
	11th to 20th	Day	24% 24% 34% 14% 3 1	441
	21st to end	Day	30% 20% 30% 13% 6% 1	506
April	1st to 10th	Day	21% 22% 31% 17% 7% 2	460
	11th to 20th	Day	21% 21% 25% 20% 10% 3	452
	21st to end	Day	22% 21% 34% 14% 7% 2	460
May	1st to 10th	Day	24% 15% 35% 16% 6% 4	463
	11th to 20th	Day	22% 14% 25% 23% 10% 5%	487
	21st to end	Day	12% 9% 29% 24% 16% 9%	529
June	1st to 10th	Day	13% 9% 29% 27% 17% 6%	574
	11th to 20th	Day	11% 7% 26% 33% 17% 6%	680
	21st to end	Day	12% 8% 27% 31% 15% 7%	680
July	1st to 10th	Day	12% 4 19% 29% 21% 15%	671
	11th to 20th	Day	7% 5% 25% 33% 21% 9%	657
	21st to end	Day	12% 4 19% 32% 23% 10%	532
August	1st to 10th	Day	9% 4 19% 29% 24% 15%	486
	11th to 20th	Day	11% 5% 23% 31% 19% 11%	479
	21st to end	Day	8% 6% 17% 26% 29% 14%	539
September	1st to 10th	Day	11% 4 18% 30% 29% 7%	490
	11th to 20th	Day	11% 2 18% 28% 29% 13%	490
	21st to end	Day	18% 10% 20% 24% 20% 7%	490
October	1st to 10th	Day	10% 7% 26% 30% 20% 7%	490
	11th to 20th	Day	13% 16% 34% 21% 12% 3	490
	21st to end	Day	24% 16% 39% 17% 3 1	349
November	1st to 10th	Day	14% 20% 34% 21% 9% 2	300
	11th to 20th	Day	21% 24% 27% 20% 5% 2	300
	21st to end	Day	27% 16% 37% 17% 3	300
December	1st to 10th	Day	23% 23% 35% 14% 5%	294
	11th to 20th	Day	22% 23% 38% 11% 5%	270
	21st to end	Day	31% 24% 29% 14% 3	297
January	1st to 10th	Night	22% 20% 33% 20% 4	950
	11th to 20th	Night	26% 20% 35% 17% 2	916
	21st to end	Night	26% 18% 33% 21% 2	836
February	1st to 10th	Night	26% 14% 36% 21% 2	760
	11th to 20th	Night	22% 22% 35% 19% 2	743
	21st to end	Night	28% 17% 34% 19% 1	628
March	1st to 10th	Night	22% 18% 34% 23% 3	760
	11th to 20th	Night	20% 16% 38% 19% 6% 1	760
	21st to end	Night	24% 13% 31% 25% 6%	836
April	1st to 10th	Night	18% 14% 36% 26% 7% 1	576
	11th to 20th	Night	21% 12% 29% 26% 10% 1	563
	21st to end	Night	20% 10% 39% 21% 9% 1	570
May	1st to 10th	Night	20% 11% 35% 23% 11% 1	570
	11th to 20th	Night	18% 6% 27% 29% 17% 3	561
	21st to end	Night	10% 3 26% 31% 27% 3	617
June	1st to 10th	Night	10% 4 24% 42% 20% 1	570
	11th to 20th	Night	8% 4 22% 40% 22% 2	570
	21st to end	Night	6% 3 26% 39% 23% 2	570
July	1st to 10th	Night	11% 3 16% 32% 30% 8%	555
	11th to 20th	Night	7% 4 17% 37% 31% 4	552
	21st to end	Night	11% 4 13% 39% 29% 4	624
August	1st to 10th	Night	8% 3 14% 36% 35% 4	567
	11th to 20th	Night	8% 1 16% 39% 31% 5%	564
	21st to end	Night	6% 2 16% 33% 38% 6%	627
September	1st to 10th	Night	14% 4 15% 33% 30% 5%	636
	11th to 20th	Night	9% 3 16% 34% 33% 6%	760
	21st to end	Night	16% 6% 24% 29% 23% 3	760
October	1st to 10th	Night	12% 7% 23% 32% 22% 4	760
	11th to 20th	Night	16% 11% 28% 29% 14% 1	760
	21st to end	Night	20% 13% 33% 23% 10%	836
November	1st to 10th	Night	16% 11% 32% 27% 14% 1	780
	11th to 20th	Night	20% 17% 29% 25% 9% 1	800
	21st to end	Night	24% 16% 26% 29% 5%	800
December	1st to 10th	Night	25% 17% 33% 21% 5%	795
	11th to 20th	Night	23% 15% 35% 21% 5% 1	760
	21st to end	Night	29% 19% 31% 18% 4	1026

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